Before I go into detail on any of the two stage to orbit (TSTO for the uninitiated) approaches that I mentioned in my post last week, I'd like to briefly discuss what I think is the key issue that drives the design and development tradeoffs for reusable TSTO launch vehicles. That issue is: how do you get the first stage back after a mission, and ready to fly again?

This article will focus on the key tradeoff that stems from this question: whether to try and recover the first stage downrange, or whether to try and perform some sort of return to launch site maneuver. The answer to this question is probably the number one driver of what approach one takes for developing a TSTO vehicle.

RTLS vs. Downrange Recovery
As I pointed out in my brief discussion about SSTO vs. TSTO approaches in Part I of this series, attaining orbit is mostly about building up a lot of horizontal velocity, and only a little bit about gaining vertical altitude. For performance optimized TSTO ELVs, the first stage often imparts a significant portion of the overall delta-V (especially for ELVs delivering satellites to GTO or GSO). This means that it ends up coming in hot, fast, and a long way downrange from the original launch site. Now, there are several different approaches to deal with this problem (or avoid it altogether).

One option is to just let the stage come down where it wants to, and recover it downrange. Downrange recovery can take several forms including recovering a stage out of the ocean after a splashdown, landing the stage at a downrange site and ferrying it back (either by rocket flight, a carrier plane, or by truck, train, or barge), or it could involve mid-air recovery of part or all of the first stage. While downrange recovery may is the general approach that probably imposes the smallest performance penalty, each of the actual approaches to down-range recovery have some pluses and minuses.

Splashdown Recovery
Let's take splashdown recovery first. Falcon-1 is an example of the splashdown recovery. The stage separates where a typical ELV would want to have a staging event, and then (hopefully) it's fished out of the ocean and refurbished for reuse. Some of the benefits of splashdown recovery:

1. Splashdown recovery is probably one of the easiest and best understood methods for recovering a traditional ELV-like first stage.
2. There's a large experience base to use as a foundation for carrying out such a design.
3. Even if your flight rate is low enough that it isn't saving you much money, you're still able to learn a lot from being able to perform post-flight inspection on the propulsion hardware. Thus, even if you aren't flying enough to save a lot of money via recovery, it will help your reliability.
4. Ocean splashdowns don't require anywhere near as heavy of recovery equipment as land parachute landings.
   

But they also have several drawbacks:

1. Trying to make a complicated rocket engine sea-water compatible, especially a turbopump-fed rocket engine, is not a trivial task. Material selection, and getting the stage out of the salt water (and cleaned out) as quick as possible are all required.
2. There's a lot of time and labor involved in hauling the stage back, cleaning it out, making sure nothing got damaged on reentry or splashdown, testing everything to make sure it's still in working order, etc. This fundamentally limits how frequently you can refly a given stage. It also translates into a lot of extra personnel and labor-hours required above and beyond what you would normally need to just build, test, and fly an expendable vehicle.
3. The wear and tear from ocean recovery, splash down, etc. are likely going to limit the number of reflights you can get on a stage or engine before major overhaul or outright replacement.
4. Your potential launch sites are limited, since you need a large body of water on which you can drop big heavy hardware. Most likely (for US entities) that means flying out of one of the existing ranges like Wallops, Vandenberg, or Canaveral. These locations, while excellent for flying missiles, and while also improving their commercial friendliness over time, are still a long way from the environment you want to be operating a reusable launch vehicle out of.
5. While it's possible to design a launch vehicle splashdown recovery first stage in such a way that a first stage failure doesn't necessarily imply the loss of your cargo, it is much harder to design such systems for graceful abort modes. Unless the upper stage is also designed for splashdown recovery (with the payload designed for it as well), a stage failure probably will result in loss of payload. This loses you one of the big potential advantages of reusability--graceful and intact aborts.

Mid-Air Recovery
The idea behind mid-air recovery is that instead of allowing the stage to crash down into the water, you instead snatch it (or a high-value part of it) out of the air using a helicopter or other sort of aircraft aircraft. This is similar to how Genesis was supposed to be recovered, and was the method used for recovering a lot of the film capsules from early spysats. There are actual serious players looking at this idea, but I don't know if it's supposed to be public knowledge yet, so that will have to be a post for another day. There was also a paper floating around by a company that does mid-air recovery work, including work for the SpaceHab ARCTUS project. If I can dig it up again, I'll probably post about that as well.

Anyhow, here are some of the benefits of mid-air recovery:

1. No salt water contamination in the rocket hardware! This greatly cuts down on the amount of work that needs to be done to turn a stage around. No need for decontamination. No need for stripping down hardware. Probably eliminates the need to "requalify" the propulsion system before reflight.
   
2. Gentle, low-shock recovery is much less likely to damage stage or propulsion hardware, also making it more likely that the hardware can just be reused after some inspection.
3. There are companies that specialize in this sort of thing, and you can just rent their services instead of trying to do this in-house. They aren't cheap, but they're a lot cheaper than building a new stage every time.
4. Your propulsion system is going to be in about as close to the same condition as it was when the engine shut down as you'll get for any recovery technique--this makes it a lot easier to get good reliable data on wear-and-tear on the engine, so you can improve the quality over time.
   

But here are some of the challenges:

1. Complex recovery technique. Sure, you can practice it a whole lot for not too much money, but there is some increased risk of failing with the rendezvous or recovery operations, which could occasionally cost you a stage.
2. Weight limits. Even with the latest techniques, which can recover payloads up to 80% of the maximum cargo capacity of the helicopters, you're still limited to around 22klb or less. Depending on the size of your stage, this may mean that you can only recover part of the stage (like say the engines). That'll still likely save at least some money, but it's not as big of a win as getting the whole stage back intact.
3. There may also be issues with trying to recover a big, but fluffy stage. Depending on the weight distribution, there could be some real oscillation issues (like what happened when they tried to move the Roton ATV under helicopter).
   
4. Range issues. Depending on how far downrange your stage comes back, you might need to also rent not just a helicopter, but some sort of barge to operate the helicopter off of. This will increase the amount of time it takes to turn a stage compared to if you could just fly it back.
   
5. Like with splashdown recovery, this method of recovery still doesn't give you graceful and intact recovery methods in the case of a first-stage failure. With dump valves and two helicopters, and a mid-air recoverable upper stage, you might be able to recover the payload over part of the trajectory, but you'll still have zones where a failure means sure loss of the payload (or a launch escape abort if you're flying people). It isn't a showstopper, but it does reduce the upside somewhat.
6. Due to challenges #1 and #5, you probably still need to launch out over the ocean, which means that once again you're still going to face the issue of launching out of an existing missile range. Basically, since there's a chance you could biff the in-air recovery, you have to do this over an unpopulated area. And since your vehicle doesn't likely have graceful failure modes, it's more like an existing ELV than a more traditional RLV, and will probably be treated as such by the FAA and the ranges. Not a showstopper either, and it might just be possible to pull this off with an over-land launch if you can find a sufficiently deserted area, but definitely a challenge.

Mid-air recovery is probably too weight constrained for something like a complete (but dry) Falcon IX first stage, but might be an interesting option for recovering the Falcon IX upper stage or the Falcon I first stage. It'd also probably be just the right size for recovering the first stage if they hadn't canceled the Falcon V. Other than the weight limit, there's some real benefits of this approach over the traditional splashdown technique.

Downrange On-Land Recovery
This type of recovery can take several forms. It could be a powered VTVL landing at a downrange pad. It could be a powered or glide landing for a HTHL. It could be a parachute and airbags landing (like Kistler, just downrange). But basically you have the thing land, on the land, downrange, and then fly the thing back, or ship it back.

Here are some of the benefits:

1. Much more efficient, performance-wise, than any of the RTLS approaches. You can still stage at the most optimal staging velocity, therefore making your upper stage design a lot easier. You also get a lot more payload per given takeoff (and dry) mass.
2. At least some of the RTLS approaches can also sometimes use this as a performance enhancing option--in case you need to launch a bigger payload than you can handle with a normal RTLS trajectory.
3. Unlike mid-air recovery, this recovery approach can scale up to fairly large sizes.
4. In emergency cases for RTLS approaches, you may want to be able to land your vehicle at alternative downrange sites anyway.
5. Unlike the other two downrange recovery options, this option is a lot more compatible with intact and graceful aborts.

And here are some of the challenges:

1. A given launch site will typically have its launch azimuths (directions in which you can launch) restricted a lot more for downrange land recovery than it will for an RTLS vehicle. This is because you need to have a suitable place downrange where you can actually land. This makes downrange recovery vehicles less flexible than RTLS capable vehicles.
2. You need facilities at both ends, especially if you intend to fly the stage back after landing.
   This may entail having almost as many launch support people at the downrange site as at the initial site, which greatly increases the fixed costs of such a system. Probably not quite double (since you don't have payload processing facilities there), but it's a non-trivial expense.
   
3. If you do a rocket powered return, you've now effectively halved both your flight rate (as you have to do two launches, two landings, two ground preps, etc. per a single paying flight), and halved the number of revenue generating flights you can get out of a given airframe. Both of these directly affect the bottom line.
   
4. If the return flight is a rocket-powered suborbital flight (as per AST's definitions), I think that each of your downrange sites will need to be an FAA licensed launch site, and you will need launch licenses for all of the return flights. Now, once you have one launch license to base things off of, getting additional ones should be easier, but its still extra paperwork. Also, your Ec and MPL calculations are going to be different for the return flight, because your IIP will move at different rates over different areas under your groundtrack for the two trajectories (not to mention mission-critical operations will occur with your IIP over a different location). All of this stuff has to be taken into account.
5. If you have a jet powered return (either using a carrier aircraft, or if the stage has built-in jet engines), you now need to deal with the aircraft side of FAA, which may entail getting the vehicle type-certified. I'm not certain, but having a vehicle that operates under both regimes is likely going to make things a lot harder, not easier. Being unusual is not a virtue when dealing with regulators. If you're using an existing carrier craft, that'll make things easier however, as it is purely a subsonic aircraft, and thus a lot closer to what FAA is used to dealing with.
   
6. If you try to return the stage via trucking or train, now the stage has to be "roadable". Which means making it skinny enough to fit on existing transports. While this may be feasible for some smaller, dense-propellant RLV stages (after all I think that Falcon IX is roadable), it is a constraint on the size of stage. And the aspect ratio roadability forces you into is not as ideal for VTVL stages. VTVL stages want to be shorter and squater than typical rocket stages.
   
7. If you return the stage via trucking or train, you now need heavy moving equipment at any downrange sites, experienced heavy equipment personnel there, and it's going to cost you a lot of extra time. All of these things add cost, and slow down your turn time.
   

Conclusions: The Case for RTLS
Now, I probably ought to clarify something. I don't think any of these downrange recovery ideas are stupid. If done right, they can save a lot of money compared to a purely expendable system, while also increasing reliability by allowing for post-flight inspections and the like. Especially with the downrange land-landing techniques, you can get all of the benefits of traditional RLVs.

In other words, while there are some challenges with downrange recovery, there are often some real benefits. There are some cases where using these downrange recovery approaches really is the best choice. SpaceX and the others looking at these approaches aren't being foolish by pursuing them. I just think that the inherent limitations of this sort of reuse (especially the first two options--splashdown and mid-air recovery) will probably prevent it from being a revolutionary as opposed to a modest, evolutionary improvement over a purely ELV approach. Now, in the near to medium term, even when RLVs first start flying, they'll likely be relatively quite small compared to the EELVs (for reasons I'll go into in a later post). Which means that approaches that allow existing ELVs to become somewhat more reusable, and improve their economics somewhat are actually useful. I think that while small RLVs will bump ELVs out of the people, light cargo, and propellant markets very quickly after they enter the field, there'll still be payloads that are too big for RLVs that are small enough to be economically viable in the near-to-medium term. The ULA's, SpaceX's, and Sea Launch's of the world will still have a useful role to play for some time yet. Particularly for launching bigger payloads like Bigelow stations, transfer stages, etc. So, having ways to improve them is good, even if they aren't necessarily going to change the world all by themselves.

As for the last options--land recovery downrange, it actually does have the potential to be revolutionary. But the approach still has some serious economic and regulatory drawbacks that are sufficient to make one start looking at RTLS approaches, even though they may be less optimal from a purely performance-based standpoint. There are four primary (and one somewhat oddball) RTLS techniques/trajectories: pop-up, glideback, boostback, flyback, and once around return. Of these five approaches, the most well known (until recently) and thus most thoroughly studied is the flyback approach. However, the first three are the ones (pop-up, glideback, and boostback) that I think are the most promising and relevant to near-term orbital RLV endeavors, and thus will get the bulk of my focus for the remainder of this series (Parts III-V). But I'll probably spill a few electrons discussing the last two as well. They are interesting after all.

The story broke yesterday that a group of scientists, astronauts, and other space enthusiasts is going to be meeting at Stanford next month to discuss an alternative to the Vision for Space Exploration. Clark and several others have already commented, but I figured I ought to throw in my two cents.

Basically, I'm skeptical.

While there are some good things in the plan, such as supposedly more commercial involvement, and destinations that tap better into some of the supposedly more pressing space-related concerns of the US populace (ie planetary defense against near-earth objects), there's still a lot to be concerned about.

Are we going to see a repeat of what happened with the VSE, where there were all sorts of wonderful platitudes about commercial involvement at the start, that end up being effectively nothing in the end? I mean, COTS is great and all, but NASA spending $10B on its own in-house solution (that's going to end up competing with COTS for ISS cargo/crew delivery when Ares V never gets built), while giving only $500M for more commercial approaches is not what we were led to believe back in the early days of the VSE. Once NASA gets its hands on this new plan, how much commercial content will really survive? If the only commercial involvement ends up being renting an extra Bigelow module or two, it won't be a complete waste, but that's not saying much.

At least from what I've seen, they still are talking about "giving America the Shaft", and wasting countless billions on Ares V, and EDS. If they do that, they're still going to have all the extremely expensive shuttle infrastructure that will have to get paid for every year. Sure, they'll save a little on the edges by not having an LSAM line running, and possibly save a tiny bit by cutting back on the mission tempo (only one manned mission per year or every other year maybe)--though with how much of the money will be going to fixed infrastructure, the savings won't really be that great.

What this new approach probably won't do (any more than what we're getting with the ESAS implementation of the VSE) is actually be relevant to the commercial development of space, or helping our civilization become a truly spacefaring one.

I guess people just get way too hung up on the destinations. Quite frankly, where NASA goes over the next 20 years is of almost trivial importance compared to how it goes there. For all I care, they could set their sites on performing manned exploration of Europa, just so long as they do it in a way that actually helps promote the development of the infrastructure we need to become a truly spacefaring society.

I know I keep hitting on these concepts over and over again, but that's because while the meme is spreading, it still hasn't really sunk in among those in power. There's nowhere in the solar system that's of such pressing importance as to justify a NASA designed and operated transportation system.

On the flip side, almost anywhere in the solar system is a good enough destination if NASA were to go with a truly commercial transportation system. One using commercial propellant depots in orbit that buy propellants from whoever can launch it cheapest, and sell it to whoever wants to move something around in space (both NASA, commercial entities, and other governments). One where NASA "astronauts" are passengers flying on commercial vehicles alongside cosmonauts, taikonauts, UKnauts, Koreanauts, ESAnauts, private (or government) customers going to Bigelow stations or on CSI or Space Adventure operated trips around the moon. One where NASA only builds and operates the actual spacecraft, not the launch vehicles. Because if NASA helps build up a commercial industry like that, we'll end up getting not just whatever the destination de jour is, but everywhere else as well. Maybe NASA ends up spending most of its resources focused on putting boots on Mars, but with a propellant depot on orbit, and NASA acting as an anchor tenant with enough demand to help close the business cases for future RLVs, you're going to see space travel cheap enough that a lot more people can get in the game, and a lot more destinations may be visited. While NASA's off planting flags on Mars, some groups will be exploring NEOs, others will be offering tourist trips to and around the Moon, and others might even be building cloud colonies on Venus.

Anyhow, I think you get my point. We'll see what this new group comes up with. They might surprise us, but for now I remain skeptical.

When I first saw the title of this book, I read it as "The 4-Hour Work Day," and I still thought it was outrageous. But no, this entrepreneur-turned-author really does aim to show you how to work a 4-hour work week. I'm a great believer in the power of books to change your life, and I just couldn't pass this one up. Turns out it's well-written, funny, and full of good ideas. And, yes, it has Michael Belfiore

Since SpaceShipOne spun through 29 rolls flying out of the atmosphere in 2004, the ship's designer, Burt Rutan, has been promising a new and improved design for his follow-on ship--one that won't shake, rattle, and roll paying passengers quite so much. SpaceShipTwo, to be owned and operated Richard Branson's Virgin Galactic, has been coming together behind closed doors, with its exact Michael Belfiore

Since the release of Rocketeers last summer, I've been retooling for my next project, a book about DARPA, the Pentagon's research arm. And what a long arm it is, sponsoring technology research and development that effects our lives in ways most people can hardly imagine. The computer network you're connected to, for instance, started as a DARPA project. Watch this space for updates on the Michael Belfiore

Ok, before I go into this latest blog post, I want to put a disclaimer up front:

• This idea is crazy.
  
• I'm not posting this because I think it's the greatest idea since sliced bread.
• I don't think that this the one and true way to get to space.
• Don't try this at home.
• I don't necessarily thing this is better than .
• Oh, and did I mention I thought this idea was a bit crazy?

But even if it's crazy, it is interesting. And it was developed by a guy with the same last name as me, Allen Goff of Novatia Labs (Sacramento, CA), so while it's a bit crazy, it's definitely clever.

The idea is what Allen called Fleet Launched Orbital Craft, or which another author calls "Separated Ascent Stage Launch Vehicles" (I'll call it by Allen's term "FLOC" from here on out). The idea is somewhat related to bimese and trimese launch vehicles. In a bimese launch vehicle, you have a TSTO vehicle where both stages are identical to each other. Both stages ignite for a vertical takeoff, and the ideas typically use propellant crossfeed to guarantee that one stage is still full when the other one's propellants are almost depleted. At that point the empty stage separates and returns home while the other one proceeds on to orbit. Trimese approaches are similar, but use three identical stages. The theory being that by designing just one stage and using it several times, you can get a lower development cost, even if both stages are somewhat suboptimal.

FLOC is just the logical extension (or maybe reductio ad absurdum) of the Bimese/Trimese concept. You could technically call it an n-mese concept, where instead of only 2 or 3 similar stages you instead have "n" similar stages. John Carmack's looking at one version of the n-mese concept (like OTRAG did in the past), but this one is different from John's modular concept. The big difference between FLOC and a more traditional n-mese approach like John's is that for FLOC, not all of the stages are attached when the vehicles are on the ground. An illustration from Chris Taylor's AIAA article on the economics of the approach (AIAA 2006-4783) might clarify things a bit:


Basically, you have 2^n stages at takeoff, each of them paired together into a bimese configuration. They all takeoff together, all from right near each other (ie they probably all launch within 1-2 seconds of each other, and within 1-2km of each other). They fly as close together as is physically safe. They use propellant crossfeed to guarantee that one stage on each of the bimese pairs is still full when the other one runs dry. When those stages run dry, each bimese pair stages. The "empty" stages all return to the launch site (possibly using an airbreathing engine once back down to subsonic speed to cruise back). The full stages then perform a...wait for it...exoatmospheric rendezvous with each other, mechanically hook-up so they can operate as a new bimese pair, reestablish propellant crossfeed, and then continue on their way. You then lather, rinse, and repeat until your final stage ends up in orbit.

I mean, what could possibly go wrong?

Seriously though, as Chris points out in his AIAA paper (linked above), while the idea is totally crazy, it does have some interesting ramifications. Chris points out that using a fleet of 8-32 launch vehicles, you can place extremely large payloads into orbit using stages that have a propellant fraction similar to a 747 without having to use cryogenic propellants. Also, you can "tune" your payload to orbit (or your suborbital performance) by adding more or less bimese pairs. Using a 747 sized launch vehicle, they were predicting up to 200 metric tonnes (!!) of payload to orbit in a single launch campaign using 32 vehicles. That's about twice as much as a Saturn V, with a stage design that's so low performance it's more like a commercial airplane than a rocketship. As I said previously, smaller payloads could be done as well using smaller numbers of stages.

How hard can doing a dozen exoatmospheric rendezvous be? I mean, you have about a 2 minute window for each rendezvous operation. That's plenty of time....

Needless to say, there would need to be a lot of operations development and practice before such a system could become practical. The rendezvous happen outside of the atmosphere, which definitely helps a lot. And the launch vehicles can all launch from the same area at the same time, which also helps a lot. It's a lot easier to do a rendezvous when you're already flying along an almost identical trajectory at the same time. You'd probably need some sort of automated hold-down mechanism for the bimese pairs, or some way of starting up all the rockets in idle and automatically checking to make sure things are working on each vehicle before you commit to launch. You'd probably also want to do things like building in more thrust than the vehicles actually need (so that underperformance on one engine, or a premature engine shut down doesn't risk the mission). Having a redundant bimese pair or two (depending on the number of stages you're launching already) could also help. Another point that should be made is that in the 2 minute rendezvous window, you only have to get the vehicle pairs mated back together--you can actually hook up the propellant crossfeed after the engines have lit, so long as it happens before you burn about say 1/3 of the propellant of the combined vehicles. This allows you to take the process in two steps instead of having to do both all at once.

More importantly, I'd also like to see the rapid rendezvous and mating demonstrated a lot of times on a subscale basis before trying this for real. You could demonstrate at least some of the basics using cold-gas thrusters on armed robots on something like a Zero-G flight. That would at least allow you to get some of the basics of grappling and mating down in an environment where you could easily get 50-100+ attempts for only a few tens of thousands of dollars. The next step would be demonstrating with two subscale suborbital vehicles that you could consistently do the full rendezvous and mating operation exoatmospherically. With suborbital vehicles, you could start out with a more relaxed rendezvous window of say 5 minutes, and work your way down as you get the bugs worked out.

The good news is that a single one of these stages (or a single bimese pair) should have enough performance to perform a suborbital flight with a decent sized payload. You can then slowly work your way up from there. With a 4 stage configuration, you should be able to at least get to orbit with some payload (something light, probably in the 1-2 tonnes range) once you've demonstrated and debugged doing a single exoatmospheric rendezvous mission. After that, it's mostly operations from there, working your way up to the point where you have enough reliability to reliably pull off larger missions.

Anyhow, for more details, read the paper. I just thought that while this idea was crazy, it was a very fun and interesting form of crazy, and does actually get you thinking.

Anyway, enjoy!

I'm glad to see that my first "orbital access methodologies" post has received as much attention as it has already. As I mentioned in my original post last month, I intend to talk about a few other promising approaches that I discussed for that guest lecture at UND in November. The rest of these approaches are TSTO approaches, including the "pop-up" VTVL approach favored by Armadillo (and discussed in the excellent book, "The Rocket Company"), the "glideback" approach that I think is the most likely approach someone like XCOR would take, and the "boostback" approach, which is similar to what Kistler ended up pursuing in the end.

I've ended up fleshing these out to a lot more detail than what I went over in my presentation at UND (going from two 3x5 cards worth of notes to 12 pages worth of text), so it's been taking me a lot more time to write these than I had originally intended. I thought at first that I'd be able to bang these out, one per evening, when in reality it's a lot closer to a full week worth of evenings.

In addition, I have at least one Jonny/Jimmy blogging post, one politics post, one family update, a follow-on to the Thrust Augmented Nozzles post, and a post about reusable lunar landers in the queue....

I wanted to give you all a heads up on what I'm working on, but also to ask for patience. This stuff takes me a while. Thank you all though for the kind words and support!

As I mentioned last month, I would like to briefly discuss in a series of blog posts some of the more promising potential approaches for reusable orbital transportation. There is often a tendency among engineers to completely dismiss any idea other than ones own preferred approach as being unrealistic, naive, flawed, impossible, inefficient, etc. However, the more I've studied the problem, the more I've come to the conclusion that there are probably several technical approaches that can be made to work for providing reliable, low-cost access to orbit. Each of them has its own set of strengths, challenges, unresolved questions, and operating characteristics. By their nature, this means that different approaches may lend themselves better to different potential market niches and different development paths.

The first such approach I would like to introduce for discussion is epitomized by a proposed design (illustrated below, credit: Teledyne Brown) that was brought to my attention about a year ago. This proposed design, termed "Spaceplane" was developed at Teledyne Brown by Dan DeLong (who later became one of the founders XCOR Aerospace and is currently their Vice President and Chief Engineer, and who also currently owns all the rights to the Spaceplane design). Dan's proposed concept was a winged, "assisted" single-stage to orbit (SSTO) design that was launched off of the back of a converted 747. The LOX/LH2 stage, powered by 1x SSME and 6xRL-10s would theoretically be capable of delivering ~14klb of unmanned cargo to a 400km circular orbit. The vehicle would be reusable, using an Inconel-foil over fiberglass insulation concept for its reentry TPS, and using a runway landing for its recovery method.



While the specifics of Dan's proposed design are now a bit dated (the concept was proposed back in the late 80's), the general approach still merits investigation.

To Stage or Not to Stage: That Is The Question

Now, before I go into the specifics of this approach, I know at least a few of you are probably already thinking things along the line of "SSTO? He can't be serious. Everyone knows that SSTOs are totally unrealistic!" While to be honest, I'm mostly a TSTO guy myself (as is Dan DeLong these days), but I think there's a real danger in how quickly and without contemplation people tend to buy into new conventional wisdoms.

The fundamental reason why anyone would even want to stage a rocket vehicle has to do with the physics of the rocket-powered flight. The rocket equation, says that the change in velocity due to a rocket in flight is linearly proportional to the specific impulse of the propulsion system and proportional to the natural logarithm of the vehicle's mass ratio (the ratio of the mass at ignition to the mass at shutdown of the engines).

DV = Isp * g * ln (MR)

Another way of looking at this equation is that the required mass ratio of a vehicle is exponentially proportional to the required velocity change divided by the vehicle's specific impulse:

MR = e^(DV/(Isp * g))

The inverse of the mass ratio is the dry fraction of the vehicle, ie. the percentage of the vehicle's gross takeoff weight that can be allocated to structures, propulsion, payload, recovery systems, controls, power, life-support, etc, etc. The rest is fuel. Rewriting it in terms of dry fraction (df), we get:

df = e^-(DV/(Isp * g))

Now this is a fairly simplistic way of viewing things (ie. the Isp actually varies quite a bit with time based on the altitude at a given time, the engine throttle level, if you're using thrust augmentation, etc, etc.), but shows the crux of the problem. The total delta-V needed to attain a low earth orbit can range anywhere from ~8-10+ km/s, while you'd be lucky to get a mission-averaged Isp much higher than ~400-440s even using the highest Isp propellants in service, LOX and LH2. Now there are all sorts of subtle nuances that we could go into. Things like how dense propellants typically require lower overall delta-V because they end up having less gravity and drag losses, or that depending on what latitude you're launching from you can get a small "boost" due to the earth's rotation. But the crux of the matter is that for a single-stage system, you're dealing with a dry fraction of less than 10% (and typically quite a bit less than 10%).

That 10% has to cover all those categories mentioned above while still providing a high enough payload fraction that your system doesn't have to get too gargantuan to deliver a sufficiently sized payload. And it has to be robust enough to be reused many times. And your system needs to be maintainable. And it needs to have graceful failure modes, and safe abort modes throughout the flight path. And it needs to be buildable on a realistic budget and timeframe.

All of those issues make the concept of staging very desireable. By staging you get to drop off some of your dry mass along the way, instead of having to lug it all up to orbit. This tends to relax the required mass ratios substantially, which makes it a lot easier to do all those things that make a reusable vehicle truly reusable (as opposed to recoverable, refurbishable, or scavengeable).

But that staging comes at a price. Staging creates a lot of complexity, and introduces some potential failure modes that can be hard to actually check-out on the ground. Staging is one of the single highest risks of failure for existing launch vehicles. Additionally, with a TSTO, now you're really designing three vehicles, not just one. A first stage, an upper stage, and a combined entity. You now have to come up with abort modes for all the different configurations.

Probably one of the biggest headaches for TSTOs is how to recover and reuse the first stage. Getting to orbit is only a little bit about going up, and mostly about hurtling yourself sideways fast enough to "throw yourself at the ground and continually miss". Doing so entails gathering quite a bit of horizontal velocity with a first stage, which means that the first stage gets quite a bit of horizontal distance between it and the launch site by the time it releases the upper stage. Most of the TSTO approaches I'll discuss later revolve around how to get that first stage back. This is a real challenge for TSTO vehicles, though as Dan put it about SSTOs, they have their own challenges with getting the stage back (mostly due to trying to pack a robust heat shield and a robust structure into such a limited available mass budget).

So, in spite of the real challenges of developing SSTOs, there is a reason why some sane and rational people still look at them from time to time. There are real drawbacks to all approaches, and if an SSTO can be technically feasible, it might actually be desirable economically.

With that in mind, I'd like to get back to the topic of this post: air-launched "assisted" SSTOs.

The Benefits of Air Launching

One of the lessons I've learned as an engineer is that many times the best way to solve a really nasty and intractable-looking problem is to find a way to not actually solve that problem, but to replace it with an easier problem, and solve that one instead. In the case of an SSTO, trying to make a ground launched, horizontal takeoff and landing SSTO is a horrible challenge. You have very little dry mass to start with, and ground launching requires landing gear rated for the fully loaded weight of your vehicle, wings that have to be able to produce sufficient lift at very low speeds for takeoff, engines that can operate near sea level while still being efficient in vacuum (which entails either really high pressure designs, altitude compensations, or carrying around different engines with some optimized for high thrust at low altitudes, and some optimized for high efficiency in vacuum), and several other challenges. According to Dr Livingston, a Boeing engineer several years ago suggested that such a system was just not technologically feasible with modern materials and propulsion systems. While there have been some improvements on both fronts since he made that comment back in the mid-90s, I wouldn't be surprised if a ground takeoff HTHL SSTO is still unrealistic.

So the real engineer finds a way to cheat.

And a good way to relax all of those constraints is to not try taking off from the ground, but to start at a reasonable altitude, by using a subsonic airbreathing carrier aircraft. Starting, as SpaceShipOne did, at a reasonable altitude gives several distinct advantages over ground launch (the following list comes from Dan DeLong, with some thoughts from me [in brackets]):

1. The airplane carrier contributes to the overall altitude and velocity. These advantages are small. [Total savings are probably on the order of 100-200m/s. While this is a small fraction of the overall delta-V, the exponential nature of the problem means that even a small decrease in required delta-V makes a big difference.]
   

2. Meteorological uncertainties are mostly below launch altitude. Propellant reserves can thus be less. [Or this means that you can fly on a more dependable schedule, and that you can have more robust propellant reserves without paying as much of a penalty for such.]
   

3. Total integrated aerodynamic drag losses are less, as the launch is above much of the atmosphere. [This provides a bigger benefit to low density propellant combinations such as LOX/Methane or LOX/LH2, but overall could be worth several hundred m/s of delta-V, particularly for smaller vehicles]
   

4. Max Q is less, which reduces structural mass, and may allow lower density thermal insulation. [You may also be able to "split the difference" on the structural mass somewhat--allowing for a higher FOS on the structure, which allows much less maintenance/inspection, while still pocketing at least some of the mass savings.]
   

5. Engine average Isp is increased because the atmospheric back-pressure effect affects a smaller fraction of the trajectory. [This means that your mission averaged Isp is going to be much closer to your vacuum Isp than is typical for a booster engine.]
   

6. Engine expansion ratio (non-variable geometry assumed) can be greater because overexpansion is less problematical. [For instance, IIRC, you can light an RL-10 at 30,000ft without risk of unsteady flow-separation caused by overexpansion. This can make a huge difference, as it means you can use an engine with a much higher vacuum Isp. Possibly a benefit of as much as 5-10%, with greater improvements seen by lower pressure systems that often have higher reliability than the ultra-high pressure staged combustion engines preferred for booster applications these days. When combined with benefit #5 above, this can have a large impact on the required propellant fraction due to the exponential nature of the rocket equation.]

7. Wing area can be smaller because the wings do not need to lift the gross weight at low subsonic speed. Air launch Q is greater than runway rotation Q.
   

8. Wing airfoil shape need not be designed to work well at high gross weight and low subsonic speeds.

9. Wing bending structure need not be designed for gross weight takeoffs or gust loads. Wings can reasonably be stressed for 0.7 g working plus margin. This is a large weight advantage made possible by the carrier aircraft flying a lofted trajectory and releasing the orbiter at an initial angle of at least 15 degrees. (25 degrees is much better but not crucial, more than 60 degrees has no value) This initial angle decays in the first 10 seconds of flight but picks up again as propellant is burned and the constant wing stress trajectory yields a better lift/weight ratio. The thing to keep in mind is that the wings are sized and stressed for landing, and that insofar as they exist, are used to augment launch performance. [A comment I heard from a professor of mine back at BYU was that many people try to use composites as "black aluminum", i.e. they don't try to understand the nuances of the material, and thus miss out on most of the benefits. I think that that may often be the case with wings on rocket vehicles--if you design a vehicle to take the maximum advantage of your wings, you can negate some or all of the supposed "penalty" for carrying them in the first place. And that's coming from a VTVL guy!]

10. Thrust/weight ratio can be smaller because the low initial trajectory angle does not have large gravity losses. This allows a smaller engine, propellant feed, and thrust structure mass fraction. I found 1.25 at release to be about optimum. This is a bigger advantage in air launching because total integrated aerodynamic drag losses are less and the trajectory need not get the orbiter out of the thick stuff as fast. [Lower gravity losses due to the flight angle reduces the required delta-V somewhat, and is probably a bigger benefit once again for high performance, low-density propellants, which typically suffer from higher gravity losses. Lower required thrust-to-weight is also big because your propulsion system is often a large part of the dry mass of an SSTO, so being able to get away with a lower required T/W ratio for the vehicle can make a large difference.]
    

11. The lower mass/(total planform area) yields lower entry temperatures. I assumed inconel foil stretched over fibrous blanket insulation for much of the vehicle undersurface. Titanium over blankets, or no insulation worked on the top surface. Payload bay doors peaked at 185 F. [Having a better ballistic coefficient (the relationship of mass to planform area) means that your vehicle starts decelerating at a higher altitude where the atmospheric density is lower. Basically, drag force is proportional to area, while since F=ma, the acceleration is inversely proportional to mass.
    
    In other words, "Fluffy" is good for reentry vehicles, which means that by necessity, a fixed geometry SSTO is probably going to have gentler reentry heating loads than a fixed-geometry TSTO. This is increased by the fact that many of the benefits/constraints of air-launching push vehicles towards lower density propellant combinations like LOX/Methane or LOX/LH2. This is a good thing, because an SSTO has a lot less mass to cram that TPS system into. This is also good, because lower temperatures and more robust TPS systems mean lower maintenance, lower costs, and higher "availability".]
    

12. Mission flexibility is greater. For example, the carrier airplane can fly uprange before release to allow a wider return-to-launch-site abort window. Good ferry capability, etc. [The other major benefit for missions to specific orbital destinations, like say a Bigelow station, is that the carrier airplane can move the launch point around. By being able to place the launch point at just the right position relative to the station, you can provide for first-orbit rendezvous opportunities even if your launch site isn't directly underneath the given station. The ability to move the launch point also potentially opens up longer launch windows. Lastly, being able to move the launch point allows options like operating out of an airport closer to "civilization" while still launching out of an area with low population density, like say over an ocean or a desert.]

13. [Update: A commenter noticed that Dan and I both forgot to include an important additional benefit of this approach--landing gear for an air-launched SSTO can be designed based on landing weight instead of takeoff weight. This is a big deal for SSTO designs. Boeing had another proposed design, RAS-V that used a trolley for takeoff, but would probably be pretty dicey for an abort. Dan also mentioned the point I forgot to bring up that the RL10s on his design could be used to establish a subsonic cruise of a respectable distance, so you wouldn't actually dump propellants, you'd burn them off in your smaller engines. All in all this ability helps Mass Ratio substantially since the landing gear for a ground takeoff HTHL SSTO is typically a large chunk of the dry weight of the vehicle.]
    

As can be seen from this list, by "cheating" a little bit on the boundary conditions, assisted SSTO approaches can avoid many of the typically largest drawbacks of ground-launched SSTOs. What was a probably intractable problem before (ground-launched HTHL SSTO) becomes a lot more feasible by adding the air-launch "assist". Now, technically you could say that the carrier airplane in an air-launched "assisted" SSTO is really a stage, and therefore the idea isn't really SSTO--and you would be technically correct. But, I do think there is a fundamental difference between an airbreathing carrier plane and a true first stage, such as: no worries about TPS for the carrier plane, no need for RCS systems, no need for rocket propulsion (probably), no need for high propellant fractions, etc.

So all in all, there's a fairly compelling case that if you're interested in developing a SSTO vehicle, and a winged one at that, that air-launching is a big win over ground launching.

The Constraints, Challenges, and Drawbacks of Air-Launching

But as with everything in engineering, air-launching is not without its constraints, challenges, and drawbacks. While I'm sure that someone like Dan DeLong, or Antonio Elias of OSC could probably do better justice to this section than I could, I'll try to touch on some of the high-points:

1. There are a limited number of existing aircraft designs that can be used for air launching. What this means is that the design space for gross takeoff weight vs. carrier price is not a smooth continuous function. If you are near the upper limits of a given carrier craft, even a small increase in takeoff weight might end up forcing you to use a much larger carrier craft.
2. Most existing aircraft aren't that great for air-launching large vehicles. If you drop the vehicle from beneath most commercial aircraft, you're very limited on maximum volume beneath the wings or the hull. If you launch off of the back of an aircraft, you now need to have a higher L/D wing (or light the engines before separating) so as to not collide with the carrier after separation. Also, if you use a top-launched configuration, now you have to mount the stage on top of your carrier, which requires a substantial amount of ground handling equipment (compared to a bottom-dropper).
   
3. Due to needing to fit on an existing carrier aircraft, air-launched SSTOs are a lot more Gross Take-Off Weight (GTOW) limited than ground-launched SSTOs (which can grow to arbitrarily big sizes).
   
4. Related to point #3, there are certain systems on a launch vehicle that don't scale down very linearly. There are also minimum gage issues. These two realities mean that as an SSTO gets smaller, the maximum achievable mass ratio for the system gets worse and worse. Below some minimum size, it's no longer possible to reach orbit with any appreciable payload at all. I'm not positive where that exact point is (and it probably depends on a *lot* of details, but it is probably in the ~50klb range.
5. This is still an SSTO, and even if you cheat by air-launching, you still have a very demanding mass ratio to meet while still making the system robust enough for reuse.
6. Air-Launching a cryogenic propellant stage requires either very good insulation, or some sort of propellant storage capabilities on the carrier craft, or at least some sort of propellant conditioning equipment (ie something to pull heat out of the propellants and prevent them from boiling off). Or possibly all of the above.
7. Due to upper limits on the size of available carrier craft, this concept is unlikely to be scalable to payloads much bigger than 20-25klb.
   

Now, none of these are necessarily deal-killers, but its important to know a design choice's drawbacks.

Potential Enabling Technologies

There are a couple of recent technologies that could make a vehicle like this a lot more realistic than back when Dan DeLong first developed the concept. Specifically, cryogenic composite tank materials, some advanced cryogenic insulation techniques that are under development, the White Knight series of carrier aircraft, thrust augmented nozzles, and orbital tugs.

First off, cryogenic composite tank materials (such as XCOR's "NonBurnite" flouropolymer matrix composites) allow for somewhat lighter tank masses, allow for cryogenic "wet wings" if desired, and allow for insulation and the tank to be integrated into the vehicle structure.

The advanced cryogenic insulation technique I mentioned would help a lot with reducing/eliminating boiloff issues for cryogenic propellants (particularly LH2 if you go that way). I can't really go into the specifics on this approach quite yet. I had written an SBIR proposal for pursuing this technology (along with some teaming partners in industry), but we barely lost out, so it may take a lot longer before the idea is proven out. Suffice it to say that it could cut down on boiloff substantially in gravity, and even moreso in microgravity. Keep your fingers crossed.

The benefit of the White Knight series of carrier aircraft should be obvious. Having a large carrier aircraft with a high undercarriage that is purpose-built for carrying large rocket powered vehicles is immense. I don't have exact specs for WK2 (I figured it would be really bad form to try and pump my friends on the Scaled Propulsion team for such info), but my guess is that its at least 40klb, and possibly as much as 60-80klb. Depending on the exact numbers it might be just barely big enough for a fully orbital SSTO, though I'm not sure how much payload you could get with a vehicle that small. I really don't have a great feel for how the scaling performance for the SSTO works. There have also been several rumors (from all sorts of sources) about the possibility of a White Knight 3 down the road. T/Space showed such a vehicle in their original presentations. That would likely be capable of carrying a booster in the 300-500klb range, which is about the weight of Dan's original "Space Plane" proposal. The benefit of using a White Knight 2 or 3 for your carrier plane (above and beyond being able to buy an airplane that is purpose built for air-launching) is that the SSTO wouldn't be the only customer for the carrier aircraft. Which means the SSTO would only have to pay a fraction of the amortization costs of the WK2/3 development. More importantly, if you can get away with something like WK2, there may very well be several of these built for Virgin Galactic (and other customers), which means that the unit price of the airplane will be lower, parts will be more available, there will be a larger operational/maintenance experience base for it, and depending on the required flight-rate, it might even be possible to just rent a WK2/3 from a SS2 operator instead of having to own one outright.

Ok, I'm sure I'm starting to sound like I have a bit of a hobbyhorse thing going, but I think that thrust augmented nozzles would be a very good match for an air-launched SSTO. Especially if they were running in a "tripropellant" configuration (ie with the fuel in the thrust augmentation section being a denser fuel like kerosene, methane, or subcooled propane). The first big advantage is that it would allow an engine with a much higher thrust to weight ratio compared to a more traditional engine. This would allow for a much lighter engine to be used, which directly translates into more mass for the rest of the vehicle (and the payload). Another benefit is that depending on the fuel used (and the construction technique for the wings), a "wet-wing" tank could be used for the TAN fuel, which would allow a lot more fuel to be carried at almost no extra dry-weight. Combine this with the fact that the LOX tank would be bigger, and the LH2 tank smaller, and it ends up giving you a much higher achievable Mass Ratio for a given construction technology. Using TAN, you can also get away with a larger expansion ratio on the nozzle, giving better Isp after the TAN propellants burn out. Also, if the TAN injectors are broken up into quadrants with separate valves, they could possibly be used for Liquid Injection Thrust Vector Control. This would eliminate the need for the gimbal, and possibly allow for the now much bigger rocket engine to package better into the rocket vehicle. Lastly, if the thrust augmentation is light enough, it might allow for the possibility of keeping some "go-around" propellant for increased landing reliability. While adding the denser TAN propellant doesn't give quite the same drag and gravity loss benefits as it would for a vertical ground launched vehicle, it would still likely increase the payload fraction for the vehicle at a slight increase in GTOW. Aerojet was estimating, IIRC, a 3x increase in payload for a less than 50% increase in GTOW.

Lastly, space tugs (possibly based on the Orbital Express design, or possibly based on the Loral/Constellation Services tug designs) could greatly help such a system if it turns out to have lower performance than hoped for. Instead of taking the payloads all the way to their destination, a tug could possibly allow the SSTO to place payloads into a much lower temporary orbit (which would increase payload mass). Having a tug would also reduce the mass and complexity of the SSTO as it would no longer need its own rendezvous and docking hardware. Also, having a tug means that the cargo (or propellant) could be stored in generic containers, which would simplify ground handling and payload installation. A pressurized tug would be necessary if you wanted to fly people on the spaceplane, but that isn't too unreasonable.

All of these new technologies, most of them which have only come out in the past 5 years or so, make a system like this a lot more feasible today than back in 1986.

Preferred Instantiation

[Update: I also forgot to include this section in the original post]

While I think Dan's original design provides a lot of useful ideas, I think that my preferred instantiation of an air-launched "assisted" SSTO would be a lot smaller. After Space Plane, Dan also went the direction of a smaller vehicle--one he called "Frequent Flyer". I don't recall the exact specs for that design, but they were around 40-50klb GTOW, and required a solid strapon "0th stage" to provide enough thrust. I'd go instead with a wet-wing tripropellant design using kerosene in the wings burned in a single RL10 modified for LOX/Kero thrust augmentation. The gross weight would go up a bit, probably to up around 70-75klb (which is hopefully below the upper limit of what White Knight 2 can carry--I don't know for sure), but you'd get a better mission averaged Isp, would have a fully reusable system, and would probably increase the payload a bit over the Frequent Flyer. But Dan would be in a much better position to say. My goal with this instantiation would be basically either two people, or 1-2klb worth of cargo to LEO. If a vehicle this size works, and if it can fit on WK2, it would be possible to do a larger follow-on using something like a WK3 down the road.

That's just my opinion though.

Remaining Unknowns and Some Potential Paths Forward

So the question becomes, where are we at now with regards to this concept? What unknowns are that we currently know about? Where do we go from here?

The key "known unknowns" I can think of include:

1. TPS design and reentry aerodynamics--is it feasible to make a reusable TPS system that will work for this vehicle that is robust enough, and what moldline/airfoil design will provide the best balance of needed subsonic performance, and workable hypersonic aerodynamics?
   
2. Cryogenic propellant tanks and insulation--can tanks be designed that are both light enough and robust enough for the application? Can long-lifetime cryogenic composite tanks be built that work at LH2 tempeatures? Can an insulation technique be found that is adequate enough to prevent boiloff during the ferry to the launch site? Do we need to use some form of subcooling, propellant conditioning, or "top-off tanks" on the carrier plane?
3. Thrust Augmentation--can thrust augmentation actually deliver enough of an advantage to justify its use in this application? Can an existing engine (such as an RL10 variant) be readily modified for use with thrust augmentation? What is the optimal augmentation level? Does the better payload fraction provided allow you to use a smaller vehicle? What TAN fuel is best? Kerosene? Subcooled Propane? LH2? Can the thrust augmentation be combined with an LITVC system? Does that gain you anything? Can you adequately control the CG shift during flight with the TAN fuel in a "wet wing"? Could an RL10 type engine operating on "vapors" in turbine bypass mode provide enough of a core flow to ignite the thrust augmentation for a go-around burn at landing? Or would you need separate go-around thrusters?
4. Vehicle Sizing--what's the smallest vehicle size that can reasonably deliver (with margin) the payload in question? What are the actual carrying capacities of WK2 or 3? Would such a minimal vehicle be small enough to fit under WK2, or would WK3 be necessary?
5. Mass Ratio--what mass ratio would be required for the vehicle? Based on existing technologies, how feasible is that mass ratio to attain? Is the required mass ratio more doable using denser propellants, and if so, can a denser propellant vehicle still keep a low enough GTOW to fit on potential carrier planes?

To me, the most critical questions that are also the most unknown, are the ones regarding the TPS and reentry aerodynamics. Most of the other questions, while important, are much more straightforward to answer.

As for the path forward, I think there are multiple prongs that can be taken.

First off, for the carrier plane, WK2 is mostly built and will probably be flying this year. More exact information about its maximum carrying capacity can probably be had in the relatively near future. Trying to find a way to make a vehicle that closes using WK2 would be the most preferable option.

Second off, the TPS/Reentry Aerodynamics. Some of this can be worked on the "traditional way" using CFD and special wind tunnels (at places like NASA Ames). However at some point, it would probably be worthwhile to move on to subscale models launched from suborbital vehicles. Basically, a suborbital vehicle with a "nanosat launcher" upper stage could probably put up a small, instrumented reentry model to nearly orbital speeds. A lot of care would be necessary in designing the experiment and analyzing the data to get the actual data you want, because there are all sorts of scaling laws going on a the same time. Things like different reynolds numbers, the fact that the standoff distance of the bow shock is going to be proportional to the linear dimensions of the vehicle, so a subscale model is likely going to see more intense heating, etc. It should be possible to design a series of low-cost experiments though that can at least retire some of the risk in advance before trying to build an operational version.

As for overall vehicle integration and Mass Ratio control issues, an HTHL vehicle like Xerus actually provides a useful starting point for working ones way up to an SSTO. Now, the XCOR people aren't SSTO fans. And they're especially not LH2 fueled SSTO fans. But, the best approach for trying this would probably be to hire someone like XCOR to try and build a lower-performance iterative prototype first to test out some of the key functionality, and then work your way up to the performance needed for SSTO. The first prototype might use just a traditional LOX/Methane engine with as high of a mass ratio as possible. Make sure that the handling and basic aerodynamics work out right. Test out the cryo insulation, and air-launch cryo-propellant handling procedures. Make sure that the TPS functions as expected in the suborbital (though relatively high velocity) environment that such a vehicle could provide. Upgrade the engine to a TAN system and get some experience operating that and making sure that the LITVC scheme works. Test out RCS functionality. Test abort modes.

Do a second iteration that has LH2 as well as the TAN propellant. Develop and test out a TAN-modified RL10. Get experience using such an engine. Get in-flight performance data. Make sure the cryo insulation still works. Make sure the tank can handle the cold cycles. See how close you can get to the mass ratio. Instrument the crap out of your vehicle and figure out where you can shave weight, and how robust/reusable the TPS is in an almost orbital situation. Figure out if you need to scale up the vehicle, or what other changes will be needed to reach a sufficient payload target. Start expanding the envelope to orbit.

Now some of these steps might be skippable depending on how previous steps go. Some of them might be doable as part of other programs (for instance figuring out the cryo composite tanks for LH2 or the special insulation system might benefit other projects, developing flightweight propellant conditioning hardware that can fit on a WK2 or 3 might also be useful for other projects). But these are just some thoughts on what has to be done from a technical standpoint to get "there" from "here".

Conclusions

In spite of the bad reputation that SSTOs have earned during the last decade, there are at least some versions, like the air-launched SSTO that aren't entirely crazy. They still might not make sense, but if any SSTO RLV design ever makes it, my guess is it would likely be something like this.

I know that Clark and Keith have already mentioned this, but I have to congratulate NASA for finally obeying the law and purchasing commercial services from ZeroG instead of just running their own in-house NASA proprietary Vomit Comet. Whoever pushed this through deserves congratulations.

Peter D. and ZeroG also deserve kudos for not giving up (and for building a business that was AFAIK doing well even without NASA as a customer).

Now if only we could get the guys at ESMD to follow their lead...

[Note: I came up with this idea a couple of weeks ago, but it got left on the backburner over the holidays. Oh, and welcome to anyone coming here from the Carnival of Space.]

One of the biggest mixed blessings of lunar transportation is the lack of an appreciable atmosphere on the moon. While this is a big benefit as far as propulsion efficiency and deep throttling goes, it is also a big drawback for crew safety. Basically, a VTVL vehicle lives or dies on its propulsion system. However, in an atmosphere, even if you have a complete propulsion failure (say catastrophic loss of power, propellant tank rupture, etc.), there is still the option of using an emergency ballistic chute (if your vehicle has one), or "bailing out" and using your own parachute. While this is by no means foolproof, it sure beats the alternative.

The problem is, on the moon there is none of that nice, draggy, "air" stuff that are somewhat non-optional for parachutes.

The traditional solution to this problem has been to use a two-stage lunar lander, and treat the upper stage (the ascent stage) as an escape capsule. But such TSTO designs make reusable lunar transportation a lot more difficult. And you're still stuck with the tricky situation of what happens if your ascent engine fails?

While a good reusable lunar lander is probably going to borrow heavily from operations, design, and maintenance experience from terrestrial VTVL suborbital vehicles, there's still the reality that cislunar space is a more dangerous place. Not only are there environmental factors such as micrometeors, radiation, etc. that make failures more likely. But the effects of those failure modes are more severe. There's a reason why most of the predicted risk for a lunar mission center on the transportation phases to and from the lunar surface. The Moon really is a "Harsh Mistress".

Ejection Seats: aka "Attempting Suicide to Avoid Certain Death"

So here's a crazy idea: what about using some sort of ejector seat that used pure rocket power instead of parachutes? Basically it would be a propulsion system with a main engine, and possibly some RCS engines, and some sort of minimal GN&C system. Assuming that either some sort of hypergolic combination (or something like a scaled up version of what Digital Solid State Propulsion is working on) were used, and that the package was sized for say 1200m/s, you're only talking about a couple of hundred pounds per spacesuited crew member.

Assuming 400lb for the crew member in a space suit, 100lb worth of dry mass (chair structure, engines, tanks, any pressurization systems, valves, etc--probably quite doable), you're talking about ~250lb of propellant. For a total weight of about 350lb for the ejector seat (at least some of which may have been needed for a non-ejectable landing seat anyway). If you go much higher than 1200m/s, the propellant fraction starts growing fast enough that the system would probably weight too much to make sense, but at an extra cost of only about 250lb per crew member, it might not be too crazy. Especially if it allows you to save weight elsewhere by going to a higher performance but non-hypergolic main propulsion system, for instance.

Since there's about 2km/s of Delta-V between a low lunar orbit and the lunar surface, the worst case failure would occur partway through the retro burn (or the orbital ascent burn), where you have about half of the delta-V left. 1200m/s gives you a little bit of margin, plus some propellant for RCS ops, off-nominal operations, etc. If you're most of the way down, or only part of the way up, you abort to the surface (using your legs as landing gear like parachutists do on earth). If you're more than half way up, or less than half way down, you abort to orbit.

Also, 1200m/s can probably give you a pretty decent suborbital hop. Unfortunately the lack of atmosphere means that once again you have to decelerate as well as accelerate. But we're still probably talking about a several hundred km range in case your lander malfunctions during a sortie mission.

Lastly, having each crew member have a maneuverable emergency seat like that also makes rendezvous failures between the lander and the lunar orbital station (or CEV or whatever you're using to get back to earth in) much less likely.

Are there some dangers in such a system? Probably. Ejection seats kill ground crew on a regular basis here on earth. But they still save enough lives on net (in spite of how much more reliable even combat aircraft are compared to rockets) that they still get used. It's like a launch escape tower. Even if it only has a 75% chance of working, and a nonzero chance of going off at the wrong time, it will probably cut back on overall fatalities by a substantial amount.

Now, obviously, in order to do any good, you obviously need the crew in spacesuits during orbital descent or ascent maneuvers. Also, you need a way of getting the seat out of the ship without undue risk to the crew members. Shrapnel from something like a shaped charge that on earth might just risk causing an ugly injury could cause a loss of pressure integrity in the spacesuit with predictably bad results. Fortunately, due to the lower gravity, lack of aerodynamic forces, etc. it may be easier to make such a system safe and reliable than it would be for say an ejection system for a supersonic jet fighter.

Potential Benefits
One of the main potential benefits of having something like a lunar ejection seat, is that it frees up the design of your lunar lander a lot more. For instance, you can now use a single-stage (and thus more easily reusable) lunar lander without having to take as much risk of losing the crew. Also, you can pick propellants for the lunar lander more on performance and economics criteria instead of having to use hypergolics on your ascent stage because you're trying to shoehorn the thing into being an escape capsule.

Now, I don't know if the idea makes total sense on balance, but I think it's an interesting one at least worth looking at. What do you all think?

by guest blogger Ken


Since this is the pause and reflect season, I'm going to pause and reflect on what a space-y holiday it's turning out to be. In large part because of the ongoing success of a couple of small projects I've been cultivating at NSS of North Texas, but other projects as well.

One of my favorites is the children's reading library at the Frontiers of Flight Museum. The local Civil Air Patrol Cadets put together an awesome wooden airplane bookcase. The wing is up front on top and acts as a reading surface. The pilot's area is open so a child can sit in it and steer the plane. The sides are hollowed out into bookcases all the way back to the empennage.

The Museum routinely drops by library booksales to try to bulk up the stock, but the reality is that books will walk from the museum, perhaps to placate any number of irascible children. The staff reminds me of this fact every time we try to give them books. So what we do is incribe "A gift to the Frontiers of Flight Museum from the North Texas chapter of the National Space Society for all the children of the Metroplex" on the inside cover of each one, and then hand them over. The museum saves the best of the lot for their classroom programs, and sets the rest free in the children's area to the vicissitudes of fate and the cruel hands of countless children.

This year's haul was phenomenal. 23 books all told, with a lot of really nice ones in the mix. To give you an idea:

-Usborne's "On the Moon" by Milbourne & Davies
-Simon & Schuster's "The Moon" by Seymour Simon
-Kingfisher's "The Best Book of the Moon" by Ian Graham
-Children's Reading Institute's "What the Moon is Like" by BRanley & Kelley
-"Apollo Moon Rocks" by Marcus & Lillian Langseth
-Bright & Early Books' "The Berenstain Bears on the Moon" by Stan & Jan Berenstain
-Firefly's "New Atlas of the Moon" by Legault & Brunier
-HarperCollins' "I want to be an Astronaut" by Byron Barton
-Childrens Press' "Spacelab" by Dennis Fradin
-Golden Books' "Glow in the Dark Trip to the Planets" by Hammond & Jordan
-Funk & Wagnall's "Charlie Brown's Cyclopedia Vol. 3 - Blast Off to Space: Astronauts, Rockets & Moon Walks"
-DK's "Revealed: Space" by Alex Barnett
-"Engineer's at Work: Space Rocket" by Tim Furniss
-Firefly's "Final Frontier: Voyages into Outer Space" by David Owen
-"The Space Atlas: A Pictorial Atlas of Our Universe" by Couper & Henbest
-Barron's "Discoverology: Voyage Through Space" by Ian Graham
-Disney Learning's "Wonderful World of Space" by Andrew Fraknoi
-Kingfisher's "My Book of Space" by Ian Graham
-Random House's "There's No Place Like Space!" by Rabe & Ruiz
-"Time for Learning: Stars & Planets" by Rick Morris
-"Spacebusters: The Race for the Moon" by Philip Wilkinson
-DK Readers' "Astronaut: Living in Space" by Kate Hayden
-DK Readers' "Rockets and Spaceships" by Karen Wallace

That should hold them for a while. I hope. And of course, yours truly provided all of the Moon books.

We also had a great turnout for our Santa Space Toy Drive, and I had a back seat full of space shuttle toys and model rockets and planet puzzles and space games and plastic spacemen. One complaint that does comes up frequently is that space toys are hard to find, so folks go online. I took a walkthrough of a local Toys'R'Us to see if I could find anything and settled on a Lego Mars Mission mobile laboratory (what some might call a MOLAB). There really wasn't a whole lot of anything, spacewise. If they'd had any "Moon in my Room"s those were long gone. There was a Fisher Price 'PlanetACE' figure, but for some reason I didn't go for that one.

It is rather discouraging. The usual fantasies are there - cowboys, dinosaurs, pirates, trains, princesses, athletes, pop tarts. But not really any spacemen.

And that makes it difficult when you're trying to get people to be interested in space as a place for us to live and work, not just look at pretty pictures of. There's not a lot of material out there locally to enable it. Which makes it easy to overlook. And easy to ignore. We need more space toys stocked in our local stores!

All told we put together three fair-sized boxes of stuff, and I dropped them off at the big drop-off point downtown for the Santa's Helpers program to help disadvantaged youth. I also made a point to talk to one of the 'official' Official Elves (as opposed to the volunteers, which I might just have to help out with next year) to get some contact information and to see about getting more visibility for our efforts. Which admittedly pale in comparison with well-established local projects that do things like drop of 60 bikes for the kids. It's also hard to put out collection boxes when the USMC has all the best locations for their 'Toys for Tots' program and I, for one, am not going to mess with the Marines.

Nevertheless, my sister texted me the following evening to inform me that NSS of North Texas was specifically thanked during the 6pm newscast. Woo Hoo!

On the education side of things I just got my grade for the online Moon class I took starting in September. This was the first time this sort of thing was offered, so it was a learning experience for everyone. It wasn't really an in depth scientific look at the Moon, but rather an exploration of teaching about the Moon and how just because we tell someone something about the Moon, they may not necessarily 'hear' what we say. The documentaries "A Private Universe" and "Minds of our Own" were distributed to everyone, and they were certainly eye-opening, not least because I could see much of myself in many of the gifted youngsters to whom they were talking. It certainly had an impact on some of the strategies I use in outreach. While I may not necessarily have been doing anything wrong in how I convey information about the Moon, there were certainly things I could do better. Sure most of my projects were late. The trip to China in the middle of the semester didn't help either, and I was basically playing catch-up through the end of the semester. Still, my researches in the Lunar Library give me an edge that few others have, and I managed to eke out an A. Double Woo Hoo!

Speaking of the Lunar Library, I can report that the reviews of Lunar science fiction have had page views more than quadruple in 2007. The project was begun in January of 2006, and by the end of that year I had 12,000 page views for the thread. For 2007 that's up to over 66,500 page views to date. Most of the traffic has come in the last half of the year, so I'm going to set an ambitious target of 150,000 page views by the end of 2008. Even better, the end of this phase of the project is in sight, as I only have about 50 or so books left to review, and a handful of short stories. There are over 150 reviews up to date, and I'm guessing that by the end (hopefully the end of 2008) it's going to be the most comprehensive overview of Moon-related fiction around.

Speaking of space fun, it's not too early to start thinking of a Yuri's Night event on April 12th in your locale. It's on a Saturday night in 2008! That can mean only one thing - WORLD SPACE PARTY!!!

NSS of NT has already given the go-ahead to try and put something together in 2008, and the plans are already starting to form. This could be so much fun. Kind of like a Scarborough Faire, but in reverse - future-looking instead of past looking. It could be fun, but as I sit down and plot out what it would take for what I have in mind (a phenomenal all-day space educational/party bonanza for all ages!), it seems a daunting task. Would there be enough volunteers in the metroplex space community to make it happen, and enough bucks to fund it? One benefit of the ISDC was that we got to know a lot more of our local space-related folks than we used to, and that always helps. If you're in the the D/FW metroplex and want to help out, drop me a line at my Lunadyne account at Gmail.com.

Other Space Holidays in 2008 to think about:
Hero Remembrance Week - January 28th to February 1st
Space Day - May 1st
Moon Day - July 20th
World Space Week - Week of October 4-10

My Wish for 2008? I'd like to see another success with the flight of at least a prototype tourist vehicle. Some smaller successes, like an X-Prize Cup win, and a Millenial Challenge or two, would also be good as well.

Best space wishes for 2008!

Ken