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So, What's Wrong With Capsules?

Frequent commenter "Habitat Hermit," commenting on my Space Show appearance on Sunday, wrote:

There's still plenty of room for disagreements --I have some myself (perhaps even a big one when it comes to capsules although it depends on the details, I think they've still got lots of more or less unexplored potential...

I agree that there are lots of interesting concepts for capsules and their recovery modes. But that's beside the point. The reason that I don't like capsules, of any form, is quite simple. They imply that the only part of the vehicle (at least the upper stage of it) that returns is the capsule. Hence they imply at least a partially, if not fully expendable launch system. I don't believe that we are going to seriously open up space by continuing to throw hardware away.


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David Summers wrote:

I'm not sure that follows - I like capsules-like airships as a re-entry shape better than aircraft-like airships. Hypersonic lift is just too different for more normal airfoils to make sense.

So why can't you have a huge capsule - big enough to hold all the tankage, engines and all? A large capsule like that would actually make the heat shield requirements lower, being fluffy and all... an SSTO capsule might be a "challenging engineering problem", but a multistage one shouldn't be any harder than an airplane like one. (not that you can't do an airplane one, just that the difficulty difference between that and a capsule is not necessarily that large)

Rand Simberg wrote:

It sounds to me like you're redefining the word "capsule." It doesn't just mean "no wings."

David Summers wrote:

Hm, perhaps so. But if so, what would you call what I described (or what I hoped to describe - a big round heat shield with a rocket attached to the back)?

Or I guess you described it more easily - "no wings". What would you call a "no wings" SSTO CRATS rocket? (Other than vaporware...)

Rand Simberg wrote:

I don't know what I'd call it--I'm not sure that there is a single word for it, but I wouldn't call it a capsule. In space parlance, a capsule is generally understood to not have any significant propulsion capability, other than for reaction control. It is a life support system that allows reentry and landing.

Pete Zaitcev wrote:

What is a capsule then?

But even so, look at the original trunkless Dragon with Mickey Mouse ears. It looks like returning just about whole of the spacecraft except the escape tower.

Jeff Mauldin wrote:

"Hence they imply at least a partially, if not fully expendable launch system. I don't believe that we are going to seriously open up space by continuing to throw hardware away."

More to offer a contrasting view than because I disagree with this statement, I'd like to point out that if it was somehow operationally cheaper and cheaper in materials and manufacturing costs to use throwaway hardware, the fact that we have to throw away hardware is no show stopper. It is hard, however, to imagine a multitude of flights a year with throwaway hardware. Maybe composite materials that can be molded and formed very cheaply and in high quantity?

Also, if the parts in the craft other than the capsule are recoverable and reusable, would this fix the issue of throwing hardware away? One advantage of orbital fuel depots is that the craft doesn't have to be designed to lug all its own fuel into orbit. It might be a design advantage as well to not have to lug the entire launch craft into orbit.

Perhaps I'm redefining the capsule as well.

Jardinero1 wrote:

Maybe it's time to reconsider these concepts:

Edward Wright wrote:

Jeff, perhaps you should ask Burt Rutan why he didn't build 50 airplanes to fly 50 test flights, instead of only one.

You can "imagine" hardware to be as cheap as you like, but hardware costs what hardware costs, regardless off imagination. Every mature transportation system operates at around three times propellant costs. Since propellant costs pennies per pound while aerospace hardware costs hundreds or thousands of dollars per pound, it should be obvious that you cannot do that with expedable hardware.

Frank Glover wrote:

One might (espically in the case of a VTVL SSTO) have a 'capsule' that was a normally intergrated part of a larger RLV that seperated only under unusual circumstances...which would almost always mean the flight deck section in an emergency abort. Some military aircraft have been designed this way (the B-1, IIRC), but they have the unusual expectation of operating in environments where other people will be actively trying to shoot them down.

The B-58 and B-70 individual ejection 'capsules' should similarly be kept in mind, but those also existed in aircraft that one at least hopes to return intact.

IOW, we *should* have been (but clearly aren't yet) at a point in spaceflight by now, where a 'capsule' is something that serves a lifeboat or other specialized function, not the primary means of getting between the surface and LEO.

Edward Wright wrote:

Pete, an "aircraft" is a vehicle designed for controlled, usually powered, flight. They have robust landing modes and other characteristics that allow for good reuse (thousands of flights), hence economical operation.

A "capsule" is an escape device (generally single use) to return a pilot to Earth after an ejection. (In aviation, capsule's have mostly been replaced by ejection seats).

Calling a space capsule a "spacecraft" does not make it a spacecraft any more than an aviation escape capsule is an aircraft.

Yes, some capsule's are designed for imited reuse. The TKS capsule, I believe, was designed to be reused up to 7 times. That's still closer to "no reuse" than the sort of reusability required from aircraft and true spacecraft, and the high gee forces and unpredictability of parachute landing make it hard to improve that.

Then, of course, there's the matter of the booster which makes up >90% of the system.

Edward Wright wrote:

Frank, the MiG-105 spaceplane concept had an escape capsule like you describe. However, it was only intended for emergencies (such as combat damage).

During the Cold War "space race," the desire to use existing missiles and their limitations led designers to eliminate the "aircraft" and just launch the pilots in the escape capsule. That has warped our thinking ever since.

Habitat Hermit wrote:

Rand wrote:
"I don't believe that we are going to seriously open up space by continuing to throw hardware away."

100% agreement on that and I see the point about how it doesn't imply that the rocket (or other) stages also need to be reusable.

Paul Milenkovic wrote:

Leaving aside the Charlie Foxtrot going on at NASA about Orion, I remember getting into one of those male primate threat-posture contests over at Jerry Pournelle's site advancing the Arthur Schnitt "Big Dumb Booster" idea relative to Jerry Pournelle being a believer in Have Region, DC-X, and whatever else he was associated with in regard to SSTO.

I guess I was harping on his point of single-stage, one on which he was giving minimal ground. My main point is that SSTO will get you into low Earth orbit, and then what? You have your empty-tanked DC-Y or DC-whatever in orbit, but the interesting destinations are Out There someplace, anyplace -- GEO, Moon surface, Mars, asteroid belt, and so on.

Jerry Pournelle's public response is that if you have a working SSTO, you fly your DC-Y into orbit, you then make multiple "tanker flights" of another DC-Y to top off your tanks, and then your LEO-capable DC-Y becomes your Moon Shuttle.

I don't know if a refueled DC-Y is an optimal Moon or Asteroid Belt shuttle, although it has a heat shield that comes in handy for an aerobraking return to Earth orbit. The point is that you have reusable launcher to LEO and then what? While Heinlen is quoted that LEO is halfway to anywhere, it is only halfway to that anywhere, and how do you get the rest of the way.

To that end, I say don't worry about a reusable launcher to LEO, lets start worrying about the Next Step. The Next Step is on orbit propellant transfer, on orbit propellant depot, perhaps some kind of LEO-GEO or LEO-Lunar orbit shuttle, perhaps combined with propellant transfer and using aero braking.

The tragedy of NASA and of Constellation or Orion or whatever you are supposed to call it is not the retirement of the Shuttle and the giving up on a winged, reusable Orbiter. The tragedy is the absence of the Next Step. Contract out the LEO thing to COTS and let the commercial contractors figure out whether BDB's or RLV's are the way to go, and let that evolve as the market evolves. Have NASA work on propellant transfer, propellant caching, aerobraking Earth return -- the reusable upper stage as it were.

The ISS could have had a component for the Next Step, but instead it is an expensive micro-gravity science project, and it doesn't seem like micro-gravity is this valuable industrial resource, or at least at the costs to do it. The return-to-the-Moon program could have been organized around the Next Step, but instead, it is a reprise of the Russian two-launch Moon shot. The reversion to a blunt-body reentry vehicle is the least of the problem.

But I am taking this a step further. My argument is if we focus on the RLV, we will get an RLV and nothing to take us further. The really interesting stuff in space, historically, was based on at least a restartable upper stage -- the Agena and Centaur projects -- their importance to military reconnaisance and commercial comm satellites in underappreciated. The upper stage is the thing I say, and lets take the Next Step.

Jeff Mauldin wrote:

"Since propellant costs pennies per pound while aerospace hardware costs hundreds or thousands of dollars per pound, it should be obvious that you cannot do that with expedable hardware."

I'd have to say this strikes me as simple truth. So I'll agree that you can't have cheap, reliable space access where you have to throw away significant hardware every launch.

How about the other side of Rand's statement:

"They imply that the only part of the vehicle (at least the upper stage of it) that returns is the capsule."

Is that necessarily true? Imagine (a dangerous thing as Ed pointed out) a staged rocket with few or many stages, but with each stage recoverable and reusable. Years ago I saw a small bit of (entirely theoretical) research into rockets with many stages (e.g. 7 or 8 or more). The main advantage, if I recall, was that as you used up the fuel in each stage you could drop off all the hardware associated with that fuel, thus giving you less mass to boost (the same advantage of any staged approach, just taken to a greater extreme). Naturally that would seem to give more opportunity for failure as well.

Rand Simberg wrote:

Every stage you add to the vehicle adds complexity and failure modes, reducing reliability. And while not impossible, it's hard for me to imagine a reusable upper stage that has to get all the way to orbit, and then deploy the capsule, and then come back on its own. It makes more sense to just make it a single system, and bring it all back at once. In fact, right now, while we think of Ares 1 as a two-stage vehicle, it's really three if you consider it all the say to the capsule, because the (expendable) service module provides in-space propulsion.

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This page contains a single entry by Rand Simberg published on May 27, 2008 8:11 AM.

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