Jon Goff has put up the fourth installment of his survey on space transport concepts. As he noted earlier, it could be the basis of a useful textbook on the subject, with a lot more work and analysis (and accompanying graphs and figures). When I was on The Space Show the other day, David got a chat from an aerospace engineering student about when he’d learn how to design low-cost launchers, because he hadn’t seen anything about that in any of his course work. This would be the text for such a course.
As Jon notes, TPS is a common thread in making reusable entry vehicles practical and cost effective. The Shuttle tiles are too high maintenance, and risky (as we saw with Columbia). However, a lot of these issues go away if the vehicle “swallows the tank” (as Rockwell and others proposed in their X-33 concepts). No external tank dramatically reduces the risk to damaging the tiles, and containing a hydrogen (or even hydrocarbon, though to a lesser degree) fuel tank makes the vehicle much more “fluffy”* on entry, considerably reducing the heat load. Because of the ET, Shuttle had unique TPS issues that future vehicles are less likely to have to worry about. And also, as Jon notes, XCOR is in the process of building exactly the type of “X-vehicle” that will be useful to start to prove out both trajectory and TPS concepts, something that NASA should have done years ago, and probably would have had it still been NACA.
[Update late afternoon]
Notwithstanding the silly microkerfuffle in comments, I should add that when I came up with the term “fluffy,” it didn’t occur to me to apply it to a vehicle. I really intended it to apply to something that actually is fluffy (i.e., homogeneously undense, e.g., liquid hydrogen), rather than something that has low average density, but very high local density with vast volumes of low or zero density. We should probably come up with some other word to describe a large empty tank, to distinguish between the homogeneous and heterogeneous cases.
*A word I came up with years ago at Rockwell to mean the opposite of “dense.” Others may have come up with it independently.
“Fluffy” is a euphemism for “very difficult to build”. It’s a textbook example of trying to make a virtue of a necessity.
“Fluffy” is a euphemism for “very difficult to build”.
Ummmmm…no. It’s what I said it is. I’m not sure what your basis is for this (to me, strange) comment.
Just a nit-pick. The shuttle tiles worked fine for Columbia, it was the reinforced-carbon-carbon on the leading edges that failed. But I agree they are high maintenance.
Jim,
As Rand said there are many values of “fluffy” that are not particularly that hard to build. It basically means that you’re incorporating the tank into the RLV, thus driving down its ballistic coefficient. I’ve actually been using relatively conservative stage fractions for a lot of the numbers I’ve been running.
~Jon
As Rand said there are many values of “fluffy” that are not particularly that hard to build.
And you know they’re not particularly hard to build because…
It basically means that you’re incorporating the tank into the RLV, thus driving down its ballistic coefficient.
And thus making it very difficult to build. I have three suggestions:
1.) Count how many RLV designs over the last 50 years or so have been sized by a desired ballistic coefficient as opposed to providing propellant volume.
2.) Compare the ballistic coefficient of the shuttle orbiter as built as opposed to the ballistic coefficients of the shuttle orbiters that carried ascent propellant proposed during the shuttle design phases.
3.) Compare the ballistic coefficient of the Apollo CM to the ballistic coefficient of the Chrysler SERV SSTO proposed during the shuttle design phases.
I’ve actually been using relatively conservative stage fractions for a lot of the numbers I’ve been running.
And you know they’re conservative because…
Count how many RLV designs over the last 50 years or so have been sized by a desired ballistic coefficient as opposed to providing propellant volume.
Who has proposed sizing the vehicle to the ballistic coefficient? Who are you arguing with?
The only point is that, if one incorporates the tank into the vehicle (as is required to make it fully reusable) it will have (as a side effect) a lower ballistic coefficient. But no one has claimed that a low ballistic coefficient is the design goal, or that it should drive the design.
And why is it, again, that a low-ballistic-coefficient vehicle is difficult to build? You’re going to have to actually make an argument, rather than an assertion.
Jim,
Because I’ve seen and touched a hardware from a reusable aerospace structure that had far better mass fraction than was needed for most of these proposed approaches? And I know enough about the manufacturing processes to make such structures, that I feel it safe to extrapolate that any space firm that actually had enough money to develop said RLV could also afford to use said manufacturing process?
~Jon
The only point is that, if one incorporates the tank into the vehicle (as is required to make it fully reusable) it will have (as a side effect) a lower ballistic coefficient.
The point you’re missing is that including propellant tanks is not an efficient means of lowering ballistic coefficient.
And why is it, again, that a low-ballistic-coefficient vehicle is difficult to build?
I’m sure it’ll come back to you the next time that Jon or someone else (properly) extols the virtues of dense propellants but to tide you over until then I’ll point out that including the propellant tanks means longer load paths.
You’re going to have to actually make an argument, rather than an assertion.
Physician, heal thyself.
Longer load paths, true… but fewer load *concentrations*. The propellant tanks, incorporated properly into the structure, are marvelous deep-section beams, with excellent stiffness, and the propellant acceleration loads only have to be coupled into the engine thrust structure once, not cantilevered off to the side, carried through the orbiter, and finally coupled to the engine truss (gah). Now THAT is a long load path.
If you have a coherent criticism, make it. Deliberately opaque oracular comments are merely annoying.
Jim,
Sure, there are other ways to get a lower ballistic coefficient. However, including tanks in the vehicle is pretty much a prerequisite for making the thing truly reusable, and as an added bonus it helps lower the ballistic coefficient compared to using drop tanks. What exactly is wrong with making a virtue of a necessity? Isn’t that typically one of the goals of good engineering?
~Jon
Jim,
To be fair, I did come across a discussion by Paul Dietz suggesting that if metallic TPS doesn’t work out (it isn’t a guaranteed solution), that a higher ballistic coefficient might be better using say a transpiration cooling TPS system. I guess to be honest I really don’t know which TPS approach will be more workable. My only point was that at least from a fabrication and design standpoint, making a reentry vehicle that is fluffy due to internal tanks is probably one of the easier tasks in getting an orbital RLV working.
~Jon
The point you’re missing is that including propellant tanks is not an efficient means of lowering ballistic coefficient.
And the point that you’re missing is that no one has proposed a goal of lowering ballistic coefficient. We, or at least I, am getting tired of this particular straw man. You’ve kicked it so much there’s almost no stuffing left.
And no one has disputed that a lower ballistic coefficient vehicle is more difficult to build. We’re simply disputing your contention that it is “difficult” (by which you seem, in context, to mean “impossible”) in absolute terms, and therefore a poor design choice.
I’m still trying to figure out how TPS tiles are risky when the issue is the ET shedding debris on them?
I’m still trying to figure out how TPS tiles are risky when the issue is the ET shedding debris on them?
It’s the main issue, but not the only one. They’re also susceptible to damage from things like bird strikes, and micrometeroids on orbit. Also, if they were more robust, the vehicle might have better all-weather capability.
They’re also susceptible to damage from things like bird strikes, and micrometeroids on orbit.
Bird strikes? Geez, fighter jets crash all the time due to bird strikes. They neither use TPS tiles or travel at orbital velocities. If you have a design that’s immune to bird strikes, then I think you are trying to sell the Titanic as unsinkable.
MMOD, ok. But that’s a pretty broad issue, and tile does pretty well in the orbital MMOD environment.
All-weather, in terms of reuse, is another story. Tile does require treatment for reuse, which does go back to your “high maintenance” point.
Fighters crash from bird strikes either because it takes out an engine, or breaks a canopy, I suspect. Unless the vehicle has an airbreathing engine on it, that won’t be a problem for a launcher.
So you are suggesting a launcher with an internal tank has nothing to worry about in regards to a birdstrike, so long as the engine is not airbreathing? I’m still thinking unsinkable Titanic.
Anyway, it appears that even the risky and flaky ET TPS can handle a birdstrike.