How hard is it to build a reusable launch vehicle?

Hello! Newcomer, my first post, with a question that I'm quite certain has been answered here many times before in various ways (in the TSTO thread particularly), but - how hard is an RLV?

I ask this because in many of the threads here that I've read through, and indeed in the literature generally, it seems as though thermal protection is really the critical issue for an RLV and the reason why we don't have one yet. However, given that McDonnell Douglas seemed to reckon they could push straight to Phase III after the DC-X and build a fully reusable SSTO - even if they were optimistic, surely a TSTO RLV is much easier? Considering the propellant fraction requirements.

I'm obviously a rather naive non-expert when it comes to launch vehicle technology and I thought I'd ask this here and see what answers I got. Could a TSTO RLV, reusable enough to be flown basically like an aeroplane (whether VTVL, VTHL, HTHL or something else), be built today?

I think it's very difficult.. also what's the exact defenition for you.. how reusable is reusable ? and how large does it need to be...

I really don't know the answers, and I hope others can answer them to you.
Personally I don't find it easy to talk about how difficult or how much it costs.. since it's hard to guess.. or not to forget all expensive details..
Maybe we can post some "opinions" of what the development costs might be..
I think 1 billion us$ is +- right... IF it's build efficiënt.. it can easily take 10 billion or more..

Btw, welcome on the forum

I think there is no easy answer to your question as it is not just a matter of money or available technology, political will and vested interests pay a big role as well.

Take Boeing or Lockheed for example, I suspect either of these goliaths could produce a TSTO RLV for a couple of billion dollars but neither would do so because they have a vested interest in using expendable launch systems. Until someone comes along with their own RLV neither will do anything and then all they may do is try to buy them.

Companies like Scaled Composites, SpaceDev and SpaceX either are too immature or dont have the cash at the moment to attempt it, maybe in another 10 years this will change. T-Space's CXV is pretty close to a TSTO RLV with only the upper stage being expendable, but it dosent look like anyone is going to fund it.

We do not live in a climate that will enable a TSTO RLV to be built, unless you've got a few billion dollars of course.

Quick answer: sure, it can be done.
Long answer: sure, it can be done, but nobody wants to pay for it.
A general rule of engineering (one that I'm quickly learning) is that if something is physically possible, it is possible to build -- it just might be really expensive.

My main problem with a TSTO as opposed to SSTO is sheer simplicity: to provide the TPS (thermal protection system) for an SSTO, you simply need to cover the leading edges and bottom of the vehicle with some kind of insulating material, and/or install an active cooling system (think steam or other coolant lines running through the hull). In order to build a TPS for a TSTO, you need to cover the leading edges and bottom of two vehicles, along with providing some way for the two to connect together without interfering with the TPS (this way the docking mechanism doesn't turn into slag on every re-entry, possibly resulting in destroying the rest of the vehicle -- remember STS-107?).

Speaking of the Space Shuttle, it's actually nothing more than an attempt at a compromise, with a partially-reuseable vehicle. The idea is to make the expensive stuff (the Orbiter and the two SRBs) reuseable, while leaving the cheap stuff (the main fuel tank) to burn up in the atmosphere. Not a bad idea, but the turnaround time (several months at best) is what kills it. The STS designers realized that they didn't have the technology at their disposal to build a true SSTO RLV, so they did what they could with what they had on hand (and especially with what they had to put up with from Management).

Now, if you want to keep the first stage in the atmosphere entirely, you've still got to provide some method of keeping it in one piece after it's done falling about 50 miles. So either you need wings and some streamlining on it (meaning possibly a disposable fairing for joining the two stages), or you need to build the thing out of solid diamond and hope that you don't land on (read: crash into) the neighbor's farmhouse.

Outstanding question, Centrillium, and welcome to the boards! Now let's see what the resident experts (I'm thinking of SawSS1 and Peter) say.

I'm gonna publish a white paper. Right here on this board. Someday. Really.

I'm still in the "research" phase, but I hope to attain a respectable level of credibility in this work, so I will be taking my time about it... not to mention that I have these nuisances like a career and a mortgage and a family and stuff. I expect it won't be until sometime in the spring.

The general theme here seems to be accurate, i.e. it is doable but expensive, involves fairly familiar methods in general but requires some bleeding-edge "enabling technologies" to come to fruition in any practiceable way.

There are questions about mission and specification which are germane to the funding and implementation issues, and while these will not be the focus of my paper, I will have something to say about them as well...

Please be patient, and certainly don't stop the thread to wait for me!

Thanks for the interesting question, Centrillium!

It's not hard to build one, really. It's hard to build one that has a practical use, and harder still to make it economical.

EDITED: The technical challenge of the heat shield is well understood and a number of working solutions exist - from cork as used by the Chinese, to ceramic tiles as preferred by NASA. The real question is what payload fraction you want, what launch rate, and what to drive the first stage with (rockets, jet engines or something in between; the upper stage will almost always be a high-ISP chemical rocket type).

For example, a handful of companies could build a two-stage vertically-launched RLV today, but it would cost more than a reasonable number of expendable rockets. Then, the real question is, what mission and what launch rate?

Where the Shuttle was concerned, one has to see that apart from being partially reusable, it had to fulfill a number of secondary requirements, like reaching the USA when returning from orbit (as opposed to, say, the Pacific), and actually carrying astronauts. Hence the wings.

Remark for Sawss1:

Please do! If you want a proof reader, I'm game!

Thank you all for your replies!



Yes, my original question was certainly quite vague, but really - any essentially reusable system that places something into LEO. I guess the most conservative system might be something like a big DC-X lower stage and a small DC-X upper stage, with a throwaway fairing as spacecowboy described - my naive calculations suggest that this vehicle could reach orbit with a 67% propellant fraction, surely that can be done?



I thought that the Shuttle suffers from the fact that the ET is actually the structural backbone of the vehicle and therefore ends up having to be much more than a simple 'drop tank'... might be wrong about that though. How much does an ET cost?



I'd be very curious to hear more detail on these 'enabling technologies' - I'm guessing mainly materials/thermal protection? Incidentally, are you referring to RLVs in general, or something more specific? (My fault for asking a vague question in the first place...)

Centrillium, you're doing just fine. As a newbie (there's no offense intended in that term), you came up with an intelligent question that (shockingly) hadn't been asked yet. We're happy to actually have something useful to talk about.

As far as propellant and mass fractions, I don't know. I likely will by the end of the semester, but I don't right now. See, I'm terribly lazy -- I have too much to learn (Intro AE, Statics, Thermo/Compressible Flow, and AE Design Competition) and too little time to learn it in, so I'm going to wait until somebody tells me a fact before I bother to go look it up. (Hey, at least I'll admit it.)

Now on to the STS: you know, you're probably right. The ET would have to connect the whole works (especially the SRBs). So yeah, it must have some pretty serious rigidity. Bah. Just like those idiots to take a perfectly simple idea and make it more complicated than necessary.

And yep, you're right again, he's talking about materials. Mostly carbon-nanotube-based stuff. That's what we're hoping for (although I'd much rather have my fusion drive so I can get to Mars in under a week).

Well,
This is a thread of the kind I'd like to see more of. Unfortunately I might not have much more than questions to contribute. Which I'll do right now.

How feasible would something like the DH-1 from The Rocket Company be?

For those that didn't collect the online first edition, the DH-1 was a TSTO RLV where the first stage lifted straight up out of the atmosphere and the second stange turned and accelerated into orbit. It can be called a boosted SSTO.

The first stage dropped back and landed vertically on its vernier engines. The second stage re-entered tail-first and landed on its side under a large parawing, like the Gemini was originally intended to do. Both stages were manned.

The DH-1 in The Rocket Company used custom Pintle engine and RL60s. Would a ring aerospike on both stages make them (in the long term) simpler to build and more efficient, or would it be an even bigger engineering and maintainance headache?

Cheers,
ErikM

All I know about aerospike engines is that they offer good benefits to efficiency and power, but are a pain to manufacture (although they should, I would think, be easier than a conventional bell), and require more intensive cooling systems.

It looks interesting, and looks a lot like what Centrillium is talking about.

Thanks for the input, erikm.

More replies - thanks again!



Good question, and I have to buy that book. Its on my Amazon wish list now. Thank you for pointing it out!

As far as aerospikes are concerned: My understanding is that any given bell nozzle is optimal only for a certain external air pressure/altitude, and suffers lower Isp at any other, but an aerospike can be 'tuned'. On a TSTO you probably use different nozzles on the two stages so the advantage of an aerospike is reduced. I might be wrong, but I think aerospikes are most interesting for SSTO vehicles.

The 'boosted SSTO' sounds very interesting. I need to spend some more time playing around with Excel to understand the parameters that effect TSTO vehicles... I think that the lowest propellant fraction is achieved when the velocity increment is divided equally between the two stages (provided both have propulsion systems with equal Isp), but this doesn't necessarily translate to the smallest (GLOW) vehicle for a given payload.

Centrillium:
There is one very good reason to use an aerospike on the orbital stage. The spike's expansion surface is part of the heated surface on re-entry. Since it would likely already have an active cooling system anyway (for lift-off) this would limit the surface you would need to weigh down with heat shield.

TRC also brings up other issues and one of them is economics. Since the protagonists don't want to go down NASA's path, they choose cost per pound to orbit as prime criterium for their craft. In pursuit of that they're willing to settle on slightly lower-tech but much cheaper solutions on a surprising range of issues, most of them having to do with the first stage. First stage ISP is one of them. Total mass fraction is IIRC another.

The first stage gets designed and built to a lower technical standard than the orbiter. Of course, it only has to punt the orbiter up X miles, separate, fall and land more or less where it started. The orbiter of course is built to exacting standards. Every pound of structure/fuel/etc in that is a pound less payload.

Cheers,
ErikM

A fly-back booster certainly seems the easiest to build--with less problem with great heat www.starbooster.com

The accidental loss of such a vehicle as opposed to the intentional staging of a simpler, lighter craft (i.e. its mass is all fuel, no wings) costs you more. It may not have tiles, but it will have the same kind of engine swap-out complexity as the shuttle's boat-tail if it is of any real size.

TSTO craft have twice the wing weigh, twice the landing gear, etc. Weight creep can be a real issue.

The Martin Astro-Rocket was going to be an all-hypergolic design, which kept both booster and orbiter to a more compact, mid-sized build. Hypergolics seem to be falling out of favor, with Proton to be replaced by Angara and LM-5 to a hydrogen burner IIRC.

http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld002.htm

Thanks for your interesting comments, erikm, publiusr. Particularly on the subject of the aerospike, I hadn't considered that aspect - I know that most of the studied vertical riser RLVs have re-entered base-first, which is hard to do with bell nozzles. IIRC, McDonnell-Douglas' Delta Clipper never was intended to do so though. Speaking of which...

This book was a really lucky find for me in the library here at university, its extremely interesting. I was just today reading the section on the Delta Clipper, and if its to be believed, the 11.4 tonne payload / 590 tonne GLOW design on paper included a '15% margin on the dry mass, a 20% margin on engine mass, and a 2% margin on engine specific impulse, plus oversized propellant tanks in case more propellant is required for any dry mass growth'. The account suggests that Delta Clipper was actually designed quite conservatively, and yet it was to go to orbit on 440-second Isp H2/O2 bell nozzles in a single-stage with 180+ m/s on-orbit and re-entry delta-vee, plus propellant for a rocket-powered descent and landing.

Okay - that was a bit technical, but the point is, SSTO can't be that easy...? I mean, 'easy' being a relative thing here, but still. Maybe the LH2 tank would have turned out as a sticking point - it was to be built from carbon composites like the X-33's, I think. Or maybe McDonnell-Douglas were just optimistic with their estimates? More generally - could Delta Clipper have been built?

Sorry, I keep asking questions. Maybe one day I'll have answers for someone else's...

We could do an SSTO today with a lot of money, but it wouldn't get a lot there. 11 tonnes is virtually no payload.

The biggest benefit of staging is the amount it adds to the mass fraction. To get a worth while SSTO it would have to be massive and thus currently out of reach.