NASA Eyes Shuttle Replacements

jonerik writes "According to this article at Space.com, NASA yesterday released a status report on the first year of NASA's Space Launch Initiative; the search for a space shuttle replacement, currently planned to begin operating ten years from now. The competing contractors - Boeing, Lockheed Martin, and a team consisting of Northrop Grumman and Orbital Sciences Corp. - have their work cut out for them. NASA is looking for both a ten-fold improvement in per-pound launch costs (from $10,000 per pound to $1,000) and massive improvements in crew survivability." In related news, Rubyflame writes: "Aviation Now has a story about four new kerosene-fueled rocket engines being developed by Aerojet, Pratt & Whitney, Rocketdyne, and TRW. These are engines that will produce a million pounds of thrust, intended to outdo Russian designs in reliability and launch cost, and one of them may power a new reusable launch vehicle. Kerosene has the advantage that it's denser than hydrogen, so the fuel tanks can be smaller."

Comments

Re:about time

[Jubedgy]

Well, I wouldn't call it embarassing since:

There's been only one major accident (challenger) in those 30 years

No one else has a reusable launch vehicle (that I know of...I don't think russia does, pretty sure no one else does either

--Jubedgy

Multi-stage Launch

[jchawk]

This article is light on details but does mention that all of these systems that they are working on are two-staged.

At first you may think that two-staged launches are a waste of money, but some of it does at least look promising.

The design from Boeing is an interesting one. It looks like a smaller shuttle attached to a jumbo jet. It's then flown near the limits of space where the top ship would then come apart and finish it's journey into space on it's own.

The jumbo jet would then return to the launch site.

I must admit that I would love to see a 1 stage space craft. :-)

Re:Multi-stage Launch

[AJWM]

If you really are a rocket scientist, as your sig states, then you should know that "efficiency of the engines" (I assume you mean Isp) has damn near bugger all to do with it.

Ion engines are wonderfully efficient in converting mass to thrust, but they won't get you off the ground. The key issue for launch vehicles is the ratio of total impulse (ie, thrust x time) to system weight, where the system weight is not just the propellant mass but also that of the tanks to hold the propellant (this is where LH2 loses out, too bulky), the engines, thermal shielding, etc.

At least you qualified with reusable single-stage launchers. Several simple thought experiments using existing technology provide examples of workable (but not necessarily reusable) SSTO vehicles. E.g. a Shuttle External Tank with six SSMEs. Or a Saturn-II stage (with the bulkhead moved for the different mix ratio) with the 5 J2's replaced with an SSME.

Of course those are both LH2/LOX examples -- high Isp on the engine but crappy structural weights because of the size of the hydrogen tanks. Convert the engines to something like a LCH4/LOX (liquid methane/lox) cycle (easily do-able with RL-10s, probably require a massive redesign of something like SSME) and you lose some seconds of Isp but gain back because the CH4 is so much denser than LH2 that the tankage is much smaller.

Cheap, one-shot boosters, designed for low cost rather than reusability

This sounds good, but the problem is that, unless they are way overengineered (ie heavy) or you're willing to accept a few blow-ups, "cheap" and "one-shot" are mutually contradictory since a one-shot is inherently untestable, therefore you have to inspect quality in.

Max Hunter (rocket scientist, designer of the Delta's daddy, Thor) beat all this to death years ago, didn't anybody listen to him?

Re:Multi-stage Launch

[prisoner-of-enigma]

>performance rocket engine they had to use, couple
>with the experimental composite cryogenic fuel
>tanks.

No. This isn't the case; I was talking to some engineers that worked on the Roton just last weekend. They indicated that they knew of no problem that would have precluded the design from working. The composite cryogenic fuel tank THEY used (as opposed to the X33 debacle) - it worked fine in all testing; including something like 50 pressure cycles IRC.


It's good to hear that someone's worked on the problem a bit. Still, I'm sure it's quite expensive.


>you need a bigger rocket, which means more fuel,
>then a bigger rocket...you get the idea.

No, the simulations converge- SSTO is definitely possible. I've seen atleast 2 hard and fast designs for SSTO vehicles- the Roton and Mockingbird. The Roton would have carried 3 tonnes to LEO; the Mockingbird design didn't have a payload of any note, but was really tiny (1.5 tonnes), and cheap. I've studied both concepts extensively; they both appear workable.


I will point out that the payload capacity you're speaking of is about a tenth of what one Saturn V can hurl into LEO. It's like comparing an old big-block V8 with a 4 barrel carb versus a high-winding multicam, turbocharged, intercooled 4 cylinder engine. Both will make gobs of horsepower, but the latter is going to be much more expensive than the former AND generally more prone to failure. The Shuttle main engines are a case in point with their trouble-prone turbopumps. The J5 engines on the Saturn only had to work once, thus were much cheaper AND more reliable.


>What we need is a way to extract more energy from
>whatever fuel we use.

Another thing I saw on the weekend- I was at a presentation by a guy talking about a laser powered launch system. The idea is you take a large bank of lasers and point it at a hydrogen powered launch vehicle, which has a heat exchanger it uses to heat the hydrogen. The ISP is about 600 seconds, which is plenty for reaching orbit. The laser bank was priced at about $1 billion but its dropping at about 30% a year currently- only cheap semiconductor lasers are needed, and they're getting cheaper and cheaper.


I've seen this concept demonstrated with computer simulations, but I'm still skeptical of it. Ground based lasers will always be subject to thermal blooming due to atmospheric attenuation. By the time you're 50 miles up, that's got to be a tremendous power loss, meaning you'd have to have incredibly powerful lasers chewing up gobs of power. You'd need a nuclear power plant on site just to power the darn thing most likely.

Re:Multi-stage Launch

[Moofie]

You're absolutely right, I was using some verbal shorthand right there. I was really considering the specific thrust, the thrust per unit mass. Lighter engines which require less reaction mass are more "efficient" in the context I was using it. Aerospike engines might be incrementally more "efficient" than SSME's, but I don't think they're better enough to give really inexpensive SSTO travel.

There are two contentions that I'm making.

1) SSTO doesn't make sense, because you have to schlep a lot of dead weight into orbit with you. Once the fuel tank is empty, it's just drag and extra mass. Pitch it.

2) Reusable hasn't yet made sense, because it's almost less expensive to build a new one than to re-certify one for flight (where "one" is "whatever launch system we're discussing"). It may be that in the future, we'll have engineering technology such that we can re-certify the spacecraft with less intrusive inspections, but I don't think that the expense of that process is warranted, particularly for a system that's just for throwing big stuff into orbit.

Your point is well taken. Manned vs. unmanned shots require very different engineering constraints. I think it's unfortunate that NASA seems to be totally hell-bent on making one system that does both things well. And you're right, a large payload return module would be a useful thing to have in the inventory. But there's no reason to take it up there every time you launch.

Pics

[Anonymous Coward]

More pics here. Dig the one with 6 jet engines.

Re:Kerosene?

[Waffle Iron]

I understand the space savings advantages of kerosene, but how does the thrust produced per unit weight compare to that of the current SRB/LRB compare? Having to (hypothetically) double the fuel weight to double the thrust seems like a waste of money to me.

On another article a few weeks back, someone posted an answer that cleared this up for me. (I'm too lazy to track down the posting now.)

Bottom line is: hydrogen is like a high-horsepower, high-RPM turbo racing engine; it's best for driving light vehicles at high speeds (upper stages). Kerosene is like a high-torque diesel truck engine, good for getting a lot of weight moving from a dead stop.

The difference has to do with the physics of exhaust density, speed, momentum, etc.

Re:Crew survivability?

[Bios_Hakr]

You also forgot to mention the fact that before every launch, an explosive demolition team arms a large satchel of c4 in the nose of the SRBs. Gee, I'd hate to be the one to press that big red button when the shuttle deviates from its flight plan.

Aren't both Boeing stages identical?

[Ars-Fartsica]

From what I could see from the photos, tthe Boeing stages appear to be identical (?), which would be a huge cost-savings in terms of parts reuse, interchangeability, etc.

Its true though that all of the designs share some characteristics...one stage to get you off the gorund, one to get you into orbit. Obviously this isn't by accident...the physics of the problem and materials/fuel presently available must dictate this design.

Don't mix passengers and freight

[Richard Kirk]

Who remembers the Rolls-Royce HOTOL proposal? It must be over 25 years ago now. It was to be a reuseable airbreathing horizontal take-off thing. It looked like an aircraft, but it had no crew, and it was aimed at the space bulk freight market. This would have saved a bunch on all the life support and pressurization stuff for the early models. If it had been found reliable after, say, 50 flights, then there was the option to add and extend a pressurized cabin, toilets, lemon-scented towels in individual sachets, and other comforts.

Okay, Britian has a long history of telling people what they ought to have built without actually putting very much together themselves. But it still strikes me as the right solution.

Laser launchers.

[Christopher Thomas]

Ground based lasers will always be subject to thermal blooming due to atmospheric attenuation.

Interesting. Is this caused by the lasers or just natural artifacts of the atmosphere? Incidentally power is the cheap bit in the equation, and you need less of it delivered at altitude due to g-limiting anyway; so it may not matter.

Atmospheric. You have two effects happening. One is that minute particles in the atmosphere scatter the laser beam. This is unavoidable, and causes exponential attenuation over long distances. The second is that the atmosphere absorbs some of the light you're sending, and heats up. This causes optical mayhem that defocuses the beam.

Compounding the problem is the fact that you'll have to fire through a *lot* of atmosphere. Your craft needs most of its velocity to be tangential, and you want as long an acceleration path as you can get away with to keep the acceleration to something that a) you can provide and b) won't damage your cargo. This means a grazing path through the atmosphere, which means your lasers will be firing through hundreds or possibly thousands of kilometres of air (i.e. as far as you can manage).

The only practical scheme I can think of for very long distances is to have multiple stations along the flight path and to fire a converging beam, so that heating problems are only significant for the last little part of the beam path.

On a couple of other points: You'll be using a laser array, not a single laser, so the cost will be directly proportional to the power required. More power means more cost.

Also, I have doubts about a heat-exchanger system working. Throughput tends to be low compared to the power flow required to get high ISP, and a heat exchanger means a heavier craft. The most practical craft design I've seen suggested, which has flown in small-scale tests, has the bottom of the craft being a curved mirror with a central projection. The laser is focused by the mirror and heats air immediately below the central projection, which is shaped to force the air to move away from the craft.

Laser launchers are a neat idea, and avoid the problem of carrying most of your reaction mass when set up in jet mode, but there are formidable engineering problems to overcome before they're practical.