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

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: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.

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.

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.