vulture4":1dfe8krg said:
An illuminating article!
As the article on nasapaceflight rightly points out, the X-33 was doomed by NASA management's decision that every new technology they could think of had to be tested at once. The composite tank, built by ATK (funny how they keep turning up) was a rather poor design and failed, because of its shape and its construction. The vehicle could have flown with the Al-Li tank, and would have generated much useful data. The inexplicable decision by Ivan Bekey that it should fly with a composite tank or not at all, and his unrealistic insistence that Lockheed pay much of the cost, resulted in cancellation. Basic research is the government's responsibility.
If Mr. Bekey is still around maybe he could post an explanation. But we are certainly much worse off for the loss; there were some very successful new ideas in the design which were essentially abandoned, such as the eminently maintainable metallic tiles. And let it be a lesson for the next generation of RLV. Don't try to do everything at once with one vehicle, or assume that it can be done with private capital. We need to test each idea in flight before we can choose a complete optimal design.
Personally I think a two-stage design is the logical first stage (so to speak) in developing a practical RLV. There's no reason not to purse the air-launched and vertical-take-off concepts simultaneously, as we were doing in the 1990's.
Here is a thread on Nasaspaceflight discussing RLV technical issues:
http://forum.nasaspaceflight.com/index. ... ic=19127.0
An interesting poster is Danny Dot, an old aerospace engineer who was one of the workers trying to design an SSTO RLV, the holy grail of rocket engineers.
He says re-entry issues are design breakers. His wrote the first reply in the thread. He talks about the need for materials that can endure re-entry. Numerous times he says materials able to take the heat are about as strong as chalk. The thermal protection system should also be light weight.
Another interesting blog is
Cuddihy's Cut
His first blog entry discusses rocket fuels liquid hydrogen vs RP-1. As you may know, higher ISP or exhaust velocity generally means lower thrust. He talks about how the first stage means more thrust and so hydrogen isn't the optimum fuel. Also, while hydrogen is energy dense in terms of mass, it's not so energy dense in terms of volume. Which adds to surface area. This increases drag as well as mass for structure.
The
Nasaspaceflight thread does discuss TSTO RLVs, but not enough to make me happy. Like you, I'm also interested in TSTO RLVs. If I remember right, the thread also includes some discussion of Skylon and the Sabre engine. Another interesting possibility.
Perhaps RLVs from earth's surface are possible. I don't know. But I don't regard it as a given that throwing enough money at this notion will make it happen.
My favorite reusable vehicles notion are spacecraft that never land on the surface of a planet or large moon. The delta V between Earth-Moon-Lagrange 1 (EML1) and Deimos is less than 4 km/sec. Such a craft has a low delta V budget and doesn't have to deal with TPS and re-entry issues.
A reusable ascent/lander spacecraft designed for travel between the moon and EML1 is doable. About a 5 km/sec delta V budget for round trip and, again, no TPS or re-entry issues. I also believe a reusable ascent/lander spacecraft designed for travel between Phobos and Mars surface is possible.
My notion of interplanetary reusable vehicles presupposes an enormous investment in infrastructure: propellent depots in LEO, EML1, Phobos and Deimos as well as propellent extraction on the moon, Phobos, Deimos and Mars. If we continue doing just the rare sortie missions, this investment isn't worth it. Should we get serious about human settlement of space, this investment is a prerequisite, in my opinion.
LEO depots could also aid RLVs designed for travel between earth's surface and LEO. If such vehicles could use propellent to shed velocity, we wouldn't have to rely exclusively on aerobraking to lose the 8 km/sec LEO orbital velocity. This could ease the TPS and re-entry requirements.
Here is a delta V map of the propellent sources and depots I was talking about:
Units are kilometers/second. The red lines are one way trips using aerobraking.