The meeting has picked up again at 4 PM with a discussion by Mitchell Burnside Clapp on what Rocketplane is up to in Oklahoma.

Mitchell starts off by explaining that he has left Rocketplane Limited over "creative differences." Apparently Chuck Lauer will give the Rocketplane Limited talk tomorrow.

Pioneer Rocketplane Corporation is not Rocketplane Limited, and he is still Pioneer Rocketplane, looking for what to do now. He has a new rocket engine cycle that doesn't involve combustion, using differential temperature between the propellants. By running a heat engine between the two, you can generate the enough shaft work to pressurize the propellants, which makes for a much simpler pressurization system. He likes it because it involves no chemistry--just physics. "Worst thing that can happen when starting a jet engine is starting a fire, which you can run away from. A rocket engine can generate an "earth-shattering kaboom." He thinks it's a good thing to be able to test the pressure condition of the powerhead before ignition, and this would allow that. The idea is to allow a non-catastrophic engine start sequence. Looking at Stirling, Brayton and Rankine cycles (he currently favors Brayton). This kind of technology would mitigate his concern about vertical takeoff/landing.

They're also doing a lot of work on hot metal structure. Inconel, stainless steel and aluminum have similar strength/weigh ratio, but elastic modulus is different. Nonetheless, he thinks that one can learn a lot about Inconel behavior by building airplanes out of stainless, so he's doing some research in that area as well.

Talk was quite entertaining (as always), but he talks too fast to do the raconteurage justice on a typed blog.

In questions, a discussion about why Brayton is preferable to Stirling. In short, better power density. Preferred working fluid is supercritical nitrogen.

Hmmm Sounds like an expander cycle to me. I do believe that the RL-10 has been around for more than 40 years.

It's sometimes a sign of startup maturity when they kick the founders out in favor of people who can run the company. Sometimes.

And the engine cycle sounds a bit different from the expander cycle in that the heat comes from the other propellant rather than the chamber, so you don't need to start the engine to get the pump running.

Finally, there's a long history of jet engines doing much worse things than catching fire, of which UA 232 is only one of the more famous.

I'm not so sure if kicking the founder out is such a good thing in this case. Mostly, I'm skeptical of the people who replaced him. Until they prove themselves otherwise, I regard anyone who ever took the X-33/Venturestar seriously as tremendously tainted, regardless of their credentials. What that program lacked in pure outright thievery, it made up for in sheer incompetance and fundamentally bad design. I sincerely hope that they can redeem themselves, but in the meantime it pains me to think that the same people who ran that program are now in charge of Mitchell Clapp's baby.

I'm a bit surprised about the supercritical hydrogen claim. Sure, it stays gaseous at LOX temperatures, and has a great thermal conductivity, but wouldn't the speed of sound be a bit high at the 'hot' temperature? (This wouldn't be a problem if he's considering using a piston-driven pump instead of a turbine-driven pump.)

If that is an issue he might consider adding helium, or even deuterium, to the hydrogen to raise the average molecular weight. A bit of neon might be even better if it will stay gaseous through the cycle. The solar dynamic system that had been proposed for the space station added xenon to helium to increase the average MW, but that wouldn't work in a cryogenic system.

I misheard or Mitchell misspoke. It's supercritical nitrogen.

Ah, nitrogen makes more sense. The speed of sound in it at room temperature is close to that of air, so the tip speed of the turbine shouldn't be excessive. If the thermal conductivity is too low he might consider a helium/argon mix.

Supercritical hydrogen would also have presented a fire hazard, and made material selection more difficult.

He might consider the application of this kind of scheme to nuclear or laser thermal rockets. Cycles like his could extract additional work from the propellant before it is injected into the thrust chamber, without the need for radiators (the process of dumping cold propellant into a hot thrust chamber generates lots of entropy; for systems in which limits are set not by the energy content of the fuel itself, but by the maximum temperature of the reactor components, this irreversibility could be wasteful).