Re: rotons again

Doug Jones (
Thu, 01 Jul 1999 07:53:01 -0700

Spike Jones wrote:
> Doug, I read the writeups today about RR in Av Week and Aerospace
> Daily.
> I am having a hard time believing Av Weeks contention that the
> rotary engine was dropped because of investor confidence, yet the
> helicopter landing system was kept. That seems like a contradiction
> to me. The engine was what gave me confidence in RR, not the
> landing system. {8-[

I gave up trying to read Gary's mind months ago. He's a visionary and leader, not an engineer, a crystal ball gazer, and I can't follow him. That damn fastrac engine ain't gonna inspire anyone.

> I am still nursing the concept of a two stage to orbit, both
> stages rotary, neither recoverable, where the second stage is
> parked in a 600 km circular orbit, so that it can be used later
> as raw materials to make something useful. Ethane and lox, lox
> tank initially pressurized to about 1.5 to maybe 2
> atmospheres (absolute), the rest of the chamber operating
> pressure provided by rotation. The initial tank pressure is to
> keep the temperature of the lox below the freezing point of ethane,
> since my concept has the two tanks nested.

A coupla points- I'm sure you meant, higher pressure to keep the LOX *above* the freezing point... and propane is a better choice than ethane (it actually has a lower gel point, and a eutectic mixture of the two goes even lower). Isothermal LOX/propane hjas long been a favorite of mine.

Alas, even subcooled propane has poor density, and the pumping pressure of a rotary engine is directly proportional to density. Kerosene is a *lot* easier to use.

As for two rotary engines- remember that low pressure engines have only a trivial (less than 1%) Isp hit in vacuum, with the same expansion area ratio. Make the upper stage pressure fed.

If you put back the insulation between the tanks, you can Vapack both propellants (use their vapor pressures to drive the fluids), eliminating helium bottles, regulators, and heaters altogether. The RCS runs off the vapors left after the main engine burn.

The main problem is that regen cooling can't be used- the coolant film boils in the coolant passages (think of a drop of water on a hot skillet) giving lousy heat transfer. The cooling would fail and the chamber would melt. Happily, low pressure ablators last a good long while (at 1500+ psi, they erode frighteningly fast). This would make the engine, inherently an expendable, cheaper than a regen system.

> I have convinced myself that the manufacturing costs of a
> system could make this concept a go, since you would have no recovery
> costs. Anyone down there working on something like this? spike

There are a few still grinding away... I feel this would be great for freight, and with an escape rocket on the crew cabin, could be used to haul people.

Doug Jones, Freelance Rocket Plumber