Hi gb

Cool points. As Zuppero's website indicates he was talking about BIG space vehicles. Small, near-term Moon missions or whatever commercially will probably need your fill-up stations - until we get water from the Moon or the NEOs.

I wrote to him about the reactor performance needed for the exhaust velocities he quotes. Jim Powell did all the computation a few years ago and considering his nucleonics experience I'd trust his figures...

Steam Rockets:

Jim Powell at Brookhaven calculated it would take about 800 Celsius to get Isp of ~ 215 seconds specific impulse. He used nearly perfect nozzle expander estimate. In practice one can get 95% perfect, but I am not sure what the 95% means: energy efficiency or Isp efficiency. Powel had tested some steam reactor elements at 1500 Kelvin. He believed we should say "235 seconds" so he could use his particle bed reactor pellets he tested.

Dennis Pellacio at SAIC and various places calculated quite similar operating temperatures.

Bruce Schnitzler at the Idaho National Lab found that the cladding experiments had already been done at 1005 Kelvin. They found erosion was minimal for relativelty short duration (days to weeks at most) of space travel, vs years of electric power reactors.

As a result, I could not decide whether to use SiC coated particles in a particle bed reactor like Jim Powell, or a standard fuel-pin/fuel-rod water reactor like Schntzler would do. The best answer would be to test claddings and concepts in a real reactor.

And neither NERVA nor DUMBO would do because they are focused on essentially unclad elements using hydrogen propellant. The hydrogen would exit to space along with a small number of sparklies that flake off the fuel. Since space is already "radioactive" (galactic cosmic radiation, ~many MeV particles), the nuclear sparklies in the exhaust would be non detetectable within the first few million miles of the rocket. The NERVA designers opted for radioactive exhaust. I tended to not favor that one for policital correctness purposes.

Recall that the NERVA approach runs at very not 2500 Kelvin or hotter. They could not run for years, and did not expect to.

So the reactors wouldn't run as hot as I suggested, else they'd degrade in a hurry. Steve Howe reckons modern fuel elements wrapped in zirconium oxide shouldn't produce a radioactive plume.

Adam