On 12/23/2020 6:20 PM, wrote:
“Nuclear propulsion would be advantageous if you want to
go to Mars and back in under two years,” says Jeff Sheehy, chief engineer in
NASA’s Space Technology Mission Directorate. To enable that mission
capability, he says, “a key technology that needs to be advanced is the fuel.”"


I think the key thing to keep in mind here is that by fuel they mean
what and how constructed is the fissionable material in the reactor. For
now we can forget fusion unless you are using fission-fusion-fission
bombs for propellant ala the nuclear Orion type spacecraft, which I
don't think is under serious re-think at least now with all the arms
treaties in place. So what exactly makes up the best FISSIONABLE
material, how is it constructed and clad are all research items. There
are pre-existing examples of how to build a Nuclear Thermal Rocket. In
the 1960's the NERVA project (long since defunct) developed a working
prototype rocket engine that was actually tested in the desert decades ago.

My main point is that it is important when talking about a nuclear
rocket to distinguish between FUEL (reaction-able (i.e. fissile)
material) vs. PROPELLANT. One of the easiest propellants to feed a
nuclear thermal rocket is water. Water is so handy in space it's hard to
overemphasize its importance. In fact if water is super flash heated in
a reactor it might also be possible to dissociate it and then combust
the hydrogen and oxygen gases for additional propulsion, although
chemically you are still not going to get the ISP you would from just
flashing the water into extremely high pressure steam and jetting it out
the nozzle. The other nice characteristics of water in space is that it
provides a very good radiation barrier for that cone-of-safety between
the crew compartment and the reactor, doesn't require cryogenics to
store (although it might require some heating to maintain a liquid state
at *reasonable* pressures), and can be electrolytically converted into
hydrogen and oxygen for combustion for other purposes.

The other means of nuclear propulsion is nuclear electric. Where the
reactor provides enough electricity to power an ion propulsion system
that ultimately probably also uses water as the the source propellant,
but a far far smaller quantity. Nuclear ion is low thrust but can be
sustained for a much much longer period of time, allowing to the
spacecraft to reach tremendous velocities given enough time. Sort of the
high mileage version of nuclear propulsion vs the drag racer that is
nuclear thermal. I view nuclear electric as a much much harder task,
certainly in my view more mechanically complex. Maybe a second
generation of nuclear propulsion?

Dave