Douglas Eagleson writes:

On Monday, December 28, 2020 at 10:11:01 AM UTC-5, David Spain wrote:
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
...

[explanation of Nuclear Thermal vs Nuclear Electric propulsion elided
for brevity ...]


Thank you for clarifying the meaning of fuel in a reactor engine.

The nuclear engine powered aircraft designs used air as fuel.

That would be quite a trick of transmutation. I think (again) you meant
propellant, since the heat was intended to be derived from a nuclear
fission fuel in a reactor as opposed to combustion of a
hydrocarbon-based jet fuel.

https://en.wikipedia.org/wiki/Nuclear-powered_aircraft

For a feel of what a nuclear powered jet aircraft would look like see
the uncannily weird and at the same time, semi-accurate water propelled
SST featured in the first episode of the made for TV adapatation of the
Phillip K. Dick novel, "The Man In The High Castle" (Amazon Prime
Video). Note the cooling steam coming off the engine moderators as it's
parked on the tarmac. Stay goodly distance back.... :-)

In space a duel use fuel is as you said. Chemical fuel,
reactor thermal and/or electric boosted.

I vote for simple hydrazine plasma boosted. The reactor
design needs plasma cooling though. Radiant heating
lowers the allowed power density.

Putting some kind of radiators on the reactor is allowed.
The shield using fuel is maybe an option, but what happens
on approaching the Earth with low tanks.

Well you can siphon off water from multiple tanks if you are expending
water as propellant. The cone of protection narrows as you empty tanks,
with the one closest to the crew compartment being consumed last. That
would be for nuclear thermal or NTR. Nuclear electric would use a closed
cooling system and the only time water would be expended is during
thrust. And only a very small amount comparied to NTR. The trick is
keeping the reactor cool. In the original design of the interplanetary
nuclear electric ion-propelled spaceship Discovery (2001 A Space
Odyssey), the ship had giant fins that extended away from the reactor
compartment to allow radiational cooling into space. The fins were
deleted in the final design because many thought it made the vaccum-only
spacecraft look too much like it had aerodynamic fins. An obvious
pseudo-scientific non-sequitur. Keeping the reactor far away on the
x-axis of the spacecraft helps as well. Even without the radiative fins,
I've always thought Discovery made for a pretty decent spacecraft
design.


Think big and have a thousand foot long ship and use
water to shield. This is human supply water. Coming back
with empty water tanks will have a lower ship mass to
return with. Lowering the engine power demands. And
the crew radiation dose. Added hydrazine shielding
is a bonus.

I like water because as you pointed out above it's triple
use and non-cryogenic. Propellant, bio-hydrate and a (derivative) fuel
source, assuming you have nuclear electric.

Dave