In article ,
says...

Are nukes the future of space travel?

"For all the controversy they stir up on Earth, nuclear reactors can produce
the energy and propulsion needed to rapidly take large spacecraft to Mars
and, if desired, beyond. The idea of nuclear rocket engines dates back to the
1940s. This time around, though, plans for interplanetary missions propelled
by nuclear fission and fusion are being backed by new designs that have a
much better chance of getting off the ground.

Crucially, the nuclear engines are meant for interplanetary travel only, not for
use in the Earth?s atmosphere. Chemical rockets launch the craft out beyond
low Earth orbit. Only then does the nuclear propulsion system kick in.

The challenge has been making these nuclear engines safe and lightweight.
New fuels and reactor designs appear up to the task, as NASA is now
working with industry partners for possible future nuclear-fueled crewed
space missions. ?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.?"

See:

https://spectrum.ieee.org/aerospace/...travel-to-mars

I'm still skeptical of nuclear propulsion the near term. Hydrogen is
the reaction mass of choice for nuclear (highest ISP), but liquid
hydrogen is bulky, requires large tanks, and must be kept at very low
cryogenic temperatures. Long term storage of liquid hydrogen in space
is a technology not yet developed.

To get the necessary hydrogen into the stage you're going to need to
perfect in orbit cryogenic refueling. That's a technology which would
also be beneficial to a conventional liquid fueled rocket engine.

In-situ propellant generation at Mars requires you find water to crack
into H2 and O2. With nuclear, most of the O2 will go to waste (you only
need so much O2 for breathing). Also, if you find water, that could
also be used to make liquid methane and liquid oxygen. Liquid methane
and liquid oxygen aren't as deeply cryogenic as hydrogen and are
therefore easier to store on Mars.

I think you see where I'm going with this. In the short term, liquid
methane and liquid oxygen based chemical propulsion is a lot easier and
can get the job done for Mars missions. That's not to say that nuclear
won't have a place in the future, I just don't want to put nuclear on
the critical path to Mars.

Jeff

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