For nuclear-electric systems, it's not just the reactor, but also shielding, all the power conversion machinery to turn thermal power into electrical power, radiators, etc. And the mass does still matter in space.Mr.Tucker wrote:Ok, ok, so I've just had an exciting idea for space travel, and I thought that this would be a good place to share it. Some opinions (maybe there are some nuclear engineers or such browsing this page) are welcome.
Some background: first, I was watching the presentation the liquid fluoride thorium reactor (good idea to watch it, so you know where I'm coming from):
https://www.youtube.com/watch?v=YVSmf_qmkbg
And was intrigued when mister Sorensen presented the products that such a reactor would produce. He mentioned medical molybdenum, that fascinating bismuth-213 isotope used for targeted alpha therapy, plutonium-242 for deep space RTG missions, all the good stuff.... and xenon. Which is something of an annoying byproduct (acts as a neutron poison). And it produces quite a lot of the stuff, apparently (which tends to be a problem in solid-fuel rod pressurised reactors):
https://i.imgur.com/xjfA8.jpg
So I thought: hmph, loads of power (as one tends to get with nuclear) and xenon byproduct...what needs loads of power and uses xenon?....
This does:
https://en.wikipedia.org/wiki/Dual-Stage_4-Grid
A ship that uses a LFTR reactor to power a beefed up ion thruster with specific impulse in the tens of thousands of seconds, AND doesn't need to carry much propellant (perhaps an initial charge before the reactor spools up) because it gets produced in the reactor itself. Separate it from the salt (which is easy; one is a gas, one is a liquid), and feed it to the ion drive. An NEP with a mesmerising potential.
Some would say: "ah, but reactors tend to be heavy". Firstly, it's space, so that is less of a problem. But, and more importantly, MSR's were designed during the 50's and 60's to power NUCLEAR BOMBERS:
https://en.wikipedia.org/wiki/Aircraft_ ... Propulsion
If you give an aero engineer a list of potential types of reactors he can use to power his jet engines, he will invariably choose the one that is most compact and light (I should know, I'm an aero engineer...). So, although I have no proof whether that was actually the case, I suspect that this type of reactor is extremely light and compact compared to it's opponents.
So...am I talking nonsense?
As for the xenon, just consider the relative mass of the reactor fuel and a typical xenon propellant tank. A reasonable craft might carry a few tens of kg of fissile fuel and burn only grams of that during any given trip, while carrying multiple tons of propellant...even the Dawn probe carried 425 kg of xenon. You're just not going to get enough xenon to be worth the equipment needed to collect it.
There are conceptual designs for fission fragment rockets that actually directly use the fission products as reaction mass. These have specific impulses in the range of 100k to 1M seconds, but due to the geometric issues of allowing the fission fragments to escape the fuel, they wouldn't be liquid salt reactors...the fuel would be something like fine wires, thin disks/ribbons, or suspended particles of dust.