Posted by rbanffy 10/26/2024
Of course, humanity being what it is, we'll never trust each other with the idea of building thousands of nuclear bombs with the "firm promise" that they'll only be used for space travel.
[1] https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
I think the idea was for the bomb to vaporize a certain amount of propellant. According to wikipedia, the propellant was supposed to be tungsten, but I imagine that any substance would do. For example ice. The vaporized propellant hits the pusher plate and is reflected, resulting in an exhaust jet of very high velocity. The ISP was initially calculated to be between 4000 and 6000 seconds (so 10 times higher than the Space Shuttle), but later when they did the calculations with fusion bombs they concluded that an ISP of 75000 seconds is possible.
Starship is all about a lot of mass to Earth orbit. This is a little mass somewhere between interplanetary and interstellar design parameters. Yes, Starship could put it into Earth orbit, when expendable probably interplanetary, sure.
And it may well help launch it, but that’s where its relevance ends, it’s at the other end of the spectrum of what you would design for. For example, at this corner of the design space, chemical propellants aren’t a thing.
You can expand in parallel. If Starship works you can launch 100 spacecraft instead of 1, or in other words, you can expand your system by a factor of 100 "in parallel" and hence increase whatever you get from that small payload margin by a factor of 100.
Besides that there are other approaches like laser array + photon sail which more directly benefit from mass to LEO.
With 3.5% enriched uranium, about 1/8 of that mass on the power supply is fuel.
Yeah, it's not impossible. But nuclear reactors aren't usually anywhere near 7 times heavier than their fuel.
On not on earth, but Soviet spatial reactors[1] weren't too far from that:
> The fuel core of the reactor was 0.24 m in diameter, 0.67 m long and weighed, as an assembly, 53 kg,[1][2] and contained 35–50 kg of enriched uranium. The entire reactor, including the radiation shielding, weighed 385 kg.
The problem is yield, out of 100kW of thermal power it was only able to generate up to 3kW of electricity due to lack of efficient cooling in space.
But then you'll get into severe storage and control problems. And that thing has to work for 13 years, untouched. There's a maximum somewhere on the middle.
Anyway, I don't think reactors on earth are anywhere close to 140 times the mass of the fuel either. And they don't have to use radiative cooling.
Remember that on Earth nuclear reactors create electricity by boiling water to turn turbines. Such a system will be far more difficult to design for space.
Ok, a bit sci-fi but I had to say it.