[VLM]

In all fairness modern naval reactors like the A1B are pushing more like 700 MW thermal (unclassified, probably higher) and float in what amounts to an infinite heatsink of excellent liquid coolant. Of course lifetimes and decades of experience produce the A1B, The First Torchship is not going to do as well.

The real problem for a spacecraft isn't generating 700 MW of thermal heat, which is pretty small and light, but radiating it away continuously. The ISS EATCS radiates about 70 KW and each radiator is in the thousands of sq ft and thousands of pounds. So four more digits would be tens of millions of sq ft and millions of pounds, VERY superficially. However the reactor probably doesn't have to be optimized for human temps, so maybe ten times to hundred times better? It would be quite large at any rate.

There are certain engineering optimizations you make if you have an infinite liquid heatsink like a naval reactor vs incredibly expensive cooling like a space reactor. If you're willing to boil sodium your condenser can radiate a lot more per sq ft than an ammonia based refrigerator for ISS HVAC. That's a little far fetched but the VHTR/HTGR design goal was a cool 1000 C, so the radiators can run quite a bit hotter and smaller than ISS HVAC systems.

[baybal2]

Space is an infinite heatsink if, and becomes an actually very good one once your radiators can reach 2000K+

Not a problem for a glowing plutonium plasma.

Here is a quote on one project from sixties:

> a spaceborne electric powerplant dubbed EU-610, with an electric output of circa 3.3x106 kWt, specific power 0.7x105 kWt/kg/sec (sic), relative mass 18.7 g/kWt, length 10000 mm.

[giantrobot]

Space acts as a thermal insulator most of the time because radiation is the only means for a spacecraft to get rid of waste heat. Thermal radiators also only work if they can be oriented that they don't face the Sun.

The exhaust of a nuclear thermal rocket isn't waste heat. The heat from the reactor that heats everything that is not propellant is waste heat. Getting that heat to radiators without cooking people or melting/weakening load bearing structures is the challenge with nuclear reactors in space. On Earth we use literal tons of water and air to the job.

[baybal2]

Yes, the trick there is that only MHD coils, and few chunks of tungsten carbide will be anywhere close to the plutonium jet, and most of thermal radiation will be emitted to space.

[VLM]

You'll need some electricity and that's a mere heat generator.

The Carnot heat engine efficiency is not nice at a cold side of 2000K and the working fluid will be a problem. Maybe gaseous helium or argon to reduce leakage, I guess.

The only place I found EU-610 was a Russian thermal rocket. Which is kind of like saying all you need for a coal electrical plant is a lightweight pile of charcoal.

Its a different type of technology, sorta like the difference between burning in a cast iron stove to boil tea, vs using a microwave oven to boil tea.

[LargoLasskhyfv]

Sounds like https://en.wikipedia.org/wiki/Nuclear_lightbulb

[rbanffy]

If we are in the 700 MW range, we can go with an NTR as well and solve the cooling problem with propellant.