Do RTG reach the >2170K necessary for this?
(Note the reason given for this limit is to guarantee that, if this capsule accidentally falls to Earth, the cladding has the mechanical performance to stay intact on impact. If iridium gets too hot, its strength is permanently degraded (so it says). I suspect if you voided this requirement -- if you created a separate of class of spacecraft "no longer capable of impacting earth" -- the limit could be raised much higher).
[0, pdf] https://ntrs.nasa.gov/api/citations/19910015359/downloads/19... (2.2 "Temperature Constraints")
Cooling things in space turns out to be hard. There's no convection or conduction, so all cooling occurs via radiation. I believe the ISS radiators are rated at about 1 watt per m^2, which means that you can calculate the radiator area (and mass) given your power budget.
To a first approximation, the ISS radiators are about the same size as its solar panels.
This is a factor missing from virtually all sci-fi representations of space ships. I had this realisation a few years back talking about 2001 --- the Discovery should have had a huge set of radiator fins given its nuclear propulsion (ion drive I believe, in the story universe). Wikipedia states that there were radiators in the book, though I don't recall that element.
MMRTG [0] dissipates 2,000 W, in a package that only has several m^2 of radiator fins. That's >100x more efficient than the ISS (1 W/m^2). ISS' radiators have to operate below room temperature; MMRTG fins [1] are in the 100 °C - 200 °C range. Higher temperature -> higher heat dissipation per fin area.
(By the way, the comment we're replying to asks about nuclear quadcopters on Titan [2], which is in dense atmosphere not a vacuum!)
[0] https://en.wikipedia.org/wiki/Multi-mission_radioisotope_the...
And yes, Titan / cold atmosphere would make for highly-efficient thermal gradients.
Oh well, I was briefly excited but there's a big enough gap that it probably won't work as is