Reply To: EmDrive + Focus Fusion = Space Access for all?

[texaslabrat]

belbear wrote:

Radiating waste heat into deep space isn’t so difficult, on condition you shield your radiator from the sun. After all, space shows us a 3 Kelvin black-body and that’s really cold.

Right, but that is purely radiative cooling, which as I understand it isn’t nearly as efficient as conductive or convective cooling — there’s a reason that thermos bottles use vacuum flasks. As “cold” as space may be, you can cool things far more efficiently on earth by, for example, dumping heat into a lower temperature fluid. (I’m sure that some one with way more technical expertise could clarify what sized radiator would be needed to dump 5MW of heat into space.)

Sure, radiative cooling is less efficiënt than convective/conductive, but in space that’s all you’ve got, unless you vent precious matter (such as the flash-evaporative cooling the Space Shuttle uses when its payload doors are closed)

Radiator size is not the only issue here, temperature is one too. Radiative cooling becomes more efficiënt with rising temperature (look at the Sun, that’s a very efficient radiator), so all you need to make your radiator panels smaller is to make ’em hotter.

blackbody radiation is proportional to T^4 (where T = absolute temperature…eg Kelvin or Rankine).

I don’t imagine a FF spaceship at full trust using the kind of big, shiny radiators like the ISS has, I imagine them being rather small, sturdy and glowing hot. Using high-temp coolants such as molten metal rather than ammonia. Since energy is not such a scarce commodity on board a FF vessel as it is in a solar-powered one, a two-stage cooling process that steps up the temperature using some sort of refrigerator cycle can be used.

Why in the world would you want to do that? Who cares how big the panels are *IN SPACE*? There’s no drag, and surface area has very little correlation to mass in the absence of large dynamic loads. You say that energy isn’t scarce with FF..I beg to differ. Purposely *wasting* energy on an active cooling mechanism such as you’ve described necessarily means a bigger power source, and a bigger cooling system, and bigger radiator panels anyway. It’s basically the rocket equation to some degree. Efficiency = good. Passive cooling = efficient. Therefore, Passive cooling = good 🙂

Actually both types of radiators would be needed, one high-temperature for the high-grade waste heat from the reactor and one low temperature for low-grade waste heat from electronics and life support.

p.s.: you got your quotes wrong…

If you make the panels big enough, you’d only need a single set. The hull can also be used as a heat-sink/radiator with proper design. Again, simplicity is the key.