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

[HermannH]

belbear wrote:
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.

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.

Wouldn’t it be nice if we could just suspend the laws of physics when it suits us!

The laws of thermodynamics dictate that the temperature of the coolant and therefore the radiator must be lower than the maximum operating temperature of the device you need to cool. You cannot ‘step-up’ the temperature.

Well, you could build a device that cools the primary coolant and heats a secondary coolant to a temperature that is even hotter. One such device is a Peltier. However, you need energy to operate that device and you will generate additional heat that needs to be dissipated as well. This additional heat would be of a relatively low temperature and would require even bigger radiators.

In other words, heat voluntarily only flows ‘downhill’ from a hot body to a body that is colder. If you want to reverse that natural flow you need to supply energy and you will generate even more heat in the process. This is the reason why you won’t find a portable air conditioner that you can just put in the middle of your room. Its waste heat needs to be dissipated to the outside; otherwise you end up heating your room not cooling it.