Using thermal radiation as propulsion

[vitekc45c]

So lets asume that a spacecraft has a heat exchanger at the front, that absorbs incoming heat radiation from space, and radiator at the back that emits heat radiation. The spacecraft is powered by nuclear reactor(s). Could this type of propulsion work? and if it could, would it be practicaly usable?

[Camacha]

How are you going to make sure the exchanger in the front absorbs and the exchanger in the back emits?

[EzinX]

Why bother absorbing radiation? Conservation of momentum means that even if you could make the exchangers one way, any radiation absorbed at the front slows you down by the same amount that re-emitting that radiation out the back speeds you up.

What you are describing actually does work just fine as an engine, but a nuclear reactor won't cut it. You need to tame a small black hole and convert matter directly to gamma rays via Hawking radiation. If this can even be done (there are...issues....with such a thing) you could get actually acceptable rates of acceleration running on pure photon thrust (1/10-1/100 of a g or so)

Or you need to carry a lot of antimatter onboard and do the same thing. Good luck not blowing up if a micrometeorite punctures a fuel tank, though.

[undercoveryankee]

So lets asume that a spacecraft has a heat exchanger at the front, that absorbs incoming heat radiation from space, and radiator at the back that emits heat radiation. The spacecraft is powered by nuclear reactor(s). Could this type of propulsion work? and if it could, would it be practicaly usable?

It is possible to get a net force out of a thermal gradient. Solar heating of a rotating object sets up thermal gradients that transfer net momentum perturb its orbit in what's called the Yarkovsky effect. Similar thermal-gradient forces are now generally believed to be the cause of the Pioneer anomaly.

As an intentional propulsion system, the "front" wouldn't be so much absorbing photons as minimizing what it emits. Absorbing a photon that hits the front still transfers some momentum in the wrong direction, but you would cool the front surface to reduce its self-radiation and darken it to minimize reflections because reflecting transfers more momentum than absorbing.

The rear would be the opposite: heated and surfaced to maximize radiation, and designed to reflect as much ambient as possible without compromising its emissivity in the fat part of its emission spectrum.

The laws of thermodynamics limit how steep a gradient you can set up: active cooling generates waste heat that has to be radiated from somewhere, and the bigger the temperature difference between the cold and hot sides, the less efficient the cooling becomes. You might be able to accelerate by about 1 m/s per year without excessive wasted energy. At most, you could bend an orbit enough to save yourself a midcourse correction.

[Guest]

Probe core | battery | ASAS | Old mattress (actually IR reflector) | [A load of blutonium] ->>> thrust

-> Solar arrays

"Will be there not too soon" spacecraft

No moving parts except ASAS gyros and solar arrays actuators