# Spaceship propelled by Stefan's law of heat radiation?

1. May 3, 2015

### Jip

Hi,
I got this fun idea. Has anyone ever consider that heat radiation could be an efficient propellant for a spaceship?
Imagine a spherical spaceship, half of which is hot and with a large emissivity coefficient (say almost 1), and the other half is cold and with low emmisivity. A naive calculation with the approximations above gives me a net power of this engine:
P almost equal to sigma 2 Pi R^2 T_hot ^4

which can be quite large. Of course, inside the spaceship, energy must be produced to keep the hot surface at constant temperature. One the main advantage I see here, is that you can use nuclear energy to provide with the required heat, and this is much more efficient than chemical energy.

So it seems to be a not so crazy idea! :) :)

2. May 3, 2015

Staff Emeritus
You might calculate how much thrust you get. To make things a little more practical, you might want to envision it as a hot sphere at the focus of a cold parabolic mirror.

3. May 3, 2015

### vemvare

Such effects are measurable and play a role in the trajectory of interplanetary probes and in the orbits of Satellites and space junk.

The pressure of light isn't very powerful, though, each 300MJ would at most provide 1N of thrust. This is a classical case of thrust versus specific impulse, the specific impulse is extremely high, but the thrust is low and thereby the spaceship would accelerate very, very slowly. It isn't a very practical solution. Since the specific impulse (or more accurately the light pressure equivalent of it) squared and divided by two is higher than the energy content per mass unit of any conceivable fuel (except antimatter), there really is no point in building this thing, it would go faster and accelerate faster if the specific impulse was lower.

4. May 3, 2015

### Jip

Yup, I think I found it
http://en.wikipedia.org/wiki/Nuclear_photonic_rocket

Is it a correct intuition to say that it produces such a low acceleration because you emit pure momentum/energy but no mass (compared to standard fuel)? Is that the main reason?