Comment by aetherspawn
8 years ago
That's a cool thought. You could perhaps use the atmosphere of planets to accelerate at very high velocities with the energy stored between each body (which would be a lot .. ie 60 days of 24/7 solar harvesting).
The question is whether the thrust you produce is roughly linear with the energy you expel? Or does it taper asymptotic? What if the power system on the craft is titanium batteries that are designed to deliver 1 MW for say 2 minutes? Will that give you the needed acceleration in a given planets atmosphere? What if you use planetary lasers and don't need batteries at all?
Solar light isn't that strong once you go beyond mars.
Earth gets 1400 W/m^2, at Saturn only 16 W/m^2 and on Neptune maybe 1.5 W if you get lucky.
60 days of continous harvesting, assuming the spacecraft doesn't use any power (which is not true in reality), is about 2 kWh at Neptune. Not that much. Saturn would be 23 kWh.
Yuck, that’s miserable.
It's the inverse square law that bites you here as the same amount of energy gets stretched out into a larger sphere as it travels outwards (at earth the energy is 1.4kW for a square meter, when going outwards, this square meter gets stretched)
Double the distance and you get 1/4th the energy.
Saturn is 9AU or 9 times as far as earth; 1/81th the energy. (1400 / 9^2 = 17, so math checks out; roughly)
We're quite lucky to be close enough for solar energy to be a viable source of energy.
[*]: https://en.wikipedia.org/wiki/Inverse-square_law
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It may not work on any planet. A fundamental design challenge is that increased size of solar panels create more drag, and increasing the height to reduce drag means it must go faster which further increases drag. While the Solar Impulse has demonstrated an equilibrium of speed-to-size can be maintained at normal altitude and low speed, we'll have to wait to see if something can be built to sustain equilibrium at these heights.