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Ask HN: How can you turn solar energy into thrust?

3 points by joshlegs ↗ HN
I was just reading about energy sources for satellites, etc., and I got curious: How might a person/company/government/mankind convert solar energy into thrust? I found this website (http://energy.nd.edu/research/transformative-solar/), but it doesn't really provide a lot of substantive information on how such a thing might be achieved. I didn't find much else through a cursory Google search. (Well, found this, but am not quite catching on yet: http://cstec.engin.umich.edu/)

update: I did find this website, which gives a little more info on it -- http://www.physicsforums.com/showthread.php?p=2793009

So, in short: in idiot terms, what kind of technology would be required to convert solar energy into thrust? I would think you could have, for example, some sort of air thruster that works off of electricity that works like an air compressor, but don't know if that's really efficient, etc.

Sorry if this is a dumb question, I just was thinking about it while at work and wanted to pose it to HN.

7 comments

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alphaBetaGamma
Conservation of momentum gets in your way: you need to "throw" something out of the back of the satellite to get some momentum in the forward direction.

Though you may read on solar sails though. http://en.wikipedia.org/wiki/Solar_sail

joshlegs
Thanks for the link. I read a little about solar sails in some of the other stuff I saw. This is a bit more comprehensive on the topic. I would imagine, though, that converting it is theoretically possible, since thrust is just the expenditure of energy, which the sun provides. I just didn't know if there was some technology that would allow for it.
mchannon
For space applications, you can use the aforementioned solar sail, or charge an ion engine (though this still requires you to relinquish some matter).

For air applications, the old-fashioned propeller is your ticket to thrust. Something like a dirigible or a hot-air balloon, which is able to keep itself in the air without continuous thrust of a minimum force, would be less difficult to make work.

Interestingly enough, weight is more important than efficiency when dealing with aircraft PV. A company I spoke with that was working on "stratellites" (basically high-altitude blimps that stayed in one place indefinitely) used thin film PV instead of high efficiency cells because of their flexibility and high power-per-weight rating.

joshlegs
I guess I didn't mention space being the focus in the first post. Obviously a propeller wouldn't work there, but I'm not sure something like compressed air would work either, because you'd have to create more air, I believe, for it to be successful.

I'll hvae to read more about the ion engine though.

dgunn
Thrust implies that you're pushing off of something. For a propeller say on a helicopter, you're pushing off a column of air that "sits" on the ground. In space, thrust typically means you're pushing against gas you're providing in the form of burnt fuel. I have no clue how solar energy could create true thrust in a vacuum.
lewisgodowski
An ion thruster would probably be the best way, although it still requires matter to shoot out the back. In any method, the solar power will only be used to power whatever method you use to shoot matter out of the back. Because of this, you're still limited by the amount of fuel you can get into space at launch.

http://en.wikipedia.org/wiki/Ion_thruster