# How come we don’t put a huge windmill-like machine in space? [closed]

I am new to this sight, but since space being a vacuum there is little to no air resistance at all making a windmill spin forever. There is no wind in space however, if you spin it once, it should keep spinning for a long long time. Now to create energy in a windmill you need the propeller blades to turn a rotor and with almost unlimited spinning time on the propellers you could constantly make energy.

I don’t understand space that much and I hope it is not a stupid question, so please take in mind that I little to nothing about space and this might be too idiotic of a question to answer.

• If you pull energy out of the windmill, something has to replenish it to keep it going. Google perpetual motion. – Don Branson Jul 13 '18 at 19:50
• If there is no friction and air resistance at all the windmill will spin forever only if you don't try to make energy out of it. The almost unlimited spinning time will be short as soon as you get some energy out of it. – Uwe Jul 13 '18 at 22:38
• It is not entirely true that there are no winds in space. There is no wind as you would conceive of here on Earth but, there are solar winds. – Willtech Jul 14 '18 at 3:32
• Why a windmill without wind? Why not a huge, forever-rotating peanut-butter-jelly toast? The sky is the limit. – henning Jul 14 '18 at 16:18
• @DonBranson No problem, just point a fan at the windmill. A fan driven by the dynamo powered by the windmill. – Reinstate Monica Jul 15 '18 at 2:20

As far as I understand you want the "windmill" to drive a dynamo. Have you ever tried turning a dynamo? It takes some force to do so, and that force is then (partially) turned into electricity. So the dynamo is braking your windmill and unless there is some power input to the windmill (wind), it will eventually come to stop. But there is no wind so your windmill quickly runs out of momentum.

But there's another problem: the former paragraph assumed the windmill is able to actually drive the dynamo. But there is nothing "holding" your dynamo still so the windmill would just turn together with the dynamo and no energy is transferred.

• And it would have been such a fun experiment running up the cable to it also. – Willtech Jul 14 '18 at 3:31
• "the windmill would just turn together with the dynamo and no energy is created." Not necessarily true - counter rotating multi rotor 'windmills' could overcome that. Not that it would produce much useful energy, but it might make interesting orbital art installations. Solar cells are much better if getting energy is the point. – Ken Fabian Jul 14 '18 at 23:36

Consider the windmill as a system.

• If there is no wind blowing on the windmill, there is no energy being input into the system.

• If you pull power out of the windmill, energy is being output from the system.

With an output and no input, whatever energy is in the system will be drained and not renewed.

• ...well, presumably you could use solar wind to drive it, but I'm not sure OP knows about that, given the rest of the question. – Clockwork-Muse Jul 15 '18 at 4:22

Oh, but we are doing that. We call that windmill "Earth". Our first big windmill was just called "Moon" but we took so much energy out of its rotation that it has stopped spinning and is now permanently facing "Earth" with the same side.

However, we still use "Moon" as a tool for getting the energy out of the much more ambitious "Earth" by driving mountains of water called "tides" through tidal power plants and other contraptions on the surface of "Earth".

Taking energy out in that manner, however, slows "Earth" down so its speed of rotation went down several seconds per day in past millennia.

Note that we don't actually catch wind with our mills so there is no point in constructing them in the usual manner with rotor blades.

• Tidal friction caused an increase of the length of a day of 17 to 23 microseconds per year. That is 17 to 23 milliseconds per millenium, much less than several seconds per day. In wikipedia you find 1.7 to 2.3 milliseconds per century. Do you have a source for "several seconds per day in past millennia"? – Uwe Jul 14 '18 at 18:02
• @Uwe Looks like 1 second per 50 millenia, he actually didn't specify how many millenia he meant – Erbureth says Reinstate Monica Jul 14 '18 at 18:41
• @Uwe The real issue is that the relative rate of slowing (angular acceleration divided by angular velocity) has dimensions of 1/T. When you say 17 to 23 ms per millennium, that is really 17 to 23 ms per day per millennium (T/T^2). And when user26501 says several seconds per day in [read "over"] past millennia, they are saying several seconds per day per N millennia. I think you were somehow reading it with an implicit "day" denominator, as several seconds per day per day. – nanoman Jul 14 '18 at 20:54