Science Questions

Could a large enough fan propel a space shuttle?

Sun, 27th Jun 2010

Part of the show What's the point of eyebrows?


Tony, Pennsylvania asked:

Could a large enough fan propel a space shuttle?


Andrew -   Okay.  Let’s tell this stuff one bit at a time.  First of all, if space really were completely empty then a fan would be useless to try and propel you through it.  Although the conservation of angular momentum is important here, what it would actually mean is that if you weren’t very careful about how you constructed this, you would switch a fan on, and you would find that this spacecraft started rotating very fast in the opposite direction.

Chris -   This could be uncomfortable for the people inside.

Andrew -   But actually, space isn’t completely empty.  So if you could get over this conservation of angular momentum problem perhaps by having two fans which were counter rotating then you could use the – if you’re near the Sun – something like 5 to 10 protons in every centimetre cubed of space or if you're a bit further away from the sun, somewhere else in the galaxy, something like half a proton on average every centimetre cubed.  If you got a really big fan, you could in principle use those protons to thrust against, but I've been just scribbling away, doing a really rough calculation here, and I've made some pretty generous assumptions and I wouldn’t swear by this if NASA were asking me, but this is what I reckon.  If you could take a really well designed spacecraft, say it was 10 tons.  That’s fairly light and it had a 100 metre radius fan and blades that were 1 metre deep, then I reckon you would get sufficient acceleration to get from 0 to 60 miles per hour in the age of the universe, so in about 14 billion years.

Chris -   It’s a pretty efficient system then.

Andrew -   Well I think we should certainly be funding, looking into this to see what the possibilities are, yeah.

Chris -   And this assuming that you had overcome, although you presumably wouldn’t have overcome with one fan, the spacecraft spinning around the opposite way for the hectic ride for the people aboard.

Andrew -   No.  I think you need counter rotating blades definitely.


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Tony asked the Naked Scientists: Hello Naked Scientists, My name is Tony, and I am in Tobyhanna, Pennsylvania, US. The father of one of my friends proposed the idea of putting a giant fan on the back of a space shuttle as a sufficient way to generate thrust. My friend believes that it would do nothing as space is fairly empty. Based upon my rudimentary phsyics knowledge, I think that the shuttle would accelerate slowly due to the Conservation of Angular Momentum. Is it feasible (or even possible) to propel a shuttle with a giant fan? How would the slanted blades of the giant fan affect the system? What about the size of the fan and fuel considerations to power the fan? What do you think? Tony, Thu, 17th Jun 2010

Once in space the fan will do nothing to propel the craft at all. A fan or propeller work by propelling the air backwards and thereby the craft forewards. No air - no thrust. There is no effect from conserevation of angualr momentum except that if you satr the fan going it will cause your craft to rotate in the opposite direction. graham.d, Thu, 17th Jun 2010

Although the spinning of the space shuttle, if cylindrical with a fan on top or bottom, would provide some amount of gravity for the passengers on board. This is one of the leading ways to provide gravity to the vacume of space without a massive object. I think it would be best to increase the fans speed to an ammont that would cause the shuttle to have a gravity of 9.8 meters per second squared. This would provide the passengers with the acceleration of earth's gravitational quantity, thereby removing the risk of bone and muscle degeneration. If this also propeles the ship so be it. Spencer W., Thu, 19th Mar 2015

I'm wondering, what would happen if you put a sail in front of the fan and generated your own wind tube to propel your space vehicle? Jacob centeno, Tue, 21st Jun 2016

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