Could a large enough fan propel a space shuttle?

27 June 2010

Question

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.