Science: Spacecraft: Propulsion
Nanotube fuel   (+5, -4)  [vote for, against]
Using nanotubes as fuel

OK, only a brief description, since that other idea with the space elevator seems to be more interesting.

The idea is to place lots of nanotubes (or nanotube bundles) radially around a core rotating cylinder. Imagine the whole thing in a vacuum container. You start to slowly rotate your packed cylinder and go up to higher and higher speeds. Since you can reach very high perimeter speeds with nanotubes, you could go up almost to the tensile limit, then release the tubes one by one with that high speed out to the back of your rocket accelerating it just like with chemical propulsion.

The trick about it is that you place all the energy into the system in form of rotation before you lift off the ground.

But since it could take some time until nanotubes are cheap enough it is more like a theoretical suggestion.
-- Fledi, Jan 25 2005

Flywheel Rocket Flywheel_20Rocket
[xaviergisz, Mar 24 2007]

How would you anchor and release the tubes? Also, would you have to expend fuel in certain increments to avoid unbalancing the wheel?

What about using a wheel like this to accelerate a more traditional fuel?
-- tiromancer, Jan 27 2005

The tubes would have to be anchored by winding them around a core cylinder, also made of nanotubes. Then you can use a laser placed so that it cuts through the tubes at the required position.

When you strain the tubes almost to the limits it should also be possible to have some steel pieces mounted on the accelerated end and use a magnet close to the tip of the rotating tubes to tear them at the desired position.

Accelerating traditional fuel like rats for example is a good way to have a high ISP substitute to ion engines with the ability to generate much higher thrust for a short time. Unfortunately the mass ratio of nanotubes/rat fuel is so high that you can't get to LEO with that (from earth surface).

The unbalancing of the wheel IS the thrust that has to be led in to the spacecraft structure by the central cylinder, so the higher thrust you want to have, the stabler that cylinder will have to be made including the bearing.

It would be a good propulsion for space however, where you can reload te tubes and slowly get them back to full speed with power from solar cells.

When I first had this idea for some days I thought I had found a way to increase the fuel ratio by taking two nanotube wheels and placing the fuel on a rotating band (like an assembly line or a conveyor belt) and hooking it onto the tubes at the two rotating ends.

Unfortunately there is a force resulting from the unbalanced situation of having a hooked up band on one side and nothing but the rotating tubes on the inner side of the conveyor belt. You need to connect the two rotating cylinders' axis with a nanotube to keep it from tearing, which brings us back to the starting mass ratio.
-- Fledi, Feb 01 2005

//accelerating it just like with chemical propulsion//

Well, no. In fact it’s not accelerating at all.

//The unbalancing of the wheel IS the thrust//

That’s different.

//release the tubes one by one//

Wouldn’t it be better to release one, then release two by two...?
-- Shz, Feb 01 2005

Bad science. Suppose I spin a ball of yarn really fast; I have put energy into spinning it. Now I laze off a bit and watch it fly off. The part that flies off will keep its momentum, the ball of yarn will become lighter, and nothing else will go anywhere.
-- Voice, Mar 24 2007

[Voice] The fact that you put energy in to spin up the ball is the point - that's how the energy is stored. Now you release something and it goes flying off in one direction - the remaining mass has to be accelerated in the opposite direction.
-- MaxwellBuchanan, Mar 24 2007

I think [Voice] is right. Nothing will move. If I accelerate a ball and throw it, I will be pushed in the opposite direction. In this idea, the acceleration has already happened since this ship or whatever it is gets its rotating drum up to speed before leaving the ground.

I wonder, though, if one could use rotational stored energy to power a ship in space. If one suddenly stopped the rotation, where would the vector point. mmm this is reminding me of [Ling]'s gyroscope-powered vehicle ideas. They make my head hurt. In that good can't- keep-from-picking-it way.
-- bungston, Mar 24 2007

// I think [Voice] is right. Nothing will move//

No, you and [Voice] are both wrong. It doesn't matter when or how the energy was put in, any more than it matters how long a rocket has been standing with fuel in it, or how long a spring has been held compressed for.

If it helps, just imagine this whole apparatus inside a black box, with a hole in one side. Now imagine that something comes flying out of the hole at high speed. Of COURSE the box is going to be accelerate in the other direction. It doesn't matter what the source of energy was (in this case, kinetic energy in the spinning mass).

It's basic physics a la Newton.
-- MaxwellBuchanan, Mar 24 2007

[Frank] that is exactly why I put the spinning mass *in* the box in the first place - do pay attention.

If you have a black box that spits something out one way, the box will move in the opposite direction. It doesn't matter a badger's bollock what is in the box and what it is doing, as long as it's inside the box. This is called "conservation of momentum", and it sits very nicely alongside the conservation of angular momentum.
-- MaxwellBuchanan, Mar 24 2007

I don't think this is going to work, on the grounds of that carbon nanotubes do not have enough mass to launch a presumably 180 pound person off terra firma, nevertheless a spaceship. In fact, I can't think of anything that can be reasonably launched besides a rocket that will lift it off.
-- croissantz, Mar 24 2007

[Croissants] - I think the plan may have been to use this for propulsion in space, rather than lifting off from earth (compare, for example, ion drives and other low-impulse drives).

Also, the force produced would depend on the number and velocity of nanotubes in question.

Also. you mean "less still" or "much less", not "nevertheless".
-- MaxwellBuchanan, Mar 24 2007

Sounds like a sort of workableish energy storage mechanism. A battery. As a battery, it should work. Charge it up on the ground, release the energy in one direction. To figure out the energy storage and thrust you would need figures that I don't have, I could look it up, but it's late and I won't because I'd probably get it wrong. I'd probably get it wrong if it wasn't late.
-- wagster, Mar 25 2007

when the first tube is released from the wheel, the whole system will get a pullback because of the imbalance inflicted on the wheel. - the tube will go, let's say "up", the centrifugal force imbalance will be oriented towards "right" then a little lower, then a little lower, then left and low, and in the end, the tube corespondint to the released one will be in the same position in which the previously released tube was and the balance will then be reinforced by a new release. My question is - how do you plan to overcome the effects of these vibrations, since their force is as powerful as the propulsion force?
-- sweet, Mar 25 2007

[sweet] The "pullback" or the "vibrations that you mention are the propulsion. They all act in the same direction. [Fledi] The use of rats would be a little better(I'd have bunned it!), or you could just take a look at the flywheel link.
-- the dog's breakfast, Mar 25 2007

//the "vibrations that you mention are the propulsion// No, that is wrong.

The net propulsion comes solely from the ejected mass (the spat-out nanotube); you would get exactly the same net propulsion if you just fired a nanotube out of the device using a gun, at the same velocity.

The vibration (which arises simply because you're running an unbalanced centrifuge) will cause no net motion; it will just make the device vibrate.

This vibration will be superimposed on the net motion caused by ejection of the fibre. A little elementary physics goes a long way (albeit slowly).
-- MaxwellBuchanan, Mar 25 2007

Yes [Max] net propulsion comes solely from the ejected mass and that is where the vibrations come from. When the mass is stripped away so as to escape in the same direction, it is the remaining system being continually unbalanced but in the same direction=motion. You need to unbolt your centrifuge from the floor...
-- the dog's breakfast, Mar 26 2007

Interesting. The nanotube which should be released is at 90 degrees before pointing backwards. The release of the nanotube causes a change in the centre of rotation (assuming the whole thing is rotating in space, with no bearings). The change in the centre of rotation should not be allowed to increase the radius too much or some of the other nanotubes might experience too much tension. Not sure, but a change in the centre of rotation will bring about a change in rotational speed, to get constant angular momentum.

The details are complicated, but if mass is fired in one direction, the rest must go in the other direction.
-- Ling, Mar 26 2007

I think it must move forward, but I think it will also spin. If you take your black box, sitting quietly in space, but instead of launching your rubber bands out the back you just put the breaks on the spinning mass you'll end up with a spinning black box. This is conservation of angular momentum - even though you can't see angular momentum in your black box from the outside, it has plenty of the stuff.

Since you can't destroy angular momentum, and since you aren't losing any when you shoot rubber bands out of the back, each time your device loses angular momentum it will be added to the ship.

Of course, this doesn't have to be a bad thing. Make your ship comparatively massive, and counter that momentum when you start your next batch of carbon tubes spinning.
-- Worldgineer, Mar 26 2007

I prefer to think that the spinning mass of tubes spins faster, to keep everything constant. Look at it the other way around...a stream of water hitting a paddle wheel, or those wheels, in a partial vacuum, that rotate with sunlight. Where does the rotation come from?
-- Ling, Mar 27 2007

Angular momentum gets me every time. You're right, of course. If done exactly right you should be able to keep your ship from rotating. Angular momentum will be conserved around a point near where the tube is fired.
-- Worldgineer, Mar 27 2007

Just put two lots of spinning nanotubes back-to-back, one clockwise and one esiwkcolc. Then their angular momenta cancel and life becomes simpler. I think this was already done here, though (but not with nanotubes). [EDIT - it was, and is the "Flywheel Rocket" already linked]
-- MaxwellBuchanan, Mar 27 2007

I just realised that the nanotube, that is released, is not only going in a straight line, but it is spinning too! That means angular momentum is conserved for that piece, and also the rest of the spinning mass. The cente of mass is shifted a bit, but nothing need spin faster.

Phew, now the Universe can rest easy.
-- Ling, Mar 29 2007

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