A modern, high-tech flywheel is capable of storing more energy, for its weight, than an electric battery. It can also store more energy per pound than a great many chemical compounds. Flywheels are in many ways ideal for use in outer space, as they can spin nearly forever in a vacuum. Some man-made satellites will soon use flywheels instead of batteries while passing through the Earth's shadow. (See NASA link.)
It is possible to propel a spacecraft with a flywheel indirectly, by taking energy from it as electricity to power an ion engine. I propose to propel a spacecraft with a flywheel directly, by gradually throwing off the flywheel's rim as reaction mass.
Any particle shed from a flywheel's rim is going to travel away at a tangent from the wheel, in a straight line, at the speed of the circumference's rotational velocity. The mass of the remaining wheel will move in the opposite direction. (I thought for a while that there would be no reaction, which would make a neat pistol.)
The rim would have to be shaved perfectly, all the way around, to prevent imbalances developing, and all the shed particles directed in one direction, to make a drive, and the bearings would have to take the load. (Otherwise it would be a flywheel bomb, which would be sneaky, but is already done in science fiction.)
I considered several methods of shaving a flywheel. A cutting tool would slow the wheel, laser beams take electricity, and two flywheels rubbing rims would need to be very precise. I thought of making a flywheel rim out of tape, and cutting the end loose. I even thought of building a centrifugal-fan shroud around the wheel, and directing the particles with electricity, somehow.
Then I saw a ball-throwing machine that featured two car tires about a foot apart (thank you, MXC!) and considered using the flywheel(s) as a mass thrower. That took a lot of balls, and left the slowed-down flywheels in the craft, which is mass to be decelerated. So I settled on a hybrid of shaving and mass-throwing.
My best flywheel rocket design is this: Two flywheels arranged almost rim-to-rim, like a ball-thrower. The nozzle of a bottle of liquid is pointed at the place they almost touch. A tiny drop of the liquid is sent into the gap, and touches both rims at once. The droplet is instantly disintegrated and the teeny bits of it thrown away from the craft. The rims of the wheels are very slightly eroded and the bits of them also thrown away from the craft. The wheels push against the bearings for an instant, moving the spacecraft, and slow a little in their rotation.
The thrust of a flywheel rocket will be limited by the thrust load on the bearings. Stronger bearings than are presently available will have to be developed. Imbalance of the wheels from irregular shaving is a very great risk, and must be considered in design.
Flywheels, shaving methods and mountings will vary, of course, and require experimentation and development. For instance, one possible way to make an erodible flywheel rim is to immerse a hub in a supersaturated solution and rapidly spin the entire arrangement while crystals form radially outward from the hub.
The uses for a flywheel rocket are limited to low-thrust, long-term operation, although a strong particle stream might have uses as a weapon. A flywheel rocket is by nature a stealthy craft, as it emits no flame or light, and little electrical energy. The flywheels can even be used to stabilize the craft. A flywheel-equipped spy satellite could loiter undetected, then move away quietly, or be launched by a conventional rocket and slow to a stop on arrival.-- baconbrain, Sep 14 2006 NASA's Aerospace Flywheel Development http://space-power..../projects/flywheel/Power storage and stability for spacecraft. [baconbrain, Sep 14 2006] Using an imbalance as propulsion Nanotube_20fuel [Shz, Sep 14 2006] Conservation laws http://en.wikipedia...ki/Conservation_law [ldischler, Dec 19 2006] SX.SE: Would a grinding machine be a simple and workable propulsion system for an interplanetary spacecraft? https://space.stack...system-for-an-interRelevant question from today that I spotted in HNQ while deliberately reading but not contributing to the Stack Exchange network [notexactly, Apr 01 2019] A halfbaked propulsion system if ever I saw one. One small problem I found - to be a propulsion system, the unit would need to be exposed to the cold vacuum of space, therefore you may have trouble finding a liquid to use. It might be better to use tiny solid spheres (down to the limit of buckyballs, I s'pose) and have the gap between the flywheels monitored to make sure that it is exactly the right size for one sphere. Using something solid would also make it easier to control the propulsion level (ie. particle flowrate). Other than that, a commendable effort! [+]-- neutrinos_shadow, Sep 14 2006 Cool idea. For the rim of each flywheel to release it's outer layer evenly you may be able to use an ablative material for the flywheel's construction and heat the outer rims at a specific point to release exact amounts of material. (+)-- 2 fries shy of a happy meal, Sep 14 2006 (+)Just a question, how are you going to spin up the flywheels in the first place?-- Galbinus_Caeli, Sep 14 2006 However NASA does theirs. Electric motors or maybe gas turbines. I left out an idea for using regular rockets to launch a flywheel craft with a clamp-on solar panel and motor to spin up the flywheels while close to the sun, to be dropped before using the flywheels to decelerate out past Pluto.
I thought that it would be bad to push around a set of spinning flywheels with a regular rocket, but bearings may be designed to take the stress. Maybe.
I didn't specify the liquid, but liquid helium would never freeze.
Whoops. I didn't see that Nanotube Fuel idea (or the others) when I looked for similar ideas. Well, this is a little different, so I'll leave it up.-- baconbrain, Sep 14 2006 It is different. I linked that as contrast. Where yours is concerned with preventing imbalance, that one intentionally creates it. Just something to think about.-- Shz, Sep 14 2006 NASA is using theirs for energy storage when in shadow. Basically for orbital craft on board systems. Beyond earth direct drive should be practical and avoid the extra mass of the flywheels and the energy loss from storage and recovery.-- Galbinus_Caeli, Sep 14 2006 If, instead of one flywheel, there were two, sheering against each other, with the hubs on sliding bearings to enable them to close in as they wore away, thus reducing the gap, would that solve some of the vibration problem?
Also, could the additional sparks be diverted to heat up this croissant? +-- xenzag, Sep 14 2006 //two, sheering against each other, with the hubs on sliding bearings to enable them to close in as they wore away//
Very good. That's what I meant by "two flywheels rubbing rims would need to be very precise". I don't like the thought of sliding the flywheels while under load, but it would be a lot simpler. If the axes were slightly skewed, there would be a lot more grinding going on.-- baconbrain, Sep 15 2006 With a precision flywheel, you should be able to store .05 MJ/kg. Compare this specific energy density to that of other methods of propulsion: A solid rocket is about 60 times greater, a liquid rocket some 200 times greater, and an ion thruster 9000 times greater.-- ldischler, Sep 17 2006 Thanks, ldischler. I didn't know how to check. My math skills are not that good. Maybe if I made the wheels and the liquid react with each other . . ..
That's why I mentioned the only advantage of this, which is stealth. I had some idea of throwing a small flywheel craft in a direction that will pass an enemy spaceship, and, after all their sensors tell them that it is not powered, it quietly veers to intercept.
As has been said elsewhere in the 'Bakery, this is a solution looking for a problem.-- baconbrain, Sep 17 2006 //precision flywheel//
As mass is shed (from one point, remember?) it becomes unbalanced and wobbles out of control.
This is a solution that breeds more problems, looking for a problem.-- neelandan, Sep 18 2006 why do you need it to be a liquid... use a soft but heavy solid (depleted uranium?) which will be sheared into small bits as it passes between the wheels.-- arvin, Sep 18 2006 //The flywheels can even be used to stabilize the craft. //Counter-rotating wheels negate the stabilization.-- ldischler, Sep 18 2006 //Counter-rotating wheels negate the stabilization.// Wha? I thought they'd cancel out precession and all. Have you a source for that?
//why do you need it to be a liquid// So it squirts itself out of a pressurized bottle with only one moving valve. A flywheel rocket is such a punk drive that weight and complexity has to be kept to a minimum. But yes, a soft solid would work, if the balls were small.
//As mass is shed (from one point, remember?) it becomes unbalanced and wobbles out of control.// If the mass is shed because a droplet drifts into the wheel, it will not be shed from one point. The wheels are whizzing around so fast that a slow-moving droplet would be hit by all parts of the circumference many times. I stated clearly that even shaving would be needed, and addressed that issue in the best way that I could.-- baconbrain, Sep 18 2006 //Have you a source for that?//This is basic physics: If you're just talking about one axis, you add up the angular momenta as scalar quantities. So much positive and an equal amount negative, and the sum is zero. If you have zero angular momentum, you can twist the object as if there were no flywheels in it. (There will still be forces created on the bearings and the support structure, but these will be internal to the spacecraft.)-- ldischler, Sep 18 2006 Oh. Thanks. (Now my head hurts.) Okay, the counter-rotating flywheels add no stability.-- baconbrain, Sep 18 2006 I don't like the idea of gradually disintegrating flywheels leaving a trail of space junk. If the wheels were made of some material that would eventually evaporate in the vacuum of space, that would be better. Nevertheless, a [+].-- AntiQuark, Sep 18 2006 The counter-rotating flywheels add no stability to your craft, but they add enormous loads into their respective bearings during maneuvering. Their net effect is zero, but each wheel itself will need very sturdy mounts.-- elhigh, Sep 24 2006 Even launching from space your platform will have effects of pushing itself against the launch.If you have enough solar panels and ion power to reverse this then you have my vote.NASA's launchers are considered dispensable, at 75 miliion apiece (if they even work).I don't consider that science,it's economic genocide.-- SuperHole, Sep 24 2006 Kinetic energy does not radiate. So, no.
Something spinning in a perfect vaccum will spin until the end of time.-- Galbinus_Caeli, Dec 19 2006 //Does the rotational kinetic energy of a flywheel ever radiate out into the cold of space?//If the flywheel is not attached to anything, no. If it's part of a system, yes--the flywheel will eventually spin down with respect to the rest of the spacecraft due to friction, even if the friction is very small. While the angular momentum of the entire spacecraft remains unchanged, kinetic energy is converted into heat, and the heat is radiated away.-- ldischler, Dec 19 2006 //Has anyone ever done an experiment in the cold of space to prove this? //Look at the earth moon system. The earth is a flywheel, and the moon is effectively another flywheel, but turning much slower. Tidal interactions generate heat on the earth, which slows its rotation while throwing the moon out into a more distant orbit. The total angular momentum of the earth moon system is unchanged, but the kinetic energy is slowly dissipated as the days get measurably longer and the moon gets measurably further away. (See the link on conservation laws.)-- ldischler, Dec 19 2006 If you're squirting liquid in between the flywheels, won't it spray out in a fan? Since the fluid would adhere to the flywheels, some of it would be pulled out to the sides. I think using particles like bucky balls might work better, but some of them still might scatter to the sides.-- discontinuuity, Dec 19 2006 // Sitting in my bathtub today, it finally hit me why this idea can't work.//A sort of reverse eureka moment, eh?-- ldischler, Feb 18 2007 //reverse eureka// indeed.
Sorry, [Brau], but I wasn't making the assumption nor working with the idea that you mention. The power goes out with the shed bits, and the flywheels slow down and stop, probably all too soon.-- baconbrain, Feb 19 2007 Assuming a perfectly frictionless lazer beam is used to knock particles loose off the flywheel, and perfectly frictionless axle bearings, the maximum amount of kinetic energy that this spacecraft could shave off of a flywheel would be: Rotational KE = (1/2) I w^2 where I=(1/2) m r^2.... also, where rw<c and m=m./sqrt(1-(rw/c)^2) due to relativity theory.
This would mean that the more rotational KE you put into the wheel, the more massive it gets, but then the problem is in how to spin it that fast in the first place. Perhaps a nuclear turbine could do the trick for our "flying saucer" thing.... a nuclear turbine might just give it enough spin to launch it to distant planets, that is, assuming the particles that make up the disk itself don't gain enough KE to break the molecular bonds and fly off in all directions at once and the laser etcher is high power enough to etch stuff moving at relativistic speeds (maybe a nuclear turbogenerator should be incorporated to run the laser and other electronic payloads).-- quantum_flux, Feb 19 2007 The release of momentum of the particle would launch the wheel in the opposite direction with equal momentum (as with a rocket). The unbalanced force resulting from this release of momentum (and thereby causing the acceleration of the craft in this case) would be an impulsive snap due to the centripetal force that was keeping the particle from flying off in the first place being released.-- quantum_flux, Feb 21 2007 Would superconductors and high strength magnets provide enough of a bearing to keep the other equipment around the flywheels?
Can we think of other methods of erosion rather than physical contact? e.g. decomposition under UV light?If the heavy component of the flywheel was powdered, held in place by a UV unstable compound, then applying UV to one area, would release particles just after.-- Apothecary, Nov 07 2007 //I thought of making a flywheel rim out of tape, and cutting the end loose.//
Which could be the same as wire, yes. I just couldn't figure what would happen--the first part of the wire/tape might just pull the rest off, or something. Plus there'd be no way to stop/start/throttle the thing.
I like the UV idea.-- baconbrain, Nov 07 2007 If you're heading for a place with an atmosphere can you use the last of the flywheel yo not only propel but spin the craft? If so can you use Bernoulli's principle to counter-rotate the craft against the direction of entry, thereby providing lift and giving the craft more time to slow?-- Voice, Apr 04 2019 Do you mean the Magnus effect?-- notexactly, Apr 04 2019 Yeah, that.-- Voice, Apr 04 2019 random, halfbakery