Rocket flywheel instead of battery/generator (crazy idea)?

Question

The Electron rocket is launched using a battery instead of generator...

I saw some video about research into using flywheels for energy grid buffer storage, I think that they could reach higher efficiencies and energy density than batteries, so.. essentially any mass could be turned into a flywheel, what about some component of the rocket? Or even the battery itself, doubling as a flywheel until the momentum is drained and then being used as the battery it is?

Would the gyroscopic effect mess things up?

Answer 1

A flywheel is so efficient because it is big and heavy, both of which you don't want to add to a spacecraft. As for spinning an existing component, the only parts of a rocket that really have any significant mass (would justify the additional bearings and generator) would be fuel tanks. Spinning a fuel tank with liquid fuel in it would be tricky though, especially adapting the piping and fuel lines to work while spun at thousands of RPMs is a significant engineering challenge.

Also, most rockets don't really suffer from a lack of electrical power. Of all modern rockets, the Electron is basically the only one which uses batteries as "fuel" and discards them as the rocket gains altitude. Additionally, while flywheels may reach high densities, they are still nowhere near the immense energy density of liquid rocket propellants. Syphoning off a tiny portion of the fuel is often done with a small turbine which then powers the pumps. In the Electron, this turbine is replaced by batteries for mechanical simplicity.

Once at higher altitudes or in space, electricity can also be generated with reliable solar power which precludes the need for large stores of electrical power.

As for your second question, a large spinning part of a rocket could actually be beneficial as it provides spin-stabilization. Early rockets and long range missiles actually spun the entire rocket to make control easier and the rocket fly straighter without the need for complex control loops.

Answer 2

In principle, this should be possible. The question is, is it practical?

• An example flywheel used in car racing (i.e. a high-performance, cost-no-object application) weighs 18 kg and stores 400 kJ = 400 kWs, is 111 Wh, or 6.1 Wh/kg.

• A Li-ion battery has a specific energy of 100–265 Wh/kg, 16-44 times higher.

So the flywheel solution needs far more weight to store the same energy. And this is all dead weight: it has to be carried to orbit, and can't be jettisoned on the way up.

I also can't see a way to use part of the rocket as flywheel mass without causing complications. A rocket is mostly empty space (the fuel tanks), with some engines at the bottom. You'd have to spin the entire tank section relative to the engines, but you can't do that because there are multiple propellant lines between these sections. Plus the structural strength requirements on such a construction.

To convert between J and Wh:

1 J = 1W.s
A 1.9 MJ flywheel stores 1.9E6/3600 = 527 Wh, 527/500 = 1.05 Wh/kg

Answer 3

The other answerers have failed to do their research correctly. NASA and China are both researching this. The idea is it helps increase the life of the battery on board since there are less charge cycles - but a battery is still used because by just using the flywheels for energy storage, the life of the flywheel bearings is reduced. Using both provides the longest lifetime for an unmanned satellite.

Well, the fly wheel to be used for power grid storage is efficient because it's lightweight; not heavy that the other answerers believe. To be fair to them, JET uses 2 775 ton fly wheels, which spin up to 225 rpm, but they were built 30 years ago, and getting slow heavy wheels is significantly easier than fast light ones.

Energy stored is linear with mass, but squared with velocity - so by having a lighter wheel of half the mass that can spin double the speed without tearing itself apart, double the energy is stored.

These fly wheels are suspended by magnets in a vacuum; and allowed to spin to huge velocities.

So in principle - putting them in space would actually make life much much easier for the vacuum side of the problem. The magnets to stop it hitting the ship aren't particularly light, but can clearly be scaled down for a ship rather than a city; And you could counter any imbalance of rotation forces by having 2 spin in opposite directions.

The problem is the amount of damage that can be done if things go wrong on a manned ship. If a battery goes wrong on a manned ship and you have a fire, you can implement emergency measures, seal the area with the batteries, and eject them. Granted not ideal, but it solves the problem. If something goes wrong with a flywheel spinning at 10k rpm, you won't have a space station to try and save; nor the time to try and save it.

The other issue is that (from the nasa interview) flywheels are good to store large amounts of energy, but not small amounts... Making them impractical for small craft.