11
$\begingroup$

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?

$\endgroup$
  • 3
    $\begingroup$ Storage duration is the key here. Capacitors works well between nano seconds to a few seconds, with the lowest energy density. Battery works well on the order of hours, with very high energy density. Fuel cell works well on the order of hours and days, with even higher energy density. Fly wheel works fine around a few minutes, hence all it's current applications. Unless your satellite carries an electric laser gun, currently none of the space application requires a minute-long high power burst. $\endgroup$ – user3528438 Jan 9 at 18:24
  • 1
    $\begingroup$ @user3528438 if you do your research, you'll find that flywheels have very long storage time, with some calculated to hold energy for decades. $\endgroup$ – UKMonkey Jan 9 at 19:03
  • $\begingroup$ it is not just a research project, companies have been selling flywheel energy units for years, but as user3528438 alluded to their typical application in the current marketplace is for handling extremely high energy bursts in a very short time. their main problems are levitation and friction, which arent as big problems in space. it makes me wonder...... $\endgroup$ – don bright Jan 10 at 4:23
  • $\begingroup$ Added a new tag per meta discussion. $\endgroup$ – called2voyage Jan 12 at 17:37
25
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ "flywheels may reach higher densities than conventional batteries" -erm , no. $\endgroup$ – Hobbes Jan 9 at 18:10
  • $\begingroup$ @Hobbes whoops, fixed $\endgroup$ – Dragongeek Jan 9 at 19:07
  • $\begingroup$ Many suborbital (sounding) rockets still use spin stabilization as a means to reduce dispersion without adding the complexity of guidance. $\endgroup$ – Adrien Jan 9 at 20:46
  • $\begingroup$ Spin stabilization is still used, for example on the Minotaur V. Its not a thing of the past. DAWN even used Yo-yo despin as late as 2007. $\endgroup$ – Polygnome Jan 10 at 17:57
12
$\begingroup$

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

$\endgroup$
  • 1
    $\begingroup$ That's still only 10 Wh/kg. $\endgroup$ – Hobbes Jan 9 at 18:57
  • 1
    $\begingroup$ Additionally, the racecar flywheel is connected with a what is essentially mechanical clutch, on a rocket you'd need to include the mass of a generator to convert the spin to electrical power. $\endgroup$ – Dragongeek Jan 9 at 19:10
  • 1
    $\begingroup$ Another negative is that the flywheel could add gyroscopic effects to attitude changes, both resisting attitude changes, and causing torque induced precession. Also, attitude changes would cause a the gyroscope to lose angular momentum. But I don't know how significant these effects would be, given how heavy fueled boosters are. $\endgroup$ – Wayne Conrad Jan 9 at 20:04
  • 1
    $\begingroup$ No. That 1.9 MJ requires 500 kg of flywheel, so that's 3.8 J/kg. My calculation was off, 10 Wh/kg is a factor 10 too high. Flywheels are severely limited by material strength; at high speeds you need very strong materials to keep the flywheel from disintegrating. $\endgroup$ – Hobbes Jan 10 at 9:29
  • 4
    $\begingroup$ No. As quoted, a battery contains 100-250 Wh/kg. I've got a 500 Wh battery sitting on my desk, it weighs less than 3 kg. $\endgroup$ – Hobbes Jan 10 at 9:45
4
$\begingroup$

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.

$\endgroup$
  • 3
    $\begingroup$ Name one current satellite that uses flywheels to store energy. $\endgroup$ – Hobbes Jan 9 at 18:08
  • 4
    $\begingroup$ And flywheels are anything but lightweight. I've provided numbers, if you know better provide a reference. $\endgroup$ – Hobbes Jan 9 at 18:09
  • 1
    $\begingroup$ No current satellite uses magnetic bearings, as far as I know, and while the paper linked describes a theoretical way to integrate energy storage and attitude control, few or no satellites have yet implemented this. (And, of course, if you have a functional set of flywheels, using them also for energy storage costs no extra mass. This doesn't mean it would be worthwhile to scale them up to use nothing else, or worthwhile to use them on a launch vehicle.) $\endgroup$ – Nathan Tuggy Jan 9 at 18:11
  • 2
    $\begingroup$ @Hobbes You're right, they're not currently in use, theyre in current research. The point is, they're not a crazy idea... And I've updated the answer to reflect $\endgroup$ – UKMonkey Jan 9 at 18:47
  • 3
    $\begingroup$ note that the question was about launchers, not satellites, i.e. a lifetime of 10 minutes to 4 hours and no recharging. $\endgroup$ – Hobbes Jan 9 at 19:02

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.