Flywheel Rocket

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.