# Would space ship/station rotation for artificial gravity cause the “axis” to also rotate?

Would space ship/station rotation for artificial gravity cause the "axis" to also rotate similar to that of a helicopter with out a tail rotor?

Additionally would a second rotating section going in the opposite direction be required to negate the rotation due to inertia?

• Additionally would a second rotating section going in the opposite direction be required to negate the rotation due to inertia? – Gip Apr 12 '18 at 14:58
• There are helicopters with out a tail rotor that do not rotate. – Uwe Apr 12 '18 at 14:59
• They typically have a second rotor someplace else, or some other mechanism to stop the rotation of the aircraft. en.wikipedia.org/wiki/NOTAR – Gip Apr 12 '18 at 15:04
• @Gip it might be better to move the "Additionally..." back into your question if it is part of your question. Click edit. Once you are comfortable, you can delete your comment if you feel like it. – uhoh Apr 12 '18 at 15:08
• I do not fully understand your question. Can you restate? The main rotor of a helicopter continuously interacts with the air, causing a torque on the fuselage that is counteracted by the tail rotor or other means to hold the fuselage steady. In space the torque comes from tangential rockets, the changing speed or orientation of a momentum hweel, an interaction with an external magnetic field, et cetera. After these torques stop, the station angular momentum ceases to change. – MBM Apr 12 '18 at 16:42

## 2 Answers

tldr: No, once an object is rotating, its axis will remain fixed in the absence of external forces.

## Helicopters

A typical helicopter's rotor experiences drag which would causes it to slow down (transferring angular momentum to the surrounding air). In order to keep the craft aloft, the helicopter's motor needs to provide a constant torque to the rotor. This causes the body of the helicopter to rotate in the opposite direction at an increasing rate.

Since it's difficult to pilot a helicopter while vomiting, a tail rotor is used to provide a reverse torque on the body of the helicopter to keep it steady. The reverse torque is adjusted to match the torque of the motor (slight changes in the reverse torque allow us to yaw the helicopter).

## Rotating Spacecraft

Unlike a helicopter, a Spacecraft operates in a vacuum and therefore does not experience angular drag on its rotating parts thus no torque is required to keep it spinning.

The gyroscopic effect (a consequence of conservation of angular momentum) ensures our axis of rotation remains fixed. This is very useful for real-world spacecraft if we want to keep their solar panels and antennae pointing in a constant direction. It is also useful if we want to spin our entire vessel - as with a gravity ring.

## Caveats

Unfortunately, the real world often gets in the way of idealised physics. Here are some problems you may encounter:

• Our ideal model assumes that there is no (net) external force on our spacecraft. In deep space this is a good approximation, but things like solar winds, atmospheric drag and tidal forces can produce a small net torque. This can result in torque induced precession where your rotation axis vector traces a circle in space.

• Unless your rotation axis is perfectly aligned with one of the principal axes, you will experience torque-free precession. This is a constant periodic 'wobble' that can be seen when throwing a frisbee. This becomes virtually guaranteed we consider that a manned spacecraft will always have its internal mass distribution changing - people moving around inside will change the moment of inertia and cause an off-axis rotation.

• If you choose to spin on the wrong principal axis, you will encounter the intermediate axis theorem and your spin will be unstable, causing the rotation axis to flip periodically. This video provides an excellent demonstration of how to get spacesick.

• If our craft has a rotating part and a non-rotating part, friction at the joint will conspire to spin-down the former and spin-up the latter.

All of these can be mitigated either with very careful design (see spin stabilised space probes), active management of the mass distribution or active attitude management using on-board thrusters.

Counter-rotating parts can be used to avoid the first three problems by having zero net overall angular momentum, but it still suffers badly from the fourth issue of friction.

If they have no contact, and in fixed speed in space, the answer is No (almost impossible).

But if they are connected, yes. There is no perfect zero-friction device. You are dragging one against the other.

You may need a torque adjustment (maybe some arms or ropes with adjustable length for calibration) to keep the axis stable, or maybe you may want to have the axis counter-rotating and get two habitats (main and axis) with gravity to avoid such a device.

Note you will have a gyroscopic effect if in orbit.