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Comment by dgoodell | Hacker News Reader

I work at NASA. We had a big review meeting for a space brayton (turbine engine) concept. One manager-level engineer absolutely thought that the spinning rotor would apply a constant torque to the spacecraft that would have to be constantly counteracted. I tried and failed to explain conservation of angular momentum to him.

I studied physics and it always surprises me how many good engineers really don’t have a very good intuitive understanding of basic physics principles like conservation laws.

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I’m just checking my intuition here. An accelerating or decelerating rotor would apply an angular force (first derivative), while a constant rotation would not?

Yes. The time derivative of angular momentum is torque.

Great, sounds like our tax dollars are hard at work then.

I'm confused... you are right that it takes no additional torque to keep a frictionless rotor rotating at the same angular velocity, but in the real world there is friction and air resistance where you have to keep applying torque to keep something spinning at the same rate. and if you are applying torque to someone, then that thing is applying an equal and opposite torque back... is it not?

> if you are applying torque to someone, then that thing is applying an equal and opposite torque back

Yes, just like with a force.

> but in the real world there is friction and air resistance where you have to keep applying torque to keep something spinning at the same rate

Yes, but I think you are forgetting one thing: If the friction is between the rotor and the body, then this friction does not only act on the rotor, but also on the body. I.e. the friction creates a pair of torques which the engine can perfectly counteract with it's own pair of torques.

If the friction is between the rotor and the air, then yes, the spacecraft needs some kind of counteraction or it will start turning.

If rotor has 0 friction mounting then sure, no constant torque. Otherwise small constant torque that depends on friction.

See, in physics, you can assume there is no friction. Engineers often forget this one weird trick.

>"See, in physics, you can assume..."

In theoretical model of ideal XXX. Not when asked to actually calculate something practical unless the ideal model is good enough for constraints.

I am not an engineer. Actually MS in physics however ancient ;)

It's an old engineering jibe, of course :)

I suspect that the engineer in the GP's story might not remember the equations for angular acceleration, but has an intuitive sense that if a spinning something is in contact with a non-spinning something, you need a good reason for why some of the energy won't get transferred from the former to the latter.

Which is balanced by the torque applied by the working fluid that's being shoved through the rotor to keep it spinning.

Friction with the helicopter doesn't matter, friction with the air does.

Let me see if I got this: for instance, a helicopter can turn off their anti-torque tail rotor if the speed of the main rotor is constant?

No, because the main router isn’t applying a pure downward thrust. It’s also moving air in the direction of rotation. A turbine has internal baffles and the ducting itself to direct the air in one direction (?)

without friction with the air/medium it would have to turn it off or it would start spinning

Yes and no. The torque in helicopter is primarily a side effect of power being applied to the rotor. When a helicopter auto rotates (such as after an engine failure), the pilot has to remove anti torque forces with the pedals. This remains true even when you change pitch to slow your descent to the ground. So as long as no power is being applied, if you are in a free descent with a constant rotation on your rotor, then no, you will not need anti torque.

Source: I am a helicopter pilot.

Fantastic, thank you!

What about friction? Wouldn’t that cause torque?

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