So how much can rotors take?

[Zorlond]

I'm messing around with a very large build (it's an asteroid eater), and I've run into a bit of a snag. There's a big swinging arm involved to move all the drill bits in a big circle. And it's all rotating on a single rotor. The arm itself is ~300,000kg, the drill bit head is about 230,000kg. And that's after slicing big chunks off. already.

It moves at about one degree every five or six seconds. So. Excrutiatingly. Slow.

Have I gone way past what a rotor can reasonably manage? Should I just swap all the heavy blocks for light and hope it doesn't snap like a twig? Do I have an answer here?

[Zorlond]

Actually, scratch the plea for help... Just after writing this, the arm managed to reach the ticking mechanism meant to move the whole array forward slightly, and once it did, the entire arm started spinning wildly, promting me to dash for the control to turn down the settings from absolute maximum. :P

Obviously, something jostled loose in there, dunno what. But the origional question still seems valid. Just how much can a rotor take before just being unable to move?

[Tal Maru]

http://en.wikipedia.org/wiki/Torque

There you go :D

[Zorlond]

Considering my experimental ship is jittering back and forth between blitz-fast (just after it triggers the forward motion) and slug-slow (all the rest of the time), I think it's safe to say RL calculations aren't in effect here. :P

[TrueGlory]

tl;dr: Depends on 3 things. Is there a force? Where is the force and how big is it? And what is the 'Moment of Inertia' of the object you're trying to turn?

It depends on a few factors: One factor is the forces acting on the object the rotor is supposed to be rotating. For example, are there thrusters on the sides of the object that would resist rate of change of displacement? Or is the object affected by gravity in a way that could resist torque from the rotor?

A second factor would be where the force is acting upon the object. Think of the 'torque' of the rotor as the 'moment' it inflicts on the object. If the other force acting on the object creates a moment equal to the torque that the rotor can provide, then you have reached the motor's limit. For example, a force acting on the end of a beam will produce a higher moment than a force acting near the center of rotation. To help you out with this, here is a handy equation:

T = Fx, where T = Torque caused by a force, F = Force on the object and x = distance of Force from center of rotation.

The first two factors are examples of more simple physics and a very quick number crunch can work those two problems out.


The third factor is noticeably more complex. It firstly depends on the tolerance on a design. What I mean by this is what the engineer deems an acceptable movement, for example, a design that takes 3 minutes to rotate an object at maximum speed might be deemed acceptable, yet a design that takes 3 years to rotate an object at maximum spee might not be. In both cases, the object is capable of moving, but the second moves so slowly it can be considered as 'unable to move'.

Again, the torque comes into this problem. However, the acceptability of the design depends on a variable called 'angular acceleration'. In laymans terms, this is the rate of increase of rotationary speed. This variable is in turn dependant on two other variables, the Torque on the object and the objects 'Moment of Inertia'. To further relaborate, 'Moment of Inertia' is not the same thing as 'Torque'. In simple terms, the 'Moment of Inertia' of an object is the objects 'resistance' to angular movement. The equation relating Torque of angular acceleration is:

T = I*alpha, where T = Torque, I = Moment of Inertia and alpha = angular acceleration.

Torque is easy to figure out. The complexity in this third factor lies in calculating the Moment of Inertia. This can be mathematically calculated by integration, however this only proves useful for simple shapes, where calculations are relatively simple. More complex shapes, although can be worked out mathematically, can take large amounts of time, and are better off measured experimentally.

[TrueGlory]

Also, bugs tend to occur when you immediately attempt to use the rotor at maximum speed in some cases, or set the Torque to maximum without any speed. Turn the rotor off and back on and it should run smoothly. I'm more than happy to have a look if needs be.

[Zorlond]

Go ahead and take a look:

http://steamcommunity.com/sharedfiles/filedetails/?id=261675846

Note that the one and only thing that should be affecting the main arm is the rotor it's attached to. It does have a Gravity Generator at each end, but they are only there to affect the mass blocks attached to the sled, thus pulling it forward every half rotation. There's nothing else out there for those Grav Gens to act upon. So unless Grav Gens can suddenly pull laterally, I don't see how they could affect anything.

And of course there's no Gyroscopes or Thrusters on the mining arm. It's supposed to rotate, those would kill any rotation that would happen.