[gizmo686]

In principle this would work. If they had a spinning mass, then moved the mass closer to the center of rotation, its speed would increase. The reason they do not do this is that it is just not that useful in this context.

One of the major limitatinos of the SpinLaunch approach is the g-forces. As it is, objects launched by SpinLaunch experience a g-force on the order of 10,000G. For a constant velocity, this force goes up as the inverse of the radius. As a result, you want the mass to be as far away from the center as possible so that the g-force experienced is minimal.

The other problem is that this approach is not actually an efficient way of gaining speed. Conservation of energy is still a thing. The rotating mass experiences an apparent centrifugal force pushing it away from the center of rotation (the 10,000Gs that I mentioned above). In order to move move the mass closer to the center, you must counteract this apparent force. At that point, you are likely better off taking the energy you would spend pulling the mass towards the center and simply apply it directly towards increasing the rotational speed.

[redprince]

> In principle this would work. If they had a spinning mass, then moved the mass closer to the center of rotation, its speed would increase.

Actually it wouldn't speed up. The speed of the mass traveling on it's circular path around the center remains the same. The orbit just becomes smaller and thus the path becomes shorter. The mass now makes more rounds in the same time. It is thus spinning faster around the center but traveling at the same speed on its path around it.

[generuso]

It is a common and understandable error to assume that the linear velocity should remain constant.

If the object goes in a circle, the radial force always acts perpendicular to the direction of motion, and cannot change the linear velocity of the object. The centripetal force does not do any work, the energy of the rotating body remains fixed.

But there is a subtle difference when one does pull the object closer to the axis. First, it is easy to notice that one does expend work. This energy must go somewhere, and there is nowhere else for it to go except into the kinetic energy of the moving object.

But how exactly is this energy transferred? If you think about it, when the object is pulled in, it no longer goes in a circle, but follows a spiral. Its velocity is no longer strictly perpendicular to the direction of the force. If you integrate this seemingly small effect, this is precisely what makes pulling on the sting to increase the velocity of the object.

Ignoring the mechanism, a formal calculation, from conservation of angular momentum or conservation of energy would immediately tell how much the linear velocity of the object would increase when it is pulled closer to the axis.