Hacker News new | ask | show | jobs
by andechs 1652 days ago
The object is not a point mass at the end of the arm, it's a line. Different parts of the line are moving at different relative velocities & accelerations relative to the centre of rotation, and thus when released the rocket not going to go straight.
2 comments
feoren 1652 days ago
The longer the centrifuge arm relative to the length of the projectile, the less of an issue this is. We're already talking about an extremely long centrifuge arm.

Attach the payload at two points in a narrow V, equidistant from the center of mass. In the instant before release, fire pistons that slightly extend the upper arm and retract the lower arm, applying the exact amount of force you pre-calculated was needed to counteract the differences in rotation between those two points. Release while this (hopefully uniform) force is still being applied.

Yeah, you're jolting the hell out of your payload, but that's peanuts compared to what it's about to experience anyway.

link
NovemberWhiskey 1652 days ago
But if the object is rigid, and the acceleration is through its center of mass, then there isn't any rotational component, right?
link
feoren 1652 days ago
GP has a point: consider the limiting case of an extremely long payload on an extremely short centrifuge arm. The payload is basically rotating around its own center of mass already, before release. Put your left index finger on the middle of your right arm, rotate the whole thing, and release -- you'll intuitively see that your right arm must keep rotating.

But see my other response above -- this does not seem like one of the bigger issues when the centrifuge arm is much longer than the payload. And we already know this launch system will only ever work for small, non-human, durable payloads.

link
NovemberWhiskey 1652 days ago
>GP has a point: consider the limiting case of an extremely long payload on an extremely short centrifuge arm. The payload is basically rotating around its own center of mass already, before release.

But isn't the payload rotating around its own center of mass because the centripetal force acting on the "front" of the payload is not parallel with the centripetal force acting on the "rear" of the payload? As soon as you cease to apply the centripetal force (i.e. release the projectile) you're no longer going to generate any torque.

Imagine the payload being held by ropes on the back and on the front and then both ropes releasing exactly as its center of mass passes through horizontal.

link