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Comment by curiousfab | Hacker News Reader

If it sounds too good to be true, it usually is.

Spinlaunch: BUSTED!: https://www.youtube.com/watch?v=9ziGI0i9VbE Spinlaunch: BUSTED (Part 2): https://www.youtube.com/watch?v=ibSJ_yy96iE

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Man, I have my opinions on Spinlaunch, too, but I find thunderf*t’s videos (especially the BUSTED ones) unenlightening & unhelpful to rationally discussing the pros and cons of some technology.

Scott Manley’s video (if video is the format you prefer) on Spinlaunch was much better. Scott Manley also has a much better understanding of rockets and space technology than thunderf*t. https://youtu.be/JAczd3mt3X0

Thunderf00t can show great science at times, but most of his channel is just opinion pieces disguised as science. Most of his arguments are sound, which is not very difficult considering some of the things he attacks, but some are more debatable, or even just wrong. And he never back down, and he has a community of mindless followers which is more fitting to a cult leader than to a man of science.

His analysis of trajectory (the shadow of the missile against the terrain) was laughable.

Thunderf00t is a contrarian with a cult audience.

The tumbling in this video is a good demonstration of the angular momentum problem. The projectile has a carp ton of angular momentum after release, and there's no system in place to fix that. Maybe you could cancel it using the atmo and aerodynamics except that would probably melt the fins.

And angular momentum is just one of the challenges that appear to be unsolvable. When you hit orbital height, your velocity is in the wrong direction. How do you add enough horizontal velocity? How do you construct a rocket engine that can survive the forces involved in initial launch? How do you prevent the bearings from melting? How do you construct a launch chamber which survives the explosive compression when the projectile penetrates the vacuum barrier into the atmo?

And all with a Theranos-level of founder knowledge.

>The tumbling in this video is a good demonstration of the angular momentum problem. The projectile has a carp ton of angular momentum after release

I'm not sure I follow; if the object is held rigidly to the launch arm until the point of release, it's going to have a lot of angular velocity, sure, but that doesn't imply rotation, does it?

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.

But if the object is rigid, and the acceleration is through its center of mass, then there isn't any rotational component, right?

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.

>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.

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.

> When you hit orbital height, your velocity is in the wrong direction.

Wait, why is your velocity in the wrong direction? You're right that you need to add a whole hell of a lot of horizontal velocity to reach orbit, but you do have ~400 meters per second of horizontal velocity borrowed from the surface rotation of the Earth. You're not going the wrong direction -- there's nothing you need to counter-act. You just need your 2nd stage (assuming we count the initial throw as a 1st stage) to provide all that velocity. The fuel you need to do that was launched up with you. What's the problem? It's a hell of a lot easier than the way we do it now.

> How do you ... survive forces involved

Yes, that is the real problem.

> The projectile has a carp ton of angular momentum after release

No, the centrifuge has a crap ton of angular momentum, some of which is converted to linear momentum in the projectile upon release.

Yep, the "wrong" velocity is what you suggest: the horizontal velocity is missing. Keep in mind the horizontal velocity is the critical detail that distinguishes "orbits" from "crash landings."

There is no conversion to linear momentum. The projectile is rotating once per turn of the centrifuge. Nothing they do eliminates that angular momentum. That's why in the linked video the projectile is, on close examination, rotating wildly. It's also why the projectile's nose exits the apparatus at a very different location than the tail.

I play Kerbal Space Program, so I know a thing or two about orbital mechanics! (I'm only 30% joking -- many of the answers on the Space Exploration Stack Exchange say "Go play Kerbal Space Program, then you'll understand). Yes, that horizontal velocity is hard to attain, but that's what 2nd stages have always been for. We're only talking about replacing the 1st stage, which is by far the most expensive. You're still way better off already being at orbital height, never having to accelerate directly against gravity and through all that air resistance. And the speed you need to reach orbit is less the higher up you are.

> The projectile is rotating once per turn of the centrifuge.

I keep trying to challenge this, but I think you must be right. I was thinking the angular velocity would be smaller than the arm, but it must be the same. Still, the angular momentum can be much smaller. And I'll paste my proposed (possibly infeasible) solution to that from another answer:

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

The projectile is rotating at the same rpm as the arm it was attached to.

Random other idea: just put your entire payload inside a spherical fairing and launch the whole ball into space. Once past atmosphere, eject the fairing and stretch out. Like a dancer extending their arms, this itself will slow your rotation down. Then fire RCS thrusters anti-rotationally or use a reaction wheel (can we pre-load the reaction wheel with oppositional angular momentum before launch?) until you're steady.

Bonus: can you use the rotation of the ball to help guide you, curveball-style, into the direction of orbit?

Thunder started out great but now he just likes to rationalize the dominant perspective. It’s a loss.

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