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Comment by usefulcat | Hacker News Reader

This is fascinating, but I can’t believe he didn’t really address the elephant in the room (for me anyway). The instant the payload is launched, the launching mechanism will still be rotating at several hundred RPM but will no longer be balanced. I don’t see how it wouldn’t proceed to immediately and spectacularly tear itself apart. So they must have figured out how to rebalance it almost immediately. THAT is what I’d really like to hear about; that seems like the hardest aspect of the whole process.

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He mentioned it toward the end, but did not describe the solution. I'm thinking the counterweight on the other end would need to move a very precise distance in effectively zero time.

EDIT: hmm, or a supplemental weight on the payload side, moving outward the right distance as the payload releases.

It seems like you'd generally want the non-payload rotating bits to vastly outmass the payload, so its release perturbs the whole system to the least amount possible. And then you use regenerative braking to reclaim the energy .

You'd need to reclaim a lot of energy very quickly because after the projectile's exit the atmospheric pressure is going to slow you down right quick

You could slam a blast door closed behind the exiting projectile. It should be possible to place it in the exit tunnel so that it is fully closed before the inrushing atmosphere gets to it.

Plausible. Do you know how fast such doors can close?

I wonder why they don't just have an inert concrete mass on the other side and an exit port that opens into a sand bunker, sure it's a lot of energy but they don't have to contain it, only limit back splash from the mass hitting the sand. Doesn't even need to be enough sand to stop the mass completely, just enough to catch fragments, the mass can continue into the earth with no consequences

Yes, that's an intriguing idea.

What I came up with was this: the launch arm has a second mass inside it, that will move (via centrifugal force) away from the center, in the same moment the payload is released.

Of course, just having two payloads launch in opposite direction as you propose maybe has much less implementation problems (though I wonder how you stop that second dummy payload in a way that does not cause an earthquake or explosion :)

Sand will melt with all the energy absorbed. Water is much better.

The idea I heard was to just yeet a counterweight straight into the ground. Their system does not appear to have symmetrical exits though.

Seems like the simplest solution, throw it in to a hole in the ground.

It could work, assuming you can shield yourself from the inevitable explosion once that mass impacts something.

Maybe they can redirect the ejected mass away from the launch site with a huge curved underground tube Mythbusters' style: https://www.youtube.com/watch?v=73FFitene58

Scott Manley talked on youtube about the centrifugal force on the wheel being around 10 thousand g (in the full sized version). So how do you build something that touches a tunnel at hypersonic velocity with many thousand tons of force without disintegrating? It's not like you can just add wheels or skates to your ejected mass (or the tunnel).

You got me curious, so I dug up their patent. This is as close as they come to addressing it.

> Although not shown in the previous figures, the circular mass accelerator structure 150 may comprise a second exit port directly opposite the exit port 115 to capture the counterweight 135 that is released simultaneously with the launch vehicle 105 to minimize an imbalance on the motor at the time of release. The counterweight 135 may be a solid material, or a liquid such as water.

The use of a liquid is a curious idea. Perhaps it could be dispersed in such a way as to spread the force of the counter weight being released across a wide surface area? Like a small explosive forces the liquid out in all directions?

https://patents.google.com/patent/US10202210B2/en

Whether liquid or not, running the formula for kinetic energy of the counterweight, I'm getting something around 1 GJ (gigajoule) for a weight of 1 ton at 1500 m/s [1].

According to this site [2] that's equivalent to around 200 kg of TNT. Even with the counterweight being mostly water, that's quite a lot of energy to disperse. How does one evenly spread out the water to a surface the size of a football field? Would that even be enough area to prevent a shock wave being reflected back at the launch equipment?

[1] https://calculator.academy/joule-calculator/#f1p1|f2p0

[2] https://www.convert-me.com/en/convert/energy/tntkg.html?u=tn...

There's a slide in Manley's video that says gross vehicle weight is 11,000 kg. And 450 RPM @ 100 m diameter, so 2 km/s. Kinetic energy for vehicle is 22 GJ. Equivalents:

* 5 metric tons TNT

* 1/3000 Little Boy atomic bomb

* 4 barrels of crude oil (!)

* Boil 2,300 gallons of water

* 0.25 mg of matter converted to energy

* Enough energy to melt two 11,000 kg iron counterweights, with 2 GJ left over

* A magnitude 3.7 earthquake [1]

[1] https://www.volcanodiscovery.com/earthquakes/energy.html

Perhaps it would be possible to use liquid carbon dioxide. The change from liquid to gas inside the vacuum chamber might help disperse the energy.

Yeah, I didn't see any explanation for that either. With that much kinetic energy around I wonder if it's designed to simultaneously launch a portion of the counterweight.

It's got a lower lever so the overall momentum might be low enough to make that recoverable. Maybe if it's 10% of the rocket's momentum, going into... yeah, it's hard to imagine that being recoverable but maybe it's just weights and that's good enough.

I'd love to see a real explanation.

I was guessing, its water in a container with bomb bay like doors, when the payload is released the doors open and the water goes into some complex structure that takes the energy out of the water. but only a guess.

Isn't the simplest solution to just always launch things in pairs?

If you take the simplest design then the second object would be launching into the ground, which is basically what they're doing with the counterweight. Alternatively you could have a contrarotating system, but I'd then be worried about two high speed objects travelling close together. The best solution would probably be a metal counterweight, so that you can regeneratively decelerate it from it's exit port.

Build the thing on a sea platform. Launch the counterweight into water. Make it sharp-nosed like the payload, so it could potentially survive impact with water and be recovered.

>so it could potentially survive impact with water and be recovered.

Doubtful even if pointy. Water is way denser than atmosphere

Hey! Deep sea and space exploration at the same time! (You just have to get complementary mission designs.)

Something like 10,000g to 0 in an instant—it does seem to be the largest engineering problem with a full-scale design.

You can just launch a dummy weight in the opposite direction into a body of water.

He did mention that in some detail.

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