Spinlaunch on the Moon

[lamontagne]

A small array of spinlaunchers close to a lunar factory.  Would need to have more for the full output of the factory..

The towers are 25 m tall, 1.5m in diameter, about 1 cm thick aluminum.  The rotors have a radius of 25m.
These ones have 150 hp motors and launch 50 kg every two hours, 500 kg per hour for the array, ot about 2000 tonnes per year.
Spinlaunch without the box, basically.

Awesome! I like the detail of the factory. And I also appreciate the brown tones in the regolith. Too many space artists just assume that moondirt is just pure grey.

Two things come to mind:

First, though this is likely artistic licence, the spacing of the launchers so close to each other and the rest of the plant. A failure on these things is likely to be catastrophic. Close to release, a broken tip is going to hit the ground at ~2000m/s and then kick up a spray of regolith at similarly high velocities, not to mention a LOX payload rupturing. So you want a good deal of space between them. Or use fewer, beefier ones with higher mass at lower rates?

Second, aluminium might not be the best choice since it has quite a coefficient of expansion and has lifetime fatigue issues (the arms will be rotating quite fast, with a wobble when the payload has departed). On that note, I expect you'll have some really DEEP anchoring. How are tensegrity towers for this kind of application? You could make the tower a tripod, pentapod shape or whatever because you just need to clear the arm (failure modes notwithstanding).

For the last mile problem catching, you could probably use some kind of electrostatically charged net. Payload has a positive charge, and so does the net except near the middle.

-Spacing was more or less based on wind turbines.  They probably wouldn't be all aligned in the way I showed them.
-Perhaps just stronger arms.  The arms mass is a kinetic energy storage and the energy is not lost.  so they can be more massive with little energy cost.
-The tower will probably have problems with whiplash.  Need to redesign I think.  I used wind turbine towers,  but I think I'm wrong on this.  There's probably a reason why the large Spinlaunch is on the side of a mountain!
-Perhaps a guyed tower would help.  This puts a lot of emphasis on the foundations and attachment points, however.

[lamontagne]

https://selenianboondocks.com/2009/12/compound-tethers/

Decade and a half ago compound tethers were suggested. Tether with a tip speed of half of final has a secondary tether hub at the tip with similar velocity. Tips hit final velocity on one part of the circumference but reach zero relative on the opposite point. Could handle cargo without spindown/spinup.

Cool concept.  This solves one problem and, perhaps, causes another :-)  I think it increases the acceleration, that is one of the limiting factors.  It adds a latching step, that must happen real fast.  And isn't it going to be a very large device?
It also adds rotating joints that are operating under a few thousand gees, that might be a design difficulty.  Perhaps for generation 2?
I think you need to spin up and spin down anyway, since the work done to accelerate up the payload will reduce the overall velocity of the arms by momentum transfer.  After launch, you will need to speed up back to the '0' velocity.  After launch the grapples will go by the catch point with non zero velocity, until they are accelerated back up again.

[lamontagne]

Second, aluminium might not be the best choice since it has quite a coefficient of expansion and has lifetime fatigue issues (the arms will be rotating quite fast, with a wobble when the payload has departed). On that note, I expect you'll have some really DEEP anchoring. How are tensegrity towers for this kind of application? You could make the tower a tripod, pentapod shape or whatever because you just need to clear the arm (failure modes notwithstanding).

For the last mile problem catching, you could probably use some kind of electrostatically charged net. Payload has a positive charge, and so does the net except near the middle.

-Good point.  I'll go for carbon fiber matrix for the few first ones then, and after that switch to locally produced steel.

-Should have paid more attention during my dynamics classes :-)  It's really an energy transfer and damping problem at that point.

-I really need to do my homework and circularize the orbit.  Instead of waving my hands around and talking of cold gas thrusters.

-A cool thing is that once the design is working, it can be increased to much larger payloads with almost the same physical footprint.  Just need a bigger motor, beefier arm, stronger mast and deeper foundations.

[redneck]

With the moon being in vacuum, would it make sense to spin horizontally? A small hill might suffice for tower. Possibly artificial with the structure built and then ballasted with regolith. Possibly allow multiple destination trajectories?

[DanClemmensen]

With the moon being in vacuum, would it make sense to spin horizontally? A small hill might suffice for tower. Possibly artificial with the structure built and then ballasted with regolith. Possibly allow multiple destination trajectories?

If your launcher is on a mountain or the top of a tall tower, horizontal should work well. This should allow the use of tethers instead of a rigid arm. If you can launch at more than lunar escape velocity (2.38 km/s), you may be able to launch off both ends of a symmetric tether. surround your tower with a circular mag lev and take as much time as you want to build the speed up to launch velocity, then release simultaneously. The tower will see a net zero launch force at release time, but the tension relaxation in the tethers will be fairly impressive. with a track circumference of 23.8 km, you need one revolution every 10 seconds, or 6 RPM. centripetal acceleration is
    a=v²/r
    r=28.3/(2*pi)= 4.5 km
    a= 1.26 km/s²
    a = 1260 m/s²
or about 120g.

Of course, you will need catchers out there to catch both of those payloads. If you stay just below escape velocity, those two points will be quite close to each other.

[lamontagne]

With the moon being in vacuum, would it make sense to spin horizontally? A small hill might suffice for tower. Possibly artificial with the structure built and then ballasted with regolith. Possibly allow multiple destination trajectories?

If your launcher is on a mountain or the top of a tall tower, horizontal should work well. This should allow the use of tethers instead of a rigid arm. If you can launch at more than lunar escape velocity (2.38 km/s), you may be able to launch off both ends of a symmetric tether. surround your tower with a circular mag lev and take as much time as you want to build the speed up to launch velocity, then release simultaneously. The tower will see a net zero launch force at release time, but the tension relaxation in the tethers will be fairly impressive. with a track circumference of 23.8 km, you need one revolution every 10 seconds, or 6 RPM. centripetal acceleration is
    a=v²/r
    r=28.3/(2*pi)= 4.5 km
    a= 1.26 km/s²
    a = 1260 m/s²
or about 120g.

Of course, you will need catchers out there to catch both of those payloads. If you stay just below escape velocity, those two points will be quite close to each other.

The whole point of the rigid arms was to get rid of the tethers    We had a long thread on the subject a few years ago, and I had given up on them at the time.
I've found that tethers are extremely tiresome to design effectively, and that they get suprisingly complicated vey fast.
The rigid arms are much simpler, easier to deploy and have the great benefit that some are being built at this very moment by Spinlaunch.

I have a spreadsheet on the subject.  I've just added circularization to it, but it's something of a work in progress.  Turns out that I will need to use about 5 kg from my 50 kg for propulsion.

[lamontagne]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

[redneck]

I vaguely remember a discussion where the payload slid down the arm as it was rotating such that it was accelerated down as well as around.  Seem to recall that theoretical payload velocity could reach 1.4 times arm tip velocity   

[catdlr]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

I'm on board with this lamontagne.  I like it.

[Exastro]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

What's the reason for making the arms turn about a horizontal axis rather than a vertical axis (so the motion is in a plane parallel to the ground)?  Seems like the latter would be easier to do since it wouldn't need such a high tower.

[lamontagne]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

What's the reason for making the arms turn about a horizontal axis rather than a vertical axis (so the motion is in a plane parallel to the ground)?  Seems like the latter would be easier to do since it wouldn't need such a high tower.

Instinct?
-I think loading the upper arm while the lower arm is rotating might be a problem.  It already looks fairly unsafe as is.
-I was a bit worried about sag, but I guess the answer to that would be to calculate it.
-The vertical one should be easier to aim if you are off the equator, by turning the top a little.

[lamontagne]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

I'm on board with this lamontagne.  I like it.

Thanks, something to go with your icon.

[Proponent]

What's the reason for making the arms turn about a horizontal axis rather than a vertical axis (so the motion is in a plane parallel to the ground)?  Seems like the latter would be easier to do since it wouldn't need such a high tower.

A vertical axis would also allow arbitrary launch azimuths.

[catdlr]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

I'm on board with this lamontagne.  I like it.

Thanks, something to go with your icon.

I like this too

[lamontagne]

What's the reason for making the arms turn about a horizontal axis rather than a vertical axis (so the motion is in a plane parallel to the ground)?  Seems like the latter would be easier to do since it wouldn't need such a high tower.

A vertical axis would also allow arbitrary launch azimuths.

So three other reasons to go.

[lamontagne]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

I'm on board with this lamontagne.  I like it.

Thanks, something to go with your icon.

I like this too

Aye, but can AI do this?

[catdlr]

An updated system.  Two rotating arms per tower.  As one spins up the other spins down.

They are a bit shorter, as I've lowered the height of the target orbit, and hence the velocity.

I'm on board with this lamontagne.  I like it.

Thanks, something to go with your icon.

I like this too

Aye, but can AI do this?

Touche my friend.

[MickQ]

Isn’t there going to be a problem with balance once the payload is released ??

[Proponent]

Well, the actual terrestrial SpinLaunch seems to be able to cope with that.

[lamontagne]

Isn’t there going to be a problem with balance once the payload is released ??

Needs to be calculated.  The launch vehicle is much lighter then the arm(s).  I think the arm heats up a bit from the reduced tension.  The arm itself is a really big gyroscope, so probably something to do with angular momentum comes into play