Spinlaunch... Can it succeed?

[bdunham7]

Could you put a rocket nozzle on the launch side of the arm, facing inward (towards the axis of rotation). At the moment of launch, you start that nozzle firing, which should stimulate the centripetal force of what the vehicle was exerting.

You would need the nozzle to basically fire instantly. Not sure if some sort of cold gas type thing would be quick enough.

You could have the counter balance be the tank of propellent potentially.

If the projectile is 1000 lbs at launch, and given the estimated dimensions of the launcher there is 7000g at full speed, you'd need about 7 million pounds of thrust, about 5 Space-X Merlin 1D engines.

[Marco]

The counterweight could actially be a liquid holding object, IE a sealed container of water which would get released into the chamber at the opposite/down vector at release.  The inside of the vacuum chamber can have a fins and pipes pattern to gradually disperse the trajectory of that water at multiple angles over a vast distance, all within/part of the vacuum chamber.  Basically, a huge pipe straight down which has many gradual Y splits eventually hitting shock absorbers.

That's a lot of bother and one unholy wet mess near precision machinery just to keep one steel water container reusable.

[Nusa]

Well, the unholy part is easy to solve. Just have the water blessed first.

[MazeFrame]

Just my "spinning things at the end of rope with badly tied knot"-experience: This sounds like a headache and an interesting accident investigation.
Maybe put some effort into launching a rocket from a carrier aircraft instead, that has at least been proven to mostly work.

[Nusa]

Just my "spinning things at the end of rope with badly tied knot"-experience: This sounds like a headache and an interesting accident investigation.
Maybe put some effort into launching a rocket from a carrier aircraft instead, that has at least been proven to mostly work.

Carrier aircraft is Virgin Galactic's thing. Although they do share the same spaceport with Spinlaunch.

[mc172]

Is it even worth drawing a vacuum or are they just doing it because it's the in thing at the moment? *cough* Hyperloop BS *cough*  woooo it uses vacuum technology, invest in us!
The expense of drawing and maintaining a high vacuum vs the work to overcome the air resistance encountered...? If left at ambient pressure, the air inside the disc isn't going to be stationary relative to the moving parts for long as it'll be stirred around by the rotor arm so won't really be an absolute air resistance per se, like it will be once the projectile leaves the launcher.

Oh dear, at those speeds?  I've worked with vacuum systems in the past, things like turbo-molecular pumps, and yeah, you want the air out of there before you wind that thing up.  Having it circulate with the rotor isn't any sort of solution because it will be a very turbulent flow, which means you lose energy and thus the air will never keep up with the rotor.  And if you did manage all that, you'd still have to cover the launch tube part on the inside because otherwise you'd have a resonance there--a sort of hypersonic pipe organ.  It's not a hyperloop type of situation, you need a vacuum.  And the vacuum is probably the easiest problem to solve out of all the complications in this scheme.

What has a turbo-molecular pump got to do with it? Those pumps are optimised for running at ultra high vacuum with extremely tight clearances, there are other pumps (such as turbos on cars) that run in atmospheric conditions at tens and sometimes hundreds of thousands of RPM with housings that also happen to be similar in shape to the Spinlaunch housing. But the rotor arm and projectile assembly isn't a pump, it's the opposite, it's probably one of the most inefficient pumps you can build.

How do you seal such a large aperture (the end of the launch tube) without causing significant drag and potentially introducing large trajectory errors when the projectile has to break through it? The seal isn't going to be a bin bag like the one you see in the video, holding back an atmosphere isn't easy. If you can seal one end of the launch tube, why can't you seal the other instead with the same type of seal, just with different geometry?
Also, if you're worried about the launch tube becoming a pipe organ, why does the launch tube need to be so long? It serves no purpose, why even have a launch tube and not just a hole in the main housing?

I never said drawing such a vacuum would be a significant technical challenge, just incredibly energy intensive.

[Haenk]

I never said drawing such a vacuum would be a significant technical challenge, just incredibly energy intensive.

Compared to the required launch energy, that's probably next to nothing.
However reaching (exit speed plus friction loss) will require some crazy object speed - without air, that speed will be reached faster. However that little bit of air left in there will still heat up the projectile; exiting the vacuum into plain air will then probably vaporize the projectile, no matter what it is made of.

[Fred27]

I can't see an easy way to deal with the imbalance after launch. Whatever happens it would certainly be better to try to recover some of that energy rather than dissipate enough to lift a rocket into space. It's a shame you can't fire two rockets in opposite directions to keep it all balanced and get twice the bang for your buck.

[bdunham7]

What has a turbo-molecular pump got to do with it? Those pumps are optimised for running at ultra high vacuum with extremely tight clearances, there are other pumps (such as turbos on cars) that run in atmospheric conditions at tens and sometimes hundreds of thousands of RPM with housings that also happen to be similar in shape to the Spinlaunch housing. But the rotor arm and projectile assembly isn't a pump, it's the opposite, it's probably one of the most inefficient pumps you can build.

Turbochargers and other high-speed pumps that operate at atmospheric pressures (jet engine compressors, for example) all take tremendous continuous energy to operate and generate huge amounts of heat.  Turbo-molecular pumps operate with a near vacuum at both ends and spin quietly at 60-100kRPM with a fairly low energy input and almost no heat.  Which model works for the case at hand?

I never said drawing such a vacuum would be a significant technical challenge, just incredibly energy intensive.

Why do you think it would be energy intensive compared to the energies required for the rest of the operation?

[T3sl4co1l]

Well what is it, like 10 m^3 or something?  Easy trick to remember, pressure is also power density.  So 10 m^3 of vacuum at 10^5 Pa is also 10^6 J.  Whereas the rotor consumes some MW for minutes to spin up, so is a good hundred times or so more energetic.  Or wait was that hours for the small one, I forgot already and their website is truly unnavigable so screw 'em.

Tim

[mc172]

Turbochargers and other high-speed pumps that operate at atmospheric pressures (jet engine compressors, for example) all take tremendous continuous energy to operate and generate huge amounts of heat.  Turbo-molecular pumps operate with a near vacuum at both ends and spin quietly at 60-100kRPM with a fairly low energy input and almost no heat.  Which model works for the case at hand?

I see your point but atmospheric and vacuum pumps have deliberate features conducive to pumping action or doing maximum work to a gas that the rotor does not.

I mentioned turbochargers as a direct response to the following:

I've worked with vacuum systems in the past, things like turbo-molecular pumps, and yeah, you want the air out of there before you wind that thing up.

...which I interpreted as "spinning things quickly in ambient conditions isn't a good idea". I agree that spinning up a TMP in ambient conditions is probably not a good idea, but that's tangential and for reasons not shared with the Spinlaunch rotor arm.

Why do you think it would be energy intensive compared to the energies required for the rest of the operation?

I didn't say anything about the energy requirements of the entire system, I said:

The expense of drawing and maintaining a high vacuum vs the work to overcome the air resistance encountered...?

To be honest it was just idle musings about how everything seems to require some sort of vacuum element in there these days to be a viable "future technology".

Well what is it, like 10 m^3 or something?  Easy trick to remember, pressure is also power density.  So 10 m^3 of vacuum at 10^5 Pa is also 10^6 J.

This is assuming the pumps do not become increasingly less efficient as the pressure in the chamber reduces.

[madires]

Would a satellite survive the huge centrifugal force in that centrifuge?

[bdunham7]

I agree that spinning up a TMP in ambient conditions is probably not a good idea, but that's tangential and for reasons not shared with the Spinlaunch rotor arm.

Actually the reasons are exactly the same.  If the TMP is already spinning and you expose it to atmospheric pressure, you get a lot of heat.  If you try to spin it up before you have the system pumped down by the roughing pump, you find that it takes several orders of magnitude more torque to even get it going at low speed--and that it is totally impossible to bring it to full speed.  BTDT on a system where we were trying to minimize the back diffusion of roughing pump oil into the vacuum chamber--the bright idea was to start the TMP sooner. 

In the case of the SpinLaunch, the launch velocity is about Mach 6.  Whether you are moving the arms and projectile through the air at Mach 6, the air is moving around the perimeter at Mach 6 or you have some hyperloop-like compromise where the air moves at Mach 3 and the relative motion of the projectile to the air is Mach 3 (ignoring turbulence, which you actually can't--it will be huge), you still have ludicrous amounts of energy being expended and the heat of that will likely melt the chamber and ablate the projectile to oblivion.

 

[mc172]

That's a fair point. I'm with you now!

[Rick Law]

What would the centripetal g force on the payload be compared with simply shooting it out of a very large gun pointed skyward?

In comparing one force verses another acting on the same mass, since f=ma are true for both cases (Newton's second law of motion), mass cancels out leaving you just comparing acceleration.

For the object orbiting around a center, V is the linear velocity while orbiting, and it equals the velocity at release:
Centripetal /centrifugal acceleration is a = V²/R, here R is the distance to the center of rotation.

For the object being shot out of a barrel (or rail of a rail-gun), V is the exit velocity of the object when leaving the barrel
Shooting out of a gun is difficult to say since acceleration by gas-explosion is not constant while in the barrel.  Let's assume it is constant acceleration like you can do with a linear motor catapult or rail-gun.  With such assumption, while in the barrel, then:
Linear constant acceleration would be  a = V²/2L, here L is the length of the gun barrel.

So, if your gun is half the radius of the centrifuge, your acceleration is the same and thus the force are the same.  Of course, radius is just 1/2 the width (diameter) of the centrifuge.

I don't know about these SpinLaunch folks...  I think building a rail-gun 25 meters in length would be easier than building a centrifuge 100 meters in diameter, and a linear rail is far easier to aim.
 

[bdunham7]

I think building a rail-gun 25 meters in length would be easier than building a centrifuge 100 meters in diameter, and a linear rail is far easier to aim.

By my estimates, that rail gun is going to require about a 1 GJ of energy and 40GW of power--30X the 1.21GW required to get a DeLorean into interchronous subspace!  Maybe that's doable with a lot supercaps.

[Nusa]

I don't know about these SpinLaunch folks...  I think building a rail-gun 25 meters in length would be easier than building a centrifuge 100 meters in diameter, and a linear rail is far easier to aim.

With a gun, you need to have the entire energy cost of accelerating the mass to terminal speed in a storage device capable of total dump in a fraction of a second, not to mention the massive equipment required to actually convert that energy to kinetic energy. What would the linear g force be, and how much of the projectile's mass is required to support the launch?

The beauty of the centrifuge is that the energy can be input over a long period straight off the power grid, and the projectile itself is storing almost half of that energy. Sure, there are a few details to iron out, but that's what the 1/3 scale demo model is for.

[SiliconWizard]

Would a satellite survive the huge centrifugal force in that centrifuge?

It's in their FAQ: https://www.spinlaunch.com/faq#p2
10 000 G

As The Breeders were singing:

I know you're a cannonball
I'll be your whatever you want

[Rick Law]

I don't know about these SpinLaunch folks...  I think building a rail-gun 25 meters in length would be easier than building a centrifuge 100 meters in diameter, and a linear rail is far easier to aim.

With a gun, you need to have the entire energy cost of accelerating the mass to terminal speed in a storage device capable of total dump in a fraction of a second, not to mention the massive equipment required to actually convert that energy to kinetic energy. What would the linear g force be, and how much of the projectile's mass is required to support the launch?

The beauty of the centrifuge is that the energy can be input over a long period straight off the power grid, and the projectile itself is storing almost half of that energy. Sure, there are a few details to iron out, but that's what the 1/3 scale demo model is for.

(bold added)

Since f=ma, The linear g force is f=mV²/2L.

What you said about the huge amount of energy in a short span of time is true and it is a huge problem.  That is the problem they are trying to solve by accelerating it slowly in a circular motion.  But in that circular motion, they added other problems, a greater centrifugal force (as compare to linear) being one (f=mV²/R).  Friction heat is another that is worst than linear acceleration.  Slow=time for heat to spread.  So on.

That huge amount of energy is going to be needed one way or the other.  The kinetic energy carried by the projectile is the same whether you accelerate it by moving in a circle, or moving in a straight line.  It is the amount of time you have to inject that energy that is the issue.  Of course, efficiency is a factor too.  Is fast one-time injection cheaper (less waste) or is slow injection cheaper?  What are the other problems (such as system stability) going to cost...

I am not convince that going circular will solve more problems than it costs -- but then I am not an investor so it is not my worry.

[BrianHG]

Thunderfoot part 2:

[m98]

Oh come on, why does anyone still listen to Thunderfoot regarding spaceflight-topics? Aimlessly scrolling through Wikipedia doesn't make an aeronautical engineer out of a (still extremely capable) chemist.

Regarding rail guns. Rail guns don't work, at least not practically, and even the US military is on the verge of totally giving up on that technology.

[mfro]

Hmm.

With the final numbers from the video (100 m diameter, 450 rpm), the payload would reach an effective velocity of about 2300 m/s (roughly a third of earth' escape velocity).
If this worked, it would surely save a lot of fuel for the launch.

Unfortunately, to achieve launch velocity, the payload also had to sustain 11000 g (!) (centripetal acceleration). Far from like 4 g a classic rocket launch would cause.

I would actually assume that this is unsuitable for anything that isn't hard solid (like a bullet). Considering that the device you "throw" still needs a rocket engine to reach space (that is at least a partly hollow mechanical structure and contains compressible fluids), I would even assume it's not suitable at all for anything (other than shooting down the ISS maybe, that is).

[madires]

This is another totally infeasible mess:
- With the poor control of trajectory shown in the test run, how do they want to control something much faster and more heavy?
- When the "rocket" is launched what is the impact of the counter weight still rotating? Bearing killer?
- Are any satellites available which can sustain >10000 g for hours?

[AVGresponding]

Oh come on, why does anyone still listen to Thunderfoot regarding spaceflight-topics? Aimlessly scrolling through Wikipedia doesn't make an aeronautical engineer out of a (still extremely capable) chemist.

Regarding rail guns. Rail guns don't work, at least not practically, and even the US military is on the verge of totally giving up on that technology.

Since you're such an expert, perhaps you could explain why Thunderfoot is wrong, using the relevant mathematics and physics?

[thinkfat]

At some point the projectile will have to leave the vacuum chamber. It will hit the atmosphere at supersonic hypersonic speed. That will be like hitting a wall. It's going to be quite a sight.