What makes Spin Launch a promising alternative to traditional rocket launches?

[sophiecentaur]

TL;DR Summary

Spin Launch is a proposed method of launching small, rugged, payloads into orbit. So far, there's not a lot of ready information about it. But does it have legs?

Spin Launch is a proposed method, as a first stage for launching small payloads into orbit, using a slingshot method. There seems to be a video of a successful one-third scale test.

Would the numbers add up, though and would it be better than an ordinary first stage vehicle? One advantage could be that the system would involve a large reusable static 'rotor', driven by electric motors which could launch small (second stage) rockets with small payloads, as opposed to massive first stages and sometimes many separate small experiments. i.e. possible economies of small scale.

Questions could include the total energy involved and the losses of passing through the lower atmosphere at higher speeds than those involved in conventional rocket launches.

Somebody has actually committed to actual hardware because the video shows it operating.

 

[berkeman]

I wasn't able to find the g-forces involved. Have you seen them?

 

[sophiecentaur]

I wasn't able to find the g-forces involved. Have you seen them?

There are a number of figures quoted, in amongst the purple passages about Mankind and stuff, I wasn't sure which figures apply to full or third scale example. Certainly the g forces would exclude use by humans. The v²/r formula imposes a sort of limit, depending on what radius you'd be wanting and what friction losses could be tolerated. In the limit, you'd be into the rail gun in a vacuum alternative.

But horses for courses. If the spin launch would work for small individual satellites then fair enough. I was hoping for some of the vast potential of PF to help me with this.

 

[anorlunda]

Prominent space tech engineer Scott Manley pretty much debunks the spin launch idea.

 

[hutchphd]

Here's one way to think about it. Almost all the additional energy needed for Low Earth Orbit is the kinetic so roughly $$ \frac {v^2} {R_{Earth}}=g$$ The centrifuge of radius r requires a radial acceleration $$\frac {v^2} {r_{centrifuge}}=a_{centripetal}$$ so $$a_{centripetal}=\frac {R_{Earth}}{r_{centrifuge}} g$$
This would seem limiting even for LEO.

 

[sophiecentaur]

pretty much debunks the spin launch idea.

Imo, he's more gentle than that with it. He does use some figures and points out problems. Suggesting the use of the spin launch from the Moon sounds interesting.
His final line is that it would be nice to find a system that doesn't rely on rockets, which is how I feel too.
Just imagine ameliorating the imbalance problem by sending two vehicles in opposite directions at the same instant. That could reduce the angular momentum impulse problem with one vehicle going to a higher solar orbit and another vehicle going to a lower solar orbit.

 

[berkeman]

Prominent space tech engineer Scott Manley pretty much debunks the spin launch idea

Not at all; he seemed quite impressed and hopeful that it will keep moving forward.

And it looks like the orbital launch system will involve about 10,000g of centripital acceleration applied to the rocket (max just before launch)...

 

[sophiecentaur]

This would seem limiting even for LEO.

Yes but that's been accepted. The proposal is to use spin launch as a first (sub orbit) first stage to lift a small rocket for the final stage.

 

[anorlunda]

The ambitions for Spin Launch sound very similar to the Pegasus Rocket that launches from an airplane, and then carries small payloads to LEO.

https://en.wikipedia.org/wiki/Northrop_Grumman_Pegasus#Partial_successes

However, runaway costs seem to have swallowed Pegasus.

Pegasus is terribly overpriced in today's market, they don't get many contracts these days (all launches in the last 15 years were from the US government). You can launch half that mass with Electron for ~$6 million, or over 20 times the mass with Falcon 9 for 50-60 million.

I wonder what the payload for Spin Launch would be accounting for the weight of an ablative heat shield, plus rocket motors, fuel and guidance.

 

[sophiecentaur]

accounting for the weight of an ablative heat shield

What sort of Energy would the heat shield need to dissipate? A small fraction of re-entry energy, no?

 

[Vanadium 50]

I wonder what the payload for Spin Launch would be

Ketchup.

 

[anorlunda]

What sort of Energy would the heat shield need to dissipate? A small fraction of re-entry energy, no?

I don't think so. Spin Launch needs maximum velocity where the atmosphere is most dense. Reentry is the opposite; minimum velocity at low altitudes.

See the Sprint missile tests from the 1970s. And Sprint did not need maximum velocity at ground level.

https://en.wikipedia.org/wiki/Sprint_(missile)
Sprint accelerated at 100 g, reaching a speed of Mach 10 (12,300 km/h; 7,610 mph) in 5 seconds. Such a high velocity at relatively low altitudes created skin temperatures up to 6,200 °F (3,430 °C), requiring an ablative shield to dissipate the heat.

 

[berkeman]

I don't think so. Spin Launch needs maximum velocity where the atmosphere is most dense. Reentry is the opposite; minimum velocity at low altitudes. See the Sprint missile tests from the 1970s. And Sprint did not need max velocity at ground level.

Sorry for the dumb question, but how come hypersonic tactical missles don't require ablative heat shields?

 

[hutchphd]

I know the X-15 required the Pink spray-on ablative for its 4,500 mph highest speed flight and melted badly enough to never fly again. But I'm sure duration will matter etc.

 

[berkeman]

I know the X-15 required the Pink spray-on ablative for its 4,500 mph highest speed flight and melted badly enough to never fly again. But I'm sure duration will matter etc.

But wasn't that at 100k' or something?

 

[hutchphd]

Yes so the heating will be worse lower down. Where do hypersonic weapons do their maneuver thing? Isn't it lower (and thereby worse for heat?) I'm certain there are boondoggle chasers willing to $$$olve the problem!
There were holes in X-15

 

[berkeman]

Thread moved from the Astrophysics forum to the Aerospace forum with a redirect left behind.

Paging @boneh3ad

 

[anorlunda]

Sorry for the dumb question, but how come hypersonic tactical missles don't require ablative heat shields?

Because they don't achieve maximum speed until they reach higher altitudes.Here is a capture from that Scott Manley video.

If you watch the video of a Falcon 9 launch, you'll hear the term "Max Q" at about 90 seconds. That's maximum dynamic pressure. It doesn't occur at launch because of lower speeds. It doesn't occur later because the atmosphere is less dense.

Max Q for Spin Launch would be at ground level by definition. No calculations are needed for that. So the skin would be most stressed and rate of change of temperature would be greatest at ground level.

Scott Manley said that it could probably work as an engineering project, but the big question is can it do it profitably? That comes down to $/kg payload in LEO. In an earlier post, I quoted @mfb saying that the Pegasus LEO launch system died because of too high $/kg. That makes things like heat shield and strength of the external skin critical because it subtracts from payload.

Here's SpinLaunch's claim that remains to be proved.

https://interestingengineering.com/spinlaunch-catapulting-nasa-payload
SpinLaunch says it will eventually be able to send about 440 lbs of payload to orbit at a fraction of the cost of other satellite launch services, such as those provided by SpaceX, ULA, and other space companies.

 

[berkeman]

Because they don't achieve maximum speed until they reach higher altitudes.

 

[hutchphd]

I just don't see this centrifugal thing. Too much watching of Punkin Chunkin I fear


Of course I don't understand all the hubbub about hypersonic weapons either. There are ICBMs that can MIRV the heck out of any city with precision Megaton delivery and we are concerned that an aircraft can manuever at 5 Mach. Oh my good Lord now I'm scared...

 

[Vanadium 50]

The Orbital Punkin Chunkin (or the Vernean Cannon) has an acceleration of v²/2s. Here v has to be at least orbital velocity (more because of frictional losses to atmosphere) and s is the ramp length. At 7500 m/s and 100m, it's 28000 g's.

Using a circular "slingshot" design let's s get larger so a gets smaller.

 

[mfb]

You can't launch directly to an orbit from the ground - that orbit would intersect the ground. You need a rocket anyway. SpinLaunch wants to reach ~2.2 km/s, enough to get through the atmosphere, with most of the speed coming from a rocket engine once it's approaching space. With a radius of 50 m the target acceleration is 10,000 g.

Drag is worse for smaller rockets, which sets an effective lower limit on the size of ground-based launches. The experimental record is a 2.6 tonne SS-520 rocket delivering 3 kg to orbit. If no rideshare option works then you'll need to buy a multi-tonne multi-million (USD) rocket to go to orbit, even if your satellite is just 1 kg. If a rocket "launches" in vacuum it can be much smaller. Suddenly your 1 kg satellite can be launched by a ~30 kg rocket and some ablative heat shield (to be discarded before/when the rocket ignites), which will be much cheaper. The centrifuge costs more than the big rocket, sure, but that is easy to reuse many times.
The big downside is the giant acceleration the system needs to withstand. It's not unheard of - we have guidance units withstanding 15,000 g.

The idea is interesting, but no one knows if the market is large enough to make this profitable - assuming they can overcome the engineering challenges.

There are also concepts to launch rockets from a balloon. They come with their own challenges.

 

[hutchphd]

Using a circular "slingshot" design let's s get larger so a gets smaller.

So I don't understand.
$$v^2_{punkin}=r a_{centripetal}$$ but assuming the gun fires uniform acceleration,$$v^2_{gun}=l a_{gun}$$
Seems similar.
Anyhow that's a pretty strong pumpkin.
The KE of 1kg in LEO is 32 MJ which is a few dollar's worth of electric energy. Certainly is tempting

 

[Vanadium 50]

You can't launch directly to an orbit from the ground -

Is that true with an atmosphere? I'm thinking that "bouncing" off the atmosphere on the downward arc may circularize the path.

 

[hutchphd]

I think one could do that but the cost would be an even faster (than orbital) initial speed. What is best lift to drag ratio for a quasiorbital object in the upper atmosphere?
As I consider it further, I am less convinced that it is possible without a reaction motor onboard. How would it work?

 

[Vanadium 50]

Because you're going up through the atmosphere you need to start off faster than you would on an airless world anyway.

Imagine after exiting the atmosphere the first time someone places a large rubber sheet over the entire upper atmosphere. That will clearly work. So it seems to me that the question is one of practicality, not possibility. It's a lot easier to skip a stone once than a hundred times.

 

[hutchphd]

Imagine after exiting the atmosphere the first time someone places a large rubber sheet over the entire upper atmosphere. That will clearly work.

I don't see it. Next time around it still bounces
Of course if the sheet is omnipresent then the required orbital speed is reduced to zero but that is not the problem at hand.
I believe your proposed bounce (even if perfectly elastic) will only shift the intersection point with the surface and not circularize the orbit.

 

[Rive]

I'm thinking that "bouncing" off the atmosphere on the downward arc may circularize the path.

As far as I know that kind of 'bouncing' eats up a lot of kinetic energy, and the result will be still an ellipsoid orbit with the next reentry scheduled. So - it may give time to the engines to do some trick, but cannot really 'circularize' in itself.

Regarding SpinLaunch itself: interesting idea, but with the reusable rockets at full swing I don't think it has much future left. Maybe some military application might be considered? Regarding the peak/average power demand it's far better than those hyped railguns.

 

[sophiecentaur]

I believe your proposed bounce (even if perfectly elastic) will only shift the intersection point with the surface and not circularize the orbit.

The craft would need some 'flight control'.

Also, every 'bounce process' involves loss of energy which doesn't help with achieving LEO. I think there's a fair bit of urban myth about what's involved with re-entry. We were always told by the press that if the craft bounced then it would leave Earth orbit for ever. Whilst this could apply to a very eccentric orbit approach, with the low energy associated with Low (or even not quite low) orbit, the crafty would come down eventually - just not in a convenient place- having lost energy at each bounce. Loss of craft and crew, of course.

From the estimated figures( in the various links) of the full scale launch, the implication seems to be that at least half the launch energy would be lost through friction, low down. Comparisons between the required normal re-entry braking and the low drag design of a spin launched craft need to be made carefully. I remember, way back, posting a PF question about why not glide slowly to Earth on re-entry and it being pointed out that, for a slow glide, the net temperature rise inside the craft could be lethal; the system relies on a quick re-entry with ablation and atmospheric cooling near the ground and good insulation. The ablative heat shielding requirement would be less dramatic than 'those tiles' .

 

[mfb]

So I don't understand.
$$v^2_{punkin}=r a_{centripetal}$$ but assuming the gun fires uniform acceleration,$$v^2_{gun}=l a_{gun}$$
Seems similar.

There are two different accelerations for a slingshot. The circular acceleration you calculated is "free", you only need to compensate friction. The acceleration in tangential direction - increasing the speed of the payload - can be done slowly, over something like an hour with an acceleration well below 1g, lowering the peak power. A cannon can't do that, it needs an extremely high peak power and needs to provide all the energy for the high acceleration.

Is that true with an atmosphere? I'm thinking that "bouncing" off the atmosphere on the downward arc may circularize the path.

Conceptually yes, but I can't see how this would work with our atmosphere. To limit drag in the atmosphere you need to launch with a significant angle. SpinLaunch wants to use 35 degrees. That angle decreases a bit while crossing the atmosphere but you still come in at a very steep angle after the first orbit. That needs to be an absurdly efficient lifting body to stay in orbit. And you are not done after the first hop. Your perigee is still low, and your only good way to change the orbit is close to perigee where changing the perigee is very inefficient. Ultimately you want to circularize your orbit using the atmosphere at the height of your final orbit - where you then want the atmosphere to be thin enough to stay in orbit for a while.

For orbital velocity you probably want to be far higher, at the very least on top of a tall mountain. StarTram made some calculations that a capsule could survive a ~9 km/s shot at ~6 km altitude and 10 degree angle. ~20 g initial deceleration from the atmosphere, 800 m/s total velocity loss. That would need a relatively small circularization burn (~0.5 km/s) after half an orbit.

 

[hutchphd]

The acceleration in tangential direction - increasing the speed of the payload - can be done slowly, over something like an hour with an acceleration well below 1g, lowering the peak power.

So what? The centripetal acceleration will make you just as flat. The power will be less as you say.
If you want to talk about the rate of change of the acceleration (known as the "jerk" where I come from) then you you may have a point but I am not convinced it is very salient.

 

[Grinkle]

This interview / discussion / 'factory tour' was very interesting. I recommend it to anyone with interest in the spin launch physics, engineering and associated launch economics.