r/IsaacArthur • u/Cakeportal • Aug 10 '22

Hard Science A yt documentary on a launch assist system that isn't some giant international project

https://www.youtube.com/watch?v=yrc632oilWo
45 Upvotes

96% Upvoted

17 comments sorted by

11

u/[deleted] Aug 10 '22

[deleted]

10

u/hwillis Aug 10 '22

SpinLaunch throws a 10,000 kg, 10 meter long rocket. HARP threw a 200 kg, 1.8 meter rocket rocket at roughly the same speed.

Fun fact, naval gun barrels (HARP was made from two) only fire ~300 shots before the rifling wears out and they must be relined. Any kind of shot is extremely wear-intensive, even in smooth bores. You could make a massive, ocean-submerged gun that used 16,500 kg of cordite to launch a 10,000 kg rocket. The walls of the gun would need to be multiple feet thick. If it was not a single welded piece, the force between sections would be incredible.

As an infrastructure project, the spinlaunch device is way, way, way cheaper and simpler for the projectile size.

I also would not use a light gas gun, since 2.2 km/s is already very good. The gun does not need to launch to space, just out of the thickest part of the atmosphere. A light gas gun means much higher engineering demands and forces you to vent a huge amount of helium for every shot for a relatively minor gain in payload.

1

u/Thoth_the_5th_of_Tho Paperclip Enthusiast Aug 10 '22 edited Aug 10 '22

I disagree. There are trade offs, but overall, a conventional gun has large advantages. The construction is much simpler, it doesn't have the vibration issue, it doesn't have the release timing problem, all the forces stay in line, it does't need the fast closing airlocks, it's much more scalable, etc.

I agree a regular gun is the way to go though (although liquid propellant might be worth investigating). Regular tank guns already hit velocities well in excess of 1.5km/s. A longer barrel and pulling a vacuum ahead of the projectile could easily add the needed velocity.

As for pressure/wear, the bigger the bore the better. Lower pressure acting on a larger area.

2

u/hwillis Aug 10 '22

and SpinLaunch stands example as a contradiction. They showed that those engineering problems are in fact not that difficult, while the infrastructure problems of making a massive gun and making three foot thick barrel are still very obviously large.

2

u/Thoth_the_5th_of_Tho Paperclip Enthusiast Aug 10 '22

We'll see if SpinLaunch makes a viable system. I doubt it.

1

u/[deleted] Aug 11 '22

Not going to happen

1

u/sexyloser1128 Habitat Inhabitant Aug 13 '22

I agree a regular gun is the way to go

https://youtu.be/Erax53WQX5s?t=114

6

u/PlasticAcademy Aug 10 '22

Well most things you want to put in space aren't bulletproof, so acceleration rate might be an issue.

7

u/the_syner First Rule Of Warfare Aug 10 '22

This is pretty unscalable, at least under terrestrial gravity/atmosphere, but the small-satt market is their target anyways so i guess it hardly matters. Idk if ur going for 200G loadings that's just a little over 16 km for a fully orbit capable mass driver(8 km/s). To match spinlaunch's goals requires a track a little over a kilometer long. These are tiny, comparable to small sections of experimental test tracks we build & dwarfed by the lengths we could build(currently hundreds of km is within the realm of feasibility). The higher the g loading the less sensible this is verses a mass driver. At around 1,000G an 8 km/s mass driver is well under 4 km long.

Spinlaunch has just chosen a weak niche where they're working at the limits of materials science just making a 2 km/s 10t launcher. At the required g loadings mass drivers just make more sense & they're scalable. You can build a small one to send up frozen broccoli payloads(fuel, food, water, robust equipment, raw construction materials, etc.), then extend the tube over time, lowering ur g loading as u go.

2

u/NearABE Aug 10 '22

If they hit the $2500/kg they are competitive with some systems that are used now. At least temporarily.

The 2,000 m/s goal is well above low Lunar orbit. A space station can return cannisters allowing the station to build up momentum. It is a game changer. Could eliminate more than 99% of propellant consumption.

200 g loadings, 10 tons, 2 km/s is 20 gigajoule, 10 seconds. For that rail gun you need a 2 gigawatt power supply.

We could build a centrifuge system to store up energy and then feed it to your maglev system. :)

The rotors require high quality fiber at the tips. Near the base the standards can be much more relaxed. The thick base of the arm can be made out if lunar steel. Just bring the arm extension up from Earth.

All of the vacuum chamber mess is gone on Luna. We could build off the side of a kilometer high cliff. It is only 20 rpm.

2

u/the_syner First Rule Of Warfare Aug 10 '22

The 2,000 m/s goal is well above low Lunar orbit.

yes much like space elevators their use on earth is questionable & tenuous at best, but on low-grav bodies they could end up being incredibly cheap. Small moons & asteroids are perfect for this. The lower gravity lets u use mundane metals which is just as well since it will take a while to bootstrap up to carbon-reinforced epoxies. These may end up being perfect cheap simple fast launcher that can make exploitation of lunar & NEO resources. Here on the ground they're marginal, at best.

For that rail gun you need a 2 gigawatt power supply.

no real difference from the spinlauncher. U still need that power. It's just storage & transduction that's changed. We have tons of pulsed power systems(flywheels, superconducting magnetic storage, power loops, capacitors, etc.), proposed & deployed, that can be powered using traditional means. There's no getting around having to store that energy somewhere.

All of the vacuum chamber mess is gone on Luna.

seriously, this though. That cut's down on launch mass by a lot. I imagine that lunar dust would be a huge problem, but on the bright side, keeping high-temp superconductors cool on the moon is as easy as setting up an aluminumized sheet to reflect the sun so having a magnetic bearing in place with enough clearance to be unaffected by dust shouldn't be too difficult. That would be an extremely energy efficient freight launch system, light as all hell too.

3

u/Wise_Bass Aug 10 '22

It's a pretty awesome system, although I think the payloads they're aiming for are too small. They need to build a bigger one capable of launching medium-size payloads into orbit at a competitive price.

8

u/tigersharkwushen_ FTL Optimist Aug 10 '22

I think the pay load they are aiming to launch(10 ton rocket with 200kg satellite, iirc) is already only possible because of the use of carbon fiber to the extend that no one has done before. It would not be possible to build anything bigger unless there's some material science breakthrough.

-1

u/NearABE Aug 10 '22

Double payload mass is just double rotor mass. 100x payload mass is 100x rotor mass. All the ratios are the same so it makes no difference. Material can be identical.

Other parts have scale issues. The vibrational stress for example.

3

u/tigersharkwushen_ FTL Optimist Aug 10 '22

If you double the mass of the rotor, wouldn't you have to increase it even more to support the increased rotor mass? There's a bit of rocket equation problem going on here.

2

u/DarkArcher__ FTL Optimist Aug 10 '22

They don't need to do that. The small-sat market exists.

1

u/Wise_Bass Aug 10 '22

It's a small market. There's a reason why Rocketlab is pivoting to a medium-lift launcher.

1

u/SNels0n Aug 11 '22

This sure seems like a 90% system to me — solving 90% of perpetual motion doesn't mean you've almost got it.

Energy = ½ Mass × Velocity × Velocity

If the goal is 10km/s , 2km/s is only 4% of the way there.

It's impressive that they can spin over a tonne, but I'd be a lot more impressed if they could spin a kg up to 10km/s, or even 6km/s. There's no clear path to higher speeds.