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Comment by mikewarot | Hacker News Reader

If we're pushing "out of the box" ideas, why not just use hydrogen balloons to hold up a railgun for the first 20,000 meters of altitude? The ambient pressure at the end would be about 1/10th of that at sea level. You could have outriggers with a very thin high voltage power line to enable station keeping via thrusters (repurposed quadrotor parts?)

I wouldn't be surprised it the ambient electrostatic field from the atmosphere were sufficient to power station keeping, or at least some of the instrumentation.

If that works, I'm sure they could extend it to twice as long, almost into space.

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Relatedly, have you ever heard of skyhook? Not the CIA one but the orbital infrastructure concept -- lets you literally just take a supersonic capable plane with no rocket engines whatsoever into space.

Always love that concept with how it's actually engineer-able with current materials and sounds like it shouldn't work until you look closely.

This?

https://en.wikipedia.org/wiki/Skyhook_(structure)

Interesting idea, but if I'm understanding correctly, how do you stop the thing you hooked from swinging around and back down? Would you need to reel in 50 miles or whatever of cable?

You can just let go of it when it's high enough, it picks up (ground) speed on the way up.

The main cost of getting to low Earth orbit isn't gaining altitude, it's gaining speed. Even if you could levitate a spacecraft to, say, 200 km altitude at zero cost in energy, you would still need about 97% of the energy to get it to orbital speed, that you would have needed to launch it from the Earth's surface.

thats what the railgun is for. Altitude is to reduce friction and energy needed.

> thats what the railgun is for

A railgun that can provide a delta-v of 8 kilometers per second?

Yes, that would be the proposal.

Existing ship mounted railguns already shoot projectiles around 3km/s. It is generally thought that they could be scaled up to 8 km/s and larger size. The main problem is that if you fire human sized objects that fast at sea level you end up with a plasma ball due to air friction. This isn't an issue with chemical rockets because they start at 0 and accelerate to Vmax, whereas the railgun projectiles start at Vmax and decelerate.

Higher altitude would remove much of the friction problem.

Existing light gas guns can already hit 8 km/s: https://en.wikipedia.org/wiki/Light-gas_gun

The biggest advantage of very high altitude reducing the friction problem isn't just that it stops your projectile from turning into plasma, it is that the size of the minimum viable projectile is reduced. People keep focusing on getting people or vehicles into orbit, but the main advantage of a gun-to-orbit system is that the packet size can be made very small. Think machine gun instead of cannon. This means a smaller gun and thus smaller capital investment.

Instead of the balloon-supported long gun that mikewarot suggested, imagine a balloon at 30 km altitude supporting a 5 meter long gun and a few tons of both propellant and <1 gram pellets. A target satellite is shot in a long continuous burst, leaving the satellite with a lot more mass and a slightly disturbed orbit. Later shots from a different location and direction can be used to compensate for the added impulse to the satellite's orbit.

This could be a cheap way to get a bunch of metal into orbit, or possibly even fuel and oxidizer.

You want to fire thousands of bullet size pieces of metal into low earth orbit? Holy Kessler Cascade, Batman!

While that's not my intention, if you actually wanted to damage satellites and cause a Kessler Syndrome scenario, it's hard to find a system that would have a bigger bang-for-your-buck.

I was rather envisioning a target satellite that is designed to absorb the pellet as an inelastic collision. It would probably resemble something like this: https://en.wikipedia.org/wiki/Black_body#/media/File:Black_b... . Of course, if it didn't work as designed, then it could be a problem.

The point was that a high altitude low-mass-per-shot gun system could get mass into orbit with a similar energy cost as a space elevator with significantly less upfront capital cost and no need for miracle materials.

> Existing ship mounted railguns already shoot projectiles around 3km/s.

Projectiles of what mass? And with what acceleration? As I understand it, with feasible railgun lengths the acceleration is well above the limit of human tolerance, and the projectile mass is significantly smaller than a typical payload put into LEO by conventional rockets.

You sent me down a fun rabbit hole for rail-guns, nano-tubes, super conductors, velocity requirements, megajoules required to launch 1,000KG payload into space:

Imagine a U/C channel of carbon nano tube and corresponding super conducting magnets that act as the rail-gun in the same way an aircraft carrier does...

(The maths in this thread are pretty cool):