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Comment by charlesju | Hacker News Reader

I think the interesting use case would be to put things into space for a lot less money. Imagine putting one or many of those new Space X internet satellites into a rail gun that goes straight up.

It doesn't need to go all the way into space, it just needs to get pretty high, then the cost to break into orbit can require a lot less fuel and make it a lot more cost effective to do space stuff.

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This has been discussed before but to summarize, it doesn't work that way.

I realize you didn't say "into orbit" but it is a reasonable place to start. The disadvantage of 'gun' type launch systems are that they only get one shot at putting energy into the system and they have to do that at low altitude where the air is thickest (the biggest impediment to going fast) as a result your space craft leaves the gun and goes into the soup of the atmosphere and burns up[1]. This is a problem with the rail gun as well as hypersonic projectiles that have active payloads have to deal with friction heating and remain functional at the point where they arrive. The secondary problem is the g-force as a function of the acceleration of going from full stop to 7.9 kM/s in say 10 milliseconds. Your basic physics will tell you that is an acceleration of 80 thousand 'Gs'. If you are launching a payload you need a container that can not crush its contents. Of course all that air resistance will bleed off energy/speed so you actually have to start much faster than 7.9 km/s, as much as 10 - 12 km/s to "coast" into orbit. Now you're over 100,000 Gs to start.

[1] This is not unlike shooting a bullet into water (other than water has higher viscosity and the bullet has a lower velocity) but the effect is the same.

Is there a reason you couldn't make the railgun bigger? For example, build it up the side of a high mountain. Then you can spread the acceleration across multiple kilometers and by the time you reach the top the air is thinner.

> Imagine putting one or many of those new Space X internet satellites into a rail gun that goes straight up.

Orbit doesn't work like that. Getting 200, 300 miles up is the easy part - the vast majority of the required energy comes from going sideways at 7800m/s.

I think they were suggesting that you could save fuel for once you're out of atmosphere, so that you're not fighting air resistance when you start a burn to get into orbit.

You could save a decent chunk of your fuel this way. Maybe 25% or so.

In exchange for that, you have to build your rocket and satellite to withstand 10,000 gees of acceleration and hypersonic flight through the lower atmosphere. And somehow build a rail gun that can fire something the size of an office building.

You use an extremely long track, not high acceleration.

Hard to do if you’re aiming straight up.

You'd have to re-purpose something like a deep mining shaft from a diamond mine, then stick a giant rail gun in it, almost like Germany's V-3 cannon[1] but on steroids.

[1]https://en.wikipedia.org/wiki/V-3_cannon

No we aren't aiming straight up because we need to get into orbit. We need to go sideways as fast as possible. As long as you're not aiming at the ground you will always aim "up" but not straight up.

“Imagine putting one or many of those new Space X internet satellites into a rail gun that goes straight up.”

Firing sideways could save you much more fuel but also means you’re in the lower atmosphere at much higher speed and for much longer.

Could the targeting system account for the sideways movement?

Do you mean in order to use this as an ASAT or ABM system? In that case, probably - though you'd still need some sort of terminal guidance to actually achieve a hit.

If you mean "account for the sideways movement" as "give the satellite the horizontal component of the required orbital velocity" this doesn't work for a variety of reasons - #1 is that the railgun is 2000m/s and orbit is 7800m/s, and #2 is that things going 7800m/s (17450mph) at or near sea level tend to get very, very hot very, very quickly which is not particularly great for railgun projectiles and utterly disastrous for things like solar panels.

Maybe I should rephrase and emphasize that I think this is interesting not necessarily immediately practical. From what I understand about rail guns, they're not even practical yet to deploy for military ground to ground purposes, let alone to get things into space.

I think I'm taking a lot of liberties to imagine a future where we figure out a ton of challenges to get all the engineering to match the theory of making this happen. And in general, I believe we should take an optimistic approach to the future, given that getting humans into space in scale would be a marvelous accomplishment for the human race.

Two problems with that: going up is only 3% of the energy that is required. The other 97% is "going fast enough sideways to stay up". The other problem, even if you angle your shot, is that you need to raise the perigee so that your orbit doesn't intersect the launch site anymore. These two things combined mean that you would have to railgun a rocket that is still about half the size of a rocket that takes of normally. And liquid filled rocket would NOT like or withstand a rail gun launch. But you have should bought yourself TWO major technological problems: rocketry and rail guns. That is not economical at all. Much easier to make the rocket a bit larger (you need to develop a rocket anyway) and get rid of the complexity introduced by the rail gun.

Ronald Reagan was way ahead of you (and everyone else). Back in 1987, the Strategic Defense Initiative (SDI, also known as "Star Wars") featured this concept.

The press excoriated him (hence the derogatory name "Star Wars" being affixed to the program).

The part you mention was called "Brilliant Pebbles", and featured space-based weapons as a cornerstone of the initiative. [0][1]

[0] - https://en.wikipedia.org/wiki/Brilliant_Pebbles

[1] - https://www.nytimes.com/1989/04/25/science/what-s-next-for-s...

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