Comment by _bpgl | Hacker News Reader

Comment by _bpgl | Hacker News Reader

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> "In contrast to cargo-only chemical space-gun concepts, a mass driver could be any length, affordable, and with relatively smooth acceleration throughout, optionally even lengthy enough to reach target velocity without excessive g forces for passengers."

Wikipedia can generally be pretty good, but the above text from the subsection On Earth shows what can go wrong when deluded space enthusiasts go nuts on an article; it's pure wishful thinking. No major construction project of "any length" is "affordable," especially not one of the sophistication of a mass driver. At about 3g, about 30 m/s^2, it would take you about 270 seconds to get to 8 km/second, during which time you are traveling at an average speed of 4 km/second. This is a structure, with attendant control system and safeguards, about 1080 km, or 670 mi, in length. This is not affordable. By anyone.

What might be affordable (sticking my wet finger in the breeze) is getting 1 or 2 km/s at 5 or 6 g, or higher, and then doing the rest with a rocket, provided that you're firing continuously to amortize your initial cost. Skimming a couple km/s off the high end of the rocket equation brings your mass ratio way down, especially considering the reduced gravitational losses. (I'm assuming, in a fit of witless optimism, that we're launching from a high enough altitude that the increased aerodynamic losses are mitigated to the point that gravitational loss savings dominate, and that this adds no cost.)

Mass drivers are a fun idea because they seem to get around the rocket equation, but this affordability nonsense is just over the top.

"This is a structure, with attendant control system and safeguards, about 1080 km, or 670 mi, in length. This is not affordable. By anyone."

The Hyperloop[1] is a proposal for a 350 mile structure that actually sounds remarkably similar to a mass driver, at a projected cost of $6 billion -- quite affordable for governments, corporations, and even some individuals.

[1] - https://en.wikipedia.org/wiki/Hyperloop

A train is 150+ year old technology that could not be simpler and yet the cost of building out high speed rail is 10 times the number you quoted at least. You're delusional if you think experimental technology is going to be cheaper.

I figure mass drivers are for non-atmosphere situations. In atmosphere it seems more likely that other exotic ideas are more feasible. The longer ones seem to be for a civilization that has a construction pipeline so automated that building a multi-hundred-mile structure isn't as insane as the idea is today.

The atmosphere is nice in a lot of ways, but it is not helpful in getting to space today.

The major problem with mass drivers in Earth atmosphere is that anything like escape velocity combined with atmosphere introduces a lot of G forces and a ton of heat. (For intuition, imagine reentry, except even faster at lower altitudes.)

It might be feasible to use a mass driver to substitute for a first stage, especially if the mass driver mouth is high enough up to substantially reduce air resistance on the things coming out - the top of Mt. Kilimanjaro say. But that would be at most a few km/s of the 8 you need to get into low Earth orbit.

Yeah, you need a 2nd stage to make it remotely viable. A single stage mass driver would need to have escape velocity minus drag loss at the exit mouth, which is vastly faster than any real chemical rocket is going at ~10-15 km elevation, where the air is still reasonably dense. Also, 10-15 km is around the range of Max Q for real-world rockets and given the equation, q = 0.5 p v², a higher velocity is going to result in some extreme structural stress.

Run it in a tunnel (cut and cover to reduce costs) with a reduced atmospheric pressure that goes up the side of an equatorial mountain. Mt. Kenya is the best prospect from what I remember the last time I looked at this. It is just south of the equator so you get max angular momentum from the spin of the earth, it is about 5km above sea level so you are at half the atmospheric pressure as sea level, and it is relatively close to a downrange ocean in case the launch does not go as planned.

The tunnel can't just be cut and cover, because you can't take sharp turns of any kind and there's no straight-line gradient up a mountain. Yes, a termination at a high equatorial peak helps, but there's still a lot of air there that you're immediately buffeted by as soon as you leave the tunnel.

The Moon is a Harsh Mistress by Robert Heinlein speaks about this. Rebels on the moon list it as a demand from the Earthside governments to build a mass driver to lower costs of shipping and make moonside life more equitable.

They even list a couple of ideal locations including I believe Kenya.

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