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Comment by daedalus_j | Hacker News Reader

Using a mass driver to launch, if youkeep the G forces down to the levels NASA wants I think you need something like 100 miles of track?

You can build them much shorter, but they could never be used to launch living cargo. My guess would be that we'll see more investment in them once we have a big rocket (BFR/New Armstrong) capable of launching larger habitation units into higher orbits. Once you have more than 10 humans living up there you start having to think about how to supply them, and then a rail gun looks pretty good, but in the meantime we need to be able to lift large (dimension) things as well as humans, and rockets are actually a pretty good choice for that.

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Don't rocket-based vehicles end up burning a significant amount of fuel just to lift the weight of other fuel? What if there was a hybrid approach of using a mass driver to launch fuel containers into orbit, and then the rocket vehicle carrying people (and other more delicate cargo) could rendezvous with and pick up the previously launched fuel containers?

Would certainly make for some interesting failure scenarios to consider. Such as, what happens if the engines fail to start? Presuming they are intended to start after the vehicle is some distance down the rail.

Edit: oops, misread your comment.. sorry for the non-sequitur.

How about launching the personnel carriers this way?

https://en.wikipedia.org/wiki/Laser_propulsion#Laser_Thermal...

Then send cargo up that's hardened for 30g in a vehicle launched from a 20 mile long mass driver that's tilted 45 degrees. We should be able to build such a structure by starting several miles underground, then building up the side of a mountain. 30g over 20 miles yields a final speed of about 9735 miles per hour. With a boost like that, the vehicle wouldn't need a 1st stage and could probably get to orbit without staging.

If it's tilted at 45 degrees, how high will it be after twenty miles????

Make the bottom of the launcher a tube connected to some really deep part of the ocean and use massive floats to help support the weight of the structure then you have less structure to support in open air. You load the pods from sea level, then drop them down the rail-mortar and let computers and robotics handle the rest

At 45 degrees...it would be 14 and half miles high. If I did my math right triangled

I am sure you are correct, but I was reading the Saturn V wiki yesterday, and while it has a huge fuel load, that fuel load is completely consumed just to get the rocket to the point where the first stage can drop. It burns 400 tons of fuel per second!

Seems plausible, although I suppose it depends on what you want to do once you're in space and where you want to go.

If you could get a mass driver system to work very reliably (more reliably than chemical rockets), I could see it being useful to deliver the critical assemblies of RTGs into orbit. Plutonium is pretty resistant to high acceleration. One of the main objections to putting a lot of RTGs in space (or even better, actual nuclear reactors) is what happens during a launch failure. A system that doesn't involve any fuel, and thus won't blow up and scatter the contents around, or require tons of containment, might be useful.

Nuclear reactors have a pretty nice energy-density profile, particularly if what you want is long-duration energy for interplanetary missions.

Still hard to get around the capex problem though.

This is exactly the problem that SpaceX (And Blue Origin eventually) are trying to solve with re-usability. Think of a SpaceX launch, the "big" part of the rocket is thrown away just minutes into the flight, once it's fuel is expended. There's really no way to shortcut that, you couldn't refuel that part in-flight. Refueling that part after it lands again is much less expensive than building a new one entirely.

Meanwhile the payload on the top is now "in orbit" but operating in a vastly different environment than the larger component, with different engineering constraints, quite likely using different types of fuel.

If you could couple the initial launch vehicle being "cheap and easy" with another system for moving around to different orbits or getting into even higher orbits (https://en.wikipedia.org/wiki/Skyhook_%28structure%29 or a "space tug" type orbital vehicle) now you have access to high-orbit/cislunar space, and you can start assembling your larger inter-planetary vehicles or space stations there. (Perhaps using a mass driver to move heavy but necessary things like water and other raw materials to the site at this point, but that greatly depends on what the cost-per-pound of launch can get down to, and what resources you can scrape up out of the solar system)

It comes down to the numbers. Falcon 9's first stage accelerates at about 12.5m/s^2. That's about 1.25g's. It does that for 161 seconds, and first stage separation happens at an altitude of 100km, at Mach 6. That'd be A LOT of track to build straight up. The troposphere ends at 20km. People have trouble breathing before you get to there.

Pilots try to keep plane flight take-offs below 1g. You could possibly increase the g load but not everyone is conditioned like a fighter pilot. Even supposing you could get up to speed faster, you'd have a lot more drag in the lower atmosphere. You can't really get up to any significant speed until you exit the troposphere.

And that means building at least 20km, straight up.

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