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Comment by jcims | Hacker News Reader

Cool to see some new ideas here. Saturn V burned nearly 200 tons of propellant by the time it cleared the launch tower. Tsiolkovsky got us to the moon but he won't get us to the stars.

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It's enticing because what if you could use this to put fuel and/or parts into orbit to assemble a second stage rocket for interplanetary travel? That'd be sick.

I suppose it partly depends on how high you can reach (LEO?) with this method and whether your spun rocket can carry a fuel payload itself.

Payload needs to include a rocket or else it will come back down. Orbit requires going sideways really fast.

The launch mechanism doesn’t have to point straight up; it could point at an angle. Of course that will lengthen the distance traveled through the atmosphere, but maybe it could still work if the launch vehicle can withstand enough heat.

Doesn't matter what angle you throw it. An object in orbit always returns to its starting point. When the object is released, its orbit intersects the ground (unless it reaches escape velocity). If you want to change the orbit so it no longer intersects the ground, you must impart a huge velocity change to the object after it clears the atmosphere, and the only practical way to do that is to include a rocket engine and propellant and avionics in the payload.

That's a very clear argument. Can the effect of air resistance usefully change this, since it's a force acting on the object after it's released? It seems like it's quite bad that the force always points directly opposite the velocity, and I imagine there's a straightforward argument that this can't "improve" the orbit, but I'm not sure offhand how to incorporate that into the analysis.

Air resistance applies a force opposite to the direction of travel (retrograde). At any given instant, the effect is to not change your direction of movement, but decrease your speed of movement. A retrograde force will always lower every point of your orbit (except the exact point where the force was applied, which would remain on your orbit). Since your orbit already has a point that intersects the surface of the planet, your new orbit must also intersect the surface.

One minor complication: Technically, air resistance applies a retrograde force relative to the direction of the wind. At low speeds this might actually increase your orbital velocity. However, the speeds where this is relevant are so small that they are not worth consideration.

I wonder if it would somehow be possible to very temporarily decrease air density on the flight path. Heating it or firing a vortex or something.

I think you might be right, the atmosphere might be able to help you a little. However, any orbit you obtained by interaction with the atmosphere would still intersect the atmosphere, and so would decay rapidly. You'd still need a rocket to boost up afterward. Maybe it could reduce the amount of propellant necessary.

Why not just build couple extra O’Neil cylinders on orbit, run checkouts while populating them over 2-3 generations, and shoot it out? No reasons the majority of interstellar crews has to be Earth born.

Solid rockets had more carrying capacity at one sixth the cost of Saturn 5 at the time. Is the ultimate launch system rated human-safe a combination of spinlaunch, solid rocket boosters to reach highspeed for airbreathing ram/scramjets and lastly transition from airbreathers to liquid propellants?

No. Humans don't like 10 gs not to mention 10000.

Why aren't rockets preaccelerated with maglev sleds up mountains? No one wants to invest?

Maglev trains aren't science fiction anymore.

I wonder if you could keep a fuel line attached and keep refuelling until it cleared the tower when the line would detach?

If you could pump 200 tons of propellant in ten seconds, sure.

Just did some quick stubby pencil work, a 2 foot diameter pipe would have to be pumping the propellant at around 70 mph in order to move that much liquid.

Just the lateral deceleration of the liquid slowing down in the tank would probably provide a couple tons of thrust.

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