Landing on a Space Ring

[ajtribick]

Space Rings, e.g. the Bishop Rings of Orion's Arm fame, make it easy to launch a spaceship from the ring, especially if the ring orbits within the Ecliptic, you just "drop" the spaceship "downwards" off the ring.

Landing/docking with the ring would be harder, as the ring is constantly rotating to provide Earth gravity... what would be the best way of doing this?

[granthutchison]

Larry Niven's solution was to use a linear accelerator in reverse. Place your spacecraft in orbit at the edge of the ring, and let an accelerator assembly catch up with you, scoop you up, and accelerate you to match the ring's velocity.

Grant

[Guest]

That's right; the side walls of the Bishop Rings are 50km high at Arkab Prior, and they have a linear decelerator arranged in a loose spiral attached to the outside, terminating in a space ledge.

As the rings are small compared to a ringworld, the decleration of a massive spaceship represents quite a large transfer of energy; so the launches are generally arranged in pairs, one inward, and one outward (where the ship simply falls through a trap door arrangement).

[selden]

Well, certainly somebody has to make up the energy deficit.

I can see several problems that would have to be overcome with either method. However, I think the major problem would be the magnitude of the damage that could be caused by accidents. In other words, it'd be better to use low-mass vehicles to transfer passengers and cargo, not high-mass, high-energy-content spacecraft. Presumably it'd be most cost effective to have spacecraft designs optimized for this kind of operation, with passenger and cargo modules that would be separable from the propulsive section of the spacecraft.

[granthutchison]

As the rings are small compared to a ringworld, the decleration of a massive spaceship represents quite a large transfer of energy; so the launches are generally arranged in pairs, one inward, and one outward (where the ship simply falls through a trap door arrangement).

Hmmm. That's not going to work, though, is it?
Your arriving spacecraft robs the ring structure of angular momentum, but your departing spacecraft is neutral in terms of angular momentum (it doesn't interact with the ring structure as it departs).
To keep the angular momentum budget equalized, you'd need to use the stored energy from the arrival of a spacecraft to launch a similar spacecraft back into the same orbit used by the incomer.

Selden's scheme would work with this approach ... spacecraft settling into rim orbit, and then dropping or picking up smaller cargo/passenger loads which the ring launch system accelerated or decelerated as required. Only spacecraft assembled on the ring (or prepared to pay a very high "launch tax") would be permitted to leave via the trapdoor system.

There's no such thing as a free launch, after all ...

Grant

[eburacum45]

However, I think the major problem would be the magnitude of the damage that could be caused by accidents.

The craft that dock there are small, inter-ring craft piloted by competent AI; humans aren't allowed to perform the landing procedure on an inhabited ring. So the insurance rates are low.

but your departing spacecraft is neutral in terms of angular momentum (it doesn't interact with the ring structure as it departs).

Many of the rings have substantial interstellar-capable spacecraft attached in pairs opposite each other under the ring floor; they detach simulataneously, and the ring's rotation is not affected; but this can't be done very often, as it robs the ring of mass-

as in fact all the trapdoor launches do-

is that different to loss of angular momentum?

[granthutchison]

is that different to loss of angular momentum?

Yes - angular momentum is a proportional to mass and angular velocity. When you drop a ship out a trapdoor there's no exchange of angular momentum between the ring and the ship - the ring keeps on rotating at the same speed, and the ship retains the angular momentum it had when it was just sitting on the ring floor.
But you do exchange angular momentum if you land a ship using a linear accelerator.
Here's how it goes:
1) Ship is in a circular orbit around the ring ... ring is sweeping past the ship with an angular velocity that exceeds orbital velocity (in order to generate "centrifugal gravity").
2) Ship is picked up by the linear accelerator landing system ... ship gains kinetic energy and angular momentum as its angular velocity is increased to match the ring rotation, while the ring loses some angular momentum and kinetic energy because it's exerting a force on the ship to accelerate it, and is therefore is slowing a little itself.
3) Ship is launched through a trapdoor. Ring keeps on rotating at its (slower) rate, and the ship now benefits from the increased momentum it harvested during landing - it climbs away from the ring on a hyperbolic orbit, whereas it arrived in a circular orbit.

Your ships are effectively stealing rotational energy from the ring every time they land and then launch, and the ring just keeps on getting slower and slower.

An alternative to the strategy I mentioned above would be to require your ships to arrive on a hyperbolic orbit that synched with the ring rotation at closest approach ... the ring would then just latch on to them as they passed, without exchanging angular momentum. But there would be major fuel penalties for a ship that missed an approach and had to go around, and there would be added potential for making alignment errors.

Grant