Diff for "OrbitalRings"

Orbital rings are extremely similar to launch loops, but have some differences.

They have the following advantages:

• they have no high-g turnaround sections, so there's fewer problems to be solved in R&D, the cable/rotor has similar g-loading along its length

• they have less change in altitude on the active cable.
• they may be easier to start, it's theoretically possible to build the first one on the surface of the Earth around the equator including the ocean and then spin it up- it will lift off all by itself if the spokes are let out in a controlled way. Later ones may be built on orbit and lowered down.
• failure modes are mostly at altitude and this gives the energy a chance to dissipate in the upper atmosphere
• probably somewhat less susceptible to lighting as the magnetically active material is above the atmosphere (launch loop needs extra mass to deal with this)
• orbital rings can pass over more points on the Earth and can provide intercontinental transport.
• There's very little chance of a significant air leak, if/since the active cable is above the thick bits of the atmosphere.

They have the following disadvantages:

• the initial costs are higher as the minimum size/mass is several times larger than the launch loop in terms of magnetic material and linear cable length
• the motor to make the rotor turn has to be placed at altitude, rather than the ground
• if started from ground level, the cable is subject to accidental or deliberate attack from humans and animals (e.g. sharks) until it has left the ground (on the other hand cables are routinely laid anyway). If started on orbit, launching the materials is currently extremely expensive, but may be bootstrapped.
• orbital rings are much longer, and hence are more likely to be damaged by orbital debris.

The following things are notable, but are similar to launch loops

• a high altitude launch loop above 100km or so is highly susceptible to space debris
• orbital rings need the same sort of cable sheath as launch loops both to control the cable dynamics as well as to reduce air drag if used within the atmosphere.

Keith writes:

Good points.

Note that orbital rings cannot be started up from the surface, because there is no circular path with uniform height around the earth to start them from. Curving them over mountains will require complicated paths and higher deflection forces, leading to larger magnets that somehow have to be removed while the system is brought up to speed. Of course, you will be changing from ground motors to at-altitude motors as well. The deployment of launch loops from the ground is difficult, but much more difficult with orbital rings.

All these systems need tracks and control cables. With only periodic deflection structures, a few millimeters per second radial velocity error at a deflection structure translates to large "miss distances" at the next deflection structure, perhaps beyond the control distance of that structure. Vehicles will be a large source of deflection error.

"Orbital rings" may be a misnomer, since building more than one is problematic. All rings will need to run in parallel planes - they can't be deployed in different inclinations, or they will intersect at some point during deployment or powerdown. When one ring fails, it may take out other rings - without a ground-based rotor dispersal system, the pieces of a failed ring stay in orbit and eventually may hit something, including a replacement ring.

Debris is indeed a problem, which is why the loop is located at 80 kilometers altitude and not higher. The launch path may have "meteor bumpers" spaced a few tens of centimeters to the sides (where most of the orbiting manmade debris will come from) to deflect some of the debris, but we are counting on the atmosphere to bring down material most likely to impact it.

It is interesting to speculate about systems that would raise or lower the track a few tens of centimeters to avoid tracked debris chunks. I doubt the chunks would follow a predictable trajectory when they are in 80 kilometer atmosphere, so I've ignored that speculation, though it is common among orbital ring and space elevator aficionados.