Sled Return

The westbound segment of the launch loop closes the loop at altitude to provide extra lift for the stations, and close the loop without interfering with shipping channels in the ocean below.

The westbound segment can be used for a third purpose; returning launch sleds from For perspective, 2000 sled uses per year is 10,000 tonnes to high orbit per sled per year. the east station at the end of the launch run, back to west station for reeuse. If the unloaded sleds accelerate westward at full thrust, they can reach orbital velocity (and zero vertical track loading) much faster than an eastbound sled loaded with a vehicle. On the other hand, there will be less rotor velocity change and less power dissipated if acceleration is limited to three gees, like launch. The next two sections consider these two possibilities; a production launch loop designed by a clever engineering team will probably be better than either of these alternatives.

Two alternatives considered (and now superseded) are:

• (1) lowering the sled from east station to the eastern surface platform, then flying it back to the western surface platform,

and

• (2) releasing the sled just above orbital velocity at east station into a fractional orbit, then reentering it (with a heavy heatshield), deploying a parachute, and snagging it "Corona Spysat Film Cannister Style" with an aircraft near west station. This is a flyback sled return. It is risky, expensive and wastes energy, but should be mentioned and discarded before someone else stumbles across the same silly idea and pesters busy engineers with it.

Assume a 2500 km long launch loop. This is longer than the 1986 version to provide extra room to slow down the sled.

Faster Sled Return

A 2 tonne (estimated) launch sled is capable of launching a 5 tonne vehicle at 30 m/s², producing 210 kN of thrust for both. After vehicle release, the sled decelerates to a stop at east station, after a launch run, with this this same thrust; a 10.5 gee sled deceleration.

It should be possible to launch the 2 tonne sled westward on the return track with the same 210 kN thrust, until the sled reaches orbital velocity: 7.86 km/s in the fixed frame, or 8.33 km/s relative to the track (which rotates eastward, with the rotating Earth, at 471 m/s and 80 km altitude). This acceleration will take 80 seconds and use 340 km of the track, followed by 1800 km and 220 seconds of zero-acceleration coasting, followed by 340 km and 80 seconds of 10.5 gee acceleration. This brings the sled back to west station 380 seconds after it leaves east station. With a quick robotic refurbishment at east station, and a quick payload assembly for a subsequent launch at west station, a sled might be reused 20 times a day, perhaps 7000 times per year.

However, I do not recommend attempting the highest possible rates; the loop is expensive and a small error made in haste could be catastrophic. 6 uses per day of a $500K launch sled, a 20%/year cost of capital, is less than $5 capital cost per trip; a city bus probably costs more to operate.

Low Stress Sled Return

Consider instead returning a 2 tonne sled at 30 m/s², with 60 kN of maximum stress. The sled will reach orbital speed after 280 seconds of acceleration, then start slowing down at 30 m/s² almost immediately. This will take perhaps 600 seconds, adding less than four minutes to the complete launch-and-return cycle.

The lower accelerations create less rotor displacement from nominal, requiring less rotor velocity and displacement correction at the western surface turnaround.

For perspective, 2000 uses per sled per year is 10,000 tonnes to high orbit per $500,000 sled per year, five times as much as 5 space shuttles lifted to orbit in 135 missions over 30 years (program cost 14 astronaut deaths, 5 ground crew deaths, and $210B 2010 dollars).