Loop Port

An Arnold/Kingsbury Spaceport fed by the Launch Loop

The original Spaceport idea is a reverse mass driver in low earth orbit. Vehicles are launched by rockets up to perhaps half of orbital velocity, to be captured at 5 gees in a long decelerator tube oriented horizontally in Low Earth Orbit. Packets of material are launched from the Moon into highly elliptical orbits that enter the Spaceport from the other end, adding the angular momentum lost from the Spaceport.

An interesting idea, with a few pesky problems:


A LoopPort (please don't call it Spaceport 2.5, Roger Arnold owns the right to name and number Spaceports!) is a spaceport designed for the apogee of a Construction Orbit, a highly elliptical 86164 second geosynchronous (but NOT geostationary!) orbit timed so that apogee ( 75950 km radius ) always occurs about 11.57 hours after a 10.2 km/s (surface-relative) launch from a launch loop. This apogee is timed to occur exactly the LoopPort (and associated construction station) arrives at its 1021 m/s apogee.

Loop exit radius is 6378+80 = 6458 km perigee; the construction orbit perigee is 8378 km, a 2000 km altitude. The delta V from the loop launch transfer orbit to the construction orbit is 114 m/s. A 2 gee spaceport merely 330 meters long can supply that relatively small delta V.

Where does the makeup angular momentum come from? The cheapest way to add angular momentum is with a small rocket delta V at very high radius. So, launch (acceleration tolerant) dumb cargo vehicles from the launch loop to perhaps 300,000 km radius apogee, just inside the radius of the Moon (at times of the month when the Moon is far away and tidal effects are manageable). That is a loop launch velocity of 10.52 km/s (earth relative), and an apogee velocity of 237 m/s.

At apogee, the dumb cargo vehicle is rocket-accelerated to a velocity of 926 m/s, raising perigee to 75950 km, where it is timed to arrive at the LoopPort with a prograde velocity of 2894 m/s and a relative velocity of 1873 m/s. We "slow it down" in a separate 1000 meter long, 90 gee tube.

If the ratios of "quick trip slow gee" mass to "dumb cargo high gee" mass is 1873/114 or 16:1, the momentum balances. So, some small/fast/dumb/tardy cargo (like cryo-frozen chocolate bars, or propellant) balancing far more low-gee traffic arriving 12 hours after launch from Earth.

The "average" rocket delta V to the construction station is ( 114*16 + 926 ) / 17 or 162 m/s, and the major rocket delta V is entirely applied to the cargo, not passenger vehicles. If a passenger vehicle misses a capture, it re-enters 24 hours after launch. Disapppointing, but not lethal or even catastrophic, presuming it has a heat shield. An 114 m/s accelerator (or rocket) firing retrograde at the construction orbit apogee will reenter a vehicle 12 hours later.

This is an important feature of construction orbits; emergency returns (perhaps to get an injured worker down to a world-class hospital) always come down at the same location 12 hours after apogee, so the worst case delay between a life-threatening injury and world-class hospitals should never be more than 1.5 orbits. A permanently inhabited construction orbit station must have an onboard mini-hospital and robots that can be operated by doctors on the ground, but hospitals are gigantic and complex systems that cannot be completely duplicated in orbit, and recalibrated for zero gee. Maybe someday ...

Two Quibbles

LoopPort (last edited 2018-06-11 22:38:30 by KeithLofstrom)