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:

• All the elements of the system ( the spaceport, the earth surface launch site, and the Moon mass driver ) must be in the same orbital plane, and timed so the Hohmann orbits between the source and the spaceport are synchronized.
  • The Earth constraint is relatively easy; if the spaceport and launch site are both equatorial, and the spaceport orbital period is an integer subfraction of a 84164 second stellar day, the lineups will occur once per day.
  • The Lunar constraint is difficult; the Moon's orbit does not cross the Earth's equatorial plane at exact integer-ratio-multiples of a stellar day. Fairly hefty rockets will probably be needed, to accomodate azimuth changes and large delta Vs needed to change orbits from the inconvenient Lunar orbit to a Spaceport-intersecting one (exact in six orbital elements, three position parameters and three velocity parameters).
• The spaceport must be fairly long - to add 3500 m/s of delta V at 5 gees requires a 125 km acceleration tube.
• The spaceport must remain horizontal, though arriving vehicles from above and below will accelerate radially against the tube and turn it.
  • It might be partly stabilized with a "vertical" plumb-bob connected by a network of suspension wires, but the wires constitute a large target for LEO space debris, and a debris-parted wire will whip around unpredictably, possibly damaging the spaceport or the other suspension wires.

LoopPort

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

• I did NOT choose the 300,000 km cargo orbit perigee carefully. A different radius will be required for a half-orbit synchronous construction orbit arrival. I suggest a 1.5 orbit arrival (more complicated to compute), so that the cargo orbit can be tweaked a bit for exact arrival time, and the thrust stage can be jettisoned into earth reentry or solar orbit escape. Delaying the arrival of chocolate is a crime, I know, but getting hit by misaimed 1873 m/s chocolate cargo is not a welcome treat, either. Better safe than sated.

• The best place for the first launch loops is 8 degrees south, 120 degrees west, where the weather is the most boring on Earth. That launches into an 8 degree inclined orbital plane. A launch to the perigee of a cargo orbit (in the opposite direction of construction orbit perigee) will occur 12 stellar hours later, into an orbital plane tilted 8 degrees the other way. So, a 16 degree plane change will be required as the cargo arrival orbit passes through the equatorial plane. That will be somewhat expensive. Yet another "CargoPort" can orbit in the opposite orbital plane, with an accelerator providing the plane change, but that is too baroque for me.