High Apogee Construction Orbit with a 5 Tonne Launch Loop

The goal for this example will be assembling large microwave-transmitting space solar power satellites (SSPS). The same process can be used to assemble smaller 183 GHz millimeter wave SSPS, lunar landers, deep space probes, or very large vehicles for interplanetary missions.

Launch loops can scale up to enormous size (and cost), but don't scale much smaller than 5 tonne vehicles at 3 gees, a consequence of winds in the atmosphere they rise through.

The very low cost of loop launch into high apogee orbits enables the assembly of large spacecraft and structures from 5 tonne components. If the apogee is very high, then a small Δv at apogee can raise the perigee of the orbit well above relatively crowded LEO orbits. For this discussion, assume 2000 km perigee altitude is adequate, and a 1 stellar day delivery cycle, hence a 84328 km -(6378+2000 km) = 75950 km apogee.

Launch from the Loop

The launch loop will be located south of the equator for gentle and steady weather. 8 degrees south latitude, east of French Polynesia and west of South America may be the best region for launch loop deployments.

The earth rotates once per stellar day (relative to the fixed stars) every 86164.0989 seconds. The launch loop rotates under the perigee of an orbit at exactly this rate. In order to add another component to an orbiting assembly, it should be launched as the assembly is near perigee, overhead, timed within milliseconds. This can only happen if perigee is synchronized with the Earth's stellar day rotation, with corrections for tidal effects and the equatorial bulge.'

The minimum launch loop is designed for 5 tonne vehicles at a 45 second cadence. We will presume one vehicle per 1 stellar day cycle.

Note: While it may be possible to launch groups of vehicles more closely timed than 45 seconds, each vehicle adds tension and deflection to the track/ The tension is maximum at low speed near west station. Tight grouping will increase stress over the entire track, and reduce total throughput. In 45 seconds, the earth turns 0.19 degrees, and apogee "turns" 780 kilometers. Since higher orbits are slower orbits, it may be possible to send a string of vehicles to a series of cascaded higher orbits that intersect the construction orbit at somewhat higher velocities, and maneuver them towards rendezvous a few orbits later, permitting higher total throughput during a multi-month construction program. I'll leave such complexities to future mission designers.

A fully-powered launch loop can launch a 5 tonne vehicle every 45 seconds; that is 1920 vehicles per solar day, 5 less than that per stellar day. That can feed thousands of construction orbits and thousands of projects.

There should be at least three launch loops, providing redundancy in case of failure. If two operate on the same latitude and different longitudes, that can double the vehicle delivery rate per construction orbit.

Because of the equatorial bulge, perigee will precess westward by a few seconds per day, and the construction orbit and launch time should be lengthened to match; better orbit designers than I will compute the exact amount of precession and adjust the numbers presented here. The slightly shorter "days" used by launch and construction crews will be inconvenient in a 24.0 hour world. Multi-thousand tonne sub-structures may require more than year to assemble, and be passed between "red", "green", and "blue" teams on the ground. The substructures can be assembled into megatonne-scale structures in GEO, synchronized 24.0 hour ground team schedules and independent of launch loop scheduling.

An 80,000 tonne space solar power satellite might be assembled in GEO from 80 large subcomponents, 1000 tonnes each. Each subcomponent would be assembled from 250 five tonne loop launches (estimated four tonnes of payload and one tonne of propellant and thrust stage) from two loops over 4 months. Two loops at 75 percent capacity can launch 2000 of these 1000 tonne components per year.

Ballistic Trajectory from Loop Launch to 75950 km

Leaving the loop, vehicles will climb out through the upper atmosphere and encounter significant drag; they will need a sharp hypersonic nose cone with a heat resistant ablative spherical cap. However, the dynamic pressure and gee forces will be small, permitting the use of low-cost materials. An amusing possibility is "nanowood", ordinary wood from trees that has been treated to remove most of the "live" material and leave only strong structural cellulose. As strong as ordinary wood, less dense and more insulating than styrofoam.

Should the apogee insertion rocket fail, vheicles will re-enter somewhere to the west of the loop, probably exploding in the lower atmosphere. Passenger vehicles must be capable of safely re-entering, of course, making them vastly more expensive than cargo vehicles. Vehicles will also briefly pass through the center of the van Allen radiation belts,

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Construction orbit

All components will need some initial thrust to raise perigee above LEO. At the first apogee, the initial perigee radius of 6378+80 = 6458 km is raised to 8378 km with a relatively small and inexpensive thrust package, producing 114 m/s of delta V. It is more important that this initial thrust device is inexpensive, high thrust, reliable and accurate, rather than high I_ISP. Vehicles should arrive ready for collection.

The first component launched will be a thrust "tractor", with enough precision ΔV capability to rendezvous with subsequent vehicles. Tanks of fuel will be delivered next; these will double as radiation shielding for construction workers and radiation sensitive components.

The construction orbit will have a 86164.0989 second period, one stellar day, with perigee at 8 degrees south latitude. The launch loop (also at 8 degrees south latitude) will rotate underneath perigee once per day, to add another 5 tonne component to the assembly.

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Transfer orbit 1

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Transfer orbit 2

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Circular GEO

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