Lunar Tether Architectures LEO to Earth-Moon L2 via Lunar Swingby
♦ ΔV at perigee (3048 m/s) injects payload towards Moon. ♦ ΔV at perilune (192 m/s) reduces orbital energy relative to Moon. ♦ ΔV at L2 (143 m/s) captures payload at Earth-Moon L2. ♦ Rendezvous and docking operations at L2 facility (~10 m/s). ♦ Total ΔV from LEO to L2 via lunar swingby: ~3390 m/s 143m/s
192 m/s
3048 m/s Lunar Tether Concepts
♦ Momentum-exchange tethers orbiting the Moon have a number of unique aspects relative to tethers around the Earth • No atmosphere allows a rotating tether to take payloads all the way to lunar surface and retrieve payloads from lunar surface. • Much lower gravity allows “anchored” tethers (elevators) to be built of conventional high-strength materials. • Essentially no man-made orbital debris • No atomic oxygen to degrade tether polymer materials
♦ A large disadvantage is that electrodynamic reboost of lunar tethers is not possible • No lunar magnetic field of any appreciable strength • No ionosphere to collect free electrons
♦ Thus, the management of orbital energy and orbital angular momentum is absolutely key for the long-term success of any lunar tether • Must balance upmass and downmass, or, • Use rocket propulsion (chemical or electric) to restore or remove orbital energy
♦ Unlike the rotating momentum-exchange tether, tethers on or anchored to the lunar surface can extract orbital energy and angular momentum from the Moon itself • Rotating surface tethers are short (2-6 km) and have favorable mass ratios (~5:1) • Rotating orbital tethers are ~100 km long and have masses of 30-100 MT. • Anchored lunar tethers are very long (80,000 – 100,000 km) and require large tether mass (100s of MT) and huge countermasses (1000s of MT)
♦ In the end, the combinations of the two may be the best solution. L1/L2 Lunar Space Elevators Lunar Surface Slingshot Lunar Sling Launch to Elevator Catch
♦ A payload launched from the lunar nearside (by mass driver or surface tether sling) can be caught along the L2 elevator tether at zero relative velocity at a point about 2/3 of the distance to L2. • At this distance, the received mass would exert little additional acceleration on the tether. • This would be an attractive way to collect the necessary ballast mass to build a larger and more extensive elevator. ♦ This capability could enable enormous amounts of lunar material to be collected at the Earth-Moon L2 location with any propulsive expenditure, either for launch or station-keeping.
Earth-Moon Launch site at 17.75° E longitude Lagrange point 2 (Mare Tranquilitatis, west of the Zero-velocity rendezvous at 37,480 Apollo 11 landing site) km above the farside surface
38.94 hr from launch to catch
This trajectory was calculated (in the restricted three-body system) by releasing a mass from the elevator at zero relative velocity and propagating backwards in time until the nearest approach to the lunar surface is reached. The release distance was varied until a location was found that grazed the lunar surface at zero altitude. Elevator Release to Trans-Earth Injection
Perigee 41.9 hr from release to perilune
250 m/s ΔV at perilune (290 km) for trans- Earth injection Zero-velocity release from elevator at 39,010 km above the farside surface
♦ An elevator release can be coupled with a powered lunar swingby to effect very low ΔV Earth return trajectories from lunar space.
191.2 hr (~8 days) from perilune to perigee Trans-Lunar Injection to Elevator Catch
♦ The reverse trajectory is also feasible: • LEO to trans-lunar injection (ΔV = 3059 m/s). • Impulsive maneuver at perilune to decelerate (ΔV = 248 m/s). • Elevator catch at 39,000 km. • Descent to lunar surface along elevator. ♦ This technique has a very low post-injection ΔV requirements, but requires an elevator strong enough to carry a payload all the way to the lunar surface. ♦ The high accelerations of the lunar sling (>50g) make it infeasible to “run in reverse”.
236.7 hr (~9.9 days) Zero-velocity from injection to catch rendezvous with elevator at 39,000 km above the farside surface Trans-lunar injection into highly-elliptical trajectory that will reach lunar 248 m/s ΔV at vicinity on return leg. perilune (180 km) for lunar capture