ZTC Tether-Sling Transportation System
The ZTC Tether-Sling Transportation System project uses momentum exchange tethers for nearly propellant free transfers of payloads between solar system bodies.
It is expected that some propellant will be used by fine-tuning of transfer trajectories. Testing has shown that for lunar transfers corrections of less than 25m/s are typically required.
The basic principle of a momentum exchange tether is to take momentum from the tether's orbit and transfer it into the payload's orbit. The momentum of an Earth based tether can then be recharged over time using solar powered electrodynamic propulsion to allow it to prepare to transfer further payloads.
Both NASA and Tethers Unlimited have done significant research into momentum exchange tethers over the years. NASA's most recent Technology Assessment Group report confirmed that the principle behind the project is sound however significant but feasible advances in materials would be required for the project to become a reality.
Electro-dynamic propulsion uses the interaction between an electric current in the tether with the Earth's magnetic field. The thrust generated is a result of the Lorentz force on the electrons flowing in the tether. The Earth's ionosphere is used as a distributed return path for the tether current by way of anodes and cathodes on each end of the tether.
A simplified model of the Earth's magnetic field will be used for the purpose of simulating the electro-dynamic propulsion.
Tethers are planned for Earth (Ananke for lunar transfers and Ananke-MX for Mars transfers), Moon (Khronos) and Mars. Development work is progressing on Ananke at present.
Ananke is a 120km long tether for lunar transfers that orbits the Earth in an elliptical (eccentricity approx 0.45) equatorial orbit. An equatorial orbit is used so the regression of the ascending nodes does not over complicate the planning of lunar transfer trajectories. The periapsis of the tether is approximately 340km altitude and this places the catcher mechanism at 250km altitude for rendezvous with the payload.
The tether rotates at approximately 1 degree per second. The resulting centripetal force on the payload is approximately 2.8gees.
The design payload mass is 20-25 metric tonnes.
One of the key challenges of the system is affecting a successful rendezvous between the payload and the catcher on the end of the tether. The rendezvous is more challenging than a typical space rendezvous due to the rapidly rotating tether forcing the capture window to be very small in both space and time. Specialised guidance tools for the rendezvous of the payload with the catcher mechanism are being developed.
Ananke-MX is a stretched version of Ananke for transferring payloads to Mars. MX is short for Mars eXpress.
Khronos is a rotovator type tether design to be able place and pickup payloads directly from the surface of the moon. Khronos will most likely be placed in a near polar orbit to maximise rendezvous opportunities with payloads coming in from Earth.
Tortoise Class Lunar Transfer Vessel
The Tortoise Class Lunar Transfer Vessel is designed to be transferred from low Earth orbit to the lunar surface and back again. Capture and landing at the moon is by way of the Khronos tether. Capture at Earth is by way of aerobraking.