In order for commercial endeavors to step beyond their current tenuous foothold in LEO and develop a truly robust, sustainable presence in space, the costs of launching payloads into Earth orbit must first be greatly reduced from their current levels. A number of concepts for lower cost, reusable launch vehicles have been proposed, but the simple and unforgiving nature of the rocket equation and the limitations of presently known chemical propellants make achieving those cost reductions with rocket technologies alone a very unlikely prospect. TUI's Momentum-Exchange/Electrodynamic-Reboost (MXER) Tethers may provide a method for assisting launch vehicles to deliver payloads into orbit. This launch assist capability can combine with reusable launch systems based on chemical rockets or other advanced technologies to achieve the order-of-magnitude reductions in launch costs needed for a viable space economy to develop.
The Tether Launch Assist concept combines the techniques Tarzan used to swing through the jungle with the principles of a simple electric motor to create a system capable of repeatedly picking payloads up from a suborbital trajectory and boosting them to higher orbits. In this concept, illustrated in the figure to the right, a long high-strength tether is deployed from an orbiting facility, and the tethered system is set into rotation so that, at the bottom of its swing, the tip of the tether is moving much slower than the center of mass of the system. A grapple system attached to the tip of the tether can thus reach down below the facility and rendezvous with a payload moving in a slower, suborbital trajectory. The grapple would then capture the payload and pull it into orbit along with the tether system. Later, it could release the payload at the top of the swing, tossing it into a higher orbit. When the tether system captures and releases the payload, it transfers some of its momentum and energy to the payload. As a result, the tether facility’s orbit will drop. To enable the system to boost additional payloads, the tether system can restore its orbit in between payload boost operations by using electrodynamic tether thrusting. Electrodynamic thrusting would be accomplished by using onboard power supplies to drive current along the length of the tether. This current will interact with the Earth’s magnetic field to generate Lorentz JxB forces, much as the currents in an electric motor generate force. If the tether current is properly modulated as the tether rotates, these Lorentz forces will produce a net thrust that will restore the tether’s orbit. Because the electrodynamic reboost technique utilizes the mass of the Earth, coupled through its magnetic field, as its reaction mass, it does not require expenditure of propellant. This enables the tether system to repeatedly boost payloads into orbit without requiring resupply of propellant.
The net benefit of the Tether Launch Assist is that it can significantly reduce ΔV that a launch system such as a reusable launch vehicle (RLV) must provide to the payload. Due to the exponential scaling behavior of the rocket equation, even small reductions in the ΔV the RLV must provide can translate into large reductions in launch vehicle size. Alternatively, if the RLV system size is held fixed, the Tether Launch Assist can greatly increase the payload capacity of the launcher.
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