Analysis of Modern Materials as Space Tethers in Upper Stage Propulsion Concept
Michelle L. Nadeau
American University, NASA Glenn Space Academy 2011
NASA Glenn Research Center
Presented in this work are the results of an investigation of the use of modern materials in a rotating momentum-exchange tether propulsion system. Space tethers offer an alternative method of boosting payloads into higher orbits, requiring far less propellant outright than the chemical combustion stages on launch vehicles. The model indicates that modern materials are insufficiently strong, even disregarding a safety factor for the tether load. The forces experienced by the tether are at least thirty times greater than the tensile strength of the materials modeled. Including a safety factor of three, the materials would need to be two orders of magnitude stronger for the proposed application.
Sending anything into space expends a huge amount of energy. For example, the space shuttle orbiter is only about 5% of the entire shuttle’s launch mass due to its giant fuel tanks. The solid rocket boosters provide the initial thrust to get the shuttle airborne. Once empty, they detach at a low enough altitude that they fall back to Earth and are reused. The external fuel tank then takes over to provide fuel to the engines but when it detaches, it burns up in the atmosphere. A revolutionary upper stage for any launch vehicle – not just the space shuttle – that required little or no propellant could conserve fuel and throwaway fuel tanks.
Previous work has suggested placing in low-Earth orbit a rotating momentum transfer tether with a mass on each end. In place of a conventional propellant-driven upper stage, a launch vehicle could attach itself to one of the masses and use the tether’s centripetal motion to launch itself in a new direction and with increased speed. One problem with this method is the difficulty for the launch vehicle to safely approach, attach to and clear the rotating mass. Plus, placing this system into orbit consumes a huge amount of propellant itself.
Research Goals and Benefits:
The goal of this project is to conduct trade studies on a momentum-exchange tether that is deployed from the launch vehicle itself. This built-in tether eliminates the dependence on the tether in low-Earth orbit. Instead the tether transfers momentum from the depleted booster stage to the payload by swinging the payload around the booster stage. Suppose the tether is initially 10 km long and the payload has a tangential velocity of 5 m/s. Due to conservation of angular momentum, if the booster stage reels in the payload to 10 m, the payload is now moving at 5 km/s. Using this system, the payload is propelled without the expenditure of propellant.
Specific project goals include:
1. Investigate the interaction of payload and booster stage in terms of energy and momentum
2. Establish the most appropriate energy supply for the winch.
3. Calculate whether energy can be added fast enough that the payload does not fall back to Earth.
4. Determine whether the tether is worth the added mass to the launch vehicle.
5. Check if answers are physically realizable with currently available materials and technology
The Research Associate will learn and build upon previous knowledge of both science and engineering concepts such as:
1. Trade studies
2. Classical Mechanics
3. Orbital Mechanics
4. Energy Transfer
5. Conservation of Angular Momentum
 Petty, John Ira. “Space Shuttle Basics.” NASA. Updated 15 Feb 2005. Accessed 10 June 2011. <http://spaceflight.nasa.gov/shuttle/reference/basics/index.html>.