A momentum exchange tether is a kind of space tether that could theoretically be used as a launch system, or to change spacecraft orbits. Momentum exchange tethers create a centripetal force on the end-masses of the system causing the object to experience the pseudo-force known as centrifugal force. While the tether system rotates, the objects on either end of the tether will experience continuous acceleration; the magnitude of the acceleration depends on the length of the tether and the rotation rate. Momentum exchange occurs when an end body is released during the rotation. The transfer of momentum to the released object will cause the rotating tether to lose energy, and thus lose velocity and altitude. However, using electrodynamic tether thrusting, or ion propulsion the system can then re-boost itself with little or no expenditure of consumable reaction mass. A non-rotating tether is a rotating tether that rotates exactly once per orbit so that it always has a vertical orientation relative to the parent body. A spacecraft arriving at the lower end of this tether, or departing from the upper end, will take momentum from the tether, while a spacecraft departing from the lower end of the tether, or arriving at the upper end, will add momentum to the tether. In some cases, momentum exchange systems are intended to run as balanced transportation schemes where an arriving spacecraft or payload is exchanged with one leaving with the same speed and mass, and then no net change in momentum or angular momentum occurs.
Tether systems
Tidal stabilization
Gravity-gradient stabilization, also called "gravity stabilization" and "tidal stabilization", is a simple and reliable method for controlling the attitude of a satellite that requires no electronic control systems, rocket motors or propellant. This type of attitude control tether has a small mass on one end, and a satellite on the other. Tidal forces stretch the tether between the two masses. There are two ways of explaining tidal forces. In one, the upper end mass of the system is moving faster than orbital velocity for its altitude, so centrifugal force makes it want to move further away from the planet it is orbiting. At the same time, the lower end mass of the system is moving at less than orbital speed for its altitude, so it wants to move closer to the planet. The end result is that the tether is under constant tension and wants to hang in a vertical orientation. Simple satellites have often been stabilized this way; either with tethers, or with how the mass is distributed within the satellite. As with any freely hanging object, it can be disturbed and start to swing. Since there is no atmospheric drag in space to slow the swing, a small bottle of fluid with baffles may be mounted in the spacecraft to damp the pendulum vibrations via the viscous friction of the fluid.
Electrodynamic tethers
In a strong planetary magnetic field such as around the Earth, a conducting tether can be configured as an electrodynamic tether. This can either be used as a dynamo to generate power for the satellite at the cost of slowing its orbital velocity, or it can be used to increase the orbital velocity of the satellite by putting power into the tether from the satellite's power system. Thus the tether can be used to either accelerate or to slow an orbiting spacecraft without using any rocket propellant. When using this technique with a rotating tether, the current through the tether must alternate in phase with the rotation rate of the tether in order to produce either a consistent slowing force or a consistent accelerating force. Whether slowing or accelerating the satellite, the electrodynamic tether pushes against the planet's magnetic field, and thus the momentum gained or lost ultimately comes from the planet.
Sky-hooks
A sky-hook is a theoretical class of orbiting tether propulsion intended to lift payloads to high altitudes and speeds. Simple sky-hooks are essentially partial elevators, extending some distance below a base-station orbit and allowing orbital insertion by lifting the cargo. Most proposals spin the tether so that its angular momentum also provides energy to the cargo, speeding it up to orbital velocity or beyond while slowing the tether. Some form of propulsion is then applied to the tether to regain the angular momentum.
Bolo A Bolo, or rotating tether, is a tether that rotates more than once per orbit and whose endpoints have a significant tip speed (~ 1–3 km/s or 2,200–6,700 mph or 3,600–10,800 km/h). The maximum speed of the endpoints is limited by the strength of the cable material, the taper, and the safety factor it is designed for. The purpose of the Bolo is to either speed up, or slow down, a spacecraft that docks with it without using any of the spacecraft's on-board propellant and to change the spacecraft's orbital flight path. Effectively, the Bolo acts as a reusable upper stage for any spacecraft that docks with it. The momentum imparted to the spacecraft by the Bolo is not free. In the same way that the Bolo changes the spacecraft's momentum and direction of travel, the Bolo's orbital momentum and rotational momentum is also changed, and this costs energy that must be replaced. The idea is that the replacement energy would come from a more efficient and lower cost source than a chemical rocket motor. Two possible lower cost sources for this replacement energy are an ion propulsion system, or an electrodynamic tether propulsion system that would be part of the Bolo. An essentially free source of replacement energy is momentum gathered from payloads to be accelerated in the other direction, suggesting that the need for adding energy from propulsion systems will be quite minimal with balanced, two-way, space commerce.
Rotovator
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