## Cable Catapults

The Cable Catapult System is an idea of the late Robert L. Forward (he died in September 2002) of Tethers Unlimited, a company working on the development of advanced space tether concepts. Forward was also a famous science-fiction writer who featured space tethers in many of his books; the cable catapult appears in his novel Camelot 30K (1993), but it is more than just fantasy.

The Cable Catapult System uses a long tether as a launch rail in combination with a so-called linear motor. A linear motor is a type of electric motor that makes use of electromagnetic forces without requiring any moving parts. On Earth, linear motors are for instance used in magnetic levitation (maglev) trains. The tether, which may be orbiting Earth or another planet, is extended in space and pointed in the right direction for the launch. The linear motor, with the to-be-launched spacecraft attached, then "climbs'' forward along the tether and accelerates. Because of the lack of moving parts and because it is electromagnetically suspended and thus does not actually touch the tether, the linear motor and its cargo can reach very high speeds. When the required velocity has been reached, the payload is released to fly to its destination. The motor is subsequently decelerated on a shorter section of the tether, so that the whole launch system can be used again (its orbit will have changed, however, because as the payload accelerates and flies off it pushes the tether backward, following Newton's famous principle of action equals reaction). According to Tethers Unlimited, launch velocities of up to 100 km/s (60 miles/s) may be possible, which would enable interplanetary transfer flights with durations of months rather than years (Fig. 1.13).

Figure 1.13: The principle of the Cable Catapult System.

In Forward's novel, scientists launch themselves to the outer solar system using a cable catapult to investigate an alien civilization found there. The power supply for the catapult is a nuclear-thermal-electric system (using the heat of a nuclear reactor to produce electrical power). The energy is used to generate a sustained burst of radiofrequency energy, which travels down the long conductive cable to be absorbed by the launching motor. This linear motor then uses a magnetic coupling to pull on the conducting cable and accelerate ("like a monkey climbing a rope," as Forward writes in the book). Just before the motor reaches the power supply, which is located at an optimum point along the cable, it releases the payload capsule. Then it slows down to a stop on the shorter length of the cable, on the other side of the power supply. Using a fixed power supply rather than one on the linear motor minimizes the mass that needs to be accelerated, and therefore the amount of energy required. Due to the electric resistance of the cable, part of the energy flowing through it is lost in the form of heat. Locating the power supply at an optimum point along the cable minimizes the total electrical energy "line losses'' during the acceleration and subsequent deceleration of the launch system.

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