orbit is important or for on-station maneuvering, the cost per kilogram to place a spacecraft on orbit is likely to be a key parameter. In this case, the cost to place a spacecraft on orbit includes such items as total propulsion cost, booster system requirements, command and control costs during orbit raising, and contingency for spacecraft loss because of propulsion failures.

The trend to high power for several classes of satellites is causing electric propulsion to be considered. Commercial satellites are typically designed and programmed to perform for very specific lifetimes. The principal electric propulsion application for commercial satellites is station keeping using only the available power used for the main mission power (Sackheim and Byers 1998).

An Example: Cost Savings Achieved by Dual Mode Operation

Of the several electric propulsion systems competing for high power and orbit transfer applications, two offer high efficiency and long life at attractive specific impulses: gridded ion engines and Hall effect thrusters.1 Both devices accelerate noble gases, such as xenon and argon, to velocities in the 10 to 40 kilometers per second range. Xenon is safe, dense, and easily stored at ambient conditions. The Hall thruster is used here to illustrate electric propulsion payoff. A 25 kilowatt Hall thruster can be expected to operate as follows:

  • Isp=15,680 meters per second (1,600 seconds),2

  • Efficiency=62 percent (58 percent after lead losses and power processing),

  • Thrust: F ~ 1.6 Newtons (0.36 pounds force),

  • Xe flow=0.12 grams per second.3

The changes in velocity for station keeping are less demanding than changes in velocity encountered during orbit transfer. Station keeping changes in velocity can be accomplished by a variety of mature electric propulsion systems, including:

  • Arcjets,

  • Electrothermal monopropellant systems, and

  • Pulsed plasma thrusters.

Although arcjets are not as efficient as Hall thrusters, they have the cost advantage of using hydrazine fuel, which is already required to be onboard the spacecraft for other propulsion (Sackheim and Byers 1998).

To achieve a mass and cost comparison, a 100 kilowatt electric propulsion system made up of four 25 kilowatt Hall thrusters is fueled to match the total impulse of the Thiokol Star 75 motor, a state-of-the-art solid propellant space motor.4 To a first order approximation, the propulsion mass saved by the electric propulsion system will be considered as revenue producing payload. The Star 75 is 1.9 meters in diameter and contains 7,518 kilograms of propellant. The rocket provides approximately 200 kilo Newtons (45,000 pounds force) of thrust over 105 seconds. The cost is approximately $3.5 million. The equivalent electric propulsion system using four 25 kilowatt Hall thrusters and powered by a 100 kilowatt electric system would thrust at a combined total of 6.4 Newtons for about 33 days.

Economies of Scale

To place a kilogram of payload into low Earth orbit (LEO) costs between $6,000 and $10,000. The cost to reach geosynchronous Earth orbit (GEO) is at least $20,000 per kilogram and may go as high as $40,000, depending on the mission. When a chemical propulsion system is used, 60 to 70 percent of the mass that reaches LEO is the propulsion system needed to get the payload to GEO. Most of the mass consists of the propulsion system propellant. Using electrical propulsion, the ratio of propulsion mass to payload mass can be reversed. There are additional benefits if the power used for electric propulsion during orbit raising is also available and required for the main mission, thus creating a dual mode system. However, the lower thrust of the electric propulsion systems increases the orbit transfer time from hours to weeks. A LEO to GEO (1,500 to 36,000 kilometer) transfer with a 29 degree plane requires a satellite velocity increase of 3,500 meters per second using chemical propulsion and 4,050 meters per second using electric propulsion. The greater change in velocity required for electric propul-

2  

A lower Isp is selected to shorten trip time.

3  

Xe is xenon, the propellant generally used for gridded ion engines and Hall effect electric propulsion.

4  

Total impulse is the integral of thrust over the thrusting time.

1  

The rocket engine figure of merit is specific impulse (Isp), which in SI units is the velocity of the propellant exiting the nozzle. Meters per second is equivalent to thrust per rate of mass discharge or newtons per kilogram-second.



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