karn@petrus.UUCP (03/22/85)
I'd like to ask all you rocketry types out there for some help on a practical design problem. One of AMSAT's upcoming satellites, PACSAT (Packet radio satellite) is planned to use a low thrust, electrically powered thruster to raise its orbit from a low altitude, high inclination shuttle orbit to something approximating a sun synchronous orbit (if we can go from Vandenburg) or at least something that won't fall out of the sky in 3 months (like 800 km circular from a 57 deg Spacelab mission). We are hoping to use anhydrous ammonia as the propellant because it gives the best performance of all alternatives we've studied for the kind of thruster we envision. The engine simply boils the propellant with a heater and squirts it out the back. Thrust would be somewhere around 2 millinewtons, while power consumption might be 50-100watts. Part time bursts over a period of a couple of months should be sufficient to raise the orbit to 800 km or so. The question arises as to what value of specific impulse we should design the thruster for. Before you say "as high as possible", let me explain that in a rocket where the propellant and the energy source are separate (unlike conventional chemical combustion engines) there is a tradeoff between the power required to develop a given amount of thrust and the propellant mass that must be thrown overboard per unit time. Impulse increases linearly with the velocity that is imparted to the propellant, but the kinetic energy increases as the square of the velocity. The result is a linear increase in the power needed to produce a constant amount of thrust as the exhaust velocity (specific impulse) is increased. An example: The hypergolic kick motor on AO-10 developed 400 N of thrust with an exhaust velocity of about 2770 m/sec. The kinetic energy imparted to the propellant was therefore 400 * 2770 = 554 KW. However, if a cesium ion engine with an exhaust velocity of 50,000 m/sec could be developed, it would have to impart 10 MW to its exhaust to develop the same 400N of thrust. On the other hand, less propellant would have to be carried to achieve the same delta-vee, so more mass would be available for a power supply. Given weight figures for the solar panels and a time limit on reaching altitude, there must be some optimum specific impulse that balances electrical generation mass with propellant mass. Has anyone looked at this general problem and done the formulas? Phil