Using aluminum for space propulsion

The combination aluminum and water was theoretically analyzed to assess its performance potential for space propulsion, in particular for microrocket applications. Heat of reaction, impulse density, and handling safety are features making this combination interesting for chemical thrusters, especially since thrust is higher than typical of satellite electric thrusters and whenever a compact package is desirable. Ideal specific impulse (Isp), thrust coefficient, adiabatic flame temperature and combustion products were calculated for chamber pressures 1 to 10 atm, nozzle area ratios 25 to 100 and mixture ratios (O/F) 0.4 to 8.0. Isp reaches up to 3500 m/s. Also the effect of hydrogen peroxide addition to aluminum and water on performance was explored. This combination improves performance slightly at the expense of simplicity, making it less attractive for microrocket engines. Ignition delay times were conservatively estimated assuming aluminum coated with its oxide, and ignition occurring after melting of the aluminum oxide. To this purpose heating and kinetics times were evaluated, the first by a 1-D physical model, the second by a reduced scheme. Results indicate the heating time of a 0.1 mm diameter aluminum particle may be of order 0.4 ms, while overall kinetics takes 10 ms: thus, the Al/water combination looks in principle practical for micro-rocket chambers, characterized by short residence times.