Design criteria for film cooling for small liquid-propellant rocket engines.

Results of a study to determine the applicability of film cooling to rocket engines in the 10to 1000-lb-thrust range using earth-storable, space-storable, and cryogenic propellant combinations are presented. The accuracy of the analytical model was verified with test data using the propellant combinations of nitrogen tetroxide/monomethylhydrazine and fluorine/monomethylhydrazine (N2O4/MMH and F2/MMH). Data are presented which illustrate the interrelational effects of mixture ratio, specific impulse, wall temperature, and the percent of film cooling. The studies show that N2O4/MMH and C1F5/MMH and other similar earth-storable propellant combinations are readily adaptable to small film-cooled spacecraft engines. The space-storable and deep-cryogenic systems OF2/MMH, OF2/B2H6, OF2/CH4(LPG's), F2/MMH, O2/H2, and F2/H2 will require the use of film cooling and high-temperature materials if the potentially high performance they possess is to be realized. Otherwise, the systems have to be operated fuel rich with high percentages of fuel film cooling, which will compromise the deliverable specific impulse and system weight. For example, if a 100-lb-thrust F2/H2 engine operating at 100-psia chamber pressure was needed to provide 400 sec of /sp at a mixture ratio of 8.0 or greater, the minimum percent of film cooling and minimum wall temperature capability of the chamber material must be 40% and 5000QF, respectively.