242. Evaluating the Effect of Tank Construction on Propulsion Systems Utilizing Cryogenic Liquids

Paper

Abstract

The pressures in an unstiffened tank are usually dependent on external loads; the pressures in a stiffened tank are usually independent of external loads. Hence, a change in tank construction, which results in a change in the required pressures within the propellant tank, affects:
1) tank weight
2) pressurization gas weight
3) pressurization system weight
4) feed system weight
5) weight of turbine drive propellants
6) system specific impulse
Generalized methods of analyses are presented for evaluating these changes. The following three types of side wall construction are analyzed for the case of a cylindrical tank subjected to small angles of attack during the boost phase:
1) an unstiffened (balloon) side wall
2) a frame stiffened side wall
3) a stringer-frame stiffened side wall
Tank pressures for the three types of side wall construction reflect pressures based on side wall stability requirements, side wall strength requirements, pump net positive suction head (NPSH) requirements, and common-bulkhead stability requirements. Also, by consideration of the pressure history curves for a balloon-tank version and a stiffened-tank version of a typical bipropellant tank configuration, the effect that the vapor pressure of a cryogenic liquid has on operating pressure is discussed. Pressurization systems and pressurization gases are analyzed from the standpoint of stored-type and bleed-type pressurization concepts. Generalized gas weight equations based on the principle of conservation of energy are presented. Also presented are general equations for the weight of spherical storage bottles for storage of the pressurization gas in either gas form or liquid form. Propellant feed systems using primary-pump only and primary-pump plus boost-pump are considered. Both stored-type and bleed-type power sources are considered for the turbines driving the boost pumps. Lastly, by consideration of the entire propulsion system, the subject of system specific impulse is discussed.
As in all weight evaluation studies, the degree of confidence in a study is strongly dependent on the quality of the available analysis methods. The analysis of tank structure was based on the fundamentals of structural design; analysis of pressurization gases and turbine drive propellants was based on the fundamentals of thermodynamics. As a result, the analysis material is presented in the simplest form possible. The small error associated with such a basic approach is felt to be consistent with preliminary design practices.
The primary conclusion of this study is that an evaluation of the effects of tank construction on a given propulsion system is a systems problem rather than a structures problem. Although the change in system specific impulse is generally negligible, the weight changes may be appreciable.