SPACECRAFT PROPULSION - KTH

- 23 -Further, for vaporising liquids, the mass of liquids in a tank is:mP V P(31)In order to allow certain ullage, the volume of propellant is a certainfraction (0.5 to 0.9) of the available tank volume. Hence, with V P =CV T and Eqs. (27) and (31) for the combined mass of tank andpropellant we get:mPSS mT mP mP pop1 (32) CKAgain, with regard to the non-impulse dependent system mass, m HW ,as for compressed gas systems, it can be included as a mass fractionx (%) of the impulse dependent system mass m PSS . Hence, Eq. (32)can be expanded to include m HW :mPSS pop mHW mT mP mHW mP 11 CKx(33)With Eqs. (21), (23) and (33) the system-specific impulse forVAPORISING LIQUIDS becomes:IsspmPI mtotT mHWve pop1 1 CKx(34)From Eqs. (30) and (34) it is obvious, that the thruster exhaustvelocity, v e ≡ I sp , the non-impulse dependent system mass x as wellas the type of propellant and the tank performance factor K