SPACECRAFT PROPULSION - KTH

- 57 -5 PROPULSION SYSTEMS SELECTIONCRITERIAA detailed procedure for the selection of propulsion systems forgiven spacecraft mission requirements is beyond the scope of thisbooklet. The process for selecting and sizing the elements ofpropulsion systems is detailed in 17.However, in general, an important consideration for the selection of asuitable propulsion system will be the trade-off between its impulseor velocity increment (v) capability and the system mass.Consequently, when selecting a spacecraft propulsion system forgiven mission impulse demand, primarily the system will have tomeet the impulse or delta-v requirement with highest possiblespacecraft payload mass. Therefore, an important requirement of thespacecraft designer will be that the mass of the propulsion systemshall be a minimum or at least shall not exceed a certain percentageof the overall mass of the spacecraft. As already mentioned inChapter 3.2.2 above, the performance of propulsion systems cannotbe assessed only by the specific impulse I sp , but requires also takinginto account the system- specific impulse I ssp .To assess the suitability of spacecraft propulsion systems forspacecraft mission impulse requirements, value ranges of I sp and I ssp(see Eq. (16)) of various actual built systems can be derived frompublished data as summarised in Table 10. In addition, with Eq. (19)the dependence of the propulsion system mass fraction m PS /m S/C , onmission velocity increment v can be derived for any given value ofI sp (v e ) and I ssp . Curves of m PS /m S/C, plotted as a function of v fordifferent propulsion system designs with typical value ranges of I spand I ssp from Table 10, are shown in diagram Fig. 20 below.When suitable systems are selected, a refinement of the selection hasto be carried out. This process takes into consideration additionalparameters such as cost, operability, complexity and reliability, etc.