System-specific impulse

Abstract: For many space missions, both a main propulsion subsystem and additional attitude control (AOCS) propulsion subsystem are required. These subsystems normally use different propellants, hence require separate tanks, different flow control units (FCU) and, in case of solar electric propulsion (SEP), separate power processing units (PPU). This leads to increases in total mass of the spacecraft and complexity while reducing system specific impulse. One possibility to alleviate this problem would be to develop a main and an AOCS propulsion technology which could be integrated, sharing some of the components required for their operation, hence reducing system mass. A spacecraft employing such combined technologies as part of an SEP system is referred to as an “All-electric-spacecraft” (Wells et al., 2006). In this chapter, the system design for an all-electric-spacecraft will be presented. A gridded ion engine (GIE) is proposed as a main propulsion subsystem with hollow cathode thrusters (HCT) considered for the AOCS propulsion subsystem. The mission considered during this study is the ESA European Student Moon Orbiter (ESMO), which the University of Southampton proposed to use SEP for both attitude control and main propulsion. During the ESMO phase-A study, a full design of the SEP subsystem was performed at QinetiQ as part of a wider study of the mission performed in conjunction with QinetiQ staff and funded by ESA. The output of this study will be here presented to explain the concept of the all-electric-spacecraft, its benefits, drawbacks and challenges.

Abstract: : The system specific impulse (Iss) was determined to be the equivalent of specific impulse (Isp) of an idealized system having the same net effectiveness as the actual system but with zero mass of all structural equipment except the payload. THE Iss for chemical and nuclear propelled launch vehicles is compared on the basis of specific impulse, structural factors, equivalent velocities, and number of stages. With Iss, it is possible to separate the system net effectiveness of a multistage vehicle from the downgrading of payload-carrying capabilities introduced by the staging and to recognize that the system effectiveness for each stage is proportionate, or that it is a simple summation if the staging is non-proportionate.