S.1 Spacecraft Propulsion Systems Chapter 1: Introduction to ...

S.12 Summary
Fundamentals of rocket propulsion:
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- Thrust force is generated by expelling mass (initially stored in the spacecraft) from the
spacecraft at high velocity;
- ‘Basic Rocket Equation’ trades off exhaust velocity ve with spacecraft mass fraction R;
Propulsion performance is determined by ‘Specific Impulses’:
- Thruster-specific impulse, Isp (Ns/kg), which is numerically the same (If defined with SI
units of m/s) as the effective exhaust velocity ve (m/s), - although called ‘thruster
exhaust velocity’ hereinafter. The exhaust velocity increases with increasing gas
temperature and decreasing molar mass.
- Propulsion System-specific Impulse, Issp (Ns/kg), which is the total impulse, Itot (Ns)
delivered by the system, divided by the system total mass, mPS (kg).
Mass of propulsion systems can be determined with help of the overall ‘Propulsion System
Mass Fraction’, mPS/mS/C.
Power is the major constraint for electric thrusters on spacecraft. Therefore thrust levels of
electric propulsion will be