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CHEMICAL vs. ELECTRIC ROCKET PROPULSION CHEMICAL • Always thermal • ELECTRIC Some are thermal (resistojet, arcjet) – Some are non-thermal (ion, hall effect, MPD) • Energy / propellant initially together • Energy source / propellant separate initially • Energy source is combustion • Energy source onboard or external – Solar, Nuclear – Energy conversion equipment onboard (heavy) • Energy limited – Best propellants, Ue ~ 4,500 m/s – Isp ~ 100-500 s – Thrust ~ 0.1 N-10 MN • ‘Unlimited energy’ – Very high Ue (10,000-100,000 m/s) – Isp ~ 1,000-10,000 s – Thrust ~ mN-N • Propellant and Energy decrease with time • • Propellant decreases with time Energy remains constant with time • High Thrust, Low Isp, High mdot, Fast ao, Short tburn – Thrust independent of tburn – tburn ~O(seconds) • Low Thrust, High Isp, Low mdot, Slow ao, Long tburn – Thrust inversely proportional to tburn – tburn ~O(days-months-years) • Earth to orbit missions, low DV • Station keeping, deep space exploration, high DV Origin of Thrust: Eject Stored Mass Backwards → Use Reaction Force to Propel Vehicle Rocket Equation is Always True