Long Term Cryogenic Propulsion for Interplanetary Missions

Abstract

Interplanetary missions impose high velocity increment requirements and complex trajectory strategies; the use of chemical storable propellants implies limited specific impulse and consequent high propellant mass. The state of the art of cryogenic storage technologies is now mature to consider such systems as possible alternatives to storable propellants for such applications. The combined usage of advanced power sources such as fuel cells and very high specific impulse systems such as electric propulsion could provide a further advantage. A fast track technology feasibility study was performed at ESA/Estec Concurrent Design Facility (CDF) adopting a concurrent engineering approach, with the aim to study the potential of such technologies. A selection of potentially attractive and representative missions was performed; soon the use of fuel cells as power source was found not applicable when considering the fuel cells as a primary power source or as a secondary power source during specific mission phases. Electric propulsion concepts were as well excluded from the study due to the specific system requirements imposed by their high power demand. Study baseline was full cryogenic propulsion concept adopting LH2/LOX as propellants. Sample missions selection was performed bearing in mind general mission cost aspects:adoption of enabling technologies already under development in Europe to limit non recurring costs and use of small-medium launcher to challenging missions that would not meet the velocity increment requirements with traditional storable propellant systems. The system design cases studied revealed a good advantage versus storable propellants but still the most challenging missions were hardly reachable in terms of compliance with the reference target P/L masses due to the complexity of structures and thermal systems. With the same boundary conditions, a liquid oxygen/hydrocarbon propellant solution was preliminary analyzed; in spite of the reduced mass specific impulse, the increased volume specific impulse and reduced thermal control needs provided sensible advantages, provided attractive results versus those obtained with storable propellant solutions.

A liquid propulsion panorama

“…New concepts, like low thrust cryogenic propulsion, may enable to extend the domain of cryogenic propulsion to lower propellant masses and smaller launchers (orbital stage or upper stage of micro launchers with payload below 300 kg) [10,14,17,43].…”