Dual-mode propulsion systems for SmallSats

Dual-mode propulsion is the integration of two propulsive modes into a single propulsion system with the key attribute being shared propellant between the propulsive modes. Dual-mode propulsion is to be contrasted with hybrid propulsion, which has no commonality or shared resources between propulsive modes. In addition to providing mission flexibility and adaptability, dual-mode propulsion can reduce system dry mass and volume compared with hybrid propulsion because of common shared hardware and resources between the propulsive modes. This is especially advantageous for small spacecraft like CubeSats where mass and volume are a premium. While dual-mode propulsion concepts that are only chemical and only electric have been investigated, recent research has also focused on concepts that integrate chemical and electric propulsion. Analyses of dual-mode chemical-electric spacecraft suggest that for near-Earth transfer missions the highest transfer rates are possible when the ratio of electric-to-chemical mode specific impulse is between four and eight, and this ratio increases with decreasing chemical mode performance. A major challenge with any dual-mode propulsion concept is propellant compatibility between modes, and the most advanced concepts make use of chemically-inert propellants like xenon and water. In this chapter, we describe three different dual-mode propulsion systems that are each at a different level of technology readiness. First, the ion thruster and cold gas thruster unified propulsion system (I-COUPS) is a dual-mode propulsion system consisting of eight xenon cold-gas thrusters and a single xenon gridded ion thruster, and was flown onboard PROCYON, a 50-kg-class micro-space probe launched in 2014. Second, laboratory tests have measured performance and demonstrated the promise of a dual-mode system pairing an electrothermal resistojet and electrostatic ion thruster both fed with water propellant. A prototype system is already under development and planned for flight demonstration with ion thruster and resistojet specific impulse performance of 600 and 70 seconds, respectively. This ratio of electric to chemical performance (8.6:1) is close to the optimum for many near-Earth missions. Third, proof-of-principle experiments have demonstrated a monopropellant-electrospray concept that relies on a shared green hydroxylammonium nitrate-based monopropellant and a shared thruster that can be switched between modes. Modeling predictions suggest monopropellant and electrospray performance of 180 and 800 s are possible.