Combined Chemical-Electric Motor

When it comes to in-space propulsion, what we ultimately want is a highly robust engine that can do it all - low thrust, high thrust, and everything in between. Unfortunately, modern propulsion technology is not quite there yet. Instead, we can look towards combining several engines together. One such example is the combined chemical-electric motor (CCEM). The CCEM is a propulsion system capable of generating thrust by switching between a chemical and an electrical propulsion method. The idea is to switch between low-thrust and high-thrust engines depending on mission profile. It does not give us an entire range of thrust, but it is the right step forward.

Let’s consider for a moment some applications for this sort of engine. Obviously, launch vehicles demand great amounts of thrust so we can rule them out right off the bat. However, for in-orbit and other interplanetary spacecraft, a CCEM could be very advantageous. This is because these types of spacecrafts requires both high thrust burns and low-thrust burns. High thrust burns are used for orbital transfers and gravity boosts while low thrust burns are needed for station keeping and attitude correction. A light-weight system that can do both while maintaining good fuel efficiency could be truly a gamechanger.

On first thought, we could try the purely chemical method and throttle-down the engine during a low thrust maneuver, but this is not going to help, as fuel efficiency will decrease together with thrust. If we go the purely electric route, we could try to boost the thrust output of the electric engine (such as a hall thruster) but then we would quickly find out that there is no modern power source to supply sufficient electric energy.

The only remaining option is to try to slap a chemical rocket engine onto an electrical thruster (powered by solar panels). Agile Space is leading the way in creating this specific type of system (as seen by the main image of this article). To minimize the overall weight of the combined engine, Agile proposes to have a Hydrazine (toxic rocket fuel) tank feed both the chemical and electric portions of the engine. Unfortunately, Hydrazine is not a very efficient propellant whether it is used on a chemical or an electrical motor. In the end, as with many "combined mode" designs, it is tough to achieve high efficiency for both modes at the same time.

A better configuration does exist; however, its advantages over the Hydrazine system are marginal. For this concept we would need liquid Hydrogen and Oxygen storage. An ordinary chemical engine would be used to generate the high thrust when needed. For a low-thrust maneuver, an onboard fuel-cell would be used to generate electricity to power a highly efficient hydrogen ion thruster. Such a combined system eliminates the need for solar-panels as chemical energy is used for both the chemical and electrical portions of the system. However, water (a byproduct of the fuel cell reaction) is lost into empty space and becomes unusable for the propulsion system. Furthermore, hydrogen is not a great propellant for ion thrusters, as electrical engines benefit more from heavier elements such as Xenon and Argon.

The conclusion is simple, combining a chemical and an electric motor into the same propulsion system is not very practical. This does not mean that combined system technology is dead-end. In the future, as nuclear technology becomes more accessible for space applications, we will certainly see some dual-mode engines. These engines may rely on fission, fusion, or some combination thereof to produce a high-efficiency jets across a wide range of thrusts - an ultimate goal for a propulsion designer.