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Completed TRL 1 (started at 1, targeting 3)
The proposed technology for in-space propulsion offers several benefits over traditional technologies:
The proposed effort centers on three key areas:
We propose a collaboration on the development, testing, and characterization of an electrolysis propulsion system in the 5-22N thrust range and ultimately advance the technology to TRL-5.
Electrolysis propulsion is a hybrid of chemical and electric propulsion. Therefore, it can be compared in some ways to other electric propulsion technologies, such as ion thrusters. Although the specific impulse is much lower here, the thrust-per-unit-power is much higher. An electrolysis propulsion thruster requires 7.2 kW per N of continuous thrust; contrast with the NSTAR ion thruster which requires 23 kW per N of continuous thrust. For pulsed operations (the operations concept for the existing flight prototype), as little as 2W is sufficient to produce gaseous H2/O2 to fill the current combustion chamber in about 20 minutes. The larger chamber proposed here will be at least 4x the volume, requiring higher power or simply longer electrolysis time, but with the added benefit of longer thrust pulses and therefore higher Isp.
In the area of in-space propulsion, water-based propulsion offers extensive scalability. Electrolysis propulsion could be scaled up to the 1000N level, or greater, depending on mission requirements. The system, if large enough, could even include technologies like temporary cryogenic storage to take advantage of its high specific impulse while minimizing leak rates with the rest of propellant remaining in water and gaseous H2 and O2 storage. With the proper development, electrolysis propulsion could help the DRA 5.0 (humans to Mars) architecture close–providing an attractive alternative to cryogenic fuel storage concepts–as well as deep- space probe missions such as those in the New Frontiers program.
The Cornell Space Systems Design Studio already has extensive experience designing, manufacturing, assembling, and testing a 1N electrolysis propulsion system in a pulsed thrust configuration. This system is baselined for the CisLunar Explorer mission, an entry in the CubeQuest Centennial Challenge, that will launch on SLS EM-1. Building on this success, we propose to understand further the improvements possible at larger scale thanks to MSFC’s expertise, to characterize the system in MSFC’s superior facilities, and to inform small spacecraft missions in collaboration with MSFC. In order to do so, we propose working with MSFC to develop and test a prototype capable of 5N pulsed and continuous thrust and run a series of characterization tests on it.
We propose to design, analyze, and then test this 5N system under continuous thrust as well as additional pulsed thrust modes. During these firings, power usage, specific impulse, and thrust will be assessed. These experiments will validate the analysis that identifies the optimal length of thrust bursts to maximize Isp while minimizing the required electrical power and energy. It is known that continuous thrust will yield higher specific impulse values, but the question then becomes will this greater efficiency be worth the size, weight, and power (SWaP) cost to operate the electrolyzers at a higher rate? These tests will yield invaluable data that could provide answers to the above questions and will inform the future small-sate mission architecture of interest here and other spacecraft architectures that implement electrolysis propulsion.
At 500-1500 m/s delta-V and 5N of thrust, electrolysis propulsion offers significant advantages to traditional propellant technologies for in-space propulsion. Its higher specific impulse and lower dry mass enable it to outperform even ion propulsion. The propellant cost is negligible, and the low-pressure tanks and plumbing are inexpensive and available off-the-shelf for the most part. The green nature of this propellant also means that far more vendors and prime contractors—small companies, universities, et al.—can integrate and test small spacecraft with propulsion, broadening who can contribute to NASA’s exploration and science objectives safely and cost-effectively. Furthermore, the modular nature of the current prototype system would carry over to the scaled up version and allow for the efficient division of design and assembly tasks between MSFC and Cornell.
Finally, the intricate geometries that optimize the combustion chamber H2 and O2 storage tanks at are best realized with advanced additive manufacturing. By leveraging MSFC’s abilities in additive manufacturing, some system components, such as nozzle geometry can evolve over time, independent of the rest of the system, cutting costs and increasing flexibility.
The current system is a 1N thruster that operates in pulsed bursts. It will launch on the EM-1 mission of the SLS as part of the CisLunar Explorers Lunar CubeSat project, and is TRL-5. The system components are as follows:
The system functions via passive mechanical control and limited active electronic actuation. As electrolysis occurs, gaseous hydrogen and oxygen flow through the storage tank up to a solenoid valve connected to downstream piping that leads to the combustion chamber. Once the pressure from constant water electrolysis is high enough, as read by the pressure gauges in the fuel tank, the solenoid valve opens and allows the gaseous hydrogen and oxygen to travel through a flame arrestor/check valve and enter the combustion chamber. Once the pressure in the combustion chamber is adequate, the solenoid valve is closed, and the glow plug ignites the H2/O2 mixture. The resulting spike in pressure goes beyond the cracking pressure for the check valve of the nozzle (150 psi currently), thus allowing the combustion products to flow through the converging-diverging nozzle and out of the system.
In order to prevent water flowing from the fuel tank to the combustion chamber, on Earth, the system is stored in a cylindrical tank with plumbing from the top. The water remains at the bottom of the tank, while the less-dense pressurized hydrogen and oxygen remains at the top of the container, and can flow out of the tank to the combustion chamber. In order to prevent water ingestion on orbit, the spacecraft rotates about the nozzle’s thrust axis. This spin field causes the hydrogen and oxygen gas to separate from the water, once electrolyzed When the check valve between the tank and the combustion chamber opens, these gases flow to the combustion chamber.
Current data from pulsed thrust testing have yielded the following results: Using only the 2W available from body-fixed solar panels, this prototype has demonstrated a sustained specific impulse of 450 seconds, with short 1 Ns pulses achieving a net specific impulse of 300 seconds.
Scaling the current 1N system up to be in the range of 5N to 22N requires specific hardware changes while keeping the same basic system architecture. Currently, the spacecraft relies on rotation to allow for H2, O2, and H2O storage in the same tank. To remove the current rotation requirement on the spacecraft, additional tanks can be added to store the H2 and O2 separately from the water. On higher-thrust tanks, it is also advisable to separate the H2 from the O2 prior to combustion, as a risk-reduction precaution. Higher mass flow from the system and a higher exit nozzle area will result in higher thrust, so changes to both the combustion chamber and nozzle geometry will be required. We hope to leverage MSFC’s state-of-the-art 3D-printing capabilities using the EOS M100 to print the required larger combustion chamber and optimized nozzle geometries. It may also be employed to develop small, discrete, efficiently spaced H2, O2 and H2O storage tanks, in one single body. Furthermore, the modular design of nozzle attachment to the combustion chamber means that if multiple nozzle designs require testing, they can easily be switched out on one singular system architecture.
The proposed technology development approach is to achieve the following goals:
These goals, if achieved, would mature the 5N Electrolysis Propulsion System to the broadest possible infusion potential.
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