The Innovation Platform Editor Georgie Purcell spoke with NASA’s John W Dankanich to find out how the space agency is using plasma propulsion to improve its missions.
Plasma propulsion is an advanced form of electric space propulsion that uses electric and magnetic fields to ionise a propellant into plasma – a charged gas of electrons and ions – which is then accelerated to extremely high velocities, far exceeding chemical rockets. This solution offers a range of benefits for space missions, including increased efficiency and reduced costs.
NASA has utilised plasma technologies for a variety of activities and missions in its history and is continuing to do so in an effort to optimise efficiency, cost effectiveness, and capability. To learn more about the role of plasma propulsion within NASA’s work, Georgie Purcell spoke with John W Dankanich, In-Space Transportation Systems Capability Lead at NASA.
Can you identify some of the key uses of plasma propulsion within NASA’s missions?
Plasma propulsion, or what we generally refer to as ‘electric propulsion’ (EP), encompasses a diverse set of propulsion solutions, including Hall-effect thrusters, gridded ion thrusters, pulsed plasma thrusters, electrosprays, etc.
Electric propulsion has facilitated various science missions at NASA. For example, the Dawn planetary science mission used a gridded ion thruster called the NASA Solar Technology Application Readiness (NSTAR). That mission was led by the Jet Propulsion Laboratory (JPL) for the exploration of Vesta and Ceres, with the thruster technology developed by the Glenn Research Center. It was the first science mission to use primary electric propulsion, and the EP system also enabled it to be the first mission to stop and explore two different destinations with the same spacecraft.
We continue to use electric propulsion for planetary science today within the NASA Psyche mission, which is ongoing. The mission, which is enabled through the use of a Hall-effect thruster, launched in 2023 and is due to arrive at the metal-rich asteroid Psyche in July 2029. It is an exciting mission because Psyche may be the exposed core of a protoplanet, which can help us understand planetary formation.
NASA has seen the number of uses for plasma propulsion missions continue to grow, especially as we continue to mature lower-power and lower-cost solutions – some of which are enabling high-performance, low-cost missions. We have a project under the Space Technology Mission Directorate (STMD) called Sub-Kilowatt Electro Propulsion (SKEP). We also have partnerships with industry to develop systems for small spacecraft, such as ESPA-class electric propulsion. In the near-term, this facilitates things like geo-centric science missions and orbit servicing, but we’re also working towards lower-cost planetary science missions.
Our biggest EP mission is Gateway – humanity’s first space station around the Moon. Gateway is part of the Artemis architecture, along with other systems like the Space Launch System, Orion spacecraft, human landing systems, spacesuits, and more. These are some of the systems that will help NASA explore the Moon’s South Pole. Gateway supports NASA-led Artemis missions to return to the Moon for science discovery, and also helps chart the path for the first human missions to Mars and beyond. It demonstrates many of the technologies that we want in terms of system aggregation and the scaling up of systems. That small space station is going to be a multi-purpose outpost that supports lunar science missions, even allowing us to test technologies in lunar orbit. It’s for human exploration-built partnerships with our commercial and international partners.
As you might imagine, it can take an enormous amount of propellant to put large systems at distant destinations. One part of the Gateway programme is the Power and Propulsion Element (PPE). PPE is also managed by the Glenn Research Center and is currently undergoing final assembly at Lanteris Space Systems. PPE includes three 12.5 kW advanced electric propulsion system thrusters, manufactured by L3Harris Technologies, and four 6 kW Busek-built BHT-6000 thrusters. Those are all Hall-effect thrusters that will result in a high-power electric propulsion system that enables a single-launch architecture with the efficient transfer of Gateway to its near-rectilinear halo orbit (NRHO) around the Moon.
How has NASA’s use of plasma propulsion advanced in recent years?
NASA’s work is predominantly focused on driving success for our industry partners. Commercial space has dominated the use of electric propulsion in recent years, leveraging a lot of the advancements over the past several decades.
NASA has also been increasing the diversity of its plasma propulsion options. A key example is the advancements made to the Gateway Advanced Electric Propulsion Systems (AEPS) thruster. We now use magnetically-shielded Hall thrusters – enabling extremely long-life capabilities. We’ve also seen that technology used within a wide set of missions at different scales, with international partners now even adapting the technology for their own systems.
Alongside these extended life capabilities, other advancements include driving down the cost and working to improve our manufacturing capabilities.
What advantages does plasma offer over other types of propulsion?
The big advantage of electric propulsion is its propulsive efficiency. The efficiency of the propulsion system is often measured in a specific impulse, which is a function of the thruster’s exhaust velocity. Therefore, the faster the exhaust goes, the more efficient the momentum transfer will be, and the more efficient the spacecraft will be. Chemical propulsion systems are limited to the energy that can be released from breaking down chemical bonds through burning, typically. However, electric propulsion generally ionises a propellant and then accelerates that plasma, often to an order of magnitude higher exhaust velocity or an order of magnitude higher specific impulse.
Fuel efficiency can be everything for some of the missions that we want to pursue. For example, as I said, the Dawn mission was the first ever to stop at two different destinations with the same spacecraft. We often think about fuel efficiency on the ground. Obviously, we want good fuel mileage in our cars, and this is the same for us at NASA. The challenge we have at NASA is that, often, every time we want to travel somewhere, we need to buy a new vehicle. Fuel efficiency is everything for us, particularly in the case of stopping at multiple destinations and getting to really challenging destinations. With the Gateway mission, for example, when we are trying to move such a large mass, the efficiency of the electric propulsion system enables us to put Gateway in its orbit with only a single launch architecture.
What are the main challenges associated with using plasma propulsion technology? How are you working to overcome these?
NASA often has unique high-performance requirements. While commercial space has launched thousands of plasma thrusters in recent years, their utilisation is primarily for orbit insertion, orbit maintenance, and, eventually, de-orbit with relatively low delta-v mission requirements. For their application, the goal is to drive down the cost of those systems while meeting the minimum requirements. Some of those missions may only need to run for hundreds of hours or use a few to tens of kilograms of propellant. Gateway, however, will launch with thousands of kilograms of propellant and is designed to operate for tens of thousands of hours. It’s also designed to be serviceable for extended life operations. Even demonstrating the thruster through its qualification programme can be quite costly, especially when scaling up thrusters that can stress the limits of test facilities.
It’s also important that we work on very high reliability while also at a very low cost for these high-performing solutions, especially for CubeSats. High-reliability, high-performance solutions for CubeSats have been elusive at low cost, but we are making significant progress with multiple small business innovative research (SBIR) investments for things like pulsed metal thrusters, electrosprays, and even multi-mode systems, for example.
Thermal challenges are also always an issue for us as we move to higher-power density systems.
It’s important that, as we scale these thrusters to much higher power, we keep the mass down, which essentially drives us to these higher power density solutions.
Cost is another major challenge for electric propulsion systems. As well as the thruster, the plasma propulsion systems often need high power for the spacecraft and also a power processing unit (PPU), which can increase the overall propulsion system costs beyond what’s included in a chemical propulsion system. Both JPL and the Glenn Research Center have licensed electric propulsion systems to commercial partners to help enable their missions and push down the cost. We even recently funded a sequential SBIR with a company called HiFunda – a small business that is developing a castable inorganic composite potting material (CICPM) for high-temperature electromagnets. This investment should increase repeatability in the manufacturing of Hall thrusters, reduce lead times, reduce rework, and increase quality control whilst leveraging automation in the process. Those types of investments are helping us work towards lower-cost solutions.
The propellants can also add up for some of these missions. Therefore, NASA and our partners have been investing in alternative propellants. NASA has traditionally used xenon for its performance, but this is relatively expensive compared to alternatives like krypton or argon. We’ve even explored higher-density solutions such as iodine, bismuth, and zinc.
What does the future look like for plasma propulsion at NASA?
I think it’s an exciting time to see where we are and where we are going with plasma propulsion. This applies for both scaling up – for things like nuclear electric propulsion, which could enable some of the Mars architecture missions that we’re looking at – and for scaling down systems for things like CubeSats and Small Sat applications.
It’s amazing to be able to increase accessibility to the world, subsequently increasing the pace of innovation and what can be done in space. As you start to increase performance and enable these new capabilities with very low-cost missions, you see a diversity of missions that enable greater science and greater economic activities.
The future looks bright for electric propulsion, and bright in a broad spectrum. I mean this in the diversity of the types of thrusters, the scale of thrusters, and the propellant that we add to the portfolio as we expand utilisation of EP across all classes of NASA missions for science and exploration.