In a few years, as part of the Artemis Program, NASA will send the “first woman and first person of color” to the lunar surface. This will be the first time astronauts have set foot on the Moon since the Apollo 17 mission in 1972. This will be followed by the creation of permanent infrastructure that will allow for regular missions to the surface (once a year) and a “sustained program of lunar exploration and development.” This will require spacecraft making regular trips between the Earth and Moon to deliver crews, vehicles, and payloads.
In a recent NASA-supported study, a team of researchers at the University of Illinois Urbana-Champaign investigated a new method of sending spacecraft to the Moon. It is known as “multimode propulsion,” a method that integrates a high-thrust chemical mode and a low-thrust electric mode – while using the same propellant. This system has several advantages over other forms of propulsion, not the least of which include being lighter and more cost-effective. With a little luck, NASA could rely on multimode propulsion-equipped spacecraft to achieve many of its Artemis objectives.
The paper describing their investigation, “Indirect optimal control techniques for multimode propulsion mission design,” was recently published in Acta Astronautica. The research was led by Bryan C. Cline, a doctoral student in the Department of Aerospace Engineering at the University of Illinois Urbana-Champaign. He was joined by fellow aerospace engineer and PhD Candidate Alex Pascarella, and Robyn M. Woollands and Joshua L. Rovey – an assistant professor and professor with the Grainger College of Engineering (Aerospace Engineering).
To break it down, a multimode thruster relies on a single chemical monopropellant – like hydrazine or Advanced Spacecraft Energetic Non-Toxic (ASCENT) propellant – to power chemical thrusters and an electrospray thruster (aka. colloid thruster). The latter element relies on a process known as electrospray ionization (ESI), where charged liquid droplets are produced and accelerated by a static electric field. Electrospray thrusters were first used in space aboard the ESA’s LISA Pathfinder mission to demonstrate disturbance reduction.
By developing a system that relies on both that can switch as needed, satellites will be able to perform propulsive manuevers using less propellant (aka. minimum-fuel transfers). As Cline said in a Grainger College of Engineering press release:
“Multimode propulsion systems also expand the performance envelope. We describe them as flexible and adaptable. I can choose a high-thrust chemical mode to get someplace fast and a low-thrust electrospray to make smaller maneuvers to stay in the desired orbit. Having multiple modes available has the potential to reduce fuel consumption or reduce time to complete your mission objective.”
The team’s investigation follows a similar study conducted by Cline and researchers from NASA’s Goddard Spaceflight Center and the aerospace advisory company Space Exploration Engineering, LLC. In a separate paper, “Lunar SmallSat Missions with Chemical-Electrospray Multimode Propulsion,” they considered the advantages of multimode propulsion against all-chemical and all-electric approaches for four design reference missions (DRMs) provided by NASA. For this latest investigation, Cline and his colleagues used a standard 12-unit CubeSat to execute these four mission profiles.
“We showed for the first time the feasibility of using multimode propulsion in NASA-relevant lunar missions, particularly with CubeSats,” said Cline. “Other studies used arbitrary problems, which is a great starting point. Ours is the first high-fidelity analysis of multimode mission design for NASA-relevant lunar missions.”
Multimode propulsion is similar in some respects to hybrid propulsion, where two propulsion systems are combined to achieve optimal thrust. A good example of this (though still unrealized) is bimodal nuclear propulsion, where a spacecraft relies on a nuclear-thermal propulsion (NTP) and nuclear-electric propulsion (NEC) system. While an NTP system relies on a nuclear reactor to heat hydrogen or deuterium propellant and can achieve a high rate of acceleration (delta-v), an NEC system uses the reactor to power an ion engine that offers a consistent level of thrust.
A key advantage multimode propulsion has over a hybrid system is a drastic reduction in the dry mass of the spacecraft. Whereas hybrid propulsion systems require two different propellants (and hence, two separate fuel tanks), bimodal propulsion requires only one. This not only saves on the mass and volume of the spacecraft, but makes them cheaper to launch. “I can choose to use high-thrust at any time and low-thrust at any time, and it doesn’t matter what I did in the past,” said Cline. “With a hybrid system, when one tank is empty, I can’t choose that option.”
To complete each of the design reference missions for this project, the team made all decisions manually – i.e., when to use high-thrust and low-thrust. As a result, the trajectories weren’t optimal. This led Cline to develop an algorithm after completing the project that automatically selects which mode would lead to an optimal trajectory. This allowed Cline and his team to solve a simple two-dimensional transfer between Earth and Mars and a three-dimensional transfer to geostationary orbit that minimizes fuel consumption. As Cline explained:
“This was an entirely different beast where the focus was on the development of the method, rather than the specific results shown in the paper. We developed the first indirect optimal control technique specifically for multimode mission design. As a result, we can develop transfers that obey the laws of physics while achieving a specific objective such as minimizing fuel consumption or transfer time.”
“We showed the method works on a mission that’s relevant to the scientific community. Now you can use it to solve all kinds of mission design problems. The math is agnostic to the specific mission. And because the method utilizes variational calculus, what we call an indirect optimal control technique, it guarantees that you’ll get at least a locally optimal solution.”
The research is part of a project led by Professor Rovey and a multi-institutional team known as the Joint Advanced Propulsion Institute (JANUS). Their work is funded by NASA as part of a new Space and Technology Research Institute (STRI) initiative. Rovey is responsible for leading the Diagnostics and Fundamental Studies team, along with Dr. John D. Williams, a Professor of Mechanical Engineering and the Director of the Electric Propulsion & Plasma Engineering Laboratory at Colorado State University (CSU).
As Cline indicated, their work into multimode propulsion could revolutionize how small spacecraft travel between Earth and the Moon, Mars, and other celestial bodies:
“It’s an emerging technology because it’s still being developed on the hardware side. It’s enabling in that we can accomplish all kinds of missions we wouldn’t be able to do without it. And it’s enhancing because if you’ve got a given mission concept, you can do more with multimode propulsion. You’ve got more flexibility. You’ve got more adaptability.
“I think this is an exciting time to work on multimode propulsion, both from a hardware perspective, but also from a mission design perspective. We’re developing tools and techniques to take this technology from something we test in the basement of Talbot Lab and turn it into something that can have a real impact on the space community.”
Further ReadingL University of Illinois Urbana-Champaign, Acta Astronautica