In a few years, NASA will send the “first woman and first person of color” to the lunar surface as part of the Artemis program. This will be the first time astronauts have set foot on the moon since the Apollo 17 mission in 1972. A permanent infrastructure will then be created that will enable regular missions to the surface (once a year) and a “sustainable program of lunar exploration and development.” This will require spacecraft that regularly shuttle between Earth and the Moon to carry crews, vehicles and payloads to transport.
In a recent study supported by NASA, a team of researchers from the University of Illinois Urbana-Champaign examined a new method of sending spacecraft to the moon. It's known as “multimode propulsion,” a method that integrates a high-thrust chemical mode and a low-thrust electrical mode – using the same propellant. This system has several advantages over other forms of propulsion, not least of which is that it is lighter and more cost-effective. With luck, NASA could rely on multi-mode propulsion spacecraft to achieve many of its Artemis goals.
The paper describing their investigation, “Indirect optimal control techniques for the design of multimode propulsion missions,” was recently published in Acta Astronautica. The research was led by Bryan C. Cline, a graduate student in the Department of Aerospace Engineering at the University of Illinois Urbana-Champaign. He was joined by aerospace engineer and graduate student Alex Pascarella, as well as Robyn M. Woollands and Joshua L. Rovey – assistant professor and professor in the Grainger College of Engineering (aerospace engineering).
Artist's impression of the ESA LISA Pathfinder mission. Photo credit: ESA–C.Carreau
To break it down, a multimode thruster relies on a single chemical monopropellant – such as hydrazine or ASCENT (Advanced Spacecraft Energetic Non-Toxic) fuel – to power chemical thrusters and an electrospray thruster (also known as a colloid thruster). The latter element is based on a process known as electrospray ionization (ESI), in which charged liquid droplets are created and accelerated by a static electric field. Electrospray thrusters were used in space for the first time on board ESA's LISA Pathfinder mission to demonstrate interference reduction.
By developing a system that relies on both and can switch when necessary, satellites can perform propulsion maneuvers with less fuel (also known as “minimum fuel transfers”). As Cline said in a Grainger College of Engineering press release:
“Multimode drive systems also expand the scope of performance. We describe them as flexible and adaptable. I can choose a high-thrust chemical mode to get somewhere quickly, and a low-thrust electrospray mode to perform smaller maneuvers to stay in the desired orbit. The availability of multiple modes has the potential to reduce fuel consumption or shorten the time to achieve your mission objective.”
The team's investigation follows a similar study conducted by Cline and researchers at NASA's Goddard Spaceflight Center and aerospace consulting firm Space Exploration Engineering, LLC. In a separate paper titled “Lunar SmallSat Missions with Chemical-Electrospray Multimode Propulsion,” they examined the advantages of multimode propulsion over 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 carry out these four mission profiles.
.Earth-Mars trajectory with minimal fuel when the CubeSat is idling, as well as in Mode 1 – Low Thrust and Mode 2 – High Thrust. Photo credit: UIU-C
“We have demonstrated 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 good starting point. It 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, in which two propulsion systems are combined to achieve optimal thrust. A good (although not yet realized) example of this is bimodal nuclear propulsion, in which a spacecraft is based on a nuclear-thermal propulsion system (NTP) and a nuclear-electric propulsion system (NEC). While an NTP system relies on a nuclear reactor to heat hydrogen or deuterium as a propellant and can achieve a high acceleration rate (Delta-V), an NEC system uses the reactor to power an ion engine that provides constant thrust.
A key advantage of multimode propulsion over a hybrid system is the dramatic reduction in the dry mass of the spacecraft. While hybrid drive systems require two different fuels (and therefore two separate fuel tanks), bimodal drives only require one. This not only saves the mass and volume of the spacecraft, but also makes its launch more cost-effective. “I can choose to use high thrust at any time and low thrust at any time, and it doesn’t matter what I’ve done in the past,” Cline said. “If a tank is empty in a hybrid system, I cannot choose this option.”
To complete each of the design reference missions for this project, the team made all decisions manually – e.g. B. when to use high and low thrust. As a result, the flight paths were not optimal. This led Cline, after completing the project, to develop an algorithm that would automatically select which mode would result in 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 a completely different matter where the focus was on the development of the method and not on the specific results shown in the paper. We have developed the first indirect optimal control technique specifically for multimode mission design. This allows us to develop transfers that obey the laws of physics while achieving a specific goal, such as minimizing fuel consumption or transfer time.”
“We have shown that the method works on a mission that is relevant to the scientific community. Now you can use it to solve all kinds of mission design problems. The mathematics is independent of the respective mission. And because the method uses calculus of variations, what we call an indirect optimal control technique, it guarantees that you get at least a locally optimal solution.”
Artist's impression of an Artemis astronaut exploring the lunar surface during a future mission. Photo credit: NASA
The research is part of a project led by Professor Rovey and a multi-institutional team called 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 with Dr. John D. Williams, professor of mechanical engineering and director of the Electric Propulsion & Plasma Engineering Laboratory at Colorado State University (CSU), is responsible for leading the diagnostics and fundamental studies team.
As Cline suggested, their work on multimode propulsion could revolutionize the way small spacecraft travel between Earth and the Moon, Mars and other celestial bodies:
“It is an emerging technology as it is still developing on the hardware side. It allows us to complete all kinds of missions that we could not accomplish without it. And it's rewarding because if you have a specific mission concept, you can do more with multimode propulsion. You have more flexibility. You have more adaptability.
“I think this is an exciting time to be working on multimode propulsion, both from a hardware perspective and a mission design perspective. We are developing tools and techniques to transform this technology from something we test in the basement of Talbot Lab into something that can have a real impact on the space community.”
Further ReadingL University of Illinois Urbana-Champaign, Acta Astronautica
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