Hall thruster a serious contender to get humans to Mars

February 19, 2016 by Katherine Mcalpine
An X3 thruster that AERO Professor Alec Gallimore's team has been working on in conjunction with NASA in the Plasmadynamics & Electric Propulsion Laboratory in Ann Arbor, MI on April 28, 2015. The thruster is being built and tested with hopes of it powering humans to Mars in the near future. Credit: Joseph Xu, Michigan Engineering Communications & Marketing

The spacecraft engine that will help take humans to Mars may be based on a University of Michigan prototype.

NASA gave this dream new credibility by funding a spaceflight to be built around a tabletop-sized thruster developed by Alec Gallimore, the Richard F. and Eleanor A. Towner Professor of Engineering and an Arthur F. Thurnau Professor in the U-M Department of Aerospace Engineering.

The agency selected the thruster as part of its Next Space Technologies for Exploration Partnerships, or NextSTEP program. NextSTEP encompasses a set of projects aimed at improving small satellites, propulsion and human living quarters in space. These are milestones toward sending humans into orbit between Earth and the moon in the 2020s and to Mars the following decade.

NASA awarded $6.5 million over the next three years to Aerojet Rocketdyne for the development of the propulsion system, dubbed the XR-100. Gallimore's thruster, the X3, is central to this system, and his team at U-M will receive $1 million of the award for work on the thruster. Aerojet Rocketdyne announced the grant today.

The XR-100 is up against two competing designs. All three of them rely on ejecting plasma—an energetic state of matter in which electrons and charged atoms called ions coexist—but the back of the thruster.

Hall thruster a serious contender to get humans to Mars
The chamber that holds an X3 thruster that AERO Professor Alec Gallimore's team has been working on in conjunction with NASA in the Plasmadynamics & Electric Propulsion Laboratory in Ann Arbor, MI on April 28, 2015. The research team added the eyeballs as a fun decoration, but have yet to name the chamber. Credit: Joseph Xu, Michigan Engineering Communications & Marketing

But the X3 has a bit of a head start. For thrusters of its design power, 200 kilowatts, it is relatively small and light. And the core technology—the Hall thruster—is already in use for maneuvering satellites in orbit around Earth.

"For comparison, the most powerful Hall thruster in orbit right now is 4.5 kilowatts," Gallimore said.

That's enough to adjust the orbit or orientation of a satellite, but it's too little power to move the massive amounts of cargo needed to support human exploration of deep space.

A Hall thruster works by accelerating the plasma exhaust to extremely high speeds. The process starts with a current of electrons spiraling through a circular channel. On their whirlwind journey from the negative electrode at the exhaust end to the positively charged electrode on the inner side of the channel, they run into atoms (typically xenon gas) that are fed into the chamber. The collisions knock electrons off the xenon atoms and turn the xenon into positively charged ions.

The electrons' spiraling motion also builds a powerful electric field that pulls the gas ions out the exhaust end of the channel. Just enough electrons leave with the ions to keep the spacecraft from accumulating a charge, which could otherwise cause electrical problems.

"When they're ionized, the can shoot out at up to 30,000 meters per second, which is about 65,000 mph," Gallimore said.

The X3 contains three of these channels, each a few centimeters deep, nested around one another in concentric rings. The nesting is what allows the Hall thruster to operate at 200 kilowatts of power in a relatively small footprint.

Aerojet Rocketdyne will build two major components around the X3. The full-fledged propulsion system will need a power-processing unit to deliver electricity from a spacecraft's solar arrays to the thruster and a system for feeding xenon gas from high-pressure tanks into the channels where the action happens.

Scott Hall, a doctoral student in Gallimore's lab, will use the funding to put the X3 through a battery of tests, running it up to 60 kilowatts in the Plasmadynamics and Electric Propulsion Lab at U-M and then up to 200 kilowatts at the NASA Glenn Research Center in Cleveland.

Meanwhile, another doctoral student, Sarah Cusson, will investigate a tweak that could allow the X3 to remain operational for five-to-10 times longer than its current lifetime of a little over a year.

"If we do our jobs over the next three years, we can deliver both projects," Gallimore said. "If I had to predict, I would say this would be the basis for sending humans to Mars."

Explore further: Image: Solar Electric Propulsion engine's ionizing Hall thruster

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Sonhouse
not rated yet Feb 19, 2016
Does anyone know the actual thrust produced by that 200 kilowatt energy expenditure?
Osiris1
not rated yet Feb 19, 2016
That is the b-i-i-i-i-i-i-g seeeecret!! The actual thrust in pound-force units
Eikka
not rated yet Feb 19, 2016
Does anyone know the actual thrust produced by that 200 kilowatt energy expenditure?


The higher the thrust the lower the fuel efficiency, so you have to constrain the question a bit more in terms of specific impulse.

200 kW can technically be used to generate any arbitrary amount of thrust - the only question is "for how long".

If you compare it to other "industry standard" Hall thrusters, 200 kW would get you about 5 Newtons of thrust.
Eikka
not rated yet Feb 19, 2016
Or rather, between 5-10 Newtons of thrust.
Whydening Gyre
not rated yet Feb 19, 2016
Or rather, between 5-10 Newtons of thrust.

And that compares to what in foot/lb o thrust?
TechnoCreed
5 / 5 (3) Feb 19, 2016
Or rather, between 5-10 Newtons of thrust.

And that compares to what in foot/lb o thrust?

Give a man a fish, and you feed him for a day. Teach a man to fish, and you feed him for a lifetime.

There are so many tools freely available online to help people with these easy questions that giving you the answer would do you great disservice.

http://www.conver...t+pounds
NoStrings
5 / 5 (1) Feb 19, 2016
I worked on researching a part of a similar project 30 years ago. That's correct, a bit under 10N. However at this power and required delta V, they will need quite a bit of propellant, it would be more economical and require less volume to use cesium for propellant, not xenon. It will add a bit of complication to design, like cesium evaporator, which is not a problem if the thruster will be used virtually continuously. It will not impede reliability.
rrrander
not rated yet Feb 20, 2016
There is only one viable propulsion system to get humans or large cargo to planets and that is Project Orion. Even at the height of nuclear testing in the 1960's, the contribution of those tests to overall human radiation exposure was only 1/500th the normal dose per year. Project Orion, if launched from the surface of the planet, would generate even less residual fall-out. The most important thing is that long-term exposure to cosmic rays is detrimental to human life so you cannot have years long missions to (for example) Mars via our primitive, weak chemical rockets.
EyeNStein
not rated yet Feb 20, 2016
To characterise the thrust of this unit:-
Given that 10N will only accelerate ~1kg to 1g (Comfortable earth normal) and the thruster itself weighs more than that: There is clearly a lot of work to be done on its thrust to weight ratio before ion thrusters, and their huge power sources, will be ready to routinely shuttle people around the solar system. But its definitely already better than carrying more fuel than cargo for long haul spacecraft.
EnsignFlandry
not rated yet Feb 20, 2016
That is the b-i-i-i-i-i-i-g seeeecret!! The actual thrust in pound-force units


There are numerous places to get the conversion information. But you can go ahead and use metric units if that's not too difficult.

Does anyone know the actual thrust produced by that 200 kilowatt energy expenditure?


How long will the burn last?
Eikka
3 / 5 (2) Feb 20, 2016
Given that 10N will only accelerate ~1kg to 1g


1 g of acceleration would be far more than anyone could hope for. That would be seriously fast.

Suppose you have a 10 ton spacecraft with 10 N of thrust. Two weeks of acceleration gets you going a kilometer per second which is on the scale of what is needed to make or break an orbit. Earth orbital velocity is about 8 km/s and escape velocity is 11 km/s.

The real problem is getting 200 kW continuously for a month to do it.
TechnoCreed
5 / 5 (2) Feb 20, 2016
I am bringing in some more info on this thruster. Let's see if it is possible to have some discussion on the basis of this paper: http://pepl.engin...-125.pdf
TechnoCreed
5 / 5 (2) Feb 20, 2016
Given that 10N will only accelerate ~1kg to 1g


1 g of acceleration would be far more than anyone could hope for. That would be seriously fast.

Suppose you have a 10 ton spacecraft with 10 N of thrust. Two weeks of acceleration gets you going a kilometer per second which is on the scale of what is needed to make or break an orbit. Earth orbital velocity is about 8 km/s and escape velocity is 11 km/s.

The real problem is getting 200 kW continuously for a month to do it.

And here is what you get with a 100 days burn. https://www.wolfr...+seconds
rick_cavaretti
3 / 5 (2) Feb 20, 2016
Ft-lbs? Seriously? Who uses those old Fred Flintstone units anymore?
javjav
not rated yet Feb 21, 2016
The real problem is getting 200 kW continuously for a month to do it
I agree. This is an "Energy" problem, the "what engine?" is a secondary aspect. Solve how to get the energy, then design the engine that can use it better.

TechnoCreed
5 / 5 (1) Feb 21, 2016
The real problem is getting 200 kW continuously for a month to do it
I agree. This is an "Energy" problem, the "what engine?" is a secondary aspect. Solve how to get the energy, then design the engine that can use it better.


I would bet that with 60 of these X3 thrusters and a small nuclear power station 13MW to 15MW we could put the ISS in orbit around Mars. That would be a fitting purpose for this facility which is expected to retire soon anyway.
daqman
1 / 5 (1) Feb 21, 2016

I would bet that with 60 of these X3 thrusters and a small nuclear power station 13MW to 15MW we could put the ISS in orbit around Mars. That would be a fitting purpose for this facility which is expected to retire soon anyway.


Yes, that's what I was thinking. These are quite small and you don't need to limit yourself to just one. Then you need to power it. About 7000 square meters of solar panels will get you a megawatt and it's never cloudy in space. Solar panels are fairly light and there is no drag to worry about so it doesn't matter that you have thousands of square meters of the things sticking out either side of your craft.
You could use a nuclear plant but converting the energy from a fission reactor into electricity and then converting that back into thrust seems a bit of a waste. I'd throw my propellant into the fission reactor to generate the plasma directly, combine that with some plasma confinement tech from fusion research to make a bottle with an open end...
javjav
1 / 5 (1) Feb 21, 2016
@dagman That is not feasible. The cost to lunch and assemble 7000 square meters of solar panels and 60 Hall engines would be higher than the cost of the ISS itself , also duplicating it's weight. And what is the purpose of having the ISS on Mars? I convinced we need the opposite approach: Lunch just two guys in a small but fast ship, doing the round trip in 2 to 3 months, including one week on Mars surface. The first manned mission will be provably in that style, the 2 year approach is too expensive and too complex.
viko_mx
1 / 5 (1) Feb 21, 2016
Again pink humanist dreams presented as science. The man never will set foot on Mars. And there is no reason to do so. Some people are too ambitious to be realistic. In practice the men is very limited and dependent upon physical environment and never will master another celestial body. The men has no physical capabilities and knowledge about for this and the darkness in today's science conquered by metaphysicians gives no hope for it. The purpose of human life is very different from this.
TechnoCreed
not rated yet Feb 22, 2016
Yes, that's what I was thinking. These are quite small and you don't need to limit yourself to just one. Then you need to power it. About 7000 square meters of solar panels will get you a megawatt and it's never cloudy in space. Solar panels are fairly light and there is no drag to worry about so it doesn't matter that you have thousands of square meters of the things sticking out either side of your craft.

Ok but that would set your limits to around 50 metric tons using 5 thrusters.
TechnoCreed
not rated yet Feb 22, 2016
You could use a nuclear plant but converting the energy from a fission reactor into electricity and then converting that back into thrust seems a bit of a waste. I'd throw my propellant into the fission reactor to generate the plasma directly, combine that with some plasma confinement tech from fusion research to make a bottle with an open end...

The way I look at it, According to this paper, http://ntrs.nasa....9914.pdf my idea is possible and would only add 45 tons to ISS.
TechnoCreed
not rated yet Feb 22, 2016
@javjav
And what is the purpose of having the ISS on Mars? I convinced we need the opposite approach: Lunch just two guys in a small but fast ship, doing the round trip in 2 to 3 months, including one week on Mars surface. The first manned mission will be provably in that style, the 2 year approach is too expensive and too complex.

I am just having fun making wild speculations about the possibilities that this technology could bring.
Eikka
not rated yet Feb 22, 2016
I would bet that with 60 of these X3 thrusters and a small nuclear power station 13MW to 15MW


You need about the same area of radiators as you'd need solar panels to shed the heat from a nuclear reactor in space.

Nuclear rockets make sense if you're dumping the heat into the propellant, but making electricity with them in large amounts becomes a complex and very massive problem, largely because the energy conversion efficiency is so low over so many steps.
TechnoCreed
not rated yet Feb 22, 2016
You need about the same area of radiators as you'd need solar panels to shed the heat from a nuclear reactor in space.

Nuclear rockets make sense if you're dumping the heat into the propellant, but making electricity with them in large amounts becomes a complex and very massive problem, largely because the energy conversion efficiency is so low over so many steps.

The paper that I had linked gives you the basics to produce an optimal power to mass ratio, in the order of 2kg/kW of electrical power, for the given design. It tells you that the optimal temperature of the radiator should be around 600K and tells you that you would end up with something like 22% efficiency.

tbc
TechnoCreed
not rated yet Feb 22, 2016
...

So, if I have a nuclear reactor that delivers 60MW of thermal energy, I would end up with 13.2 MW of electric power and I would have to radiate 46.8MW of thermal energy. If you know Boltzmann law: At 600K you radiate 7348.8 W/m². It means that I need 6370m² of radiating surface to take care of the exceeding thermal energy. That is not so bad.
Eikka
not rated yet Feb 28, 2016
It means that I need 6370m² of radiating surface to take care of the exceeding thermal energy. That is not so bad.


It is, when you consider what it would actually take to transfer the heat load to the radiator, and the radiator itself. You forget that the radiating surface can't be folded up in any sort of accordion-shape because it would absorb the radiation from the adjacent folds - all the surface can see is dark space, so it needs to be flat.

6370 square meters is something like two football fields of radiators and pipes and pumps and molten salt to transfer the heat, plus the struts and structures that hold the whole thing together against the acceleration from your engines.

That stuff gets seriously heavy.

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