Space

Improved ion engines will open up the outer Solar System

Improved ion engines will open...
An ion engine test for Deep Space One (Photo: NASA/JPL)
An ion engine test for Deep Space One (Photo: NASA/JPL)
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Diagram of a Hall effect ion engine (Image: Wikipedia)
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Diagram of a Hall effect ion engine (Image: Wikipedia)
An ion engine test for Deep Space One (Photo: NASA)
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An ion engine test for Deep Space One (Photo: NASA)
An ion engine test for Deep Space One (Photo: NASA/JPL)
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An ion engine test for Deep Space One (Photo: NASA/JPL)
Deep Space 1's Ion Engine (Photo: NASA/JPL)
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Deep Space 1's Ion Engine (Photo: NASA/JPL)
Deep Space 1's Ion Engine (Photo: NASA/JPL)
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Deep Space 1's Ion Engine (Photo: NASA/JPL)

The phrase "engage the ion drive" still has the ring of a line from Star Wars, but these engines have been used in space missions for more than four decades and remain the subject of ongoing research. Ion engines have incredible fuel efficiency, but their low thrust requires very long operating times ... and therein lies the rub. To date, erosion within such an engine seriously limits its operational lifetime. Now a group of researchers at NASA's Jet Propulsion Laboratory (JPL) has developed a new design that largely eliminates this erosion, opening the gates for higher thrust and more efficient drives for manned and unmanned missions to the reaches of the Solar System.

Ion engines of various types have been used on space missions since at least 1964, when NASA flew the suborbital Space Electric Rocket Test I mission. Many classes of space missions can benefit through using fuel efficient ion engines during some phase of their mission. For example, several communication satellites have been raised into their final geosynchronous orbit using ion thrusters. The European Space Agency's SMART-1 lunar mission was placed in geosynchronous orbit by conventional means, and then made the transfer into lunar orbit using an ion engine.

Deep space missions, however, is where ion engines could really shine. Three missions, NASA's Deep Space One and Dawn, and the Japan Aerospace Exploration Agency's Hayabusa have partially or entirely obtained their post-Earth-orbit propulsion from ion engines. Their ion engines operated for several years with only an occasional panic attack while providing a few hundredths of a Newton (perhaps 0.4 oz) of thrust.

How does an ion engine work?

There are many varieties and more proposals (the VASMIR engine comes to mind), but the operating principle is quite simple. There are two basic styles of ion engines, electrostatic and electromagnetic. An electrostatic ion engine works by ionizing a fuel (often xenon or argon gas) by knocking off an electron to make a positive ion. The positive ions then diffuse into a region between two charged grids that contain an electrostatic field. This accelerates the positive ions out of the engine and away from the spacecraft, thereby generating thrust. Finally, an neutralizer sprays electrons into the exhaust plume at a rate that keeps the spacecraft electrically neutral.

An electromagnetic ion engine also works by ionizing a fuel. In this case a plasma is created that carries current between the ionizing anode and a cathode. The current in turn generates a magnetic field at right angles to the electric field, and thereby accelerates the positive ions out of the engine via the Lorentz force – basically the same effect on which railguns are based. Again a neutralizer keeps the spacecraft electrically neutral.

Powering a serious spacecraft

This all takes a good amount of electrical power – about 25 kW per Newton (3.6 oz) of thrust. So what thrust levels are needed to push, say, a 100 ton spacecraft throughout the solar system? (Forgive me – I like to dream!) It depends on the mission, of course, but 1000 N of thrust would put that spacecraft in orbit around Jupiter in about 10 months and in Neptunian orbit in just under 1.5 years. Obviously this is pretty far down the pike technologically, but let's see what is needed.First, a supply of electrical power that delivers about 25 MWe (megawatts of electric power) more or less constantly. Clearly, we're talking about nuclear power – a lot of nuclear power from a reactor system that fits within a 100 ton spacecraft. Fortunately, there is currently a good deal of effort going into designing compact nuclear reactors for power production here on Earth.

In addition, NASA and DOE are collaborating on the Fission Surface Power Project, where the goal is to make tiny nuclear power reactors for bases on the Moon and Mars. The design goal is to produce a reactor that will provide 40 kWe for 10 years, fold into a 3 x 3 x 7 meter (10 x 10 x 23 feet) space and weigh in at 11,000 lbs (5000 kg). This is quite a ways from what is needed for a 1000 N ion drive, but with molten salt reactors and efficient conversion of heat into electricity, it seems within the bounds of possibility. Besides, I said I was dreaming.

Design roadblock

If the above were worked out satisfactorily, could we build a 1000 N thrust ion engine? There are some minor technical problems with efficiently ionizing the fuel and cooling the engines, but the biggest roadblock of which we are currently aware is that the large ionic current passing through the engine will cause enough erosion to destroy the engine. This is not a materials problem – it is a design issue. This is the roadblock recently demolished (at least partly) by NASA researchers at the Jet Propulsion Lab in Pasadena, California.

Diagram of a Hall effect ion engine (Image: Wikipedia)
Diagram of a Hall effect ion engine (Image: Wikipedia)

You can see in the cross-section diagram above that the fuel plasma fills the anode and gas distributor. At low thrust, the small plasma density is accelerated by the Lorentz effect of the crossed magnetic and electric fields. However, at large thrust, the plasma density becomes large enough to distort the fields, resulting in positive ions being accelerated directly into the anode walls.

When these ion energies are large enough, they will erode material from the walls in a process called sputtering. To make matters worse, in the quest for better ion engines, one desires both larger thrust and larger exhaust velocities (which require less fuel). Changes made to meet both of those goals greatly increase the rate of erosion.

This problem is made more difficult because the electrodynamics of the fields and the plasma are seriously nonlinear, making it difficult to predict the effect of a change in engine design on the erosion of the engine.

The obvious approach was to magnetically shield the walls from the energetic ions. The NASA team accomplished this by shielding the boron nitride walls so that the magnetic field from the inner and outer magnetic coil would pass around the end of the anode annulus. Properly done, the magnetic field no longer penetrated the walls. As a result, the magnetic field lines, rather than penetrating the walls at angles close to perpendicular, are nearly parallel to the walls. This causes the positive ions to be accelerated away from the walls, and as a result the walls are effectively the coolest part of the internal engine surfaces.

The result of experimental tests of the new magnetically shielded configuration showed the rate of erosion was reduced by a factor of 500-1000. This highly successful demonstration took place in a six kW Hall effect ion thruster.

While there will no doubt be more challenges to overcome as work proceeds in the further development of large-scale ion drives, this new research looks to have solved the nearest and most visible problem. Manned deep space missions are one step closer, and having dreamed of space travel all my life, I'd like to live to see it happen.

Sources: Applied Physics Letters, NASA-JPL

34 comments
PrometheusGoneWild.com
While this is very encouraging, the real problem with space travel is the cost of getting materials out of the Earths gravity well. While this new technology will help with unmanned craft, I do not see a real long range manned system happening until we can launch every heavy sections of a craft to be assembled in space. I have a running bet with a Physics Professorial friend, I am betting on electromagnetic launch he thinks the space elevator is going to win the day. I have 1$ riding on it:)
n2liberty
The cost to orbit using chemical rockets is not as bad as you might think. The majority of cost is the cost of the rocket. If we can create a reusable rocket the fuel cost is quite reasonable. The fuel for a resupply launch to the space station only costs about $200,000. The big cost is in the rocket we have to throw away now. We are not talking about a Space Shuttle approach that was a total financial disaster costing more to refurbish after each launch than simple throw away rockets cost. We need a system that can use a simple low cost ablative heat shield for reentry and deacceleration of the booster until it reaches terminal velosity with a controlled fall through the air. With out all the fuel the crafts density is lower than that of a human being so terminal velosity is about 100 to 200 mph. When you get close to the ground restart the engine and make a controlled landing. Preferably near the next launch location. Refuel add a new heat shield and payload ready to go again. SpaceX is working on this and I believe they will succeed.
Astro Rosaire
The author is very optimistic about space nuclear power's funding level, as of late the Fission Surface Power (FSP) Project has been zero funded following Obama's cancellation of the Constellation Project (from which it received its funding). All of the nuclear systems he speaks of are PAPER reactors (meaning academic studies and computational work)! Very little has been done in terms of actually building and operating one of these little gems, which is unfortunate. It's nice to talk about the engines that get you to the outer planets, but until the US concertedly funds a space nuclear project to power these devices, they are just an engine for making pretty lights on earth.
Kwazai
MontyPython... How prevalent is elemental hydrogen in deep space? I'm assuming the electro-negativity of the ions is proportional to the sputtering problems (like nickel/platinum nucleate sites for brown's gas production). With a source of fuel in space it would only be necessary to magnetically(electrostatically?) push off of it for thrust (like a jet engine intake/exhaust). (recombining to regain fuel and repeat? hydrogen 1.2eV.) Passivating the material only delays the oxidation(sputtering). Maybe something like molygraphene(?molybdenum-?sulfide??) used for the magnets in the one shown in the pic. Might be lighter weight too.
Les LaZar
Is an Ion engine the most efficient way to turn the energy of a nuclear reactor into thrust? The reactor generates heat. Then some mechanism (therocouples or a fluid driven turbine) converts that heat into electricity with efficiency losses. The ion engine + fuel then converts the electricity to thrust. Isn't there some way to combine the heat of the reactor with the ion engine fuel to get thrust and skip the heat to electricity and electricity to thrust steps?
PrometheusGoneWild.com
n2liberty, I love Space X and hope they do make their rockets reusable and self landing (they do more in a year than NASA does in a decade). I have been watching them closely and love what I see. However, rocket launching is dangerous and even if they land their rockets for reuse, you then have the issue of how many times do you reuse a part before you replace it or rebuild it. This leads to all new maintenance and safety issues. With electromagnetic launch you would still use a rockets. Very safe, low thrust rockets to keep accelerating the "truck" space vehicle into orbit. Once the vehicle drops off its load, it returns under powered flight. When I propose the idea people laugh and say "That would have to be huge!". Yes I am talking a couple miles of flat and then up the side of a mountain. We are talking billions (still cheaper than the Shuttle....) There would be only one. And the first country that made one would put everyone else out of business.....
Simon Sammut
Les, I completely understand where you are coming from. The problem is that each module Is designed separately then the modules are assembled together to form a working system. The efficiencies you are suggesting would imply that each project is designed completely customised using no 'off the shelf' modules. This would me insanely expensive. The modular approach is cheaper, but the "one module fits all" approach is never optimal.
Daniel Moreno
Gwyn Rosaire, one of the nuclear reactor types that you say are are just 'Paper Reactors' is in fact not a 'Paper Reactor.' The Molten Salt Reactor Experiment at ORNL ran for several years and was poised for commercial viability until politics gutted the project. No paper designs and math here, it's a real, usable piece of technology.
Nairda
"When these ion energies are large enough, they will erode material from the walls in a process called sputtering. " Is the rate of sputtering proportional to thrust output? Just an idea. Reaction chamber damage is not easily predictable. So why waste material reinforcing the chamber when you can just repair on the fly. Can the sputtering effect be offset by a material that repaired itself. Or more simplistically, can a directional nozzle inside the reaction chamber spray a coating of material to repair walls affected by sputtering. In this way you can run the ion drive at maximum thrust, and every few week/months/years power down, assess the damage, repair, and power back up again. Likely at a certain thrust level, there would be a distribution of errosion in just a few spots.
Ben Selinger
@Les LaZar: Sure. You can cut out the middle man entirely and just use the reactor core as a primary heat source. Store the reactor core inside a pressure vessel, and connect to it an exhaust jet. Just flush the toilet and off you go. The problem however, is that we're now talking about a steam powered space ship. While that's a ridiculously cool idea (and I hope someone does it some day), getting around in space is all about weight to thrust ratio, and weight to thrust comes down to exhaust velocity and mass. When discussing efficiency in propulsion, there's more at play than simply mechanical efficiency. We have to consider that not only do we need energy, but also fuel mass. Fuel mass ultimately is the most important consideration to propulsion in a vacuum, as is by far the most scarce, and the more you bring, the more you have to burn to move it. A nuclear/steam powered rocket nozzle might (i'm pulling numbers out my ass here) provide an exhaust velocity of 1km/s. I doubt it, but let's call it 10km/s just to be safe. Conversely, the theoretical limit to the exhaust velocity of an ion thruster, is limited only first by the energy available and eventually by the speed of light. 10km/s to 300,000km/s. Assuming even 10,000km/s, you require 1,000x less fuel than our totally unrealistic steam thruster. But that's not even true because you need fuel to push that 1,000x. I believe this works out to some magical math that has to do with inverse squares and some X's and Y's or something. So in terms of mechanical efficiency, the nuclear steam engine is ridiculously efficient when compared to a nuclear generator providing electricity. In terms though, of propulsion efficiency, with fuel mass brought into the equation, the steam engine feels like a horse drawn carriage being compared to ... well, an ion thruster I guess.