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All ion thrusters are quite small. Having read through the more approachable information about them, as they use physics pretty beyond me, I don't understand what it is about them that makes it necessary they be so small. Are they fundamentally limited in some way? Is there reason to hope what limits them now could be overcome at some point so their high Isp could be used for higher acceleration on larger spacecraft?

Edit (next day): The answers so far have helped me understand the basic issue that the power needs of these engines are huge. In some ways I think this deserves to be treated as a separate issue from the scalability of the engine itself - this article makes the point that energy could be beamed to the craft, for instance, meaning you don't need to carry the mass of the power source with you. And the point has already been made that nuclear reactors could be used, which have the required power density, if the details of how to make use of that was figured out (see supercapacitor conversation in comments under EchoLogic's answer).

The delta V imparted for their weight is very high. NSTAR engines weigh 48 kg including the thruster, power processing units, xenon feed system, and control interface. Three of those and 475 kg of xenon will impart delta V of over 10 km/s to the Dawn probe, itself weighing about 600 kg without those thrusters.

So if the power issue was handled, could you just make great big ones to push a large vessel fast, and take advantage of that efficiency?

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    $\begingroup$ They don't need to be scaled up. You can cluster as many of them as you like, assuming you have the power to support them (e.g. a nuclear reactor). What you'd really like is to increase the power density, so that you can ameliorate the area scaling problem. That increase in power density could be with the same size thrusters or larger thrusters. In any case, yes, you can scale them up, which you might do mainly for cost. The current 4.5 kW Hall thrusters are being scaled up to 12-15 kW for ARRM. $\endgroup$ – Mark Adler Dec 14 '15 at 1:15
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    $\begingroup$ @MarkAdler - power density was really what i meant, though there is now too much water under the bridge to go editing the question to say that. At any rate, all of the perspectives in the different answers add up to an interesting overview of doing more with them, by whatever means. $\endgroup$ – kim holder Dec 14 '15 at 1:34
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Assuming you mean "quite small" in terms of mass as well as thrust output. Fundamentally, current ion drives are limited by the amount of power available to them - it takes many, many kilowatts of input power to provide tiny amounts of thrust.

As you know from the answer you linked, the Dawn spacecraft is powered by 3 NSTAR ion engines which generate a maximum thrust of 90mN, and each has a mass itself of ~25.5kg. What that answer failed to mention is the enormous amount of input power Dawn requires to operate its ion engines. It has an enormous 36.4m2 array of solar panels which when stretched, make the entire probe 19.7m across (Source).

Dawn undergoing testing

At 1AU, they generate a huge 10kW of power, but due to the inverse square law, this drops off to 1300W at 3AU. This is a problem inherent to all solar-powered spacecraft - in fact, the 2007 NASA meeting which produced the above linked document on spacecraft solar power estimate it would take a 250m2 solar array to provide just 400W to a Uranus orbiter. This entire situation is acerbated when your propulsion source requires kilowatts of power to run - limiting ion engines' usefulness far from the sun.

The NSTAR ion engine design reference calls for 2.3kW of power specifically. Much of this energy is needed to provide a voltage distance to accelerate the ions across (therefore producing the thrust).

The rest of an ion-engine is mainly solid stage and digital, with few moving parts overall. This contributes to its low mass. This low mass isn't inherent to Ion engines either - the physical Hydrazine thrusters themselves commonly found on spacecraft don't weigh that much either, it's the mass of the fuel which is the problem.

Solutions

Most attempts to scale up ion engines inevitably either require large solar arrays or nuclear fission reactors (respectively named Solar Electric Propulsion & Nuclear Electric Propulsion). Neither power source is entirely ideal. The closest scale up attempt of ion-propulsion was a plan to attach a VASIMR engine to the ISS (which has since fallen though) to provide occasional reboosts to the station's altitude to compensate for its orbital decay. This would use 200kW of power to generate 4N of thrust - the problem is this is actually more than the ISS' entire power output. Their solution was to trickle charge a set of batteries allowing the engine to fire intermittently.

Ad Astra, the company developing VASIMR, also proposed a mission architecture where one could reach Mars in 39 days, but such a mission would require a multi-megawatt nuclear reactor. The current technical and political climate renders this a non-starter.

In summary...

Fundamentally, ion engines are small and deliver small amounts of thrust because the size, mass, and power required to run them at higher thrusts is currently in-feasible.

Until launch costs drop, and energy solutions can deliver more power to spacecraft, ion engines look set to be confined to low thrust applications.

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  • $\begingroup$ 'Enormous' is something of a relative term. Compare the size/mass of that solar array with the upper stage booster you'd need to launch DAWN from Earth orbit to interplanetary trajectory. Then figure the solar array does double duty: once the spacecraft arrives at its destination, it provides power for the onboard instruments. $\endgroup$ – jamesqf Mar 30 '15 at 5:29
  • $\begingroup$ It's not just the need for a multi megawatt reactor that prevents 39 day VASIMR Mars trips but also the need for unbelievably great specific power. 2 kilowatts per kilogram isn't doable. $\endgroup$ – HopDavid Mar 30 '15 at 6:55
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    $\begingroup$ @CeesTimmerman Capacitors are useful for many watts over a very brief time. A "fast discharge rate" as your source puts it. An ion engine for a 39 day trip to Mars would need a 2 kW/kg power source that lasts much longer than a typical capacitor discharge. $\endgroup$ – HopDavid Mar 30 '15 at 15:55
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    $\begingroup$ @CeesTimmerman Okay, let's say 100 Wh/kg. As OrbWeaver says 39 days at 2000 watts is 1872000 Wh. So your 2000 kW power source masses 18720 kg. To get the power density I mentioned, it needs to mass 1 kilogram. $\endgroup$ – HopDavid Mar 31 '15 at 2:50
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    $\begingroup$ @CeesTimmerman It doesn't matter if it's 700 Wh/kg, it's still nowhere near enough. The Dawn spacecraft has three of these thrusters. Dragging along 8000+ kg of deadweight is simply not feasible for a 1240kg craft. The whole point of ion thrusters is to reduce mass. $\endgroup$ – user8812 Mar 31 '15 at 12:28
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Leaving aside the power consumption problems (which have been well discussed in other answers) and returning to the part of the question where you asked:

Are they [Ion Thrusters] fundamentally limited in some way? Is there reason to hope what limits them now could be overcome at some point so their high Isp could be used for higher acceleration on larger spacecraft?

There’s some very interesting research being done at Princeton’s Electric Propulsion and Plasma Dynamics Lab (EPPDyL) into just that question using Lithium Lorenz force accelerators. The research is currently still in progress, but the results published so far look encouraging.

This graph from Scaling of Effciency with Applied Magnetic Field in Magnetoplasmadynamic Thrusters (Page 5): enter image description here

Shows the thrust of their experimental thruster against it's current supply at different Li feed rates. The thrust to power ratio (if I'm reading the graph correctly) shows an efficiency of 1N/37.5kW at optimal configuration. About twice that quoted above for the VASIMIR device.

So there's definitely progress being made, but there's still a way to go before ion thrusters begin to replace chemical engines on larger craft.

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While their ISP is insanely good, ion thrusters have miserable T/W ratio, less than 1/1000th of unity; you simply cannot take off from any large body with them, no matter how many of them you use.

The primary limiting factor, I think, is power consumption. The thrusters used on Dawn and DS1 require 24kW per newton of thrust. You'd need 1.21 gigawatts to run a 50kN thruster. Ion thrusters currently in use are small just because you'd need ridiculously large solar arrays to make use of larger ones.

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    $\begingroup$ Couldn't help to giggle when I read 1.21 gigawatts ;-) $\endgroup$ – Dennis Mar 30 '15 at 16:11
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    $\begingroup$ How much would a vehicle weigh if 50kN is enough to accelerate it to 88 MPH in a parking lot? $\endgroup$ – dotancohen Mar 30 '15 at 17:32
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    $\begingroup$ 40m/s, 40 meter parking lot, 20m/s^2 acceleration (~2g), 2.5 tons, 2 seconds. $\endgroup$ – Russell Borogove Mar 30 '15 at 17:39
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    $\begingroup$ A base model deLorean masses about 1.25 tons, by the way, so the ion thrusters and nuclear power plant would make up the other half of the mass. $\endgroup$ – Russell Borogove Mar 30 '15 at 17:42
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    $\begingroup$ "1.21 gigawatts! 1.21 gigawatts. Great Scott!" = Dr. Emmet Brown ;-) $\endgroup$ – Kirkaiya Mar 30 '15 at 17:44
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What most of the other answers are failing to recognise is that ion thrusters are small BY DESIGN.

It is true that you COULD scale up your ion drive if you hook it up to a similarly scaled up power source. But you wouldn't.

The reason that Ion drives are used it because they have great ISP. The reason they have great ISP is that they have great exhaust velocity. The reason for that is that a HUGE amount of kinetic energy is put into a TINY amount of fuel.

1/2 mv^2 = KE

Scaling up the ion drive would simply increase the amount of fuel being dumped out the back. Which, yes, increases the thrust, but at the expense of ISP.

The fact is scaling up a ion drive runs counter to the entire concept of an ion drive.

EDIT:

Okay, there isn't so much as a "hard" limit to most ion drives on the ISP. However there are significant engineering problems. Lets take the classic linear accelerator style ion drive. At some point you need to accelerate ions towards an electrode. The more energy you put into the the ions, the heavier they slam into the electrode (should they hit). This causes damage to the electrode.

This style of problem exists in all ion drives. Fundamentally you are trying to make the flimsiest possible engine, which produces the hottest flame. At some point your flimsy engine will leak the hot flame, and stop working efficiently.

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  • $\begingroup$ But isn't it that the more fuel you are thrusting out, the more energy you need to use to maintain the Isp? If you have the energy for the usual exhaust velocity, can't you maintain it? In the worst case scenario, couldn't you simply strap 10,000 small thrusters to the same ship? Though surely it can be done more efficiently than that - my question can perhaps be boiled down to how much fuel can one ion drive spit out at one time with that same great Isp. $\endgroup$ – kim holder Mar 31 '15 at 15:21
  • $\begingroup$ @briligg My point is that it does work. But its completely counter to the point of an ion drive. Instead of strapping 10,000 small thrusters to your ship, you would use the SAME thruster, and put MORE power into each unit of fuel. There is a limit there, and that is to do with the erosion of the drive due to high energy particles. Further more, you would NOT strap 10,000 small thrusters to a ship because you would LOSE Delta-V due to increase of the DRY WEIGHT. $\endgroup$ – Aron Mar 31 '15 at 16:08
  • $\begingroup$ @briligg Basically, you would only strap 10,000 small thrusters to a ship if you had physicsless parts from KSP. $\endgroup$ – Aron Mar 31 '15 at 16:09
  • $\begingroup$ "There is a limit there, and that is to do with the erosion of the drive due to high energy particles." - tell me more about that! If you could add it to the answer that would be great. Because "use the SAME thruster, and put MORE power into each unit of fuel" is the goal, if it can be done. $\endgroup$ – kim holder Mar 31 '15 at 16:32
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    $\begingroup$ @kimholder: tl;dr version: put to much power and not only propellant is being accelerated out of the engine... the engine begins dismantling itself atom by atom. Ion engines of decent thrust are still quite propellant-hungry, and its supply is the limiting factor, so instead of more engines consider just packing more propellant... $\endgroup$ – SF. Dec 13 '15 at 23:35
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Ignoring the energy question that has been covered very nicely already...

Ion engines are "low thrust" engines, which means their thrust-weight ratio is much lower than 1.

Yes, ion engines are scalable! You can make them any size, but you can't really increase the thrust-to-surface area ratio. So if you want to double the thrust you have to make the engine twice as wide, or use two engine right next to each other. Practically this means that we can 1) scale up an ion-engine spacecraft and 2) add even more engines to get a better thrust-weight ratio but it's still going to be a fundamentally "low thrust" spacecraft with those graceful spiral trajectories. That doesn't mean they aren't useful, it just means whoever operates them needs patience.

Volume increases with a cube of size, and area with the square of size. So if you make a spaceship 10 times longer than Dawn, you'd end up with something like 1000 times the weight. To get the same thrust-to-weight ratio, you'd need engines with 1000 times the area, or sqrt(1000)=32 times larger. Also, the solar panels would need to be 32 times as long for the same shape. There's no limit but eventually your ship is all solar panels and engines!

Now this is pure speculation... an ion engine is a particle accelerator, and electrostatic accelerators are a very basic way to do the job. A small cyclotron might give you a better thrust-weight ratio, but it would still be "low thrust"!

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    $\begingroup$ Could you please edit to include some references for claims made here? Some of what you say is stated as a matter of fact when it's far from it, for example that you can make any size ion engines or that a cyclotron would give a better thrust-to-weight ratio. Thanks! $\endgroup$ – TildalWave Dec 13 '15 at 20:12
  • $\begingroup$ You could take existing engines and stack them side by side as an array of any size. As far as the cyclotron thing, I said it was pure speculation, didn't I? $\endgroup$ – Grant Entwistle Dec 19 '15 at 14:42
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I think using 24 Kjoules of energy to produce the equivalent of 1 Joule of momentum is the killer here.. If the ion engine is ever going to be truly usefull, it'll need to go up.. And in that case I think even 2 Joules of momentum or 10, wouldn't be a breakthrough, but a fraction of efficiency that is measured in the percentage range would be, instead of the current of 41.67 per million..

(even if the ISP would drop..) (but still, if the ISP is but a fraction, while the efficiency would go up several decimals, it would be worth it..)

More fuel in this case yes, but much less engine weight, much less reactor weight..(or solar panels for that matter..) On the latter I'd suggest a solar sail rather than panels.. Also, a solar sail would be much less prone to efficiency loss due to damage.. A solar sail would still be useful even at 10+ au..

(the solar sil would function very similar to a solar panel, but would be flexible, with much bigger cells..unlike the standard definitions of a solar sail, which would yield momentum through the physical impact of 'solar particles' onto it's surface, rather than electrical current..)

(thinking of that now, it may also be possible to use both, at least to certain extent, being moving away from the sun..)

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  • $\begingroup$ Ion engines are certainly "truly useful" already. See for example directory.eoportal.org/web/eoportal/satellite-missions/a/… See also here and here. Also, momentum is not measured in Joules. $\endgroup$ – uhoh Jan 16 '18 at 9:33
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    $\begingroup$ Ion engines produce roughly 1 newton of thrust, not 1 joule of work, continuously for the cost of 24 kilowatts of power. Those are very different units, and confusing them has similar effects on one's conclusions as confusing acres and arc-seconds. $\endgroup$ – Nathan Tuggy Jan 16 '18 at 10:07
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I have actually put much thought into this and I devised 2 ways to do this. A solar array attached to the ion engine could be used to focus light or laser into a parabolic nozzle that would farther focus the light into the neck of the thruster creating the energy needed to ignite or create plasma from the fuel.

2nd was to shoot a laser in to the parabolic mirror and again the shape of the thruster nozzle would focus the laser into a fine point.

enter image description here Towards the Sun. My illustration differs from the below picture. Mine focuses light into a fine point for heat.

enter image description here http://www.geoffreylandis.com/lightsail/Lightsail89.html This illustration is focusing light for electricity.

Not sure if my method would be better.

Away from the Sun.

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    $\begingroup$ This is not an answer to the question about scaling up ion thrusters. $\endgroup$ – Uwe Mar 3 at 10:27

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