Why throttle an ion engine by changing the level of electrical charge?

[T.C.]

In the description of Dawn's ion engines here, NASA says "The thrusters work by using an electrical charge to accelerate ions from xenon fuel to a speed 10 times that of chemical engines. The electrical level and xenon fuel feed can be adjusted to throttle each engine up or down."

I can understand why the fuel feed changes for throttling, but why does the electrical charge change? Shouldn't the optimal charge remain constant regardless of fuel feed and throttle setting?

-TC

[tutike2000]

Maybe they meant that the total power consumed by the engine is less when you use less xenon. Not that the charge / unit mass of xenon changes.

[T.C.]

Maybe they meant that the total power consumed by the engine is less when you use less xenon. Not that the charge / unit mass of xenon changes.

Yes, I'm sure you're right.

When I asked the question, I had my electrical concepts mixed up. In retrospect, it makes sense that what should remain constant is the potential difference which accelerates the ions; the current required to sustain that difference should, of course, vary with the feed rate. I withdraw the question.

-TC

[Guest]

To accelerate ions, you need first to create ions. Xenon or argon fresh and cold from the feed system are neutral atoms.

[YNM]

Probably more charge = more ions. And stronger magnetic field... ?

[Nuke]

you have a lower volume of gas and therefore need less power to ionize it.

also one of the reasons to throttle an ion engine is to make it use less power for situations where running it at full power is not possible, such as while in a planet's shadow or while too far from the sun have full power from solar arrays.

[Hcube]

Why would you want to throttle down an ion engine anyways ? Ion engines' thrust is often measured in milliNewtons so i don't think that throttling down is ever necessary

[Tim_the_Plumber]

Any idea why it has to be very very expensive Xenon?

Cheap stuff not good enough?

[SargeRho]

Because it's easy to ionize and doesn't corrode the engines.

[beabop]

Depending on where you are in orbit, a certain amount of thrust and Isp will be optimal. The ion engine orbits I've seen are this crazy patchwork of turning the engines on and off.

Specifically, your exhaust velocity, thus Isp, depends on your grid voltage. Power is limited to what your solar panels can make - grid voltage times exhaust current (it's a charged flow) can't exceed this limit. So at max power, you trade off Isp for thrust. All ion engines can do this, which makes VASIMR less impressive.

Also, you tune your power plant size for the mission. There's no point in saving fuel if you end up needing a greater mass of solar panels to do so. Specific power (watts per kilogram) is what limits ion engine Isp.

Edited June 24, 2015 by beabop

[Streetwind]

Any idea why it has to be very very expensive Xenon?

Cheap stuff not good enough?

Xenon is a giant element, and a noble gas to boot. You can store and handle it super-conveniently at all sorts of temperatures, it comes into the engine as single atoms, and because of the sheer number of electrons it carries, stealing one in order to ionize it requires very little energy. And then, the heavy atom gives a (relatively) big 'push' when it is accelerated out the back.

These advantages, combined with the fact that you need only very small amounts because ion drives are so efficient, outweighs the disadvantage of high price in many cases. However, there are also electric engines in development that aim to run on other, cheaper noble gases, like Krypton. The VASIMR is such an engine.

[T.C.]

Specifically, your exhaust velocity, thus Isp, depends on your grid voltage. Power is limited to what your solar panels can make - grid voltage times exhaust current (it's a charged flow) can't exceed this limit. So at max power, you trade off Isp for thrust.

Beabop, Thank you for the excellent explanation.

For those that missed it, I had speculated that there should be an optimal potential difference (grid voltage) in an ion engine. That seemed reasonable to me, because I assumed it would always be desirable to maximize Isp. And if you're keeping the voltage tuned for optimal Isp, then it may be technically correct, but nevertheless seems odd to say that you're adjusting the electrical level to throttle the engine. Thus, my initial question.

In reply, Beabop confirmed that Isp depends on the grid voltage, but also explained that ion engines intentionally trade off Isp for thrust. Therefore, there are two aspects to engine performance -- thrust and Isp -- and it makes sense that there must be two inputs -- feed rate and voltage -- to control the performance. It no longer seems odd to say that the electrical level can be adjusted to throttle the engine.

-TC

[SomeGuy12]

Do you need either nobel gas? Could you make an ion engine that ran off of hydrogen or something far more common like that, instead? Those nobel gasses sound a bit hard to come by if you had an ion powered spacecraft that landed on a comet or asteroid and you wanted to refuel.

[YNM]

Other gases are diatomic - you'll need to break the bond first before ionize them which means more power requirement. Xenon is the last (heaviest) noble gas that's not radioactive (radon does).

[Nuke]

mpd can run hydrogen but its power requirements are through the roof. its on the big heap of engines we cant use because we aint got the power supply.