Dawn Probe - three ion engines, but only use one at a time?

[mellojoe]

Something I never realized before is that the Dawn Probe has three separate Ion engines, but only uses one at a time.

Why do they have three? Is it redundancy? A backup in case the primary fails? The original plans for the craft used triple gyroscopes for attitude / directional control, so I can't imagine them designing triple main engines also for directional control. What's the deal?

[MinimumSky5]

I assume it's to get triple redundancy, and because while Dawn may be able to power three engines at 1AU, it may only be able to power one engine in he asteroid belt.

[smjjames]

Well, according to this: http://dawn.jpl.nasa.gov/technology/ion_prop.asp Only two are actually needed and one is a spare, plus only one is used at a time and all three have been used. I'm not sure if they share the same tank or they have their own separate tanks, so it could be to keep the craft balanced or to keep the thruster usage around the same so that they can get as much use out of the engines in case one of them breaks.

[Nuke]

they also kind of use it for load balancing. with a known mean time before failure, its better to run one at a time and cycle between them such that each engine has been run for more or less an equal amount of time, rather than run one till it fails and then switch up. its better to have 3 slightly used, broken in engines than 2 brand new unproven engines.

[NovaSilisko]

I'm not sure if they share the same tank or they have their own separate tanks,

Why would they possibly have separate tanks? If that were the case, the failure of one engine would immediately convert its propellant to dead weight. Not to mention 3x the mass for the tank structures.

[Nuke]

tankage isnt the problem, i think the main issue is the danger of grids burning out.

[Frida Space]

Indeed, one engine failed at the end of June, and Dawn has sinced used its other engine(s?).

[mellojoe]

tankage isnt the problem, i think the main issue is the danger of grids burning out.

Elaborate? What grids can burn out?

[sal_vager]

Elaborate? What grids can burn out?

Some ion engine types use a grid to accelerate the ions, but this is damaged over time due to electrons colliding with it.

[Mr Shifty]

Something I never realized before is that the Dawn Probe has three separate Ion engines, but only uses one at a time.

Why do they have three? Is it redundancy? A backup in case the primary fails? The original plans for the craft used triple gyroscopes for attitude / directional control, so I can't imagine them designing triple main engines also for directional control. What's the deal?

The thrusters are spaced 45° apart along one axis and can be gimballed 8-12° on the other two axes, so the 3 engines and spacing are used for thrust vectoring. The reaction wheels are primarly used for orienting the solar panels correctly, and the RCS was used to de-spin the reaction wheels, but now the engines themselves are being used for attitude control as well, since a couple reaction wheels have failed.

[mellojoe]

The engines themselves deteriorate?

I mean, I understand standard wear and tear, combined with the extreme conditions met while travelling through space. But, I guess I never realized that the Ion engines deteriorated this way, just gradually during standard usage. Does this mean they have a limited lifespan separate from the amount of fuel onboard?

((edit))

Just did some reading. Found this:

Grid erosion cannot be avoided and is the major lifetime-limiting factor. By a thorough grid design and material selection, lifetimes of 20,000 hours and far beyond are reached

More design process are leading to better engines, though, and it looks like the next generation are yielding lifetimes of upwards of 40,000 hours (4-and-a-half years continuous thrust).

Edited August 26, 2015 by mellojoe

[Nuke]

there are a lot of electric engines that have parts which are subject to corrosion over time. mpd engines have electrode wear, like the grid wear on electrostatic engines. though there are a number of engines being developed that do away with the wearable parts. i think that is actually one of the main (and seldom mentioned) advantages of vasimr since it uses rf heating.