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Since the L2 point is unstable, JWST needs engines to maintain its orbit. It uses mono-propellant engines which have given it a 5-year minimum lifespan.

Why weren't ion engines used instead? Wouldn't they provide much more Delta-v for the same mass of fuel?

Are they not reliable enough? Or is it just too much extra complexity?

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    $\begingroup$ I think this is an excellent question! As a follow up, I think you could ask next "How would conventional thrusters vs electric propulsion compare for station keeping of a space telescope in a Sun-Earth halo orbit? What would be the tradeoffs for a system being designed today?" $\endgroup$
    – uhoh
    Dec 19 '21 at 21:13
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Since the L2 point is unstable, JWST needs engines to maintain it's orbit. It uses mono-propellant engines which have given it a 10 year (maximum) lifespan.

The JWST uses bi-propellant engines (hydrazine and dinitrogen tetroxide) to perform midcourse corrections on the way to the Sun-Earth L2 pseudo-orbit, to perform orbit insertion into that pseudo-orbit, and to perform orbit maintenance maneuvers. It only uses monopropellant engines for coarse attitude maneuvers and reaction wheel desaturation.

The midcourse corrections, orbit insertion, and orbit maintenance maneuvers require a good amount of delta V that can be delivered quickly. Even a monopropellant hydrazine engine wouldn't suffice. Adding monopropellant engines and a bit more hydrazine for use by those monopropellant engines apparently made more sense in the initial design than did adding a whole new set of tanks and thrusters for ion engines.

While some initial design decisions can be changed, thrusters are amongst the many items on a spacecraft that are pretty much frozen from day one. Moreover, that initial design occurred about 20 years ago, when ion thrusters were just past the experimental stage. And they certainly weren't going to switch to ion thrusters. Deep Space 1 was launched in December 1998, and it worked, kinda-sorta. DS1 had significant problems and was not a complete success. It's ion thrusters didn't work at first and the vehicle performed poorly on some planned fly-bys.

It is possible in another world that the designers of the JWST could have modified their spacecraft 20 some years after the initial design to use ion thrusters for some aspect of vehicle control. Then again, they could have also redesigned the vehicle to use more modern computers, more modern solar arrays, etc. They didn't do so in part due to the immense amount of conservatism that is baked into spacecraft design.

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    $\begingroup$ While SpaceX has shown NASA that it makes sense to be a bit more Agile (maybe a lot more Agile), that happened well into the construction phase of the JWST. The JWST is a prime example of space exploration done in the old school way, with the concomitant old school space exploration price tag. $\endgroup$ Dec 19 '21 at 20:37
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    $\begingroup$ @NgPh JWST, for one. The Mars rovers, for another. The rovers recently sent to Mars use computers from the previous millennium, and those computers from the previous millennium are downscaled to run about half as fast as their earth-side equivalents ran. On the other hand, the Mars helicopter, which has experimental rather than operational status, uses (nearly) brand new stuff. $\endgroup$ Dec 20 '21 at 16:53
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    $\begingroup$ @NgPh The JWST followed the traditional requirements review / preliminary design review / critical design review / ... / flight readiness review paradigm. That is waterfall, to the nth degree. There is very little backing down from very early concepts in that paradigm. $\endgroup$ Dec 20 '21 at 17:12
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    $\begingroup$ @NgPh I wrote "Agile", not "agile". Agile was initially conceived about 20 years ago as a way to escape waterfall for software development. In the last seven years or so the concept has spread well beyond software development. Waterfall (e.g. a process that follows the requirements review / preliminary design review / ... / flight readiness review paradigm) is inherently and immensely conservative. SpaceX is very Agile (and very agile). Some old space companies have adopted some aspects of Agile, but they remain a bit non-agile. Whether Agile makes an organization agile is not guaranteed. $\endgroup$ Dec 21 '21 at 7:37
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    $\begingroup$ @NgPh The TL;DR response to That quality assurance process doesn't tell me which part of the JWST is "immensely conservative". is every single bit. The conservatism is built-in. It is entirely possible to build something that is "better" in almost all regards by simply foregoing the fantasy of being able to foresee all the gotchas from day one, which is the essence of the waterfall model. $\endgroup$ Dec 21 '21 at 8:26
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@DavidHammen's answer goes a long way towards answering, especially in that the space telescope's bus was finalized quite a long time ago when ion propulsion was much less a proven long-term reliable technology.

It says:

...and orbit maintenance maneuvers require a good amount of delta V that can be delivered quickly.

They may not be strong enough to recover from a short interruption in station keeping

Coms satellites in GEO are now becoming "all electric" for station keeping and many are doing GTO to GEO with electric propulsion as well.

So we might ask if the space telescope were to be designed today if it would use electric propulsion for station keeping or not.

That the space telescope's halo orbit is exponentially unstable and if a few of the bi-monthly station keeping maneuvers are missed for some reason it may get harder and harder for low thrust to be able to bring it back to station.

Once the acceleration away from the halo orbit along the unstable manifold exceeds the acceleration provided by electric thrusters (which is always pretty darn small) all is lost and it will spiral out towards a heliocentric orbit.

There is no analogy to this exponential instability in the station keeping of comms satellites in GEO. These are very well gravitationally bound to Earth, and the won't wander off into a heliocentric orbit if left unattended.

But wait, there's more (weight)! (potentially)

The question considers the weight of ion propulsion propellant being lower than that of conventional thruster propellants, but ion thrusters have heavy magnets and power supplies to produce the plasma.

It's very possible that once the full electric propulsion system is designed and weighed, one strong enough to return JWST to its halo orbit if a few bi-monthly station keeping iterations were missed, that it doesn't end up any lighter.

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  • $\begingroup$ @OrganicMarble but I forgot to mention the electrical power demand which may actually turn out to be the final nail in electric propulsion's coffin. Hopefully the OP (or someone) will ask the followup mentioned here and then someone can get out their spherical cow envelope backs and work the problem semi-quantitatively. I'll bet that the solar panels will have to be a lot bigger and perhaps contribute more solar photon pressure as well, which could be a bad or good thing considering how it uses this thrust as part of station keeping. $\endgroup$
    – uhoh
    Dec 19 '21 at 21:41
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    $\begingroup$ "It's very possible that once the full electric propulsion system is designed and weighed, one strong enough to return JWST to its halo orbit if a few bi-monthly station keeping iterations were missed, that it doesn't end up any lighter." - Maybe not lighter, but still much-longer-lasting. $\endgroup$
    – Vikki
    Dec 19 '21 at 22:14
  • $\begingroup$ @Vikki good point! Ya I shot myself in the foot there; I was trying to be cautious and not say "much heavier than" outright. $\endgroup$
    – uhoh
    Dec 19 '21 at 22:17
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    $\begingroup$ The traditional spacecraft design, development, and implementation process follows the rather outdated waterfall model, which is not anywhere close to being Agile. Spacecraft development could be much more Agile, but that would require massive changes to how NASA and ESA do business. $\endgroup$ Dec 20 '21 at 13:32
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    $\begingroup$ I don't understand the relevancy of these discussions: why not make it last longer, why not make it lighter, ...? There are already so many "firsts" (and correspondingly failure points) in JWST's scientific instruments and platform elements (e.g. the sun shield). The mission lifetime is constrained by these risks. The technology and sizing of the propellant & thruster are non-critical in every aspects. It's not at all like in a traditional GEO satellite where the propellant mass is almost half of the satellite wet mass at launch, and hence EP is game-changing. $\endgroup$
    – Ng Ph
    Dec 20 '21 at 18:09

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