There are two kinds of thrusters, the ones used for mid-course correction and for stationkeeping, and the ones used for attitude control. The course correction and stationkeeping thrusters use hydrazine fuel and dinitrogen tetroxide oxidizer. The attitude control thrusters use only hydrazine. So any potential refueling mission would only need to bring hydrazine.
Attitude control is predominantly done with reaction wheels, but thrusters are necessary for periodic momentum dumping of the reaction wheels. The reason is that as the telescope points in various directions, solar pressure on the large sunshield will typically be off-center with regard to the center of mass, and will create a torque that has to be offset with the reaction wheels.
Momentum dumping would seem to be a good place to use high-efficiency electric thrusters; although doubtless they wanted to keep something conservative and proven for JWST.
Thanks for this. Those would all be liquids? Would the delivery system need to keep them heated to stay liquid? Or would the problem really be keeping them cool from the sunlight?
Why would they not need the oxidizer? Wouldn’t they still need to use the stationkeeper thrusters?
(I have this image of a robotic fuel system: “Should . I . wipe . the . mirrors . for . you . while . the . fuel . pumps . in?” )
Quite right. That was a Wolfpup Logic Fail! BOTH attitude control/momentum dumping AND stationkeeping are mission-critical, so both hydrazine and the oxidizer would have to be re-supplied. Hydrazine can be used as a monopropellant without oxidizer, because it doesn’t have to “burn”, it can be passed over a catalyst to produce a powerful exothermic reaction. But the stationkeeping thrusters were designed to be used with oxidizer.
I would take that bet. The most recent public statement on JWST suggested that, thanks to its so far very smooth launch, its fuel could last as long as 20 years.
If that’s really the case, I would hope that by the time it’s ready to take its forever nap, we’ll be talking about launching a gamma-ray-dark energy telescope, and are surprised that JWST is still a thing.
I have no cite handy, but I read somewhere that if the fuel lasts 20 years, the rest of the satellite will be near it’s end-of-life anyway, so refueling it would be pointless.
The extended mission life is great news, but here’s a bit of perspective. The JWST cost around $10 billion. By contrast, the Large Hadron Collider, the enormous international particle accelerator that is doing so much to advance our knowledge of quantum mechanics, is also touted for its awesome cost – $9 billion. It’s amazing the anyone would even consider the JWST to have been a “throw-away” in as little as 10 years. The Hubble is over 30 years old and still working.
Yeah…but…
LHC was finished in 2008, it would cost a lot more now (and its proposed successor is currently being estimated at about twice the cost of JWST. That’s the current optimistic estimate with nothing started yet).
And in terms of science…I think it’s fair to say the LHC was a bit of a disappointment. I mean, it ruled out a bunch of hypotheses and confirmed the Higgs, which is not nothing.
But it’s still pretty close to the most pessimistic predictions when LHC was being built.
Will JWST lead to more advances and discoveries? Who knows, but based on the current programme and its resolving power I’d, well, once again I’d take that bet.
I don’t want too sound to negative about the LHC though. We were right to build it, and particle physics arguably has better prospects for earthly applications.
Actually, IIRC the most pessimistic predictions were that the LHC would destroy the world (or possibly the entire universe, I forget). There may still be a website out there somewhere put together by a Russian with a poor command of English that “proves” this will be the case.
Besides the Higgs boson, which was a vital component of the Standard Model, the LHC has discovered more than 50 new hadrons and done a great deal to advance quantum physics. The difference from the JWST is that much of what CERN does is obscure for the general public – no awesome pictures or detecting-life-on-other-planets potential. But it’s still important science, including testing and ruling out hypotheses, which is as important to scientific progress as confirming them.
As I recall most descriptions were something like “Well, first it will confirm or refute the Higgs field – it will be a massive shake up if it can’t find the Higgs boson! – then it will look for supersymmetric particles, the particle(s) that make up dark matter, and generally any way that we can extend the standard model eg WIMPs, sterile neutrinos, micro black holes, and lots more besides!”
And, as it happens, we basically got the “first” fragment of that paragraph and nothing else.
As I say, I’m not knocking it. My job involves utilizing particle accelerators for medical use.
And we couldn’t have known how much new science we’d get from the LHC – it was a sensible investment.
It’s just a strange pick IMO, to put perspective on a $10 billion telescope launched in 2021 to compare it to an accelerator from 2008 that cost almost the same (*before* adjusting for inflation) and its bang for buck was so poor that the successor project is struggling to get funding.
But it was justified on sound scientific principles and as you point out, no one knew for sure how much valuable science it would produce. And it has, in fact, produced a lot. $9 billion + inflation adjustment worth? Who can say? It may be that particle physics research is more expensive than anyone thought.
The biggest argument abainst extending JWST, IMO, is that space launch and operations are now improving so fast that we should, by end of life of JWST, have the capability to put up much more capable instruments for a lot less money.
JWST is very much a device defined by the limits of space launch when it was designed. Those limits are changing dramatically.
There may still be a website out there somewhere put together by a Russian with a poor command of English that “proves” this will be the case.