What puzzles me is the maneuvering fuel. When that fuel runs out, the telescope can no longer orient itself. This ended the life of the Kepler telescope.
But aren't there other ways to orient in space?
1. use pressure from the solar wind
2. have 3 electric motors on 3 axis. Wouldn't spinning those motors rotate the craft? Electric power to do it would come from solar panels, giving it plenty of fuel.
JWST, like many other spacecraft, has reaction wheels to orient itself. The reason Kepler ran out of fuel when it did was that it was expending more maneuvering fuel for attitude control because two of its four reaction wheels failed. Hubble and the ISS also had similar failures.
We're getting better all the time at building more reliable components (including reaction wheels and cryocoolers) though. Until a few years ago, the life of something like JWST would be limited by the amount of liquid helium on board to cool the components. Modern cryocooler technology (aka a space grade refrigerator) is good enough to cool it indefinitely. Solid state cryocoolers, previously unachievable, are now apparently available for some applications (important not only for reliability but also to reduce vibrations).
Reaction wheels can be used for attitude control but they still have to be unloaded by thrusters after maneuvering for a while. You're right that you could use a rudder (probably two rudders would be required for 3d attitude control) and have to have a balanced solar wind profile (JWST does actually have a solar wind balancing flap, but I don't think it's adjustable like a rudder). But solar wind won't act fast enough if you want to quickly change attitude for observations. And you can't use reaction wheels for stationkeeping. It very much matters where the telescope is, since if it drifts too far away from Earth it will be much harder to send high bandwidth data, and if it's too close to Earth, Moon etc. it will have no way to orient without heating up or blinding itself with the IR sunlight reflected by them.
> The reason Kepler ran out of fuel when it did was that it was expending more maneuvering fuel for attitude control because two of its four reaction wheels failed. Hubble and the ISS also had similar failures.
And to calm down anyone afraid of JWST sharing the same fate - construction of reaction wheels have been changed some time ago to make them significantly more reliable. The source of issues on Hubble, Kepler, FUSE, Hayabusa, Dawn and TIMED was electrical arcing between metal parts of reaction wheels. Static charge was building up like when you rub a ballon against your head. That charge caused arcing that in turn caused metal pitting and increased friction leading to failures. That failure mode was understood only in late 2007, when Kepler was already fully build and ready for launch.
JWSt uses new generation ceramic bearing in its reaction wheels, they have been used in spacecrafts since 2010 with great performance.
> JWST, like many other spacecraft, has reaction wheels to orient itself.
It should be noted that reaction wheels can saturate when the motor reaches its top speed. One then needs to spend fuel to provide a counter-force while the wheel to spins down.
So even with reaction wheels running off solar panels or similar you need fuel, though much less.
Perhaps in a future design the reaction wheel could be unloaded by solar rudder, reducing the need for maneuvering fuel (although station keeping fuel would still be required without an outright solar sail).
They've already mentioned having more than the of amount of manoeuvring fuel (or, as we should really be calling it, delta-v) they had planned to have left at this point. Space craft have a limited life time anyway (CCD's wear out, semiconductors are subject to electron migration, solar panels degrade), so having a limited amount of fuel to stay at Sol-Earth L2 is just part of the whole lifetime equation.
Pressure from solar wind will constantly put some torque onto telescope as its center of pressure is offset from center of mass. This torque will be counteracted by reaction wheels but they have maximum rotation speed and need to be unloaded using thrusters periodically.
Momentum flap will bring average CoP closer to CoM but CoP will shift as JWST is rotated to point at different targets. Unwanted torque can be minimised by carefully choosing observations targets so it mostly cancels out but it cannot be done perfectly.
> As an additional challenge, the JWST observation schedule in the next 21-day period will not be known at the time of SK [station keeping] maneuver planning. A planned observation schedule one week ahead will be available, but the actual observation schedule will be event-driven. If a ‘target of opportunity’ arises then the schedule can be changed within 48 hours to point at the new target.
As far as I can tell it only really needs the fuel to maintain an L2 orbit, which is important because if it's too far away we can't really communicate with it effectively (i.e. actually download much of the data it's generating). For orientation it uses reaction wheels as you mention, and then there's a general plan to desaturate these momentum wheels by managing the average orientation of the telescope (it's effectively like an inverted pendulum: the solar wind will push it further away from having its back to the sun), but this might intefere with some observations so they may burn some fuel to maintain orientation in certain circumstances.
Kepler used a similar strategy (though I don't know what its desaturation strategy was): it only ran out of fuel very quickly after its reaction wheels failed.
"Unlike Hubble, Webb isn't designed to be fixed by astronauts. But it can be refueled robotically. Zurbuchen says that 'once this telescope is deployed, I'm going to put all the effort towards developing that technology, and so within the 10-year lifespan, we can go refuel it'"
The astronauts had some serious, unexpected difficulties fixing the Hubble in orbit, and had to improvise. I'm skeptical that blasting a fuel coupling into space, and then trying to figure out how to connect it to a robot, will end well.
I don't know. If the fuel coupling design is compatible with robotic access, it might be doable. That would seem easier than the dozens of repairs that astronauts did with Hubble. I assume we learned a lot with the Hubble servicing missions, some of which will be applicable to robotic work. The full list of Hubble service/repairs is quite remarkable if anyone is interested:
not sure about 1, but it already orients itself using reaction wheels, which are basically what you are describing with 2.
The issue is that its position at the lagrange point L2 is an unstable equilibrium, which requires occasional adjustment using thrusters. In terms of gravitational potential energy, its position in space is a saddle point, not a local minimum.
It matters a lot, L2 is unstable point so if it ventures beyond L2 there would be no way to bring it back and it would enter a heliocentric orbit. Communication between Earth and telescope would become impossible after some time as gimballed antenna can only rotate so far and stray light reflected from Earth would limit its field of view.
Due to that JWST will always be on 'close side of L2' and technically in slow freefall back to Earth and boosted up periodically, but always a bit short of passing to the other side.
You are right, it is fixable. It was fixed by adding active station keeping to the telescope.
> Or you can rotate the telescope.
There are limits on its rotation with respect to the Sun, dark side must be kept away from sunlight at all times. It can rotate 5 degrees "pitch down" toward the Sun and 45 degrees "pitch up". Gimballed antenna has enough authority so that it can communicate with Earth at whatever valid rotation telescope is so that science operations are not interrupted for transmitting data.
> The earth and the moon are nearby anyway. What about that reflected light? And heck, what about the sun limiting its field of view?
That light is reflected back by sunshield as all 3 bodies are behind it. Sun is limiting field of view but area of exclusion changes as telescope orbits and it can image every point in the sky at least every 6 months and 39% of the sky at any given moment.
Actually you can't rotate the telescope; they managed orientation even through the launch because if some of the science instruments get pointed towards the sun for too long they'll be destroyed. Now that it's deployed it's even more important that the cold side stays cold.
But aren't there other ways to orient in space?
1. use pressure from the solar wind
2. have 3 electric motors on 3 axis. Wouldn't spinning those motors rotate the craft? Electric power to do it would come from solar panels, giving it plenty of fuel.