They’ve given up on reaction wheels, then?
Reaction wheels are surely being used, but they become saturated, and you need RCS fuel to spin them down. One of the things kerbal glosses over.
Anyone hear the first place they will direct the telescope to view once fully calibrated? I’m pretty sure that time on the telescope has been fully booked down to the minute several years out already.
From Wikipedia: in addition to two pairs of thrusters (one pair for orbital insertion, one pair for stationkeeping) there are also eight monopropellant rocket engines (MREs) that are used for attitude control and momentum unloading of the reaction wheels. JWST has six reaction wheels for attitude control without propellant, as well as a set of gyroscopes:
More info on the attitude control subsystem:
That’s mentioned upthread. Some objects in our solar system, I think partly for calibration reasons.
So I listened to part of the Jan 8 press briefing – someone asked how may of the 344 failures have not been retired. Answer was 49, but 15 of those are instrument related. An example would be a filter wheel – if it failed it wouldn’t be a mission failure but an instrument failure.
Brian
The JWST is 143,000+ miles from its L2 orbit. It has traveled 755,000+ miles so far. JWST is currently deploying its 18 primary mirror segments. This will take several days. After all individual mirror segment deployments are completed, the detailed optical mirror alignment process begins which is about a 3 month process.
It’s also slowed to virtually a crawl – moving away from earth at barely 700 mph, which for a spacecraft is practically standing still!
Some of the instruments have had preliminary checkouts and look good. That includes the Fine Guidance Sensor, which is critical for stable images. If I’m interpreting this correctly, the FGS – along with an advanced spectrograph – are two major pieces of instrumentation provided by the Canadian Space Agency.
Yes, this process has been interesting. It’s a bit like trying to toss a ball up onto the top of a pedestal so it just barely clears the edge and lands there without bouncing or rolling off.
We can all be thankful the Ariane rocket performed so accurately!
The analogy I used with my kids is, “Imagine there’s an eyelash stuck to the ceiling. You want to toss a ball up exactly hard enough that it brushes the eyelash loose, while not actually touching the ceiling.” They thought that was pretty cool.
Sounds easy enough. It’s not rocket science. Oh wait a minute. Yes it is.
With the precision involved here, maybe we’ve found something that really is rocket surgery.
A few whimsical semi-serious comments …
The bad news is, if the intent is to have it reach a velocity of zero just at the L2 orbital insertion point, it will never get there! Cite: any of the numerous variants of Zeno’s Paradox.
The good news: Zeno apparently spent too much time philosophizing in hot baths while consuming fermented Phoenician grape juice. 
The key is that zero velocity (more or less) is not approached asymptotically, but in response to a steady negative acceleration that is nominally constant (within any short span of distance). So the approach to zero velocity is no less timely than a baseball reaching the top of its arc and starting to come down again.
Except that this particular baseball isn’t coming back down because of the wonders of the L2 Lagrange point, where it’s in effect in a solar orbit with the earth’s gravity providing the extra pull that allows it to orbit with the earth’s orbital period while being a million miles further out. Also, as I noted in post #87, it’s not going to be parked at L2, but actually in a very slow and large orbit around L2.
The mission lifetime will likely be much longer than the initially projected ten years because of the accuracy of the initial Ariane rocket boost, perhaps twice as long. The slow and gentle nature of the L2 orbit means that the necessary orbital adjustments use very little fuel, so every little bit saved goes a long way.
Having been born nearly into the Space Age (a little while before Sputnik), I’ve heard “rocket science” used as a metaphor for complex science or engineering my whole life. Since I studied physics and spent my career as an industrial scientist, I’ve tended to look askance at this. It’s like those bottle launchers that pressurize plastic bottles partly filled with water. Stuff squirts out the bottom and it makes it go up. Big deal! Saturn V was just a much bigger and noisier bottle!
But more recently I’ve needed to understand how compressed air squirting out a little hole works to transfer heat. And I had no appreciation for how much is going on here! There are different kinds of temperature, different kinds of pressure, different kinds of conservation laws, and on and on. De Laval nozzles. Overexpanded and underexpanded exhausts. Mach diamonds.
I worked for a little while with a guy who helped work out rocket fuel pumps and cavitation. Did you know that the Rocketdyne F1 engine has a fuel pump with 55,000 shaft horsepower? And Saturn V has five of these damn things! It operated with open loop control, but the Space Shuttle has a complex system that transitions from open loop to closed loop, and somehow manages not to blow up in the process.
OK, I’m impressed.
It’s also making an insertion burn when it gets there. If it didn’t, then it would either almost make it to the L2 area, then start coming back down, or overshoot it and keep on going.
Trick is to use as little fuel as possible in the process.
Those are about the coolest damn thing I’ve seen in a while.
Correct! However the intent is to arrive at L2 such that the minimum delta-V is needed to insert into an orbit at L2.
The station-keeping plan for the James Webb Space Telescope is zero velocity in the x-component at the fourth successive crossing of the XZ plane of the rotation libration point frame. A differential corrector is employed to determine the necessary delta-v. Maneuvering along the position component of the stable eigenvector of the monodromy matrix produces a minimum delta-v solution. The techniques developed to determine the minimum maneuver direction in a full ephemeris model, along with strategies to cope with the attitude constraints imposed by the sunshield that prevents the ability to maneuver along the stable eigenvector, are examined in this study.
Seconded. Trés kewl.
Scott Manley has a whole bunch of videos about Apollo in general and Saturn V in particular. I think he’s impressed by the damn thing. For me, one of the most interesting is the complex sequence of events that have to happen ju-ust right between “Ignition sequence start” and lift off.
I just happened upon this tidbit in the Wikipedia JWST page, about serviceability:
“Since the successful launch, NASA have stated that limited accommodation was made to facilitate future servicing missions, if any. This included: precise guidance markers in the form of crosses on the surface of JWST, for use by remote servicing missions, as well as refillable fuel tanks, removable heat protectors, and accessible attachment points.”
So, there’s some glimmer of hope.