James Webb Space Telescope successfully launches

Well, duh! Everyone surely knows that maneuvering along the location parameter of the precise garminschteiller of the polygamy loci would be complete lunacy.

I think we should kidnap Bullitt. He knows things. Things that make us go.

  • Grebnedlog

Copy and paste, copy and paste.
Even I can sound smart when I copy and paste.

Duh!

Point is, associated to these halo orbits are “tubes” along which lie trajectories that get you to and from these orbits, with no or little delta-v if you know what you are doing.

Yes, in fact the “orbital insertion burn” is actually the same event as mid-course correction 2 (MCC2), and its main purpose is a small speed boost and slight orbital adjustment.

And we’re getting close! Less than 90,000 miles to go, at this moment exactly 90% of the way there. But it’s now only moving at the speed of a jet airliner.

This is most likely a stupid question, but…

The Arianne gave the JWST an initial velocity to place it in the L2 position.

It has been speeding through space, which is effectively empty, on its way. What is slowing it to mere jet aircraft speed? Gravity gets less and less strong as it gets further, and friction from gasses must be negligible.

(I do understand that it needs to slow so it does not overshoot)

You answered your question.

Yes, the force of gravity decreases with (the square of) distance. But it doesn’t go to zero.

Gravity gets weaker as the inverse square of distance, but at JWST’s distance it’s still nowhere near zero. JWST has been coasting uphill this entire time. Put it this way – when it gets to the L2 orbit point, the earth’s gravity will still be strong enough to allow it to orbit with the earth’s period while being nearly a million miles farther out from the sun, where its orbital velocity should be much slower. Were it not for the earth’s gravity, centrifugal force would pull it up into a much higher solar orbit. That’s the balance that creates the L2 Lagrange point.

Thanks to you both.

I’m liking the new Mirror Segment Deployment Tracker screen, but does anyone
know what this means …
“NOTE: Segment A3 and A6 will be moved separately at the end of the process because their position sensors are read out in a different way.”

My understanding is that A3 and A6 each have a known faulty position sensor called a linear variable differential transformer (LVDT). The LVDTs are supposed to have two transformer coils to deal with temperature differentials; in A3 and A6 one of them is faulty. This was known before launch and apparently was not practical to correct, and felt to be unnecessary because readouts can be done with just one coil, using a different procedure.

Interesting !
Thanks.

If you’re interested in the super nitty-gritty details, here’s a paper that talks about the mirror actuators:

This part is particularly relevant:

Each actuator’s LVDT is made of up of two coils which the ADU electronics normally use in a differential form to provide temperature-independent position data. When a move is commanded the MCS calculates the expected actuator length from the coarse step count (CSC) of the motor. An algorithm then converts this length into the expected LVDT reading using a calibrated set of coefficients. If the LVDT reading in telemetry matches the predicted value to within a calibrated tolerance, the commanded move is confirmed.

On two separate PM Segments, one of the two coils on an LVDT is faulty, so a method to read those LVDT positions with only one coil was developed and tested at OTIS CV and used to confirm mirror moves in this Single Sided Operation Mode.

Unfortunately there’s no explanation for why they didn’t just replace the faulty LVDTs. I assume it was probably because they’re deeply integrated and the whole mirror assembly would have had to be thoroughly re-tested. The telescope was already years late.

Woah ! …TMI !!!

Thanks !

Since we’re going through a slow-moving boring period right now, I thought I’d chime in with some updates for those interested who may not be following things all that closely.

The slow phase right now involves the deployment of the 18 segments of the main mirror. This is not the adjustment or “fine-tuning” phase – that comes later. The main mirrors were stowed in a protected configuration for launch, whereby three pins on the back of each one were set into corresponding holes in the telescope structure. The deployment involves moving them 12.5 mm away from the structure so that the mirror segments are free for subsequent fine alignment.

The actuators that perform the fine alignmnent move with an accuracy of 10 nanometers, about 1/10,000th the width of human hair. Consequently they move very slowly. Since these are the only actuators available, they are also being used for the one-time 12.5 mm move from the stowed to the operational position, and it’s taking a long time! Although they appear to all be moving simultaneously, they are actually being moved one at a time in short bursts to avoid heating issues.

The latest update arrived just now. Except for A3 and A6 as noted previously, they are now 2 mm from their final operational position. Less than a week to go before the JWST enters L2 orbit.

That’s very interesting. Do you know anything more about these actuators? Fine actuators tend to be low displacement–for instance, piezoelectric actuators can operate at a resolution of nanometers, but have a travel in the micrometer range. Something like a geared linear actuator can have high displacements, but is nowhere close to nanometer precision. Seems like it must be something special for it to work at both high precision and large displacements.

Please see section 1.2, “Actuator Description”, from the paper I cited. I have no further information beyond what is cited in that paper.

Designed by Ball Aerospace, the linear activators are capable of sub-10 nano-meter motion accuracy over a 20 mm range both at ambient and cryongenic conditions, as is required for precise control of mirror positions after deploying 12.5 mm out of the mirror launch restraints …

Interesting, thanks. So it is a pretty standard ball screw style actuator, but with a twist–it has a “compound flexure”, essentially a lever system to increase precision at the expense of displacement. But it also has a kind of clutch system to disengage the ball screw, essentially switching it from coarse to fine adjustments. It looks like the fine adjustment can’t be disengaged, so presumably they just accept that it’ll move at the same time. The fine adjustment is on a cam, so it should just oscillate up and down as it moves.

I do wonder why it’s so slow to move. Seems like the coarse adjustment should be pretty quick. Maybe they’re just trying to be gentle.

I’m surprised they can get that kind of precision out of that system. I guess that as long as they can prevent any backlash in the system, the fine adjustment can handle the rest.

Yay!! Full deployment of all main mirror segments is now complete! As planned (per previous discussion) A3 and A6 were the last to be done. All 18 segments are now in operational position.

Still chugging along at just a little over 500MPH and about 2 Earth Circumferences to go!