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I gather that the James Web Space Telescope (JWST) has just deployed its "aft momentum flap". However I cannot find much other information. It is described as the "aft" momentum flap as if there might also be a "fore" momentum flap, but I don't think that's the case. It's also likened to a trim tab on an airplane which can be adjusted to keep the plane level in flight. But this trim tab appears to be fixed in position and not adjustable or retractable.

Presumably there is a set of detailed calculations that calculates the expected center of pressure on the sun shields in various telescope orientations. Is this documented anywhere?. The torque and build-up of angular momentum arises from the mismatch between the center of pressure and the center of mass. The momentum flap is set at a particular angle, is this to help keep the center of pressure and center of mass aligned over the wide range of orientations required for observations? What is the momentum flap made of? What are the moments of inertia of JWST? etc.

This is an interesting topic, so if anyone can point me to more detailed description, I'd appreciate it.

[edit] added picture showing stability from solar sail with dihedral angle concave towards center of mass. For small angles this reduces to a restoring torque proportional to the rotation $\theta$. If the dihedral is the opposite, then it's a destabilizing torque.
solar pressure on sail with dihedral

I see there are some papers being published by SPIE that might be relevant, but they're behind a paywall.

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    $\begingroup$ Thanks for a great question. The flap shows up on numerous illustrations, but I could not find any description of its functionality. I look forward to seeing an answer $\endgroup$
    – Woody
    Dec 31, 2021 at 2:15
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    $\begingroup$ NASA says "Spring drives rotate the flap to its final position after being released by hold-down devices." Sounds like it is fixed once deployed. It is in an anti-dihedral conformation which should de-stabilize its attitude from solar pressure, not stabilize it. Curiouser and curiouser. $\endgroup$
    – Woody
    Dec 31, 2021 at 2:51
  • $\begingroup$ researchgate.net/publication/… says "The shield is designed with a curved dihedral shape for attitude stability". Maybe the sunshield has too much stability and something is needed to partially cancel the restoring forces. $\endgroup$
    – Roger Wood
    Dec 31, 2021 at 3:16
  • $\begingroup$ @Woody there's some discussion here but it's old slideplayer.com/slide/13607342 $\endgroup$
    – Roger Wood
    Dec 31, 2021 at 3:32
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    $\begingroup$ The sunshield is asymmetrical about the J-2 axis. The tab may have been added to balance this asymmetry. $\endgroup$
    – Woody
    Dec 31, 2021 at 4:17

2 Answers 2

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I do spacecraft guidance and control. The momentum flap has nothing to do with passive stability per se, like anhedral or dihedral would with regard to roll stability on an airplane. The 6 momentum wheels have more than enough torque to overcome any environmental torques, as well as to torque the fairly massive satellite. But passive attitude stability is just not a concern, because there is no time when its attitude will be allowed to drift passively.

What it does is, as others have mentioned, balance the solar torques over large spans of time. As someone mentioned, this is essentially equivalent to trying to match the center of pressure (in this case solar radiation pressure) to the center of mass as close as is possible. Any mismatch between those along the vector from the sun is not important, because that will create no torque about the center of mass.

An interesting consideration is that the solar torque is constant over time assuming a steady attitude, and thus the integral of that, the angular momentum, will build up linearly over time. But the buildup is best thought of as occurring in an inertial coordinate frame. That is, if JWST is pointed in the +V direction (let's call that in the direction that Earth moves around the sun) making an observation, and let's say that the aft momentum flap is slightly under-sized, then the front of the telescope will be constantly pushed away from the sun more, and a reaction wheel will have to be steadily (yet gradually of course) increasing its speed to counteract this (the actual algorithm that partitions the torques and momenta between the 6 wheels is something I've looked for but not found. For now, assume it all goes into one wheel that is aligned fortuitously for this example). Now, after a few hours of this, if the JWST makes a 180 degree rotation around the axis that goes from the telescope to the sun, so that the telescope is pointed in the -V direction (direction Earth is traveling away from), then the solar torque will happen in the same direction in the spacecraft coordinates, but in the opposite direction in inertial coordinates, thereby allowing that one wheel (which is now spinning in the opposite direction from what it was previously- that direction change happened during the 180 degree slew, in cooperation with other wheels) to slow back down at roughly the same speed that it sped up previously.

The observation planning team will be characterizing the environmental torques on the JWST to see what its properties actually are and which direction torques actually build up on it, and then forecasting these, and then scheduling observations in order to allow solar torques to dump this angular momentum so that it's not necessary to use propellant to do so nearly as often. Spending a little extra time slewing between observation attitudes is worth the time spent if it lengthens the mission by an even greater amount of time.

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  • $\begingroup$ thanks - that makes sense. I think rather than center of pressure you mean metacentric height en.wikipedia.org/wiki/Metacentric_height It's desirable to keep the center of mass and the metacentric height close together to minimise the solar torque and accumulation of angular momentum. To achieve this, the flap has to be the right size and at the correct angle. Is that right? $\endgroup$
    – Roger Wood
    Apr 4, 2022 at 6:07
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Here is what I've found so far. It's not much, and the flap is not mentioned on either the Sunshield nor the Bus pages.

You are in good company likening it to a trim flap! In Figure 2 of Stationkeeping Monte Carlo Simulation for the James Webb Space Telescope (found here) it is labeled as "Trim flap; helps stabilize the satellite" :-)

But sadly so far I can't find anything within there that calls it out by name. There may however be some helpful sources cited.

From The James Webb Space Telescope Observatory

The Observatory of the James Webb Space Telescope is comprised of three elements: the Integrated Science Instrument Module (ISIM), the Optical Telescope Element (OTE), which includes the mirrors and backplane, and the Spacecraft Element, which includes the Spacecraft Bus and Sunshield.

The Spacecraft Bus provides the support functions for the operation of the Observatory. The bus houses the six major subsystems needed to operate the spacecraft: the Electrical Power Subsystem, the Attitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

The momentum flap balances the solar pressure on the sunshield, like a trim flap in sailing. It's not adjustable on orbit, but it is while it's on the ground.

That page goes on to link further to the following, but surprisingly neither mention the flap!

JWST diagram from https://www.nasa.gov/mission_pages/webb/observatory/index.html

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    $\begingroup$ Thanks, I've added a comment in response to @Ng Ph 's comment. I think the function of the trim flap is actually to reduce the stability and make JWST barely stable so that the the angular momentum buildup during observations is small. $\endgroup$
    – Roger Wood
    Dec 31, 2021 at 19:54
  • $\begingroup$ @RogerWood The center of mass is well "above" the sunshield, so whenever it is not exactly normal to the incident light there will be a torque. So I don't think that with the sunshield alone there is any stability in attitude to begin with except when the telescope is pointing perpendicular to the Sun-Earth line. Having a second reflector at a different angle will modify the no-torque angle to something different, and I'll bet that angle is closer to the average direction that it will be pointing. Just a hunch though... $\endgroup$
    – uhoh
    Dec 31, 2021 at 20:13
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    $\begingroup$ added picture to illustrate stable configuration $\endgroup$
    – Roger Wood
    Jan 1, 2022 at 2:10
  • $\begingroup$ "Not adjustable in space" I assume they have accurate radiation heat transfer modelling so that they know where the infrared radiation and it's recoil torques will go. $\endgroup$ Dec 1, 2023 at 5:12
  • $\begingroup$ @KevinKostlan it's hard to imagine NASA doing anything without accurate models that are cross-calibrated to reality using scale models, vacuum chambers, solar simulators, etc. Consider the aft momentum flap a first order correction only. It doesn't have to be perfect to be tremendously helpful. I guess it's a bit like a trim tab on an aircraft, but it's a ten billion dollar aircraft that had ten "extra years" of development. I think they had sufficient time and resources to model the heck out of it! $\endgroup$
    – uhoh
    Dec 1, 2023 at 6:23

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