James Webb: A $10bn machine in search of the end of darkness
- Thread starter weltweit
- Start date
2hats
Dust.
The second course correction burn, MCC-1b (9.5 minutes duration, just over 2 m/s delta V), has just been completed. JWST has already passed lunar orbital distance. Next up is deployment of the palletised structures as a prelude to unfolding the sunshield membranes (which they carry and support).
Steel Icarus
we move
You know I bet all those space scientists and experts haven't taken this into account ;-)Won't that sun shield act as a solar sail and blow it off course?
2hats
Dust.
The sun shield will indeed 'sail' solar radiation pressure (SRP) to a degree (that will vary widely according to spacecraft orientation which is of course dictated by science targets), but that has been factored in.Won't that sun shield act as a solar sail and blow it off course?
First of all, the L2 libration point 'orbit' (unstable by its very nature) is ever-so-slightly (sun-)biased to compensate for this additional force (trade it off against the gravitational forces). The trajectory (because it's not really an orbit as such) has to be corrected every ~21 days with a station-keeping burn anyway to compensate for the instability ie N-body gravitational forces (same type of SCAT thrusters as are being used for these mid-course corrections en route to L2), otherwise the observatory would either 'decay' back to to a geocentric orbit (interaction with the Moon making it somewhat more complex than a highly eccentric Earth-centred ellipse) or alternatively be perturbed out into a heliocentric orbit.
Secondly, SRP will also create a torque about the centre of mass (the sun shield won't always present full plane perpendicular on to the Sun radius vector). The reaction wheels will compensate for this, though in the process they accumulate angular momentum themselves, the rate depending on orientation of the spacecraft with respect to the Sun and duration spent in that attitude. So periodically they will have to be despun (there is naturally a limit on how much angular momentum they can accumulate - they can only spin so fast). To minimise this, astronomical observation sequences will be planned in such a way that the spacecraft will change observing direction to alternately load and then unload reaction wheels as much as is possible.
Obviously there is a balancing act between this engineering need and the astronomical science programme. The reaction wheels can only be unloaded in the aforementioned manner to some degree. So periodically the observatory also uses the thrusters to permit reaction wheels to be fully despun.
Additionally the spacecraft features a momentum trim tab (or flap) that helps balance out SRP (working in tandem with the reaction wheels). This 'paddle' uses the SRP to help reduce reaction wheel angular momentum build-up and fuel burn that would otherwise be spent on maintaining attitude (countering the torque).
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moochedit
I'm not feeding seagulls
Well at least it's not yodelUPS delivery system.
petee
no gods, no malarkey
i don't come here to read filth like this.
SpookyFrank
A cheap source of teeth for aquarium gravel
Reaction wheels, that's really fucking clever in a low tech sort of way.
And I suppose if you're looking for the edge of the observable universe it doesn't really matter which direction you're pointing the thing. The edge of the observable universe being by definition equally far in all directions from any given observer.
moochedit
I'm not feeding seagulls
It can't point in a direction that the sun is visable as it would damage it.
2hats
Dust.
Confirmation from NASA trajectory analysis of the JWST spacecraft that Ariane 5 orbital insertion was so precise that (potentially) they could extend the science mission lifetime well beyond the original 'hoped-for' ten years courtesy of fuel savings on the way to L2.
2hats
Dust.
The Deployable Tower Assembly, on which the telescope and science instruments sit, has successfully been extended above the main spacecraft bus, creating space for the sun shield to be deployed, and facilitating thermal isolation.
Next up: momentum flap deploy, then retraction of the sun shield covers.
Next up: momentum flap deploy, then retraction of the sun shield covers.
sleaterkinney
Well-Known Member
Both sun sheild arms have been deployed, I was worried when there wasn't an update yesterday.
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2hats
Dust.
The delay was due to sensors failing to confirm the sun shield cover had fully retracted. They waited for the next step (boom deploy) until they had temperature and gyroscopic data that clearly indicated that the covers were where they should be.
2hats
Dust.
Two issues have arisen over the past couple of days.
First an electrical power issue - the solar arrays were configured for a (factory) preset duty cycle prior to launch. Once flying free in space this resulted in a battery drawdown and necessitated reconfiguration to optimise (this was anticipated). The Webb team paused the deployment sequence in order to better understand what temperatures the battery and arrays were experiencing and could operate at on orbit, characterising the duty cycle in the process. Subsequently they have tuned the panels and battery to optimise performance, and thus power provision, staying within comfortable operating temperature limits.
Second, sun shield motor temperatures were a little on the high side (327K rather than a preferred 320K). So they elected to repoint the observatory to cool them, gaining a more comfortable margin on the operating temperatures before proceeding with the next stages of sun shield deployment.
Having addressed those issues, tensioning of layer one of the sun shield will start today, followed by tensioning of layers two to five over the next 2-3 days. If this proceeds according to plan then later in the week they hope to commence deployment of the secondary mirror assembly and perhaps unfold the primary mirror wings.
Once the sun shield tensioning exercise is complete something like 75% of the original mission single point failures will have been obviated.
First an electrical power issue - the solar arrays were configured for a (factory) preset duty cycle prior to launch. Once flying free in space this resulted in a battery drawdown and necessitated reconfiguration to optimise (this was anticipated). The Webb team paused the deployment sequence in order to better understand what temperatures the battery and arrays were experiencing and could operate at on orbit, characterising the duty cycle in the process. Subsequently they have tuned the panels and battery to optimise performance, and thus power provision, staying within comfortable operating temperature limits.
Second, sun shield motor temperatures were a little on the high side (327K rather than a preferred 320K). So they elected to repoint the observatory to cool them, gaining a more comfortable margin on the operating temperatures before proceeding with the next stages of sun shield deployment.
Having addressed those issues, tensioning of layer one of the sun shield will start today, followed by tensioning of layers two to five over the next 2-3 days. If this proceeds according to plan then later in the week they hope to commence deployment of the secondary mirror assembly and perhaps unfold the primary mirror wings.
Once the sun shield tensioning exercise is complete something like 75% of the original mission single point failures will have been obviated.
2hats
Dust.
Exactly like a ball thrown straight up in the air as it closes in on the maximum height in its trajectory.
Secondary mirror assembly now confirmed as latched in place. Next on to the instrument radiator and primary mirror wings.
2hats
Dust.
Aft instrument radiator now confirmed as deployed this afternoon. Each wing of the primary mirror now scheduled to be deployed tomorrow and Saturday. If all goes to plan they will be on course to start exercising the individual mirror segments next week (prelude to tuning them to work in concert, to align them to produce an optimal image as part of the optical telescope commissioning phase).
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