JWST Countdown To Terror 😳

[dannybgoode]

5 hours ago, Chaz2b said:

Really? Have you seen the speed a certain Tesla is travelling at?!

It's currently travelling pretty slowly in spacecraft terms.  It's only doing 2500km/h or so - Concorde could do 2100km/h by way of comparison and Apollo 11 on the way to the moon over 40000 km/h and it will keep on slowing down (the odd correction burn aside) until yes, it will pretty much just stop and enter L2 orbit.  The maths to work out the required velocity etc is beyond my tiny brain for sure!

[Zermelo]

19 hours ago, kirkster501 said:

The biggest test is to come

The BBC news has also been saying that the current stage is "JWST's biggest test". Out of interest, do you know if anyone has rated the umpteen different critical steps/components in terms of their relative risk?

 

[Zermelo]

Apologies if this has already been asked (and answered).

I'm wondering how a satellite can "orbit" L2, given that L2 is only a metastable point. It would not keep true in the direction perpendicular to the sun-earth axis, so would require constant course corrections? Different for L4/L5.

[kirkster501]

6 minutes ago, Zermelo said:

The BBC news has also been saying that the current stage is "JWST's biggest test". Out of interest, do you know if anyone has rated the umpteen different critical steps/components in terms of their relative risk?

 

Not sure there is a “ranking” as such but I think the generally perceived wisdom is that the sunshield port and starboard extension phases and the subsequent tensioning is the biggest risk simply because there are hundreds of moving parts to it. Then the secondary *has* to deploy. There is zero mission if it doesn’t.

No news as yet from NASA about the telescopic extension of the sunshield today so far. 

[Mr Spock]

I picked this up off fb. It shows how the orbit will work.

[kirkster501]

3 minutes ago, Zermelo said:

Apologies if this has already been asked (and answered).

I'm wondering how a satellite can "orbit" L2, given that L2 is only a metastable point. It would not keep true in the direction perpendicular to the sun-earth axis, so would require constant course corrections? Different for L4/L5.

Think of it like a saddle with huge, deep wells in spacetime caused by the Sun and the Earth.  There are peaks and troughs in this gravity field and the peaks are L1 and L2 allowing a spacecraft to orbit them.

 

[Stu]

6 minutes ago, Zermelo said:

Apologies if this has already been asked (and answered).

I'm wondering how a satellite can "orbit" L2, given that L2 is only a metastable point. It would not keep true in the direction perpendicular to the sun-earth axis, so would require constant course corrections? Different for L4/L5.

Isn’t that the point, it has a certain lifetime governed by the amount of fuel available for the course corrections required to keep it in orbit around the L2 point? It is not a stable orbit so needs to be maintained.

The good thing is that they have more fuel than they hoped, so the lifetime (assuming everything goes smoothly from here) should be longer than 10 years.

[kirkster501]

2 minutes ago, Stu said:

Isn’t that the point, it has a certain lifetime governed by the amount of fuel available for the course corrections required to keep it in orbit around the L2 point? It is not a stable orbit so needs to be maintained.

The good thing is that they have more fuel than they hoped, so the lifetime (assuming everything goes smoothly from here) should be longer than 10 years.

Yes. An orbit of L2 cannot be maintained without the expenditure of some fuel because the points are subject to disturbance by other solar system objects - especially The Moon and Jupiter.

[ONIKKINEN]

3 minutes ago, Stu said:

Isn’t that the point, it has a certain lifetime governed by the amount of fuel available for the course corrections required to keep it in orbit around the L2 point? It is not a stable orbit so needs to be maintained.

The good thing is that they have more fuel than they hoped, so the lifetime (assuming everything goes smoothly from here) should be longer than 10 years.

Yes, some lagrange points are not very stable and need regular maintenance to stay there. Which is why we dont see L2 orbiting asteroids.

L4 and L5 are stable though and in the case of Jupiter-Sun Lagrange points are full of asteroids in the form of Trojans and Greeks.

[Mr Spock]

They thought it was going to last 5 to 10 years, but the excess fuel means it will be over 10 years.

[Zermelo]

4 minutes ago, kirkster501 said:

Think of it like a saddle

Yes, I was ...

Just now, kirkster501 said:

Yes. An orbit of L2 cannot be maintained without the expenditure of some fuel because the points are subject to disturbance by other solar system objects - especially The Moon and Jupiter.

OK, I hadn't realized the propellant would be needed for that purpose, I thought it was mostly being used for the imaging pointing.

12 minutes ago, Mr Spock said:

I picked this up off fb. It shows how the orbit will work.

 

Nice, I've not seen that one. And it includes the word "syzygy".

5 minutes ago, ONIKKINEN said:

full of asteroids

Yes, I think there are even the odd one or two in the sun-earth L4/L5 points.

 

Thanks all.

 

 

[Mr Spock]

1 minute ago, Zermelo said:

And it includes the word "syzygy".

Great word isn't it. I must try and work it into some conversations 😜

[kirkster501]

6 minutes ago, Mr Spock said:

They thought it was going to last 5 to 10 years, but the excess fuel means it will be over 10 years.

Let's hope so Michael.  Wouldn't that be wonderful?  Also they will no doubt learn new ways to minimise fuel consumption too.

If there is to be a refuel mission it needs to take place before the fuel runs out because when it does if will not just freeze and stay there; anything orbiting L2 without the fuel to maintain the orbit will drift off into a solar orbit into interplanetary space and at the mercy of Jupiter.  Remember that JWST will be in a solar orbit, not an earth orbit.

[kirkster501]

2 minutes ago, Mr Spock said:

Great word isn't it. I must try and work it into some conversations 😜

Let's meet up at the pub mate  

[Zermelo]

3 minutes ago, kirkster501 said:

they will no doubt learn new ways to minimise fuel consumption too.

and you mentioned previously that they will sequence the successful observation requests in the most fuel-efficient order. It sounds a bit like a travelling salesman type problem.

[Captain Scarlet]

9 minutes ago, kirkster501 said:

Let's meet up at the pub mate  

Pub is the perfect opportunity actually

”syzygy egg n chips please”

sorry I’ll go now

M

[kirkster501]

15 minutes ago, Zermelo said:

and you mentioned previously that they will sequence the successful observation requests in the most fuel-efficient order. It sounds a bit like a travelling salesman type problem.

Yep.  Even Noble laureates such as Dr Mather who want to study objects that the JWST has to slew a long way to will have to wait until most of the "slew" is accomplished by the JWST's rotation around the sun in its orbit.    The amount of slewing of the JWST is massively controlled and subject to huge chains of approval to minimise the use of the limited fuel.  There is a Youtube video on this somewhere.

[Zermelo]

28 minutes ago, Mr Spock said:

Great word isn't it. I must try and work it into some conversations 😜

... or into a festive game of Scrabble

[dannybgoode]

30 minutes ago, Zermelo said:

OK, I hadn't realized the propellant would be needed for that purpose, I thought it was mostly being used for the imaging pointing.

Positioning is largely done via gyroscopes which are electrically powered so largely non-fuel dependent. They may need to use a little though for this purpose

[kirkster501]

16 minutes ago, dannybgoode said:

Positioning is largely done via gyroscopes which are electrically powered so largely non-fuel dependent. They may need to use a little though for this purpose

The positioning itself is still done by small thrusts of rocket engine lighting.  The gyroscopes - and these are much more reliable than the HST's -  provide the reference for a 3-D model of the sky that is constantly calibrated by the alignment of Canopus and Vega.  What the JWST team do not want to be doing is slewing all over the sky between research projects. i.e M82 and then M42 and then M87, for example.  The projects are scheduled scope time dependent on proximity to each other in the sky as a major factor, amongst some other things, to minimise fuel consumption.

[ONIKKINEN]

29 minutes ago, kirkster501 said:

The positioning itself is still done by small thrusts of rocket engine lighting.  The gyroscopes - and these are much more reliable than the HST's -  provide the reference for a 3-D model of the sky that is constantly calibrated by the alignment of Canopus and Vega.  What the JWST team do not want to be doing is slewing all over the sky between research projects. i.e M82 and then M42 and then M87, for example.  The projects are scheduled scope time dependent on proximity to each other in the sky as a major factor, amongst some other things, to minimise fuel consumption.

Isnt it reaction wheels doing the fine pointing and gyroscopes "guiding" or making sure the orientation is correct?

Reaction wheels saturate if left running and need to be de-spun with external forces. Propellant is used for this most of the time, but the HST uses magnetorquers that somehow utilize the Earths magnetic field to provide the de-spin torque (no idea how). I guess L2 is far enough out that its not an option?

Also, light pressure will probably be considerable since JWST is basically a sail with the fully extended bits so reaction wheels probably need more desaturation.

Edited December 31, 2021 by ONIKKINEN

[Ags]

Desturation. I like it, sounds very Star Trek. I suppose it means spinning faster and faster with no way to slow down ( without making JWST spin in the opposite direction)?

[ONIKKINEN]

15 minutes ago, Ags said:

Desturation. I like it, sounds very Star Trek. I suppose it means spinning faster and faster with no way to slow down ( without making JWST spin in the opposite direction)?

Yes, and the wheels can be allowed to spin only so fast without risk of damage. At some point all the wheels are spinning so fast that any changes in orientation take ages to do and they must be de-spun to maintain control. If you just let the wheels de-spin unpowered JWST would be doing cartwheels as the force has to go somewhere, which is why some form of external torque is needed (usually reaction control thrusters).

[Zermelo]

1 hour ago, kirkster501 said:

these are much more reliable than the HST's

That's a relief.

[kirkster501]

Happy New Year everyone.  The party at Kirkster towers is over and just checked in to NASA - as you do 😂- and seen the port side sunshield has been deployed 👍🏻
https://blogs.nasa.gov/webb/2021/12/31/first-of-two-sunshield-mid-booms-deploys/