paw Ars Tribunus Militum 21y 1,984

dj__jg said:
I guess ESA has a shot at being a role model at de-orbiting stuff, since they sure aren't being a role model at putting stuff into orbit considering the delays and expendable nature of Ariane 6.

Let's not dump on ESA too much re being a role model. Ariane 5's outstanding launch of JWST, doubling its lifetime, should not be overlooked.

Honest question: have any NASA launches exceeded expectations by that much? //

Cloudgazer Ars Tribunus Angusticlavius 8y 15,517

paw said:
Let's not dump on ESA too much re being a role model. Ariane 5's outstanding launch of JWST, doubling its lifetime, should not be overlooked.

Honest question: have any NASA launches exceeded expectations by that much?

I'd love to know what the private opinion of the NASA team was about that launch. One way to view it is that ESA doubled the lifespan of JWST. Another is that they came within 30 m/s of disaster. //

Cloudgazer Ars Tribunus Angusticlavius 8y 15,517

Shiranui said:
How do you mean? How do you turn overdelivering on estimates into pessimistic relief?

Either I'm missing something about Arianespace having taken unnecessary risks to achieve this feat (which I have not heard of so far), or that's a very "glass half empty" perspective.

NASA had an estimated life based on ESA delivering JWST into the expected trajectory, JWST would then need to use its on board thrusters to get the perfect insertion into L2. There was never any doubt that Ariane had the grunt to get JWST into that orbit, or indeed beyond that orbit, but it was imperative that they not overshoot, because if they did JWST was lost.

The targeted trajectory NASA requested from Ariane left room at the top because of that. ESA ate into that margin which delivered a 'better' outcome, but the final adjustments by the JWST were a mere 23 m/s. Had they 'over delivered' by another 23m/s which they were quite capable of doing there would be no JWST.

Publically this was all praised as a great success, but I can't imagine it was quite the same story behind the scenes.

Think of it like shooting the proverbial apple off your wife's head. More points if you hit lower on the apple. This doesn't mean if you aimed for the middle and hit right at the bottom then your wife is going to be entirely happy, because a little lower and you're not a hero - you're William S Burroughs. //

Cloudgazer Ars Tribunus Angusticlavius 8y 15,517

Dan Homerick said:
While reading this, I was thinking "But couldn't JWST have rotated around and burned retrograde to correct a small overshoot?" And to answer that thought, I presume the answer is no, because then it'd be flying through it's own thruster plume, which would fog up the mirrors.

That right?

Kinda, that's half the story ..

More Than You Wanted to Know About Webb’s Mid-Course Corrections! – James Webb Space Telescope
https://blogs.nasa.gov/webb/2021/12/27/more-than-you-wanted-to-know-about-webbs-mid-course-corrections/

Webb has thrusters only on the warm, Sun-facing side of the observatory. We would not want the hot thrusters to contaminate the cold side of the observatory with unwanted heat or with rocket exhaust that could condense on the cold optics

So you're right about not wanting to fly through the plume, and that (along with other considerations) resulted in thrusters only on one side of the vehicle. But as a result of that design decision it's even worse than just contaminating the instrument

Webb’s Journey to L2 Is Nearly Complete – James Webb Space Telescope
https://blogs.nasa.gov/webb/2022/01/21/webbs-journey-to-l2-is-nearly-complete/

“So, why did the Ariane not give Webb more energy and why did Webb need course correction? If the Ariane had given Webb even a little bit too much energy than needed to get it to L2, it would be going too fast when it got there and would overshoot its desired science orbit. Webb would have to do a significant braking maneuver by thrusting toward the Sun to slow down. Not only would that big burn cost a lot of propellant, it would be impossible because it would require Webb to turn 180 degrees in order to thrust toward the Sun, which would have exposed its telescope optics and instruments directly to the Sun, thus overheating their structures and literally melting the glue that holds them together.

Like the enterprise in star trekkin the JWST is always going forwards 'cause they can't find reverse.