All hail the Ariane 5 rocket, which doubled the Webb telescope’s lifetime

https://arstechnica.com/science/2022/01/all-hail-the-ariane-5-rocket-which-doubled-the-webb-telescopes-lifetime/

by fabian2k • 5 years ago

52 12 5 years ago

garaetjjte3• 5 years ago

This doesn't make much sense, Webb was deliberately underboosted for L2 orbit because it has thruster only on one side and cannot correct for any overboost [0], so if Ariane performed as expected there should be exact calculated amount of fuel left. If there would be more fuel left in Webb than predicted it would actually mean that Ariane overboosted from programmed trajectory, which is bad thing. I think this is just NASA doing its usual underpromise-overdeliver rather than better-than-expected performance from launch vehicle.

[0] (supposedly it cannot rotate its cold side to the Sun, though I don't know what difference it makes when it's not taking images at the time?)

dandelany2• 5 years ago

there should be exact calculated amount of fuel left

Nothing is exact :) Ariane upper stage boosts JWST on a certain trajectory +/- some error in all directions, and then detaches. Along the way, they figure out the error and JWST corrects it with trajectory correction burns. This is somewhat independent of any fuel left for the "underboosting" you're talking about.

A mission designer looks at the performance specs/error distribution for Ariane and tries to include enough fuel so that, 99.X% of the time there will be enough for trajectory correction maneuvers AND a 10-year lifetime. When the (Ariane) burn is precise, errors are small, and less fuel is wasted on TCM, leaving more for the mission itself.

What this (10->20 year lifetime adjustment) really means is that NASA's left quite a lot of margin for TCM here... Which, for something this expensive, was a good thing :)

garaetjjte3• 5 years ago

The point is, if Ariane slightly overboosts requested trajectory it increases JWST lifetime, but it's not indicative of exceptional Ariane performance. In other words, perfectly precise Ariane burn yields shorter lifetime than slight overboost, thus I'm surprised that article attributes increased lifetime to especially precise launch vehicle.

jmole• 5 years ago

It's not just velocity, it's angular accuracy. In other words, where are we pointing?

When you're course correcting, it's more like "steer this way" or "steer that way", not "dammit we're not going fast enough".

Getting the wrong velocity would have been a complete disaster.

garaetjjte3• 5 years ago

True, but I think most of course correction delta-v budget is due to velocity rather than direction changes, especially with that deliberate underboost. Additionally direction is controlled during burn which is several minutes long, while velocity depends on precise timing of shutting down relatively big thruster.

nabla9• 5 years ago

After reading the article I learned the reason why Ariane 5 rocket performed better than expected. Really cool.

ensan• 5 years ago

It's unfortunate that, at least in the mind of almost anyone who has taken an embedded systems course, Ariane 5 is associated with that very expensive overflow bug.

idontwantthis• 5 years ago

Wasn't that Ariane 4?

aw1621107• 5 years ago

The system was originally from the Ariane 4, but the failure itself occurred on the Ariane 5's maiden flight [0].

[0] https://en.wikipedia.org/wiki/Ariane_flight_V88

GenerocUsername• 5 years ago

Bad title.

All hail the Ariane 5 rocket, which doubled the Webb telescope’s lifetime

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