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Webb will issue a final course correction burn to insert it into the orbit around L2. It arrives from earth so the approach velocity should be higher than the desired orbit velocity and the vector should point away from earth and sun. It needs negative delta-v.

How can JWST brake without turning around? Turning around is not an option, because the telescope's dark side may not be exposed to solar radiation. The engine is located on the hot side, pointing to where Webb came from. So how can it decrease velocity?

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    $\begingroup$ it's actually confusing to refer to an L2 "orbit", however widely the terms are used together. it's more like stationkeeping. $\endgroup$
    – Fattie
    Jan 13 at 0:34
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    $\begingroup$ @Fattie According to Wikipedia's article about Lagrange points an orbit is very well possible. The limitation is that L1, 2 and 3 are pulling in objects only in tangential directions, not in radial direction. So Webb in L2 has to stabilize itself radially (in the sun / earth / Webb axis) using its own thruster but can orbit naturally in the plane perpendicular to that axis. L4 and 5 are in fact stabilizing in all directions. They allow for full natural orbits and can hold trojan bodies. $\endgroup$
    – Jpsy
    Jan 13 at 6:56
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How can JWST brake without turning around?

Answer: JWST does not.

From More Than You Wanted to Know About Webb’s Mid-Course Corrections!:

One interesting aspect of the Webb launch and the Mid-Course Corrections is that we always “aim a little bit low.”

The L2 point and Webb’s loose orbit around it are only semi-stable. In the radial direction (along the Sun-Earth line), there is an equilibrium point where in principle it would take no thrust to remain in position; however, that point is not stable. If Webb drifted a little bit toward Earth, it would continue (in the absence of corrective thrust) to drift ever closer; if it drifted a little bit away from Earth, it would continue to drift farther away.

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. This means the thrusters can only push Webb away from the Sun, not back toward the Sun (and Earth). We thus design the launch insertion and the MCCs to always keep us on the uphill side of the gravitational potential, we never want to go over the crest – and drift away downhill on the other side, with no ability to come back.

Therefore, the Ariane 5 launch insertion was intentionally designed to leave some velocity in the anti-Sun direction to be provided by the payload.

MCC-1a similarly was executed to take out most, but not all, of the total required correction (to be sure that this burn also would not overshoot). In the same way, MCC-1b, scheduled for 2.5 days after launch, and MCC-2, scheduled for about 29 days after launch (but neither time-critical), and the station-keeping burns throughout the mission lifetime will always thrust just enough to leave us a little bit shy of the crest.

We want Sisyphus to keep rolling this rock up the gentle slope near the top of the hill – we never want it to roll over the crest and get away from him.

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    $\begingroup$ This is the reason why the ESA spokesperson lauded the Vulcain engine they used for its precision, see this article on ARS Technica. I wondered why it was the precision that was lauded, and not performance. Your answer, however, makes it perfectly clear. If you aim to give the payload just that tiny bit too little a push, then you need a precise engine. $\endgroup$
    – Dohn Joe
    Jan 12 at 8:54
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    $\begingroup$ Very enlightening answer! Does that mean that JWST will actually need much longer than 30 days to reach its "final" orbit? If you try to roll to a stop on a flipped parabola as close as possible to the top the last "meters" can take very long. My intuition says that it can take extremely long if you try to get to zero relative velocity very close to the top. $\endgroup$
    – Jpsy
    Jan 12 at 10:10
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    $\begingroup$ Yes, I believe they said that the first burn, MCC-1a, was time critical, MCC-2 was seen as 'flexible'. $\endgroup$ Jan 12 at 10:43
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    $\begingroup$ @DohnJoe You need precise GN&C (guidance, navigation, and control) more than precise engines. I've yet to work on flight software development of / work on simulation development of a vehicle for which engine performance (thrust, specific impulse) was expected to be within 1% of predicted values. Precise engines help, but it's the Ariane 5 GN&C software and sensors that made the performance as good as it was. $\endgroup$ Jan 12 at 12:48
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    $\begingroup$ @blobbymcblobby I've also read that (about MCC-2 being flexible), but I've also read that MCC-2 is the orbit insertion burn. That would make MCC-2 time critical, but the exact timing would have been something they couldn't predict prior to launch. $\endgroup$ Jan 12 at 12:56

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