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Is the launch from the ESA spaceport because of agreement or an orbital insertion advantage from that facility?

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    $\begingroup$ It may be helpful to note that the rotational velocity at the KSC is 408.456 m/s while Kourou spaceport is 463.21 m/s, a difference of 54.754 m/s, which might be a lot of propellant saved considering the JWST has less thruster fuel and is more massive than other payloads launched by the Ariane 5 ECA. $\endgroup$ Dec 23, 2021 at 18:14
  • $\begingroup$ An addendum to my previous comment - This answer notes that "an additional 161 kg of payload" could be added with the same launch vehicle launched from Kourou instead of the KSC. $\endgroup$ Dec 25, 2021 at 17:49
  • $\begingroup$ @fasterthanlight consider that the entire manoevering fuel capacity for the telescope is about 160 kilograms - that's just two average adults (or one larger adult) its not a lot but it is absolutely significant ! $\endgroup$
    – Criggie
    Dec 26, 2021 at 6:39

2 Answers 2

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The Ariane 5 launch vehicle and all the work associated with the launch is ESA's contribution to the JWST project. In return, ESA gets at least 15% observation time:

ESA's participation in the JWST mission was formally approved by the ESA Science Programme Committee in 2003. The four major European contributions to the mission are formalised in the Memorandum of Understanding on JWST signed by NASA and ESA in 2007. These contributions are:

  • provision of the NIRSpec instrument;
  • provision of the Optical System of the MIRI instrument through special funding from the ESA Member States;
  • provision of the Ariane 5 launcher and all launch services;
  • provision of staff to support mission operations.

In return for the European contributions, ESA gains full partnership in JWST and secures full access to the observatory for astronomers from ESA Member States, on identical terms to those of today on HST. European scientists will be represented on all advisory bodies of the project and will be able to win observing time on JWST through a peer review process, with an expectation of a minimum ESA share of 15% of the total JWST observing time.

(emphasis mine; source: https://sci.esa.int/web/jwst/-/45728-europe-s-role)

Overview of contributions to JSWT

(source: ibid)

It appears that the capabilities of Ariane 5 (with or without the added benefit of being closer to the equator) did not play a role in the decision, at least not initially: on page 17 of this presentation (direct link to 7.3 MB PDF) from 2003, the Atlas V was considered as launch vehicle, with "ample mass margin".

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It turns out that there is only one currently operating rocket that has the width (5.4 meters), length, and the payload capacity (6200 kg to a C3 of ~ -0.75).

Atlas V 551 is very close, it has the fairing and is just a bit undercapacity, only being able to carry 6150 kg. Vulcan will be able to, I think Falcon Heavy modified with the larger fairing and vertical integration would likely work. Delta Heavy has too small of a payload fairing. Ariane 5 is really the only rocket that can do it, and that launches from French Guiana.

This is all in addition to the agreement that ESA had to be a part of the JWST development, and others. But it turns out the more fundamental reason is only Ariane can lift it today.

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    $\begingroup$ I think that Ariane being the only launcher able to do it is more of a result of the choice of using Ariane than inherent to JWST. I found a presentation (see link in answer below) from 2003 where Atlas V was still considered, with "ample mass margin". Seems JWST just grew into the clothes it was offered. $\endgroup$
    – Ludo
    Dec 23, 2021 at 19:27
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    $\begingroup$ Beside the answer, afaik another argument for the Ariane was, that although it is not reusable, it is enough cheap, and it has an astonoshingly good success probability (maybe a single failed launch to more than 100 successful one). If you want to launch a $20billion satellite, actually even a much more costly Ariane would be reasonable economically. $\endgroup$
    – peterh
    Dec 23, 2021 at 20:35
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    $\begingroup$ @peterh 95.5% success. 106 successful out of 111 tries. Not sure it's very "cheap" though. $\endgroup$ Dec 23, 2021 at 20:40
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    $\begingroup$ @OrganicMarble Uhm, that is not so good. If the worth of the JWST is \$20bn, \$500mn would be the net value of the launch risk... $\endgroup$
    – peterh
    Dec 23, 2021 at 20:41
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    $\begingroup$ @peterh though they’ve had 83 consecutive successes between 2003-2017, a single partial failure in 2018, then like 14 more successful ones. All but one failure is pre-2003. I feel like counting all launches instead of recent ones is a metric prone to exaggerate failure rate for rockets that fail early and then fly well after the issues are fixed. Counting since 2003, they’re at 98.9% success rate (97/98 launches) $\endgroup$ Dec 24, 2021 at 2:48

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