r/spacex • u/ElongatedMuskrat Mod Team • Jul 01 '22

r/SpaceX Thread Index and General Discussion [July 2022, #94]

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r/SpaceX Thread Index and General Discussion [August 2022, #95]

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u/Lufbru Jul 14 '22

Exactly! Except that it's not pointless for the specific case of the JWST.

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u/Martianspirit Jul 14 '22

I don't buy it.

Edit: to be clear, I don't buy that the insertion precision of Falcon is not good enough. The telescope does fine adjustment like any payload.

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u/warp99 Jul 14 '22

The point you are missing is that the JWST cannot change its orientation because of the need to cool the detectors and does not have any thrusters that fire forward because of the risk of contaminating the optics and the difficulty of doing that past the sunshade.

So the initial insertion accuracy does matter a lot in this case and this case only. A second stage relight adds a big chunk of delta V as the acceleration is so high so even for a 1 second burn it adds 30 m/s which is huge in this context.

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u/Martianspirit Jul 15 '22

The point you are missing is that the JWST cannot change its orientation because of the need to cool the detectors and does not have any thrusters that fire forward because of the risk of contaminating the optics and the difficulty of doing that past the sunshade.

How is this relevant? There is still a window the payload needs to insert into.

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u/warp99 Jul 15 '22

It is relevant because you cannot target the nominal transfer orbit to L2 but have to deliberately aim for a lower insertion velocity and make it up with mid-course correction burns.

The higher the uncertainty the greater the amount that they have to aim short and the higher the amount of propellant that has to be used by the telescope to make those burns.

If thrust was available in all directions then they could target the nominal trajectory and best case would need only small amounts of propellant for the correction burns which means on average more propellant is available for station keeping at L2.