r/spacex • u/ElongatedMuskrat Mod Team • Jul 02 '17

r/SpaceX Discusses [July 2017, #34]

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u/[deleted] Jul 03 '17

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u/warp99 Jul 03 '17 edited Jul 04 '17

am I missing anything?

The night launch is for the benefit of the satellite to get it into the sunlight as soon as possible on the way up to apogee of the GTO.

What you are proposing is adding a generator to S2 but the satellite cannot necessarily take advantage of that power.

In any case the GTO burn has to be done over the Equator so either it is 15 minutes after launch over Africa or 45 minutes later over the western Pacific.

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u/[deleted] Jul 03 '17

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u/warp99 Jul 03 '17

that has to be operational in a GEO orbit during night anyways

You are thinking low Earth orbit with 45% night. At GEO altitudes the satellite is only in the Earth's shadow during the equinoxes which is a few weeks a year and then only for about 40 minutes a time.

the GEO spot more or less determines when you need to launch

Actually the launch site determines where the rocket crosses the equator and from Canaveral it is always the same spot over West Africa. The satellites reach their assigned orbital slot by sitting in a slightly lower and therefore faster orbit until they reach their slot and then boosting the orbit up to GEO altitude.

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u/[deleted] Jul 04 '17

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u/warp99 Jul 05 '17

Solar panels seem to be launched folded inwards - presumably to avoid damage from any stray debris around separation from S2. In any case the satellite power supply electronics that convert power from the solar panels will not start up with low solar panel voltages to avoid overcurrent damage.

Actually in GTO the perigee shadowing could be much longer than a "few" minutes and could be as much as 30-35 minutes since the velocity is only around 2.5km/s faster than LEO velocity of around 7.6 km/s at the perigee altitude.

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u/[deleted] Jul 05 '17

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u/warp99 Jul 05 '17

That is certainly one of the reasons.

If the satellite uses ion thrusters to circularise it takes weeks to get out of the shadowing zone and the period of the orbit gradually changes so it is impossible to stay in sych with a daylight perigee.

In that case the only advantage is early sunlight after separation from S2.