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I have not been able to find information online to chart out the SpaceX Starlink launches to see which have announced instantaneous launch windows and others are not announced as such, leading to my assumption that some Starlink launches do not have an instantaneous launch window. Which leads to my question - if Starlink satellites have their own thrusters to maneuver into proper orbits once deployed, why would there be a need for an instantaneous launch for them?

I could see a need for specific orbital-position launches to be instantaneous (such as a manned launch to rendezvous with the Space Station where time/position in orbit needs to be very precise right off the launch), but for Starlink, there doesn't appear to be that time criticality, and they seem (at least to the novice observer) to be quite independently maneuverable and time to final position isn't nearly as important.

But maybe all Starlink launches are instantaneous, and just aren't all announced as such.

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I think it is because SpaceX is maxing out the performance on the Falcon 9. 60 satellites at 227 kg each (Spaceflight Now - SpaceX releases new details on Starlink satellite design) is a grand total of 13,620 kg (not including the supporting structure they are stacked on).

NASA Launch Services Program-Launch Vehicle Performance Website gives the capability of Falcon 9 at ~15,000 kg (extrapolating from image below) to the ~260 km parking orbits (Jonathan's Space Pages):

NASA F9 performance

This is when landing on the droneship which all Starlinks have at least tried to do so far according to Wikipedia's launch history of Falcon 9.

Being near max capacity limits the amount of leeway (timewise) they have with launch times if they intend to place the satellites in a specific orbital plane (which is the whole point of the constellation they are building). They may be able to have a finite, non-instantaneous, launch window but the tradeoff would be the satellites using more propellant to reach the intended orbit, limiting their (revenue generating) lifespan.

Some caveats/speculation:

  • It is not clear if the performance numbers given are for the current Block 5 booster or earlier Block 4 boosters.
  • 400 km is the lowest altitude the website given for Falcon 9
  • Most Starlinks go to a 53° inclination orbit but there are limited options on the performance website
  • There have been a few first stage landing failures in Starlink launches that might support the claim of being at maximum capacity. I have also heard that they were testing the limits of the first stage recovery system so this could be unrelated.
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