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The Falcon 9 has a capacity of 22,800 kg to LEO.

A Starlink satellite has a mass of 227 kg.

That means that one Falcon 9 can launch 100 satellites.

So why does SpaceX launch 60 instead of 100 satellites at a time?

Later edit: at 260 kg per satellite, the capacity is 87 instead of 100. Still greater than 60.

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    $\begingroup$ Starlink satellites version 0.9 are 227 kg but version 1.0 are 260 kg. $\endgroup$
    – Uwe
    Jun 20, 2020 at 16:05
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    $\begingroup$ Payload volume restrictions. Can't fit 100 in the fairing. $\endgroup$
    – Anton Hengst
    Jun 20, 2020 at 19:51

2 Answers 2

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To begin with, Starlink sats have a mass of ~260kg not 227kg. Secondly, the reusable Falcon 9 LEO payload has a limit of around ~15600kg; the 22,800kg payload is for expendable Falcon 9.

60sats*260kg = 15600kg.

This mass limitation can be verified in the fact that for Starlink L8, SpaceX had to remove 2 Starlink sats to add 3 Planetlabs Skysat with a mass of 110kg each, which produced a payload mass of 15410kg. (an additional starlink sat would've pushed the mass to 15670kg)

So basically SpaceX launch 60 at a time because of mass limitations.

The other main limitation is fairing volume, because 60 sats is likely at the limit of the fairing volume.

enter image description here

But if they were develop a bigger fairing, would they launch on an expendable Falcon 9, as it would be able to deliver more sats to orbit? No, because it's cheaper for SpaceX to launch in reusable mode. Current estimates put the internal cost of an expendable Falcon 9 launch at ~50 mil US vs the reusable Falcon 9 at a cost of ~30mil US. This puts the launch cost per sat at 570,000 US/sat for expendable and 500,000 US/sat for reusable. (this isn't mentioning that a reusable Falcon 9 can support a higher launch rate)

If SpaceX wanted to launch 100 sats, they would probably use fully reusable extended fairing Falcon Heavy. Assuming that Starlink is at the limit of fairing height with a stack of 30x2, that would give you a starlink sat height of around 22-23cm. That means you could fit 2 stacks of 54 sats in an extended fairing, which gives a payload mass of 28,080kg. (Fully reusable Falcon Heavy has a LEO payload mass that could reach up to ~28,000kg.)

anu

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  • $\begingroup$ There should be at least some 10 if not some 100 kg for the structure to stack 60 sats of about 15 metrical tonnes. $\endgroup$
    – Uwe
    Jun 20, 2020 at 13:27
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    $\begingroup$ @Uwe That structure is probably built into the Starlink sat mass, though I can't say for certain. Looking at this deployment, youtube.com/watch?v=5h2t9Oyg2o0, there isn't an internal structure to support it, the sats support themselves. The only structure that I can clearly identify that is seperate from the sats is the tension rods. $\endgroup$
    – Barry Jenakuns
    Jun 20, 2020 at 13:32
  • $\begingroup$ Thanks. How did you get the 15,600 number? I cant find IT on SpaceX web neither on Wikipedia $\endgroup$
    – Joe Jobs
    Jun 20, 2020 at 14:03
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    $\begingroup$ @JoeJobs: If you throw away the rocket, you have to include the cost of the rocket in the launch cost. $\endgroup$
    – Jörg W Mittag
    Jun 20, 2020 at 15:44
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    $\begingroup$ @JoeJobs The cost of an expendable launch is higher than a reusable one. They don't actually list the cost of the expendable launch, but I think it's around 50% higher, although I could be wrong. Would probably be worth its own question if you wanted to continue. $\endgroup$
    – PearsonArtPhoto ♦
    Jun 20, 2020 at 18:13
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The structure neccessary to transport and release 60 individual satellites as payload has a weight too. This structure should work under enhanced gravity during launch as well as in zero gravity of LEO.

The container structure should be launched into LEO and therefore reduces the possible payload weight left for satellites.

There is no astronaut there unloading the satellites. Actuators are needed for release of the 60 satellites. All that working in space reliable.

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    $\begingroup$ I wonder if there is even room for more than 60 in the existing fairings $\endgroup$
    – uhoh
    Jun 20, 2020 at 12:40
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    $\begingroup$ @uhoh A new longer fairing with more room for more than 60 will weigh more. May be we will hear about launches of about 80 in future. The question is: is there any space and weight left to launch more than 60 using the existing fairing. Recently they launched only 58 together with 3 other satellites. It seems this was done because of weight limits and not of space limits. $\endgroup$
    – Uwe
    Jun 20, 2020 at 13:09
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    $\begingroup$ Actually, the whole point of doing the deployment the way SpaceX does, is that there are no actuators, no pushers, no springs, no nothing. Just good old centrifugal force. $\endgroup$
    – Jörg W Mittag
    Jun 20, 2020 at 15:42
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    $\begingroup$ Have you ever seen a Starlink deployment? There is no "controlled release", they cut the stack loose and let them sort themselves out. They're just built to tolerate bumping into each other a little bit in the process. $\endgroup$
    – Christopher James Huff
    Jun 20, 2020 at 17:25
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    $\begingroup$ Starlink has almost no support structure. It's basically just a mounting frame at the bottom of the stack and a set of straps to hold the stack in place. Total weight can't be more than two to four Starlinks. $\endgroup$
    – Mark
    Jun 20, 2020 at 23:35

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