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It’s common knowledge that SpaceX goal is to create a fleet of spacecraft shuttling from Earth to Mars.

One milestone they intend to achieve beforehand is having a fleet of 1st stage boosters shuttling from Earth to LEO/GTO. They'll use their two designs as they reach maturity, the Falcon 9 (also used as the heavy boosters) and Falcon Heavy core (with partial cross-feed). This will allow SpaceX to have firsthand experience on how to manage a fleet of 1st stage boosters.

Together with this information:

  • Client flight manifests are piling up, 25 launches for 2016 that are bound to spill over into 2017.
  • In partnership with Google (and its $1B influx), doubling the Internet capacity with 4000 new satellites over 15 years.
  • Current 3 weeks average launch pad and new build turnaround.
  • Ideally need 12 flights yearly to keep the company financially afloat.

  • Reusability in its infancy, with Elon currently saying they've passed the 70% recovery success rate mark and ~90% in 2017 (was at 40% just 7 months ago). It’s not a factual metric, just Elon’s educated guess. With the goal of being well at ~100% before 2024, obviously.

  • Their yearly production capacity is of 40 1st stage reusable -9 boosters (and 20 2nd stage but with no intention of reusability), bringing it to 10 new builds for Falcon 9 and Heavy a year.

  • The Merlin rocket has a limited number of re-ignitions in them (engine cycle). Making the boosters reusable up to 10 times.

If the mature aircraft industry can be of an analogy here, fleet size to destination ratio is slightly below 2:1 (Although a commercial aircraft is reusable 40 000 times ‘metal fatigue gets it after 30 years’ and has a turnaround of well under 1 day.)

Lower bound answer: That would mean that their 1st stage booster fleet could be as low as 2 Falcon 9s and 2 Falcon Heavies, provided the turnaround is at least as low as the demand of the fleet to be reflowed. Bearing in mind that those 8 boosters will need to be replaced at a yearly rate.

Upper bound answer: The other extreme is that they will keep producing to their max capacity, ever expanding their fleet. But I find that unsustainable as economically they will be producing higher than the demand (only two destinations). Also they would need at some point to shift some of this manpower into building the BFR instead.

Which brings me to ask, how big of a booster fleet will SpaceX be settling for? They’re anticipating an even demand for Falcon 9 and Heavy. Did SpaceX provide any info on the subject?

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    $\begingroup$ Take any timeline Elon promises, especially if not a very short term promise, and multiply it by 2-3. That's about where it'll end up. But still... $\endgroup$ – PearsonArtPhoto♦ Feb 10 '16 at 19:57
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    $\begingroup$ That largely will depend on demand. With economy of scale, the price will keep dropping, so the demand will keep rising as more firms will be able to afford their own satellites. With demand rising and a steady income stream the fleet will grow too - until the break-even point where the price can't go much more down and there's simply no more demand for satellites at that price. But it's really hard to predict how this mutual relationship will progress (price depends on demand, demand depends on price, fleet size tries to follow the demand as income (related to price*demand) allows.) $\endgroup$ – SF. Mar 1 '16 at 22:00
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May 2018 Update on this:

30 - 50 Falcon 9 boosters

After the successful launch and recovery of the Falcon 9 on the Bangabandhu Satellite-1 (May 11th at 4:14 p.m. EDT), the 'Block 5' version has become operational and is advertised to be ready for rapid reusability.

According to Elon Musk's pre-launch briefing (emphasis mine):

Musk: I think probably, just to be safe, we'll probably have about 30. Call it 30 to 50 [Block 5 boosters... in rotation]. It totally depends on what number of customers insist on launching a new rocket.

That currently appears to be the closest we have to an official word on how many Falcon 9 first stages will be used. This seems to be a large number for a vehicle that is supposedly

...designed to do 10 or more flights with no refurbishment between each flight

There are, however, several factors worth noting:

  • SpaceX plans to launch primarily from Cape Canaveral, but also from Vandenberg AFB in California. Clearly the time required to transport across the US would not allow for 'Rapid reusability' thus necessitating a larger fleet.
  • At this point, it appears the Falcon Heavy core booster is not compatible with standard Falcon 9 launches as it requires various structural changes and reinforcements, although this may change.
  • Also (pure speculation here), the goal of 10 flights without scheduled refurbishment may be overly-optimistic. I imagine a thorough inspection/refurbishment schedule to be more regular than that, especially in the early days of the Block 5 program.

Full transcript of briefing here - relevant pages 8/9

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    $\begingroup$ Hi, welcome on the Space SE! Funny idea: the rockets could go home to CC also on their own "feet"... of course it would require fuel, and it would be yet another flight, but it would be surely fast. $\endgroup$ – user259412 May 22 '18 at 11:15
  • $\begingroup$ Thanks @peterh! Yeh a site-to-site launch for transporting first stages seems like a smart idea - Elon certainly wants to do it for the BFR. However, from memory the barge landings are typically ~500km downrange, so even ignoring all the other technical hurdles, VAFB to CC wouldn't be feasible. But with a lightweight dummy payload for aero, who knows! $\endgroup$ – Jack May 22 '18 at 11:24
  • $\begingroup$ I think if the first stage can go from point A to point B with payload, then it should be able to go back from B to a without payload. I think the lack of payload could be even optimized to delay the wearing of the rocket (lower power, etc). Btw, I would find it really funny if the SpaceX would finally end up as a cheap, superfast sub-orbital transport company. :-) $\endgroup$ – user259412 May 22 '18 at 12:52
  • $\begingroup$ @peterh oh I thought you meant flying unassisted between launch sites instead of by road, my bad. Yeah, imagine waiting in line at the Falcon 9 stop down the end of the street :p $\endgroup$ – Jack May 22 '18 at 13:05
  • $\begingroup$ :-) I think, also it could be transferred back with transfer helicopters. An empty first stage is only 25 tons. The largest transfer helicopter (well, it is Russian) can uplift 28 tons. $\endgroup$ – user259412 May 22 '18 at 13:23

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