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For every Falcon 9 launch to date, the process has been:

Each Merlin is test fired/acceptance fired at McGregor by themselves. The entire stage is test fired at McGregor. The entire stage is hot fired before launch. Launch Re-entry, 3 engines, then 1 engine, then 1 engine burn to land.

With the recovery of the F9 S1 from the CRS-8 mission, Elon Musk said at the post-launc press conference, (Not on shitelonsays.com yet to quote the transcript) they will take the stage to LC-39A and test fire it ten times. Now he was not clear if that was 10 hot fires, of a second or three each, or if it was a full 2 minute mission duration burn.

Regardless, that is quite the number of firings. In the past we have heard numbers like 40 firings as the design limit for a Merlin. Has this changed? Will this be a limitation in reuse, if they need to do so many test burns, before launch, and before each re-lauch?

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    $\begingroup$ There should not be as many test fires in the future recovered boosters, they just want to test this one very well. LC-39A is not a test stand so probably not prepared to handle full duration burns. $\endgroup$ – jkavalik Apr 12 '16 at 16:17
  • $\begingroup$ @jkavalik Of course. And the 10 tests may be to simulate 10 missions, so the next test is like the 12 mission, not the second. But still, that is a lot of test burns. $\endgroup$ – geoffc Apr 12 '16 at 16:18
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After the first few landings of the first stage, Elon Musk said that the first core (landed on land) from the Orbcomm OG2 mission will become a static display outside the SpaceX factory in Hawthorne. (Adding to their collection of the first Dragon to reenter, a Merlin engine hanging in the ceiling. I guess a first stage is too big to hang indoors, maybe in the Tesla factory? Or the BFR factory?).

The GTO mission stage from JCSat 14 mission was the most intense landing to date (being one of the heaviest GTO missions to date) and Elon said they would refire the stage 10 more times after the recovery and use it as a class leader for understanding the ability of the engine to be reused.

As of August 2016, they had test fired it (full duration, 2+ minutes) three times in three days!!!! Which is amazing.

Gwynne Shotwell at the Small Sat 2016 conference at U of Utah in Logan, UT was noted to have said that they will try and slip the JCSAT-14 booster onto the test stand in between the need to test stages for launch until they get to 8-14 test firings.

Thus it seems like they do not actually know the number of firings, but they designed for a large number (planning on reuse, and as detailed in the question, taking lots of firings to launch and land) and plan on stress testing a stage to find out the actual answer.

In terms of practical use, the Block 3 and Block 4 Falcon 9 first stages are being used only 2 times, and then discarded/expended on second launch. The claim is that Block 5 model (due in spring of 2018) will be rapidly and cheaply reusable, up to 10 times before major work. Thus the number of firings per Merlin should be going up quite a bit.

But it will be the penultimate (for now!) Merlin 1D that can perform in this fashion, and it took iterations through 1A->1B->1C->1D and then many tweaks based on 1D experience to get there.

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