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I was doing some back of the envelope calculations to see improvements SpaceX would have to make to reach a cost target of $100/kg into orbit. After doing some basic calculations, it seems that SpaceX would need to increase the cycle life (number of ignitions or stress cycles) that the engines go through. I tried back calculating what the current cycle life of the SpaceX Merlin 1D engine is, but couldn't find the relevant data (a weight breakdown of the engine components) to calculate it. Does anybody know what it is or where to find it?

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    $\begingroup$ $100/kg doesn't seem feasible without full reusability of both stages — which is only planned for the future methalox Raptor-based rocket. $\endgroup$ – radex Aug 26 '15 at 8:53
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No Merlin 1D engines have been reflown, per se, to date. I qualify with 'per se' since every Merlin engine does qualification tests at the McGregor, Tx test stand singly, then as a group of 9. However many exact firings that is, is not entirely clear. (2? More?) Tom Mueller has said that as part of acceptance testing of a Merlin engine (in the 1C days) that they drop in a 5/16th nut that has to be injested and survived.

Then on a typical reuse profile, there is: Hot Fire 2-3 days before, only 1-2 seconds, but full ignition cycle Launch - 3 minutes of flight (not all at full power, 2 engines are shut down, and the rest throttled later in flight to control G loads) Recovery - 2 or 3 burns - First with three engines, second and third with center engine.

Thus before a single reuse attempt is made, a Merlin 1D in the center position is likely to have fired (ignition to full power, regardless of duration) at least 7 times.

Much beyond that is proprietary information of SpaceX.

How will this change with the full thrust Merlin 1D to be used on all future launches (Post Jason-3 as of this writing. After return to flight, there is one older Falcon 9 left, everything after that is Full Thrust Falcon (or Falcon v1.2, but they are avoiding the name due to certification issues)).

On Falcon Full Thrust recovery missions, they have experimented with 3 engine firings during the reentry and landing burns in order to minimize fuel usage fighting gravity.

The JCSAT-14 core after landing was retested in the summer of 2016 at least 3 times in McGregor by the end of July. Gwynne Shotwell at the Smallsat 2016 conference in Logan, UT said their aim is to run that stage 4-10 more times until they find the lifespan of a stage. She suggested they would slip the JCSAT-14 booster onto the test stand, every time it is vacated and not needed by a production booster test. The interesting question this raises is how busy the test stand is, how long it takes to mount/dismount a stage, and then run a test.

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