three computers in each processing unit, each constantly checking on the others, to instantiate a fault-tolerant design. One processing unit is part of each of the ten Merlin engines (nine on the first stage, one on the second stage) used on the Falcon 9 launch vehicle. Turbopump The Merlin LOX/RP-1 turbopump used on Merlin engines 1A–1C was designed and developed by Barber-Nichols. It spins at 36,000 revolutions per minute, delivering . Gas generator The LOX/RP-1 turbopump on each Merlin engine is powered by a fuel-rich open-cycle gas generator similar to that used in the Apollo-era Rocketdyne F-1 engine. Production , SpaceX was producing Merlin engines at the rate of eight per month, planning eventually to raise production to about 33 engines per month (or 400 per year). By September 2013, SpaceX total manufacturing space had increased to nearly , and the factory had been configured to achieve a maximum production rate of up to 40 rocket cores per year, enough to use the 400 annual engines envisioned by the earlier engine plan. By October 2014, SpaceX announced that it had manufactured the 100th Merlin 1D engine and that engines were now being produced at a rate of four per week, soon to be increased to five. In February 2016, SpaceX indicated that the company will need to build hundreds of engines a year in order to support a Falcon 9/Falcon Heavy build rate of 30 rocket cores per year by the end of 2016. Each Falcon 9 booster uses nine Merlin engines, and the second stage uses one Merlin vacuum engine. The second stage is expended, so each launch consumes one Merlin Vacuum engine. SpaceX designed the booster with its engines to be recovered for reuse by propulsive landing, and the first recovered booster was reused in March 2017. By 2020, only five of the 26 Falcon 9 launches used new boosters. These 26 launches used 25 new Merlin Vacuum engines and 45 new sea-level Merlin engines. By 2021, only two of the 31 Falcon 9 launches used new boosters. In the first half of 2022, there were 27 Falcon 9 launches, two of which used a new booster. See also SpaceX Draco – SpaceX RCS thruster for SpaceX Dragon SpaceX Kestrel – SpaceX small upper stage engine for Falcon 1 SpaceX Raptor – SpaceX methane/LOX engine for the Starship Falcon 1 – First rocket powered by Merlin 1A Comparison