The injector at the heart of Merlin is of the pintle type that was first used in the Apollo Lunar Module landing engine (LMDE).

The initial version, the Merlin 1A, used an inexpensive, expendable, ablatively cooled carbon-fiber-reinforced polymer composite nozzle and produced 340 kN (76,000 lbf) of thrust.

The Merlin 1A turbopump used a unique friction-welded main shaft, with Inconel 718 ends and an integral aluminum RP-1 impeller in the middle.

[11] The turbine upgrade was accomplished by adding additional nozzles, turning the previously partial-admission design to full admission.

This engine differs from the Falcon 9 first-stage variant in that it uses a larger exhaust nozzle optimized for vacuum operation and can be throttled between 60% and 100%.

It was fired with a full mission duty firing of 170 seconds in November 2007,[12] first flew on a mission in August 2008,[15] powered the "first privately-developed liquid-fueled rocket to successfully reach orbit", Falcon 1 Flight 4, in September 2008,[15] and powered the Falcon 9 on its maiden flight in June 2010.

[17] The Merlin 1C chamber and nozzle are cooled regeneratively by 45 kg (100 lb) per second of kerosene flow and are able to absorb 10 MW (13,000 hp) of heat energy.

In discussing the failure, Elon Musk noted: "The flight of our first stage, with the new Merlin 1C engine that will be used in Falcon 9, was picture perfect.

SpaceX's internal review found that the engine was shut down after a sudden pressure loss and that only the aerodynamic shell was destroyed, generating the debris seen in the video; the engine did not explode, as SpaceX ground control continued to receive data from it throughout the flight.

The primary mission was unaffected by the anomaly due to the nominal operation of the remaining eight engines and an onboard readjustment of the flight trajectory,[21] but the secondary-mission payload failed to reach its target orbit due to safety protocols in place to prevent collisions with the ISS.

Shortly before the scheduled second flight of the Falcon 9, two cracks were discovered in the 2.7-meter-long (9 ft) niobium-alloy-sheet nozzle of the Merlin Vacuum engine.

The modified engine successfully placed the second stage into an orbit of 11,000 km (6,800 mi) altitude.

[30] On November 24, 2013, Elon Musk stated that the engine was actually operating at 85% of its potential, and they anticipated to be able to increase the sea-level thrust to about 730 kN (165,000 lbf) and a thrust-to-weight ratio of 180.

In May 2018, ahead of the first flight of Falcon 9 Block 5, SpaceX announced that the 845 kN (190,000 lbf) goal had been achieved.

[33] The March 18, 2020, launch of Starlink satellites on board a Falcon 9 experienced an early engine shutdown on ascent.

In a subsequent investigation SpaceX found that isopropyl alcohol, used as cleaning fluid, was trapped and ignited, causing the engine to be shut down.

[34][35][36] On October 2, 2020, the launch of a GPS-III satellite was aborted at T-2 seconds due to a detected early startup on 2 of the 9 engines on the first stage.

[37] On February 16, 2021, on Falcon 9 flight 108 launching Starlink satellites, an engine shut down early due to hot exhaust gasses passing through a damaged heat-shielding cover.

[41] The increase is due to the greater expansion ratio afforded by operating in vacuum, now 165:1 using an updated nozzle extension.

[42] Transporter-7 mission launch debuted a new Merlin Vacuum engine (MVac for short) nozzle extension design or variant aimed at increasing cadence and reducing costs.

[2] By September 2013, SpaceX total manufacturing space had increased to nearly 93,000 square meters (1 million square feet), 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.

At the American Institute of Aeronautics and Astronautics Joint Propulsion conference on July 30, 2010, SpaceX McGregor rocket development facility director Tom Markusic shared some information from the initial stages of planning for a new engine.

SpaceX’s Merlin 2 LOX/RP-1-fueled engine on a gas-generator cycle, capable of a projected 7,600 kN (1,700,000 lbf) of thrust at sea level and 8,500 kN (1,920,000 lbf) in a vacuum and would provide the power for conceptual super-heavy-lift launch vehicles from SpaceX, which Markusic dubbed Falcon X and Falcon XX.

At the 2011 Joint Propulsion Conference, Elon Musk stated that SpaceX were instead working towards a potential staged cycle engine.

[59] Since the initial announcement of Raptor, Musk has updated the specification to approximately 230 tonnes-force (2,300 kN; 510,000 lbf)—about one-third the original published figure—based on the results of optimizing for thrust-to-weight ratio.