Falcon 9 is powered by Merlin engine. It has the highest thrust to weight ratio of any boost engine ever made, while still maintaining the structural and thermal safety margins needed to carry astronauts. Merlin engines use RP-1 and liquid oxygen as rocket propellants in a gas-generator power cycle. The Merlin engine was originally designed for sea recovery and reuse. Propellants are fed via a single shaft, dual impeller turbopump. The turbopump also provides high-pressure fluid for the hydraulic actuators, which then recycles into the low-pressure inlet. This eliminates the need for a separate hydraulic drive system and means that thrust vectoring control failure by running out of hydraulic fluid is not possible.
Merlin 1A engine was the first among the series. This engine was used in the Falcon 1’s first stage rocket. Hence, it became the base for further development of more advanced and sophisticated Merlin engines. Merlin 1B was developed next, which was an upgraded version of Merlin 1A. It provided higher thrust as compared to Merlin 1A. It was planned to be used on Falcon 9 but, eventually never used as work on Merlin 1C had started.
The Merlin 1C uses a regeneratively cooled nozzle and combustion chamber. The turbopump used is a Merlin 1B model with only slight alterations. A Merlin 1C was first used as part of the unsuccessful third attempt to launch a Falcon 1. 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.” The Merlin 1C was used in the successful fourth flight of Falcon 1 on September 28, 2008.
Merlin 1C has another variant called Merlin Vacuum 1C, it has a larger exhaust and nozzle for increased efficiency in space (as density of air very less).
Merlin 1D is the latest member of Merlin family. It is the first among its types in many aspects. It has almost twice the thrust that of Merlin 1A. It has a distinct feature, to throttle down to 70% of its maximum thrust from 100%. Like Merlin 1C, it has another variant called Merlin Vacuum 1D which can throttle down to 39% of its maximum thrust. The design goals for the new engine included increased reliability (increased fatigue life and increased chamber and nozzle thermal margins), improved performance (thrust design objective 140,000 pounds-force (620 kN) and 70-100 percent throttle capability), and improved manufacturability (lower parts count and fewer labor hours).
Falcon 9 rocket in total has 10 Merlin engines: 9 of them in first stage and 1 in the second-stage. Whereas, Falcon heavy has 28 engines, 9 each in the two side boosters making it to 18 followed by 10 in the main rocket.