The Merlin engine is a rocket engine developed by SpaceX that uses a combination of rocket-grade kerosene and liquid oxygen as fuel. It is designed to provide high thrust and reliability for use in the company's Falcon 9 and Falcon Heavy rockets.

The Merlin 1D engine represents a significant improvement over its predecessor, Merlin 1C. It features a number of enhancements that make it more powerful, reliable, and efficient. The engine produces 190,000 pounds of thrust at sea level and 210,000 pounds of thrust in vacuum, which is more than 50% greater than the Merlin 1C engine.

The improved performance of the Merlin 1D engine is due to a number of factors. It has a higher chamber pressure, which allows for more efficient combustion and higher thrust. It also has an expanded nozzle, which allows for better expansion of exhaust gasses and higher thrust in vacuum. In addition, the engine features a more advanced turbopump that can deliver more fuel and oxidizer to the combustion chamber at a faster rate.

Development

SpaceX began development of Merlin 1D between 2011 and 2012. The design goals for Merlin 1D were increased reliability, improved performance and improved manufacturability. Of which all primary design goals were met, if not exceeded.

First Flight

CASSIOPE launch | Credit: SpaceX

SpaceX's first launch of the newly improved Merlin 1D engine was during the maiden flight of Falcon 9 v1.1 with the CASSIOPE mission on September 19th, 2013 from SLC-4E at Vandenberg Space Force Base (then Air Force Base).

The flight was a complete success with the CASSIOPE being deployed in its target orbit.

Testing

Merlin 1D static fire test | Credit: SpaceX

SpaceX tests every Merlin before it is fitted to a Falcon 9 or Falcon Heavy rocket. This is done at the company’s facility in McGregor, TX. Each Merlin undergoes a static fire test, typically lasting 142 seconds (slightly below the full duration of a Falcon 9 first stage burn). SpaceX tests many Merlin engines each week to keep up with the ever growing launch demand for Falcon 9 and Falcon Heavy.

SpaceX’s rigorous testing of Merlin, especially during early development, played a large role in the reliability of the engine and the rockets it powers.

Reusability

Falcon 9 returns to Earth after successful flight | Credit: SpaceX

Reusability was a major factor during development of Merlin 1D. The engine is capable of multiple restarts, making it suitable for multiple missions. In addition, SpaceX recovers their first stage of their Falcon 9 and Falcon Heavy rockets where possible. This involves landing the first stage on a landing pad or drone ship after it separates from the second stage. 

By reusing the first stage and the engines, SpaceX can reduce the cost of each launch and make access to space more ‘affordable’. 

While reusability is a driving force to lowering costs, it also plays a factor in the incredible reliability of the Falcon family. When reused, Falcon first stages gain flight heritage which reduces risk of component failure.

Variants

Merlin 1D

Merlin 1D was used exclusively on the Falcon 9 Block 2 (V1.1). It had a sea level thrust of 654 kN, a vacuum thrust of 742 kN, an ISP at sea level of 282 seconds and an ISP in the vacuum of space of 320 seconds.

Merlin 1D+

The first major upgrade was called Merlin 1D+. This Merlin 1D version was used on Falcon 9 Block 3 (V1.2). It had a sea level thrust of 845 kN, a vacuum thrust of 914 kN, an ISP at sea level of 288 seconds and an ISP in the vacuum of space of 312 seconds.

Merlin 1D++ (Current)

The last major Merlin 1D upgrade was called Merlin 1D++. Merlin 1D++ was used on Falcon 9 Block 4 & 5. It has a sea level thrust of ~990 kN, a vacuum thrust of ~980 kN, an ISP at sea level of 290 seconds and an ISP in the vacuum of space of 314 seconds.

The listed stats here aren't the only things to change over time. Other changed stats include the propellant mix ratios, chamber pressure, and pressure expansion rate.

Merlin 1D Vacuum is a vacuum-optimised version of SpaceX's Merlin 1D engine used on the Falcon 9 and Falcon Heavy second stages. It's often referred to as MVac.