*Falcon Heavy launches NASA’s Europa Clipper mission to Jupiter (Credit: SpaceX)*

400 flights. 90% of Earth's annual mass to orbit. And, of course, partially reusable. SpaceX's Falcon family of rockets needs no introduction.

But what did it take to get here?

In this article, we look at the beating heart that powers America's most reliable launcher. Here is a brief technical history of SpaceX's Merlin Engine.

Conception

A rocket's development usually begins with its engine, and the Falcon 1 was no different. And who better to enlist than rocket propulsion wizard Tom Mueller? Having had experience at TRW with building the Low-Cost Pintle Engine (otherwise known as the TR-106), he joined SpaceX in 2002 as a founding employee.

*SpaceX Employee #1: Tom Mueller (Credit: Impulse Space)*

*A TR-106 during a hot-fire on a test stand (Credit: NASA)*

Like most engines, Falcon 1's first stage engine utilised lessons taken from other engine development programs. Most notably, the turbopump assembly (responsible for flowing fuel into the engine's combustion chamber as quickly as possible) was delivered to SpaceX in 2003 by Barber-Nichols, the contractor which helped develop NASA's cancelled low-cost Fastrac engine. It was at this point that Mueller decided to name the engine. Below is Mueller's story (which he posted on Quora) on how it was named “Merlin”:

"When we first started SpaceX we just called our booster engine the 60 K engine, but after we started running it Elon told me to come up with a name for it that wasn’t numbers and letters (like RD-180, RS-68, etc.). One of the people working on the turbopump from Barber Nichols was a Falconer and she suggested we name it after a Falcon. I thought that sounded good so I asked her what are some Falcon names. She named off a bunch and I can’t recall them all but I do remember that the Kestrel is the small one, the Merlin is a medium size Falcon and the Peregrine and Gyrfalcon are large Falcons. I thought great, we’ll name the small second stage engine Kestrel and the medium sized engine the Merlin. Elon liked the naming so it stayed."

Merlin-1A

By 2002, the first of the gas generators (basically a small rocket engine used to drive the Merlin-1A's turbopump) had been built. SpaceX decided to test the gas generators at the Mojave Air and Space Port in California. However, later in 2002 the decision was made to change engine testing locations to McGregor, Texas (still in use today), due to a lack of restrictions at McGregor. The rest of the Merlin-1A was built, blown-up and iterated upon within the next 2 years, with the first turbopump tests occurring in July 2003, and the first complete engine test-firing in July 2004.

On March 24th 2006, the Merlin-1A flew for the first time on Falcon 1 Flight 1, firing for a total of 33 seconds before failing. The Merlin-1A flew its final flight on March 21st 2007 on Falcon 1 Flight 2, where it performed nominally through the first stage burn. However, Flight 2 still failed to reach orbit for reasons unrelated to the performance of the first stage.

*The SpaceX Falcon 1 in flight (Credit: SpaceX)*

The Merlin-1A produced 340kN of thrust at sea level, with a specific impulse of 244 seconds. One notable design feature of the Merlin-1A was that the exhaust of the gas generator was able to move to provide roll control for the Falcon 1, which would otherwise not have been possible due to having only a single engine on the first stage. It also had an ablative thrust chamber, which considerably increased the mass of the engine.

Merlin-1B

Before the first flight of the Falcon 1, SpaceX was already drawing up plans for an upgraded version of the Merlin-1A. Dubbed the Merlin-1B, it was originally slated for SpaceX's unrealised Falcon 5 concept. Changes from the Merlin-1A were minimal: the turbines now produced 2,500 horsepower instead of 2,000, and the gas generator exhaust was now fixed in place (multiple engines on the first stage of Falcon 5 meant that the engines themselves could work together to provide roll control). It also produced 380kN of thrust at sea level (up from 340kN), and had a specific impulse of up to 261 seconds. The most notable change was the move from torch ignition to pyrophoric ignition using triethylaluminum-triethylborane (TEA-TEB). This method of ignition is still used today, and is the source of the characteristic green flash during engine ignition of the current Falcon 9 and Falcon Heavy rockets.

*TEA-TEB being used to ignite a hybrid rocket engine (Credit: NASA/ResearchGate)*

Ultimately, lessons learnt leading up to and after Falcon 1 Flight 1 meant that SpaceX opted to expedite development of the regeneratively-cooled Merlin-1C. Thus, along with the cancellation of Falcon 5, the Merlin-1B never saw flight.

*Falcon 5 concept, with 5 Merlin-1Bs and 2 Kestrel engines (Credit: SpaceX)*

Merlin-1C

As mentioned before, the Merlin-1A had an ablative thrust chamber. SpaceX believed this design choice would be the simpler and faster option for engine development. However, it was extremely finicky and increased the mass of the engine, decreasing its performance. For this reason, after Falcon 1 Flight 1, SpaceX opted to pursue the regeneratively-cooled Merlin-1C over the ablatively-cooled Merlin-1B. Even though the turbopump was taken straight off the Merlin-1B, due to having a regeneratively-cooled thrust chamber capable of absorbing up to 10 MW (13,000hp) of heat energy, the Merlin-1C was capable of producing up to 422kN (up from 380kN) of thrust at sea level, with a specific impulse of 275 seconds.

Three versions of the Merlin-1C were produced: A sea-level variant for Falcon 9's first stage, a vacuum-optimised variant for Falcon 9's second stage, and a version for Falcon 1's first stage. It first flew on Falcon 1 Flight 3 on August 3rd 2008, where it performed perfectly until stage separation. Just 8 weeks later on September 28th, the Merlin-1C successfully powered Falcon 1 Flight 4, the first privately-developed liquid-fuelled rocket to reach orbit. The Merlin-1C would go on to fly on the fifth and final flight of the Falcon 1 and the maiden flight of the Falcon 9. In total, the Merlin-1C would fly 5 flights on Falcon 9 V1.0. Its last flight was CRS-2, on March 1st, 2013.

*Falcon 9 V1.0 lifts off from SLC-40 at Cape Canaveral Air Force Station, FL (Credit: NASA)*

Merlin-1D

The final major iteration of SpaceX's Merlin engine was the Merlin-1D, with development lasting from 2011 to 2012. Initial changes included a simplified engine design, an even lower cost per unit, and an increase in thrust from 422kN to 620kN. It also had the ability to throttle between 70% - 100%, which meant it was no longer necessary to shut down 2 of the 9 first stage engines before stage separation to limit G-forces. Its first flight was on September 29th, 2013, and it was also the first flight of the improved Falcon 9 V1.1 rocket.

SpaceX did not just stop there however. Further improvements meant that by 2016, the Merlin-1D could throttle down to 40% of its rated performance, and output a maximum thrust of 733kN. This upgraded Merlin-1D variant first flew on December 22nd 2015, on the first launch of the Falcon 9 Full Thrust. This was also the launch in which SpaceX successfully performed the first ever propulsive landing of an orbital-class booster.

*The first launch of the Falcon 9 FT & the first ever propulsive vertical booster landing (Credit: SpaceX)*

Finally, by May 2018, it was announced that the Merlin-1D had achieved a thrust output of 845kN, ahead of the first launch of the Falcon 9 Block 5 variant. More recently, SpaceX has employed a short nozzle extension for some Merlin Vacuum engines on Falcon 9 second stages, where the extra performance is not needed. This short nozzle gives 10% less thrust, but requires 75% less niobium metal, so SpaceX can launch over 3 times as many missions with the same amount of niobium. This reduces cost further, making the Falcon family of rockets even more cost effective than ever before.

*Stubby M-Vac engine firing on Falcon 9’s 2nd stage (Credit: SpaceX/Eutelsat)*

Summary

On February 23rd 2024, SpaceX launched a Falcon 9, carrying Starlink group 7-15 to Low-Earth Orbit. But this was no ordinary Starlink Mission. One of the 9 first stage Merlin engines was flying for its 22nd time, making it the single most-flown rocket engine ever, beating out even the most-flown Space Shuttle RS-25s...

Test. Fail. Iterate. From a turbopump that initially generated 2,000 horsepower, to one that now generates 12,000. From an engine that initially output 340kN (75,000lb) of thrust, to one that now outputs over 845kN (190,000lb) of thrust. From an engine that could barely survive a single flight, to an engine that's capable of rapid reuse.

Over the last 2 decades, SpaceX has squeezed out every little bit of performance out of its Merlin engine, simplifying the design as they went. And the results have been nothing short of incredible. It now holds the record for the highest thrust-to-weight ratio of any rocket engine ever built, and has started to fulfil SpaceX's goal of rapid reuse of its Falcon family of rockets.

With the help of Merlin, the Falcons are opening up space for all mankind.