Merlin Engines

Merlin Engines

Photo of nine Merlin 1D engines firing on Falcon 9 launch 2018; credit: SpaceX

Merlin is a family of rocket engines developed by SpaceX. They are currently a part of the Falcon 9 and Falcon Heavy launch vehicles, and were formerly used on the Falcon 1. 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, but since 2016 the entire Falcon 9 booster is recovered for reuse by landing vertically on a landing pad using one of its nine Merlin engines.

Merlin 1D sea level and vacuum; source: https://everydayastronaut.com/

Engine Types

The Merlin engine family includes several variants, each tailored for specific applications:

Merlin 1A: The initial version of the engine, used in the early Falcon 1 rockets.

Merlin 1C: An upgraded version with increased thrust and improved efficiency, used in later Falcon 1 and early Falcon 9 rockets.

Merlin 1D: The current workhorse engine, featuring further enhancements in thrust, reliability, and manufacturability. Used in Falcon 9 and Falcon Heavy rockets.

Merlin Vacuum: A variant optimized for operation in the vacuum of space, with an extended nozzle for improved efficiency. Used as the upper stage engine on Falcon 9 and Falcon Heavy.

Specifications and Features Summary

Thrust, vacuum: 914 kN

Thrust, sea-level: 845 kN

Specific impulse (vacuum): 311 seconds

Specific impulse (sea level): 282 seconds

Weight: 470 kg

Fuel: RP-1 (highly refined kerosene)

Oxidizer: Liquid oxygen (LOX)

Step by step:

Merlin 1A

The first Merlin engine used an expendable ablatively cooled carbon-fiber-reinforced polymer composite nozzle and produced 340kN of thrust.

This engine has flown only two times. First on March 24, 2006, and second on March 21, 2007. The first attempt resulted in a fire and fuel leak soon after launch. The second attempt was a success. Both engines were attached to a Falcon 1 rocket.

Merlin 1B

The second engine, the Merlin 1B was intended for the Falcon 1 launch vehicle capable of producing 380 kN of thrust at sea level, and 420 kN in vacuum, and performing with a specific impulse of 261 seconds at sea level and 303 seconds in vacuum. 

It proceed from 1A’s torch ignition to pyrophoric ignition. After initial testing, SpaceX decided to focus on development of the Merlin 1C. 

Merlin 1C

Using the same turbopump developed for Merlin 1B, this regeneratively cooled engine was launched in 2007 and flew its first successful mission, in the Falcon 1 in 2008 making it the first privately-developed liquid-fueled rocket to successfully reach orbit.

Three versions of the Merlin 1C engine were produced. The Merlin engine for Falcon 1 had a movable turbopump exhaust assembly, which was used to provide roll control by vectoring the exhaust. The Merlin 1C engine for the Falcon 9 first stage is nearly identical to the variant used for the Falcon 1, although the turbopump exhaust assembly is not movable. 

Finally, a Merlin 1C vacuum variant is used on the Falcon 9 second stage. 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%.

Merlin 1C Vacuum engine at Hawthorne factory in 2008, source: Wiki

Merlin Vacuum (1C)

A variant of the Merlin 1C Vacuum engine features a larger exhaust section and a significantly larger expansion nozzle to maximize the engine's efficiency in the vacuum of space. Its combustion chamber is regeneratively cooled, while the 2.7-meter-long niobium alloy expansion nozzle is radiatively cooled. The engine delivers a vacuum thrust of 411 kN and a vacuum specific impulse of 342 s (3.35 km/s). 

The first production Merlin Vacuum engine underwent a full-duration orbital-insertion firing (329 seconds) of the integrated Falcon 9 second stage on January 2, 2010. It was flown on the second stage for the inaugural Falcon 9 flight on June 4, 2010.

Merlin 1D

The Merlin 1D engine was developed by SpaceX between 2011 and 2012, with its first flight in 2013. In 2011, performance goals for the engine were a vacuum thrust of 690 kN  a vacuum specific impulse (Isp) of 310 s (3.0 km/s), an expansion ratio of 16 (as opposed to the previous 14.5 of the Merlin 1C) and chamber pressure in the "sweet spot" of 9.7 MPa. 

Merlin 1D was originally designed to throttle between 100% and 70% of maximal thrust; however, further refinements since 2013 now allow the engine to throttle to 40%.

The basic Merlin fuel/oxidizer mixture ratio is controlled by the sizing of the propellant supply tubes to each engine, with only a small amount of the total flow trimmed out by a "servo-motor-controlled butterfly valve" to provide fine control of the mixture ratio.

Merlin 1D Vacuum

A vacuum version of the Merlin 1D engine was developed for the Falcon 9 v1.1 and the Falcon Heavy second stage. As of 2020, the thrust of the Merlin 1D Vacuum is 981 kN with a specific impulse of 348 seconds. The increase is due to the greater expansion ratio afforded by operating in vacuum, now 165:1 using an updated nozzle extension.

The engine can throttle down to 39% of its maximum thrust, or 360 kN.

As SpaceX continues to push the boundaries of spaceflight, the Merlin engine is expected to evolve further. The company’s focus on the continuous improvement and innovation of the engine suggests that future versions of the Merlin engine may feature even higher thrust, improved efficiency, and enhanced reusability. In addition, it seems the the development of the Merlin engines will influence the design of the Raptor engine, which powers the Starship spacecraft. 

The Raptor engine uses methane and liquid oxygen as propellants, represents a significant difference from the kerosene-fueled Merlin. However, many of the design principles and techniques developed for the Merlin should be expected to be applied to the Raptor. Let's study more. 

Why Nozzles On Vacuum Optimized Rocket Engines Are Bigger Than Those On Sea Level Engines?

A little-known fact about rockets is the size of their second or upper-stage engine nozzles, which are substantially bigger than those used by sea-level engines. And there is a good reason for this difference in size.

The rocket engine nozzles visible below any launch vehicle during liftoff, whether one or multiple nozzles, are all part of the first-stage section of a rocket. These engine nozzles are all optimized to operate low in the atmosphere at high ambient pressures.

Hidden up and below the surface structure of a rocket is placed the second-stage rocket engine, which is connected to a much larger nozzle. And this nozzle is optimized to operate in the upper atmosphere as well as the vacuum.

Basically it is important to say, that it is essential for a rocket engine to operate at its most efficient level. It is significant for the hot gases exiting the engine nozzle to match the external air or ambient pressure. It is quite difficult as the rocket is travelling through the Earth's atmosphere, as the air pressure is decreases significantly from the sea level to space. 

Luckily for us, the principles of fluid dynamics teaches us that the pressure of rocket exhaust gases decreases as the surface area of a nozzle increases without a loss in velocity. This means that the bigger (or more expanded) a rocket nozzle is, the lower the gas pressure. As a result, the first-stage engine nozzles of a rocket which are visible under the rocket during the launch are smaller than the upper-stage nozzles, which are significantly bigger to allow the pressure of the gases exiting the nozzles to decrease enough to match ambient pressures. Second-stage rocket booster typically deploys at a height of about 50 to 80 km above the Earth’s surface. At this altitude, the air pressure is 0.1% or less than the air pressure at sea level.

Diagram showing the different effects an under-expanded, ambient, and overexpanded nozzle has on the way a rocket engine’s exhaust plumes interact with the external air pressure. source: https://headedforspace.com/

As the diagram above illustrates, a nozzle can be in one of three states, depending on nozzle size and the external air pressure at a given altitude:

Under-Expanded Nozzle

Ambient Nozzle

Overexpanded Nozzle

An Under-Expanded Nozzle produces exhaust gases with a higher pressure than the ambient air, resulting in the exhaust plumes extending beyond the edge of the nozzle, which decreases efficiency. Vacuum-optimized nozzles typically display this characteristic.

A Saturn V rocket’s nozzles became under-expanded at high altitude; Source: https://headedforspace.com

An Ambient Nozzle produces exhaust gases with a pressure equal to the ambient air, resulting in the exhaust plumes extending in a relatively uniform fashion, which allows the engine to operate at maximum efficiency.

An Over-expanded Nozzle produces exhaust gases with a lower pressure than the ambient air, resulting in the exhaust plumes being compressed by the external air pressure, which decreases efficiency. Many sea-level optimized nozzles display this characteristic.

Naturally it is really impossible for an engine nozzle to match the external air pressure throughout its ascent, it is clear that keeping the exhaust gas pressure as close as possible to the ambient pressure results in the most efficient engine operation, producing the most efficient thrust.