How Do SpaceX’s Merlin Rocket Engines Work?

SpaceX’s Falcon 9 rocket lifts off from LC-39A at the Kennedy Space Center in Florida Photo: SpaceX

The force required to propel a giant 1.3 million pound rocket into space, defying the laws of gravity is quite incredible, and an engineering feat like no other. The raw power of the 200 foot jet of fire roaring from the bottom of the vehicle is mesmerizing. And with their destination being the final frontier of outer space, rockets are the most spectacular and beautiful machines ever created.

The most common rocket engine right now is the chemical type although one day, hopefully in the not too distant future, it will be replaced by more environmentally friendly propulsion systems. Since the beginning of 2020 alone, Space X has launched seventeen Falcon 9 missions with fourteen successful booster landings for a total of 169 chemical rocket engines being used! Aside from SpaceX, numerous other companies have launched chemical rockets such as United Launch Alliance (ULA) and Northrup Grumman. Clearly, the use of chemical rockets is in full swing, but how exactly do they work, and what makes them different from the engine powering your car?

A SpaceX Falcon 9 first stage booster returns to earth to perform a powered landing Photo: The Verge

To understand the differences between rocket engines and your everyday internal combustion engine in a car, first, you might need a general understanding of the principles behind an internal combustion engine here on earth. Combustion occurs when some kind of fuel mixes with oxygen and ignites, causing an explosion that releases energy. In a car engine, fuel vapor is mixed with air in a combustion chamber and then ignited by a spark. The force from this mini-explosion pushes down a piston, which turns a shaft that spins the wheels. In actuality, the mechanics of a car engine are more complicated than that, but for the purpose of this article, all you need to understand is fuel + air + spark = explosion = energy. The key here is the oxygen in the air because for combustion to occur there has to be an adequate presence of oxygen. As a rocket enters the upper sections of the atmosphere and eventually the vacuum of space, there is no oxygen. This means that rockets have to carry their own oxidizer (liquid oxygen) in addition to fuel.

Nine Merlin engines at the base of SpaceX’s Falcon 9 Rocket Photo: Spaceflightnow

In this article, I am going to focus on the SpaceX Falcon 9 rocket and the Merlin 1D chemical engines that power the first stage of the Falcon 9 rocket. Although other companies make and use chemical rockets, Space X, a silicon-valley type aerospace company, is the most exciting and has been shaking up the rocket launching industry since billionaire Elon Musk founded the company in 2002. With the lofty goal of sending humans to Mars, SpaceX is the only commercial company to send astronauts into space and to the International Space Station. It continues to challenge longtime government contractors such as ULA by providing more reliable and cheaper rockets, the Falcon 9 being its main workhorse rocket. SpaceX achieves a smaller price tag because of what is perhaps their greatest achievement to date: the first stage of the Falcon 9 rocket is reusable as it is capable of landing autonomously either on land or on a drone ship stationed in the ocean after it sends the second stage on its way and re-enters the atmosphere.

Falcon 9 vertical on SLC-40 at Cape Canaveral Air-Force Station Photo: SpaceflightNow

So how does the Merlin rocket engine in the Falcon 9 work?

The Merlin engine is a gas-generator open-cycle engine. This means that the propellant is burned in a gas generator and the resultant hot air is used to power the engine’s pumps. The gas is then exhausted. Because something is “thrown away” this type of engine is also known as an open cycle.

As mentioned before, oxygen is required for combustion to occur, so rockets must carry it with them. Merlin engines use a turbopump assembly to feed oxygen (liquid oxygen) and fuel (ultra-refined kerosene or RP1) into the combustion chamber. Both are stored in separate tanks in the first and second stages of the rocket. The turbopump assembly consists of a preburner, a turbine, a shaft, and two pumps.

Diagram of an open-cycle gas generator engine by Tim Dodd

The preburner is essentially a miniature rocket engine. The exhaust from the preburner spins the turbine. The turbine is connected to the shaft which spins the two pumps which regulate and pump fuel and oxidizer into the combustion chamber.

As you can see from the diagram, the preburner uses the same fuel and oxidizer as the main engine and is fed by two smaller pipes of fuel and oxygen that are connected to the pumps. Because the preburner powers the pumps and the pumps control the fuel and oxidizer flow to the preburner, you may be thinking how does the engine start if each component requires the other in order to work?

To solve this problem of ignition, the Falcon 9 carries a small amount of triethylaluminium-triethyl borane (that’s quite a mouthful!), a chemical compound that ignites spontaneously when it comes into contact with any form of oxygen. The Merlin engines feed a small amount of this compound and liquid oxygen into the preburner using a separate pipe system. (not seen in the simplistic diagram above) The compound spontaneously ignites, causing the turbine to spin. The pumps start flowing and the cycle begins, resulting in a constant flow of fuel and liquid oxygen to the combustion chamber. When fuel and oxygen reach the combustion chamber, a similar system is used to ignite them. Once ignited, constant combustion is sustained due to the constant flow of fuel and oxygen. The combustion of a Merlin engine produces 190,000 pound-feet of thrust. The exhaust from the preburner is expelled using a separate and much smaller nozzle.

Because the ignition sequence is so complicated, SpaceX begins it at T-minus 2 seconds to allow time for the engines to start. On the Saturn V moon rocket, the system was so complicated, it took a little more than 6 seconds for the engines to arrive at maximum power!

SpaceX’s Merlin 1D Engine being tested at their facility in Texas. Note the black exhaust on the right which is coming from the preburner. Image from Wikipedia

Once the first stage has completed its burn, it is released and heads back to earth to attempt a landing using the center 1 of the 9 Merlin engines to re-enter the atmosphere and touch down. The M Vac engine (not described here) on the second stage ignites and carries the payload of the rocket to its desired orbit.

Key Takeaways:

  1. Combustions is the process by which fuel and oxygen mix together and are ignited by some type of spark
  2. On earth, combustion can occur with just fuel because of the oxygen already present in the atmosphere. Because there is no air in space, rockets must carry their own oxygen as well as fuel
  3. Gas-generator or open-cycle engines such as SpaceX’s Merlin engines use a turbopump assembly consisting of a preburner, a turbine, and two pumps
  4. The preburner is a mini rocket engine that uses a portion of the fuel and oxidizer mixture to spin the turbine and deliver rocket fuel and oxidizer into the combustion chamber by controlling the pumps
  5. Because the preburner powers the pumps but the pumps control the flow of fuel and oxidizer into the preburner, Merlin engines use a separate chemical to ignite the preburner.

Sources: How Do SpaceX’s Merlin Engines Work: Sources