Space/Engineering
The F-1 Engine: Engineering Marvels of the Engine That Powered the Saturn 5 Moon Rocket
Examining what exactly made the F-1 engine one of the most powerful ever built.
“We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard.”- John F. Kennedy.
When President Kennedy uttered these immortal words at Rice University on September 12, 1962¹, he commissioned not only NASA but the entire nation to accomplish the seemingly impossible; get to the Moon. To do this, people needed to think of things that had not yet been thought of and invent things that had not yet been invented. Over the course of the next seven years, the tireless work of thousands of people would culminate in the largest and most powerful rocket that has ever launched: The Saturn 5.
Standing at 363 feet tall, and weighing in at roughly 6.1 million pounds when fully fueled, the Saturn 5 needed some extremely powerful engines to get it off the ground and give it enough velocity to get to the Moon². The first stage of the launch vehicle was equipped with five F-1 engines, manufactured by Rocketdyne² (hence the name “Saturn 5”). Each engine produced 1.5 million pounds of thrust, for a total of 7.5 million pounds of thrust produced at lift-off²; making them the most powerful single-chamber rocket engines ever built². To understand what made the F-1 engines so special, we must first have a basic understanding of how liquid-propelled rocket engines work.
The general premise of a rocket engine is based on Newton's Third Law³, which states that for every action; there is an equal and opposite reaction. If we push an object one way, that object will exert a force on us that is equal and opposite to the force we induced on it. So if we ignite the fuel, speed it up to some velocity, and push it out of our engine; the same force that the fuel is leaving with is being exerted back on the rocket giving it motion, this force is called thrust³. To do this involves a complex system in which we will only delve into a few key aspects. There are multiple different types of engines and engine cycles; for purposes of the F-1, we will only be looking at the Gas Generator, or Open, cycle.
Powering the mighty F-1’s was a mixture of RP-1 (a type of refined Kerosene, Rocket Propellant 1) as fuel, and Liquid Oxygen (LOx) as the oxidizer⁴. The oxidizer serves the role of, well, providing oxygen (hence the name) in order to facilitate the combustion of the fuel⁵. From the tanks, we need a high-pressure system that will get the fuel and oxidizer to the combustion chamber so we can ignite it, this is where the turbopumps come in. They are called turbo-pumps because a small turbine is used to power them⁶. Now, we need something to drive the turbine that will then power the pumps. Can we use simple electric motors? We could, but that requires A LOT of power which would need to be provided by A LOT of motors in order to fulfil the energy requirements of an orbital rocket. That energy would need to be stored in batteries; those batteries, as well as their motors, would take up space and increase mass to a degree that is simply not feasible⁷. So, engineers came up with something called the Gas-Generator (or Pre-burner). This is essentially a small rocket engine that uses some of the main fuel and oxidizer. The exhaust from this area is really really hot gas that is used to spin the turbine, which then powers the turbopumps, which then transport the fuel and oxidizer to the combustion chamber⁷.
The reason why a Gas-Generator Cycle engine is also called an Open Cycle engine is that normally, the exhaust from the gas-generator after being used to spin the turbine is “dumped overboard” out of the system because it is too “sooty” to be used in the combustion chamber⁷. Soot is a black, tar-looking substance that is formed as a result of the “incomplete combustion of hydrocarbons”⁸. Kerosene is very much composed of hydrocarbons⁹, but the reason this “sooty” exhaust is produced from the gas generator and not in the main combustion chamber, is because the gas generator is run “fuel-rich”¹⁰.
The gas generator does not utilize the same cooling techniques as the main combustion chamber, so it cannot be run at the same fuel-to-oxidizer ratio which produces the most optimal fuel efficiency and thrust¹⁰. Therefore, running the gas-generator “fuel-rich”, meaning using a much larger amount of fuel compared to the oxidizer, will keep the temperatures inside at a reasonable operating level, and produce the hot gas exhaust needed to spin the turbine¹⁰.
However, this means that all the fuel that hasn’t been completely burned in the gas generator is essentially wasted as it is being “dumped” out of the engine⁷. The brilliant engineers who were constructing the F-1 did design an exhaust manifold that leads from the gas generator into the engine nozzle extension. So, the unburned fuel in the exhaust from the gas generator, is now pumped into the exhaust of already-ignited fuel from the main combustion chamber. This performed something called “Film Cooling” which was one of the two cooling methods used for the F-1 engine to ensure it didn’t melt under the extreme temperatures¹¹.
The F-1 engine utilized two methods of cooling: Regenerative Cooling, and Film Cooling⁴. Temperatures inside the combustion chamber/nozzle of the engine reached up to 6000°F¹²; roughly half as hot as the surface of the Sun which sits at around 10,000°F¹³. To, ensure the engine did not melt under extreme heat, Regenerative cooling was used in the body of the Thrust (Combustion) Chamber, and Film Cooling was used in the nozzle extension⁴. The Second Law of Thermodynamics essentially states that, naturally, heat will ALWAYS flow from areas of higher temperature to areas of lower temperature¹⁴. The process of regenerative cooling utilizes this fact by pumping the much cooler fuel through a series of tubes that make up the walls of the combustion chamber/nozzle before it gets to the combustion chamber¹¹. Once that fuel is ignited and begins to flow out of the nozzle, the cooler fuel that is still flowing along the walls of the chamber draws heat away from the insanely hot exhaust; keeping the temperatures at the proper operating level¹¹. The tubes were made of a Nickel-alloy called Inconel-X750, which performed extremely well at very high temperatures¹⁵.
The nozzle extension of the engine was cooled using the method of film cooling⁴. This method involves injecting a cooler fluid, typically fuel, in between the walls of the nozzle and the exhaust from the combustion chamber. This prevents the intense heat of the exhaust from melting the nozzle by forming an insulative barrier between the two. There are two types of film cooling: Liquid, and Gas¹¹. We already know that the F-1 pumped the gas-generator exhaust into the nozzle extension to help cool it. What we don’t know is how exactly it did this. Remember what we said before about the exhaust from the gas generator? How it was really “Sooty” because it consisted of a lot of unburned fuel? Some might think “Well why can’t we just pump this into the combustion chamber and burn the rest of that fuel?”
The converging-diverging shape of a rocket engine nozzle is shaped this way for a reason. Pressure, like temperature, ALWAYS flows from areas of high pressure to areas of lower pressure¹⁶. As the area of the nozzle decreases at the throat; the pressure and velocity of the exhaust increase, yet as the area of the nozzle increases further down the nozzle; the exhaust temperature and pressure decrease¹¹. So, we cannot pump the exhaust from the gas generator directly into the combustion chamber, because not only is it at a lower pressure than the main exhaust; it would cause that exhaust it to go back and up through the engine (causing a very big boom), but the soot would stick to the fuel injectors and cause the engine to fail⁷. We need to pump the pre-burner exhaust into a place where it will be at a higher pressure than the main combustion exhaust, and where it can successfully withstand the temperature¹¹.
The brilliant engineers who figured this out determined that regenerative cooling could be stopped at some length down the nozzle where the lower pressures and temperatures existed, and film cooling via the gas generator exhaust could start. When injected, the pre-burner exhaust forms an insulative barrier between the main exhaust and the walls of the nozzle; due to it being so “sooty”¹¹. So, a disadvantage in one area was turned into a huge advantage in another.
The F-1 engines that powered the mighty Saturn 5 Rocket were truly marvels of engineering and were the culmination of the hard work of thousands of incredible individuals. They were undoubtedly the workhorse of the Apollo Program, and enabled humanity to accomplish the “impossible”.
References
[1] “Address at Rice University on the Nation’s Space Effort.” Address at Rice University on the Nation’s Space Effort | JFK Library, https://www.jfklibrary.org/learn/about-jfk/historic-speeches/address-at-rice-university-on-the-nations-space-effort.
[2] “Rocket Engine, Liquid Fuel, F-1.” Homepage, https://airandspace.si.edu/collection-objects/rocket-engine-liquid-fuel-f-1/nasm_A19700271000.
[3] “Liquid Rocket Engine.” NASA, NASA, https://www.grc.nasa.gov/www/k-12/rocket/lrockth.html.
[4] Fey, Tom. “One Second in the Life of the Rocketdyne F-1 Rocket Engine.” Rocket Propulsion Evolution 8.13, 23 June 2021, https://www.enginehistory.org/Rockets/RPE08.11/OneSecond.shtml.
[5] Ashish, Jauhari, and Kandasubramanian Balasubramanian. “Effect of Ammonium Perchlorate Particle Size on Flow, Ballistic, and Mechanical Properties of Composite Propellant.” Oxidizer — an Overview | ScienceDirect Topics, 2019, https://www.sciencedirect.com/topics/engineering/oxidizer#:~:text=Chemically%2C%20an%20oxidizer%20accepts%20electrons,is%20prevalent%20in%20the%20atmosphere.
[6] Weagly Last Modified Date: October 25, Jordan. “What Is a Turbopump?” About Mechanics, 25 Oct. 2022, https://www.aboutmechanics.com/what-is-a-turbopump.htm.
[7] Dodd, Tim, director. Rocket Engine Cycles: How Do You Power a Rocket Engine? YouTube, The Everyday Astronaut, 28 Apr. 2022, https://youtu.be/Owji-ukVt9M. Accessed 12 Nov. 2022.
[8] “Soot.” Wikipedia, Wikimedia Foundation, 5 Sept. 2022, https://en.wikipedia.org/wiki/Soot.
[9] Gad, S.C., and T Pham. “Kerosene.” Kerosene — an Overview | ScienceDirect Topics, 2014, https://www.sciencedirect.com/topics/medicine-and-dentistry/kerosene#:~:text=Kerosene%20is%20composed%20of%20aliphatic,and%20benzene%20and%20naphthalene%20derivatives.
[10] Blanche, Fran, director. Insane Engineering Of The Saturn F-1 Engine. YouTube, Fran Blanche, 7 Oct. 2020, https://youtu.be/Z37MdvcSaFY. Accessed 12 Nov. 2022.
[11] Dodd, Tim, director. Why Don’t Rocket Engines Melt? How Engineers Keep Engines Cool. YouTube, The Everyday Astronaut, 13 Jan. 2022, https://youtu.be/he_BL6Q5u1Y. Accessed 12 Nov. 2022.
[12] “Welcome to How Things Fly.” | How Things Fly, 15 Sept. 2014, https://howthingsfly.si.edu/ask-an-explainer/what-partcomponent-rocket-engine-reaches-highest-temperature-during-operation#:~:text=A%3A,C%20(6%2C000%C2%B0F).
[13] “Our Sun.” NASA, NASA, 15 Oct. 2021, https://solarsystem.nasa.gov/solar-system/sun/in-depth/.
[14] “Second Law of Thermodynamics.” Wikipedia, Wikimedia Foundation, 10 Nov. 2022, https://en.wikipedia.org/wiki/Second_law_of_thermodynamics.
[15] “Rocketdyne F-1.” Wikipedia, Wikimedia Foundation, 11 Oct. 2022, https://en.wikipedia.org/wiki/Rocketdyne_F-1.
[16]“Why Does Wind Blow?” NOAA SciJinks — All About Weather, https://scijinks.gov/wind/#:~:text=Gases%20move%20from%20high%2Dpressure,is%20the%20wind%20we%20experience.
