F-1

The F-1 is the rocket engine that lifted humans toward the Moon. To this day it is the most powerful single-combustion-chamber liquid-fuel rocket engine ever flown.

Quick facts

• Builder: Rocketdyne, for NASA’s Marshall Space Flight Center.
• Propellants: liquid oxygen (LOX, the oxidizer that lets the fuel burn) and RP-1 (a refined kerosene, the fuel), in a ratio of about 2.27 parts oxygen to 1 part fuel.
• Thrust: about 1.5 million pounds (roughly 6,770 kilonewtons) at sea level, rising to about 1.75 million pounds in the vacuum of space.
• Size: about 5.6 m (18.5 ft) tall and 3.7 m (12.2 ft) across, with a dry mass near 8,400 kg (18,500 lb).
• Burn time: roughly 150-165 seconds per flight.
• Turbopump power: about 55,000 horsepower (around 41 megawatts) at roughly 5,500 rpm.

What it is and how it works

The F-1 is a liquid-propellant engine, meaning it carries its fuel and oxidizer as liquids and mixes them as it runs. It uses a “gas-generator cycle.” A small amount of the LOX and kerosene is burned in a separate chamber called a gas generator. That burst of hot gas spins a turbine, a kind of pinwheel, which drives a turbopump. The turbopump force-feeds the main propellants into the combustion chamber at enormous pressure and flow, tens of thousands of gallons every minute. Total propellant flow reached about 2,577 kilograms per second.

Inside the chamber the LOX and kerosene burn, and the hot gas is squeezed and accelerated out through a bell-shaped nozzle to produce thrust, much like a garden hose throwing water harder when you narrow the opening. Keeping the engine from melting took two tricks. The upper section was “regeneratively cooled,” with fuel flowing through tubes in the walls before it burned, carrying heat away. The lower nozzle extension was film-cooled: the cooler, fuel-rich exhaust from the turbine was poured down the inside surface as a protective curtain, visible as the darker outer ring of the exhaust plume.

Why it matters

The F-1 made the Apollo Moon landings possible. It delivered the brute force needed to lift the roughly 3,000-tonne Saturn V rocket off the launch pad. Building it pushed engineers into new territory. Their hardest problem was “combustion instability,” violent high-frequency pressure waves inside the chamber that could destroy an engine in fractions of a second. They beat it by redesigning the injector (the part that sprays in propellants) again and again, and by deliberately setting off small bomb charges inside running engines to prove the design could calm any disturbance within about a tenth of a second. That bomb-test method became a landmark engineering practice.

The gas-generator cycle is simpler and tougher than fancier designs, but a little less efficient, because the turbine’s exhaust is dumped overboard rather than fully burned. That gives the F-1 a modest “specific impulse” (a measure of fuel efficiency, like miles per gallon for rockets) of about 263 seconds at sea level. Kerosene is dense and easy to handle, allowing compact tanks and huge thrust, which makes the F-1 a high-thrust first-stage workhorse rather than an efficient upper-stage engine.

Where it was used

Five F-1 engines powered the first stage of the Saturn V, known as the S-IC. Clustered together they produced about 7.5 million pounds of liftoff thrust. On the rocket, the four outer engines could pivot (gimbal) to steer in pitch, yaw, and roll, while the center engine stayed fixed.

• Apollo 4 (Nov 9, 1967): first flight of the F-1-powered Saturn V, an uncrewed test.
• Apollo 11 (1969): launched the first crewed lunar landing; F-1 engines powered all six successful Apollo Moon landings through 1972.
• Skylab (May 14, 1973): the engine’s final flight, lofting the Skylab space station.

Across 13 Saturn V launches, 65 F-1 engines flew with no in-flight engine failures. Uprated and modernized versions were studied later, the F-1A (about 1.8 million pounds of thrust, never flown) and the 21st-century F-1B, but the original’s single-chamber thrust record still stands. Several recovered engines, including some retrieved from the Atlantic Ocean floor, are now preserved in museums such as the Smithsonian National Air and Space Museum.