The Saturn V That Would Send Apollo 11 Toward the Moon

Added: 2026-05-25

[00:00:00]Before Apollo 11 could reach the moon, it first had to ride the most powerful launch vehicle ever approved for a crude lunar mission. That vehicle was the Saturn 5. The Apollo 11 spacecraft would be boosted first into Earth orbit and then sent onto a trajectory toward the moon by the sixth Saturn 5 launch vehicle.

[00:00:24]Standing 281 feet tall, the Saturn 5 was not simply a large rocket. It was an integrated launch system built to lift a fully assembled Apollo spacecraft and its upper stages into space. At liftoff, it generated enough thrust to place a payload of about 125 tons into a 105 nautical mile Earth orbit. And for the lunar mission, it could send roughly 50 tons onward toward the moon. The Saturn 5 was developed by NASA's Marshall Space Flight Center, the organization responsible for turning the launch vehicle from engineering concept into operational hardware. But one of the most important decisions in the Saturn 5 program was not just how the rocket was built. It was how it was tested.

[00:01:18]The Saturn 5 underwent research and development testing in what NASA called the all up mode. That meant the rocket was not tested one live stage at a time over many separate launches. From the very first Saturn 5 launch, all stages were active. Every stage was live. Every major part of the launch vehicle had to work together as a complete system.

[00:01:45]This was a bold approach. It saved time, but it also placed enormous pressure on the engineering teams because the first flights were not partial demonstrations.

[00:01:56]They were full Saturn 5 missions.

[00:02:00]And the result was remarkable. The Saturn 5 was considered man- rated after only two launches.

[00:02:08]The third Saturn 5 known as AS503 carried Apollo 8 and its crew on the first crude mission to lunar orbit. That flight proved that the Saturn 5 was not just capable of reaching Earth orbit. It could send human beings all the way to the moon.

[00:02:29]Before Apollo 11, Saturn 5 rockets had already launched five times. The first launch came on November 9th, 1967.

[00:02:39]The second came on April 4th, 1968.

[00:02:43]Those first two Saturn 5 missions were unmanned.

[00:02:47]Then came December 21st, 1968 when Apollo 8 rode AS503 to lunar orbit. After that, on March 3rd, 1969, Apollo 9 launched on a Saturn 5 for an Earth orbital mission that tested the Apollo spacecraft systems. And on May 18th, 1969, Apollo 10 launched as the final full rehearsal before the first lunar landing attempt.

[00:03:18]So, when Apollo 11 stood on the pad, it was not riding an unproven machine. It was riding the sixth Saturn 5, a launch vehicle whose design had already been tested through unmanned flights, crude Earth orbit operations, and a crude mission around the moon.

[00:03:39]The rocket that would carry Apollo 11 was the product of an aggressive test philosophy, enormous propulsion capability, and a launch record that had moved from first flight to lunar missions in less than 2 years.

[00:03:54]And now that same machine was ready for the mission it had been built for from the beginning to send astronauts toward the first landing on the moon.

[00:04:08]The Saturn 5 was built to carry astronauts to the moon. But it was also built around a hard reality. If something went seriously wrong during powered flight, the first priority was not the rocket. The first priority was the crew.

[00:04:25]During the launch vehicle's powered flight phase, an emergency could develop so quickly that the mission would have to be aborted. In that situation, the command module and its crew had to be removed from immediate danger. That was the purpose of the launch, escape, and abort capability. It gave the crew a way to separate from a failing launch vehicle before the situation became unservivable.

[00:04:51]But once crew safety had been addressed, there was still another problem. The rest of the Saturn 5 might still be flying. It might still be intact, and it might still contain enormous quantities of fuel and oxidizer.

[00:05:06]If that remaining vehicle became a danger to areas downrange, the range safety officer had authority to take further action. This was not about destroying the rocket for its own sake.

[00:05:18]It was about preventing an uncontrolled launch vehicle from becoming a hazard to geographic areas beneath its flight path.

[00:05:26]Each propulsive stage of the Saturn 5 was equipped with a propellant dispersion system. The purpose of that system was to terminate the vehicle's flight in a safe location and disperse the propellants in a controlled way. The design goal was not to create the largest explosion possible. It was the opposite. The propellants were to be dispersed while minimizing the probability of ignition.

[00:05:52]The system used ground commands. First, a transmitted command would shut down all engines. Then, a second command would activate explosive charges that opened the fuel and oxidizer tanks. This allowed the propellants to disperse rather than remain concentrated inside an intact, uncontrolled stage.

[00:06:13]The tank openings were not made randomly on each stage. The cuts were made in non-adjacent areas. That detail mattered. By opening the tanks in separated locations, the system reduced the chance that fuel and oxidizer would immediately mix in the same place. Less mixing meant a lower probability of ignition. So even the destruction system was engineered with restraint. It was not simply a destruct button. It was a safety system designed around sequence, separation, and risk reduction. First, protect the crew. Then, if necessary, stop the vehicle, then disperse the propellants in a way that reduced the chance of ignition.

[00:07:00]And when the system was no longer needed, the stage propellant dispersion systems could be saved by ground command. That meant the range safety system itself was also controlled, monitored, and deliberately managed.

[00:07:16]The Saturn 5 was not only a launch vehicle. It was a machine surrounded by layers of decision logic, crew escape, range protection, engine shutdown, propellant dispersion, and safing commands.

[00:07:32]Every part existed because during a Saturn 5 launch, failure was not allowed to become uncontrolled.

[00:07:40]The mission could be lost. The rocket could be lost, but the crew and the people under the flight path had to be protected first.

[00:07:52]At the bottom of the Saturn 5 was the first stage. NASA called it the S1C.

[00:07:58]This was the stage that had to do the hardest and most violent part of the launch. It had to lift the fully fueled Saturn 5 off the pad, push it through the thick lower atmosphere, and begin the climb toward orbit.

[00:08:12]The S1C produced approximately 7.6 million pounds of thrust. That thrust came from five F1 engines burning rocket propellant 1 commonly known as RP1, a highly refined kerosene together with liquid oxygen.

[00:08:30]The S1C was developed jointly by NASA's Marshall Space Flight Center and the Boeing company. Marshall assembled the first four S1C stages. These included a structural test model, a static test version, and the first two flight stages. After that, the later flight stages were assembled by Boeing at the Masheed assembly facility in New Orleans.

[00:08:54]So, the first stage of Apollo 11 was not just a production item. It came out of a development program that had already built, tested, and flown earlier versions of the same basic machine. The Apollo 11 S1C was also historically important because it was the third flight booster tested at the NASA Mississippi test facility. The first S1C test at Mississippi took place on May 11th, 1967.

[00:09:21]The second was fired on August 9th, 1967.

[00:09:26]Then on August 6th, 1968, the third flight booster tested there was fired.

[00:09:32]That booster was the one used for Apollo 11.

[00:09:36]Earlier flight stages had been static fired at Marshall Space Flight Center, but for Apollo 11, the first stage had gone through the Mississippi test program before being used on the mission.

[00:09:48]Physically, the S1C was enormous. It stood 138 ft high and measured 33 ft in diameter. Its major structural sections were arranged from bottom to top. First came the thrust structure. Above that was the RP1 fuel tank. Then came the intertank structure. Above that was the liquid oxygen tank. And at the top was the forward skirt.

[00:10:16]This was not just a stack of tanks. It was a load carrying structure. At launch, the thrust structure had to accept the force from five F1 engines and feed that load into the rocket body.

[00:10:29]The tanks had to hold massive quantities of propellant while also functioning as part of the vehicle's structure, and the forward skirt had to connect the first stage to the rest of the Saturn 5 above it. Empty, the S1C stage weighed 288,750 lb. Fully fueled, it weighed 5,22,674 lb. That difference tells you what the first stage really was. It was mostly propellant.

[00:11:01]During normal operation, propellant flowed to the five F1 engines at a combined rate of 29,364.5 lb per second. That was equal to about 2,230 gall per second. Every second, the first stage was feeding thousands of gallons of fuel and oxidizer into the engines.

[00:11:24]And those engines had to burn it smoothly, evenly, and predictably while the rocket was accelerating, shaking, bending, and climbing away from the pad.

[00:11:35]The engine arrangement was simple to describe, but extremely powerful. Four F1 engines were mounted around the outside on a ring, spaced at 90° intervals. These four outer engines could gimble. That meant they could pivot slightly to control the rocket's direction of flight.

[00:11:54]The fifth F1 engine was mounted in the center. That center engine was fixed rigidly in place.

[00:12:01]So control during first stage flight did not come from fins or aerodynamic steering alone. It came mainly from the four movable outer engines. By changing the direction of their thrust, the Saturn 5 could guide itself during the most demanding part of ascent.

[00:12:19]The center engine provided raw thrust.

[00:12:22]The four outer engines provided both thrust and steering authority. Together, the five engine cluster gave the Saturn 5 the force needed to leave Earth and the control needed to point that force in the right direction.

[00:12:37]The S1C was the brute force foundation of the Saturn 5. A 138 ft booster, 33 feet across, more than 5 million pounds when fueled, five F1 engines, 7.6 million pounds of thrust, and a propellant flow so large that every second of flight consumed more than 2,000 gall.

[00:13:03]This was the stage that turned Apollo 11 from a spacecraft sitting on a launchpad into a vehicle climbing toward the moon.

[00:13:14]Above the first stage of the Saturn 5 sat the second stage. NASA called it the S2.

[00:13:22]If the first stage was the brute force booster that lifted the vehicle off the pad, the second stage had a different job. It had to keep accelerating the Apollo stack after first stage separation, carrying the vehicle almost all the way to Earth orbit.

[00:13:39]The S2 stage produced approximately 1 million pounds of thrust. It was built by the space division of North American Rockwell Corporation at Seal Beach, California. Physically, the stage was enormous, but much lighter in character than the first stage. It was 81 ft 7 in long and 33 ft in diameter. that gave it the same diameter as the first stage, allowing the Saturn 5 to keep its massive cylindrical shape through the lower and middle portions of the vehicle. The S2 was made up of several major structures. At the top was the forward skirt where the third stage attached. Below that was the liquid hydrogen tank. Then came the liquid oxygen tank. The two tanks were separated by an insulated common bulkhead. Below the tanks was the thrust structure where the engines were mounted and beneath that was the interstage section where the first stage attached.

[00:14:43]That common bulkhead was a critical part of the stage design. Instead of placing a separate wall, empty space, and another wall between the liquid hydrogen and liquid oxygen tanks, the S2 used one shared insulated structure between them.

[00:15:00]On one side was extremely cold liquid hydrogen. On the other side was liquid oxygen. The bulkhead had to separate them, insulate them, and still act as part of the vehicle's structure.

[00:15:14]The S2 was powered by five J2 engines.

[00:15:18]These were high-performance engines burning liquid oxygen and liquid hydrogen. The same basic engine type was also used on the third stage.

[00:15:28]The engine arrangement followed the same general pattern as the first stage. Four engines were mounted around the outside.

[00:15:36]The outer four J2 engines were equally spaced on a 17.5 ft diameter circle.

[00:15:44]These four engines could gimble. They could move through a plus or minus 7° square pattern for thrust vector control. That movement allowed the second stage to steer the rocket during powered flight. By changing the direction of thrust from the outer engines, the stage could control the vehicle's attitude and flight path.

[00:16:06]The fifth J2 engine was different. It was mounted on the stage center line and like the center engine on the first stage, it was fixed in position. It did not gimble. So the second stage used five engines for propulsion, but only the four outer engines for steering. The center engine added thrust along the vehicle's axis. The outer engines provided both thrust and directional control.

[00:16:34]This was the stage that took over after the giant S1C booster had done its work.

[00:16:39]By then, the Saturn 5 was already high above the pad, moving fast and climbing out of the denser atmosphere, but it was still far from orbit. The S2 had to continue the acceleration with a much more efficient propellant combination, liquid oxygen and liquid hydrogen.

[00:16:59]For Apollo 11, the S2 stage was static tested by North American Rockwell at the NASA Mississippi test facility on September 3rd, 1968.

[00:17:11]That test firing came before the stage flew on the lunar landing mission. And to reach the test site, the stage was shipped by water traveling by way of the Panama Canal.

[00:17:23]That detail shows the scale of the Saturn 5 program. These stages were not small pieces of equipment moved casually from one building to another. They were 33 ft diameter rocket stages built in one part of the country, transported by ship, and then test fired before becoming part of a flight vehicle.

[00:17:44]The S2 was the Saturn 5's middle power plant. 81 ft 7 in long, 33 feet in diameter, five J2 engines, 1 million lb of thrust, liquid oxygen and liquid hydrogen, a shared insulated bulkhead between two cryogenic tanks and four gimbled engines steering the vehicle after first stage separation.

[00:18:13]It did not lift Apollo 11 from the pad.

[00:18:16]That was the job of the first stage. But once the first stage was gone, the S2 had to keep the mission alive. It had to take the spacecraft from the violence of launch and push it almost to Earth orbit. Only then could the third stage take over and complete the next part of the journey.

[00:18:37]Above the second stage of the Saturn 5 was the third stage. NASA called it the S4B.

[00:18:44]This was the stage that had one of the most important jobs in the entire Apollo 11 mission. It did not simply help place the spacecraft into Earth orbit. It also had to restart later and send Apollo 11 toward the moon.

[00:19:00]The S4B was developed by Macdonald Douglas Astronautics Company at Huntington Beach, California. As part of the Apollo 11 mission preparation, the stage passed a static firing test at Sacramento, California on July 17th, 1968.

[00:19:17]After that, it was flown directly to NASA's Kennedy Space Center by a special aircraft known as the Super Guppy. That detail alone shows the unusual scale of the Apollo program. The S4B was too large to move like ordinary cargo. It required a specially built transport aircraft to carry it across the country and deliver it to the launch site.

[00:19:43]Physically, the S4B was smaller than the first and second stages, but it was still a very large rocket stage. It measured 58 ft 4 in long. Its diameter was 21 ft 8 in. Dry, the stage weighed about 25,000 lb. But at first ignition, when loaded for flight, it weighed about 262,000 lb. The interstage section added another 8,081 lb. Most of the S4B's loaded weight was propellant. At first ignition, its tanks contained 43,500 lb of liquid hydrogen and 192,23 lb of liquid oxygen. Together that made 235,523 lb of propellant.

[00:20:38]Like the second stage, the S4B burned liquid oxygen and liquid hydrogen. This was a high-performance propellant combination, but it came with difficult thermal problems. The two liquids were not merely cold. They were cryogenic.

[00:20:55]The liquid oxygen was about 293° below 0 F. The liquid hydrogen was even colder, about 423° below 0 F. And in this case, the liquid oxygen was relatively warm compared with the liquid hydrogen.

[00:21:14]That sounds strange at first. Liquid oxygen was already far below freezing.

[00:21:20]But compared with liquid hydrogen, it was warm enough to rapidly heat the hydrogen. If the two tanks were not properly insulated from each other, if too much heat reached the liquid hydrogen, it could begin to turn into gas. That would change the conditions inside the tank and create serious problems for stage operation.

[00:21:41]So, insulation between the two tanks was not optional. It was a mission requirement. The S4B had to preserve two extremely cold propellants at different temperatures inside one compact rocket stage while it waited for the exact moment to fire.

[00:22:01]The stage used a single J2 engine. That engine produced a maximum thrust of 230,000 lb. Unlike the second stage which used five J2 engines, the S4B used only one. But that one engine had to perform a critical dual role. During Apollo 11, the S4B provided propulsion twice. First, it fired to help place the spacecraft into Earth orbit. Then, after the parking orbit coast, it fired again.

[00:22:36]That second burn was the one that accelerated Apollo 11 out of Earth orbit and onto its path toward the moon.

[00:22:44]This made the S4B very different from the lower stages. The first stage burned once and was discarded. The second stage burned once and was discarded. But the third stage had to survive shutdown, coast in space, maintain its propellant conditions, and then restart its J2 engine when the mission required it.

[00:23:08]That restart capability turned the S4B into more than just the top stage of the Saturn 5. It became the bridge between Earth orbit and lunar flight.

[00:23:21]The S4B was 58 ft 4 in long, 21 ft 8 in in diameter, loaded with more than 235,000 lb of liquid oxygen and liquid hydrogen, powered by a single J2 engine producing up to 230,000 lb of thrust, and designed to fire twice during Apollo 11.

[00:23:47]Without the S4B, Apollo 11 could reach Earth orbit, but it could not leave Earth for the moon. This was the stage that turned an orbital mission into a lunar mission.

[00:24:01]Above the S4B third stage sat one of the most important parts of the Saturn 5. It was not an engine. It was not a propellant tank. It was a thin ring of electronics called the instrument unit.

[00:24:17]The instrument unit was only 3 ft high, but it had the same diameter as the S4B stage beneath it, 21 ft 8 in. It weighed 4,36 lb. And inside that narrow cylindrical structure was the equipment that guided, navigated, and controlled the Saturn 5.

[00:24:40]This was the brain of the launch vehicle. The instrument unit steered the Saturn 5 through launch, Earth orbit, and into the final trans lunar injection maneuver. That last maneuver was critical. It was the burn that sent Apollo out of Earth orbit and onto its path toward the moon.

[00:25:01]The instrument unit contained the guidance, navigation, and control equipment. But it also carried far more than that. It included telemetry systems, communications systems, tracking systems, crew safety systems, and its own supporting electrical power and environmental control systems.

[00:25:23]The electronics were mounted on cooling panels attached to the inside surface of the instrument unit skin. These panels were known as cold plates. They were part of the thermal control system because the electronics inside the instrument unit generated heat and that heat had to be removed.

[00:25:43]The system circulated cooled fluid through the cold plates. That heat was then carried to a heat exchanger. In space, the heat exchanger removed heat by evaporating water from a separate supply into the vacuum. So even the Saturn 5's brain needed its own carefully controlled environment.

[00:26:04]Without cooling, the electronics that guided the rocket could not be trusted.

[00:26:09]The instrument unit was divided into six major systems. The first was the structural system. The second was thermal control. The third was guidance and control. The fourth was measuring and telemetry. The fifth was radio frequency and the sixth was electrical.

[00:26:32]Together, these systems allowed the instrument unit to do much more than simply point the rocket. It provided navigation, guidance, and control of the vehicle. It measured vehicle performance and the surrounding environment. It transmitted data to ground stations. It allowed radio tracking of the vehicle.

[00:26:53]It supported checkout and monitoring of vehicle functions. It initiated stage functional sequencing. It helped detect emergency situations. It generated and distributed electrical power through the launch vehicle network. And it supported pre-flight checkout, launch operations, and flight operations.

[00:27:15]In other words, the instrument unit did not just fly the rocket. It also watched the rocket, it measured it. It communicated its condition to the ground, and it helped coordinate the sequence of events that had to happen at exactly the right time.

[00:27:33]The Saturn 5 used what was called a path adaptive guidance scheme. This was important because the rocket did not steer in the same way through every part of flight.

[00:27:45]During first stage boost, the vehicle followed a programmed trajectory.

[00:27:50]Guidance corrections were deliberately limited while the rocket was still in the atmosphere. That was not a weakness.

[00:27:58]It was protection. In the lower atmosphere, the Saturn 5 passed through winds, jet streams, and gusts. If the guidance system tried to chase every disturbance, the rocket could command movements that placed dangerous loads into the structure. A vehicle the size of the Saturn 5 could not be allowed to fight the atmosphere too aggressively.

[00:28:21]It had to fly a planned path through the dense air without overcorrecting.

[00:28:26]Only after the vehicle had left the atmosphere did active guidance become fully involved.

[00:28:34]After second stage ignition, the guidance system could begin correcting the trajectory more directly. If the Saturn 5 deviated from the optimum climb path, the vehicle did not simply try to force itself back to the old path.

[00:28:49]Instead, it derived a new trajectory and corrected toward that.

[00:28:55]This is what made the guidance system path adaptive. It could calculate where the vehicle actually was, compare that to the mission requirements, and generate a new path that still achieved the target. Those calculations were made about once each second throughout the flight.

[00:29:15]At the center of that work were the launch vehicle digital computer and the launch vehicle data adapter. Together, they performed the navigation and guidance computations.

[00:29:26]Then the flight control computer converted the generated attitude errors into control commands.

[00:29:33]An attitude error meant the vehicle was not pointing exactly where the guidance system wanted it to point. The flight control system took those errors and turned them into commands that could move the gimbbled engines and steer the rocket.

[00:29:49]But the guidance computer needed a reference. It had to know what direction was fixed in space.

[00:29:56]That reference came from the ST124M inertial platform.

[00:30:02]The ST124M was the heart of the Saturn 5 navigation, guidance, and control system. It provided space fixed reference coordinates. It also measured acceleration along three mutually perpendicular axes of that coordinate system. In simple terms, it gave the rocket a stable reference frame and told the guidance system how the vehicle was accelerating through space.

[00:30:29]Without that reference, the computer could not know precisely how the launch vehicle was moving. And without that knowledge, it could not guide the Saturn 5 accurately toward orbit and trans lunar injection.

[00:30:44]The instrument unit also had backup logic for crew safety. If the inertial platform failed during boost, spacecraft systems could continue guidance and control functions for the rocket. And after second stage ignition, the crew had the ability to manually steer the space vehicle.

[00:31:03]That detail matters. The Saturn 5 was highly automated, but it was not designed as a blind machine with no fallback. The guidance system, the spacecraft systems, the crew and the ground all existed inside a layered control philosophy.

[00:31:22]The instrument unit was built under the prime contract of International Business Machines Corporation, better known as IBM.

[00:31:31]IBM also supplied the guidance signal processor and the guidance computer.

[00:31:37]Other major suppliers provided key components. The Electronic Communications Incorporated supplied the control computer. Bendix Corporation supplied the ST124M inertial platform and IBM Federal Systems Division supplied the launch vehicle digital computer and the launch vehicle data adapter.

[00:32:00]So the instrument unit was not simply a ring of electronics. It was a complete command and control system wrapped around the top of the Saturn 5 launch vehicle, 3 ft tall, 21 ft 8 in in diameter, 4,36 lb, and packed with the systems that allowed the rocket to navigate, communicate, sequence, measure, compute, and steer.

[00:32:32]The first stage gave Apollo 11 raw power. The second stage carried it almost to orbit. The S4B could send it toward the moon. But the instrument unit told the Saturn 5 where it was, where it was going, and how to correct its path without tearing itself apart in the atmosphere.

[00:32:54]It was the thin ring that made the giant rocket fly with precision.

[00:33:01]When people think of the Saturn 5, they usually think of the five enormous F1 engines at the bottom of the first stage. And for good reason. Those engines produced the violence of liftoff.

[00:33:15]But the Saturn 5 was not powered by only five engines. Across the launch vehicle, there were 41 rocket engines. Their thrust ratings ranged from only 72 lb to more than 1.5 million lb. Some burned liquid propellants, others used solid propellant, and each type had a different job. Some engines lifted the vehicle, some helped separate stages.

[00:33:45]Some settled propellants before ignition, and some controlled the third stage while it coasted in space.

[00:33:53]The largest of all were the five F1 engines on the first stage, the S1C.

[00:34:00]These engines burned rocket propellant 1, commonly known as RP1, a highly refined kerosene together with liquid oxygen. At liftoff, each F1 developed approximately 1,530,771 lb of thrust.

[00:34:21]But that thrust did not remain constant.

[00:34:24]As the vehicle climbed, each engine built up to about 1,817,684 lb of thrust before cutoff.

[00:34:35]Together, the five engine cluster gave the first stage a thrust range from 7,653,854 lb at liftoff to 9,88,419 lb just before center engine cutoff.

[00:34:54]That means the Saturn 5 was not only getting lighter as it burned propellant, its first stage engines were also producing more thrust as they climbed.

[00:35:04]So the vehicle's acceleration increased dramatically during the booster phase.

[00:35:10]The F1 engine itself was a huge machine.

[00:35:15]Each engine weighed almost 10 tons. It stood more than 18 ft high and the nozzle exit diameter was nearly 14 ft.

[00:35:25]Before an F1 engine qualified for the Saturn 5 first stage booster phase, it underwent static testing for an average of 650 seconds. That was to qualify it for a flight run of about 160 seconds during first stage operation.

[00:35:44]In flight, each F1 consumed almost 3 tons of propellant per second. That is the scale of the first stage. Not gallons per minute, not pounds per minute. Almost three tons every second through each engine. Five engines, each one almost 10 tons. Each one more than 18 ft tall. Each one consuming propellant at a rate that is difficult to imagine. And all five had to start, stabilize, build thrust, steer, and keep burning during the most violent part of the launch.

[00:36:22]But the first stage also carried more than its five main engines. For this mission, the Saturn 5 first stage had eight additional rocket motors. These were solid fuel retro rockets. Their job was not to push Apollo upward. Their job was to help slow and separate the first stage from the second stage. Each of those retro rockets produced 87,900 lb of thrust, but only for 0.6 second.

[00:36:52]That short firing was enough to help pull the spent first stage away after shutdown so the second stage could continue the mission.

[00:37:02]Above the first stage, the second stage used a very different propulsion system.

[00:37:07]The S2 was powered by a cluster of five J2 engines. These engines burned liquid hydrogen and liquid oxygen. Each J2 developed a mean thrust of more than 227,000 lb at a 5:1 mixture ratio. During this flight, the thrust could vary from 224,000 to 231,000 lbs in different phases.

[00:37:34]Together, the five J2 engines gave the second stage a total mean thrust of more than 1.135 million lb.

[00:37:44]The J2 was much smaller than the F1. It weighed about 3,500 lb, but it was designed for a different environment and a different purpose. The J2 was built to operate in the hard vacuum of space, and because it burned high energy hydrogen, it was more efficient than the F1.

[00:38:06]That difference matters. The F1 was a seale brute force engine built to lift the fully fueled Saturn 5 off Earth. The J2 was a high efficiency upper stage engine built to keep accelerating the vehicle after the lower atmosphere had been left behind.

[00:38:26]Both the F1 and the J2 were produced by the rocket dine division of North American Rockwell Corporation. Together those two engine types formed the main propulsion backbone of the Saturn 5. The F1 lifted the rocket from the pad. The J2 carried the mission onward toward orbit and the moon.

[00:38:48]But again, propulsion was not only about the main engines. The second stage also had four solid fuel rocket engines, each producing 21,000 lb of thrust. These were the OLED rockets mounted on the S1C and S2 interstage section. Their job was to fire during staging. They helped settle the liquid propellants into the bottoms of the second stage tanks so the J2 engines could receive propellant properly. They also helped achieve a clean separation from the first stage.

[00:39:24]After that, they remained with the interstage when it dropped away at second plane separation.

[00:39:30]This is one of the details that shows how carefully staged flight had to be managed in weightlessness or near weightlessness. Liquid propellants do not automatically sit at the outlet end of a tank. Before an upper stage engine can start reliably, the propellant must be settled where the feed system can draw it. That was the purpose of olage thrust. A few small solid rockets could determine whether the next stage received clean propellant flow or whether the mission was in trouble before the main engines even had a chance to burn correctly.

[00:40:08]There were also four retro rockets located in the S4B aft interstage. This interstage did not separate from the S2.

[00:40:18]Those retro rockets were used to separate the S2 from the S4B before S4B ignition.

[00:40:27]Then came the propulsion system of the third stage itself. The S4B used 11 rocket engines for different functions.

[00:40:37]Its main propulsion came from a single J2 engine. That engine provided the thrust for the third stage burns.

[00:40:45]But the S4B also carried two jettisonable main rockets and it had eight smaller engines mounted in the two auxiliary propulsion system modules.

[00:40:57]Those smaller engines supported the third stage during its mission, especially when the stage had to coast, orient itself, and prepare for another critical burn.

[00:41:09]So, the Saturn 5 propulsion system was not one simple stack of big engines. It was a carefully timed network.

[00:41:18]Five F1 engines on the first stage, eight first stage retro rockets, five J2 engines on the second stage, four second stage ulage rockets, four retro rockets associated with S2 to S4B separation and 11 engines on the S4B, including its single J2, two main ELG rockets and eight auxiliary propulsion engines.

[00:41:47]Together, these made up the 41 rocket engines of the Saturn 5. Some burned for minutes, some burned for less than a second. Some produced more than a million pounds of thrust, others produced only tens of pounds. But every one of them had a reason to exist.

[00:42:12]The big engines gave Apollo the energy to climb. The olage rockets prepared the tanks for ignition. The retro rockets pulled spent hardware away. And the auxiliary engines helped the third stage manage its attitude in space. The Saturn 5 was not just powerful because it had enormous engines. It was powerful because every propulsion event had to happen in the right order at the right time with the right engine for the right purpose. from liftoff to staging. From staging to orbit, and from orbit to the burn that sent Apollo 11 toward the moon.