The Saturn/Apollo Stack - Introduction Content from the guide to life, the universe and everything

The Saturn/Apollo Stack - Introduction

0 Conversations

Project Apollo: The Beginnings | Mission Planning | Landing Site Selection | Earthbound Support Systems
Astronaut Selection and Training | The Saturn V | The Saturn 1B | The Apollo Spacecraft
Guidance and Navigation | Command and Service Modules | The Lunar Module
Assembling and Launching | Pathfinders | The Early Missions | Apollo 11, The First Landing
The Intermediate Missions | Apollo 15 Exploration | Apollo 16 Exploration | Apollo 17 Exploration
Skylab and Apollo-Soyuz | Conclusion

After much consideration and internal argument, NASA had by 1962 adopted the Lunar Orbit Rendezvous (LOR) mode as the most likely means to meet the 'end of decade' deadline set by President Kennedy's challenge to ' a man on the moon and return him safely to earth'. This technique dictated the design of the craft to be used for the landing attempt and the specification of the launch vehicle. LOR required the placing a two part spacecraft into orbit around the moon from where one half, a lunar module, would descend to a landing leaving the other half, a command and service module, in orbit. On completion of the lunar excursion the lunar module would take off and return to the orbiting command ship.

While undergoing assembly and readying for launching, the complete Saturn/Apollo moonship was referred to by its builders as 'the stack'. It consisted of the two part Apollo spacecraft at the top of a three stage Saturn V launch vehicle. On the launchpad the complete Saturn/Apollo stack stood 363 feet high and when fully fuelled weighed over 3000 tons. The individual component assemblies that made up the stack were each the product of separate manufacturers located throughout the United States and were brought together for assembly in the Vehicle Assembly Building (VAB), at the Kennedy Space Centre (KSC) launch site in Florida, USA. The component manufactures diverse locations ranged throughout the United States from New York State to Alabama and California.

The Saturn V Launch Vehicle

The Saturn V launch vehicle used to get the Apollo spacecraft into earth orbit and on its way to the moon was assembled from a S-IC first stage booster, a S-II second stage and a S-IVB third stage. Throughout the Apollo program thirteen Saturn Vs were launched without loss. Each vehicle took five months from the time the first components were delivered to the Kennedy Space Centre (KSC) to assemble, prepare, test and launch. In 1969, at the height of the Apollo program, three vehicles were in various stages of assembly or ready for launch, but only once was the second launch pad at KSC used. This was for the launch of Apollo 10 from pad 39B, all other Saturn V's were launched from pad 39A.

The Saturn launch vehicle design and development was the responsibility of NASA Marshall Space Flight Centre (MSFC), Huntsville, Alabama under its Centre Director, Dr Wernher von Braun. Each of the vehicle stages, instrument unit and engine programs had individual management teams and construction took place at the individual manufacturers plants.

Saturn S-1C First Stage

The first stage Saturn S-IC booster was constructed by the Boeing Co at its Michaud facility near New Orleans. It was 33ft in diameter and 138ft tall and was designed to initially lift the complete spacecraft through the lower part of the earth's atmosphere to an altitude of over 40 miles. It was the most powerful engine ever built and successfully launched. The S1-C and its fuel load made up two thirds of the stack's overall launch weight. The S-IC was capable of lifting the total launch weight and accelerating it through the speed of sound in a near vertical climb to reach a speed of nine times the speed of sound at engine cut off just three minutes after lifting off. With weight considerations of crucial importance the construction of the stage, in common with the remainder of the other two upper stages, was principally from an aluminium alloy developed by the Alcan Co. Constructed from a composite ring frame and stringer assembly, no part of the frame material used were thicker than 0.25 inches.

Two main propellant tanks formed part of the thrust and weight bearing load structure and were connected by an inter-tank section. The upper tank contained 345,000 gallons of liquid oxygen (LOX) and was initially pressurised by helium on the launchpad, but transferred to gaseous oxygen (GOX) after lift-off which was supplied by a heat exchanger drawing LOX from the fuel supply. The lower tank containing 203,000 gallons of a refined kerosene mixture (RP-1) and was pressurised with helium from a ground supply on the pad and by four bottles housed inside the LOX tank during flight. The construction of the tanks was so light that they had to be kept pressurised to prevent them from buckling under their own weight.

At the base of the stage a thrust frame mounted the five Rocketdyne F1 engines required to lift the spacecraft. The S1-C's engines were developed and built by Rocketdyne, a subsidiary of North American Rockwell Corporation. Designated the F1, each engine weighed ten tons and stood 18 feet tall with an exhaust bell exit diameter of 14 feet. Turbopumps supplied the cluster of five engines, which consumed between them over fifteen tons of fuel and oxidiser each second to develop a total thrust of seven and a half million pounds. The central F1 engine's alignment was fixed but the surrounding four engines were mounted to the thrust frame and could be pivoted up to six degrees to provide directional thrust and steering.

A stainless steel, honeycombed lower heatshield situated between the engine thrust frame and the lower propellant tank safeguarded heat critical components from the engines. Four titanium/aluminium shrouds incorporating fixed stabilising fins protected the outer engine bells from aerodynamic forces and housed the engine gimballing linkages. Also contained within the shrouds, grouped in pairs, were eight, solid fuel retro-rockets, each producing 87,900 pound thrust for a duration of 0.6 seconds when fired one second after the stage was jettisoned. These would slow the S-1C to provide separation from the second stage as its engines ignited.

Interstage Ring

An annular interstage ring separated the first and second stages and aerodynamically faired in the gap between them while providing clearance for the second stage engine bellmouths. The ring also housed four 219,000 pound thrust, solid fuel rockets, which were fired just after the first stage was jettisoned when acceleration had momentarily ceased. This provided 'ullage' of the fuel in the second stage's tanks, forcing the fuel to the bottom of the tanks ready for ignition of the second stage engines. The interstage ring was jettisoned 30 seconds after the S-1C and was referred to as a 'two plane separation'.

Saturn S-II Second Stage

The second stage Saturn S-II, built by North American Rockwell Corporation at Seal Beach California, maintained the overall diameter of the first stage at 33 feet but was 81 feet in length. As the S-IC exhausted its fuel and was jettisoned at an altitude of 220,000 feet the S-II took over to accelerate the remainder of the spacecraft, now weighing only one third of its original weight, through the upper atmosphere to a near orbital altitude of 610,000 feet.

Using five Rocketdyne J2 engines, burning more efficient cryogenic fuels, the engines developed over one million pounds of thrust between them. 260,000 gallons of liquid hydrogen and 83,000 gallons of liquid oxygen were housed in tanks separated by a common bulkhead. The mounting of the engines was similar to that of the first stage which allowed gimballing of the outer four engines for steering while the central engine's alignment remained fixed.

The S-II also housed four 219,000 pound thrust solid fuel ullage rockets to settle fuel in the second stage tanks. The difference in diameters between the second and third stages was accommodated by an interstage ring which tapered the divergent diameters and provided a step in the spacecraft's outline. Four further retro-rockets housed in the top of the interstage ring provided clean separation of the second stage from the third in a similar manner to the S-1C. This interstage ring was jettisoned still attached to the second stage.

During its development the design of the S-II proved to be the most troublesome of the three stages. Initially the fuel tanks proved liable to cracking and were redesigned. It also had a tendency to resonate on its longitudinal axis causing repeated oscillation of the framework which give rise to the phenomenon's name, 'pogo'. Modifications to the J2 engine's fuel supply system and bringing the centre engine cut-off to a minute and a half before staging, reduced the effect bringing it within acceptable limits, although the effect was never fully eradicated.

S-IVB Third Stage

The third stage Saturn S-IVB was built by McDonnell Douglas Astronautics Co at its Huntingdon Beach, California plant. It was 22 feet in diameter and 58 feet in length and powered by a single, restartable Rocketdyne J2 engine. Burning liquid hydrogen and liquid oxygen to produce almost 250,000 pounds of thrust, the S-IVB would increase the spacecraft's launch speed to 17,400 miles per hour to provide the final push to orbital speed and height. The S-IVB's second function was to carry out the Trans Lunar Injection (TLI) manoeuvre. When safely checked out in earth orbit and ready to commit to a lunar mission, it had the ability to restart its engine and accelerate the Apollo spacecraft to an escape velocity of 24,200 mph that would take it out of earth orbit and onto a path to the moon.

Pressurisation of the LOX tanks came from eight spherical helium tanks inside the liquid hydrogen tank, while ullage was provided by two further solid fuel rockets housed in external pods. Attitude control was provided by the Auxiliary Propulsion System (APS), eight liquid fuel thrusters in two propulsion system modules. The S-IVB was also used as the second stage for all earth orbital missions which used the smaller, less powerful, Saturn S-IB first stage booster.

Instrument Unit (IU)

Attached to the top of the S-IVB housed in an three foot high annular ring, mounted at the top of the S-IVB, was the guidance and control system Instrument Unit (IU). Manufactured by IBM Federal Systems Division, it contained the on-board Launch Vehicle Digital Computer (LVDC), which controlled the Saturn's flight guidance and navigation systems from lift off to earth orbit and throughout the Trans Lunar Injection (TLI) engine burn. Its functions included power supply and cooling to the Saturn's electrical and electronic systems, guidance of the launch vehicle and monitoring performance and fault diagnosis. Its communication system also returned data to the ground based mission control computers to allow ground based monitoring of its condition and performance.

Initially the IU's orientation was set with a visual reference point on the ground, and transferred to internal battery power 17 seconds before lift off, then released from ground control with five seconds to go. Monitoring the orientation of the Saturn was accomplished by a ST-124 inertial platform, manufactured by the Bendix Corporation, that measured its change of attitude through the three axes of roll, pitch and yaw while changes in the speed of the craft were measured by accelerometers. Data from the platform and other sensors was used by the LDVC to compute course corrections and adjust the engine's alignment to optimise the crafts trajectory. The IU also incorporated an Emergency Detection System (EDS) which in automatic mode could initiate the Abort Escape System (AES) abort sequence to terminate the flight if necessary. All three stages had shaped explosive charges attached to the main fuel tanks, controlled by the IU, which could be used in the event of an abort to rupture the tanks to disperse the remaining fuel. As a back up to total failure of the LVDC, the command module guidance computer which monitored the LVDC, could take over flight guidance during the launch.

Bookmark on your Personal Space

Conversations About This Entry

There are no Conversations for this Entry

Edited Entry


Infinite Improbability Drive

Infinite Improbability Drive

Read a random Edited Entry

Categorised In:

Written by

Write an Entry

"The Hitchhiker's Guide to the Galaxy is a wholly remarkable book. It has been compiled and recompiled many times and under many different editorships. It contains contributions from countless numbers of travellers and researchers."

Write an entry
Read more