The Saturn V instrument unit (IU) is a ring-shaped structure fitted to the top of the Saturn V rocket's third stage (S-IVB) and the Saturn IB's second stage (also an S-IVB). It was immediately below the SLA (Spacecraft/Lunar Module Adapter) panels that contained the Apollo Lunar Module. The instrument unit contains the guidance system for the Saturn V rocket. Some of the electronics contained within the instrument unit are a digital computer, analog flight control computer, emergency detection system, inertial guidance platform, control accelerometers, and control rate gyros. The instrument unit (IU) for Saturn V was designed by NASA at Marshall Space Flight Center (MSFC) and was developed from the Saturn I IU."Instrument Unit Fact Sheet, Saturn V News Reference." Changed December 1968. Page 2. NASA's contractor to manufacture the Saturn V Instrument Unit was International Business Machines (IBM).Roger E. Bilstein. Stages to Saturn. The NASA History Series, 1996. Chapter 8. From Checkout to Launch: The Quintessential Computer. IBM is also identified as the Instrument Unit builder on the nameplate of IU-514 at Udvar-Hazy Center. This nameplate is at the top left of location 7.

One of the unused instrument units is on display at the Steven F. Udvar-Hazy Center in Chantilly, Virginia. The plaque for the unit has the following inscription: The Saturn V rocket, which sent astronauts to the Moon, used inertial guidance, a self-contained system that guided the rocket's trajectory. The rocket booster had a guidance system separate from those on the command and lunar modules. It was contained in an instrument unit like this one, a ring located between the rocket's third stage and the command and lunar modules. The ring contained the basic guidance system components—a stable platform, accelerometers, a digital computer, and control electronics—as well as radar, telemetry, and other units.

The instrument unit's stable platform was based on an experimental unit for the German V-2 rocket of World War II. The Bendix Corporation produced the platform, while IBM designed and built the unit's digital computer.

Specifications Diameter: 260 inches (6.6 m) Height: 36 inches (914 mm) Weight at launch: ~4,400 lb (1996 kg)

Mission history There was no instrument unit for Saturn I Block I boosters (SA-1 to SA-4). Guidance and control equipment was carried in canisters on top of the S-I first stage, and included the ST-90 stabilized platform, made by Ford Instrument Company and used in the Jupiter missile.‘’Stages To Saturn’’ Chapter 8.

+ Digital Guidance Computer Saturn Launch Vehicle Launch Missions Guidance Computer Key Role/Notes Saturn I Block I (SA-1 to SA-4) None (analog/mechanical) A magnetic tape device and a cam device handled control and guidance (a non-inertial, non-digital system). Block II (SA-5 to SA-10) ASC-15 by IBM Onboard SA-5 as a test. Became the digital guidance computer for the upper stage on SA-6 and both stages on subsequent flights. Saturn IB AS-201 to ASTP Launch Vehicle Digital Computer by IBM The digital guidance computer, located in the IU and used on both the Saturn IB and the Saturn V. It controlled steering, engine ignition, and staging operations. Saturn V Apollo 4 to Skylab 1 The same computer system of the Saturn IB also served as the autopilot for the Saturn V, guiding it from launch through Earth orbit insertion and Trans-Lunar Injection.

+ Analog Flight Computer/Control Device Saturn Launch VehicleLaunch MissionsAnalog Computer or Device Function Saturn IBlock I (SA-1 to SA-4)Canister-15 Cam Device Provided a pitch-program/tilt-schedule. Tape Recorder A 6-track tape device that scheduled a precise time-dependent sequence of events (e.g., staging, cutoff).Block II (SA-5 to SA-7)Four containers and a central hub Thrust Vector Control (TVC) and Three-axis attitude stabilization.(SA-8 to SA-10)Components mounted on the wall of the IU ring Saturn IB & Saturn VAll FlightsFlight Control Computer by ECI Analog computer for processing TVC commands or calculating "Control Law" for vehicle stabilization, then outputting control signals to the engine actuator system (gimbals/steering).

The IU made its debut with SA-5, the first Saturn I Block II launch. The first version of the IU was in diameter and high, and was both designed and built by MSFC. Guidance, telemetry, tracking and power components were contained in four pressurized, cylindrical containers attached like spokes to a central hub.The Apollo "A"/Saturn C-1 Launch Vehicle System

MSFC flew version 2 of the IU on SA-8, 9 and 10. Version 2 was the same diameter as version 1, but only high. Instead of pressurized containers, the components were hung on the inside of the cylindrical wall, achieving a reduction in weight.Saturn I Summary PDF p. 36

The last version, number 3, was in diameter and tall. It was designed by MSFC but manufactured by IBM in their factory at Huntsville, and flew on all Saturn IB and Saturn V launches. This is the version that is on display in Washington, Huntsville, Houston, and the Apollo/Saturn V Center.

+ Saturn instrument unit launch history‘’Saturn Illustrated Chronology’’, Appendix H. Moonport, Appendix A. Apollo Program Summary Report, Appendix A. Program Vehicle Mission Launch date Pad IU version IU Saturn I SA-1 SA-1 27 Oct 1961 34 - Saturn I SA-2 SA-2 25 Apr 1962 34 - Saturn I SA-3 SA-3 16 Nov 1962 34 - Saturn I SA-4 SA-4 28 Mar 1963 34 - Saturn I SA-5 SA-5 29 Jan 1964 37B 1 S-IU-5 Saturn I SA-6 AS-101 28 May 1964 37B 1 S-IU-6 Saturn I SA-7 AS-102 18 Sep 1964 37B 1 S-IU-7 Saturn I SA-9 AS-103 16 Feb 1965 37B 2 S-IU-9 Saturn I SA-8 AS-104 25 May 1965 37B 2 S-IU-8 Saturn I SA-10 AS-105 30 Jul 1965 37B 2 S-IU-10 Saturn IB SA-201 AS-201 26 Feb 1966 34 3 S-IU-201 Saturn IB SA-203 AS-203 5 Jul 1966 37B 3 S-IU-203 Saturn IB SA-202 AS-202 25 Aug 1966 34 3 S-IU-202 Saturn V SA-501 Apollo 4 9 Nov 1967 39A 3 S-IU-501 Saturn IB SA-204 Apollo 5 22 Jan 1968 37B 3 S-IU-204 Saturn V SA-502 Apollo 6 4 Apr 1968 39A 3 S-IU-502 Saturn IB SA-205 Apollo 7 11 Oct 1968 34 3 S-IU-205 Saturn V SA-503 Apollo 8 21 Dec 1968 39A 3 S-IU-503 Saturn V SA-504 Apollo 9 3 Mar 1969 39A 3 S-IU-504 Saturn V SA-505 Apollo 10 18 May 1969 39B 3 S-IU-505 Saturn V SA-506 Apollo 11 16 Jul 1969 39A 3 S-IU-506 Saturn V SA-507 Apollo 12 14 Nov 1969 39A 3 S-IU-507 Saturn V SA-508 Apollo 13 11 Apr 1970 39A 3 S-IU-508 Saturn V SA-509 Apollo 14 31 Jan 1971 39A 3 S-IU-509 Saturn V SA-510 Apollo 15 26 Jul 1971 39A 3 S-IU-510 Saturn V SA-511 Apollo 16 16 Apr 1972 39A 3 S-IU-511 Saturn V SA-512 Apollo 17 7 Dec 1972 39A 3 S-IU-512 Saturn V SA-513 Skylab 1 14 May 1973 39A 3 S-IU-513 Saturn IB SA-206 Skylab 2 25 May 1973 39B 3 S-IU-206 Saturn IB SA-207 Skylab 3 28 Jul 1973 39B 3 S-IU-207 Saturn IB SA-208 Skylab 4 16 Nov 1973 39B 3 S-IU-208 Saturn IB SA-210 ASTP 15 Jul 1975 39B 3 S-IU-210 Saturn IB SA-209 Skylab Rescue Unflown S-IU-209 Saturn IB SA-211 Unassigned Unflown S-IU-211 Saturn IB SA-212 Unassigned Unflown S-IU-212 Saturn V SA-514 Apollo 18 Unflown S-IU-514 Saturn V SA-515 Apollo 19 Unflown S-IU-515 Saturn V SA-500-F Ground Test Vehicle Unflown S-IU-500F(200F) Saturn V SA-500-D Ground Test Vehicle Unflown S-IU-D

Mission profile Saturn Apollo flight profiles varied considerably by mission."Astrionics System Handbook", 1 November 1968, MSFC No. IV-4-401-1. IBM No. 68-966-0002. Section 1.3. Saturn V Mission Profile." Instrument Unit Fact Sheet. Saturn V News Reference." Changed December 1968. Pp. 5-6."Saturn V Flight Manual SA-507." MSFC-MAN-507. Changed 5 October 1969. P. 2-1 (PDF page 15). Section II. Performance. Flight Sequence. All missions began, however, with liftoff under power of the first stage. To more smoothly control engine ignition, thrust buildup and liftoff of the vehicle, restraining arms provided support and hold down at four points around the base of the S-IC stage. A gradual controlled release was accomplished during the first six inches of vertical motion.

After clearing the launch tower, a flight program stored in the launch vehicle digital computer (LVDC) commanded a roll of the vehicle to orient it so that the subsequent pitch maneuver pointed the vehicle in the desired azimuth. The roll and pitch commands were controlled by the stored program, and were not affected by navigation measurements. Until the end of the S-IC burn, guidance commands were functions only of time.

First stage cutoff and stage separation were commanded when the IU received a signal that the tank's fuel level had reached a predetermined point. Guidance during the second and third stage burns depended both on time and navigation measurements, in order to achieve the target orbit using the minimum fuel.

Second stage engine cutoff was commanded by the IU at a pre-determined fuel level, and the stage was separated. By this time, the vehicle had reached its approximate orbital altitude, and the third stage burn was just long enough to reach a circular parking orbit.

During crewed Apollo missions, the vehicle coasted in Earth orbit for 2-4 passes as the crew performed checks of systems status and other tasks, and as ground stations tracked the vehicle. During the hour and a half after launch, tracking stations around the world had refined estimates of the vehicle's position and velocity, collectively known as its state vector. The latest estimates were relayed to the guidance systems in the IU, and to the Command Module Computer in the spacecraft. When the Moon, Earth, and vehicle were in the optimum geometrical configuration, the third stage was reignited to put the vehicle into a translunar orbit. For Apollo 15, for example, this burn lasted 5 minutes 55 seconds.

After translunar injection came the maneuver called transposition, docking, and extraction. This was under crew control, but the IU held the S-IVB/IU vehicle steady while the Command/Service Module (CSM) first separated from the vehicle, rotated 180 degrees, and returned to dock with the Lunar Module (LM). When the CSM and LM had "hard docked" (connected by a dozen latches), the rearranged spacecraft separated from the S-IVB/IU.

The last function of the IU was to command the very small maneuver necessary to keep the S-IVB/IU out of the way of the spacecraft. On some missions the S-IVB/IU went into high Earth or Solar orbit, while on others it was crashed into the Moon; seismometers were left on the Moon during Apollo 11, 12, 14, 15, and 16, and the S-IVB/IUs of Apollo 13, 14, 15, 16, and 17 were directed to crash. These impacts provided impulses that were recorded by the seismometer network to yield information about the geological structure of the Moon.

Subsystems

The IU consists of six subsystems: structure, guidance and control, environmental control, emergency detection, radio communications (for telemetry, tracking, and command), and power.

Structure The basic IU structure is a short cylinder, 36 inches high and in diameter, fabricated of an aluminum alloy honeycomb sandwich material thick. The cylinder is manufactured in three 120-degree segments, which are joined by splice plates into an integral structure. The top and bottom edges are made from extruded aluminum channels bonded to the honeycomb sandwich. This type of construction was selected for its high strength to weight ratio, acoustical insulation, and thermal conductivity properties. The IU supported the components mounted on its inner wall and the weight of the Apollo spacecraft above (the Lunar Module, the Command Module, the Service Module, and the Launch Escape Tower). To facilitate handling the IU before it was assembled into the Saturn, the fore and aft protective rings, 6 inches tall and painted blue, were bolted to the top and bottom channels. These were removed in the course of stacking the IU into the Saturn vehicle. The structure was manufactured by North American Rockwell in Tulsa, Oklahoma. Edward A. Beasley was the I.U. Program Manager.