1. Home
2. Saturn Rocket
3. Saturn V Instrument Unit

Saturn V Instrument Unit

The Saturn V Instrument Unit 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 (S-IVB). It was immediately below the SLA (Spacecraft/Lunar Module Adapter) panels that contain the 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. NASA's contractor to construct the Saturn V Instrument Unit was International Business Machines (IBM).

One of the unused Instrument Units is currently on display at the Steven F. Udvar-Hazy Center in Chantilly, Virginia. The plaque for the Unit has the following description:

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.

Instrument Unit 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 (IU) for Saturn I Block I (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 Redstone missile.

The IU made its debut with SA-5, the first Saturn I Block II launch. The first version of the IU was 154 inches (3,900 mm) in diameter and 58 inches (1,500 mm) 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.

MSFC flew version 2 of the IU on SA-8, 9 and 10. Version 2 was the same diameter as version 1, but only 34 inches (860 mm) high. Instead of pressurized containers, the components were hung on the inside of the cylindrical wall, achieving a reduction in weight.

The last version, number 3, was 260 inches (6,600 mm) in diameter and 36 inches (910 mm) 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.

Mission profile

Saturn Apollo flight profiles varied considerably with mission. 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 direction. 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 manned 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 260 inches (6,600 mm) in diameter, fabricated of an aluminum alloy honeycomb sandwich material 0.95 inches (24 mm) 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 IU is divided into 24 locations, which are marked on the interior by numbers 1-24 on the aluminum surface just above the blue flange.