ASC-15
| Industry | Computer hardware, Aerospace |
|---|---|
| Founded | June 16, 1911 in Endicott, New York, United States |
| Headquarters | Armonk , United States |
| Products | ASC-15 digital computer (among others) |
The ASC-15 (Advance System Controller Model 15) was a digital computer engineered by International Business Machines (IBM) specifically for the Titan II intercontinental ballistic missile (ICBM). Subsequently, it was modified for implementation on the Titan III and Saturn I Block II launch vehicles.
Its main function involved performing navigation computations using input from inertial sensor systems and conducting pre-launch readiness verifications. The ASC-15 operated as a digital serial processor with fixed-point 27-bit word length, employing drum memory technology. Its circuitry comprised soldered, encapsulated modules housing discrete components such as resistors, transistors, and capacitors. Production occurred at IBM’s manufacturing plant located in Owego, New York.
ASC-15 in Titan II Guidance
The inaugural inertial guidance system for the Titan II missile integrated an inertial measurement unit supplied by AC Spark Plug, which was based on designs from Draper Labs, together with IBM’s ASC-15 computer. This guidance system was first utilized during the initial Titan II launch on 16 March 1962. Due to difficulties in sourcing replacement parts, the United States Air Force began phasing out this system in January 1978, substituting it with the Delco Electronics Universal Space Guidance System (USGS), which incorporated the Delco-manufactured Magic 352 guidance computer.
Encased within an aluminum frame measuring roughly , the ASC-15 featured laminated plastic covers reinforced with gold-plated aluminum foil ribbing to enhance rigidity. Internally, it contained fifty-two logic sticks, each comprising four encapsulated modules arranged around a bell frame drum memory.
The drum memory itself was a thin-walled stainless steel cylinder approximately long and in diameter, coated with a magnetic nickel-cobalt alloy. It rotated at 6,000 revolutions per minute driven by a synchronous motor and contained 70 tracks—58 active and 12 reserved spares—allocated as follows:
| Number of Tracks | Track Purpose |
|---|---|
| 34 | Instruction storage |
| 7 | Constants |
| 8 | Target information |
| 2 | Temporary data storage |
| 5 | Revolver tracks for rapid access |
| 2 | Timing control tracks |
Each track stored 1,728 bits; instructions were encoded in 9-bit units while data words consisted of 27 bits.
The drum incorporated 67 read heads and 13 write heads that hovered on a thin cushion of air above its surface during operation. To safeguard the magnetic surface during spin-up or spin-down phases, camshafts mechanically lifted the heads away from the drum using a motor-driven chain mechanism.
ASC-15 Adaptation for Titan III
Derived from the Titan II ICBM, the Titan III launch vehicle retained the ASC-15 as its navigation computer. The drum memory was expanded to include 78 usable tracks—an increase of 20 compared to the Titan II model—allocating space for 9,792 instructions across 51 tracks and 1,152 constants over 18 tracks. The data transfer speed remained consistent at 172.8 kilobits per second. Processing times were recorded as 156 microseconds for addition, 1,875 microseconds for multiplication, and 7,968 microseconds for division operations.
ASC-15 Implementation on Saturn I
During Saturn I Block I missions (SA-1 through SA-4), no dedicated guidance computer was employed; pitch control was achieved via a cam mechanism within the Servo Loop Amplifier Box, while sequencing relied on a six-track tape recorder that activated relays.
The ASC-15 made its first appearance on SA-5, marking the initial Saturn I Block II flight to achieve orbit. At this stage, it functioned passively by generating test telemetry data while active guidance systems remained unchanged from previous flights. This mission also introduced the Instrument Unit.
On SA-6, an open loop ST-90S system managed guidance of the first stage (S-I). Following stage separation, closed loop path adaptive guidance was executed by the ST-124 system alongside the ASC-15 computer for the second stage (S-IV). This adaptive guidance demonstrated robustness when an early shutdown of one S-I engine had minimal effect on overall trajectory.
The configuration consisting of ST-90S, ST-124, and ASC-15 used on SA-6 represents version one of the Instrument Unit architecture applied during missions SA-5 through SA-7.
By mission SA-7, guidance responsibilities shifted entirely to the ST-124 system controlling both stages, with the ASC-15 digital computer replacing earlier cam-based and program devices that had managed tilt programs and event sequencing.
Subsequent launches from SA-8 to SA-10 utilized version two of the Instrument Unit—a design that was unpressurized and reduced in length by two feet—thereby optimizing Saturn I launch operations.
References
- USAF Sheppard Technical Training Center. "Student Study Guide, Missile Launch/Missile Officer (LGM-25)." May 1967. Pages 61–65. Accessible via WikiMedia Commons
- Available as PDF.
External links
- Titan II missile computer – IBM Archives: Space flight chronology (archived January 19, 2005)
- A Third Survey of Domestic Electronic Digital Computing Systems – Ballistic Research Laboratories report, March 1961 (featuring image of welded encapsulated module and ASC 15 specifications)