LGM-30 Minuteman
The LGM-30 Minuteman is an American land-based intercontinental ballistic missile in service with the Air Force Global Strike Command., the LGM-30G is the only land-based ICBM in service in the United States and represents the land leg of the U.S. nuclear triad, along with the Trident II submarine-launched ballistic missile and nuclear weapons carried by long-range strategic bombers.
Development of the Minuteman began in the mid-1950s when basic research indicated that a solid-fuel rocket motor could stand ready to launch for long periods of time, in contrast to liquid-fueled rockets that required fueling before launch and so might be destroyed in a surprise attack. The missile was named for the colonial minutemen of the American Revolutionary War, who could be ready to fight on short notice.
The Minuteman entered service in 1962 as a deterrence weapon that could hit Soviet cities with a second strike and countervalue counterattack if the U.S. was attacked. However, the development of the United States Navy UGM-27 Polaris, which addressed the same role, allowed the Air Force to modify the Minuteman, boosting its accuracy enough to attack hardened military targets, including Soviet missile silos. The Minuteman II entered service in 1965 with a host of upgrades to improve its accuracy and survivability in the face of an anti-ballistic missile system the Soviets were known to be developing. In 1970, the Minuteman III became the first deployed ICBM with multiple independently targetable reentry vehicles : three smaller warheads that improved the missile's ability to strike targets defended by ABMs. However, the Minutemen III missiles were later "de-MIRVed"; since 2016 they have had only a single warhead per missile, either a W78 or W87.
By the 1970s, 1,000 Minuteman missiles were deployed. This force has shrunk to 400 Minuteman III missiles, deployed in missile silos around Malmstrom AFB, Montana; Minot AFB, North Dakota; and Francis E. Warren AFB, Wyoming. The Minuteman III will be progressively replaced by the new LGM-35 Sentinel ICBM, to be built by Northrop Grumman, beginning in 2030.
History
Edward Hall and solid fuels
Minuteman owes its existence largely to Air Force Colonel Edward N. Hall, who in 1956 was given charge of the solid-fuel-propulsion division of General Bernard Schriever's Western Development Division, created to lead development of the SM-65 Atlas and HGM-25A Titan I ICBMs. Solid fuels were already commonly used in short-range rockets. Hall's superiors were interested in short- and medium-range missiles with solids, especially for use in Europe where the fast reaction time was an advantage for weapons that might be attacked by Soviet aircraft. But Hall was convinced that they could be used for a true ICBM with a range.To achieve the required energy, that year Hall began funding research at Boeing and Thiokol into the use of ammonium perchlorate composite propellant. Adapting a concept developed in the UK, they cast the fuel into large cylinders with a star-shaped hole running along the inner axis. This allowed the fuel to burn along the entire length of the cylinder, rather than just the end as in earlier designs. The increased burn rate meant increased thrust. This also meant the heat was spread across the entire motor, instead of the end, and because it burned from the inside out it did not reach the wall of the missile fuselage until the fuel was finished burning. In comparison, older designs burned primarily from one end to the other, meaning that at any instant one small section of the fuselage was being subjected to extreme loads and temperatures.
Guidance of an ICBM is based not only on the direction the missile is traveling but the precise instant that thrust is cut off. Too much thrust and the warhead will overshoot its target, too little and it will fall short. Solids are normally very hard to predict in terms of burn time and their instantaneous thrust during the burn, which made them questionable for the sort of accuracy required to hit a target at intercontinental range. While this initially appeared to be an insurmountable problem, it ended up being solved in an almost trivial fashion. A series of ports were added inside the rocket nozzle that were opened when the guidance systems called for engine cut-off. The reduction in pressure was so abrupt that the remaining fuel broke up and blew out the nozzle without contributing to the thrust.
The first to use these developments was the US Navy. It had been involved in a joint program with the US Army to develop the liquid-fueled PGM-19 Jupiter, but had always been skeptical of the system. The Navy felt that liquid fuels were too dangerous to use onboard ships, especially submarines. Rapid success in the solids development program, combined with Edward Teller's promise of much lighter nuclear warheads during Project Nobska, led the Navy to abandon Jupiter and begin development of their own solid-fuel missile. Aerojet's work with Hall was adapted for their UGM-27 Polaris starting in December 1956.
Missile farm concept
The US Air Force saw no pressing need for a solid fuel ICBM. Development of the SM-65 Atlas and SM-68 Titan ICBMs was progressing, and "storable" liquid propellants were being developed that would allow missiles to be left in a ready-to-shoot form for extended periods. These could be placed in missile silos for added protection, and launch in minutes. This met their need for a weapon that would be safe from sneak attacks; hitting all of the silos within a limited time window before they could launch simply did not seem possible.But Hall saw solid fuels not only as a way to improve launch times or survivability, but part of a radical plan to greatly reduce the cost of ICBMs so that thousands could be built. He envisioned a future where ICBMs were the primary weapon of the US, not in the supporting role of "last ditch backup" as the Air Force saw them at the time. This would require huge deployments, which would not be possible with existing weapons due to their high cost and operational manpower requirements. A solid fuel design would be simpler to build, and easier to maintain.
Hall's ultimate plan was to build a number of integrated missile "farms" that included factories, missile silos, transport and recycling. He was aware that new computerized assembly lines would allow continual production, and that similar equipment would allow a small team to oversee operations for dozens or hundreds of missiles, radically reducing the manpower requirements. Each farm would support between 1,000 and 1,500 missiles being produced in a continuous low rate cycle. Systems in a missile would detect failures, at which point it would be removed and recycled, while a newly built missile would take its place. The missile design was based purely on lowest possible cost, reducing its size and complexity because "the basis of the weapon's merit was its low cost per completed mission; all other factors – accuracy, vulnerability, and reliability – were secondary."
Hall's plan did not go unopposed, especially by the more established names in the ICBM field. Ramo-Wooldridge pressed for a system with higher accuracy, but Hall countered that the missile's role was to attack Soviet cities, and that "a force which provides numerical superiority over the enemy will provide a much stronger deterrent than a numerically inferior force of greater accuracy." Hall was known for his "friction with others" and in 1958 Schriever removed him from the Minuteman project, sending him to the UK to oversee deployment of the Thor IRBM. On his return to the US in 1959, Hall retired from the Air Force. He received his second Legion of Merit in 1960 for his work on solid fuels.
Although he was removed from the Minuteman project, Hall's work on cost reduction had already produced a new design of diameter, much smaller than the Atlas and Titan at, which meant smaller and cheaper silos. Hall's goal of dramatic cost reduction was a success, although many of the other concepts of his missile farm were abandoned.
Guidance system
Previous long-range missiles used liquid fuels that could be loaded only just prior to firing. The loading process took from 30 to 60 minutes in typical designs. Although lengthy, this was not considered to be a problem at the time, because it took about the same amount of time to spin up the inertial guidance system, set the initial position, and program in the target coordinates.Minuteman was designed from the outset to be launched in minutes. While solid fuel eliminated the fueling delays, the delays in starting and aligning the guidance system remained. For the desired quick launch, the guidance system would have to be kept running and aligned at all times. This was a serious problem for the mechanical systems, especially the gyroscopes which used ball bearings.
Autonetics had an experimental design using air bearings that they claimed had been running continually from 1952 to 1957. Autonetics further advanced the state of the art by building the platform in the form of a ball which could rotate in two directions. Conventional solutions used a shaft with ball bearings at either end that allowed it to rotate around a single axis only. Autonetics' design meant that only two gyros would be needed for the inertial platform, instead of the typical three.
The last major advance was to use a general-purpose digital computer in place of the analog or custom designed digital computers. Previous missile designs normally used two single-purpose and very simple electromechanical computers; one ran the autopilot that kept the missile flying along a programmed course, and the second compared the information from the inertial platform to the target coordinates and sent any needed corrections to the autopilot. To reduce the total number of parts used in Minuteman, a single faster computer was used, running separate subroutines for these functions.
Since the guidance program would not be running while the missile sat in the silo, the same computer was also used to run a program that monitored the various sensors and test equipment. With older designs this had been handled by external systems, requiring miles of extra wiring and many connectors to locations where test instruments could be connected during servicing. Now these could all be accomplished by communicating with the computer through a single connection. In order to store multiple programs, the computer, the D-17B, was built in the form of a drum machine but used a hard disk in place of the drum.
Building a computer with the required performance, size and weight demanded the use of transistors, which were at that time very expensive and not very reliable. Earlier efforts to use computers for guidance, BINAC and the system on the SM-64 Navaho, had failed and were abandoned. The Air Force and Autonetics spent millions on a program to improve transistor and component reliability 100 times, leading to the "Minuteman high-rel parts" specifications. The techniques developed during this program were equally useful for improving all transistor construction, and greatly reduced the failure rate of transistor production lines in general. This improved yield, which had the effect of greatly lowering production costs, had enormous spin-off effects in the electronics industry.
Using a general-purpose computer also had long-lasting effects on the Minuteman program and the US's nuclear stance in general. With Minuteman, the targeting could be easily changed by loading new trajectory information into the computer's hard drive, a task that could be completed in a few hours. Earlier ICBMs' custom wired computers, on the other hand, could have attacked only a single target, whose precise trajectory information was hard-coded directly in the system's logic.