PGM-17 Thor
The PGM-17A Thor was the first operative ballistic missile of the United States Air Force. It was named after the Norse god of thunder. It was deployed in the United Kingdom with the Royal Air Force between 1959 and September 1963 as an intermediate-range ballistic missile with thermonuclear warheads. Thor was in height and in diameter.
The first generation of Thor missiles were rushed into service, and design mistakes resulted in a 24% launch failure rate. The competing PGM-19 Jupiter missile saw more use, but both were quickly eclipsed by the Air Force's long range ICBM program, which could be fired from U.S. soil. By 1959, with the Atlas missile well on its way to operational status, both Thor and Jupiter programs became obsolete as delivery vehicles, yet continued to be built and deployed until 1963 for political reasons and to maintain aerospace industry employment.
The missile's lasting legacy continued as the Thor and later Delta families of space launch vehicles used boosters derived from the initial Thor missile, and continued on into the 21st century.
Development
Fearful that the Soviet Union would deploy a long-range ballistic missile before the U.S., in January 1956 the USAF began developing the Thor, a intermediate-range ballistic missile. The program proceeded quickly as a stop-gap measure, and within three years of inception the first of 20 Royal Air Force Thor squadrons became operational in the UK. The UK deployment carried the codename 'Project Emily'. One of the advantages of the design was that, unlike the Jupiter MRBM, the Thor could be carried by the USAF's cargo aircraft of the time, which made its deployment more rapid. The launch facilities were not transportable and had to be built on site. Once the first generation of ICBMs based in the U.S. became operational, Thor missiles were quickly retired. The last of the missiles was withdrawn from operational alert in 1963.A small number of Thors with "Thrust Augmented Delta" boosters and W-49 Mod 6 warheads remained operational in the anti-satellite missile role as Program 437 until April 1975. These missiles were based on Johnston Island in the Pacific Ocean and had the ability to destroy satellites in low Earth orbit. With prior warning of an impending launch, they could destroy a Soviet spy satellite soon after orbital insertion.
Initial development
Development of the Thor was initiated by the USAF in 1954. The goal was a missile system that could deliver a nuclear warhead over a distance of with a Circular Error Probability of. This range would allow Moscow to be attacked from a launch site in the UK. The initial design studies were headed by Commander Robert Truax and Dr. Adolph K. Thiel. They refined the specifications to an IRBM with:- A range
- diameter, long
- A gross takeoff weight of
- Propulsion provided by half of the existing SM-64 Navaho-derived Atlas booster engine
- maximum speed during warhead reentry
- Inertial guidance system with radio backup
File:LC-26B Thor-Able.jpg|thumb|left|Thor-Able at the Cape Canaveral Space Force Museum, Florida.
On 30 November 1955, three companies were given one week to bid on the project: Douglas, Lockheed, and North American Aviation. The missile was to use existing technology, skills, abilities, and techniques to speed entry into service. On 27 December 1955, Douglas was awarded the prime contract for the airframe and integration. The Rocketdyne division of North American Aviation was awarded the engine contract, AC Spark Plug the primary inertial guidance system, Bell Labs the backup radio guidance system, and General Electric the nose cone/reentry vehicle. Douglas' proposal included choosing bolted tank bulkheads and a tapered fuel tank for improved aerodynamics.
The engine was a direct descendant of the Atlas MA-3 booster engine, with removal of one thrust chamber and a rerouting of the plumbing to allow the engine to fit within the smaller Thor thrust section.
Engine component tests began in March 1956. The first engineering model engine was available in June, followed by the first flight engine in September. Early Thor engines suffered from foaming turbopump lubricating oil at high altitudes and bearing retention issues, resulting in several launch failures. The initial Thor tests in 1957 used an early version of the Rocketdyne LR-79 engine with a conical nozzle and of thrust. By early 1958, this had been replaced by an improved model with a bell-shaped nozzle see and of thrust. The fully developed Thor IRBM had of thrust.
Phase I test launches
Thor was test launched from LC-17 at Cape Canaveral Missile Annex. The compressed development schedule meant that plans for the Atlas bunker had to be used to allow the completion of the facility in time, with launchpad LC-17B completed just in time for the first test flight.Missile 101, the first flight-ready Thor, arrived at Cape Canaveral in October 1956. It was erected at LC-17B and underwent several practice propellant loading/unloading exercises, a static firing test, and a month-long delay while a defective relay was replaced. Launch finally took place on 25 January 1957. The engine lost thrust almost immediately after liftoff, and the Thor fell onto the launch pad and exploded. A film of prelaunch preparations showed crews dragging a liquid oxygen filler hose through a sandy area, which led to the belief that debris entering the LOX, caused the failure of a valve.
Thor 102 was launched on 20 April 1957. The booster performed normally, but the flight was terminated at 35 seconds after an erroneous console readout caused the Range Safety Officer to believe that the missile was headed inland instead of out to sea. The tracking console was found to be wired in reverse. The short flight raised confidence that Thor could fly successfully.
The third Thor launch exploded four minutes before the planned launch after a defective valve allowed LOX tank pressure to build up to unsafe levels. The responsible technicians had also failed to pay attention to the tank pressure gauges. LC-17B consequently had to be repaired for the second time in four months.
Missile 104, launched 22 August from the newly opened LC-17A, broke up at T+92 seconds due to a drop in signal strength from the programmer, causing the engine to gimbal hard right. The guidance system tried to compensate, but the resulting structural loads exceeded the strength of the missile tankage.
Thor 105, on 20 September 1957, completed the first successful flight, which occurred 21 months after the start of the program. No telemetry equipment was included on this missile, with the resulting mass savings allowing a total range of.
Missile 107 fell back onto LC-17A and exploded at launch when a gas generator valve failed to open.
Missile 108 exploded around T+140 seconds without prior warning. Engineers were initially unable to determine the cause of the failure. After the first Thor-Able launch failed six months later due to a seized turbopump, it was concluded that a similar failure had occurred on 108. However, 108 did not have sufficient instrumentation to determine the exact nature of the failure.
The final three Thor tests during 1957 were all successful, but 1958 began with back-to-back failures. Thor 114 was destroyed by Range Safety 150 seconds into launch when the guidance system lost power and Thor 120's engine shut down slightly under two minutes after liftoff. The telemetry system had experienced a power failure during launch, so the reason for the engine cutoff could not be satisfactorily determined.
On 19 April 1958, Missile 121 dropped back onto LC-17B and exploded, putting the pad out of action for three months. A fuel duct collapse was believed to have been the culprit.
On 22 April, Missile 117, carrying the first Able upper stage, lost thrust and broke up at T+146 seconds due to a turbopump failure.
The Jupiter, Thor, and Atlas missiles all used a variant of the Rocketdyne LR-79 engine and all three suffered launch failures due to a marginal turbopump design. There were two separate problems with the pumps. The first was the discovery during testing at Huntsville that the lubricant oil tended to foam at high altitude as the air pressure decreased. The other was that pump shaft vibration from the nearly 10,000 RPM operating speed would cause the bearings to come out of their sockets, resulting in the pump abruptly seizing up. The Army had suspended Jupiter launches for four months until the turbopump issues could be resolved, and as a result no more pump failures affected that program.
In contrast, the USAF's General Schreiver rejected the idea of sending Thor and Atlas missiles back to the factory so as to not delay the testing program. Instead, in-field modifications to pressurize the turbopump gearboxes and use an oil with a different viscosity that was less prone to foaming were conducted. Modified bearing retainers were not installed. Subsequently, six consecutive Thor and Atlas launches failed during February–April 1958, several due to turbopump problems. The following four months did not include any turbopump failures, but the 17 August 1958, launch of the world's first lunar probe on Thor-Able 127 ended in an explosion due to a turbopump failure. A month later, Atlas 6B also suffered a turbopump failure and the Air Force gave in and agreed to replace the turbopumps in all of their missiles, after which there were no more launch failures due to a turbopump problem.
Five successful Thor tests were conducted in June–July 1958, the last one carrying a mouse named Wickie on a biological mission; the capsule sank into the ocean and could not be recovered. Thor 126 lost thrust 50 seconds into launch when a LOX valve inadvertently closed. The vehicle pitched down and broke up from aerodynamic loads. On 30 July 1958, six Douglas technicians were severely burned, three fatally, when a LOX valve failed at the Thor static test stand in Sacramento, California.