23 January 2016

What Your Kitchen Refrigerator and Ballistic Missiles Have in Common: Freon

While liquid-fueled rocket engines have been the mainstay for the satellite launch industry, the long road of technological development in solid-fuel rockets have also benefited the aerospace industry. Often times unique solutions were needed in the development of solid-fuel rockets. One of the more unusual ones was the use of liquid freon to direct the exhaust flame from solid-fueled rockets. That's right. Liquid. Freon. How? I'll get to that.

Minuteman II test launch
(National Park Service/Minuteman Missile NHS)
In the 1950s the conventional wisdom in ICBM development was that only liquid-fueled engines had the power to lift the heavy nuclear warheads of the day. The two main ICBMs in development, the Atlas and the Titan, used liquid-fueled engines. But the US Navy, seeking to put ICBMs on nuclear submarines as a sea-based strategic deterrent, considered liquid-fuels on a submarine wholly impractical and not just for safety reasons. As a result, the engineers who were developing the Polaris SLBM focused their efforts on solid-fuel rocket motors for the missile. They were storable and could be quickly fired. In addition, with enough right mix of solid propellants, the missile could be much smaller than a comparable liquid-fueled missile.

The advantages of a storable propellant and rapidity of launch made solid-fuel an attractive option for a land-based ICBM as well. In the US Air Force, General Bernard Schriever was in charge of the Air Force's ICBM development effort as the head of the Western Development Division. While he initially believed that liquid-fueled engines were the only way to power an operational ICBM, he was ably convinced by several of his engineers to look at solid-fuels as an alternative. That tangent then took on an important priority equal to that of the Atlas and Titan programs, becoming the Minuteman ICBM which was developed in the same time frame as the Navy's Polaris missile. The two weapons shared many similar characteristics due to their solid-fuel rocket engines. The first variant of the Minuteman, the LGM-30A Minuteman I, became operational at Malmstrom AFB in Montana in 1962. 
First SLBM launch, 23 July 1960.
Polaris A1 from the USS George Washington
(US Navy)
The first solid rockets used tabs that jutted into the exhaust stream to deflect the plume for directional control. It was the simplest system but to provide effective control and deflection, the tabs had to be of a size that inevitably cut into the exhaust stream's total velocity. The next solution was what the Polaris team called "jetevators". The exhaust cone of the solid rocket had an extension at the bottom of the cone that was in effect, a gimbaled extension of the skirt (rather than moving the whole nozzle assembly) and small actuators moved the whole extension. Jetevators were used on the first versions of the Polaris SLBM, the A1 and A2 variants. The main disadvantage of jetevators was they added technical complexity to the solid rocket motor as well as weight. Small jetevators could only provide slight corrections but to provide more significant directional control, larger and heavier jetevators would be needed.

Both the early versions of Polaris and Minuteman used jetevators on each of the three stages of the missiles, with the first and second stages of both missiles having four nozzles that could be differentially vectored to provide control. By 1962, however, the next versions of the missiles were already in development- for Polaris it was the A3 version (third version) and for the Minuteman it was the Minuteman II (second version, obviously). In both missiles a range increase was desired and one way to get it was to lighten the missile itself. For both new versions, the second stage switched from four nozzles with jetevators to a single nozzle that used what was called "liquid injection thrust vectoring control". For both the LGM-30B Minuteman II and the UGM-27C Polaris A3, a bigger second stage with the new liquid injection thrust control got the range increases needed. 

1964 patent diagram for liquid
injection thrust vectoring control.
(Google Patents)
Around the perimeter of the nozzle about 1/2 the way up were a series of four ports that angled slightly upward. Liquid freon was injected into one of the ports and as it did, it created a shockwave in the nozzle that pushed the exhaust stream in a direction up to 7 to 10 degress opposite from the port the freon entered. The freon didn't react with the hot plume, it merely created a thermal shockwave that pushed the plume one direction. By injecting freon into the various ports, directional control could be achieved for a lot less weight than traditional actuator-driven control mechanisms. 

On the Minuteman II, the second stage carried 262 pounds of freon in a rubber bladder to use for thrust vectoring. The Minuteman II and Polaris A3 weren't the first missiles to use this novel method of control. That honor goes to the Lance short-range battlefield missile that was used by the US Army until the 1960s. The knowledge gained from the Minuteman II and Polaris A3 in liquid injection thrust vectoring control would be used to its fullest on the large solid rocket boosters used on the Titan III and Titan IV launchers, long the mainstay of US expendable heavy-lift vehicles. Both boosters on the Titan launchers used liquid injection thrust vectoring control. If you look at a picture of a Titan III/IV at launch, you'll notice a small external tank attached to the core rocket's base, one for each booster. That's the reservoir for the liquids used for the thrust vectoring system of the solid rocket boosters.
From left to right: Polaris A1, Polaris A2, Polaris A3, Poseidon C3, Poseidon C4, Trident D5
(Federation of American Scientists)
The Polaris was superseded in the Navy's strategic deterrent by the Poseidon, which was followed by  the current missile, the Trident. The Minuteman II was retired from service and the land-based ICBM deterrent for the United States relies on the Minuteman III.

Further reading: 

Martin, the Titan I and the Titan II Ballistic Missiles
One of the Most Important Missions of the Douglas C-133 Cargomaster: Transporting ICBMs

Source: To Reach the High Frontier- A History of US Launch Vehicles, edited by Roger D. Launius and Dennis Jenkins. The University Press of Kentucky, 2002, p262-266.

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