MINUTEMAN I

Despite this preparatory work for Minuteman, the missile did not begin formal development until the air force secured final DoD ap­proval in February 1958. Meanwhile, civilian engineers employed by the air force at the Non-Rotating Engine Branch at Wright-Patterson AFB had continued efforts to develop large, solid-propellant motors. Perhaps from sources at Aerojet, they learned about adding large quantities of aluminum to a solid propellant to increase its perfor­mance. They combined this with other information they had been gathering on solid propellants while Hall was still with the branch. Bill Fagan from the branch carried this information to Hall at the Ballistic Missile Division (as the WDD had become).52

The specifics of Minuteman technology continued to evolve, but its basic concept took advantage of the reduced complexity of solids over liquids. This cut the number of people needed to launch it. Each missile could be remotely launched from a control center us­ing communications cables, without a crew of missileers in atten­dance. The air force initially considered using mobile Minutemen but ultimately decided to launch them from silos. Because solids were more compact than liquids, Minuteman IA (by one account) was only 53.8 feet long to Atlas’s 82.5, Titan I’s 98, and Titan II’s 108 feet in length. Its diameter was slightly over half that of the other three missiles (5.5 feet to the others’ 10), and its weight was only 65,000 pounds to Atlas’s 267,136, Titan I’s 220,000, and Titan II’s 330,000 pounds. All of this made the costs of silos much lower and substantially reduced the thrust needed to launch the missile. Consequently, the initial costs of Minuteman were a fifth and the annual maintenance costs a tenth those of Titan I. Moreover, a crew of two could launch 10 Minutemen, whereas it took six people to launch each Titan I—a 30-fold advantage in favor of Minuteman.53

Minuteman development used multiple approaches to arrive at individual technologies, as had been true with Atlas. Ballistic Mis­sile Division (BMD) contracted separately with Aerojet and Thiokol to work on all three stages of the missile. A later contract with Her­cules assigned it to work on the third stage, too. As firms developed technologies, parallel development gave way to specific responsi-

bilities, with Thiokol building the first stage, Aerojet the second, and Hercules the third. Space Technology Laboratories retained its role in systems engineering and technical direction at BMD. These and other companies and organizations all sent representatives to frequent program-review meetings and quarterly gatherings of top officials. They examined progress and identified problems requiring solution. Then, the relevant organizations found solutions to keep the program on track.54

One major technical challenge involved materials for the nozzle throats and exit cones. The addition of aluminum to the propellant provided a high enough specific impulse to make Minuteman fea­sible, as it had done for Polaris, and its combustion produced alu­minum oxide particles that damped instabilities in the combustion 248 chamber. But the hot product flow degraded the nozzle throats and Chapter 6 other exposed structures. It seemed that it might not be possible to design a vectorable nozzle that would last the 60 seconds needed for the missile to reach its ballistic trajectory so it could arrive accu­rately on target. Solving this problem required “many months and many dollars. . . spent in a frustrating cycle of design, test, failure, redesign, retest, and failure." The Minuteman team used many dif­ferent grades and exotic compounds of graphite, which seemed the most capable material, but all of them experienced blowouts or per­formance-degrading erosion. One solution was a tungsten throat in­sert, a compromise in view of its high weight and cost. For the exit cones, the team tried Fiberite molded at high pressure and loaded with silica and graphite cloth. It provided better resistance to ero­sion than graphite but still experienced random failures.55

Another significant problem concerned the vectorable feature of the nozzles. Polaris had solved its steering problem with jetavators, but flight-control studies for Minuteman showed a need for the stage-one nozzles to vector the thrust eight degrees, more than Min – uteman engineers thought jetavators could deliver. Thiokol’s test of the motor in 1959—the largest solid-propellant powerplant yet built—resulted in the ejection of all four nozzles after 30 millisec­onds of firing, well before the full stage had ignited. Five successive explosions of the motors and their test stands occurred in October 1959, each having a different failure mode. BMD halted first-stage testing in January 1960. Discussion among BMD, STL, and Thiokol personnel revealed two problem areas, internal insulation and the nozzles themselves, as masking other potential problems.

There followed two concurrent programs of testing. Firings with battleship-steel cases tested movable nozzles, while partici­pants used flight-weight cases to test a single, fixed nozzle massive

enough to sustain a full-duration firing. Thiokol solved the problem with insulation by summer but did not resolve the nozzle issue un­til fall. This was close to the date set for all-up testing of the entire missile. However, General Phillips had ordered Thiokol to begin manufacturing the first stage except the nozzles, permitting instal­lation of the nozzles as soon as their problem was solved.56

Among many other difficulties, a major concern was launching from a silo. The first successful launch of the missile occurred in early February 1961, which Phillips referred to as “December 63rd" since the planned date had been in December 1960. It and two suc­ceeding launches took place from a surface pad and were so suc­cessful that the team advanced the first silo launch to August 1961. The missile blew up in the silo, giving credence to critics in STL who had argued that firing a missile from a silo was impossible. As Phillips said, the missile “really came out of there like a Roman candle."57

Fortunately, team members recovered enough of the guidance system from the wreckage to find that the problem was not the silo launch itself but quality control. Solder tabs containing con­nections had vibrated together, causing all of the stages to ignite simultaneously. Knowing this, the team was able to prepare the fifth missile for flight with the problem solved by mid-November, when it had a successful flight from the silo. Previous testing of silo launches aided this quick recovery. In early 1958, BMD and STL engineers had arranged for development of underground silos. They divided the effort into three phases, with the first two using only subscale models of Minuteman. The third tested full-scale models. Most testing occurred at the air force’s rocket site on Edwards AFB in the remote Mojave Desert, but Boeing (contractor for missile as­sembly and test) tested subscale models in Seattle.

At the rocket site on Leuhman Ridge at Edwards, subscale silos investigated heat transfer and turbulence in some 56 tests by No­vember 1958. Meanwhile, Boeing modeled the pressures that the rocket’s exhaust gases imparted to the missile and silo. It also ex­amined acoustic effects of the noise levels generated by the rocket motor in the silo on delicate systems such as guidance. Armed with such data, engineers at Edwards began one-third-scale tests in Feb­ruary 1959. Full-scale tests initially used a mock-up made of steel plate with ballast to match the weight and shape of the actual mis­sile and only enough propellant (in the first-stage motor) to provide about three seconds of full thrust—enough to move the missile, on a tether, out of the silo and to check the effects of the thrust on the silo and missile. They configured the tether so that the missile

would not drop back on the silo and damage it. These tests ensured that the second silo launch at Cape Canaveral on November 17, 1961, was fully successful.58

The all-up testing on Minuteman was itself a significant inno­vation later used on other programs, including the Saturn launch vehicle for the Apollo program. It differed from the usual practice for liquid-propellant missiles—gradually testing a missile’s differ­ent capabilities over a series of ranges and tests (for first-stage or booster propulsion, for second-stage or sustainer-engine propulsion, for guidance/control, and so forth). For the first time, it tested all of the missile’s functions at once over the full operational range. Al­though the practice accorded well with general procedures at BMD, such as concurrency, its use came about in an odd way. According 250 to Otto Glasser, he was briefing Secretary of the Air Force James H.

Chapter 6 Douglas on Minuteman, with Gen. Curtis LeMay, vice chief of staff of the air force, sitting next to Douglas. Douglas insisted Glasser had moved the first flight of the missile to a year later than the original schedule. Glasser protested (to no avail) that this was not the case, and the only way he could conceive to cut a year out of the develop­ment process was all-up testing. “Boy, the Ramo-Wooldridge crowd came right out of the chair on that," Glasser said. They protested “a test program. . . with that sort of lack of attention to all normal, sensible standards." But all-up testing “worked all the way."59

Overcoming these and other problems, BMD delivered the first Minuteman I to the Strategic Air Command in October 1962, al­most exactly four years after the first contracts had been signed with contractors to begin the missile’s development. This was a year earlier than initially planned because of the speeded-up sched­ule. The missile in question was the A model of Minuteman I, later to be succeeded by a B model. The former was 53.7 feet long and consisted of three stages plus the reentry vehicle. Thiokol’s first stage included a new propellant binder developed by the company’s chemists from 1952 to 1954. Thiokol first tried a binder called poly­butadiene-acrylic acid, or PBAA, an elastomeric (rubberlike) copo­lymer of butadiene and acrylic acid that allowed higher concentra­tions of solid ingredients and greater fuel content than previous propellants. It had a higher hydrogen content than earlier Thiokol polysulfide polymers. With PBAA, a favorable reaction of oxygen with the aluminum generated significant amounts of hydrogen in the exhaust gases, reducing the average molecular weights of the combustion products (since hydrogen is the lightest of elements). This added to the performance, with Minuteman being the first rocket to use the new binder.60

But testing showed PBAA had a lower tear strength than poly­sulfide, so Thiokol added 10 percent acrylonitrile, creating poly­butadiene-acrylic acid-acrylonitrile (PBAN). The binder and curing agent constituted only 14 percent of the propellant, with ammonium perchlorate (oxidizer) and aluminum (fuel) the two other major in­gredients. The combination yielded a theoretical specific impulse of more than 260 lb-sec/lb, with the actual specific impulse at sea level at 70°F somewhat lower than 230.61

For stage two of Minuteman I, Aerojet used the polyurethane binder employed in Polaris, with ammonium perchlorate as the oxidizer and aluminum powder the major fuel. It used two slightly different propellant grains, with a faster-burning inner grain and a slower-burning outer one. The combination resulted in a conversion of the four-point, star-shaped, internal-burning cavity to a cylindri­cal one as the propellant burned, avoiding slivers of propellant that did not burn. The propellant yielded a vacuum specific impulse of nearly 275 lbf-sec/lbm at temperatures ranging from 60°F to 80°F.62

For stage three, the Hercules Powder Company used a glass- filament-wound case instead of the steel employed on stages one and two, plus a very different propellant than for the first two stages. The third stage featured four phenolic-coated aluminum tubes for thrust termination and a grain consisting of two separate composi­tions. The one used for the largest percentage of the grain included the high-explosive HMX, combined with ammonium perchlorate, nitroglycerin, nitrocellulose, aluminum, a plasticizer, and a stabi­lizer. The second composition had the same basic ingredients mi­nus the HMX and formed a horizontal segment at the front of the motor. A hollow core ran from the back of the motor almost to the segment containing the non-HMX composition. It was roughly cone shaped before tapering off to a cylinder. This motor yielded a specific impulse of more than 275 lbf-sec/lbm at temperatures ranging from 60°F to 80°F. The four nozzles for stage three of Min – uteman I rotated in pairs up to four degrees in one plane to provide pitch, yaw, and roll control.63 Minuteman I, Wing I became opera­tional at Malmstrom AFB, Montana, in October 1962.64

It would be tedious to follow the evolution of Minuteman through all the improvements in its later versions, but some discussion of the major changes is appropriate. Wings II through V of Minuteman I (each located at a different base) featured several changes to increase the missile’s range. This had been shorter than initially planned be­cause of the acceleration of the Minuteman I schedule. The shorter range was not a problem at Malmstrom because it was so far north (hence closer to the Soviet Union), but range became a problem

starting with Wing II. Consequently, for it and subsequent missiles, more propellant was added to the aft dome of stage one, and the exit cone included contouring that made the nozzle more efficient. In stage two, the material for the motor case was changed from steel to titanium. Titanium is considerably lighter than steel but more expensive. Since each pound of reduced weight yielded an extra mile of range, use of titanium seemed worth the extra cost. The nozzles also were lighter. Overall, the reduction in weight totaled slightly less than 300 pounds despite an increase in propellant weight. The increase in propellant mass plus the decrease in weight yielded a range increase of 315 miles to a figure usually given as 6,300 nauti­cal miles. There were no significant changes to stage three.65

MINUTEMAN II

For Minuteman II, the major improvements occurred in Aerojet’s stage two. There had been problems with cracking and ejection of graphite from the nozzles and aft closure of stage one. An air force reliability improvement program solved these difficulties. There had also been problems with insulation burning through in the aft dome area of stage three. Unspecified design changes inhibited the flow of hot gases in that region. Stage two, however, featured an entirely new rocket motor with a new propellant, a slightly greater length, a substantially larger diameter, and a single fixed nozzle that used a liquid-injection thrust-vector-control system for directional control.66

The new propellant was carboxy-terminated polybutadiene (CTPB), which propellant companies other than Aerojet had devel­oped. Some accounts attribute its development to Thiokol, which first made the propellant in the late 1950s and converted it into a useful propellant in the early 1960s. Initially, Thiokol chemists used an imine known as MAPO and an epoxide in curing the CTPB. It turned out that the phosphorous-nitrogen bond in the imine was susceptible to hydrolysis, causing degradation and softening of the propellant. According to Thiokol historian E. S. Sutton, “The post­curing problem was finally solved by the discovery that a small amount of chromium octoate (0.02%) could be used to catalyze the epoxide-carboxyl reaction and eliminate this change in properties with time." A history of Atlantic Research Corporation agrees that Thiokol produced the CTPB but attributes the solution of the curing problem to ARC, which is not incompatible with Sutton’s account. According to the ARC history, “ARC used a complex chromium compound, which would accelerate the polymer/epoxy reaction,

paving the way for an all epoxy cure system for CTPB polymer." The result was “an extremely stable binder system."67

It frequently happens in the history of technology that innova­tions occur to different people at about the same time. This ap­pears to have been the case with CTPB, which Aerojet historians attribute to Phillips Petroleum and Rocketdyne without providing details. These two companies may have been the source for infor­mation about the CTPB that Aerojet used in Minuteman II, stage two. Like Thiokol, in any event, Aerojet proposed to use MAPO as a cross-linking agent. TRW historians state that their firm’s labo­ratory investigations revealed the hydrolysis problem. They state that “working with Aerojet’s research and development staff," they developed “a formulation that eliminated MAPO. . . ." The CTPB that resulted from what apparently was a multicompany develop­ment effort had better fuel values than previous propellants, good mechanical properties such as the long shelf life required for silo – based missiles, and a higher solids content than previous binders. The propellant consisted primarily of CTPB, ammonium perchlo­rate, and aluminum. It yielded a vacuum specific impulse more than 15 lbf-sec/lbm higher than the propellant used in stage two of Minuteman I, Wing II.68

Although CTPB marked a significant step forward in binder tech­nology, it was not as widely used as it might have been because of its higher cost compared with PBAN. Another factor was the emer­gence in the late 1960s of an even better polymer with lower viscos­ity and lower cost, hydroxy-terminated polybutadiene (HTPB). It became the industry standard for newer tactical rockets. HTPB had many uses as an adhesive, sealant, and coating, but to employ it in a propellant required, among other things, the development of suit­able bonding agents. These tightly linked the polymer to such solid ingredients as ammonium perchlorate and aluminum. Without such links, the propellant could not withstand the temperature cy­cling, ignition pressure, and other forces that could cause the solid particles to separate from the binder network. This would produce voids in the grain that could result in cracks and structural failure.

A key figure in the development of HTPB for use as a binder was Robert C. Corley, who served as a research chemist and project manager at the Air Force Rocket Propulsion Laboratory at Edwards AFB from 1966 to 1978 and rose through other positions to become the lab’s chief scientist from 1991 to 1997. But many other people from Thiokol, Aerojet, the army at Redstone Arsenal, Atlantic Re­search, Hercules, and the navy were also involved. Even HTPB did

not replace PBAN for all uses, including the Titan III, Titan IVA, and Space Shuttle solid-rocket motors, because PBAN could be pro­duced for the comparatively low cost of $2.50 per pound at a rate in the 1980s of 4 million pounds per year, much higher than for any other propellant.69

To return to Minuteman II, however, the second major change in the stage-two motor was the shift to a single nozzle with liquid thrust vector control replacing movable nozzles for control in pitch and yaw. Static firings had shown that the same propellants pro­duced seven to eight points less specific impulse when fired from four nozzles than from a single one. With the four nozzles, liquid particles agglomerated in their approach sections and produced exit-cone erosion, changing the configuration of the exit cone in 254 an unfavorable way. The solution was not only a single nozzle on Chapter 6 Minuteman II’s second stage but also the change in thrust vector control. The navy had begun testing a Freon system for thrust vec­tor control in the second stage of Polaris A3 in September 1961, well before the Minuteman II, stage-two program began in Febru­ary 1962. The system was low in weight, was insensitive to pro­pellant flame temperature, and posed negligible constraints on the design of the nozzle. The Minuteman engineers adopted it—but one more example of borrowings back and forth between the Polaris and Minuteman projects despite the air force’s view of Polaris as a threat to its roles and missions.

Despite this pioneering work by the navy and its contractors, ac­cording to TRW historians, their firm still had to determine how much “vector capability" stage two of Minuteman II would require. TRW analyzed the amount of injectant that could be used before sloshing in the tank permitted the ingestion of air, and it determined the system performance requirements. Since Aerojet was involved in the development of the system for Polaris, probably its participa­tion in this process was also important. In any event, the Minute – man team, like the navy, used Freon as the injectant, confining it in a rubber bladder inside a metal pressure vessel. Both TRW and Aerojet studied the propensity of the Freon to “migrate" through the bladder wall and become unavailable for its intended purpose. They found that only 25 of 262 pounds of Freon would escape, leav­ing enough to provide the necessary control in pitch and yaw. A separate solid-propellant gas generator provided roll control. In addi­tion to these changes, stage two of Minuteman II increased in length from 159.2 inches for Minuteman I to 162.32 inches. The diameter increased from 44.3 to 52.17 inches, resulting in an overall weight increase from 11,558.9 to 15,506 pounds. Some 3,382.2 of this ad-

ditional 3,947.1 pounds consisted of propellant weight. Even so, the propellant mass fraction decreased slightly from 0.897 to 0.887.70

The Strategic Air Command put the first Minuteman II squadron on operational alert in May 1966, with initial operational capability declared as of December 1966. In the next few years, the air force began replacing Minuteman Is with Minuteman IIs.71

MINUTEMAN III

Minuteman III featured multiple, independently targetable reentry vehicles with a liquid fourth stage for deployment of this payload. This last feature was not particularly relevant to launch-vehicle de­velopment except that the added weight required for it necessitated higher booster performance. Stages one and two did not change from Minuteman II, but stage three became larger. Hercules lost the contract for the larger motor to Aerojet. Subsequently, Thiokol and a new organization, the Chemical Systems Division of United Technologies Corporation, won contracts to build replacement mo­tors. Stage three featured a fiberglass motor case, the same basic propellant Aerojet had used in stage two only in slightly different proportions, a single nozzle that was fixed in place and partially submerged into the case, a liquid-injection thrust-vector-control system for control in pitch and yaw, a separate roll-control system, and a thrust-termination system.

Aerojet had moved its filament-wound case production to Sacra­mento. It produced most of the Minuteman fiberglass combustion chambers there but ceased winding filament in 1965. Meanwhile, Young had licensed his Spiralloy technology to Black, Sivalls, and Bryson in Oklahoma City, which became a second source for the Minuteman third-stage motor case. This instance and the three firms involved in producing the third stage illustrate the extent to which technology transferred among the contractors and subcon­tractors for government missiles and rockets.

The issue of technology transfer among competing contractors and the armed services, which were also competing over funds and missions, is a complex one about which a whole chapter—even a book—could be written. To address the subject briefly, there had been a degree of effort to exchange knowledge about rocket propul­sion technology beginning in 1946 when the navy provided fund­ing for a Rocket Propellant Information Agency (RPIA) within the Johns Hopkins University’s Applied Physics Laboratory. The army added support in 1948, and the RPIA became the Solid Propellant Information Agency (SPIA). The air force joined the other services in 1951. After the Sputnik launch, the newly created NASA be-

gan participating in SPIA activities in 1959. Meanwhile, the navy created the Liquid Propellant Information Agency (LIPA) in 1958. The SPIA and LPIA combined on December 1, 1962, to create the Chemical Propulsion Information Agency (CPIA).

With the further development of rockets and missiles, the need had become obvious by 1962 for a better exchange of information. So the DoD created an Interagency Chemical Rocket Propulsion Group in November 1962, the name later changing to the Joint Army/Navy/NASA/Air Force (JANNAF) Interagency Propulsion Committee. Together with CPIA, JANNAF effectively promoted sharing of technology. In addition, “joint-venture" contracts, pio­neered by Levering Smith of the navy, often mandated the sharing of manufacturing technology among companies. These contracts 256 served to eliminate the services’ dependence for a given technology Chapter 6 on sole sources that could be destroyed by fire or possible enemy targeting. It also provided for competitive bidding on future con­tracts. The air force had a similar policy.72

Meanwhile, the propellant for Aerojet’s third stage of Minuteman had less CTPB and more aluminum than the second stage. The grain configuration consisted of an internal-burning cylindrical bore with six “fins" radiating out in the forward end. The igniter used black – powder squibs to start some of the CTPB propellant, which in turn spread the burning to the grain itself. The 50 percent submerged nozzle had a graphite phenolic entrance section, a forged tungsten throat insert, and a carbon-phenolic exit cone. As compared with the 85.25-inch-long, 37.88-inch-diameter third stage of Minuteman II, that for Minuteman III was 91.4 inches long and 52 inches in di­ameter. The mass fraction improved from 0.864 to 0.910, and with a nearly 10-lbf-sec/lbm greater specific impulse, the new third stage had more than twice the total impulse of its predecessor—2,074,774 as compared with 1,006,000 pounds force per second.73

The thrust-vector-control system for the new stage three was similar to that for stage two except that strontium perchlorate was used instead of Freon as the injectant into the thrust stream to pro­vide control in the pitch and yaw axes. Helium gas provided the pressure to insert the strontium perchlorate instead of the solid – propellant gas generator used in the second stage. Roll control again came from a gas generator supplying gas to nozzles pointing in opposite directions. When both were operating, there was neutral torque in the roll axis. When roll torque was required, the flight – control system closed a flapper on one of the nozzles, providing unbalanced thrust to stop any incipient roll.

To ensure accuracy for the delivery of the warheads, Minute – man had always required precise thrust termination for stage three, determined by the flight-control computer. On Minuteman I, the thrust-termination system consisted of four thick carbon-phenolic tubes integrally wound in the sidewall of the third-stage case and sealed with snap-ring closures to form side ports. Detonation of explosive ordnance released a frangible section of the snap ring, thereby venting the combustion chamber and causing a momentary negative thrust that resulted in the third stage dropping away from the postboost vehicle.

The system for Minuteman III involved six circular-shaped charges on the forward dome. Using data from high-speed films and strain gauges, the Minuteman team learned that this arrange­ment worked within 20 microseconds, cutting holes that resulted in a rupture of the pressure vessel within 2 additional milliseconds. But the case developed cracks radiating from the edge of the holes. TRW used a NASTRAN computer code to define propagation of the cracks. It then determined the dome thickness needed to eliminate the failure of the fiberglass. Aerojet wound “doilies" integrally into the dome of the motor case under each of the circular charges. This eliminated the rupturing, allowing the system to vent the pressure in the chamber and produce momentary negative thrust.74

Minuteman Ills achieved their initial operational capability in June 1970, the first squadron of the upgraded missiles turned over to an operational wing at Minot AFB, North Dakota, in January 1971. By July 1975, there were 450 Minuteman Ils and 550 Minute – man Ills deployed at Strategic Air Command bases.75