The Minuteman Intercontinental Ballistic Missile (ICBM) has been deployed for over fifty years. As one of two second generation ICBMs, Minuteman represented the ultimate solution to the concept of land-based offensive strategic weapons. The solid propellant propulsion system provided for a nearly instantaneous response while reducing maintenance efforts and costs significantly below those of the first generation cryogenic oxidizer Atlas and Titan I. Even the second generation Titan II with its storable liquid propellants and comparable response time was cumbersome in comparison.
Development of the business end of all the ICBMs, the reentry vehicles, likewise went from the first generation heatsink thermal protection system to the second generation ablative reentry vehicles enabling larger payloads (the reentry vehicle was lighter) to be carried as well as improving accuracy. This article discusses the evolution of reentry vehicle design and fabrication leading up to and including the Minuteman Mark 5 and Mark 11 reentry vehicles. Detailing the earliest efforts of the Army, Navy and Air Force reentry vehicle approaches puts the development of the Minuteman Mark 5 and Mark 11 reentry vehicles into the proper historical perspective. The discussion of the Army's effort covers only the Jupiter IRBM program and its pioneering work on ablation. The Navy's contribution was a much different approach to the heatsink concept with the discussion ending with the Polaris A-l and A-2 as the follow-on programs closely resembled the later Air Force approach. Due to classification issues caused by current world events, the third generation Air Force reentry vehicle designs are not discussed in this article though they have been described in great detail in an earlier article by Lin. (1)
While bombardment rockets have been used for centuries, it was not until the creation of the German V-2 (also known as the A-4) that the warhead needed thermal protection due to reentry into the Earth's atmosphere. (2.) Since the entire V-2 impacted the target, there was no true separable reentry vehicle. (3) The original design called for the use of a lightweight alloy of magnesium and aluminum but wind tunnel tests indicated that from an altitude of 43 nautical miles, the operational maximum altitude, reentry into the lower atmosphere at 3,345 miles per hour would result in a warhead compartment skin temperature of 1,250 degrees Fahrenheit. Therefore the decision was made to use 1/4 inch sheet steel resulting in the need to decrease the explosive payload to hold the total warhead weight to 2,200 pounds (the steel casing weighing 550 pounds). The explosive chosen for the warhead was Amatol, a mixture of sixty percent TNT and forty percent ammonium nitrate, which was insensitive to heat and shock. There was no warhead compartment insulation. (4)
Arming a guided missile derived from the V-2 with an atomic warhead was an obvious next step in strategic warfare since it was only a matter of time for atomic bomb design to catch up with guided missile delivery capability. Concerned with the vulnerability of the eastern United States to long range missiles from the Soviet Union, in 1945 the National Advisory Committee for Aeronautics (NACA) realized an urgent need to begin studying the problems of hypersonic flight (defined as greater than five times the speed of sound which is the speed at which aerodynamic heating begins to be significant). By the late 1940s, two major NACA facilities, Ames Aeronautical Laboratory (Ames), Moffett Field, California, and Langley Aeronautical Laboratory (Langley), Hampton, Virginia, responded by expanding their aeronautical work to study aerodynamic issues involved in ballistic missile flight. (5)
Theoretical research into the problem of aerodynamic heating of ballistic missiles upon reentry into the atmosphere at high speeds was first published in 1949 by Carl Wagner. (6) The first comprehensive theoretical work was begun in 1951 by H. Julian Allen and A.J. Eggers, Jr., engineers at Ames. They studied the problem of reentry heating for ballistic, glide and skip-entry trajectories. Their investigation of the three types of trajectories was driven by the need to find a flight path that could best utilize the thermal protection materials then available. Allen and Eggers dismissed the pointed nose shape, a carry over from rifle bullet design, at the start, instead focusing their calculations on a blunt, hemispherical shape, recommending that "not only should pointed bodies be avoided, but that the rounded nose should have the largest radius possible." (Figure 1)
It is important to note that these calculations were made with "light" and "heavy" missile options and no mention was made of a reentry vehicle as such. The "light" missile optimum nose shape from a heat transfer standpoint was a blunt shape; for the "heavy" missile a more slender shape was optimum. Their calculations showed that the high drag caused a detached shock wave thus the majority of the heat generated was dissipated back into the atmosphere leaving only radiated heat to contend with, unlike a sharply pointed body where the shock wave was attached to the tip, causing heat transfer and destruction of the body. Additionally the heat reaching the blunt body would be more evenly distributed, preventing hot spots more prone to burn through.
Allen and Eggers demonstrated that the maximum deceleration encountered by a reentry vehicle was a function of the angle of reentry as well as velocity and independent of the shape, size and mass or drag coefficient. The importance of shape was the amount of heat that was absorbed by the reentry vehicle. A team of Ames researchers led by Eggers and including Fred Hansen and Bernard Cunningham published a method for predicting heat transfer to blunt bodies in 1958 though the work was done and in use much earlier but not published for six years due to classification issues. (7)
In order to reach targets 4,000 to 6,000 nautical miles away, ballistic missiles would need to be accelerated to speeds of up to approximately Mach 20 (15,223 miles per hour, just short of orbital velocity), 10 times the speed of a high-powered rifle bullet. (8) Reentry into the atmosphere at these speeds would generate a shock wave in which the atmosphere is heated to many thousands of degrees, even approaching 12,000 F, which exceeded the melting point of tungsten, the metallic element with the highest known melting point, 6,116 degrees Fahrenheit. (9) At this temperature the air plasma is also highly chemically reactive. There is a transport of heat by mass conduction from the air plasma to the vehicle surface which is dependent on both the temperature and density of the air in the plasma. At high altitudes where the air density is low, the mass transport of heat is low, in spite of the very high shock wave temperature. Conversely, at lower altitudes, the higher density plasma results in a higher heat flux for equal reentry vehicle velocities(Figure 2). (10)
Before discussing individual test and operational reentry vehicles, a brief discussion of testing methods, both for ground and flight is necessary.
Reentry Research Tools
Hypersonic Wind Tunnels
While the history of the military use of ballistic missiles rightly starts with the development of the A-4 (V-2) missile, perhaps just as important was the discovery by Allied troops of two highly advanced wind tunnel facilities at Peenemunde in the summer of 1945. One had apparently been in operation, a small diameter (1.2 foot) super-supersonic wind tunnel for intermittent use up to Mach 5 and a larger diameter (3.3 foot) continuous flow super-supersonic wind tunnel designed for speeds up to Mach 10.
In 1945 the first hypersonic wind tunnel in the United States was proposed by John Becker at Langley. Design difficulties and a perceived lack of urgency by NACA and Langley administrators delayed the construction for over a year but on November 26,1947, the first tests were successfully run at Mach 6.9. (11) Eggers at Ames, proposed a continuous flow hypersonic tunnel and it was completed in 1950. Between these two facilities, hypersonic research began in earnest, mainly focusing on aerodynamic issues directed towards supersonic aircraft research.
By 1955, the three major ballistic missile programs, the Air Force Thor (IRBM) and Atlas (ICBM) and the Army Jupiter (IRBM), made reentry vehicle research a high national priority. Two flight regimes required detailed study. The 1,500 nautical-mile IRBM Thor and Jupiter warhead reentry speed would be nearly 15,000 feet per second while the 5,000 nautical mile range ICBM would be nearly 25,000 feet per second. (12) Basic ballistic shapes, along the lines suggested by Allen and Eggers were tested up to the Mach 7-10 capabilities of the early hypersonic wind tunnels, confirming their theoretical results. However, the limitations in run times and temperatures, as well as atmospheric densities, soon illustrated the need for additional testing facilities.
The first shock tube was built in France in 1899 by Vielle to study flame fronts and propagation speeds resulting from explosions. (13) The concept languished until 1946 when Payman and Shepard in Britain published a thorough description of the design and use of shock tubes in studying explosions in mines. (14)
There are many variations of shock tube design but all share a basic two chamber concept. The first chamber is separated from the second with a burst diaphragm calculated to burst when the gas in the first chamber is compressed to a predetermined value. Since 1949, shock tubes have been used to augment aerodynamic studies using hypersonic wind tunnels, in particular the use by the mid-1950's was focused on reentry vehicle design and material selection since speeds greater than Mach 10 could easily be achieved, as well as much higher temperatures. The major drawback...