THE RV-A-10 MISSILE

In the meantime, Thiokol had teamed up with General Electric in the Hermes project to produce a solid-propellant missile (initially

known as the A-2) that was much larger than the Sergeant sounding rocket. It operated on a shoestring budget until canceled, but it still made significant progress in solid-propellant technology.16

The original requirements for the A-2 were to carry a 500-pound warhead to a range as far as 75 nautical miles, but these changed to a payload weighing 1,500 pounds, necessitating a motor with a diameter of 31 inches. Thiokol started developing the motor in May 1950, a point in time that allowed the project to take advantage of work on the Sergeant sounding rocket and of Larry Thackwell’s ex­perience with it. By December 1951, the program had successfully completed a static test of the 31-inch motor. From January 1952 through March 1953, there were 20 more static tests at Redstone Arsenal and four flight tests of the missile at Patrick AFB, Florida.

230 In the process, the missile came to be designated the RV-A-10. The Chapter 6 project encountered unanticipated problems with nozzle erosion and combustion instability that engineers were able to solve.17

The four flight tests achieved a maximum range of 52 miles (on flight one) and a maximum altitude of 195,000 feet (flight two) us­ing a motor case 0.20 inch thick and a propellant grain featuring a star-shaped perforation with broad tips on the star. The propellant was designated TRX-110A. It included 63 percent ammonium per­chlorate by weight as the oxidizer. The propellant took advantage of an air force-sponsored project (MX-105) titled “Improvement of Polysulfide-Perchlorate Propellants" that had begun in 1950 and is­sued a final report (written by Thiokol employees) in May 1951. On test motor number two a propellant designated T13, which con­tained polysulfide LP-33 and ammonium perchlorate, achieved a specific impulse at sea level of more than 195 lbf-sec/lbm at 80°F but also experienced combustion instability. This led to the shift to TRX-110, which had a slightly lower specific impulse but no com­bustion instability.18

Thiokol had arrived at the blunter-tipped star perforation as a re­sult of Thackwell’s experience (at JPL) with the Sergeant test vehi­cle and of photoelastic studies of grains performed at the company’s request by the Armour Institute (later renamed the Illinois Institute of Technology). This, together with a thicker case wall than JPL had used with the Sergeant sounding rocket, eliminated JPL’s prob­lems with cracks and explosions. However, TRX-110 proved not to have enough initial thrust. The solution was to shift the size of the ammonium perchlorate particles from a mixture of coarse and fine pieces to one of consistently fine particles, which yielded not only higher initial thrust but also a more consistent thrust over time—a desirable trait. Meanwhile, the Thiokol-GE team gradually learned

about the thermal environment to which the RV-A-10 nozzles were exposed. Design of the nozzles evolved through subscale and full – scale motor tests employing various materials and techniques for fabrication. The best materials proved to be SAE 1020 steel with carbon inserts, and a roll weld proved superior to casting or forging for producing the nozzle itself.19

Another problem encountered in fabricating the large grain for the RV-A-10 was the appearance of cracks and voids when it was cured at atmospheric pressure, probably the cause of a burnout of the liner on motor number two. The solution proved to be twofold: (1) Thiokol poured the first two mixes of propellant into the motor chamber at a temperature 10°F hotter than normal, with the last mix 10°F cooler than normal; then, (2) Thiokol personnel cured the propellant under 20 pounds per square inch of pressure with a layer of liner material laid over it to prevent air from contacting the grain. Together, these two procedures eliminated the voids and cracks.20

With these advances in the art of producing solid-propellant mo­tors, the RV-A-10 became the first known solid-propellant rocket motor of such a large size—31 inches in diameter and 14 feet, 4 inches long—to be flight tested as of February-March 1953. Among its other firsts were scaling up the mixing and casting of polysulfide propellants to the extent that more than 5,000 pounds of it could be processed in a single day; the routine use of many mixes in a single motor; the use of a tubular igniter rolled into coiled plastic tubing (called a jelly roll) to avoid the requirement for a heavy clo­sure at the nozzle end to aid in ignition; and one of the early uses of jet vanes inserted in the exhaust stream of a large solid-propellant rocket to provide thrust vector control.

As recently as December 1945, the head of the Office of Scientific Research and Development during World War II, Vannevar Bush, had stated, “I don’t think anybody in the world knows how [to build an accurate intercontinental ballistic missile] and I feel confident it will not be done for a long time to come." Many people, even in the rocket field, did not believe that solid-propellant rockets could be efficient enough or of long enough duration to serve as long-range missiles. The RV-A-10 was the first rocket to remove such doubts from at least some people’s minds.21 Arguably, it provided a signifi­cant part of the technological basis for the entire next generation of missiles, from Polaris and Minuteman to the large solid boost­ers for the Titan IIIs and IVs and the Space Shuttle, although many further technological developments would be necessary before they became possible (including significant improvements in propellant performance).