CASTABLE DOUBLE-BASE PROPELLANT

The next major development in double-base propellants was a method for casting (rather than extruding) the grain. The company that produced the first known rocket motor using this procedure was the Hercules Powder Company, which had operated the govern­ment-owned Allegany Ballistics Laboratory since the end of World War II. The firm came into existence in 1912 when an antitrust suit

against its parent company, E. I. du Pont de Nemours & Company, forced du Pont to divest some of its holdings. Hercules began as an explosives firm that produced more than 50,000 tons of smokeless powder during World War I. It then began to diversify into other uses of nitrocellulose. During World War II, the firm supplied large quantities of extruded double-base propellants for tactical rockets. After the war, it began casting double-based propellants by beginning with a casting powder consisting of nitrocellulose, nitroglycerin, and a stabilizer. Chemists poured this into a mold and added a cast­ing solvent of nitroglycerin plus a diluent and the stabilizer. With heat and the passage of time, this yielded a much larger grain than could be produced by extrusion alone.

Wartime research by John F. Kincaid and Henry M. Shuey at the National Defense Research Committee’s Explosives Research Laboratory at Bruceton, Pennsylvania (operated by the Bureau of Mines and the Carnegie Institute of Technology), had yielded this process. Kincaid and Shuey, as well as other propellant chemists, had developed it further after transferring to ABL, and under Hercu­les management, ABL continued work on cast double-base propel­lants. This led to the flight testing of a JATO using this propellant in 1947. The process allowed Hercules to produce a propellant grain that was as large as the castable, composite propellants that Aero­jet, Thiokol, and Grand Central were developing in this period but with a slightly higher specific impulse (also with a greater danger of exploding rather than burning and releasing the exhaust gases at a controlled rate).24

The navy had contracted with Hercules for a motor to be used as an alternative third stage on Vanguard (designated JATO X241 A1). The propellant that Hercules’ ABL initially used for the motor was a cast double-base formulation with insulation material between it and the case. This yielded a specific impulse of about 250 lbf-sec/ lbm, higher both than Grand Central’s propellant for its Vanguard third-stage motor and the specification of 245 lbf-sec/lbm for both motors. A key feature of the motor was its case and nozzle, made of laminated fiberglass. ABL had subcontracted work on the case and nozzle to Young Development Laboratories, which developed a method during 1956 of wrapping threads of fiberglass soaked in epoxy resin around a liner made of phenolic asbestos. (A phenol is a compound used in making resins to provide laminated coatings or form adhesives.) Following curing, this process yielded a strong, rigid Spiralloy (fiberglass) shell with a strength-to-weight ratio 20 percent higher than the stainless steel Aerojet was using for its propellant tanks on stage two of Vanguard.25

In 1958, while its third-stage motor was still under development, Hercules acquired this fiberglass-winding firm. Richard E. Young, a test pilot who had worked for the M. W. Kellogg Company on the Manhattan Project, had founded it. In 1947, Kellogg had designed a winding machine under navy contract, leading to a laboratory in New Jersey that built a fiberglass nozzle. It moved to Rocky Hill, New Jersey, in 1948. There, Young set up the development labora­tories under his own name and sought to develop lighter materials for rocket motors. He and the firm evolved from nozzles to cases, seeking to improve a rocket’s mass fraction (the mass of the propel­lant divided by the total mass of a stage or rocket), which was as important as specific impulse in achieving high velocities. In the mid-1950s, ABL succeeded in testing small rockets and missiles us – 236 ing cases made with Young’s Spiralloy material.26

Chapter 6 This combination of a cast double-base propellant and the fiber­glass case and nozzle created a lot of problems for Hercules engi­neers. By February 1957, ABL had performed static tests on about 20 motors, 15 of which resulted in failures of insulation or joints. Combustion instability became a problem on about a third of the tests. Attempting to reduce the instability, Hercules installed a plastic paddle in the combustion zone to interrupt the acoustic pat­terns (resonance) that caused the problem. This did not work as well as hoped, so the engineers developed a suppressor of thicker plastic. They also improved the bond between insulator and case, then cast the propellant in the case instead of just sliding it in as a single piece. Nine cases still failed during hydrostatic tests or static firings. The culprits were high stress at joints and “severe combus­tion instability."27

In February 1958, ABL began developing a follow-on third-stage motor designated X248 A2 in addition to X241. Perhaps it did so in part to reduce combustion instability, because 3 percent of the propellant in the new motor consisted of aluminum, which burned in the motor and produced particles in the combustion gases that suppressed (damped) high-frequency instabilities. But another moti­vation was increased thrust. The new motor was the one that actu­ally flew on the final Vanguard mission, September 18, 1959. As of August 1958, ABL had developed a modification of this motor, X248 A3, for use as the upper stage in a Thor-Able lunar probe. By this time, ABL was testing the motors in an altitude chamber at the air force’s Arnold Engineering Development Center and was experienc­ing problems with ignition and with burnthroughs of the case the last few seconds of the static tests.28

The X248 solid-rocket motor consisted of an epoxy-fiberglass case filled with the case-bonded propellant. The nozzle was still made of epoxy fiberglass, but with a coating of "ceramo-asbestos." By November 11, 1958, wind-tunnel static tests had shown that the X248 A2 filament-wound exit cone was adequate. By this time also, the motor had a sea-level theoretical specific impulse of about 235, which extrapolated to an impulse at altitude of some 255 lbf-sec/ lbm, and designers had overcome the other problems with the mo­tor. The X248 offered a "considerable improvement in reliability and performance over the X241 contracted for originally," according to Kurt Stehling. He also said the ABL version of the third stage suc­cessfully launched the Vanguard III satellite weighing 50 pounds, whereas Grand Central Rocket’s third stage could orbit only about 30 pounds.29