Despite these successes, Parsons, the
project engineer of Aerojet's Solid Fuel Department, remained motivated to address the malfunctions observed during the Ercoupe tests. In June 1942, assisted by Mills and Miller, he focused his attention on developing an effective method of restricted burning when using solid rocket fuel, as the military demanded JATOs that could provide over 100 pounds of thrust without any risk of exploding. Although solid fuels such as GALCIT-27 were more storable than their liquid counterparts, they were disfavored for military JATO use as they provided less immediate thrust and did not have the versatility of being turned on and off mid-flight. Parsons tried to resolve GALCIT-27's stability issue with GALCIT-46, which replaced the former's ammonium nitrate with
guanidine nitrate. To avoid the problems seen with ammonium nitrate, he had GALCIT-46 cooled and then heated prior to testing. When it failed the test, he realized that the fuel's binding black powders rather than the oxidizers had resulted in their instability, and in June that year had the idea of using liquid
asphalt as an appropriate binding agent with
potassium perchlorate as oxidizer.
Malina recounted that Parsons was inspired to use asphalt by the ancient incendiary weapon
Greek fire; in a 1982 talk for the
International Association of Astronomical Artists Captain Boushey stated that Parsons experienced an epiphany after watching workers using molten asphalt to fix tiles onto a roof. Known as GALCIT-53, this fuel proved to be significantly more stable than the Group's earlier concoctions, fulfilling Parsons' aim of creating a restricted-burn rocket fuel inside a castable container, and providing a thrust 427% more powerful than that of GALCIT-27. This set a precedent which according to his biographer John Carter "changed the future of rocket technology": the
thermoplasticasphalt
casting was durable in all climates, allowing for mass production and indefinite storage and transforming solid-fuel agents into a safe and viable form of rocket propulsion.
Plasticized variants of Parsons' solid-fuel design invented by JPL's
Charles Bartley were later used by
NASA in
Space Shuttle Solid Rocket Boosters and by the
Strategic Air Command in
Polaris,
Poseidon and
Minuteman intercontinental ballistic missiles.