The demonstration described in this report was an effort to develop a lead-free, extrudable propellant for the Hydra weapon system. This report chronicles the beginning of this effort under the Environmental Security Technology Certification Program (ESTCP) and the continuation of the technology development under the follow-on program funded by the Environmental Quality Technology (EQT) program at the U.S. Army Research, Development and Engineering Command (RDECOM).
The technological approach was to take a lead-free ballistic modifier blend developed for castable propellants at the U.S. Army Aviation and Missile Research, Development and Engineering Center (AMRDEC) at Redstone Arsenal and to incorporate it into an extrudable propellant formulation for the Hydra rocket. Previous work done by the U.S. Navy/Indian Head and Alliant Techsystems (ATK)/Radford developed the formulation RPD-422 (also known as AA-7), which had acceptable ballistic properties but could not meet the aging requirement. An aging study of RPD-422 concluded that the cause of the premature aging was a copper-containing material in the ballistic modifier blend. AMRDEC proposed that since its ballistic modifier blend did not contain copper, it could potentially meet the ballistic and aging requirement successfully.
The ESTCP program was a joint effort between AMRDEC and the developers of the RPD-422, U.S. Navy/Indian Head, and ATK/Radford. The team developed 15 formulations that evaluated two different strategies: (1) the AMRDEC formulation approach and (2) improving aging characteristics of previous formulations (including RPD-422). This propellant formulation study had the following findings: Copper-containing ballistic modifiers were causing the premature aging but this could be overcome, the study showed that the pH of the ballistic modifier that contained copper-based ingredients affected aging, and only three formulations of the 15 that were tested were able to meet aging requirements; two of the formulations did not contain a copper-containing ingredient but one formulation did. While AMRDEC’s proposition that copper-based materials needed to be avoided in the formulation proved to be accurate, the study also showed that a copper-containing formulation could meet the aging requirement under the correct conditions. These formulations were down-selected to the propellant that had the least amount of burn rate temperature sensitivity. This formulation was one of those proposed by AMRDEC that did not have a copper-containing ingredient (referred to as RPD-517).
RPD-517 was then optimized with two goals: (1) reduce burn rate temperature sensitivity and (2) evaluate the formulations sensitivity to changes in nitrocellulose composition that can occur in a production environment. RPD-517 burn rate sensitivity was minimized so that it only contained 2% ballistic modifier blend (only half of what is required in the currently fielded Hydra motor). However, burn rate data showed that this modified formulation had more burn rate temperature sensitivity than the RPD-422 formulation. This formulation was then evaluated by examining burn rate fluctuations in the three different varieties of nitrocellulose used in production: 12.12%, 12.18%, and 12.29% nitrogen content. The 12.12% and 12.18% contents gave identical results, but the 12.29% content gave higher burn rates at the hot (150 °F) condition, so it was not included in the motor testing series. (The 12.29% variety only accounts for 10% of all nitrocellulose used in production.) ATK/Radford produced 2,400 pounds (lbs) of this propellant manufactured into propellant cartridges that were shipped to AMRDEC for static motor testing.
The EQT program was a joint effort of AMRDEC and ATK/Radford that continued the demonstration of the RPD-517 propellant with motor testing of heavyweight and flightweight hardware. ATK/Radford tested 12 heavyweight motors at hot (150 °F), cold (-50 °F) and ambient conditions. These motor tests showed that this propellant should be able to be tested at AMRDEC in flightweight hardware without over-pressurization. However, pressure data did indicate that this modified RPD-517 formulation had more burn rate temperature sensitivity than the RPD-422 formulation.
Flightweight testing was conducted on 30 of the modified RPD-517 motors at hot, cold, and ambient conditions. The flightweight motor tests showed that the motor’s performance was marginally within the thrust parameters for the Hydra weapon system at the cold condition but outside the parameters in the hot condition. However, examination of the motor cases post-test did not show any signs of charring from excessive temperature or bulging from excessive pressure. Flight modeling of the modified RPD-517 formulation showed that it should meet velocity requirements.
The Hydra project management office was not interesting in pursuing qualification of this propellant since it has too much burn rate temperature sensitivity. The propellant has two features that could overcome this technological hurdle: (1) it only has 2% ballistic modifier content and (2) has superior aging characteristics when compared to the fielded Hydra propellant. Since it has only 2% ballistic modifier, then the motor has about 2% more propellant than it needs. 2% of the propellant could be machined out of the rocket motor and still achieve velocity and range requirements. Also, this propellant was not compared to “aged” fielded propellant. Since its aging characteristics are superior to the fielded propellant, its “aged” ballistic performance should also be compared against the aged fielded propellant in a possible future effort to make a decision on its viability as a replacement. AMRDEC has 70 of the modified RPD-517 motors available for any future efforts to investigate the development of this technology.
The Hydra Project Management Office (PMO) has been presented with the progress made under this effort and the PMO has indicated that the RPD-517 propellant’s burn rate behavior is too temperature sensitive for qualification as a replacement to the currently fielded AA-2 propellant. As was shown in Section 6.0, RPD-517 also has more burn rate temperature sensitivity than even the RPD-422 formulation. However, two features of RPD-517 offer some potential for further improvement in this area: its superior aging characteristics relative to AA-2 and the lower amount of ballistic modifier content in the formulation. Propellant aging is known to alter the burn rate characteristics of rocket propellants. An evaluation of “aged” AA-2 and RPD-517 could show that RPD-517 is closer in performance to AA-2 when compared to the “unaged” data under this effort. Also, since RPD-517 uses 2% less ballistic modifier than AA-2, the propellant could be machined from the rocket motor grain to remove 2% of the propellant, which could achieve better results by an improved propellant grain design. AMRDEC has 70 RPD-517 motors available from this effort that could be used to pursue future efforts.
The major roadblock to implementing any future extrudable propellant in the Hydra weapon system may the economics. Extruded propellant has long been recognized as a much less expensive alternative to castable propellant in configurations where the rocket motor diameter is <4 in (as in the case of the Hydra). Recently, the cost of producing extruded propellant has risen to the point that the Hydra PMO has determined that a castable propellant may be the next rocket motor concept considered for future implementation.