“By their nature, things that are energetic are usually also sensitive,“ said Dr. Christine Michienzi, Advanced Gun Propellant Development Program Manager at Indian Head Division, Naval Surface Warfare Center (IHDIV, NSWC). “In the past, to make propellants insensitive, inert ingredients were used to replace the energetic ingredients, but this meant a loss in the energy of the system and therefore its performance.“
Michienzi is working with the Swedish Defence Research Agency (FOI) and a Swedish company called EURENCO Bofors in an arrangement called a Defense Exchange Agreement (DEA). The U.S. side of the agreement is run by the Office of the Secretary of Defense and overseen by the Office of Naval Research. The participants are developing new gun propellant materials that provide a maximum amount of propulsion energy without causing excessive heating of the gun barrel, and that have a low sensitivity to factors that might cause an accidental detonation.
“The Swedes and I share information on work that is of interest to us both,“ said Michienzi. “For me, this has meant that they have shared information on a number of new ingredients that they have developed or are working on. They have worked very hard to answer any questions I had and to try to solve any issues I might encounter. They have also provided me samples of some of these materials at no cost to test in some of my gun propellant formulations. I in turn provide them with information on how the formulations worked and the testing data associated with them. This is a win-win situation—we both get something that benefits us at no cost.“
“This propellant was actually designed originally to work in the Advanced Gun System – the new gun system developed for the Navy’s new destroyer DD(X),“ said Michienzi. “They are currently using a different propellant for the initial qualification because it is off-the-shelf, but it is more sensitive than NILE propellant and also has a higher flame temperature, which causes excessive heating in the gun barrel. NILE propellant gives the same performance, but at a much lower temperature and it is less sensitive. Plans are for it to be spiraled into the program at a later date.“
The Swedish group markets the nitrogen-containing organic compound as an explosive, which usually requires a larger particle size than a gun propellant. Michienzi worked with this group to ensure that the material they supplied her would have a smaller particle size that would work with her application. The Swedes are now qualifying a propellant of their own that contains the nitrogen compound.
Michienzi realized that the NILE propellant would also work in the Navy's existing 5-inch shipboard guns. The Navy currently uses Navy Cool (NACO) propellant for those systems, which has a greater sensitivity to slow cook-off and fragment and bullet impact than is desirable. (“Cook-off“ refers to a munition's behavior under slow or rapid heating. The violence of the reaction and the temperature at which reactions occur determine the suitability of the munition for various applications.)
Although NACO propellant does not meet the Department of Defense's standards for insensitive munitions, it has a waiver for use in ammunition rounds that are already in service. Newly manufactured ammunition would require a renewal of the waiver to continue using NACO propellant, but the process has gotten stricter in the meantime. Michienzi stated that now the Navy would have to prove that there was no better alternative available.
In order to optimize NILE propellant as a replacement for NACO, the Insensitive Munitions Advanced Development (IMAD) Office funded the effort that allowed IHDIV to modify propellant properties. Now that NILE propellant is available, the Navy has decided to qualify it as a replacement for NACO in all Navy 5-inch guns, a process that should take about two years. Discussion on the funding and timeline for this effort is ongoing. Michienzi recognizes that that there is no guarantee that NILE will actually be used for this purpose, but is optimistic. ‘‘It will have gone through all the required testing procedures, and it is looking pretty good that NILE will be selected for use.”
To be qualified as a propellant, NILE will have to pass an extensive series of tests to assess its responses to thermal, mechanical, insensitive munitions, and environmental stimuli. Once the propellant is qualified, the primer that ignites the propellant will also have to be qualified, if a qualified primer is not already available. Then, the entire propelling charge (a cartridge case with the propellant, primer, and closure plug) must pass qualification tests. According to Michienzi, a good portion of this testing will most likely occur at IHIDV and its sister Division at Dahlgren.
Dr. Judah Goldwasser, ONR Program Officer for Undersea Warheads and Energetic Materials, noted that there are three types of qualification tests that the nitrogen compound must undergo: systems, type, and insensitive munitions. He stated that new propellants are validated using generic hardware as well as the specific systems in which they are intended to be used. The insensitive munitions requirements include tests for susceptibility to detonation or explosion resulting from shock, bullet or fragment impact, and cook-off under heat and fire.
Michienzi acknowledges that developing a new energetic material is always challenging. ‘‘There are always steps forward and steps backward,” she said, “But it is very rewarding taking something from the initial concept phase all the way to qualification and eventual production. When you can finally see it being used by the Fleet – it makes everyone’s hard work and effort worthwhile.”
This article was written in cooperation with the Office of Naval Research, Nancy McGuire.