We present a protocol for a laboratory-scale slow cook-off test for solid rocket propellants called the Combustion Rate Analysis of a Slowly Heated Propellant (CRASH-P) test. Confined rocket propellants are slowly heated until autoignition, and both the cook-off temperature and the violence of reaction are measured with dynamic pressure sensors.
Solid rocket propellants are widely used for propulsion applications by military and space agencies. Although highly effective, they can be dangerous to personnel and equipment under certain conditions, with slow heating in confined conditions being a particular danger. This paper describes a more affordable laboratory test that is easier to set up and was developed for screening rocket propellant ingredients. Rocket propellants are cast into sample holders that have been designed to have the same confinement as standard rocket motors (propellant volume to total volume in the container) and ensure that the propellant is not easily vented. Reaction violence is quantified by the time it takes to reach 90% of the maximum pressure after autoignition, which is analogous to blast overpressure gauges used to measure violence in a full-scale test. A positive correlation was observed between the speed and pressure produced from the reaction and the power produced by the rocket propellant during the reaction.
Solid rocket propellants are used extensively in defense, space, and gas-generating applications. They are relatively reliable fuels that perform many functions extremely well. However, many rocket propellants contain dangerous ingredients such as ammonium perchlorate (AP). Rocket propellants with these oxidizers can explode violently when slowly heated1,2,3. There have been several high-profile accidents with the slow heating of rocket propellant or rocket propellant ingredients that have drawn attention to these issues such as the fire and subsequent cook-off of munitions on the USS Forrestal4 and the PEPCON explosion1. While these are thankfully rare events, they can be devastating because of the personnel and equipment losses that occur. Therefore, there is motivation to understand the violence of these reactions and drive them down whenever possible. One of the main causes of violent cook-off events with rocket propellant is that many of the ingredients partially decompose, leaving reactive product gases behind along with the oxidizer with an enhanced reactive surface area.
One specific example of this is the ionic salt, ammonium perchlorate. The low-temperature decomposition of ammonium perchlorate is drawn out and incomplete, leaving reactive intermediate products within a propellant framework with substantial porosity and surface area available for subsequent reactions5,6,7,8,9. In addition, rocket propellants that contain ammonium nitrate and explosive nitramine compounds can have very violent reactions when heated slowly10,11,12. Slow cook-off violence is an important insensitive munition metric because many rockets are required by law to pass these tests13. Currently, the best way to determine whether a rocket propellant formulation reacts too violently under slow heating conditions is to run a slow cook-off (SCO) test on a full-scale rocket motor. These tests involve taking a full-size rocket motor and heating it slowly in a disposable convection oven.
Temperature traces are provided in multiple locations up until the reaction where the violence is then assessed based on various indicators ranging from container damage and fragmentation to simple overpressure gauges and dynamic pressure sensors for measuring blast pressure. These full-scale tests are often expensive and are not practical for investigating minor changes in propellant ingredients14. A few laboratory-scale tests have been developed that involve heating propellants or explosives in a variety of configurations and assessing container damage after the autoignition event. Although current laboratory-scale tests predict time to cook-off well and sometimes the autoignition temperature15,16,17, they are less able to predict the violence.
One commonly used test is the variable confinement cook-off test18 that slowly heats a cylinder of propellant until it ignites. The violence of the reaction is determined by the fragmentation of the chamber and bolts during the exothermic autoignition reaction. The most common laboratory tests use the final condition of the chamber to rank reaction violence, and there is a degree of subjectivity to the assessment. Small differences in reaction violence are difficult to determine. This assessment of violence is qualitative in nature, and it can be difficult to assess whether a change in a formulation ingredient altered the SCO violence. Furthermore, unlike an actual rocket motor, current laboratory tests do not confine the propellant inside a case. Product gases can easily escape, and this is important because the gases can react with the propellant heterogeneously or be reactive themselves, as in the case of ammonia and perchloric acid if ammonium perchlorate is used.
One of the best efforts in instrumenting a laboratory scale test involved the use of a dynamic pressure sensor on a small-scale cook-off bomb19. This allowed for higher resolution, quantifiable differences in reaction violence to be determined for relatively minor changes in rocket propellant formulation. However, a critical problem with this test is that it did not confine the rocket propellants in the same manner as an actual rocket motor, and numerous modeling and subscale experiments have shown this to be an important factor for consideration20. In addition, the propellant usually does not have the same amount of exposed surface area or the same free volume and is not geometrically confined in the same way as a full-scale test. The Combustion Rate Analysis of a Slowly Heated Propellant (CRASH-P) test was conceived to improve upon these previous tests. Samples between 25 g and 100 g can be tested under similar propellant confinement conditions as a full-scale test21. It also provides a means of measuring the power produced from the reaction event quantitatively through dynamic pressure sensor measurements, which is something current subscale tests do not provide. The results have been found to correlate well with full-scale SCO tests.
One of the most important parts of establishing the CRASH-P test was deciding what metric from the test would be best used to quantify the reaction violence of the rocket propellant formulations. The speed and amount of pressure produced from the reaction is directly proportional to the power produced by the rocket propellant when reacting. It is also directly analogous to the blast overpressure gauge used in a full-scale SCO test. Initially, pressurization rate (dP/dt) was used, but these data were misleading because different formulations contain different amounts of fuel and oxidizer and produce different amounts of gas with varying composition. To minimize this bias from the effects of changing the formulation ingredients, the time to 90% peak pressure was used instead, and it correlated well with full-scale SCO test violence.
Another testing operation that was found to be important is confinement. Early sample holders were made with thermoplastic materials designed to handle the high temperatures of the test. Unfortunately, while these samples did not melt, they softened and did not provide the same confinement as metal sample holders. The reaction violence for these samples was noticeably less than the reaction violence for metal sample holders. Another key finding about the test was that some rocket propellant formulations had critical sizes to autoignite reliably. Aluminized formulations had difficulty cooking off and autoigniting if they were under 50 g. This was attributed to the requirement of a threshold amount of ammonium perchlorate that was required for the violent reaction. In addition, another insight was that thermoplastic bolts did not work. The original CRASH-P sample holder bolts were made from PEEK, and this had to be changed to stainless steel. Confinement was not strong enough because of the PEEK material thermally expanding before propellant autoignition was achieved.
For some formulations that ignite at higher temperatures, mainly aluminized formulations, using an aluminum propellant holder case is desirable as they do not soften at higher temperatures. Finally, ICP dynamic pressure sensors were the original pressure sensors used. However, after ~10 tests, the results got increasingly noisy, probably from being exposed to too high a temperature. The dynamic pressure sensors were switched from ICP sensors to charge amplifier sensors. However, charge amplifier sensors lose charge if left on for too long. To minimize this effect, an in-line charge amp-to-ICP convertor was used downstream at a safe temperature region. As the maximum sampling rate of the pressure sensor is 500,000 samples/s, sampling rates faster than 50,000 sample/s could be recorded. However, there was no need for this as the events were not that fast.
The authors have nothing to disclose.
The authors would like to thank the Joint Enhanced Munitions Technology Program. Mr. Anthony DiStasio and Jeffrey Brock were instrumental in making sure this work was completed.
½ x 24 x 12’ Ceramic Insulative Blanket | Cotronics Corporation | 370-3 | Thermal Insulation for CRASH-P Chamber |
20 gauge K-Type Thermocouple | Omega Engineering | EXPP-K-20-SLE-500 | Thermocouple wire for temperature measurements |
Dynamic Pressure Signal Conditioner | PCB Piezotronics | 482C16 | Converts ICP signal to voltage for data acquisition system |
Electrical feedthrough of CRASH-P chamber | Conax | ||
GC-35 Reaction Chamber | High Pressure Equipment Company | GC-35 | Main Reaction Chamber of CRASH-P Test |
Gen 3i and Perception software | HBM Inc. | Gen3i | Main Data Acquisition System for CRASH-P Data |
High-Temperature Charge-Amplified Pressure Sensor | PCB Piezotronics | 113B03 | Dynamic Pressure Sensors used in CRASH-P Test |
In-Line Charge Amp-to-ICP Converter | PCB Piezotronics | 422E53 | Converters pressure sensor charge amp signal to ICP signal |
Mica Band Heaters | Omega Engineering | MBH00295 | Resistive Element for Heating up CRASH-P Test |
Quantum X Thermocouple Amplifier | HBM Inc. | 1-MX1609KB | Used for getting Temperature Measurements |
Teflon Insulated K-type thermocouple (0.02 inch diameter) | Omega Engineering | 5TC-TT-K-24-36 | K-Type Thermocouples |
Temperature Controller | Omega Engineering | CN3251 | PID Temperature Controller |