Laboratory Scale Slow Cook-Off Testing of Rocket Propellants: The Combustion Rate Analysis of a Slowly Heated Propellant (CRASH-P) Test

The CRASH-P test can be used to quantitatively evaluate the slow cook-off reaction violence of rocket propellants on a much smaller scale than current testing methods. The test is a less expensive and faster method for screening rocket propellants for their slow cook-off violence than current full-scale tests. It's difficult to conduct full-scale slow cook-off tests in the early stages of rocket propellant development.

The CRASH-P test can provide much of this critical information early in the formulation process. The test is fast and inexpensive enough that many of the underlying questions involving chemical kinetics of rocket propellant ingredients during slow cook-off can be explored in greater detail than ever before. The CRASH-P test requires experience in thermal fluid instrumentation, as well as setting up high speed data acquisition for temperature and pressure measurements.

A visual demonstration of this method can be helpful, as setting up the diagnostic equipment and positioning the sample can be difficult with written instruction alone. Demonstrating the procedure with Christopher Gray is Scott Sumner, a research engineer from our laboratory. For installation in the propellant sample, bolt the sample holder cover to the CRASH-P sample holder to seal the sample, then bolt the sealed sample to the steel plank attached to the chamber cap of the CRASH-P test to keep the sample in the middle of the chamber.

Place one of the thermal couples from the electrical feed throughs inside the propellant sample holder to capture any exothermic reactions. Place another Thermo couple on the steel plank pointed up to record the air temperature inside the CRASH-P chamber. Place a small amount of RTV where the thermal couple enters the propellant sample holder to seal the orifice.

Once the sample has been secured, slide the chamber cap into the body of the chamber. Use a cylindrical rod to completely thread and tighten the retaining head onto the chamber and use a pitch of nine threads per inch set screws hex bolt to install the 7/8 inch diameter screws into the chamber head. Tighten the bolts to ensure that the chamber is evenly secured and use a torque wrench to ensure a uniform sealing.

Install the chamber retainer clamps, holding clamps in place with dowel pins. To install the chamber end plate, bolt the plate to the testing table to prevent axial movement of the CRASH-P test during an ignition event. Then plug the dynamic pressure sensor coaxial cables into the dynamic pressure sensors before plugging the electrical band heaters into the outlet sockets that connect to the temperature controllers.

Turn on temperature control panel and temperature controller. Cycle through the menus to make sure everything is in its default setting. Make sure the inlet and outlet wires are plugged into the dynamic pressure signal conditioner and turn on the conditioner.

Set the temperature values on the temperature controller as needed for the 16 time intervals using the first three intervals to set up a ramp and soak period at 50 degrees Celsius for two hours. Enter the remaining intervals to supply the data points for the test to obtain a linear heating profile that does not change slope during the test and set the final temperature to 350 degrees Celsius. Turn on the electric power to the heaters on the control console and turn on the temperature controller to run the test remotely.

From the control page of the temperature controller, set the RSEN on. Then press the auxiliary button to change the test condition from standby to run to start heating the chamber. For data acquisition, in the data acquisition system software, set one test data collection region to allow measurement of the pressure by the main board that measures dynamic pressure and another to allow measurement of the temperatures by the thermal couple amplifier.

Set the system to run on a triggered suite mechanism so that after a threshold voltage is reached, the pressure sampling rate changes from one sample per second to 50, 000 samples per second. As data acquisition does not stop on its own, periodically check the test for temperature exotherm or triggered pressure responses. If either response is observed, manually stop the recording and turn off the heater, video, and temperature controller powers.

At the end of the experiment, manually export the temperature and pressure data into individual tab delimited text files to an appropriate computer for their downstream analysis. Allow the test chamber to cool for at least 12 hours before venting. The chamber is carefully disassembled to release any product gasses from the exothermic reaction.

Capture any sample container fragments of the sample holder before cleaning the chamber Here, temperature traces for the inside chamber air and internal propellant temperature acquired by the data acquisition system can be observed. Minor exothermic reactions before the ignition were measured, along with the main exothermic reaction. Dynamic pressure readings for the reaction are typically recorded for the front, back, and rear dynamic pressure gauges.

Like most laboratory cook-off events, the state of the sample container can be assessed for damage after the reaction. Quite a large degree of measured variation in the reaction violence of different propellant samples is commonly observed, allowing the violence to be quantified and compared for the different reactions. In general, faster pressurizing reactions exhibit more scatter or noise in the pressure data, which is consistent with the greater oscillation of the chamber due to a more violent response.

Be sure to place the thermocouples in the same location each time for a smooth and consistent heating profile. Also, be sure to clean the pressure sensor surfaces to reduce the noise in the reaction violence measurements. After the analysis, inspect the propellant residue and sample container for final chemical product, and check the degree of damage to the sample container from the slow cook-off reaction.

On a final note, remember that certain rocket propellant ingredients can react much differently than expected while being heated under confined conditions. Thank you for watching.