Rocket Motor Test Stand (RMTS)
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Flight Thrust-Time Measurement
Static Testing and Flight Measurement

Thrust-Time Plot 

A single stage rocket carrying a Cambridge IA-X96 accelerometer payload was flown on July 11, 1998 at North Branch, Minnesota. This experiment is part of a series of efforts to determine the effect of actual launch conditions on Thrust-Time characteristics of high power rocket motors. This test used an RMS-38/360 rocket motor and an I161-10w reload kit to propel a modified single stage Quantum Leap rocket. These tests are intended to quantify the overall effects of launch acceleration and environmental conditions on the Total Impulse of solid fuel high power rocket motors. With information on the dependence of Total Impulse on actual flight conditions a better estimate of rocket flight performance may be achieved.

The IA-X96 accelerometer on board the Quantum Leap recorded the time dependant values of acceleration during powered ascent and coast to apogee. The on-board computer of the IA-X96 integrated acceleration as a function of time to compute altitude and velocity as functions of time. Since the rocket was launched about 10 degrees off-vertical, the acceleration data was corrected in a separate computer program. Then, altitude and velocity information were re-integrated using the corrected acceleration data. A new Thrust-Time curve was generated based on the re-integrated acceleration data and the weight of the rocket using the average weight of propellant. The results of the analysis to generate corrected in-flight Thrust-Time information is displayed as Flight Data in the plot on the left. The Thrust-Time data produced by the Rocket Motor Test Stand (RMTS) are labeled as Static Test Data, while the manufactures Thrust-Time data for the rocket motor is labeled as AeroTech Data.

Ambient temperature recorded for the Flight Data of the Quantum Leap was 90 degrees Fahrenheit, and the ambient temperature for the RMTS Static Test Data was 56 degrees Fahrenheit. The ambient temperature for the Thrust-Time AeroTech Data is unknown.

Quantum Leap Liftoff

The information displayed in the Thrust-Time Plot indicate a fairly close correlation between the Static Test Data and the AeroTech Data for the RMS-38/360 rocket motor using an I161-10w reload. Both the Static Test Data and the AeroTech Data are Thrust-Time data derived from static testing. The actual Flight Data of the Quantum Leap rocket displays the effect of conditions not present during static rocket motor testing.

Total Impulse for each Thrust-Time curve is computed by numerical integration. Using the trapezoidal rule the Flight Data Total Impulse was computed to be 67.5 lb-sec. The Total Impulse for the RMTS Static Test Data is 78 lb-sec and the Total Impulse for the AeroTech Data was computed to be 75 lb-sec. A comparison of the Total Impulse for the AeroTech Data, Static Test Data, and the Cambridge accelerometer Flight Data are displayed in the following table.

Thrust-Time Data Comparison
Test Data Total Impulse (lb-sec) Difference
AeroTech Data 75.0 0%
Static Test Data 78.0 +4%
Flight Data 67.5 -10%

Cambridge Accelerometer Results

The RMTS Static Test Data Total Impulse is 4 percent higher than the AeroTech Data Total Impulse. However, the Flight Data Total Impulse is 10 percent lower than the AeroTech Data static test result. The discrepancy between the static test measurements of Total Impulse and the flight measurement of Total Impulse show that actual flight conditions effect rocket motor performance.

The burning rate of the propellant surface and its dependence on pressure, temperature, erosion, and acceleration is probably responsible for the discrepancy between the static test results and the actual flight measurement of Total Impulse. Conditions which determine the burning rate of a propellant are pressure, temperature, and erosion due to high velocity flow over the burning propellant surface. Total Impulse or the chemical energy released in combustion stay essentially constant as the grain ambient temperature is changed; only the rate at which it is released is changed. Therefore, ambient temperature is not a major factor involved in the measured discrepancy of Total Impulse. However, erosive burning caused by high velocity flow of combustion gases and unburned propellant over the burning surface is the most likely cause for the major part of the discrepancy between static test measurement of Total Impulse and launch measurement of Total Impulse. Erosive burning enhancement due to launch acceleration increases the unburned propellant mass flow during the launch of a solid fuel rocket, reducing Total Impulse by a small amount.

From these results it is reasonable to say that for the RMS-38/360 rocket motor, using an I161-10w reload, the in-flight Total Impulse is about 10 percent less than the Total Impulse measured by static testing. Further investigation will be conducted to determine if a reduction of Total Impulse by 10 percent can be expected for other high power rocket motors.

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Reference: George P. Sutton, "Rocket Propulsion Elements", 6th Edition

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