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Flight Thrust-Time Measurement
Static Testing and Flight Measurement
Thrust-Time Plot
STATEMENT OF PROBLEM
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
RESULTS
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.
Back to Rocket Motor
Testing
Reference: George
P. Sutton, "Rocket Propulsion Elements", 6th Edition |