IntroductionThis web page presents the test report detailing the fourth test firing of the Kappa solid rocket motor, as well as post-test analysis. This particular test was of the Kappa-SB version, powered by KN-Sorbitol propellant, and represents the second test firing of this version, denoted KSB-002.
This static test had two main objectives:
Motor detailsThe motor for this test was the same as that used for the previous three Kappa motor static tests (KDX-001, KDX-002, KSB-001). Certain minor modifications were made to the motor:
The completed rocket motor is shown in Figure 3.
Figure 1-- Propellant segments
Figure 2-- Pyrogen igniter design
Figure 3--Kappa-SB Rocket Motor (prior to installation of thermal-sensitive labels)
The propellant inhibitor was identical to that used for the previous test (resin-soaked cotton fabric). Average inhibitor thickness was 0.010 inch (0.25 mm). The segments were sprayed with hi-heat aluminum paint on the inhibited surfaces.
As with the previous tests, 3 strips of thermal sensitive tape (Brother M-Tape) were placed around the casing at three locations, in order to give an indication as to how hot the casing would get during firing.
Once the setup was completed, and the observers located a safe distance away from the test stand, final connections to the pyrogen igniter system were made. A continuity check was performed, which verified that the igniter element was operative. The ignition system was then armed, final positions were taken by the personnel, and the countdown was commenced. I was poised with my digital camera to record the motor firing. When the zero count was reached, the motor came to life within a half-second, thrusting strongly, with no "build up" as had been the experience with previous firings. The sound of the exhaust jet was very loud, and undulated slightly. The forceful thrusting continued for about two seconds, then rapidly tailed off. An orange flame briefly issued from the nozzle at the end of tailoff. Some blackish smoke then issued from the nozzle, which quickly reduced to a slight but steady stream of grey smoke (from the decomposing inhibitor material) which continued for a minute or so. The spent motor was then quickly approached in order to observe the strips of heat sensitive tape. The strip nearest the nozzle end of the casing was beginning to darken. The other two strips were still completely white. The motor was very hot to the touch, in particular the nozzle, which was completely blackened (as per the previous three tests). A photo of the motor under thrust is shown in Figure 4.
AnalysisExamination of the videotape showed that the three strips of thermal tape remained white during the motor firing. This indicates that the casing remained relatively cool (<250oC.), proving the effectiveness of the redesigned casing insulation. The videotape also confirmed that the motor came up to thrust very rapidly upon ignition. The pyrogen and "combustion primer" were clearly very effective in getting the motor pressurized with the grain fully burning. The footage also showed the nozzle glowing red hot at the throat region during the latter half of the burn.
When the motor was opened up for post-firing inspection, the insulating liner was found to be largely intact. Although much of the liner was charred, burnthrough had occurred in only a few isolated spots. The liner is shown in Figure 5.
Figure 5--Casing thermal liner, post-test condition (both sides shown).
The buna-N (nitrile) O-rings that sealed the nozzle and bulkhead again performed flawlessly. Careful examination of the O-rings confirmed that there was no blow-by whatsoever.
Figure 6 -- Actual motor thrust and chamber pressure as a function of time
As can be seen, the two parameters (curves) follow one another closely, as would be expected. Chamber pressure and thrust are related by the following equation:
where F is the thrust, Cf is the thrust coefficient, At is the nozzle throat cross-sectional area and Po is the chamber pressure. The thrust coefficient is an important parameter which relates the amplification of the thrust due to gas expansion in the nozzle as compared to the thrust that would be exerted if the chamber pressure acted over the throat area only. In Figure 7, the thrust coefficient is plotted. The average value of the thrust coefficient (as plotted) was 1.53.
Figure 7 --Nozzle thrust coefficient, shown over a portion of the burn regime
From the thrust-time curve, the total impulse of the motor was determined to be 1821 N-sec. (410 lb-sec.), which was less than the design impulse of 2000 N-sec. by 9 percent. The delivered specific impulse was 125.0 sec., suffering (in part) from the low average chamber pressure. This compares to a delivered specific impulse of 137 sec. for the KN-Dextrose propellant in test KDX-002 and 120.4 sec. for the previous KN-Sorbitol test (KSB-001).
Figure 8 shows a comparison of the two static test results for the Kappa-SB motor with KN-Sorbitol propellant.
Figure 8 -- Comparison of results from two tests
The shape of the two thrust curves is remarkably consistent! The only significant difference occurred during the thrust buildup phase. Undoubtedly, the pyrogen ignition and/or the combustion primer coating is to be credited. However, the overall performance difference is slight, being about 4% higher for KSB-002.
The modifications to the motor system that were intended to ensure rapid and complete ignition of the propellant segments appeared to work well, but did not affect the shape of the thrust and pressure curves. Indeed, startup was enhanced, with no "thrust buildup" period experienced, as per the previous tests. However, the overall difference in performance was slight. As such, it would appear to be unlikely that delayed grain ignition is the explanation for the odd "triangular" thrust profile obtained with the two tests utilizing KN-Sorbitol as the propellant. Interestingly, this assertion is backed up by the recent static firing of an impressive "L" class KN-Sorbitol motor tested by Paul Kelly of Australia. The propellant for this motor (BATES configuration) was also coated with a combustion primer, and was ignited by a powerful igniter. Yet, the thrust profile is very similar to my results, and is shown in Figure 9.
Figure 9 --Thrust curve for Paul Kelly's "L" class motor
Paul has also conducted static tests with a number of core burner (progressive) designs that have given unexpected thrust profiles. An examination of the test results of other experimenters utilizing KN-Sorbitol has revealed a similar trend. Peter Madsen of Denmark (on behalf of D.S.C.) had conducted several static tests of slab (neutral burning) motors that produced the expected neutral thrust curves for motors with low operating pressures (in the range of 2-2.5 MPa. or 300-350 psi), but unusual thrust profiles at higher operating pressures. As well, the enormous Phenix 100A "P" class motor developed by the Norwegian group NEAR, configured with a BATES neutral-burning grain, produced a thrust profile roughly triangular in shape. The curve for this motor is shown in Figure 10.
Figure 10 --Thrust curve for NEAR's SCA9901 "Phenix" motor
It would appear that KN-Sorbitol has an odd burn rate v.s. pressure behaviour, when burned in a motor, which did not show up in the Strand Burner tests that were done to characterize the burn rate for this propellant. One is tempted to put the blame on erosive burning. However, the observed effect is more the opposite. Erosive burning would produce an enhanced burning rate initially, when the chamber duct area is minimum. The effect would then drop off as the grain web recedes and the duct area increases. The observed phenomenon is such that burnrate is initially much lower than expected.
More research, including dedicated motor testing, as well as a closer examination of existing data, needs to be conducted to try to explain (and hopefully characterize) this unusual and puzzling phenomenon.