Testing of M7 propellant at different temperatures in closed vessel

Lot NARA 225A M7 propellant produced as tubes by Nammo Raufoss for use in M72 has been tested in a 700 cm3 closed vessel for determination of the burn rate and impetus. The propellant has been tested at three different temperatures -40 oC, room temperature and +60 oC for up to 4 different loading densities at each temperature. For M7 propellant lot NARA 225A the burn rate increases with increasing test temperature. For loading density 0.200 g/cm3 at 1400 bars the burn rate increases from 16.64 cm/s at -40 oC, 18.67 m/s at room temperature to 21.07 cm/s at 60oC. The impetus for M7 lot NARA 225A increases from 1001.5 J/g at -40 oC, 1019.6 J/g at room temperature and up to 1032.8 J/g at +60oC. These results are all as expected. For the closed vessel firings at -40 oC, the burn rate curves for the two firings of lowest loading densities 0.10 g/cm3 and 0.15 g/cm3, a breakpoint have been observed at approximately 1/3 of the maximum pressure, respectively, at 350 and 600 bars. A breakpoint that is not observed in the burn rate curves for the other firings. This phenomenon is strongest for the firing of lowest loading density. However, a breakpoint is observed at 90+20 bars for all burn rate curves. Burn rate equations have been fitted to all experimental burn rate curves at two pressure ranges from 30+10 to 90+30 bars and from 90+30 bars, to the maximum pressure for each single firing. For the firing with 0.10 g/cm3 at l -40 oC, we divided the pressure interval into three pressure ranges due to the second break point at 350 bars. Equations on three different forms; r = a + bP, r = bPn and r = a + bPn , have been fitted to the experimentally burn rate curves. However, at all pressure ranges best fit to the experimental burn rate curves is obtained for equations of the form r = a + bPn.