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51 mm Project
M-3
"An expert is a man who has made all the mistakes which can be made in a very narrow field."
1922 Nobel Laureate in Physics
Failures during the M2 test
1 Failure of the proximal thrust chamber close to the attachment of the chamber pressure monitor.
2 The microphone attached to the nitrous vent did not detect venting.
Refinements for the M3 test
1 Angling of the nitrous jets towards the centre of the fuel grain.
2 Bevelling the proximal end of the fuel grain.
3 Gluing the fuel grain to the Tufnol liner to prevent gas entering the gap and eroding the liner.
4 Abandoning the microphone system.
Test Data
Overall this third test was more successful with a longer duration of burn. The modifications to the injector end of the thrust chamber seemed to have worked. However after 3.1s there was a loss of thrust despite the continued flow of nitrous oxide.
Figure 1:
Photo 1:-Click to run video
The loss of thrust was due to the nozel retainer being ejected from the thrust chamber. This was followed by the fuel grain which can be seen on the stills from the video (Figure 1, 3.87 s & 4.17s, arrow heads). The nozel retainer was susequently found (figure 2) and was clearly very badly damaged during the test. When compared with figure 3 it is clear that most of the color which held the retainer in place has been lost. Some of the damge could be due to the retainer being ejected downwards and impacting the blast deflector of the stand. In fact the retainer is now distorted into an oval shape which would fit with this hypothesis. However inspection of a grub screw recocered at the same time reviels melting (figure 2 arrow, an intact screw is shown for comparison). This would suggest that heating of this structure was the original cause of the failure. Figure 3 shows that the retainer is in direct contact with the graphite nozel. As graphite conducts heat well, the proximetry of the nozel to the retainer would acount for the failure.
Figure2: Post firing nozel retainer and retaining grub screws. The grub screw indicated by the arrow has melted.
Figure 3: Nozel retainer and tufnol liner
Modifications for the M4
1 Redesigning the nozel reatiner to protect it from heating.
2 Fabricating a fatter oxidiser tank.