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51 mm Project
M-2
"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 M1 test
1 Overload of the hydraulic force transducer.
2 Failure of the engine casing, thought to be secondary to insufficient insulation.
3 No indication of when the nitrous tank was full.
Refinements for the M2 test
1 Problems with the stability of the amplified signal from the new strain gauge precluded its use. A new 51mm hydraulic force transducer was machined; maximum load 1400N.
2 The shoulder of the injector unit was reduced to fit into the Tufnol liner, which now extends along the whole length of the thrust chamber.
3 Once the nitrous tank is filled it vents to atmosphere. During the 38mm engine tests this venting was audible. The M series are fired from >100m away and from this distance the pitch change can no longer be heard. Nor can the nitrous liquid be seen venting. To overcome this problem a microphone was placed next to the vent. The output from the microphone was fed remotely to ear phones.
Test Data
On the day of the test the weather was appalling. Much of the set up was conducted in pouring rain. Despite this the equipment worked well. However with poor light and plastic covers to protect the cameras the video is not all it might be.
Photo 1:-Click to run video
Over the background noise of the wind and rain the new microphone was not a success. The filling tank remained open after firing and was all but emptied. No data about the oxidiser load was available.
Despite the weather ignition was successful and motor roared into life. With no data on how full the engine was at the start of the test the net force data has not been converted to gross force. From the video and the thrust time curve it is apparent that the thrust was lost after approximately 1 second. This time corresponds with a breach in the thrust chamber (marked by the arrow in photo 2). There after the engine continues to develop thrust for 6.6 seconds. After this time the test stand can be seen rotating to the right, settling back to its starting position.
The video data and the force transducer appear to be in agreement.
The peak thrust for this test was just under 600N. In the M1 trial it was proposed that the hydraulic transducer limit was exceeded. No data on oxidiser tank pressure was available, need to measure this. Due to the earlier failure of the test calculation of total and specific impulse were not performed.
It is not possible to know if the cold weather was responsible for low tank pressure or there is some other problem with the M series. The M design is based on scaled up data from the 38mm K series. May of the assumptions are based on linear relationships between thrust and fuel grain proportions and therefore might be quite incorrect.
Photo 2:-Clockwise, firing of the M2 to failure, time from ignition.
The breach in the thrust chamber is shown in photo 3 and from video in photo 4. The breach is longitudinal and the video stills suggest that it begins remote from the CP monitor attachment. It is likely that it then extended up the chamber to the attachment causing the ejection of the CP monitor seen in the video.
The breach in the chamber occurred earlier than for the M1 (3.9s). However the extent of the damage was less. The meaning of this is unclear. The fact that the chamber failed despite better insulation and that it did so early in the test suggests that heat energy is being focused at the top of the thrust chamber, rather than being carried down the engine. Adjacent to the injectors there is only the Tufnol liner to protect the engine case.
Photo 3:-Longitudenal breach of the thrust chamber. The Original position of the CP monitor is shown in red
Lower down the engine the fuel grain shields the liner and case. As seen in photo 5 areas of the liner in contact with high temperature components such as the nozzle and post combustion area become charred. However those areas containing the fuel grain are insulated and remain intact.
One possibility is that the chamber is failing because heat energy is concentrated at the top of the chamber, where the heat shielding of the case is weakest, rather than being carried down the fuel grain. The working hypothesis is that this occurs because the nitrous oxide hits the fuel grain and re calculates hot combustion products up the chamber.
Modifications for the M3
1 Angling of the nitrous jets towards the centre of the fuel grain.
2 Bevelling the proximal end of the fuel grain.
3 Moving the chamber pressure monitor video closer to the gauge by attaching it to the test stand.
4 Installing an oxidiser tank gauge.