Latest News - 4/30/2010

Something I've been meaning to do for a while is build a suitable blast shield in case the motor decides to disassemble itself due to a hard start. After some really crude analysis, I came up with this design. The sides use 12x12 inch sheets of LEXGARD MP1000 laminate with an extra 3/8 inch sheet of polycarbonate on the inside to cosmetically protect the expensive LEXGARD sheets in the event of a minor burnthrough. A washer on the bolts between the sheets should prevent moisture from getting trapped in between. For the top I used 1/2 inch hot-rolled plate steel. The whole assembly (which is a lot heavier than I intended) bolts to the front of the test stand with some extra unistrut. I spent a lot of time reading various articles about blast shields and just got a headache after a while so I settled on this design as probably "good enough". Some interesting references I found include: NASA TN D-4894 "Blast Shields Testing", SAND99-0634 "Secondary Containment Design for a High Speed Centrifuge", DOE/TIC-11268 "A Manual for the Prediction of Blast and Fragment Loadings on Structures", and BRL-405 "The Initial Velocities of Fragments from Bombs, Shell, and Grenades". Hopefully, I'll never get the chance to see how well the blast shield works.

I picked up a 0-500 psia Paroscientific Digiquartz pressure transducer from eBay that I plan to use as a calibration source for my tank and chamber pressure transducers. Paroscientific sensors are well-known for amazing long-term stability so I'm expecting the cal sheet I have from 1986 is still good. It is an absolute transducer and the zero looks to have drifted by about 0.5 psi (0.1% FS) which is reasonable over that period of time. These have a frequency output in the range of 30-42 kHz so the measurement accuracy is dependent on the accuracy of the frequency counter reference oscillator. The newer versions have both a pressure and temperature-compensating output but this older unit only has the frequency output. These have an interesting calibration curvefit and as a result, the accuracy of the indicated pressure is about 10x worse than the accuracy of the frequency counter reference oscillator. For example, a 100 ppm reference oscillator will yield about 0.1% FS accuracy of the transducer. So, to get the full 0.01% FS accuracy of the Digiquartz transducer, I need at least a 10 ppm reference oscillator. The equation is P = C(1–T0^2/Tau^2)[1 – D(1– T0^2/Tau^2)], where C, D, and T0 are from the cal sheet, and Tau is the period in microseconds of the frequency output. I don't have a real frequency counter but I do have a National Instruments PXI-6030E DAQ card and a PXI chassis with a 25 ppm clock built into it that gets me pretty close. For a 500 psi transducer, that should get me within 0.125 psi or so, certainly good enough for amateur rocket work. I also just purchased a Datum Tymserve 2100 from eBay which in addition to the NTP network port, has both an IRIG output and a 10 MHz reference clock which will come in handy for testing.

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