It's been a bit since I last posted, but for good reason. In the past two weeks I have frantically made the last adjustments and accommodations before test firing. I also got to ship out and visit Princeton, which was quite fun.
NOTE: I'm posting this from a computer other than my own so I can't upload the pictures until I get home tonight. There are pictures I promise!
The main preparations I've made are to finalize the data acquisition system and to ensure the fuel grains and ignition are ready to test. The data acquisition system initially seemed difficult to tackle, but the school allowed me to borrow a Labquest 2 unit, which greatly simplified things. The only problem is that my pressure transducer that taps into the rocket's bulkhead puts out an analog voltage signal, and the Labquest is only capable of receiving digital signal via either USB or ethernet ports. The simplest solution appeared to be to order a voltage probe for the Labquest, which the physics department promptly got done (Thanks, Mr. Clark!). Using that, I am able to plot voltage vs time on the Labquest, and in turn calculate the pressure in the rocket at any given point. Using a modified version of P=F/A (pressure is the quotient of force and area), the force produced by the rocket at different points in time will be solved (and an integral could be used to find the total kinetic energy produced).
One of the problems in using a analog pressure transducer is that it requires a steady power input to operate. The output of the transducer is between 1 and 4 volts, and the power input of the transducer should reach that. It should also be constant, as to reduce power fluctuations that would cause the analog voltage readings to not properly reflect the pressure in the bulkhead. The solution to this is to use a voltage regulator with the power supply. The power comes from a standard 9V battery, but the voltage regulator provides a power supply that is consistently a perfect 5V for as long as the output of the battery is greater than 5V.
The fuel grains are the other mater. My grains don't cure. They never cure. This is frustrating. On my last batch of grains (HTPB/Al and HTPB/Al/PVC) the composition of MDI (the agent that causes liquid HTPB to set as a solid rubber) was already 20% by weight. This is HUGE! A typical batch (albeit on a larger scale so molecular interactions over time are different, plus a typical batch is mechanically stirred for hours whereas mine are hand mixed for several minutes) is 15% by weight. Mine weren't solid even at 20. So I ripped out the semisolid, supersticky HTPB (my hands have been covereded in it for almost two weeks). This task was very difficult.
Upon making the new grains, I upped the MDI percentage to 25. This is massive. Detracts from the actual fuel weight in the grain, meaning the fuel grain stores less energy and will not burn as effectively. But this power sacrifice is necessary, because perfectly solid fuel is incredibly important. If the fuel flows in any manner, it risks plugging the nozzle and causing a dangerous pressure buildup in the case that could blow the assembly sky high.
One thing I've noticed is that I have developed an allergy to the MDI, as many people do (yes, the Swedes did a study on it, I'm not alone in this). So in addition to being a nasty irritant to skin, it also causes me to have an allergic reaction in areas of contact. It's been two weeks with a this irritatio on my forearms, back of my hands, and a little on my forehead as I stupidly wiped the sweat off my brow with such contaminated forearm. And even when I thought I started to heal, I accidentally dumped a syringe-load of MDI right back on the affected forearm. I'm stupid. But rocket science is messy.
From what I see and experience at the lab, rocket science isn't glamorous. It's hard, messy, filled with slagging metal and aluminum oxide, reactions that misbehave, and plenty else to gripe about. But that's what makes it enjoyable. The challenge is what motivates.
Next week is testing week! I'm far behind schedule with testing because of many many many unforeseen obstacles. It's long overdue, and it's going to be great.
