Theo and I tested the TR-1 motor on Saturday and unfortunately suffered a cato. It was an impressive sounding motor for that first few milliseconds though. Apparently I should have rethought my decision to use Dextrose in place of Sorbitol. On paper the change seemed fine but I had limited experience with Dextrose and never in something of this size. The actual casting of the grain segments went fine but I experienced some slumping in two of segments that I cast. At the time I decided that the slumping was due to having removed the segments in question too soon from the mould. I believe that segment slump either contributed to or was the direct cause of the cato. Fortunately I had cast an extra segment and at the end of the day, long after the firing, that segment which had been stored in the same conditions as some similarly sized Sorbitol segments that Rick had cast had slumped. The sorbitol segments remained fine. The dextrose segment had slumped so severely that the core was essentially gone. I know of many folks that have use dextrose based propellants with no reports of slumping so I’m unclear of the reason, maybe the size of the segments? For now I will switch back to Sorbitol in the TR-1 motor and save the dextrose for some much smaller scale motor testing. We had a good time regardless. The guys at FAR were very helpful, and the site had grown quite a bit since our last visit there some three years or so ago. Rick Maschek also provided lots of assitance and camera work. Rick also had a nice firing of a KNSB motor with a Double-D grain configuration. The third photo is of Rick's firing. The last photo is af an AP motor firing that anomther FAR member (Erik, I think) conducted.
Monday, September 22, 2014
Thursday, September 18, 2014
I got this Loki booster a while ago. It's a little banged up (the fins are slightly bent) but overall it's in pretty good shape since it's probably around 60 years old. This one was never fired or filled with propellant. The transition section doesn't have the roll pins installed and can be removed to revel the interior of the motor. It is also possible to see the rubber liner that protects the thin aluminum motor casing from the heat of combustion. The black spots seen at the base of the motor and on the nozzle retaining ring seem to be the same material as the liner. I'm not sure what the pin on the top of the transition section is for; it's spring loaded and maybe it's meant to help ensure a clean dart separation. The pin on the side was, I believe, to keep the dart from spinning as the rocket was spin stabilized. It's cool to think that this small booster was capable of getting a dart up to 34 miles...
Monday, September 15, 2014
Thursday, September 11, 2014
Wednesday, September 10, 2014
Tuesday, September 9, 2014
McMaster-Carr order arrived yesterday and included the o-rings, Delrin rod, screws, and pressure gauge. I didn't get a chance to assemble the motor for a final test fit, but now that I have the o-rings that's next on the list of things to do. The Delrin rod for casting the core in the propellant segments was a perfect fit in the casting base, so I'll just need to cut to length. Delrin would be an ideal choice for the casting bases too, but the stock I need would be around $60 for two bases, so I'll stick with wood for now.
Monday, September 8, 2014
This past weekend I finished a few more tasks, although it's starting to look like I may not be quite ready to test fire the motor by the 20th of this month like I was hoping to do. I was hoping to get o-rings by Saturday so I could do a final test fit of the nozzle and bulkhead but I didn't get them on Saturday like I had hoped, but I can do that this week as it shouldn't take long. I did some minor machining on the nozzle to get the two pieces to mate together perfectly. I also took the time to go back and adjust the inlet a bit by machining the inlet at decreasing inlet angles (30, 15, and 7.5 degrees) and then grinding and polishing smooth so the inlet is MUCH smoother and gradual transition to the throat. No change to the throat diameter was made.
I also made two casting bases out of scrap redwood that I had lying around. The casting tubes are held in place securely with a friction fit. In the past I have only used a 1/4" recess but these are a full 1/2" deep so the tubes seem pretty snug. The surfaces of the wood that comes into contact with the tubes/propellant were coated with epoxy. Once cured, they were machined to the final dimensions. This seals the surface from the grease used as a release agent and helps maintain dimensional stability.