From last month I left Em-Sem-Fity mostly set up for the AT K458 White Lightnin' 'load but, despite that tremendous head start, I still managed to fly only that rocket and shoot a pitiful rooker-full of photos. I'm gonna be brief today and rely on photos as proxy for 1000 words each.
I vowed to use more thermite this flying season and I'm off to a fine start. Having metered 3g of stoichiometrically-balanced CuO/Mg thermite, added one e-match, and asked Rick Frelke to push the button I finally saw fairly rapid ignition of a 98mm, 1G motor about one second after the pop. Enjoy two ignition frames 1/10th of a second apart:
And a graceful liftoff:
Once again, due to low wind, I elected to pop the main at apogee. I had a perfect heading at landing and all looked well until I had walked well past the visually estimated landing spot. Oh and... yes I did use a Com-Spec tracker but I was SO confident that I knew the landing spot that I left the receiver at my table. Jenius. Lesson: always take all that [light] equipment no matter what. As I was walking back to grab that receiver I saw Darrel looking for his own 20k President's Challenge rocket. He called to tell me he'd found mine on the north bank of the first wash heading north. For some inexplicable reason I had meandered WAY east on my outbound journey and therefore missed it.
Prior to flight I checked the static margin and it seemed way low so I added a 1lb weight to the nosecone base. This increased the liftoff weight to 19lb and that resulted in a lower altitude than last month's K1999. The K458 yielded a 6,695' barometric average (ARTS2=7,079', Raven2=6,310') with a pretty big delta between the two altimeters. Enter... data:
ARTS2: tabular, graph, motor performance (2238K374), coefficient of drag
Raven2: tabular, graph
Oh and, in leveraging last month's K1999 ARTS2 file, I realized that I somehow had entered the liftoff weight in kg rather than lbs. The above motor performance reflects accurate liftoff lubs and appears reasonable. I'll also correct the K1999 data below.
2 comments:
The ignition delay was probably caused by two things: (a) the short motor (a lot of the stuff gets blown out of it), and (b) using too much thermite -- overloading the grain with energy at ignition actually decreases the amount of thermal energy stored below the surface (I know, right?) leading to extinguishment when the flux is removed. Also, if the core is shiny, it makes the ignition delay ridiculously long. Try backing off on the thermite next time (I light Ks with 1-1.5g) and sanding the cores, and I bet your results will improve!
--david
Thanks, David. So below what threshold does the 1g per 1000N•s rule not apply? Also, I elected not to use a nozzle cap to avoid over-pressure. Might that have slowed my ignition?
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