04-10-2019, 10:14 AM
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Master Modeler
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Join Date: Jan 2009
Location: Fairbanks, Alaska
Posts: 6,507
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Quote:
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Originally Posted by tbzep
Since that Omega Cineroc flight, I've found that off axis thrust is pretty common with BP motors, especially during that critical time of high thrust the first 25 ft or so off the rod. Nozzles normally look fine after flight, so I don't know if smeared clay nozzle material, igniter residue, or something else causes it. Many of the super slow-mo videos on Youtube show this also. With that in mind, I don't know if Estes would want to start honoring off-axis thrust as a reason for flight failures.
BTW, I used to see off-axis thrust in C-slot composite motors all the time. You'd think the nozzle would straighten it out, but a nozzle throat is just a fraction of an inch long.
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I guess the high acceleration--a significant portion of whose "run" is done on the launch rod--and the quite short motor burn times (usually less than a second) of most motors might make off-axis thrust largely irrelevant in most cases. If it is that common (I don't doubt you, having seen high-speed [slow-motion] footage of model rocket launches), we don't want to risk "complaining Estes [or Quest] out of business," or eroding--no pun intended--their reputations... :-) Also:
I would never have suspected that such "off-center-burning grain" composite motors would have exhaust plumes that would actually mirror--in the rockets' flight characteristics--the internal ballistics of the burning grain inside. Maybe a sufficiently-long nozzle (or even a provided jetavator) would eliminate that, without really hurting the motor performance enough to matter. |