Larry Curcio
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Here's a comparison of the thrust curves of the C11 and D12:
https://www.rocketreviews.com/compare-estes-d12-to-estes-c11.html
-- Roger
Using the specs here as Gospel…
The *overall* effective exhaust velocities (Ve) for the D12 and C11 are 802 m/s and 733 mps respectively. The Ve in the peak is normally higher than the Ve in the plateau, because of extra thermodynamic efficiency at high pressures. The higher peak raises the overall Ve (and hence Isp) of the D12.
The difference in Ve is likely by design, because the C11 impulse at the higher Ve would come out to 9.6 ns, which is probably too close to the 10 ns limit for a BP motor.
At the same time, the plateau thrust of the C11 seems higher than that of the D12. If this height difference isn’t just random variation, it suggests the following:
1) The propellant perforation in the D12 is slightly longer than that of the C11. This creates a higher peak and overall Ve for the D12. (I grant you the perforations are small, but they have an effect. Note that a peak occurs even when no perforation pertains because of a quasi-hemispherical burning pattern.)
2) The nozzle throat diameter of the C11 may actually be slightly smaller than that of the D12. This would yield a higher plateau for the C11. Of course the plateau is shorter in duration, because there is less propellant.
And this makes vague sense on its face, as longer propellant perforations are generally associated with wider nozzle throat diameters.
Remark: I've been doing spot landing experiments using simulations to determine launch angle. Real world accuracy is much better for short-burning motors, and the C11 is a good spot landing choice in the C impulse class.
Oops! I see that Bob K. said essentially the same thing earlier. My bad!
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