Thursday, November 30, 2017

Testing a bit of the past

There are very few 3 stage rocket kits nowadays (the Estes Comanche 3 being the best well known), and those that do exist use the 24 mm Estes C11 or D12 motors in the first stage booster. This is because the first stage motor has to loft a lot of weight fast enough to have the model stable when it leaves the rod, and the current lineup of 18 mm motors doesn't have one with enough oomph to accomplish this. But 50 years ago, things were a bit different...

Back in 1964, Estes released the B3 motor, which under the English unit nomenclature of the time meant that the motor had an average thrust of a whopping 3 pounds (13.3 newtons). The B3 was a Series II motor, which meant that a) it had high thrust and b) the manufacture required an additional step - drilling a deeper nozzle into the black powder propellant. The result was a motor with a short duration (~0.3 seconds), spike-like thrust curve with a peak thrust around 7 pounds (31 newtons) - the Estes catalog described the thrust as "like a sledge hammer blow." And it was true - the B3 could move single stage rockets so fast that poorly attached fins would rip right off, and easily had the power to loft heavy 3 stage payload carriers like the Farside-X. When I first started rocketry in '68, the motor classification system had just gone metric, so the B3 became the B14. Same characteristics, different label.

Comparison of B6 and B14 thrust curves in the 1968 Estes catalog (Click to enlarge).
We swore by the B14, as it made 18 mm 3 stagers possible. But good things cannot last, and Estes, citing safety concerns, stopped producing the B14 in 1980, when it was replaced by the B8. Also classed as a Series II motor, the B8 had a thrust curve with a sharp initial spike (up to 22 newtons), which dropped into a plateau with about the same thrust as that in the B6, except that the plateau didn't last as long. Total thrust duration was around 0.5 seconds. The B8 was a less capable motor, but it was safer to make, as the motor machine could be equipped with a pintle (think small thin nail) that could produce a deeper nozzle than that of the normal Series 1 motors. The B8 was discontinued around 1997, and we currently have no high thrust 18 mm motors. This is likely to remain the case, because spokespersons for Estes have categorically ruled out any notion of restarting their production, despite the frequent demands of geezer rocketeers.

So why this foray into rocket motor history? It's because I have recently acquired some old Centuri B14 and Estes B8 motors, and I tested a couple of them - along with a current B6 - on my motor test stand last Saturday. I measured the thrust on the 50 newton maximum scale at 50 samples per second; the resulting curves are shown below. Note that the blue B14 curve is very similar to that shown in the catalog image above - a sharp spike up to 21 newtons, followed by an impulsive event at the end of the thrust (this was a booster motor, so no delay). However, there is a bit of a puzzle, as the peak thrust should have been over 30 newtons, whereas this Centuri motor produced only two-thirds that amount. I'm at a loss to explain this, except maybe to invoke motor deterioration over the 44 years since this motor was made. The red Estes B8 (manufactured in 1980) curve shows a similar sharp rise to 23 newtons peak, and then drops down to a thrust of just over 4 newtons, similar to that of the modern B6-4 (black curve). It is spent after 0.5 seconds, with the ejection charge occurring about 5.9 seconds after burnout.

Saturday's thrust measurements (Click to enlarge).
It's kinda cool to have this data, as thrust curves for the B14 are hard to come by, with most folks referring to the figures in old Estes catalogs. This is actual data, though I am going to test another one of the precious B14's to see if that one's peak thrust is also lower than expected. Even so, the availability of those motors enables me to build unmodified Estes Farside and Centuri Arrow 300 clones, secure in the knowledge that I can get them off the pad fast enough to be stable in wind, even with altimeters in the payload sections.

2018 is gonna be a fun year!

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