A8 or 6 or 5 or 3-3 or 2

Recently I wrote something that mentioned how the Estes A8 motors really are, for incomprehensible reasons, more like A3. I’ve been looking at some old Estes catalogs online (they’re here, also here) to get a little more information on the subject.

The 1961 catalog lists an engine designated A.8–3. In those days the units were all Imperial; as it says in a table in the 1964 catalog, the letters denoted:

¼A 0–0.175 lb-s
½A 0.176–0.350 lb-s
A 0.351–0.700 lb-s
B 0.701–1.20 lb-s
C 1.21–2.00 lb-s

and the average thrust number was given in pounds. (Notice the B and C impulse ranges cover less than a factor of 2.) The A.8–3 had a total impulse of 0.7 lb-s, which translates to 3.11 N-s, a burn duration of 0.9 s, and a maximum thrust of 18 oz (5.00 N). The average thrust wasn’t shown, but it’s just the total impulse divided by the duration, which comes out to 0.78 lb or 3.46 N. So the A.8 designation was consistent with those values.

In the 1967 catalog the A.8–3 was still 0.7 lb-s but the duration was down slightly to 0.83 s and the maximum thrust was up to 23 oz (6.39 N), for an average thrust of 0.85 lb (3.79 N).

1968 saw Estes’s introduction of the metric designations still used today, shown here with Imperial equivalents:

¼A 0.313–0.625 N-s 0.07–0.14 lb-s
½A 0.625–1.25 N-s 0.14–0.28 lb-s
A 1.25–2.50 N-s 0.28–0.56 lb-s
B 2.50–5.00 N-s 0.56–1.12 lb-s
C 5.00–10.00 N-s 1.12–2.25 lb-s

One thing to notice here is, by these modern standards, the Imperial A.8–3 was a B motor! On the low end of the B range, but B nonetheless.

Estes didn’t simply relabel the existing motors. Instead they revamped the motor line, and designated replacements for the old motors. The “replacements” were not equivalents but substitutes, and were often rather different from the engines they replaced.

For instance, the replacement for the old A.8–3 was the new A8–3. The “replacement” had a lower total impulse, 2.50 N-s; a burn duration roughly half the old one, 0.42 s; and a maximum thrust more than twice the old, 48 oz (13.3 N; for whatever reason, maximum thrusts were still given in Imperial units only. Present Estes catalogs give total impulse in metric and all other values in both metric and Imperial.) By calculation, the average thrust was 5.95 N. So why wasn’t it “A6-3”?

It may have simply been an error. In the 1969 catalog the total impulse and maximum thrust are the same, but the duration is 0.32 s, giving an average thrust of 7.81 N. The “0.42 s” in 1968 doesn’t seem to have been a simple typo, since the 1968 durations for the A8–5 and A8–0 were 0.42 and 0.40 s; in 1969 these became 0.32 and 0.30 s. But still, the 1968 durations may simply have been some kind of transcription error.

In any case the numbers stayed the same from the 1969 catalog to the 1996, and the A8-3 really was an A8-3. Or at least it was according to Estes’s stated specs…

But here’s where things start getting weird.

The NAR S&T sheet for the Estes A8-3 and -5 motors gives a testing date of 95-March-25, and their measurements were: Total impulse, 2.32 N-s; duration, 0.73 s; maximum thrust, 9.73 N; and average thrust, 3.18 N! So slightly lower total impulse, substantially lower peak thrust, and a burn duration more than twice as long as what Estes was publishing, resulting in 60% lower average thrust. They also measured an average time delay of 2.25 s. By the NAR test, the A8-3 was really an A3-2! The NAR thrust curve shows the peak occurring at about 0.22 seconds, followed by a plateau at about 2.5 N out to around 0.65 s, dropping to zero at 0.73 s.

And yet in 1996 Estes was still using the same numbers, while in 1997 the stated duration was increased to 0.5 s and the maximum thrust to 11.8 N. Total impulse was still shown as 2.5 N-s, for an average thrust of 5.0 N.

In 1997 Estes also included thrust curves in the catalog. The curve for the A8 peaks at about 12 N at a time of about .19 s and falling to zero around 0.55 s, a much shorter plateau duration than NAR’s, and almost twice as high — around 1.0 lb (4.4 N). The same curve appeared in 1998.

The next catalog to include engine data was 2002, and the A8 thrust curve shown there was utterly weird — rising to about 13 N in about 0.13 s, then falling to zero by 0.25 s. This had to be completely wrong. The numerical values were the same as in 1997.

From 2003 to present the thrust curve in the catalog has been fairly consistent with the 1995 NAR curve. The peak is just under 10 N at about 0.22 s, the plateau is about 2.5 N, and it drops to zero just after 0.7 s. And yet the numerical values are: Total impulse 2.50 N-s, duration 0.5 s, maximum thrust 10.7 N — inconsistent with the thrust curve shown on the facing page. From these you’d compute a mean thrust of 5.0 N, but it’s only that high because the stated duration is so much shorter than what’s shown in both the NAR and Estes thrust curves.

Here’s a summary:

Source Total impulse (N-s) Burn duration (s) Maximum thrust (N) Plateau thrust (N) Average thrust (N)
1961–66 catalogs
3.11 0.9 5.0 3.46
1967 catalog
3.11 0.82 6.39 3.79
1968 catalog 2.50 0.42 13.3 5.95
1969–96 catalogs 2.50 0.32 13.3 7.81
1995 NAR test 2.32 0.73 9.73  2.5 3.18
1997–2002 catalogs
2.50 0.5 11.8 5.0
1997–1998 catalogs (curve) 0.55 12  4.4
2002 catalog
0.25 13  0
2003–14 catalogs
2.50 0.5 10.7 5.0
2003–14 catalogs
0.72 9.9  2.5

What to conclude? I don’t know, other than (1) the A8-3 has changed over time, and (2) these data are inconsistent. Sure, there’s some variation from one motor to another and you can’t expect NAR’s test results to exactly equal Estes’s — but they differ by a lot more than that can explain, I’d say, and Estes’s numbers and curves disagree with each other. The NAR and Estes curves do seem to be in decent agreement, at least since 2003, so those probably can be relied upon. And in any case, “A8” is far from truth in labeling.



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