Yes, that's what I've always heard, that the reduced volume in the thick walled case left no room for delay or ejection charge - hence, booster-only C motors.

Doug

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YORF member #11

C8-0 1.5lb/sec = 6.67n/sec

Still a C, just not a full C.

The lack of internal space explanation does make sense. I don't know if the thinner walled casings started being used as early as 1968, though. Wish I still had some from that era. I somehow had the impression that Estes started using the current casing dimensions at around the same time that they came out with the D motors and the mini motors, in the 1970-71 time frame.

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SAM # 0011

Mark,

The C6-0/3/5/7 was first out in 1968. That would imply thin walled casings. It also coincides with the advent of the metric motor designations. So the thin walls and metrics came out together.

The minis and D's came not long after.

Doug

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YORF member #11

I am aware of the sequence of releases. I would have assumed that the modern casing was introduced with the new C6s in 1968. I thought I heard awhile back that Estes didn't start using these casings until the early 70s, though. Back in 1968 or '69 I had B and C Estes motors. I also had Series III motors and pre-metric motors from my 1967 starter set. I wish I still had them so that I could check this out. At the time, I didn't notice any differences at the time, but that doesn't mean that there weren't any. I do remember the casing walls on my first motors being quite thick, though.

I remember when Estes came out with the D motors in 1970. They were the biggest model rocket motors that I had ever heard of. I thought of them as super-motors but I never got around to buying any of them during the next year. I suspended my involvement in the hobby mere minutes (only a slight exaggeration) before Estes announced their new mini T motors in 1971. Talk about perfect timing.

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SAM # 0011

This is my memory of it as well. Most of my early flying was about 1965-1969. I built a Ranger back then and launched it with both series I and series II B's. Those were the thick walled engines and a B filled the available space, no room for enough propellant for a C with delay and ejection charge.

I remember when the Big Bertha came out, one of my friends built one. It did have the low and slow majestic flight characteristic, and I wasn't interested in it for that reason. Plus it wasn't very sleek with the blunt nosecone. Nowadays a BB would fly very well on a modern C engine. I'm actually planning to build a BT60 rocket, maybe a Vagabond, to use for D12 motors.

We built models with the BT10 and BT30 tubes in those days. I had forgotten about the BT40 tube but I still have one model made with it, the BT40 is not much bigger than the BT20 motor mount tube. I remember when the BT55 tube came out and I built a scratch built design with it. I thought this was a good tube for a B, slightly faster than a BB.

Estes didn't have C single stage engines back then but at some point in time we discovered FSI engines and they had a C. It was bigger than 18mm, maybe 20mm or 21mm. I tore the motor mount out of my BT55 model and made a motor mount just to fit the FSI engines. Now that I think about it it might have used the BT40 to hold the motor.

As a kid we had made our own launch system using dry cell batteries, so we could carry it out into my grandfather's pasture. When I launched my Ranger we had someone drive us to a different launch site and we used their car battery for power. This is another advantage of a single motor over a cluster.

I neglected to add to my previous post that in light of the explanation that Doug provided, I stand corrected regarding the introduction of the thinner-walled Estes engine casings. It obviously had to have been 1968, because the C6 motors with delays and ejection charges that were introduced that year could only have fit into casings with the current (thinner) inner diameter.

I can easily see how a BT-40 could be used as a motor tube for a 21mm FSI motor. I don't have any samples of 1960s Estes Industries BT-40 to verify it, but assuming that Semroc's version is faithful to the original, a 21mm motor could conceivably be just gotten into it with a very snug fit. I use BT-40 for homemade motor adapters for Quest D5 motors, which are nominally 20mm in diameter. I have to slit the tube down the side to get the motor to fit in, but that might be partly due to the loose-fitting paper label on it. (The ID of Semroc BT-40 is actually 0.022" smaller than 20mm, so this might be necessary anyway.) With the slit BT-40 glued inside a length of Series 085 tubing (LT-085) and the gap caused by the slit blocked with a little bit of epoxy putty or JB Weld, I have a 20mm to 24mm adapter that is dirt-simple to construct.

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SAM # 0011

There might be a way around this cluster ignition problem. Similar to the way the Estes MIRV uses a trifurcated vent to direct the single 18 mm first stage motor's hot burn-through particles into the nozzles of its three 13 mm second stage motors, a "spider pan" (as the Russians call a similar black powder vented-flame "flash pan" system that ensures ignition of all of the Soyuz rocket's thirty-two core stage and booster thrust chambers) could ignite the Ranger's three motors using just a single igniter. It could be arranged thus:

The "spider pan" could be made of ceramic or metal. A little black powder or pyrodex would be poured into it, and it would be placed on the launcher's jet deflector (held in place using a stand-off bracket that would be slid down the launch rod to keep it from falling off the jet deflector) with its three vent tubes aimed into the motors' nozzles. A standard Estes or Quest igniter would be installed inside the spider pan, with its leads protruding through two small holes in the side of the spider pan. It could be made in two threaded sections (upper and lower) that would facilitate igniter installation & removal, filling the lower section with black powder or pyrodex, and cleaning the spider pan after use.

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NAR #54895 SR

Correct, as Mark+3 previously noted. Here are the links to the catalog pages. The C.8-0 booster motor in the 1967 Estes catalog is shown as having 1.50 lb.-sec. of total impulse. The C6-x motors in the 1968 catalog are listed as having 2.25 lb.-sec. of total impulse. This translates to 6.675 N-sec. for the 1967 motor and 10 N-sec. for the 1968 C motors. Quite a difference.

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SAM # 0011

I was trying to reconcile bits of my memory and I think I jumped to incorrect conclusions. I just took some measurements and that changes my conclusions.

I thought I remembered hand rolling a paper tube to use as a motor mount. The motor tube in the rocket is thinner than normal Estes tubes, but smoother on the inside than I would have expected myself to make at age 14 or so. The ID of this tube is 0.945" and as I recall it fits the FSI engine about like an Estes BT20 fits their 18mm engines, or maybe a bit larger. That should be a good gauge of what the FSI engines required.

I knew that the other rocket that I still have used a tube somewhere in between BT20 and BT50, and I had forgotten about BT40 until I read this thread. This rocket has centering rings at the back end and a balsa adapter glued into the front end so I can't measure the ID. The OD measures about 0.905".

So obviously my FSI engine mount doesn't use a BT40.