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Old 11-27-2016, 05:26 PM
johnpursley johnpursley is offline
Somewhat Skilled Rocketeer
Join Date: Mar 2011
Location: Houston, TX
Posts: 54
Default Another Estes K-36 Tidbit

Was going to mention this in an earlier post...

I've had more than one person claim that they thought the original K-36 Saturn flew better, particularly on the D13 and D12 that what they see today (and I agree in my "recollections"). The claims and recollections are probably true. Why?

The original K-36 engine mount was almost flush with the rear of the BT-101 body tube...and the nozzle-ends of the motor(s) projected beyond that. The newer versions of the Saturn V (the current version included) recess the motor by about 3.5", presumably to help shift the CG further forward and make the model stable with only slightly oversized fins and no need for the slip on clear fins of the original Saturn V kit.

So what does the difference between "flush" engines and "recessed" engines make? Well, it has something to do with the "Krushnik Effect"...from a modeler in the 1960s who studied and defined the effect. If you fire a rocket engine into a tube that extends beyond the rear of the nozzle of the engine you (simplified explanation) create an artificial nozzle extension that approximates a grossly "overexpanded" nozzle which can very significantly reduce effective thrust.

Back in 1970 there was a launching of the Estes Saturn V inside the Houston Astrodome during half-time of the AFL-NFL Championship football game (they call it "Superbowl" now). The model needed to be quick to set up and reliable. The obvious choice was the new Estes D13 (we'd cringe today, knowing the CATO prone history of the D13). The problem was, the D13 would propel the K-36 higher than the dome clearance of something like 200 feet (the claimed altitude of an Estes Saturn V on a D13 engine was something like 300 feet...though it was probably really closer to 250). So, Estes produced a special "downrated" D13 just for that occasion. Well today's Saturn models don't go near 200 feet (on D12 motors)...something between 100 and 150 feet is more like it. It's not entirely because the modern D12 is less powerful than the D13 but more than likely because, in the newer Saturns, the Krushik Effect is reducing the efficiency of the motor and likewise reducing altitude.

I've heard from more than one modeler over the years (again, myself included) who have built Saturns with the engines that are flush or that project slightly beyond the rear of the body tube and the performance is (for lack of side by side comparison) visibly better than with recessed motors.

It's not hard at all to modify any of the line of newer Estes Saturn V models so that the motor is flush (or much less recessed) than indicated in the kit instructions. Just be sure to add adequate nose weight (inside the Apollo capsule is a good place) to keep its CG the same as for the recessed motor. An easy way to assure the CG stays where it should be is to NOT glue the internal engine tube/ring assembly in place until you finish the model. Then install the heaviest recommended motor and slip the assembly into the model until it is recessed by the amount recommended in the instructions. Install the recovery system and wadding and then balance the model to locate the CG and make note of that distance from either the nose or the tail of the model. Then, slide the internal assembly back so that it is almost flush with the rear of the body tube and add nose weight to the nose of the model to bring the CG back to the location you measured with the internals recessed...super calculations (other than measuring), no computer and no redesign (other than locating the internals 3-inches or so further rearward).
John Pursley
Accur8 Spacemodels
Accur8 Spacemodels (Website)
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