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#21
08-21-2008, 08:52 PM
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Mark: this was called originally called ganged motors in the early 60's when Doug Frost first researched the idea and later in the 70's called tandem motors.... and the reports of CATo's everytime wasn't true. Our own Trip Barber back in 1978 when tamdem motors were allowed, did an extensive R&D study(which I just so happen to have) and he found out that if they were glued together in a certain way with a outer sleeve, and the lower motor engine was either equal to or greater than the nozzle size for the upper motors, and the total thrust duration of both engines was < 2 sec approx, then the motors would work just fine. In fact in 1978, when Trip did this R&D report it just so happened that he was also Chairman of the NAR S&T and he actually allowed these tandem motors to be used in NAR competition as they had an advantage over staged models. Unfortunately, then new NAR President J. Patrick Miller, stretching NFPA regulations to say that tandem motors were "being manufactured" by the people who glued them together and statements from Estes that ths was not an intended use, immediately overruled Trip and Tandem motors were no more. I started a thread on this topic a few months back on TRF: http://www.rocketryforum.com/showpo...348&postcount=1 no need for it to be repeated here. Since the TRF post I received the Trip Barbers reports but I could nver locate Doug Frost to get his early 60's R&D report, but it was just a older version fo what Trip did with an electronic test stand versus a mechanical test stand that Doug Used. terry dean
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"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them. Last edited by shockwaveriderz : 08-21-2008 at 09:08 PM. 
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#23
08-22-2008, 12:05 AM
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Thanks, Terry. That conclusion goes against my intuition, but that's what research is for, right?
 I missed your recent thread on TRF, and I was, instead, just summarizing the information that I could recall from the Oldrockets thread. That earlier thread did stick out in my memory because I had never heard of the technique until then. At first blush, it struck me as being quite ingenious, until I read the comments about CATOs that followed in the thread. Trip Barber's research report obviously carries the most weight regarding this issue, though.One clarification about my post though. When I said "CATO, every time" I was specifically referring to the effects of a blocked nozzle exit port, and was not saying that this was the inevitable result of tandem staging. (The complete sentence was "Blocked nozzle exit port = CATO, every time.") What I did say in that post was that I thought the technique had a significant tendency to produce CATOs, but I was not saying that such a result was inevitable. This is all moot now, anyway, because you have provided me with information that has caused me to modify my earlier take on the subject. Mark \\.
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Mark S. Kulka NAR #86134 L1,_ASTRE #471_Adirondack Mountains, NY
Opinions Unfettered by Logic • Advice Unsullied by Erudition • Rocketry Without Pity
Last edited by Mark II : 08-22-2008 at 12:25 AM.
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#24
08-22-2008, 08:14 AM
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I thought the reason for the demise of tandem motors was that a certain manufacturer said that this was not "use as intended by the manufacturer"...
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#25
08-22-2008, 08:34 AM
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Yes, and that was after NUMEROUS motor failures or catos (not CATOs since the abbreviation is not an acronym). The firing of the upper stage motor through the lower stage motor reodes the inside of the lower motor casing. This results in more mass being accelerated out the lower nozzle which results in 10% higher thrust throughtout the entire upper stage motor burn. The lower casing will burn through the side with longer burning upper stage motors (C6-0 to C6-5 or -7, D12-0 to D12-5 or -7). This was a safety issue. And, yes, epoxying two motors together and epoxying a sleeve over them to form a secondary outer casing is indeed "manufacturing" and the outer casing and epoxy has not been certified/tested and there is no quality control, so it makes sense that they are not allowed/certified/covered by NAR insurance.
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-Fred Shecter NAR 20117 (L2) Southern California Rocket Association, NAR Section 430
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#26
08-22-2008, 09:01 AM
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Were these sleeves just heavy paper rolled with epoxy onto the tandem assembly to build an extra heavy wall? I guess an old BT-20 tube (modern motor mount tubes are puny in comparison) and epoxy would give some strength without having to roll anything, or maybe use a 24mm blank casing, but they fit slightly loose and you'd need the epoxy to completely seal between the 18 and 24mm casings. Just curious.
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#27
08-22-2008, 09:31 AM
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it was a sleeve of regular body tube, like BT-20 or BT-50. Obviously that presented a problem for the airframe body tube or motor mount tube for the new and larger diameter motor assy, so most folks used CMR RB-74 for the motor sleeve and RB-77 for the body tube. Thin wall cardboard tubes with epoxy slathered on by average Americans is not a reliable high quality motor casing. There is no need for this now, as there are double the power motors in the same size and weight. They are called "Aerotech".
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-Fred Shecter NAR 20117 (L2) Southern California Rocket Association, NAR Section 430
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#28
08-22-2008, 09:32 AM
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As far as I'm concerned, unless one is boring into the propellant of an existing motor to create a port burner, it should NOT be considered manufacturing and it should be allowed.
The cato rate should have zero impact on this. Gluing motors together is no different than taping them together for staging. it is NOT motor manufacturing and I'm sure even the BATFE would agree with that. Once one gets into packing BP propellant or drilling cores, THAT is manufacturing. I'd even see removing clay end plugs from E15-P, E9-P and A10-PT engines to create boosters as perfectly acceptable. One is NOT manufacturing anything. My attitude toward this is probably why I never joined the NAR with it's too-many-regs and why I almost always choose to fly on my own private land. I could care less when a motor cert expires or if has ever existed.
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When in doubt, WHACK the GAS and DITCH the brake !!! Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't ! Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY. ALL systems are GO for MAYHEM, CHAOS, TURMOIL, FIASCOS, and HAVOC !
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#29
08-22-2008, 10:07 AM
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It alters the performance.
It alters (increases) the pressure of the lower motor casing. The thin sleeve must function as part of the motor casing to retain pressure, so that makes it part of the newly re-manufactured motor assy. IF and only if a manufacturer submitted tandem motor kits including the sleeves, to a recognized certification authority and had them tested and passed, could they legally be used as Model Rocket Motors. RMS motors are tested and sold as a system. Delay adjustment is tested and approved. This is all very clear and logical to me. Quote:
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-Fred Shecter NAR 20117 (L2) Southern California Rocket Association, NAR Section 430
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#30
08-22-2008, 10:30 AM
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Quote:
Bob< that was 1/2 of the reason. The other half was as I stated above: J. Patrick Miller decided that the cat of gluing 2 motors together to make a tandem motor was 'manufacture" and that violated the NAR Safety Code and NFPA regulations. Check your copies of Model Rocketeer for 1978. In hindsight, it appears this was just a power play by a then new NAR president to show who was in charge. Its also just another example of how the NAR used its safety code and NFPA regulations to control model rocketry. terry dean
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"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them.
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