y Kenneth B. JaroschNAR 56442, TRA 10290&<strong>Baffles</strong>Mo r epart II28 MAY/JUNE 2010 Sport <strong>Rocketry</strong>In Part 1 <strong>of</strong> this article, found on page 20 <strong>of</strong> the March/April 2010 Sport <strong>Rocketry</strong>, I discussedbaffles for mid-power rockets up to 4 inches in diameter. In this part <strong>of</strong> the article,I will discuss my efforts to build ejection baffles for high power rockets with diameters <strong>of</strong>5.5 and 7.6 inches.Medi/Maxi-Magg, My First 5.5" RocketNow the fun begins: To use the design principles from Part 1 <strong>of</strong> this article to build amodular labyrinth baffle/bulkhead design in a real High Power Rocket. Not just a modelrocket or Large Model Rocket stuffed with a H motor, but a regulation High Power rocket—meaninga rocket over the 53-ounce limit <strong>of</strong> Large Model Rockets and one that requiresyou to be certified to fly HPR.For the Spirit <strong>of</strong> America 2008 (4" x 70") rocket in Part 1, I used a “cut down” planto construct the baffle/bulkhead fin can unit (by which I mean that the airframe tubing <strong>of</strong>the original kit was too long, and needed to be cut down to a shorter length to accommodatemy baffle design). This same scheme could be used with several kits in the 5.5" classserving as the basis for the build, such as the LOC I-Roc, Big Nuke, or Magnum. But fordemo purposes I decided to do the flip side and use the “build up” plan to show how theparts from a really short rocket kit could be used as the basis for a rocket using my baffle/bulkhead design. In the 5.5" class such a starting model is the LOC Minie-Magg, while inthe 7.6" range you could use the LOC Warlock or possibly the Door Knob as your startingpoint.So I started with the parts from the 5.5"-diameter LOC Minie-Magg kit as the basisfor my rocket. But first, a little back ground on the Minie-Magg. LOC calls this rocket thelargest G-motor rocket (5.5" x 37") available. That’s true, but the minimum motor is theG80-4T. Even then they used some weight reduction techniques to get the rocket flyableon a G80. First they used a short nose cone <strong>of</strong> 13" versus the 21" unit they use for their
other 5.5" rockets. The body length is only24" and the wall thickness appears to bethinner than the normal 20" payload sections.I found that out to my surprise whilesizing up the parts. The kit only uses twocentering rings on a short (10") 38mm motortube. The fins tabs are only just 1/16"through the wall, and do not extend downto the motor tube. The recovery systemuses an elastic shock cord with an epoxiedloop for the shock cord mount. All these issuesand more would have to be addressedto make this into a more advanced rocket.I bought the Magg and a 34" 38mmmotor tube at HUB Hobby. I wanted toget started and see what I needed to do.Many hobby stores ceased carrying highpower rocket parts many years ago because<strong>of</strong> the federal regulations on highpower motors. So I had to order two payloadextensions (PL-5.38), a Stiffy tubecoupler (STC-5.38), and an extra 5.38"centering ring from LOC/Precision. One <strong>of</strong>the payload kits would provide parts forthe outer chamber <strong>of</strong> the baffle/bulkheadand the 5.5" x 20" body tubing for the recoverysection. The 37" basic kit plus the20" recovery section would bring my “Medi-Magg”design to a length <strong>of</strong> 57". Withthe addition <strong>of</strong> a 20" payload extension, itwould become my full up “Maxi-Magg” at5.5" x 77".The TC-5.38 coupler from the payloadunit is 11" long. That’s quite a big jumpup from the 4" x 6" couplers used in theSpirit <strong>of</strong> America 2008 from Part 1. Still,with that huge coupler diameter, the wallthickness appears to be the same as in thesmaller units. This allowed the tube to easilydeform under a little pressure. For myfirst upgrade, I ordered the Stiffy tubecoupler stiffener (STC-5.38) from LOC toreinforce that 11" regular coupler to increasethe lateral support. After all, thatunit was going to have the leverage andweight <strong>of</strong> 40" <strong>of</strong> heavy body tube (0.080"thickness), bulkhead unit, and the noseconeto support. The STC-5.38 stiffeningcoupler is 5.25" diameter, 10-7/16" long,and has a wall thickness <strong>of</strong> 1/8". This isquite a significant reinforcement, as thisstiffener alone weighs 8 ounces. It is 9/16"shorter than the regular coupler, which allowsfor a 1/4" bulkhead at each end witha 1/16" recess at the head for epoxy fillets.Because <strong>of</strong> the 5.25" diameter, the stiffenerfits loosely into the regular coupler makingit easier to epoxy in place. The stiffener hasdeep spiral grooves in the sides to accommodatethe extra epoxy needed for maximumadhesion. More on that later.The bulkhead is 1/4" thick plywoodand came with the standard 2.0"-long1/4" eye bolt. This eye bolt looked reallypuny compared with that large bulkhead/coupler unit. So my second upgrade wasto replace the standard eye bolt. That eyebolt has a Safe Working Load (S.W.L.) <strong>of</strong>80 pounds. Going to the next size <strong>of</strong> 5/16"with a length <strong>of</strong> 3.25" only increased theS.W.L. to 90 pounds. But I noticed on thehardware store’schart that a stainlesssteel 5/16" eye bolthad a S.W.L. <strong>of</strong> 130pounds, so I wentwith the 3.25"-long5/16" stainless steeleye bolt for my upgrade,along withtwo large fenderwashers.The 11"-long5.38 coupler (withthe stiffener inside)forms theouter chamber <strong>of</strong>the labyrinth baffle/bulkhead.The38mm motor tubeand third centeringring <strong>of</strong> the motormount forms theinner chamber. Themiddle tube is epoxiedto the bulkheadand formsthe middle concentricchamber.This middle chamberis designed tobe 10" long, or 1"shorter than theoutside coupler chamber. The body tubesize choices for this middle chamber were2.56", 3.00", or a 4.0" body section 10"long. I wanted the volume change throughthe chambers to be as linear as possible.I did the volume change math from the38mm inner chamber (motor tube) to themiddle chamber and then out into the largechamber (the large coupler). The smoothestand most balanced increase occurredwith the use <strong>of</strong> the 3.00" body tube forthe middle chamber. It just so happened Ihad a PL-3.00 payload extension on handthat comes with a 10" section <strong>of</strong> BT-3.00.A perfect fit. I did a lot <strong>of</strong> test fitting alongthe way in this project. Photo 1 shows theparts <strong>of</strong> the baffle.Since this was a “build up” plan, I usedthe kit’s 5.5" x 24" body tube and addedthe extended motor tube to that length.With the outer chamber (5.38 coupler) <strong>of</strong>Above: The Medi-Maggconfiguration <strong>of</strong> Ken’s 5.5"high power rocket withzipperless/baffle.Left: The Maxi-Maggconfiguration.the ejection baffle at 11" long, themotor tube (part <strong>of</strong> which formsthe baffle’s inner chamber) had toextend into the baffle 1" less thanthe length <strong>of</strong> the baffle, or 10" <strong>of</strong>length into the baffle. Also, half<strong>of</strong> the coupler would be insidethe body tube <strong>of</strong> the fin can (thatmeans 5.5" inside the body tube). The thirdcentering ring would be placed at that locationto form the base <strong>of</strong> the baffle. The10" length <strong>of</strong> motor tube extending forwardfrom that point places the top end <strong>of</strong>the motor tube 4.5" beyond the top <strong>of</strong> thebody tube. That would suggest a requiredmotor tube length <strong>of</strong> 28.5" if the motortube were flush with aft end <strong>of</strong> the fin canbody tube. Using the three centering ringsystem, the rear and middle centering ringsare used to form a fin tab cage inside thebody (even though the fin tabs are only1/16" long in this model). But when I sizedup the screw length on the Public MissilesMotor Retainer that I planned to use, Irealized that in order for the rear centeringring to be up against the base <strong>of</strong> thefin tabs, the aft section <strong>of</strong> the motor tubewould have to be limited to 5/16" exposurebelow the 3/16"-thick rear centeringSport <strong>Rocketry</strong> May/june 2010 29
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