'Built' thread: "PulpStar" aka Project DDT* * - Dual Deployment Trainer

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David_Stack

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Since joining the BAR community a couple years ago and discovering this forum all of my rocketry to date has seen me building from kits, though even in those builds I've attempted to incorporate 'best practices' and techniques that I've seen featured in posts here. Having gained much from this community I thought it time to attempt to 'give back', and post a Built thread (since the rocket is all but ready for a maiden flight, lacking only a few last minute 'finishing touches'...

Background: though electronic deployment is not a requirement for L1 certification, the associated technology is something which is of interest to me, and when I do fly my L1 certification flight eventually I would like for that flight to incorporate dual deployment for recovery.

Inspired primarily by Nytrunner's thread "The road to L2 is paved with practice. PSII trainer fleet" (https://www.rocketryforum.com/threa...aved-with-practice-psii-trainer-fleet.140897/), I decided to follow his example and build a MPR rocket as my dual deployment trainer. Since Estes has significantly reduced their PSII fleet, I chose to amass the parts and put together a 'short kit'. I started out intending to clone the Estes Partizon as I liked the lines of the rocket, fin design, etc., and the fact that it was comprised of three 15" body tube segments meant that I could join the two lower tubes as a booster, and the third uppermost tube would serve as my payload section, with an avionics bay at that separation point.

I located a RockSim/Open Rocket file for the Partizon and started playing in Open Rocket as I knew I was going to need to account for the weight of an avionics bay on stability/CG and motor selection, all the while 'window shopping' the web sites of the various kit manufacturers (since the proper number of rockets one should own is n+1, where 'n' is the number of rockets you currently have). Split fin designs have always had a certain appeal, and it dawned on me that Tim Lehr/Wildman offers a 2.6" Darkstar kit. 2.6" is not that far-removed from the 2.5" Estes PSII tubing, the Darkstar has a 32" booster and 16" payload (as compared to the 31" 'booster' and 15.5" 'payload' of the Partizon) so after locating a .rkt/ork file for the 2.6 Darkstar and copying the fin set into my Partizon file, I now had a MPR 'Darkstar' made of cardboard and wood, hence "PulpStar".

Before going any further I wrote to Tim, asking if he had any heartache with what I envisioned building. He was fine with it, so the next step was to contact the folks at Rocketry Works about getting centering rings and fins cut (along with a baffle/bulkhead to be installed in the rear coupler that doubles as a mounting point for my recovery harness). James and his team were able to work from my .ork file and before I knew it I had those parts from Rocketry Works, plus body tubes, couplers, 29mm motor mount tube, nose cone, and 29mm motor retainer courtesy of Estes laid out on my workbench.

Time to build:

First step, locating the CRs on the motor mount tube, dictated by the threaded 'male' fitting of the motor retainer (so it bears against the aft CR), a desire to have the front centering ring close to the forward end of the MMT (leaving room for a fillet and also making it easy to shake the red cap used by AeroTech to seal the ejection charge well out post-flight), and the TTW fin tabs. i used tape to temporarily 'tack' them into position.

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I printed up a fin wrap guide from Payload bay, and marked the locations of the fins on the body tube. A length of aluminum angle was used to extend those lines up the body tube, and I also marked the fore and aft locations of the slots for the TTW fin tabs. Using the same aluminum angle secured taped to the body tube as a guide and fresh #11 scalpel blades, I cut the TTW slots.

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Since I had the CRs in place on the MMT, and the body tube slotted, I couldn't resist a dry fit...

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Satisfied, I pulled the fins out, removed the MMT assembly, and began incrementally removing the tape used to position the CRs, marking the location of each with pencil.

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I started out by gluing the forward CR in place (Titebond III for the majority of this build), to include a fillet on the forward side.

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Because I wanted to be able to do internal fillets (primarily for the learning experience), this was the only CR glued at this time, the others will go in as the fins are installed, building from the front of the MMT assembly back... (lesson learned, I marked the location of each CR though I really only needed to worry about the forward one, the position of the remainder was dictated by the TTW fin tabs)

Next post will document my work on the fins.
 
Fins and the like...

So as mentioned, Rocketry Works cut fins for me from 1/8" ply. While I could have left the leading and trailing edges blunt or rounded them over, I wanted to emulate some of the work I've seen here on the forum; notably the well-shaped tapers achieved by members like rfjustin.

Lacking a table router, I first tried to build a jig/holding fixture similar to the one John Coker has detailed, intending to use it with a table saw (no bench sander either). End result was less than I had hoped for (too much movement of the stock as it passed by the blade), so I took a more 'hands on' approach. I think it is one of CJ's build threads that depicts sandpaper adhered to a flat surface, and the fin held at the desired angle. The next series of photos depict my 'jig':

I have a length of ~ 1 x 4 aluminum extrusion to which I secured a length of aluminum angle along the edge to serve as a fence. Using transfer tape I adhered a strip of sandpaper at the edge of the fence, and at an appropriate distance from the fence and parallel to it I affixed a length of basswood (again using transfer tape) to establish the bevel angle.

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A strip of transfer tape was applied to the surface of a Great Planes sanding bar, and the fin was stuck to the sanding bar with the edge of the surface to be bevelled aligned with the long edge of the sanding bar. A piece of tape was applied to the area of the fin that contacted the basswood, to prevent any wear of the fin surface.

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With the fin secured to the sanding bar and the sanding bar edge guided by the aluminum angle fence, it was then only a matter of sliding the bar back and forth to sand the bevel, stopping occasionally to monitor my progress toward the center thickness of the fin. When that point was reached, turn the fin over to the other side and repeat. Same for the trailing edge of the aft fins (I left the trailing edge of the forward fin and the leading edge of the aft fin square).

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End result:

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With the fins now airfoiled, time to start installing them.

First I glued the MMT with the previously installed forward CR in. Applied Titebond through the forward end of the forward fin slots, rotating the tube to get a nice bead of glue all the way around. Inserted the MMT twisting as I slid it home to a position where the back edge of the CR was lined up with the edge of the fin slot. I temporarily affixed another CR on the rear of the MMT to ensure the MMT was axially aligned in the body tube. Looking down from the top of the body tube I saw a nice fillet of glue at the top of the CR.

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Once the glue was set, I started installing the fins. Fabricated a fin alignment guide courtesy of Payload Bay and foam board, applied Titebond to the root of the fin tab, the forward edge of the fin tab (that contacts the CR) and the root of the fin that contacts the body tube. Held it in place for a few seconds to allow the glue to start to set, then pulled the fin off, repeated the glue application, and reinstalled (the 'double-dip' method). Rinse and repeat for the other two fins in the days to follow.

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Next was internal fillets, again using Titebond. Likely not necessary, but again I was building this model to expand my building techniques... I used a syringe, some flexible tubing, and a piece of brass tubing to make an applicator that I could reach into the body tube from the rear. Applied Titebond in the valley between the fin and the MMT on two adjacent fins, with the booster sloped downward to allow the glue to flow along the joint.

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Once all three forward fins were filleted I installed the next CR, using the same glue applicator to apply a bead of glue around the inside of the BT, on the MMT, and on the back edge of the fin tabs then slid the CR forward along the MMT, using a piece of tubing large enough to slip over the MMT as a ram.

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Applicator in hand, I laid a bead of glue on the MMT, the inside of the BT, and installed the third CR, using it forward to align it with the leading edge of the aft fin slots.

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Rear fins were installed in the same manner as the front fins, only the fin guide was left affixed to the forward fins, and I used a pair of steel squares sandwiched on either side of the front fins and the rear fins to ensure that the fins were in line with each other.

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Applied internal fillets on the rear fins once they were all installed just as I'd done with the front ones, but did NOT install the rear CR, since I still had to provision for a rail button hard point later.

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Though the rocket is intended to be used for dual-deployment, I still wanted the ability to fly it with motor eject, and so with the help of Rocketry Works I turned the coupler joining the two lower sections of body tube into something of a baffle (hardware courtesy of Teddy at One Bad Hawk)

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Next step was installation of rail buttons. I chose to use brad-hole T-nuts installed on the inside of the body tube, with the threaded portion poking through the body tube. I drilled out the body tube to accommodate that threaded portion, and drilled the inner diameter of the rail buttons to slip fit over the T-nut. T-nut flanges were bent slightly to conform to the body tube, and were installed by applying epoxy on the flange, then pressing them up against the body tube. Nice epoxy 'rivets' were formed with epoxy pushing through the brad holes in the flange.

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With no other work needed inside the lower body tube, I glued in the aft CR, complete with a Titebond fillet

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With the fins installed, time for fillets. I used carbon paper and a fondant ball tool to mark off the fillet 'boundaries', applied masking tape up to the line, mixed up BSI Finish Cure resin and West Systems 407 filler thickened to desired consistency, poured it along the fin, and 'pulled' my fillets with the fondant tool dipped in alcohol (pulling the tape after about 10 mins). Here you can also see the threaded T-nut protruding through the body tube which the rail button will install on.

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I've read a number of posts here on TRF from members expressing concern (and in some instances, failures) associated with using the molded in loops on nosecones as attachment points for recovery harnesses, and parachutes. I decided to do as many have done, and fabricate a bulkhead inside the nose cone.

I cut off the beveled portion at the base of the nosecone, then sized a plywood bulkhead to fit up inside the nosecone, resting on the inner 'ledge' where the nosecone and the shoulder meet. I scuffed up the inner surface with coarse sandpaper, wiped it down with Isopropyl, and then bonded the bulkhead in place with BSI 30 minute epoxy (hardware again provided by Teddy / One Bad Hawk).

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Finished up the booster by joining the two body tubes with the coupler/baffle fabricated previously. I aligned the two tubes with a piece of aluminum angle to ensure they were axially 'true' while the glue set.

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With the airframe basically complete, time to build my first avionics bay. Nytrunner was invaluable, in that he had already worked up a bay for these Estes 2.5" tubes, using the Estes coupler. That coupler is only 4" long, so Nytrunner's solution was to design end caps that have the same OD and the coupler and a necked down portion that fits inside the coupler, this allows for a longer sled. He also designed a companion sled with mounting points for either a Stratologger CF or Missile Works RRC, hold-down slots for battery tie wraps, and a recess for the Apogee push button switch. The parts are designed to be 3D printed. Debt of thanks to members JonathanOtt and CWBullet for their 'fabrication services'.

Because I wanted to use a Lab Rat Rocketry pull-pin switch, I ended up fabricating my own sled from plywood and aluminum tubing. In addition to the pull-pin switch, I installed a Finger Tech screw switch in series so that I could connect the battery to the altimeter while the sled is outside the coupler, but not apply power. Aluminum 10-24 threaded rod was used as the rails for the sled, and to secure the end caps with the aid of appropriate hardware (threaded coupler nuts do double duty retaining the end caps, and also providing a means to thread the welded eyes to the avionics bay.

Charge wells are 'disposable', taking a cue from rfjustin's Punisher build: https://www.rocketryforum.com/threa...hemed-build-thread.130046/page-2#post-1525111. The wells are secured to the bulkhead/end cap with 6-32 screws, and the eMatch wire is routed through the bottom of the charge well and bulkhead and connected directly to the altimeter. Blue Tac is used under the charge well to ensure no BP residue enters the avionics bay.

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To connect the bay to the Payload section, I drilled holes for three 2-56 screws through the airframe, coupler, and recessed shoulder of the end cap. 2-56 T-nuts were set on the inside of the end cap with JB Weld. An addition hole was drilled in the coupler to provide access to the FingerTech switch.

No switch band is used, instead the three 3/32" sampling ports are drilled through the airframe right at the separation joint between the booster and payload. One of those three sampling ports was drilled oversize, so a piece of 1/8" styrene tubing could be installed. This provides a guide for the arming switch pull-pin, and is my 'locating dowel' to key the avionics bay into the booster and payload sections, so that it is always aligned. I first saw this in one of Mugs914's builds.

Finishing was my typical, Duplicolor high build primer, Duplicolor Perfect Match lacquer, and because I had the better part of a can leftover from a prior build, Montana Gold lacquer. Design inspiration courtesy of Nathan's Frenzy XL.

End result:

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