(Yet Another) Der Big Red Max build

[ep29030]

I decided to post the build process for my Der Big Red Max kit. I want to uprate the kit to use G or baby H 29mm motors, but I want to use the original balsa multi-part fins and just strengthen them with bonded paper reinforcements. I will use the original paper centering rings as well. I will also add a baffle so I wouldn't need wadding. Since the kit requires clay in the nose, I will one-up it by adding a 1/4-20 coupler nut in the nose to give it the required kit weight, plus provide the option to screw on more weight or an electronics bay. I'll cut off the bottom of the nosecone to add the coupler, which will be fun since it has an angled area.

I am not using the original 29mm motor tube. I am using a longer tube to provide length for the baffle, and to push the recovery system up closer to the nose to shift the center of gravity forward a bit more.

Anyway, time to start. The obligatory parts out of the bag.

 

[Hobie1dog]

balsa fins and paper centering rings for a baby H motor

 

[ep29030]

balsa fins and paper centering rings for a baby H motor

Absolutely. I'll explain in a post or two.

 

[ep29030]

CONSTRUCTING FINS
I always start builds by getting the fins ready for paint and filling the seams in the body tubes. I don't start constructing the kit until they have been filled and given a couple rounds of sandable primer followed by sanding. Since I want to reinforce the stock fins, I will start by reinforcing them with cardstock. I decided to use cardstock rather than normal 20-24 lb typing paper. Either one will do the job. I've done lots of balsa fins this way and it adds significant strength.

Start by sanding the edges of the fins to get them flat and remove most of the burned balsa from the laser cuts. I am using 220 grit paper glued to a flat piece of 1/2in plywood--homemade sanding block. I make blocks all the time to fit the need--just use 3M 77 spray adhesive on the paper, then bond it to the wood, simple to do and you make any shape you desire.

I used Titebond III to glue the three pieces together, then put tape across the pieces and some weight (sanding block) to keep it flat until the glue set. I then sand the flat surfaces to remove any glue bumps, etc before I bond the paper to the fins.

 

[ep29030]

CALLING FOR REINFORCEMENTS
As I said earlier, I am using 110# cardstock on both sides of the fin as a composite reinforcement. Next step is the trace the fin onto several pieces of cardstock and cut them out. I then apply Titebond to one side of the fin and spread it out with my finger. You need a generous layer of glue all over the fin surface. Place the cardstock on the glued surface then immediately turn over the fin and repeat the glue application/spread/apply cardstock process. Balsa fins will warp if glue is applied to only one side!

BTW, I am using Titebond but the process works the same with Elmer's School (white) glue or epoxy.

 

[ep29030]

ON TO THE BONDSMAN
After the cardstock is stuck onto both sides of the fin, it is time to bond the paper to the fin. My standard approach used to be stacks of books or two pieces of plywood with wood clamps. I finally made a press that gives nice results and is simple to use. It's a fancier way to use two sheets of plywood. Part of the magic is the dense foam on each side--it helps the reinforcement to wrap around the fin edges a bit. My press is 12x12 inches and covers 95% of the fins I reinforce.

Anyway, the process is to put a piece of wax paper (release material) on both sides of the fin, place it in the press and put pressure on it until it dries--I usually let it go overnight. Then remove the fins from the press and discard the wax paper and behold your bonded fins.

 

[ep29030]

SEAL THE DEAL
Once the fins are bonded, I let them sit and dry a few hours, then I start trimming the excess paper with a hobby knife. I then soak thin CA into the surface of the cardstock to seal it. The fin is now ready for edging/rounding, and smoothing the surfaces.

 

[ep29030]

FIN SURFACE PREP
To prep the fins, I used my 220 grit sanding block and also hand-sanded the fins to round the leading edges and knock down some of the high spots on the flat fin surfaces. To fill large errors, I got out the body putty (the cheap single part kind) and slathered a layer of it on each side of the fin. Once dried, I used a quarter-sheet sander with 220 grit paper to remove most of the putty. Next fin prep step will involve sandable primer.

 

[ep29030]

THE JOYS OF SANDABLE PRIMER
To continue preparing the fins for final paint, I taped the fin where it goes through the wall and where surface fillets will be, to avoid building up paint there (and to provide better glue surface. I applied three coats of Dupli-Color Sandable Primer, sanding it down between each coat. This is tedious, but it levels out the surfaces. The best results are obtained when using a sanding block on the fin surface to keep it flat and true. The rest of us use our electric sander and say, "close enough to fly".

 

[ep29030]

BODY TUBE PREP
I applied Body Putty to the tube spirals, followed by a light coat of sandable primer, then everything was hand-sanded to fill the spirals. I also wicked in thin CA on both ends of the airframe and did another round of sandable primer/sanding. Airframe should be ready for construction.

 

[ep29030]

DISCUSSION ON CENTERING RINGS
If you want to know why I can use paper centering rings with G and H motors, here is my answer. Centering rings do multiple things: they bear the loads for rocket motor thrust and parachute deployment. They also seal the compartment for ejection gases to pressurize the airframe and separate the rocket. However, when you have TTW fins, most (if not all) of the motor thrust and deployment loads are borne by the TTW fin joints. In this rocket, there are two centering rings that are glued around their perimeter for a surface of about 19 lineal inches (2 rings x 9.5 inches/ring). By applying TTW fillets, there are 36 lineal inches of surface area. (3 fins x 4 fillets x 3 inches per fillet)- that is 90% more surface area for flight stress. I intend to remove the aft centering ring and apply internal reinforcements to the fins so they will carry the thrust/deployment load. I will omit the middle centering ring--I will re-purpose it for my baffle.

Once thrust/deployment loads are transferred to the TTW fins, the centering rings are needed only for sealing the gasses, so I can go with lighter rings. If it weren't for my intent to install a baffle I could get away with only a forward centering ring for flight. BTW, if you have not read Tim Van Milligan's article on making stronger paper centering rings, it is worth the read because there are many times you can strengthen cardboard rings to do amazing things with some good engineering practices.

 

[ep29030]

NOTE on fin weights. For grins, I grabbed my balance and weighed a couple of the fins. I had already added paper to one of them, so I have only two fin weights. All numbers in grams.

Bare Fin / Cardstock / Sealed, Reinforced Fin
Fin #1: 13.74 / 5.23 / 26.58
Fin #2: 11.45 / 7.68 / 24.92
Fin #3 weighed 26.25 sealed, reinforced

 

[bad_idea]

For reference, I weighed the fins of a Launch Lab/Vanderburn birch plywood DBRM upgrade kit just now and saw 113.7g for the trio, so 37.9g apiece. I suspect your fins are stronger (though of course ply fins can be reinforced too).

 

[Trident]

Cool approach to using kit’s components and be able to fly on bigger motors. I might do the same on my DBRM. I bought Stickershock for it, so I hate to add in more expenses for upgraded fins and CRs.
I have used basswood ”spokes” on cardstock centering rings on standard Estes kits (no thru-wall fins) to beef them up to use high-thrust motors (Aerotech RMS E and F) with great success. I just add maybe 4-6 spokes, running from motor tube to edge of the CR. No real need to even consider lite-ply replacements.
And I also use 110 lb. cardstock for papering fins. If others have not used it, you can a ream of it pretty cheap at Hobby Lobby. I’ve been using ModPodge for applying the cardstock since it’s reasonably thin and brushes well.

 

[ep29030]

MOTOR TUBE & BAFFLE
I decided to add a baffle. It makes flight prep much faster. I could just add a baffle assembly but that's no fun. By adding a longer motor tube I can add a baffle and push the weight of the parachute forward to improve stability. So I discarded the provided 29mm motor tube and supplied a 10.5" tube from my stash of tubing. You could make the tube even longer if you remove the base of the nosecone and open it up.

I re-ordered the centering rings: the forward ring was moved to the center as it will seal the gases and force them forward for ejection. The (former) center ring has three slots for the fins. It will be the foreward ring. I covered one slit with a piece of cardstock then I decided to cover the internal baffle area with some aluminum foil tape for some durability. This also covers the slits in the foreward ring. I covered the body tube and rings. I chose a weird baffle: it is a long slit. I've tried several different baffle approaches and this one is an experiment. The idea is to send the gases out and force them around the body tube before they exit. I added a weird-shaped flap at the top of the slit to force the gases down and around. The flap was a scrap piece of heavy centering ring cardstock. Yes, it's an odd shape I know. But it slides up in the tube and doesn't permit gases to come out the slit and go straight up to the exit holes. Note that I trimmed off the foil at the edge of the centering rings to provide a glue area.

White glue or epoxy are better for gluing centering rings inside the airframe on thin-walled tubes because Titebond shrinks as it dries and will create a slight ring in the airframe that is visible when you use sandable primer to prep the airframe for final paint.

 

[ep29030]

ATTACHMENT POINT FOR RECOVERY
I originally intended to glue Kevlar recovery line along the outside of motor tube (ala PML kits), but decided to add a heavy wood plug with loop at the top of the motor tube. It is relatively heavy, esp. compared to the first approach I had in mind. But it permits me to replace the recovery system if needed and it puts its weight at the fore end of the airframe to improve stability. And it can easily handle my rule of building recovery components to handle 40-50X rocket weight.

For the attachment point, I found a piece of 3/8" plywood. I cut a plug out of it with a 1-3/8" hole saw, then I reduced its size to fit inside the 29mm airframe at my 1" belt sander (a great rocketry tool to own). Used a stainless steel eyelet not used on a prior project. As I recall, it has a working load around 70 lbs. Once the plug was sized and the eyelet screwed in place, I glued the plug/attachment point into the end of the motor tube.

In retrospect, this was a weighty decision. My original idea of gluing the recovery line on the motor tube would have weighed less and accomplished the purpose. On its plus side, it is rock solid and I can replace the entire recovery train at any time.

 

[ep29030]

MOTOR TUBE/BAFFLE ASSEMBLY
I used a paper punch to add gas exit holes in the foreward centering ring (formerly the middle ring of the kit). I put the majority of the holes as distant from the gas exit slit as I could get, to force the hot gases to go around the motor tube prior to exit. Centering rings were glued to the motor tube with Titebond. This completes the motor tube assembly, except the 29mm motor retention which will be added as one of the final assembly steps.

 

[ep29030]

LAUNCH LUGS/RAIL GUIDES
To keep it complicated, I decided I would add both 1/4" launch lugs and rail guides. This gives me flexibility in pad choices at launch time. In addition, I decided to add a small amount of standoff to the rail guides so the rail is less likely to rub the airframe during launches.

To attach the lugs, I used my piece of aluminum angle to draw a straight line between a set of fins. I grabbed a handy 1/4" dowel rod and used it to keep the lugs aligned as they were glued (with Titebond) and taped to the airframe.

For the rail guides, I traced their outline on a scrap piece of 3" tubing from a prior project. They were cut out and sanded to shape. I drew reference lines on the airframe between another set of fins and glued the rail guide standoffs in place with Titebond glue. I carefully covered the surface of the rail guide standoffs with tape to protect them from paint--I want a bare surface when it comes time to add the rail guides near the end of assembly.

 

[ep29030]

Cool approach to using kit’s components and be able to fly on bigger motors. I might do the same on my DBRM. I bought Stickershock for it, so I hate to add in more expenses for upgraded fins and CRs.
I have used basswood ”spokes” on cardstock centering rings on standard Estes kits (no thru-wall fins) to beef them up to use high-thrust motors (Aerotech RMS E and F) with great success. I just add maybe 4-6 spokes, running from motor tube to edge of the CR. No real need to even consider lite-ply replacements.
And I also use 110 lb. cardstock for papering fins. If others have not used it, you can a ream of it pretty cheap at Hobby Lobby. I’ve been using ModPodge for applying the cardstock since it’s reasonably thin and brushes well.

You can also get 110# cardstock at Wal-mart. That is where I got my current supply. Hobby Lobby has some really nice paper products in the art section. I need to go find some of the old cotton rag paper from typing days. It would make an excellent reinforcing material. I have a large 5.5 to 3 inch airframe reducer I made with Bristol paper and that paper was great for the purpose.

 

[ep29030]

Cool approach to using kit’s components and be able to fly on bigger motors. I might do the same on my DBRM. I bought Stickershock for it, so I hate to add in more expenses for upgraded fins and CRs.
I have used basswood ”spokes” on cardstock centering rings on standard Estes kits (no thru-wall fins) to beef them up to use high-thrust motors (Aerotech RMS E and F) with great success. I just add maybe 4-6 spokes, running from motor tube to edge of the CR. No real need to even consider lite-ply replacements.
And I also use 110 lb. cardstock for papering fins. If others have not used it, you can a ream of it pretty cheap at Hobby Lobby. I’ve been using ModPodge for applying the cardstock since it’s reasonably thin and brushes well.

ModPodge is a great idea for cardstock application.

 

[ep29030]

For reference, I weighed the fins of a Launch Lab/Vanderburn birch plywood DBRM upgrade kit just now and saw 113.7g for the trio, so 37.9g apiece. I suspect your fins are stronger (though of course ply fins can be reinforced too).

Adding composite skins to balsa cores is a nice way to keep fins light but get the stiffness we need for larger motors. I have an upscale Cherokee D (2.6" airframe) I built with 1/4" balsa fins with a skin of 1 oz/yd carbon fiber mat and epoxy. Fly that rocket on 29/240 motors all the time.

 

[ep29030]

FIN SURFACE PREP
To prep the fins, I used by 22 grit sanding block and also hand-sanded the fins to round the leading edges and knock down some of the high spots on the flat fin surfaces. To fill large errors, I got out the body putty (the cheap single part kind) and slathered a layer of it on each side of the fin. Once dried, I used a quarter-sheet sander with 220 grit paper to remove most of the putty. Next fin prep step will involve sandable primer.

I should add that I once I round the leading edge and square up the outer edge of the fins, I cover them with thin CA and sand lightly after it hardens. This gives the fin edges additional hardness to endure the abuses of travel and hard landings.

 

[DigBaddy]

Great thread!

 

[ep29030]

AIRFRAME ASSEMBLY
With the fins sanded and primered to an acceptable degree, the body tube filled and sanded, launch lugs attached and standoff for the rail guides all in place on the airframe, it is time to glue the motor tube/baffle/anchor point into the airframe and then move to fin attachment. I planned it so 3/4" of motor tube protruded below the airframe. That is a more than needed to attach Estes plastic motor retainer that comes with the kit. I want the motor a bit lower to reduce the blackness on the aft end of the rocket. I will also paint the aft centering ring black, since it becomes that color anyway.

I recommend white glue or epoxy for internal centering rings--yellow glue shrinks and will leave a visible line on the exterior of thin body tubes. Heavy high power tubes are not so much an issue. I used white glue for the forward ring, spread out with a long balsa stick--made the stick from leftover balsa sheet scraps of the kit.

I made two tape tabs on the aft centering ring so I could pull it out. I only glued the forward ring in place to fix the motor tube in the space. I removed the aft ring so I would have access to the TTW fin tabs. I will add cardstock reinforcements to strengthen all the joints since flight/recovery loads will be transferred to them. Next step is fin attachment to airframe/motor tube.

 

[ep29030]

TTW FIN ATTACHMENT
Since I abandoned the Estes plans for fin attachment, the fins will require extra shaping with sandpaper. First, the motor mount attachment edge is not flat on the fins--the bottoms must be flattened with a sanding bar or other flat surface. And once that is done, the fin tab is not tall enough to reach the motor tube. The solution is to sand the root edge of the fin where it meets the airframe to get more length for the TTW tabs to reach the motor tube.

Because the fins have cardstock reinforcement they are now too wide to fit in the fin slots. I used a 100 grit sanding stick from the hobby shop to widen the slots in the airframe. I worked one fin at a time, sanding the bottom smooth, then sanding the fin root and test fitting the fin until the TTW tab touched the motor tube. Fins were attached to the motor tube with Titebond III wood glue. Here is a place where shrinking glue is a good property--pulls the fin into the tube.

 

[ep29030]

TTW FIN REINFORCEMENT
Over time I've moved away from internal fillets toward reinforcement strips. The strips provide more surface area for about the same weight. For high power kits my favorite is fiberglass or kevlar cloth and epoxy. For this smaller rocket, I am going with cardstock and Titebond wood glue.

I drew up a sheet full of strips that are 1 inch wide--1/2 inch on each side of the joint, and 3 inches long which is the length of the fin roots. I cut the strips and used a rule edge to get a clean fold down the center of each strip. It takes 12 strips. To apply each strip, I first folded the tab as much as it would go, applied glue, spread it out with my finger across the entire surface--a generous glue layer so it's won't grab when placed. I slid the tab into place with my hand, then used a balsa stick (scrap) to push the tab down in place along its length. Works best when you first get the center fold into the corner, then work out toward the edges. It was surprisingly easy to get the tabs in place with good bonds. Happy with the final product. Set it aside to dry and harden.

 

[ep29030]

Once fins are reinforced with cardstock strips, I put exterior fillets on fins with Titebond III. I do Titebond fillets twice, then switch to white glue and do fillets another 2-3 times. The white glue doesn't shrink as it dries, so the fillets don't get the edge "bumps" they get using only Titebond. Next up will be nosecone modification.

 

[Scott_650]

Really like those internal reinforcement strips for the fins - great idea!

 

[ep29030]

NOSECONE MODIFICATION: Threaded Coupler
The kit requires clay in the nose for stability. I decided to add a coupler nut in the nose tip. Will use a 1/4"-20tpi coupler glued into epoxy. I've done this on several rockets. The threaded coupler permits the addition of nose weight, or an attachment point for a future av bay (e.g. GPS). It's a versatile addition. The epoxy + coupler will take care of the kit's weight requirement and give me options.

To accomplish this upgrade I need to open up the nose cone base. This also means I lose the nosecone attachment point for recovery system. No problem there--I'll use a piece of fiberglass/epoxy and some Kevlar strap to make a new attachment point inside the nose. To open the nosecone I carefully placed it on my miter saw and slowly lowered the blade. Bottom came off clean. I then angled the cone and repeated the process on the angled area of the cone. I could have removed it with a hobby knife or a hot knife, but this was faster. I added an Irwin laser beam to my saw so I can see where I am about to cut--this is a big advantage for this kind of cut.

Before working inside the nose, I wrapped a dowel rod with a shop towel and soaked it with lacquer thinner. This was put into the nosecone tip and rubbed around to remove any mold release and assure good adhesion. Give it 10 minutes to evaporate and we are ready.

I finally decided to give myself some additional surface area inside the nosecone to prevent the epoxy/coupler from popping out under thrust. I did that by measuring the nosecone where I wanted to add fiberglass/epoxy (see photo with approximate measurement). I transferred the dimensions into VCP (thanks, Gary Crowell) and created a cone. I did a test fit of the cone to see if my dimensions were close. Once I was satisfied, I traced the template on some 3 oz/yd plain weave fiberglass and cut it. I laid the cut fiberglass on wax paper, soaked it with West Systems epoxy (about 1 oz) , wrapped it around a dowel rod to deliver it in the nosecone, then gently unrolled it and used the dowel to put it in position. Sounds harder than it was. The final product looks good and increases the surface area of the epoxy/coupler plug. More surface area means stress is spread over a larger area.

Next step was inserting coupler nut in a depth of epoxy and keeping it centered until it sets. Before mixing the epoxy. I used some West Systems epoxy with slow hardener. I mixed up 2 oz but used about 60-70% of it--about 1.25 oz. The coupler was about 2/3 inserted into the epoxy with that amount. Prior to inserting the coupler into the epoxy I wiped it with lacquer thinner. I put a small piece of tape on the forward end of the coupler to keep epoxy from seeping into the threads. I used an 18" long piece of 1/4" all-thread to position the coupler and hold it until the epoxy hardened overnight. Next day, I removed the all-thread. First mod complete.

 

[ep29030]

NOSECONE MODIFICATION: Attachment Point

Due to removal of nosecone base, I have to create a new attachment point. My choice is a piece of the plain weave fiberglass, soaked in epoxy and attached to the wall of the nosecone. Under it is a short piece of flat Kevlar strap I have in my box. I'll use a small quick link to attach the nose to the recovery train.

With nosecone modifications complete, we can move on to paint and finish.