Der Big Red Something Build

[Bruiser]

I know there has been plenty of Der Big Red Max builds but this will be my go at it. I think it'll be a little different than the others in a few ways.

I started with the fins. I have them glued up and sanded. Now I have to decide if I want to laminate them with 1/32 bass, 1/32 ply or paper them. I am leaning toward 1/32 bass because it's lighter than ply and stronger than paper. The only negative is that there will be a seam because the bass that is available locally is 4 inches wide and that's not quite wide enough to cover the whole fin.



I also started the quasi-glassing of the body tube. Using MINWAX Polycrylic and a semi opaque knee high like I did on my Goblin bash. Looking good so far

+

I need to decide if I'll be using the cardboard rings or if I need to cut out some 1/8th ply rings. I kinda think the cardboard will be fine as this does have thru the wall fins that will also "lock in" the motor tube. They are also a little lighter. Just how concerned should I be about weight back there? I noticed there is some clay included in the kit

-Bob

 

[kenstarr]

Ooh I like the Der Something rockets! Can't wait to see what you get going
Ken

 

[Back_at_it]

For rockets where I'm going to add anything to the fins, I've moved to heavy card stock for smaller fins and use file folders soaked in epoxy for the larger stuff. It's thinner and stronger than balsa or bass and you're left with a nice smooth surface.

As for the centering rings. I'd definitely use the stock paper for the rear ring but I think I'd step up to a thin plywood upper ring if you plan to anchor the shock cord to the upper ring. I wouldn't trust a paper ring as the attachment point.

Here are my Blue Sapphire upscale fins papered using file folder.

 

[86mustang408w]

I just got mine back from 2013! The 4" dia 40" long one! 38mm here we come..

 

[Bruiser]

Last night I made the decision to go with basswood for skinning the fins. This morning I cut out 3 skins using up most of the piece of basswood I bought yesterday. I laid two pieces on the balsa core and realized that even without glue the fin was almost 1/4 inch now. I also thought about how there would be a built-in weak spot were the seams would be. You can see how I would need to add another piece of basswood because 4" stock was not wide enough. I did briefly consider staggering the seams by using 3" stock on the other side. Then I thought (based on past experience laminating) that trying to line up 2 pieces on each side of the core and get it positioned just right to dry would be quite trying...


Back to scratching my head. I thought about going with the 1/32nd ply but again the fin would end up being 1/4" wide and there is the weight penalty as well. Thought about papering using the tried and true Avery label paper method and I decided that it might not be strong enough and I really want the fins a little thicker. Around 3/16ths is the sweet spot that is stuck in my head.

I decided to laminate them with cardstock. So I cut out a fin pattern from some cardstock I had laying around and I weighted it. The full skin came in at .22 of an ounce. Then I weighed the basswood skin and it came in at .29 of an ounce and that was not a complete skin. Saving over .42 of an ounce was making me happy. Then I went to grab some more of that cardstock and I saw a piece of thinner cardstock. The piece was just big enough for 6 skins so I got to cutting. I took the first piece and tossed it on the scale and it came out to .13 of an ounce, a saving of over .96 of an ounce. That is sweet.


Next step will be my first experience with a lamination of this size. Keep your fingers crossed for me

-Bob

 

[Daddyisabar]

I know there has been plenty of Der Big Red Max builds but this will be my go at it. I think it'll be a little different than the others in a few ways.

I started with the fins. I have them glued up and sanded. Now I have to decide if I want to laminate them with 1/32 bass, 1/32 ply or paper them. I am leaning toward 1/32 bass because it's lighter than ply and stronger than paper. The only negative is that there will be a seam because the bass that is available locally is 4 inches wide and that's not quite wide enough to cover the whole fin.

View attachment 588825

I also started the quasi-glassing of the body tube. Using MINWAX Polycrylic and a semi opaque knee high like I did on my Goblin bash. Looking good so far

View attachment 588826+

I need to decide if I'll be using the cardboard rings or if I need to cut out some 1/8th ply rings. I kinda think the cardboard will be fine as this does have thru the wall fins that will also "lock in" the motor tube. They are also a little lighter. Just how concerned should I be about weight back there? I noticed there is some clay included in the kit

-Bob

No good, stinking, performance robbing nose weight included in a DRM kit? I am appalled. I am triggered! Mods severe enough to overcome the beautiful inherent stability of my beloved DRM. Sounds like an average run of the mill oddroc abomination, not a safe flying and totally gorgeous 3FNC.

Lots of weight on the hinny to require God awful nose weight. Computer simulations, swing tests, cardboard cut outs, wind tunnel testing, anything to avoid the dreaded modeling clay or epoxied lead shot.

Say it ain't so! No DRM nose weight!

 

[Bruiser]

Hmmn, not sure if you are just funin' or if you're funin' on me

That's ok though. The fins are done. Did the laminating yesterday and let them sit overnight. They looked great this morning. I trimmed the overhangs, rounded the leading edge, did some light sanding, applied thin CA around the parameter and now all three of them come in at 2.08 ounces. I can't believe how rigid yet light weight they are!


I also made my laundry shelf last night which will also be the attach point for the upper rail button.

I also made the attachment for the lower rail button. It'll glue in just like in the picture.

The laundry shelf is going to mount behind the CG as shown on the OR file by K'Tesh. Again, a little concerned about that but I think it will be offset by the nose cone mods that are coming.


That's it for now. Have a great day!
-Bob

 

[Back_at_it]

Mine required 2oz of nose weight but I had a complete plywood fins and ring upgrade. I doubt you will need to add any nose weight to make yours stable.

 

[86mustang408w]

Hmmn, not sure if you are just funin' or if you're funin' on me

That's ok though. The fins are done. Did the laminating yesterday and let them sit overnight. They looked great this morning. I trimmed the overhangs, rounded the leading edge, did some light sanding, applied thin CA around the parameter and now all three of them come in at 2.08 ounces. I can't believe how rigid yet light weight they are!
View attachment 589245

I also made my laundry shelf last night which will also be the attach point for the upper rail button.

I also made the attachment for the lower rail button. It'll glue in just like in the picture.

The laundry shelf is going to mount behind the CG as shown on the OR file by K'Tesh. Again, a little concerned about that but I think it will be offset by the nose cone mods that are coming.
View attachment 589246

That's it for now. Have a great day!
-Bob

Ok, what's the one to the left, that's completely new to me..

 

[Bruiser]

The disk with holes in it? That is the laundry shelf. It will have a screw eye in the middle. So this one serves three purposes
1) keep the parachute from moving all the way back
2) the screw eye with be where the shock cord attacks
3) the upper rail button will screw into it

There will be more to come when I finish it up and glue it in

Bob

 

[Hobie1dog]

 

[Bruiser]

Felt a little better today and got some work done.

I worked on the laundry shelf some more. I coated the bottom side with some JB weld and the top side with epoxy. In this picture you can also see where I have added a "doubler" to the shelf. This is where the upper 10/10 rail button will be screwed in. I did not apply any epoxy on the "side" of the plate and a little area around the edges bare so the epoxy would have bare wood to glue when installed in the body tube.


Here it is looking down. I pushed it in with another tube but it still somehow managed to flip around on me and made a little mess. Thankfully a 3" tube is just big enough to get my hand in so I was able to straighten it all out.


I also started epoxying the fins in. I usually use Elmer's but I decided this build needed a little more. I have two fins on so far. I went to install number three and I noticed it has a slight warp. I have it on a piece of glass with some weight on it. Hopefully it'll straighten out over the weekend.


That's it for today.

Cheers,
-Bob

 

[Bruiser]

I have been hung up with a fin. One of them has a warp in it. So as a reminder, these fins are the stock balsa fins skinned with some light cardstock of which I used all up skinning the fins.

I propped the leading edge up a bit and put some weight on it over the weekend. When I came back to it Monday nothing had changed so I propped it up a little more, put on a heavier weight and left it over night. This morning it still has not changed.


Besides just starting over the only thing I can think to do is to try removing the cardstock skin from one side, straighten the fin out and apply a new skin.

Any other ideas?

I'm gonna work on something else this morning but hope to get something going on this fin this afternoon. The other two fins are already glued on the rocket. This was the last fin to glue on and that's when I found the warp.

-Bob

 

[techrat]

I've flown with worse warping than that. That said, I find the fins for DBRM to be far too thin for the size of the rocket. I think you were better off skinning them with the basswood like originally planned. After the glue dries, sand a rounded edge into the fins and they will work fine. When you re-skin with the basswood, make sure the grain is going 90 degrees opposite the balsa grain, put the fins under several books while the skinning dries and that should straigten everything to 100% or close enough for government work.

 

[Back_at_it]

Typically the fix for warped fins is to lightly spray water on both sides of the wood then put them under something heavy for a day and let them dry.

Unfortunately I don't know how well this would work for you with the paper. You could try it but I would wrap the fin in wax paper incase any of the glue softens. I'm assuming you used some type of wood glue to do the paper fins.

 

[tsmith1315]

Besides just starting over the only thing I can think to do is to try removing the cardstock skin from one side, straighten the fin out and apply a new skin.

My first HPR had a fin like that, LOC plywood. Here’s what I did:

Glue fin tab in place.
Squirt a dab of CA on the line where the tip should be.
Carefully bend the tip into position with one hand and give it a shot of CA accelerator. (Maybe use a 3rd hand)
Finish gluing and fillet as usual.

That passed L1 and L2 certifications, and 25+ years later my L1 recertification. Still flies straight today.

 

[BABAR]

I have been hung up with a fin. One of them has a warp in it. So as a reminder, these fins are the stock balsa fins skinned with some light cardstock of which I used all up skinning the fins.

I propped the leading edge up a bit and put some weight on it over the weekend. When I came back to it Monday nothing had changed so I propped it up a little more, put on a heavier weight and left it over night. This morning it still has not changed.


Besides just starting over the only thing I can think to do is to try removing the cardstock skin from one side, straighten the fin out and apply a new skin.

Any other ideas?

I'm gonna work on something else this morning but hope to get something going on this fin this afternoon. The other two fins are already glued on the rocket. This was the last fin to glue on and that's when I found the warp.

-Bob

Balsa Fillet?

Pick which slot you are going to pt THAT fin in.
Take 1/8” balsa and cut a stick the length of the root of the fin for the outer body tube.

Glue it in place on the body tube next to the slot on the side TOWARD which the fin is warped.

When you slide the fin in the slot, the fillet should force the fin straight.

Once dry and in place, put sandpaper around a 1/4 dowel and sand the fillet down to shape.

 

[Bruiser]

Those were all good ideas but my computer went down again after I posted my question so I was left to think it thru myself (scary, lol)

I remembered reading a thread were somebody had papered fins with white glue and the fins warped. Somebody suggested using an iron to flatten them. They said the glue would soften with the heat so I tried that. It worked great but the warp came back when the fin cooled back down. I repeated the process but this time I clamped a piece of "T" aluminum to the root right away and allowed it to cool. It stayed straight when I removed the aluminum so I mixed up some epoxy and glued the fin in. It was looking great until I put the fin alignment guide on so I could eyeball the fin to make sure it was straight out from the tube. That is when I saw the fin tip was warped into a nice curve.

Hmmn, so what to do now? Ever hear the saying "in for a penny, in for a pound"? Instead of pulling the fin back out before the epoxy set up I went all in and applied an epoxy fillet to the fin and set it aside to cure. Once it set (been using 5 minute epoxy) I ironed the tip and clamped the "T" aluminum to it. I put it in the LAN room where it's air conditioned and left for the day. My office has an evaporative cooler which is probably what caused the warp in the first place.

I'll post an update in the morning. Hopefully it'll be good and I'll primer it some time in the morning. If it's warped, well, I don't know...

-Bob

 

[Bruiser]

Well here we go... I took the "T" aluminum off and it was... PERFECT!


When taking this picture I realized that I had not applied a fillet of glue around the aft centering ring so off to do that now. First coat of primer should go on this afternoon.

There's a lot to see in this pic of the nose cone mods. First the bottom of the nose cone has been cut off, along with the entire "slanted area". Next I used part of the cut off piece to fabricate a filler piece for the side of the shoulder. That was attached with Plastruct Plastic Weld. It does take a bit to solidify but it does end up being fairly strong. Then I drilled a series of 1/8th inch holes around the parameter of the shoulder to help the plywood ring get a great bond with plastic of the nose cone. Since epoxy doesn't have much of a chemical bond to plastic, the holes will provide a mechanical bond. Essentially the epoxy that flows into the holes will be similar to a rivet in function.


You can see the tee nuts, t-nuts or as Dubro calls them, blind nuts. These are not Dubro though and I will have to do some grinding on them to make them fit better. The barrel (threaded area?) needs to be ground down to be 1/8" tall so they don't stick thru the 1/8" ply ring. I am also going to grind away part of the flange so they don't intrude into the open area. I'll take a pic when done. I wouldn't need to do this if I had Dubro blind nuts but I ran out so these where sourced locally. Then there is there is the closure plate. I'll give that a coating of thinned epoxy so that it is easier to keep clean.

-Bob

 

[Bruiser]

Wow! hard to be believe I have been off the build for two months. I didn't seem that long but I am feeling a bit better so I did get some work done.

It's had it final prime and sanding. I primed this rocket like three or four times to fill the weave in the nylon. It's my fault because I applied the water based poly with a small 1/2" wide acid brush instead of a 1" wide foam brush that I have always used before. It resulted in not applying as much WBP so it required more primer to fill the weave. Lesson learned for next time.


I need to figure out if I need to add nose weight. I looked at K'Tesh's sim last night and I just couldn't figure out where the C/G is without an engine. Can somebody help me with that? I must still be a little foggy upstairs. I just can't seem to wrap my head around the numbers... This is when I need to add the weight if needed so I can touch up the primer before spraying top coat. There will be a lot of effort put into the paint scheme on this one.

I also did a little work on the nose cone attachment plate. It's all finished up and ready to be screwed in place once the nose weight is figured out


Anyway, sorry for the delay in the build. Some days there's just no energy; sometimes it's weeks. Thanks for following along.

-Bob

 

[jackiboi_11]

Looks awesome! Im thinking of either getting a DBRM soon or getting a Loc Goblin instead and getting the DBRM later!

 

[SolarYellow]

Since epoxy doesn't have much of a chemical bond to plastic, the holes will provide a mechanical bond. Essentially the epoxy that flows into the holes will be similar to a rivet in function.

I've observed that J-B Weld 50133 Plastic Bonder has kind of a solventing action that gives it a nice chemical bond with lots of plastics. Definitely works with polystyrene.

https://www.amazon.com/J-B-Weld-50133-Tan-1-Pack/dp/B009EU5ZNO/

 

[Bruiser]

The LOC Goblin doesn't have the right shape nose cone. If you are wanting a 3" Goblin, Estes will be releasing one soon or you could bash one of these rockets into one. Just need to cut some fins and get an extra piece of body tube and a coupler. That's what I did. Here's mine


Can no one help with the proper C/G, no motor?

-Bob

 

[waltr]

The CP (center of Pressure) should be about half way from top of fins to the Black band.
Then you want the CG (center of Gravity) 1 to 2 tube diameters above the CP (with a motor installed). Maybe 2 to 4 diameters without motor but ALWAYS re-check when a motor is installed.

 

[dpower]

Can no one help with the proper C/G, no motor?

-Bob

My 3" Der Big Red Max has the CG 17" from the tip of the NC, with no parachute installed, but includes a light (300#, IIRC) kevlar shock cord and nomex blanket. This is otherwise a stock build, with the included clay nose weight, and weighs 11.7 oz. I'm switching from the included stiff plastic chute (which is cool looking) to nylon, since the plastic chute didn't work too well on it's last flight with a Jolly Logic Chute Release.

At this weight, it flies nicely on an E16-4 and F15-6, from a 5' rod, though I've also flown it on an F27-4.