Double Trouble 2.0 - A HP 2-Stage Rebuild Thread

[AndrewW]

So after a second CATO in as many years I am once again going to rebuild the booster for my 2 - Stage HPR called Double Trouble. The rocket is a RW double Shot kit that had its booster extended after the first CATO. This time the motor let go near the rear closure totally destroying the fin can so my plan is to build a whole new booster.

After getting back from the last launch I went through my shop and found what I hope is enough parts to rebuild. I did order some G-10 from McMaster Carr to make new fins and got some CRs at the launch but otherwise I think I am all set.

I am hoping to have this done by the next URRG launch on Oct 20th.


My first successful 2 stage flight


The first CATO


Damage from the first CATO


The second CATO


Damage from the second CATO


Parts for the rebuild

 

[AndrewW]

I got started by first cleaning all the airframe and motor mount tubes. I then laid out the fin locations.

This is the first time I slotted my own tube and this is what I came up with. Using a 1/8” carbide bit in a Dremel plunge router attachment I set up the tube in my vise and using one of the jaws as guide slotted the tube according to my marks.





I then cleaned up the slots with a 1/8” mill file.
It am pretty happy with how the slots came out and once set up it only took about 10 minutes to complete.

 

[AndrewW]

I got working on the fins next. This is the first time I have cut my own fins but overall the process was fairly simple. I started with a 12”x24”x1/8” sheet of FR4/G10 black fiberglass that I got from McMaster Carr.


I started by cutting three blanks from the sheet stock using my table saw with an abrasive cutoff blade in it.


I next stacked all three blanks and stuck them together using carpet tape prior to cutting the leading and trailing edge angles.

I decided to change the fin shape from the original design to something a little more like the fins on a DX3 which I feel matches the fins on the sustainer a bit better.


I then beveled the leading and trailing edges using a modified tenoning jig on the saw with the same abrasive blade.


The finished product


I only put a modest bevel on these fins and left about 1/32” flat to help with durability.

 

[Flyfalcons]

Excellent work!

 

[Banzai88]

Sweet set up with the fins.

 

[AndrewW]

This is going to be a fast build so I am going to just keep moving and post pics whenever I can.

So I got started on the motor mount last night. I am using a thrust ring I machined as a proof of concept piece that I eventually used in my L3 rocket. It is just a basic stepped ring but I included a thicker section which interfaces with the fin via a slot.


Test fitting thrust ring onto MM


Cutting the fin slots in the thrust ring


Checking the fin slot


I used a 3” cardboard coupler tube to align the MM tube, thrust ring and retainer to layout the mounting holes


Drilling the retainer mounting holes


Tapping the mounting holes for 6-32 screws


Test fitting the thrust ring

 

[AndrewW]

I finished up the motor mount assembly yesterday.


I used some epoxy to laminate two CRs for the upper anchor mount. I then drilled and mounted a 1/4” SS u-bolt with a 3’ Kevlar harness on it.


Roughing up the MM with 60 grit sandpaper


I used JB Weld to attach the CR and thrust plate to the MM tube


On the fin side of the upper CR I tacked on some balsa blanks to be cut later as small fin pockets. As I will be injecting fillets these will prevent the thin epoxy from flowing around the tips of the fins. They do not provide any real structural support



Test fitting the assembled MM


Cutting the slots in the balsa blanks


The completed upper CR

After everything was fitted I installed the assembly using some West System epoxy making sure to remember to install the rear T nut and rail button after the first CR was inserted but before the thrust plate.

 

[Banzai88]

After everything was fitted I installed the assembly using some West System epoxy making sure to remember to install the rear T nut and rail button after the first CR was inserted but before the thrust plate.

There's the key right there! Can't tell you how many times I've forgotten to put that rear T nut in.....and then remembered it just as I was doing final fitting and gluing of the motor mount! Luckily I usually use Rocketpoxy, so there's enough working time to correct that mistake.

 

[AndrewW]

There's the key right there! Can't tell you how many times I've forgotten to put that rear T nut in.....and then remembered it just as I was doing final fitting and gluing of the motor mount! Luckily I usually use Rocketpoxy, so there's enough working time to correct that mistake.

That is what the piece of blue tape is for, to remind me to put it in there.

 

[grouch]

Very nice! More and more I want to try a stager.

 

[BDB]

I was there to see the successful first flight. Sorry about the subsequent CATOs, but its a great excuse to start this thread. This is fantastic work!

 

[AndrewW]

Very nice! More and more I want to try a stager.

If you are like me and are always looking for a challenge or a way to expand your involvement in the hobby you should give it a go. The work involved in designing, building and flying a two stage both safely and successfully was a whole new level for me by a large degree.

I do not have a great deal of experience in the area having only two HP 2 stage birds with only a 60% success rate (both failures where motor related and the sustainer ignition locked out which I suppose is somewhat of a success) but feel free to contact me with any questions. I certainly have learned a great deal going down the two stage road and in some regard it made my L3 seem almost easy.

 

[AndrewW]

I was there to see the successful first flight. Sorry about the subsequent CATOs, but its a great excuse to start this thread. This is fantastic work!

Thanks. It actually CATOed on my first attempt at URRG last year in September. You saw the first rebuild fly at MDRA back in February. It flew again at METRA in June before blowing up yet again at URRG this September. I am not sure what it is, September or URRG but needless to say I will not try flying this thing again out there next year around September!

 

[BDB]

I certainly have learned a great deal going down the two stage road and in some regard it made my L3 seem almost easy.

I'm the process of planning my L3. Hopefully it will fly about 1 year from now. And I completely agree your assessment. The L3 process (aside from the documentation) seems way easier than the MPR staging I've been playing with this year.

 

[AndrewW]

Got a bit more done on the fin can late last night.


Test fitting the fins


Checking alignment


Prepping the root edge of the fin by sanding with 60 grit sandpaper


As usual cleaning all surfaces to be bonded with acetone prior to bonding


Tacking the first fin in place using thick CA


End view of the alignment jig I use


Tacking the third fin


All three fins tacked and ready for internal fillets

 

[AndrewW]

Got the internal fillets done between last night and this morning.


Drilled the holes for injecting the internal fillets. I found that centering drills work really well for drilling fiberglass.


I use west systems for the internals as it is very thin and flows well. The only drawback is if you don’t get the root and edge of the fins completely tacked it will find its way through and make a mess


Injecting the epoxy. I got a five pack of these syringes on amazon and for the most part have been using the same one. When done just clean with alcohol.


Last set done. I usually add some heat while doing this a it initially helps the epoxy flow and also helps to set it up a bit quicker. After some trial and error I found the 15ml of epoxy per fillet was just about right.

 

[AndrewW]

Working on the external fillets. I am using Rocketpoxy with black dye. I got the first two sets done yesterday about six hours apart and am in the process of doing the last set this morning.


My mixing set up


Masking the airframe


Weighing the two components in a 1:1 ratio by weight


I mix a batch of this sizes resin and hardener for a full minute


After adding about 7-8 drops of dye I mix for another minute


I then let the mixed epoxy sit for 20 minutes before applying. Once I lay in the fillet I let it again sit for 3-5 minute to allow it to flow into place.


Then using a 1/2” piece of PVC dipped in alcohol and held at a constant angle I pull the fillets. I am usually able to do this in just on pull. I then immediately pull the tape. You sometimes have to watch for a little sag around the tip and root of the fin which is easy to wipe away with a rag and some alcohol.


Finished set of fillets.


From a slightly different angle.

 

[blackjack2564]

Those are good look'n fillets!
Nicely done.

 

[AndrewW]

Well I was originally in a rush to get this done for the October URRG launch but weather, work and family had other ideas.

I had some time in the past couple of days to finish this up and other than prepping it for flight it is essentially done.

Here are some pictures of the finishing steps.







I dry sand the fins, body tube, and fillets up to 400 grit to smooth everything and get rid of any deep scratches.



I then wet sand up to 1200 grit after which it gets a full wipe down with alcohol.



I then applied new vinyl lettering



I then applied 20+ coats of Bona High Gloss floor polish.


Finally I installed the Aeropac retainer


Finished!


Another pic


And another


The full stack

 

[Banzai88]

Very nicely done. I hope that it flies as well as it looks!

 

[AndrewW]

Very nicely done. I hope that it flies as well as it looks!

Me too. Out of four flights so far, only two flew successfully without blowing up.

 

[MaxQ]

Andrew, two out of four....sorry to hear that,
I came late to this party, so can you tell me the nature of the CATOs?
Cessaronis ? End closure failure was due to ? And the other one?
I haven't advanced to larger two stage projects because of the two stage gremlins on several flights, ...and they were CATOs in the booster each time (rather than electronics).
Two Stagers are a challenge, very rewarding when they work. Very frustrating when they don't. And having a CATO as a failure point...after all the testing and prep...REALLY frustrating.

 

[AndrewW]

So the first CATO was on my first attempt at flying this rocket. I was flying the booster on a 38mm CTI J595 Blue and the front closure failed. It was due to a known problem with the material used to make the closure. There was a factory recall on the closures and vendors had been replacing them I just happened to forget to check mine to see if it was one of the ones affected. Other than that the flight was sort of a success in that the altimeter locked out the ignition of the sustainer. Unfortunately the whole stack made it to 197’ which was a few feet shy of the launch detect altitude which prevented any of the deployment charges going off and there for everything came down quite hard. There was some damage to the fins, the booster air frame was toast and most of my electronics sustained some form of damage. It was a costly oversight to say the least.

The second CATO, my fourth attempt, is a little more of a mystery to me. About 2 seconds into the booster burn the motor burned through its casing just above the rear closure. This was a 54mm 4G K740 C-Star.

I am not sure what caused this but it completely destroyed the fin can. Once again I consider this a partial success as the electronics prevented the sustainer from lighting due to the parameters I had set for lock out even though the booster had reached over 2000’ at the time of its demise.

My second and third flights were successful. The first at MDRA on a CTI 54mm 3G J-760 to a CTI 38mm I-125. That combination got me to ~9700’
The third flight at METRA was a bit of a challenge as their waiver is only 4500’ and finding a motor combination that will deliver 10:1 off the pad yet stay below the waiver took some work. I flew it on a CTI 38mm 4G I-800 to a CTI 38mm 3G I-236 to about 4000’

I hope to fly the new booster at MDRA in a couple of weeks on the replacement K-740 to a J150 longburn which should put it well up over 15k.