Advice for Card Stock Motor Mount Limits

[laizalott]

Is there an accepted rule of thumb when it comes to the cardstock centering rings that are common on low and mid-power kits?

I seem to have fallen in love with the G76G 29/40-120 reload, that crazy sound and beautiful green...absolutely lovely. I would love to throw it into a Der Big Red Max that I made earlier in my return to rocketry (stock, with Tightbond wood glue), but...

The G76 reloads have an initial thrust of about 35lbs, and I'm scared that is just going to punch right through the cardstock paper motor mounts. My DBRM has survived some F22 and F35 flights without issue, but at best that's 1/3 to 1/2 the thrust this G reload will put out. I know I could always buy another DBRM kit and laser cut some plywood mounts to do with epoxy, but...that just feels wasteful, like giving up.

Am I worrying over nothing? Has anyone ever put a G80-esque motor into a card stock motor mount and had it come back unscathed?

 

[jmasterj]

It has through-the-wall fins, right? If so, you'll be fine.

 

[kuririn]

It has through-the-wall fins, right? If so, you'll be fine.

+1.
Install the motor mount with only the front and mid centering rings on.
Glue fins on then do internal fillets.
Then glue on the rear centering ring and fillet.
That's what I did with the Great Goblin.
Much stronger.

 

[terryg]

I would be concerned about the stock fins standing up to a G76 reload.

 

[bad_idea]

I have a rocket that flies on the I200 using cardstock rings. Has fiberglassed TTW balsa fins epoxied to the motor tube, with epoxy fillets to a glassed cardboard body tube, so the rings aren't asked to cope with much.

 

[SolarYellow]

Why not just laser cut one or more centering rings to double up on the card stock rings you have? A little surface prep to get adhesion and slap them in there.

 

[prfesser]

Make three rectangular gussets---think "TTW fin tabs but without the fins"---from scrap balsa, plywood, or even thick cardboard. Length = distance between centering rings. Glue onto the MMT before gluing the motor mount in place. Make sure that the actual fin tabs lie between the gussets.

 

[laizalott]

I appreciate all the construction advice. If I have to build another one, I will do definitely plan for more strength in addition to the stock instructions.

For this one, it sounds like I might just have to roll the dice unless I can somehow manipulate some wood and epoxy reinforcement to the forward centering ring; my hand is too big to fit in 3" tube, so that will be tricky for me

The bottom has an Estes 29mm mount glued in, so it would be difficult to glue extra reinforcement direct to motor tube now after the fact. The TTW fins are glued to the centering ring...but I am pretty sure I used wood glue, no epoxy at all.

 

[ep29030]

Don’t worry, just fly it. The rings will be fine.

 

[Capt. Eric]

I appreciate all the construction advice. If I have to build another one, I will do definitely plan for more strength in addition to the stock instructions.

For this one, it sounds like I might just have to roll the dice unless I can somehow manipulate some wood and epoxy reinforcement to the forward centering ring; my hand is too big to fit in 3" tube, so that will be tricky for me

The bottom has an Estes 29mm mount glued in, so it would be difficult to glue extra reinforcement direct to motor tube now after the fact. The TTW fins are glued to the centering ring...but I am pretty sure I used wood glue, no epoxy at all.

I've never tried a G with cardboard rings, but there are a couple of things you can do to strengthen what you have. Add a layer of epoxy to the front and rear rings. Not just fillets, but a layer that covers the whole surface. On the rear, make sure you sand any paint off so it adheres well, and protect the retainer threads.
The other thing you could try is to drill a small hole in the cavities formed by the rings and fins around the mmt, then inject expanding epoxy foam into it. Need to work fast and practice a little so you get the procedure and volume right. Will be messy, but you can repair the finish when done.
Good luck with it. No guts, no glory! And if it doesn't work out, you can always build another!

 

[SolarYellow]

Don’t worry, just fly it. The rings will be fine.

And if it shreds, buy a Vander Burn plywood upgrade kit and a chunk of tube and rebuild.

 

[hcmbanjo]

I sometimes add gussets to strengthen a cardstock ring engine mount.
The balsa strip gussets won't add any real weight.
If you have thru-the-wall fins, the gussets would have to be placed between the fin slot locations.

https://modelrocketbuilding.blogspot.com/2016/04/estes-hi-flier-xl-3226-build-part-3.html

 

[ep29030]

As others have mentioned, adding paper gusset supports under the ring can add lots of strength without much weight. Tim at Apogee Rockets wrote an article on reinforcing paper. Here is the link if you are interested: https://www.apogeerockets.com/education/downloads/Newsletter63.pdf

 

[techrat]

Depends upon how strong you built it. I know my DBRM will take a G76, but I over built it, even using the stock parts. Lots of glue and epxoy, and I did the "semi-glassing" on the body tube, using ladies nylon stockings. The balsa fins were also mostly doubled-up for strength and to reduce warping from having been papered. If you built it well, it should take the motor.

 

[techrat]

My DBRM has survived some F22 and F35

Try an F56 and scale your way up to the G76. That's the only way to find out.
Your rocket is a little bit different from someone else's depending upon what steps were taken during construction.
That said, if the fins are soldily glued to the motor tube, and to the body tube, the Cardboard Centering Rings shouldn't make that much of a difference. The load will be spread out and the rocket will fly.

 

[bjphoenix]

my hand is too big to fit in 3" tube

I was building a 2.6" LOC kit about 20 years ago. I was about to figure out a way to feed a shock cord through the eyebolt in the motor mount when my 12 year old daughter grabbed the cord and put her hand and arm down the tube. This year I replaced the cord by myself- I taped an Xacto knife to a dowel to cut the old cord then taped the new cord to a piece of wire and fed the cord through the eyebolt. So if you have a 12 year old handy you might try that, or engineer yourself some long tools.

I've done a lot of builds where I added fillets to the front centering ring by standing the tube up nearly vertical, dribbling in a pool of glue from the front, and rotating the tube around to distribute the glue around 1/3 of the perimeter. Let that dry then do another 1/3, etc.

 

[terryg]

The pair of 12" forceps I purchased at Amazon several years ago are great for a lot of rocketry tasks.

 

[techrat]

I've done a lot of builds where I added fillets to the front centering ring by standing the tube up nearly vertical, dribbling in a pool of glue from the front, and rotating the tube around to distribute the glue around 1/3 of the perimeter. Let that dry then do another 1/3, etc.

I thought this was a standard building trick that everyone knew. Dribble glue down the tube, let it collect, and swirl it around and around and around. Makes a nice airtight seal too -- sometimes I have to drill a hole in my Payload Bay tubes to get the nosecone to go on.

 

[Alan15578]

I have never had card stock motor mounts fail in flight. I have ruined them when friction fitting motors. Still, I find plywood rings easier to make than card stock rings.