Estes Door Knob Fix

[Lee]

My first MP rocket, and I love the way it flies on a D 12-3 – slow majestic climb off the pad.

But, and I had suspicions, the 1/8 inch flimsy fins positioned at the bottom of the BT are vulnerable when landing. Initially, I filled the grain with CWF thinking that would strengthen them enough. Not so, cracked a fin on first launch. Repaired with TBIII, then papered the fins and saturated with CA. Second launch, more damage – corner of fin crunched.



Here’s where I’m seeking advice. I am scheming to extend the MMT (strongest part of the rocket?) so that the MMT bears the landing impact. Since the MMT is epoxied in, I am not replacing it. Seems to me that an easier way is to just add (epoxy) a second retainer which adds approx. 5/8 of an inch. The photos show the dry fit of my work. At the moment, the second retainer is being held in place by a 29MM adapter.



For aesthetics, I’m adding a boat tail. To secure it, a made a centering ring sized to screw onto the threads of the retainer. I can epoxy the CR to the retainer which now gives me a secure attachment point for the boat tail, which was initially made from yellow cardstock. I can laminate the boat tail with additional layers to make it more durable.



In addition, I’m now pondering a trim to the fin corners – similar to Minie Mag – to further distance them from landing impact.



Anyone have thoughts or advice? Thank you in advance for your contributions

 

[user 35280]

Are those the stock fins from Estes or the plywood upgrade parts from,
https://www.launchlabrocketry.com/shop/p/vanderburndoorknob29mmupgradekit

 

[Lee]

Stock. I don't think the upgrades were out yet when I purchased the door knob last January

 

[user 35280]

Stock. I don't think the upgrades were out yet when I purchased the door knob last January

James at
https://www.rocketryworks.com/doorknob-plywood-fin-upgrade-set-of-4/has the plywood upgrades as well.

 

[Lee]

The balsa fins are epoxied to the MMT and CRs. I can't imagine undoing that in order to replace with plywood upgrades

 

[user 35280]

The balsa fins are epoxied to the MMT and CRs. I can't imagine undoing that in order to replace with plywood upgrades

Just a recommendation if you get another kit.

 

[kuririn]

But, and I had suspicions, the 1/8 inch flimsy fins positioned at the bottom of the BT are vulnerable when landing. Initially, I filled the grain with CWF thinking that would strengthen them enough. Not so, cracked a fin on first launch. Repaired with TBIII, then papered the fins and saturated with CA. Second launch, more damage – corner of fin crunched.

CWF does not strengthen the balsa, just fills in the grain. Papering the fins will strengthen them but that is due more to the glue, not the paper. Saturating with thin CA after papering is rendered less effective because the paper and balsa pores already have absorbed the glue you used to do the papering, thus blocking absorption of the CA.
I never combine papering and CA for strengthening fins. It's either one or the other.
TLP kits used to have standard stock balsa fins for their mid power kits. Instructions recommended painting them with thin CA then sanding smooth. Stiffens them considerably while adding very little weight.

 

[icyclops]

If you use a thicker, hard coated type of paper it will actually provide a lot of stiffening and strength. I use spray mount and coated card stock and have had hard landings on precarious placed fines with little to no damage. Have not had actual fin breakage on root edge or full fin split, just a little dentition on the fin edge….easy to cosmetically repair. Works great for most LP and some MP…but for most MP I use thin plastic styrene sheet (.20-.30) and Spraymount Super 77.

 

[Bill S]

I second using epoxy to paper. I used 65lb cardstock attached with 30min epoxy on my Executioner's fins, and despite some landings that I was sure would result in damage to those pointy fins protruding out the bottom, only surface scuffing. BUT that process basically tripled the weight of the bare fin, so take that into account when you build the kit.

 

[dpower]

What's holding the second retainer to the first retainer? This seems like the tough part, it needs to provide sufficient strength to hold the motor for deployment, without kicking the motor. Presumably you've put a small length of MMT tube in the additional CR to glue the second retainer on, but that doesn't seem like enough area to hold against the force of an ejection charge.

 

[Lee]

That is what I'm working on - gluing retainer to retainer.
1st attempt - JB Weld plastic - didn't work.
2nd attempt - Reg JB Weld - didn't work.
3rd attempt - Testors - believe it or not it's actually holding. I've bounced it 6 times on concrete floor, hasn't broken off yet.
In the mean time, I'm waiting for a reply from Estes on the type of plastic their retainers are made of.

 

[Lee]

OK - got an answer from Estes. The retainer is made from "High Heat ABS Plastic".
Anybody know the best adhesive for this?

 

[Lee]

Seems like LOCTITE® PLASTICS BONDING SYSTEM
is the answer.

 

[RokitFlyr]

Howdy!
To the original question of bonding the 2 retainers, ABS pipe cement as used for irrigation/plumbing might work.
My .02 cents worth of thoughts.

All the building/upgrade ideas are good but useless in this situation.

Adding a second retainer will add weight to the aft end and move the CG, which means adding nose weight, = heavier rocket landing = more impact. Rockets rarely land straight down, so extending the MMT to take the force probably won't help much. Balsa fins will ding/chip/dent etc. Even plywood.

Leave it as is and enjoy flying! If you want it to stay purty, put it on a shelf .

 

[Lee]

Thank you for your input - greatly appreciated.
The rest of my mod was to weigh the retainers and yes - add similar nose weight, then increase the parachute size.

The rest of my story is this: I have 3 grand kids ages 7, 8 & 11 who like rockets. Last January I came across a sale by a hobby shop - I believe in Johnstown Pa - selling Door Knobs for $23! I bought and built 4 of them before flying. Only one has flown - the damaged one. Once I perfect the modification I will duplicate with the others. The kids are chomping at the bit to paint and fly them. None of them will just sit on a shelf so I am seeking to make them as durable as possible.

 

[RokitFlyr]

Great score on the kits!
Meant no offense on the shelf comment. Just saying they all get dinged.
To strengthen wood fins on my builds, I thin 30min. epoxy with denatured alcohol, and paint them before assembly. Super strong and sands very smooth. Good Flying!