Transferring trust through a boat tail

[BDB]

I am planning to build a 150% upscale of a Binder Design Velociraptor for my L3 project. The rocket will be made of 6" Blue Tube and will use a hand-laid fiberglass nosecone and tailcone (PML). I have started working with my TAP, but I'd like to crowdsource an idea to see if anyone has done something similar.

The key question that I have is how to transfer the thrust from the motor's aft thrust ring to the airframe for a rocket with a boat tail. Surely, the fiberglass tail cone is not sufficient for the thrust of L3 motors, and I'm a little nervous about only trusting the glue holding the motor mount to the centering rings. For inspiration, I looked to the mother of all boat tail designs, the V2. Interestingly, it was designed with a "hard point" directly in front of the boat tail (highlighted).


I think I'd like to also install a thrust ring/hardpoint in my design directly in front of the boat tail and attach it to the airframe with both wood glue and screws. I would then like to create a sort of fin pocket that would abut the motor retainer and run continuously to the forward hard point.

I'm bored at work today (and my CAD skills suck), so I tried to sketch something up so you could see what I mean (I do not claim to be an artist, either):


The fin pocket would follow the shape of the boat tail, but it would have grooves to allow for interlocking centering rings. I could make it out of solid 1/4" ply, but it might also be interesting to try to cut out some of the material to reduce the weight.

So...two questions: 1) Has anyone tried something like this before? 2) If so, is there a pattern that I could cut into the fin pocket that would reduce its weight but retain its strength (think bridge design)?

 

[KenECoyote]

FWIW the Loc Precision uses 3 CRs on their 7.5 V2. Bottom, center (abuts up against the tail cone shoulder) and top (glued to the body tube).

In this pic I foamed the fin can and I plan to launch it on J-L motors. I also did a bit of customizing.

 

[Rschub]

I assume a 3" motor tube

3" X 3.14=9.42" circumference.

Assuming a 1/4" centering ring 9.42 X .25=2.355 square inches of glued area not counting fillets.

West system epoxy has around 7000 psi tensile strength, 3500 for a safety factor of 2.

3500 x 2.355=8242.5 lbs of force on the motor tube before the epoxy gives way.

I think you will be OK without the bridge reinforcement section.

 

[jmasterj]

You could put a thrust ring onto the bottom of the tailcone (why would that not be strong enough?), to which your motor retainer would then attach. You could extend your motor tube pretty far up into the airframe, giving you more length in which to place centering rings. Your TtW fins will provide a lot of strength, holding the motor tube in place.

I think you'll be fine without over-engineering something new.

Chain of custody (trust), though, is a whole different ballgame.

 

[BDB]

FWIW the Loc Precision uses 3 CRs on their 7.5 V2. Bottom, center (abuts up against the tail cone shoulder) and top (glued to the body tube).
View attachment 622722
In this pic I foamed the fin can and I plan to launch it on J-L motors. I also did a bit of customizing.

What the purpose of the double forward centering ring? Does the top one have the same ID as the motor tube? If so that's an interesting idea for transferring thrust from the motor tube to the airframe.

 

[KenECoyote]

What the purpose of the double forward centering ring? Does the top one have the same ID as the motor tube? If so that's an interesting idea for transferring thrust from the motor tube to the airframe.

It's an illusion! The very top piece is a locking plate for the MMAS (Modular Motor Adapter System) and just bolts the motor adapter (in this case 54mm to 98mm) to the top CR. the main "motor tube" is 98mm.

Only the bottom, center and top rings (with 98mm center hole for the 98mm main motor tube) are epoxied in.

 

[K'Tesh]

I am planning to build a 150% upscale of a Binder Design Velociraptor for my L3 project. The rocket will be made of 6" Blue Tube and will use a hand-laid fiberglass nosecone and tailcone (PML). I have started working with my TAP, but I'd like to crowdsource an idea to see if anyone has done something similar.

The key question that I have is how to transfer the thrust from the motor's aft thrust ring to the airframe for a rocket with a boat tail. Surely, the fiberglass tail cone is not sufficient for the thrust of L3 motors, and I'm a little nervous about only trusting the glue holding the motor mount to the centering rings. For inspiration, I looked to the mother of all boat tail designs, the V2. Interestingly, it was designed with a "hard point" directly in front of the boat tail (highlighted).
View attachment 622718

I think I'd like to also install a thrust ring/hardpoint in my design directly in front of the boat tail and attach it to the airframe with both wood glue and screws. I would then like to create a sort of fin pocket that would abut the motor retainer and run continuously to the forward hard point.

I'm bored at work today (and my CAD skills suck), so I tried to sketch something up so you could see what I mean (I do not claim to be an artist, either):
View attachment 622719

The fin pocket would follow the shape of the boat tail, but it would have grooves to allow for interlocking centering rings. I could make it out of solid 1/4" ply, but it might also be interesting to try to cut out some of the material to reduce the weight.

So...two questions: 1) Has anyone tried something like this before? 2) If so, is there a pattern that I could cut into the fin pocket that would reduce its weight but retain its strength (think bridge design)?

I have a sim for the Velociraptor... You could use it to help you understand what your need to do.



And my variation of it..





https://www.rocketryforum.com/threads/kteshs-openrocket-files.123564/post-1588085
And a little self plug for upscales of the Raptor...



https://www.rocketryforum.com/threads/binder-design-raptor-evolution-build.128811/#post-1520021

 

[OverTheTop]

Just use an Aeropack-style minimum diameter mount at the front of the motor. No need for anything pushing at the rear. That is how I do all my HPR, including my Velociraptor.

Place it for the longest available motor and make spacers for shorter motors.

 

[OzHybrid]

You could put a thrust ring onto the bottom of the tailcone (why would that not be strong enough?), to which your motor retainer would then attach. You could extend your motor tube pretty far up into the airframe, giving you more length in which to place centering rings. Your TtW fins will provide a lot of strength, holding the motor tube in place.

I think you'll be fine without over-engineering something new.

Chain of custody (trust), though, is a whole different ballgame.

Maybe he meant "In Thrust We Trust"

 

[boatgeek]

Dispute the premise. A fiberglass nose cone based boat tail is going to be at least as strong as a blue tube airframe. You’ll also have the opportunity to glue in the shoulder, which will make the connection rigid as all heck.

 

[Rschub]

Load always follows the stiffest path and everything else is just along for the ride.
Gluing everything to each other with 32 fillets just complicates the issue.
If your motor tube is fiberglass that is the stiffest part in that area. Columns are very stiff.