Implementation of rocket motor (R. Nakka's Impulser) into a minimum diameter rocket

ThisGamerAlex

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Hi! This is my first post so hopefully I wont be breaking any rule or something, sorry if I do. Recently I have seen the Impulser rocket motor made by Richard Nakka and I can't seem to figure out how to 1) transmit the thrust generated by the motor to the rocket body tube (what methods could be used?) and 2) how to retain in place that kind of motor in a minimum diameter rocket (what mechanism could you use?)

I'm aware of parts like centering rings, thrust rings, etc but I'm still new to this hobby and I still can't fully understand the work behind those techniques, how you implement them into your own rocket design... I am trying to learn as much as I can, so every bit of information/knowledge you might want to give is fully welcomed.

Resume in case it's not clear: I have doubts on the implementation of that specific design of motor (how to transmit the thrust correctly to the rocket body tube and how to retain the motor in place). I hope this post belongs to the techniques section.

Have a nice day!

Alex
 

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heada

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Most rockets have an external thrust ring on the aft end of the motor (nozzle end) that transfers thrust from the motor to the rocket. In a minimum diameter configuration, that is directly to the airframe.

As for motor retention, there are a few methods ranging from friction fit to forward retention from the forward closure to block that is glued into the airframe and threaded rod in the fillet of the fin that is used with a nut to hold the aft end of the rocket to the airframe. I'm sure there are others but those are 3 off the top of my head.

edit: a 4th is a thin motor retainer glued to the aft end of the airframe like a slimline.
 

ThisGamerAlex

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First of all, thank you for sharing with me your knowledge. Most of my doubts probably are really simple things but even though I consider having a nice level of english, the languaje barrier for some rocketry concepts/words and the lack of experience make it harder to visualize it.

The friction fit method, as far as I know, would be recomended only for not really heavy or powerful motors, right? Correct me if I'm wrong please.

The second one you mentioned would be to use a threaded closure (like the one below) on your motor, then also have installed a minimum diameter motor retainer like the red one from Apogee (or to build that piece myself, with 3D printing for example) and link them with an eyebolt, right? I have seen someone build a 3D printed rocket that had a 3D printed minimum diameter motor retainer connected to the BT itself and attached the motor with an eyebolt, acting as a motor retention system and a medium to transfer thrust to the BT. Do you think that would be strong enough to fly the rocket safely (without a thrust ring on the motor)?

54mm_Threaded_Closure.jpg da7a96c316d4558ff147e4cecd662987.image.320x320.jpg

The third one you mentioned I can't seem to understand it with just explanation so if you have any image that show what you mean (for that method or any other) I would really appreciate it.

And on the 4th, you mean to glue a motor retainer piece after I have inserted the rocket motor into de minimum diameter BT? Otherwise, if I glued it before, the motor wouldn't fit in first place.

Thanks heada
 
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ThisGamerAlex

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Okay I just thought of an idea for the motor retention. If the BT walls were thick enough as to be able to drill some screws, you could make a ring out of plywood. So you would first fit the rocket motor and then install at the aft end the plywood ring and drill the screws so that it stays in place and hold the rocket motor too. It would also not get destroyed (hopefully) from the exhaust gases or the heat. I think it is a pretty good idea, what do you think guys?
 

heada

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I am not fan of friction motor retention but there are some here that are fans. It has been used in the past to retain motors in the M class so it can be done successfully on high power motors. I always use positive motor retention so I can't advise on how to do it for high power motors.

That is the glue-in retainer that is forward of the motor. These are made from aluminum but a 3D printed version could work if the math was done to ensure the ejection charge doesn't overpower the sheer force of the glue.

That is another thread on this forum that is discussing motor retention on minimum diameter high power rockets. The threaded rod and nut was discussed in the first post. Further down there is a post that discusses the glue-on retainer from Slimline. It does increase the diameter of the rocket at the aft end but only by a few mm.
 

tsmith1315

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I've friction fit up to full-J impulse. Just a single layer of masking tape, it doesn't take as much as you think. Wrap it in a spiral with wide spacing like threads, sometimes half a turn of tape is enough. It works on adapters, too.
 

cls

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Advantage of masking tape friction fitting:. Saves a little weight vs aluminum retainer such as Aerospace

Disadvantage: adhesive on poor quality tape can get sticky and gooey from motor heat, and the motor can get really stuck!!

I've successfully used good masking tape (3M beige) up to 54mm 1706 cases, for K185, K550, etc.
 

G_T

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I've gotten some masking tape labeled 3M that is beige, but the paper is thinner than the older stuff (tears at times just removing it from the roll) and the adhesive is annoyingly tacky - and gets worse fast with time once applied to something. Perhaps I ended up with counterfeit. Or the new stuff just isn't what the old stuff was.
 

Rocketjunkie

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Build the rocket with 1/2" or so of the body behind the fins. Build up a tape thrust ring on the motor if it doesn't have one on the rear closure. Insert motor into rocket and tape over the joint between the motor tube and motor thrust ring. (This was the method recommended with the original Enerjets in the early '70s.)
 
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