HPR transitions - materials and techniques

[umcoo]

Hi all,

I'm currently designing a scratch built HPR (38mm motor mount up to J impulse) which will have a transition from 4" to 2.6". I'm looking at the best (and strongest?) method of achieving this. There is no commercially available unit our there which fits the size I need. I am considering a 3D printed unit, but does anyone have experience of 3D printed parts on a J impulse? What materials, infill etc have people successfully used? The print would also have 'couplers' at either end. Is there a general rule for their length. The actual transition will be just over 6" in length.

Alternatively, I was checking out the John Coker videos of a plywood 'frame' and using polystyrene or back-fill it with expanding foam?

The body tubes will be glassed phenolic tube, so I would also glass over the transition once the structure is complete.

My concern is with such a high powered motor, would these materials be sufficient for the stresses of flight?

Thanks very much in advance - forgive the many questions!

 

[Steve Shannon]

There have been many who have used cardboard tubes and plywood centering rings, phenolic tubes and plywood centering rings, and G10 or G12 and centering rings.
I’m not sure why you believe there are no commercial components to accomplish this. I suspect LOC Precision, PML (now owned by LOC), Wildman, Apogee, and perhaps several other manufacturers have components that can be used.
Here’s a video describing how:

 

[Seatechnerd83]

I’m not sure why you believe there are no commercial components to accomplish this.

He said, "which fits the size I need" which is a little bit different than believing there are no commercial components to manufacture your own. in my experience there is a very limited amount of transitions commercially manufactured. Apogee components makes some plastic transitions, and the only FWFG transition is a 54mm-4in one from WM. Not very many commercial options, and the techniques to learn how to build good ones aren't always the easiest to learn. (In my transition needs, I've resorted to having FWFG coupler components as structural, and then 3d printed aerodynamic "fairings" for better looks/performance.

 

[Seatechnerd83]

Hi all,

I'm currently designing a scratch built HPR (38mm motor mount up to J impulse) which will have a transition from 4" to 2.6". I'm looking at the best (and strongest?) method of achieving this. There is no commercially available unit our there which fits the size I need. I am considering a 3D printed unit, but does anyone have experience of 3D printed parts on a J impulse? What materials, infill etc have people successfully used? The print would also have 'couplers' at either end. Is there a general rule for their length. The actual transition will be just over 6" in length.

Alternatively, I was checking out the John Coker videos of a plywood 'frame' and using polystyrene or back-fill it with expanding foam?

The body tubes will be glassed phenolic tube, so I would also glass over the transition once the structure is complete.

My concern is with such a high powered motor, would these materials be sufficient for the stresses of flight?

Thanks very much in advance - forgive the many questions!

Also, if youre feeling adventurous, Wildman Rocketry has a fiberglass 4in-54mm (2.1 in) transition (except that comes with extra airframe, but you could email Tim and ask him if you could purchase only the transition without the extra airframe) so you could do a little bit of dremeling to get your part to fit.

 

[tsmith1315]

You could print the conical section as a sleeve to go over a coupler/centering ring core.

 

[Tractionengines]

Or print it undersized and coat it with a couple wraps if fiberglass. That would strengthen the print and keep the layers from separating along the print lines. Then just smooth OD to slip-fit into your body tubes.

 

[umcoo]

Thanks all, much appreciated.

I think the 3d print is the better way to go, and then internally fill with differing sizes of CR, then coat the outside in fibreglass. If that doesn't survive a J425, I'm not sure what will

 

[prfesser]

I am an inveterate scratch builder and have made transitions for high-power models using cardstock transitions generated either with a compass (old school method) or with OR. Make five or six identical ones (careful to ensure that they nest without gaps). Coat each inside and out with epoxy, wiping off the excess after they're wetted out. Nest them together, staggering the seams. Place on polyethylene and add a weight on top to compress the layers (not too much weight). Cure.

One could also use a single paper transition and cover it with fiberglass but that would mean more work finishing the glass.

 

[umcoo]

I am an inveterate scratch builder and have made transitions for high-power models using cardstock transitions generated either with a compass (old school method) or with OR. Make five or six identical ones (careful to ensure that they nest without gaps). Coat each inside and out with epoxy, wiping off the excess after they're wetted out. Nest them together, staggering the seams. Place on polyethylene and add a weight on top to compress the layers (not too much weight). Cure.

One could also use a single paper transition and cover it with fiberglass but that would mean more work finishing the glass.

Interesting, thanks. I hadn't considered this at all. I am assuming internal centering rings add some additional strength as well, or are they not required?

 

[prfesser]

Interesting, thanks. I hadn't considered this at all. I am assuming internal centering rings add some additional strength as well, or are they not required?

Yes, there is a centering ring inside the larger end, thick enough to slide into the tube it goes into. I also sand the large end flush after it has cured. Sometimes a little sanding is required on the inside of the smaller end for a perfect fit.

 

[Handeman]

I've seen several college teams try to fly 3D printed transitions. They were usually the point of failure when the flight went side ways half way up.
Using a 3D print over a solid CR/tubing structure for aerodynamics should work fine.

 

[justforfun]

This was 30+ years ago, still works today. Simple inexpensive quick. Three tools required, hobby knife, ruler and compass.

Roy

 

[umcoo]

This was 30+ years ago, still works today. Simple inexpensive quick. Three tools required, hobby knife, ruler and compass.

Roy

Interesting. Is that a veneer?

 

[justforfun]

Interesting. Is that a veneer?

1mm GL2 Finnish Birch Aircraft Plywood grade AB/B.

The transition was used on my Aerobee 300.

Roy

 

[OverTheTop]

I 3D printed an ABS transition for my half-scale Nike Apache. Flown on M2020. 3" to 4" transition.

Airframe tubes slightly overlap to provide extra strength against bending loads.

 

[OzHybrid]

Or print it undersized and coat it with a couple wraps if fiberglass. That would strengthen the print and keep the layers from separating along the print lines. Then just smooth OD to slip-fit into your body tubes.

Or print it the correct size for the shape and glass fibre it internally for strength. Less finishing.
Norm

 

[OverTheTop]

Or print it correct size for finishing and design it for the 3D printing realm. No glassing necessary if it takes the loads it needs to without breaking. Think where the flight loads will go and design appropriately.

 

[OzHybrid]

Or print it correct size for finishing and design it for the 3D printing realm. No glassing necessary if it takes the loads it needs to without breaking. Think where the flight loads will go and design appropriately.

Unfortunately, there's a lot more to printing a structural object than just printing an object. Not everyone can print a structural thing. This is just an option for success.

 

[umcoo]

I 3D printed an ABS transition for my half-scale Nike Apache. Flown on M2020. 3" to 4" transition.

Airframe tubes slightly overlap to provide extra strength against bending loads.
View attachment 577984
View attachment 577985
View attachment 577986
View attachment 577987

Thanks for all comments and suggestions. I really like the look of this and obviously it's flight proven. Do you have an STL available?

Are the holes to bolt it into the airframe too? I was curious if epoxy was enough of if it would also need some hardware to keep it secure.

 

[OverTheTop]

Will see if I can dig out the STLs. Yes, bolted to both airframes. It gives you the option to print another one if you break it during flight or transport. Epoxy would work too, if you choose. Personally I like it removable. It also allowed me to swap the upper section to a different Apache configuration for more altitude. The original Apache with the wider upper launched a mass spectrometer for NASA back in '71.