How to build a tail nozzle (other than 3D-print)

neil_w said:

This will be 24mm for sure; it'll be upwards of 6 oz with no motors.

Rear-eject is certainly possible (and would indeed protect the tail nozzle) but I just don't love the look of this one with rear eject. It would be BT50 inside BT55, pretty tight, and it would also concentrate more weight toward the rear. For this I think I'd rather just reinforce the tail and go with standard front eject.

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Ahhh, but there is a trick up your sleeve! Rear eject is possible with PLENTY of room.

I came up with this when I upsized/cloned the SR-72 DarkBird rear eject boost glider.
https://www.rocketryforum.com/threa...w-sr-73-raven-glider-and-new-gyskelion.38883/

I assume you are going with a 24 mm motor, so BT-50 motor mount. This will be long enough to hold the motor, plus say an inch. Forward of this you mount a BT-20 “chimney”, this runs all the way forward and can even go into the nose cone if you want. You can attach your nose weight to the forward end of the tube inside your nose cone.

The aft centering ring for the BT-20 in the motor mount works as your motor stop. You smear this and the aft three or four inches of the chimney with epoxy (I usually use a rolled up piece of aluminum can, Der MicroMeister used to get all upset) as the ejection flame from the 24mm engine gets toasty inside the 20 mm tube. Smear some epoxy on the inside of your nose cone tip.

At ejection, chimney ducts the gas all the way forward to the nose. You have centering rings around the BT-20 in the thickest portion of your rocket, one forward and one aft. The gas then pushing BACKWARD ON THE FORWARD centering ring. Your chute is between the centering rings, and the shock cord attachment is near the tail of the pod. The whole pod slides out, including the nose weight you put around the forward end of the Chimney.

You DO need a second shock cord attachment between forward end of the pod and the forward section near the base of the nose cone. I recommend you “daisy chain” this as it makes it a lot easier to load.

Beauty of it is that


1. the pod descends nose first. No direct impact on nozzle or the rear fins

2. The body of the rocket is really light, it is just a hollow tube, some wing fins, and a nose cone. Your nose cone weight is jettisoned attached to the pop pod.

jlabrasca said:

I think we really should be warning neil_w about the Krushnic Effect <smile>

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Yeah, I was suggesting to make the MMT go to aft end of the cone (but not further aft). Of course, there is still base drag, but that should be OK on a LPR motor.

JohnCoker said:

Yeah, I was suggesting to make the MMT go to aft end of the cone (but not further aft). Of course, there is still base drag, but that should be OK on a LPR motor.

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Krushnic would require more than a small fraction of a caliber motor indent, would it not? I’m talking a 1/8” indent in a 1.6” opening.

iirc we usually start having krushnic effect issues around one full caliber of motor being recessed.

neil_w said:

Krushnic would require more than a small fraction of a caliber motor indent, would it not? I’m talking a 1/8” indent in a 1.6” opening.

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rharshberger said:

iirc we usually start having krushnic effect issues around one full caliber of motor being recessed.

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I think this is the generally accepted rule of thumb -- however motors with a very long tail ( fast exhaust ) are likely okay to be recessed a bit further.

More importantly for most of our cases, motors with a short tail ( slow exhaust ) for very much of their burn should probably be LESS inset.

Here's the Apogee POF Newsletter article on double layered shrouds:
https://www.apogeerockets.com/education/downloads/Newsletter349.pdf

hcmbanjo said:

Here's the Apogee POF Newsletter article on double layered shrouds:
https://www.apogeerockets.com/education/downloads/Newsletter349.pdf

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I just re-read it yesterday.

I'm having a bit of a time locating 110lb cardstock. My local Walmart showed it in stock, but when I went there they had none. Am going to try again at lunch.

Food for thought... I made the rear motor nozzle removable on my lifting rocket. Then if it gets crunched it's a much easier fix.

Yes the engine will stick out the back about 1/2".. no worries

Well I fabricated a double-shroud with 110 lb cardstock and TBII, and coated it with CA:

I have to say it is quite stout. I believe this should be sufficient:

The coating of epoxy on the interior is just a bit of insurance, probably not necessary.

Looking at this to-scale picture, does anyone think there's even the slightest chance I'm going to have Krushnic issues?

neil_w said:

Looking at this to-scale picture, does anyone think there's even the slightest chance I'm going to have Krushnic issues?

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No.

Not at all.
If they are easy to make, make a bunch. Then if it crumps, you can just replace it.
I still like LakeRoadsters idea of making it removable.
Didn’t you come up with some sort of twist on thingie for nose cones a few threads Ago?

Rear eject the entire tail/fin/cone assembly so it won’t land on it. Or run an external shock cord to near the cg so it lands more or less horizontally.

BABAR said:

Not at all.
If they are easy to make, make a bunch. Then if it crumps, you can just replace it.
I still like LakeRoadsters idea of making it removable.
Didn’t you come up with some sort of twist on thingie for nose cones a few threads Ago?

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Doing a twist lock on the this part is an intriguing idea... dunno if I can make it happen though. Not much room to work with here, and there'll be interference from both the motor on the inside and the transition and fins on the front side. Will ponder though.

In the meantime, I am satisfied with the strength of the part that I have fabricated. Double-layer 110lb cardstock + CA coating = pretty impressively rigid.

neil_w said:

Consider the following rocket:
View attachment 388983

That tail nozzle goes from BT55 to BT60 size, 1.25" long. How would I fabricate such a thing? I'm comfortable doing transitions in cardstock, but that thing is going to take the brunt of landing, and I am not sure if I can imagine a cardstock shroud surviving. Or could, it with sufficient reinforcement? If so, what sort of reinforcement?

I can't think of any other way to fabricate that thing other than 3D printing. At least, no way that I'm reasonably capable of. It needs to be light, this is LPR.

Any thoughts?

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Some ideas . . .

(1) Use 1/64" Plywood, instead of cardstock.

(2) Whatever the tail nozzle is made out of, reinforce it with 1/2oz - 2oz fiberglass.

(3) Use a "Sling" to bring the rocket down horizontally.

Dave F.

neil_w said:

I dunno, pretty pricey!

Here's an updated version of what I was thinking:

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That would work but if you printed the part one wall thick it would be fairly light.

One other idea for this type of project when the amount of reduction isn't too great is to glue on the centering ring, then slather on either a bunch of either CWF or ultralight spackle , let it set a bit , then remove the excess leaving behind the conical shape. The ultralight spackle is super light. Soak in some thin CA after it dries and it will be hard as a rock but still light.

I did this to create a Nike Smoke nose cone once, starting with an existing conical come that I needed to be extended to be wider than the tubing.

Post deleted.

I neglected to check for page 2 before posting. You're way ahead of me.

+1 on the 1/64 ply. I did this nozzle with it, as well as the surface details on the tube. I soaked it with thin CA and sanded it smooth and it was ready to paint. Came out very strong!

Make it expendable, i.e. just replace it after each flight. Print them 6 or 8 to a page. You will save time and money in the long run.
Art Applewhite

For a more bell-shaped nozzle use a nose cone as a mold. Put on a coat of saran wrap or tape and wind glue soaked cord or line from the smallest dia. of the bell to the largest. Trim and finish as desired. The size of string simulates cooling tubes, or fill and smooth to copy the newer 3d printed nozzles. Different sizes and shapes of NC's give varied bells. As always, the first one or two will teach you the technique, then vary the materials to get the desired effect.

numbo said:

For a more bell-shaped nozzle use a nose cone as a mold. Put on a coat of saran wrap or tape and wind glue soaked cord or line from the smallest dia. of the bell to the largest.

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What a funky idea, I may need to try that just for the fun of it. What kind of cord do you use? I’m thinking cotton would hold white or yellow glue best.

Yes, cotton is easy in water thinned white or yellow glue. I've used nylon or monofilament on a thin bed of epoxy to give a smoother finish.

Get a carbon fiber sleeve and feed it over the nozzle then fold it in on itself to cover the inside. High temp epoxy and you are set.

Though it is a bit late to the conversation, I think I have another possible solution for Neil_w's question. I took a plastic dosing cup from my son't Allegra liquid, and lo and behold it could work.

Bottom diameter is about 1.26, top diameter is about 1.65, height of cup is 1.36. It is somewhat flexible, so I am not sure how the paint/primer on it will hold up to repeated flexing/usage (maybe use a black Sharpie to color it?). Nor do I have any idea as to how it will stand up to the heat.

Neil might have to move the rear of the engine back a bit so it isn't too deep in the nozzle, but that should be easy to do.

Picture of cup: