Things to consider before building a minimum diameter rocket.

[Senior Space Cadet]

By minimum diameter rocket I mean a rocket with a body tube the same size as the motor, such as an 18mm body tube with an 18mm motor.
My first suggestion, if you are considering it is, don't. Your mother raised you better than that.
I'm nearing completion of my first one and I wish I hadn't gone down this road.
I never built one before because the normal way of keeping the motor from flying out the back, when the ejection charge goes off, is to tape it in. Seemed like a poor solution. Still does.
In designing the rocket, you need to leave room for the tape, so you have to move the fins forward. This moves the CP forward.
Then you have to move the motor back so you have enough surface for the tape to grip onto. This moves the CG back.
Because you are using a smaller body tube and nose cone, which weigh less, you either have to use a longer body tube or add weight to the nose cone, which negates most of the advantages of using a minimum diameter in the first place.
Then there is the problem of anchoring the shock chord. Some glue it on the outside of the body. Some use the old method of using a folded up piece of paper, but now you don't have room to get your finger in the body tube. Well, maybe a child or small woman could. I wear XL gloves. My solution was to tie a big knot on the end of some Kevlar chord and glue it under an engine block. Seems pretty secure, but Murphy will have the last say.
I took some short cuts and made some mistakes in designing my rocket. The result is, I had to put a bag of concrete in the nose cone.
I was going to use streamers, but with the weight in the nose cone, I'm thinking I better use a parachute, which now means I probably have more weight in the nose cone than I need.
Other than all that, it's going to be brilliant. They will be talking about my fins, in reverent tones, for years to come. It will be a thing of near beauty. Well, I'm pretty sure you won't gag.
I might have photos in 24 hours, or less, or I might wait till launch day, which might be a while.

 

[o1d_dude]

Scots frequently say “If [fill in the blank] were easy, the English would do it.”

Continue with your project and report back with the launch results.

I have a fiberglass 18mm MD rocket. The body tube is loooong, the kevlar shockcord is tied to the motor thrust block, and I’m using a streamer. It’s a Mach 1 “Micro Mamba”. Has micro-railbuttons, too.

Several TRF’ers have built the same kit and flown it successfully.

Improbable, yes. Impossible remains to be seen.

 

[n27sb]

By minimum diameter rocket I mean a rocket with a body tube the same size as the motor, such as an 18mm body tube with an 18mm motor.
My first suggestion, if you are considering it is, don't. Your mother raised you better than that.

I think I would have to politely disagree.
Most of what I build is minimum diameter. I usually incorporate Dual Deploy, Gps, and motor retention.
I recently built and flew a 24mm 2 stage with all of the above and Electronic Air Start of the sustainer to over 5800'
on a total impulse of F or 80ns.
Most rockets beyond the basic Estes single stage kits have challenges. I think that is what keeps this hobby interesting.
I enjoy following your post because you ask great questions for discussion.
Good luck on your project.

 

[dhbarr]

Next up, take a journey down sub-min-dia, like a flying case or rack rocket! So many different ways to turn money and time into smoke and grousing ^_^

 

[Kelly]

I never built one before because the normal way of keeping the motor from flying out the back, when the ejection charge goes off, is to tape it in. Seemed like a poor solution.

I think the more normal way, is to put tape on the outside of the motor, running the length of the motor, until it fits somewhat tightly into the body tube, AKA "friction fit". No tape on the outside of the rocket. There's an art to this - too tight and it's tough to get out; too loose and the motor ejects - but this has been done since the start of model rocketry. Now you can have the motor flush (or even inset a bit!) into the end of the tube, solving your problems with Cg/Cp and fin placement.

Also, drag goes up with the square of rocket diameter, so using a longer body tube to get Cg where you want it should still be a good tradeoff, as compared to going with a bigger diameter.

 

[Flyfalcons]

If you intend to use a tape band around the outside of the motor and body tube for retention, then you really don't have to move the fins very far forward. My recent 24mm MD scratchbuild has them forward maybe 1/4". Just wrap the tape tightly, and supplement that with friction fitting the motor, and it won't go anywhere. It is also possible to run an external motor hook if you really want. I have done that on a couple 18mm MD rockets. A slight drag increase but also more convenient.

 

[NateB]

If your finger can't fit in the body tube, use a push stick instead.

 

[hcmbanjo]

Senior Space Cadet wrote:
By minimum diameter rocket I mean a rocket with a body tube the same size as the motor, such as an 18mm body tube with an 18mm motor.
My first suggestion, if you are considering it is, don't. Your mother raised you better than that.
I'm nearing completion of my first one and I wish I hadn't gone down this road.

I never built one before because the normal way of keeping the motor from flying out the back, when the ejection charge goes off, is to tape it in. Seemed like a poor solution. Still does.
Friction fit was never 100% reliable. Before masking tape was used, the oldest kit instructions suggested using rubber bands wrapped around the motor. It's like anything else, you learn through experience the proper amount of wraps before you end up crimping the body tube when pushing in the motor.

In designing the rocket, you need to leave room for the tape, so you have to move the fins forward.
As mentioned above, you only need about 1/4" clearance for the tape wrap around the body tube and extended engine.

Then you have to move the motor back so you have enough surface for the tape to grip onto. This moves the CG back.
Because you are using a smaller body tube and nose cone, which weigh less, you either have to use a longer body tube or add weight to the nose cone, which negates most of the advantages of using a minimum diameter in the first place.
The body doesn't have to be that long - Look back at classic models like the Estes Sky Hook. That model does have long, trailing fins.

Then there is the problem of anchoring the shock chord. Some glue it on the outside of the body. Some use the old method of using a folded up piece of paper, but now you don't have room to get your finger in the body tube.
Estes style Tri-Fold shock cord mounts aren't good in models smaller than a BT-50 anyway. Too small a tube and the chute or streamer ejection can be blocked by a thick, folded mount.

Well, maybe a child or small woman could. I wear XL gloves. My solution was to tie a big knot on the end of some Kevlar chord and glue it under an engine block. Seems pretty secure, but Murphy will have the last say.
Kevlar tied around the engine block is probably the best way to go in a small diameter model.

I took some short cuts and made some mistakes in designing my rocket. The result is, I had to put a bag of concrete in the nose cone.
I was going to use streamers, but with the weight in the nose cone, I'm thinking I better use a parachute, which now means I probably have more weight in the nose cone than I need.
We've all made design mistakes in our first scratch builds. A "Bag of concrete?" Even the old Estes Wac Corporal used two nose weights coming out to about 1/4 oz. How much weight did you use?

Other than all that, it's going to be brilliant. They will be talking about my fins, in reverent tones, for years to come. It will be a thing of near beauty. Well, I'm pretty sure you won't gag. I might have photos in 24 hours, or less, or I might wait till launch day, which might be a while.
You've got the right attitude about it being "Brilliant". Looking forward to seeing the pictures.

 

[modeltrains]

It is also possible to run an external motor hook if you really want.

That has been my choice.
Sort of a logic of "self-contained is good"
Tape is just another thing to have to bring and keep track of.
Tape can/will damage the body tube when peeled off.

 

[Flyfalcons]

Tape is the universal rocketry tool. It has so many uses that I never go launching without it.

 

[Jeff Lassahn]

I've built several simple minimum diameter rockets, and I find that they're not that hard once you get used to them.

For small tube diameters having a few dowels or metal rods of different diameters to push things around while gluing stuff inside is very helpful. I got a couple of sizes of the brass tubes you can buy at the hobby shop or hardware store and they've served me well for ding things like installing engine blocks.

For motor retention, I've always glued in an engine block to stop the engine sliding forward under thrust, positioned so the engine sticks out maybe 1/8 inch from the end of the tube. I've generally not needed to do anything fancy to keep motors coming out backwards, I just friction fit them and usually the motors are too tight rather than too loose and I have to roll the motor against a hard surface to crush the diameter down to fit cleanly. When they are too loose a wrap of cellophane tape around the motor solves the problem.

As long as your nose cone's fit is much looser than your motor's fit ejection will push the nose out instead of the motor.

A basic three fin and a nose minimum diameter rocket with fins at or very close to the back and maybe a little bit of fin sweep will often not need any nose weight at all. Even the ones that do, the weight needed will be less than the extra weight of a larger airframe.

I've anchored the shock cord by the classic method of wadding a bunch of folded paper and glue around it and gluing it some random place in the body tube. It's not elegant but for small rockets it works. A good metal rod to shove stuff into place and hold it while the glue sets is definitely a help here, particularly if you're building 13mm T size stuff.

It's a different set of tradeoffs than bigger diameters, and obviously you should build the rockets that make you happy. But I've enjoyed my minimum diameter rockets and have raised them to be happy well-adjusted members of the community.

 

[modeltrains]

As long as your nose cone's fit is much looser than your motor's fit ejection will push the nose out instead of the motor.

That sounds like one for the "A good point to note" category.

 

[Ronz Rocketz]

Tim just talked about shock cords in minimum diameter rockets.

 

[GlenP]

Next up, take a journey down sub-min-dia, like a flying case or rack rocket! So many different ways to turn money and time into smoke and grousing ^_^

Good point! For an 18mm power rocket, you can always use a transition/reducer to go down to a 13mm body and nose, for example, or smaller. Less room for recovery stuff and nose weight, this poses yet another fun challenge.

 

[o1d_dude]

Micro Mamba BT20 build video

Check the video clip for building a minimum diameter fiberglass rocket.

 

[AeroTech]

I’m a big fan of minimum diameter rockets. For shock cord mounting, I ran a Kevlar thread under a fin fillet.
This rocket used a Mylar tape thrust ring and a wrap of Mylar tape around the thrust ring/body tube junction for motor retention. No friction fit.

This particular rocket flew to over 13,000 feet on an H13 motor and was successfully recovered with no damage. Like Jeff said, swept fins are a good way to increase stability without having to add nose weight or increase fin size (and drag).

 

[Rktman]

I never built one before because the normal way of keeping the motor from flying out the back, when the ejection charge goes off, is to tape it in. Seemed like a poor solution. Still does.
In designing the rocket, you need to leave room for the tape, so you have to move the fins forward. This moves the CP forward.
Then you have to move the motor back so you have enough surface for the tape to grip onto. This moves the CG back.

Apogee, Estes, and other manufacturers sell screw-on type motor retainers for tubes as small as 18mm. Granted it adds a bit of drag, but you won't have to move your fins as far forward.

 

[Steve Shannon]

Tucked away in Gary’s post above is also this gem: don’t place a thrust ring up in the body of the rocket unless you will never want to use a different size motor. Instead, glue or build a thrust ring on the aft end of the motor.

 

[Quaranta]

I disagree. Slimline motor retainer is the solution...

https://www.rocketryforum.com/threads/mini-eagle-claw-mod-for-dual-deploy-build-thread.46544/
Now a shameless plug... It's for sale!!!

https://www.rocketryforum.com/threads/angry-bird-rocket-for-sale.161493/

 

[n27sb]

Tucked away in Gary’s post above is also this gem: don’t place a thrust ring up in the body of the rocket unless you will never want to use a different size motor. Instead, glue or build a thrust ring on the aft end of the motor.

I thought you were not allowed to glue anything to the motor, but you would know. Thanks for the tip

 

[Back_at_it]

I've been building minimum diameter rockets for a very long time as they are cheap and easy to build and I don't feel bad if I lose one. I don't let that fact that the motor is friction fit change my design as I simply use masking tape around the outside of the motor and slide it into the tube until it is somewhat tight. If your nosecone isn't tight, it will pop before the motor pushes out. Back as a child, most of my rockets were BT-20 based as they were cheap and we were poor so I learned friction fit pretty quickly. I still use friction fit on any rocket that the fins don't extend down past the engine hook as I like my rockets to stand on their own.

As for shock cord mounting. I'll normally add a second engine block ring with Kevlar attached to it and push it up in the body tube a few inches and pull the Kevlar out the top. Doing this gives you a strong mounting location without cutting holes in the body tube.

As for nose weight. That really depends on your fin design and how large of a motor you are running For instance, you can build an Estes Alpha as a minimum diameter and fly it on D12-7's without adding any weight. I've done this a few times without any issues. If you want to fly something like an AT F44-8 or something equally as silly you will need a 1/4 oz up top.

As for recovery, Anything smaller than BT-50 (24mm) is a pain in the @ss if you want to run a parachute. I have a Mini Mean Machine with a 9 inch parachute and it's all I can do to get that thing in there without using colorful language. My Mini Mean Machine is also built as an 18mm minimum diameter and flies really well on B6-4 and C6-7's.

 

[SolarYellow]

I’m a big fan of minimum diameter rockets.
This rocket used a Mylar tape thrust ring and a wrap of Mylar tape around the thrust ring/body tube junction for motor retention. No friction fit.
View attachment 429979
This particular rocket flew to over 13,000 feet on an H13 motor and was successfully recovered with no damage. Like Jeff said, swept fins are a good way to increase stability without having to add nose weight or increase fin size (and drag).

I really like that concept. Is there anyone making little caps like that for 18 and 24mm motors? I'm thinking the same ID and OD as the body tube, with a little ID lip to wrap over the end of the motor. I can machine (or maybe spin) something out of aluminum easily enough, but it would be easier to buy it.

 

[mh9162013]

I really like that concept. Is there anyone making little caps like that for 18 and 24mm motors? I'm thinking the same ID and OD as the body tube, with a little ID lip to wrap over the end of the motor. I can machine (or maybe spin) something out of aluminum easily enough, but it would be easier to buy it.

If I understand what AeroTech posted, there are no caps. Rather, what you see in that pic is the back of that motor (with the mylar tape around it), which has a built in thrust ring at the nozzel end.

 

[Steve Shannon]

If I understand what AeroTech posted, there are no caps. Rather, what you see in that pic is the back of that motor (with the mylar tape around it), which has a built in thrust ring at the nozzel end.

Correct, as I recall. No cap, just Mylar tape wrapped around and around until it was the size of the BT OD, then a wrap of Mylar tape that straddled the BT and thrust ring.
Gluing a piece of MMT to the base of a cylindrical motor case would work also. I’m sorry I never noticed that question two years ago, but gluing on a thrust ring is absolutely permitted.

 

[milehigh]

https://castlerocketeer.blogspot.com/2018/08/flatcat-boostglider-build-part-8-pop-pod.html?m=0
This post from my blog outlines the motor retention method I use for my minimum diameter birds. I've never had a motor eject with this system.

 

[PDawg]

I'll have to disagree. I've built C, D, E, and F min diameter rockets and they all flew great. Using tape wrapped around the Motor always works, no special Fins, standard shock cord mount, nothing special in the build. L-R.---- F Min. dia, E-min Dia, E min dia. with Quest Plastic fin unit, F Sub min dia rocket, and the crazy E rocket that wasn't supposed to have any chance of flying. Every one flew great, no nose weight added. I'm getting into slow and low rockets now, but Min diameters will always be my favorite. These rockets are awesome, but can only be launched on a very clear day. Cheap way to get Altitude and it doesn't hurt as much if it gets lost.