Question About Minimum Diameter Rockets

[mh9162013]

I'm thinking about my next project and I'm thinking building a minimum diameter rocket that uses 18mm BP engines. Right now, I'm looking at an Estes Yankee, Wizard or Viking (or all 3!). Anyways, one of my concerns is what to do if the engine gets stuck.

One thing I thought of was moving the engine block (thrust ring) back towards the end of the rocket maybe an extra 3/8" or so. So the 18mm engine sticks out the back about 5/8" or 3/4" instead of 1/4" or 3/8". The goal of doing this is to reduce the chances of the engine getting stuck and increase the chances of removing an engine that gets stuck. I figure less surface area inside the rocket b/w the engine and the rocket body tube can achieve this goal. Another benefit is more room for a payload, baffle and/or parachute (instead of a streamer)

I can see 2 potential drawbacks, however. First, it'll lower the rocket's center of gravity. Second, it might reduce the rocket's aerodynamics.

My question is: are there any other drawbacks or disadvantages to this approach that I may be missing?

 

[Spitfire222]

I wouldn't mess with changing the location of the motor, especially on these really small rockets where even small changes can affect how they fly. I've never really given thought to a stuck motor, it's simply never been on issue on these type of 18mm Estes rockets I've built. But two things I can think of:

1. If you're concerned the motor fit is tight, wrap some sandpaper around the provided yellow spacer tube and sand the inside of the body tube a bit, and use some tape during flight for peace of mind.
2. If a motor does get stuck, the 18mm Estes BP motors are easy enough to crush and pull out with some type of pliers.

You're overthinking it, build per the instructions and go fly!

 

[Lee]

Or just use a dowel and push it out from the NC end

 

[mh9162013]

Or just use a dowel and push it out from the NC end

That won't be an option with a baffle in the way (I plan on installing a baffle).

 

[kuririn]

Even MD rockets can have an engine hook installed:
https://estesrockets.com/wp-content/uploads/Instructions/002178_HI_FLIER.pdf18mm rocket (Hi Flier) with engine hook:

Or if you have at least 1/4"-1/2" tube overhang past the fins at the rear of the rocket you can do a tape wrap instead of friction fit. If the stock fins are even with the rear of the tube then just move them up a bit. Check for stability in Open Rocket with the slightly changed fin position.
13mm MD rocket (Star Trooper) with tape wrap motor retention:


I think there was an article on rear end tape wrap for motor retention in the Apogee Newsletter archives. It works, never had an engine retention failure with this method.
But you will have to decide whether doing these mods are worth the small chance of your motor getting "stuck".
Happy flying.

 

[mh9162013]

Or if you have at least 1/4"-1/2" tube overhang past the fins at the rear of the rocket you can do a tape wrap instead of friction fit.

What do you coat the bottom rim of your rocket with to prevent the tape from damaging the finish or paint on your rocket?

 

[kuririn]

What do you coat the bottom rim of your rocket with to prevent the tape from damaging the finish or paint on your rocket?

I do a wipe with thin CA before painting.
My friend Babar uses a strip of metallic tape.
Both will prevent the paper from lifting up.

 

[bjphoenix]

In the old days a lot of rocket kits were minimum diameter, none of them had engine hooks, and we never had a problem with engines being stuck. Engine retention was always by way of wrapping tape around the engine until it fit tightly enough, but not too tight.
Under normal circumstances engines don't get stuck however I recommend always removing the engine right after its flight. One of my rockets landed in water last year and was there about 5 minutes before I retrieved it. The rocket wasn't damaged but I didn't pull the engine out right away and the next day it wouldn't come out. I pulled harder and the entire motor mount came out with it.

 

[dr-ws]

That won't be an option with a baffle in the way (I plan on installing a baffle).

In the Olde Days (c. 1971), we'd take Ye Olde Metal Coathanger and, after unbending it, bend one end at about 3/16-1/4" to 90 degrees, then the balance of the coathanger to form a handle. Insert the small bent end into the used ceramic nozzle, then progressively tug until engine cried uncle.
For water-soaked engines, we'd let them dry in an arid, drafty environment for about 5-7 days, then use the coathanger, usually also successfully. If epoxy was used for the motor mount, I don't recall one ever coming free.

 

[BABAR]

In the old days a lot of rocket kits were minimum diameter, none of them had engine hooks, and we never had a problem with engines being stuck. Engine retention was always by way of wrapping tape around the engine until it fit tightly enough, but not too tight.
Under normal circumstances engines don't get stuck however I recommend always removing the engine right after its flight. One of my rockets landed in water last year and was there about 5 minutes before I retrieved it. The rocket wasn't damaged but I didn't pull the engine out right away and the next day it wouldn't come out. I pulled harder and the entire motor mount came out with it.

Yup, I have had same issue. I take the motor out while I am walking back to launch area (oooh, ow, sometime it’s a bit hot!). Usually comes out easy when warm, which seems paradoxical but has been my routine experience. Other advantage is I either toss it in a trash can or more commonly when no safe can available I stick it in a ziplock so I doesn’t stink up my car. (I do NOT love the smell of black powder in the morning on my way to work.)

As @kuririn said, some sort of protection is nice for an EXTERNAL tape wrap (which works amazingly well) but you need at least 1/4 inch or more of tube below the tail end of fin attachment. If you don’t want to mess with your paint scheme, cellophane tape is pretty much invisible. Try to wrap it the OPPOSITE direction of your habitual retention tape wrap. So it won’t come off with your retention tape.

 

[ksaves2]

Only problem with m.d. small rockets on large motors is they disappear sometimes for a very long time. Winds aloft can be at a totally different direction than the ground winds and one might not be able to get a visual on descent with the higher probability of losing the rocket. Of course if their is facility to use a small Rf tracker, that increases the odds the rocket will be found with a directional receiver.
I sometimes use big engines in well used "beater" rockets to try to get them "lost" as I don't care anymore. More often than not these "beaters" come down within sight! Oh the vagaries of the rocket gods!!!!

 

[diyaerospace]

I wouldn't worry about the engine getting stuck. I have never had a motor gets stuck in the body tube that couldn't be pulled out with some pliers.
As for the tape damaging the finish on the rocket, I wouldn't be concerned about this. If you use a primer before painting masking tape won't strip the paint.
Retaining the motor is as simple as a friction fit or tapping the body tube to the motor.
Good luck with your next rocket!

 

[rharshberger]

+1 external tape retention method, and as for tracking a reflective mylar streamer as long as you can get it works very well since it "sparkles". ASP Rocketry sells some of the best mylar streamers, but in a pinch you can cut one from a mylar "space"/survival blanket or reflective bird scare tape, the latter two options are not as goos as the ASP streamer as they are very thin and a little seems to break off every flight. I use a 10' mylar streamer on my SBR Lil' Fusion that has a 24mm instead of stock 18mm motor mount, on 25mm G's it hits nearly 3500' and has be relatively easy to track (and re-acquire visual) from apogee to landing.

A last important note about minimumn diameter rockets is that in most cases the longest delay motor ejection is still not long enough to reach apogee before ejection, so electronic based ejection may be need for full altitude.