Engine Downsizing

I seem to recall reading or hearing that it is possible to stuff a 18mm engine inside a 24mm spent engine and using it on a rocket built for 24mm. Is this true ?

After all this discussion about altitude, not to mention smaller engines being cheaper and more widely available, has me thinking about the possibilities.

Kinesthesia said:

I seem to recall reading or hearing that it is possible to stuff a 18mm engine inside a 24mm spent engine and using it on a rocket built for 24mm. Is this true ?

After all this discussion about altitude, not to mention smaller engines being cheaper and more widely available, has me thinking about the possibilities.

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Yes. Remove the clay nozzle and clean out the residue. Glue them in the old casing. You can also do the same thing with a 13mm motor inside an 18mm casing.

Most definitely true. You just need to construct a motor adapter. This is easy to do. All you need is a BT-20 motor tube, a thrust block, a standard engine hook and two AR-2050 centering rings. To use it, you place your 18mm motor into the adapter and them place the adapter into the 24mm motor mount. You will need to have a way to restrain the adapter to keep it from being ejected out of the mount. If the 24mm mount has a motor hook, you can employ it to hold in the adapter. A second motor hook on the adapter itself holds in the 18mm motor. Use the method described in the post above.

Of course, to be safe the rocket in question must be one that the smaller motor can launch safely. It has to be light enough to be within the maximum lift-off weight for the smaller motor.

I have done this several times myself. One rocket that was often flown on smaller motors was the Estes Goblin. When it was launched on its standard 24mm D motor it could often go out of sight, so many people, myself included, stepped the power down to a C6 motor, which they used with the aforementioned motor adapter. Don't worry about the ejection charge strength of the smaller motor. In any rocket that the 18mm motor can lift, the charge will be more than sufficient to deploy the recovery system.

EDIT: tbzep's method above for constructing an adapter out of a spent C11 or D12 motor works really well. In fact, it's probably the best way. I don't glue the smaller motor in, because the fit is usually so tight that I don't worry about it coming out. It never has when I have used this method. At most I have put a wrap of masking tape around the 18mm motor to make the fit sufficiently tight. I have to push the spent 18mm out with the help of a dowel and some tapping with a hammer. I can then reuse the same adapter with another motor. Don't use anything to tap the smaller motor into the adapter, though, because you can easily damage it. If you can't push it in by hand with firm, steady force, it is too tight.

Like previously stated, building an adapter is real easy.

If you have some spare BT-5 tubing, making a 13mm mount for something like a Viking or Wizard is real easy without having to drill out and/or cut a spent engine casing.

I've found that the little yellow tube Estes supplies to help push engine blocks into BT-20 tubes works great...simply cut 3 rings out of one, slice 2 of them to nestle in the 3rd one, and the whole assembly wraps perfectly around a BT-5. You can either make two of them, or use the engine block as the second 'ring'. You can then either use part of a spent 13mm engine casing as an engine block, or if you have a motor hook, that works too.

I've adapated several Vikings and Wizards this way. My opinion is why bother building a rocket that a kid in a moment of poor judgment will over engine and it vanishes....even if you have an appropriate sized field. With a permanent smaller engine mount, he may lose it because his field was too small, but never because it went too high.

Also, as a note, Viking's and Wizards flown in this configuration work fantastic...even with a 1/2A3-4T they fly about 200 feet, pop right at apogee and flutter down nicely...the reduced weight helps with a softer landing.

FC

FastCargo said:

Also, as a note, Viking's and Wizards flown in this configuration work fantastic...even with a 1/2A3-4T they fly about 200 feet, pop right at apogee and flutter down nicely...the reduced weight helps with a softer landing.

FC

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That is such a great point you made, FC. With so much emphasis on bigger, faster, higher, one can forget how much fun simple, uncomplicated rocket launching down at the neighborhood field on modest (and cheap!) 1/2A and A motors can be, just like we did back when we were kids. No worries about whether I assembled the motor correctly, did I measure out the right amount of black powder for my ejection charges and did I wire up the e-matches right, is there a good fresh battery in my altimeter, etc, etc., etc. ad nauseum. All of that rigamarole has its place and the challenge of mastering it can be fun, but geez, sometimes you just want to chuck all of it and simply go out, launch some darned rockets and have fun for awhile. Thank you for the reminder.

I'll 3rd all the above comments about building reducing adaptors...Easy, inexpensive and lots of fun!

I don't think I've flown a Cherokee-D without an 18mm adaptor since loosing my very first one built back in the 70's flown on a D motor. With such Light weight models even 18mm motor can put it up there a good ways.

Using a spend 24mm casing works but it adds a lot of unnecessary weight to the model. Cutting that same casing into "centering rings" saves both money and mass...all ya really need is about 2-3/4" of BT20 tube..the hook is really an extra..internal and external tape secure the motor to adaptor just fine

Micromeister said:

Using a spend 24mm casing works but it adds a lot of unnecessary weight to the model.

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That's true, but the whole point is to fly lower (and cheaper), so as long as the added weight doesn't ruin the stability, it actually helps keep the bird on the small field.

tbzep said:

That's true, but the whole point is to fly lower (and cheaper), so as long as the added weight doesn't ruin the stability, it actually helps keep the bird on the small field.

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Exactly. If you're downsizing from a D12 to a C6, even with the added weight of the spent 24mm casing, the adapted C6 should still be lighter than the original D12.

Doug

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Micromeister said:

I don't think I've flown a Cherokee-D without an 18mm adaptor since loosing my very first one built back in the 70's flown on a D motor. With such Light weight models even 18mm motor can put it up there a good ways.

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Back in the good old days, the biggest engine we had was a B and they would fly plenty high for us. It was somewhere around 1970 or so when Estes changed their engine production and came out with the C engines. As I recall the earlier engines were made with thicker walls and there was not enough room inside for enough propellant to make a C. Then they changed production to thinner walls, changed their designations to metric, and came out with a C. Somewhere I think I still have a few of the older engines.

bjphoenix said:

Back in the good old days, the biggest engine we had was a B and they would fly plenty high for us. It was somewhere around 1970 or so when Estes changed their engine production and came out with the C engines. As I recall the earlier engines were made with thicker walls and there was not enough room inside for enough propellant to make a C. Then they changed production to thinner walls, changed their designations to metric, and came out with a C. Somewhere I think I still have a few of the older engines.

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The newer thin wall, metric-designated 18mm motors were introduced by Estes in the 1968 catalog. They first showed up at Centuri in their 1969 catalog. I did have some 1968 dated C6-0s and C6-7s, but I used the last of those motors in 1996.

Hi Bob,

Picture makes it almost perfectly clear ... Is the heavy centering ring in the middle of the adapter a requirement? Just trying to understand why those grams of weight are there, instead of just a tape wrap around the engine hook.

Alan Roberts said:

Hi Bob,

Picture makes it almost perfectly clear ... Is the heavy centering ring in the middle of the adapter a requirement? Just trying to understand why those grams of weight are there, instead of just a tape wrap around the engine hook.

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The forward and center centering rings keeps the motor tube aligned and centered in the larger motor tube and the rear ring gives the larger motor hook something to hook onto.

mwtoelle said:

The forward and center centering rings keeps the motor tube aligned and centered in the larger motor tube and the rear ring gives the larger motor hook something to hook onto.

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Yup,

That is exactly why the center ring is there. The rear ring never makes it into the motor tube.