Getting back in the saddle...

mcgunsmith

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Hello from Marine City, MI where I am a professional gunsmith and general hobby nut. My first love has always been model aviation with a close second given to submarines. My first successfully finished and flown rocket was at age 10, c.1977 an Estes Bandit. It flew REALLY slow owing to its multi-coat butyrate dope finish and ginormous weight, but lift-off was totally cool and no wadding ?! Bonus.

I flew off and on over the years, the hit a giant dry spell until about 2010 or so. Then in '16 we had to basically empty my house (it adjoins my gunsmithing shop) and turn it into additional retail space. My hobby corner went away. BUT, now I have the entire upper floor basically one huge room and it's all mine.

The model rocketry is getting unpacked!!

I have at least a dozen kits to build, and the same number to finish. There are two PML high power kits needing a bit more paint then decals. Since I thought I might provide the local Scout troop with rocket supplies, I still have a few full bulk boxes of A, B, and C size motors, even a handful of Ds.

...and I have issues, lol. Seems I had modified a STANDARD AGM-78 from Launch Pad Models into a cluster motor mount. Must've seemed like a good idea at the time, but I'm gonna pursue this. Anyone ever compile notes on successful mods like this? I had a 3 ring binder bulging with plans, notes, other BS...it remains packed somewhere. I'm going down that path that says "how far would I have to upscale an Estes Bullpup 12D to accept a D motor? How do I balance a model for which I've designed a booster stage? CAN I do this at all?"

That's where I'm at but hey it's great to buy new tubes of glue lol. I'll take some pics of my "building" kit stash and whatnot and post here if anyone wants to see a field of un-painted paper tubes and balsa wood that look just like rockets.
 

smstachwick

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Welcome back!

By “balancing” I assume that you mean measuring CG and calculating CP (center of gravity and center of pressure, respectively) to get stable flight. OpenRocket is great for this, but if you want to do it the old-school way, just plug the relevant dimensions into the Barrowman equation to get CP. Be sure to check both the full stack and the sustainer on its own, you don’t want an unstable multistager to stage and then make a stable flight directly into somebody’s eyeball.


CG is pretty easy, just balance the complete rocket (with fresh motors, wadding, etc) and mark the balance point. Yes, both configurations here too! There is a mathematical way that’s similar to Barrowman’s method, it just uses mass instead of surface area and volume. Can’t recall that at the moment though.

Check that CG is ahead of CP by 1-2 calibers (body tube diameters) and adjust accordingly. After that, you’re ready to go!
 

mcgunsmith

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As promised a few pics of my dusty stash. Packing up your "stuff" for temporary (?) storage is the pits man, I'm here to tell ya. Only one missing fin so far, a canard off the PML Bullpuppy.
 

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mcgunsmith

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Welcome back!

By “balancing” I assume that you mean measuring CG and calculating CP (center of gravity and center of pressure, respectively) to get stable flight. OpenRocket is great for this, but if you want to do it the old-school way, just plug the relevant dimensions into the Barrowman equation to get CP. Be sure to check both the full stack and the sustainer on its own, you don’t want an unstable multistager to stage and then make a stable flight directly into somebody’s eyeball.


CG is pretty easy, just balance the complete rocket (with fresh motors, wadding, etc) and mark the balance point. Yes, both configurations here too! There is a mathematical way that’s similar to Barrowman’s method, it just uses mass instead of surface area and volume. Can’t recall that at the moment though.

Check that CG is ahead of CP by 1-2 calibers (body tube diameters) and adjust accordingly. After that, you’re ready to
Welcome back!

By “balancing” I assume that you mean measuring CG and calculating CP (center of gravity and center of pressure, respectively) to get stable flight. OpenRocket is great for this, but if you want to do it the old-school way, just plug the relevant dimensions into the Barrowman equation to get CP. Be sure to check both the full stack and the sustainer on its own, you don’t want an unstable multistager to stage and then make a stable flight directly into somebody’s eyeball.


CG is pretty easy, just balance the complete rocket (with fresh motors, wadding, etc) and mark the balance point. Yes, both configurations here too! There is a mathematical way that’s similar to Barrowman’s method, it just uses mass instead of surface area and volume. Can’t recall that at the moment though.

Check that CG is ahead of CP by 1-2 calibers (body tube diameters) and adjust accordingly. After that, you’re ready to go!
Thank you for the reply! Question: if I try the swing method on a finished, loaded kit, with the line attached at the CG, and everything balanced correctly, it should fly around in a stable condition at or near zero angle of attack, correct? And, that if the CP is NOT far enough aft of the CG, the model will fly in a visibly unstable attitude.
 

smstachwick

LPR/MPR sport flier with an eye to HPR and scale
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Correct, although the swing test does have its limitations. Low airspeed and the forces associated with circular motion can sometimes produce inaccurate results. It’s also limited to pretty small rockets, of course.

But a combination of it and other methods will give you a good idea what kind of stability you’re looking at. Contradictory readings may indicate neutral or slightly negative stability, while corroborating readings will indicate strong positive or negative stability.
 
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Rob Campbell

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As promised a few pics of my dusty stash. Packing up your "stuff" for temporary (?) storage is the pits man, I'm here to tell ya. Only one missing fin so far, a canard off the PML Bullpuppy.
And a Ringmaster thrown in for good measure!
 

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