A few questions about building techniques

[MikeyDSlagle]

I like to try different methods when building just to see if there is a better, and hopefully cheaper, way to do things. My latest comes in the form of motor retention. I am not a big fan of friction fit, though admittedly I have never used it. The commercially available retainers are too pricey. So I am leaning toward the kaplow or kaplaw clip retention method. It seems a pretty straightforward method. With the screw on motor retention, you need to leave "X" inches (3/8 or so) of the MMT hanging out the rear CR. With the clips could you not move the MMT in farther, perhaps flush? Possibly even recess the motor a smidge to give it a cleaner look. The motor is going to stick out a bit anyway. Any ideas on this? I want to move my rear CR flush with the tail of the rocket but don't want my motor and all that hardware sticking about an inch. I get that I will still see the clips and what not but not as bad.

Another question. Foamed fin can vs internal fillets. The foam stuff is pretty tough. Would it save weight to use foam rather than epoxy for internal fillets? Internals can be a real PITA at times and on minis, dang near impossible. On paper/wood models wood glues works well for internals so I don't see foam being much lighter or very beneficial there, other than for smaller diameter rockets that you can't work in the fin can like the minis.

Think that is all for now.

Edit:
As a side note. Anyone flew a Madcow 4" Cowabunga or Tembo for level 1 cert? What kind of altitude can be expected on the smallest HP motor in 29mm, I will have to get or build an adapter. Or even 38mm. I just don't want to lose the thing. I am asking for future reference as I am not ready for my L1 yet. (and then I can ask again when I am ready)


Thanks guys,
Mike

 

[rharshberger]

Lots will have opinions on what you are asking, so here is my .02:

If the Cowabunga or Tembo are possible going to be foamed, foam adds weight at the wrong end of the rocket on short fat models like those.

Kaplow Clips are inexpensive and work well, they just require tools to put on or remove, but I still use them on the majority of my 38 and 54mm motor mounts. The rear CR can be either flushed or recessed for clips, though the same can be done with some retainers depending on the diameter of the airframe vs. the diameter of the motor tube.

Foam: not really necessary on a L1 or L2 rocket, you can if you want but epoxy will probably weigh less, and wood glue will weigh even less. If the project is wood and cardboad then go with wood glue as much as possible. Epoxies are easier to get good external fillets with.

An option to fighting internal fillets is to inject them (lots of threads about that by blackjack2654 aka Crazy Jim), I do the injected fillets sometimes when space is tight like a 54mm MMT in a 3" rocket.

Fin Pockets (I have a thread devoted to this one) are another method of doing internal fillets.

A good source of Kaplow clips is to either make your own from aluminum angle or mirror clips from the hardware store, screen door screen clips also work well. The hardware store is a good place to go for inspiration especially if they have an entire aisle devoted to just fasteners (the type in the hinged lid boxes).

The left and center pics are the same rocket, 54mm MMT visible in center pic on left side (38 to 54mm adapter is on right in same center pic), Left picture is same fin can with 38mm motor casing and 38-54mm motor adapter and kaplow clips mad from 1"x1"x1/16" aluminum angle. The right pic is from my L1 Crayon rocket with a 38mm MMT and kaplow clips.

 

[RocketFeller]

I like to use a couple of T-nuts and an aluminum retention ring. I agree about a flush aft centering ring, in my opinion it looks a lot more "finished" than a mmt sticking out the back.

 

[Banzai88]

I recently completed a Cowabunga. https://www.rocketryforum.com/showthread.php?131499-Bullet-Bill-the-Cowabunga

Be aware that the Cowabunga can require a good bit of weight in the nose. I put a little over 9 ounces in mine to be stable with any 38mm 3G/360 motor to at least 1.5 calibers, but you may not want quite as much as this taking base drag into account and what maximum on motors you want to impose on yourself. I wanted something that would top out about 3Kish feet and NEVER have to worry about stability, since short stubbies have a notorious reputation.

If you move the motor aft, you're going to have an effect on the CG, so check your changes in OR, and as an aside, the model from MadCow is WAY off of where parts go and how much they actually weigh.

A baby H 38mm is going to get you 2400ish feet AGL, a 29mm with adapter a little less due to the extra weight. Remember to check your sim with the adapter weight when using a 29mm motor. You'll drift quite a bit if you open the main at that altitude, but it should be easily visible assuming you don't paint it sky blue or haze gray. (I flew a Leviathan to over 2400ft on a baby H for my cert and it drifted over a 1/4 mile down wind).

 

[DavieRockets]

Mirror clips work great, are available at any hardware store and are cheap (Like $1.59 for 4). You can use PEM nuts, T-nuts or glued-in machine nuts on the centering ring.

 

[MikeyDSlagle]

I am not going to foam either of those rockets I mentioned. That is a big hole to fill. Was just seeing what kind of altitudes I could expect is the only reason I mentioned them at all. And BonzaI answered that. I will sim them of course when I get them completed and get a final weight. 2400 feet is plenty high enough. I don't imagine I will do that often until I can get me a chute release or some such. I would like to keep my rockets low when possible, for now.

I've read other threads about kaplow clips and got the jest of it, I have some flat bar to use, I was just curious about position on the rear CR. If the motor is moved at all, it will move forward a bit, not aft.

I like the aluminum ring and have tried to cut one myself but was unsuccessful which has led me to the clips. I esp like the way the motor on the orange and black rocket looks. Nice and clean...that's what I want. I have some ideas justo gotta get on it.

I have unable to find the mirror clips locally. I will look again next day off when I have more time.

 

[RocketGeekInFL]

There was an article in Sport Rocketry not too far back that used a technique of soaking fiberglass cord in epoxy and then laying into the wedge for an internal fillet. This is a technique I have yet to try, but I am thinking about trying on a Madcow Tomach I have in my build queue.

 

[BDB]

@Dan (RocketFeller) - Where did you find the aluminum rings? They look great!

 

[RocketFeller]

@Dan (RocketFeller) - Where did you find the aluminum rings? They look great!

I actually cut them from scrap on a drill press. I screw the piece of scrap to a board, then cut the ID and finally the OD.

 

[MikeyDSlagle]

I didn't read the article but I have thought about doing something like that with either balsa, fiberglass or kevlar. For external fillets I use epoxy, usually with talc as a filler. I am building an order with us composites to get me some of their epoxy and fillers. They also have an epoxy glue, looks like rocketppxy to me, that I'm gonna try.

 

[BDB]

I actually cut them from scrap on a drill press. I screw the piece of scrap to a board, then cut the ID and finally the OD

Sorry to keep hijacking this tread....Dan, do you use your drill press like it's make-shift mill, or do have an adjustable circle cutting bit that is specific for Al?

 

[RocketFeller]

Sorry to keep hijacking this tread....Dan, do you use your drill press like it's make-shift mill, or do have an adjustable circle cutting bit that is specific for Al?

I actually used hole saws for most of them. I have cut thin aluminum with a standard circle cutting bit, the trick is to use plenty of oil.

 

[MikeyDSlagle]

When the aft CR is brought out flush, it doesn't touch the fins. Is that an issue? I wouldn't think so but thought I would ask. I already left enough MMT sticking out the back for a screw on retainer but that's not an issue. I can trim it if need be.

 

[RocketFeller]

When the aft CR is brought out flush, it doesn't touch the fins. Is that an issue? I wouldn't think so but thought I would ask. I already left enough MMT sticking out the back for a screw on retainer but that's not an issue. I can trim it if need be.

I wouldn't think it would make any difference except in extreme cases. If your fins are tearing lose from the MMT you have pushed the rocket pretty danged hard!

 

[MikeyDSlagle]

Yeah kinda what I was thinking. I got Titebond 2 internal fillets on the MMT and BT joints and I'll have epoxy externals. Shouldn't be an issue. I was able to fiddle around with the Cowabunga a bit today and made some (pretty rough looking) clips. Wasn't bad at all. Will look better when I get mores time to fool with em.
I'm still playing around with different ideas on the rear CR and retention.

Thanks for the input folks
Mike

 

[MikeyDSlagle]

Mirror clips work great, are available at any hardware store and are cheap (Like $1.59 for 4). You can use PEM nuts, T-nuts or glued-in machine nuts on the centering ring.

View attachment 282768 View attachment 282771 View attachment 282772

This center pic is nice and clean. Very nice.