Love kits with pre-slotted body tubes and through the wall fins….

[BEC]

Or, on a small model, just CA the broken fin (I've done this a bunch of times on the BMS School Rocket — the original one — which has TTW fins on a BT-50-based model. I don't think I've ever had a TTW fin break off at the side of the body tube.

 

[Paul Howard]

Slots compromise the integrity of the tube.

Ok, I'll play along here and bite the hook. My rebuttal: Nope.

 

[K'Tesh]

Attaching the fins to the inner tube and the outer tube makes the fin can final assembly so much stronger that any loss in the outer tube strength, due to slotting the outer tube, is negligible.

Presuming that the end user is competent in their building abilities.

 

[techrat]

The Estes "Green Eggs" kit, at just $17 retail (you can often find it cheaper than that) has TTW Fins, and it's a simple kit that uses a D-motor only (You can also use a C-11, but not E BP motors stock, unless they are composite (Q-Jet) types)... But if you change the motor mount, you can pretty much run anything you like, and for what it is, it's a decent kit.

 

[lakeroadster]

Presuming that the end user is competent in their building abilities.

 

[Initiator001]

AeroTech released the first mass-produced/mass-market thru-the-wall rocket kits in 1990.

They are still available.

 

[BEC]

….gee, Bob, I wonder what kit that is?

Green Eggs, as mentioned above, is another nice model with TTW fins. And Qualman Rocketry has a baffle for that odd tube size (though it’s still not on his web site, so I guess you’d have to ask).

 

[teepot]

If the fin is broken below the tube wall I use an oscillating cutter to get the fin tab out and then make a new fin. But most of the time the fins break at the root. Then it's epoxy time.

 

[Cape Byron]

Small Skill Level 1 kits benefit too. Semroc's 'Aphelion' comes to mind. Lovely little kit. Still have one for sale on Fleabay out of the dozen or so I started with six months ago.

Of course, there's one in the 'mine' box.

 

[tfrielin]

All my favorite rocket builds have been kits that come with pre-slotted body tubes and through the wall fin construction. My first build of this type was an Estes Argent and I thought “hey this is great!” My question is why don’t smaller rocket kits ever use through the wall fins? I realize they aren’t “necessary” in many applications but I still would prefer if given the choice. For example Apogee just announced their new MPR “Quick Draw’” kit but without a pre-slotted body tube and through the wall fins I am just not interested….

The Estes Super Big Bertha kit in this regard has two annoying aspects (I hesitate to use the word "flaws")

To wit: First, you have to cut the slots in your self. I found that an unecessary task to go through---Why didn't it come pre-slotted, like the Doorknob, or the Nike Smoke, both of which I bought at the same time.? And both pre-slotted?

But I dilligently cut them in. As if I had another choice.

But on top of that, the fin construction beat that for added parts and extra construction steps.

Instead of four solid balsa fins, you had a balsa frame to which you had to build up with four or five fin components. So, instead of a fin parts count of four---one single piece fin/four fins--- you had to piece together fifteen or twenty pieces to get the assembled fins.

Ok, I concede This Is A Kit---Some Assembly Required and all, but those two steps seemed a bit like busy work. And, in looking at it from a novice builder's point of view, this extra tedium might be something that turned you off the otherwise enjoyment of model building. Maybe, with kids short attention spans these days...

Did Estes's profit margins really require the lack of pre-slotting here? Or one piece fins? Ok, maybe. If so, I did my part to keep them in business.

But having bitched about the Super Big Bertha as per above, it really is a good flyer and I'm glad I built it. Yeah, those built-up fins are nearly indsetructable when it comes to hard landings.

Best of all, its famous slow take off and ascent perfectly lends it to being a great launch behicle for the strap on AstroCam. Over Thanksgiving, at my relative's farm, I got a great video of a flight from liftoff to touchdown.

So, all is forgiven.

Except, I'm still not sure I'd go to the trouble to build it again.

 

[frogglicker]

I'm a bit late to the party here but I have a question about TTW fin assembly. Having watched a few builds, I've seen a couple of different techniques, and with a larger body tube, I am curious how you do your assembly and what you feel are the pro/cons of each technique:

1. Completely build the motor mount and attach the fins, then slit the back of the BT and push in the MMT assembly
2. Build the MMT and attach it to the BT, then attach fins

Method #1 seems like you can get the most strength out of the MMT by adding your fillets on both sides of the centering rings and the root cord. However, fin alignment might be more fiddly. Also, the strength of the BT where the slit is made is questionable

Method #2 seems like a better method for ensuring fin alignment but the strength of the contact points seems less exact.

I'm only guessing, having not done a TTW build yet, but I'm building a couple of Estes Goblin kits with the Vander Burn upgrade along with the Der Big Red Max (again with Vander Burn upgrade) and would be really interested in your thoughts.

 

[techrat]

Completely build the motor mount and attach the fins, then slit the back of the BT and push in the MMT assembly

1. Build the MMT and attach it to the BT, then attach fins

 

[techrat]

1. Completely build the motor mount and attach the fins, then slit the back of the BT and push in the MMT assembly
2. Build the MMT and attach it to the BT, then attach fins

With option #1, If you don't align your fins precisely, you may find its a PITA to get them to go into the body tube slits after everything is glued in place. That said, many many people opt to do the "fin can" first and widen the slits in the BT as needed, figuring they can fill in any errors with epoxy, fiberglass or whatever, and then sand sand sand and sand some more.

Option #2 does allow you get everything aligned, at the cost of stronger internal fillets. That said, if you leave off the bottom centering ring so you have access to the MM assembly even after the fins are attached, you can goop in extra glue/epoxy to create messy, but still viable internal fillets. After that's all dry, you seal it up with the rear centering ring and it's kind of cross between 1 & 2...

But yeah, there's more than one way to build a rocket...

 

[bjphoenix]

1. Completely build the motor mount and attach the fins, then slit the back of the BT and push in the MMT assembly
2. Build the MMT and attach it to the BT, then attach fins

I've done it both ways, I prefer #1 if the fin design allows it. All mine have been scratch built so I make the fin tabs basically the full length of the fin root to get maximum strength. After the mount and fins are complete then I mark the locations on the tube and cut the slots. Of course the slots extend to the rear of the tube. If a design requires the fins to be located well ahead of the end of the tube then you may not want the slot behind the fins. Glue the front centering ring on the motor mount then glue it into the tube, sliding in the rear ring just to hold the motor tube centered. Then take the rear ring out and glue on the fins. You have access from the back end to create internal fillets. After the internal fillets are done you can glue in the rear ring.
I should point out that through the wall doesn't automatically mean that the fins extend to the motor tube. They might just go through the rube a little bit and have internal fillets.

 

[frogglicker]

Thanks, everyone. I look forward to trying both methods myself soon.

 

[Initiator001]

<snip>

Did Estes's profit margins really require the lack of pre-slotting here? Or one piece fins? Ok, maybe. If so, I did my part to keep them in business.

The answer to the lack of pre-slotted body tube is that, yes, it was done for cost reasons.
This was told to me by one of the Estes management staff when the Pro Series Super Big Bertha was released.
At the time Hobbico was failing and doing everything it could to save money/make more profit on Estes products.

In addition, the SBB was to be packaged in a bag but during shipping a case of the model the body tubes became 'out-of-round' so that's why the model comes in a box.

 

[BEC]

….Bob, I have one in a bag…

As for the discontent over the built up fins: lots of reasons to do this. The main one — they are appropriately thick for an upscale of the original — so they look right — while at the same time not being terribly heavy. This means the model performs much better. This fin approach was introduced (so far as I know) on the Mega Mosquito and it works very nicely there, too.

It may be that multiple sheets of thinner balsa are also less expensive. The certainly would be easier to laser cut, reducing rejects, too.

I do agree it was kind of disappointing to have to cut the fin slots, though.

 

[Initiator001]

….Bob, I have one in a bag…

Me too.

 

[bjphoenix]

The answer to the lack of pre-slotted body tube is that, yes, it was done for cost reasons

I don't mind cutting the slots myself. I use the same piece of aluminum angle that I use for fin marking to mark the slots. For thin tubes like Estes kits I just use a sharp hobby knife. For thicker tubes like LOC cardboard I saw the slots with a Zona razor saw. The slots don't have to be real accurate since fin fillets will cover any gaps.

 

[Back_at_it]

All my favorite rocket builds have been kits that come with pre-slotted body tubes and through the wall fin construction. My first build of this type was an Estes Argent and I thought “hey this is great!” My question is why don’t smaller rocket kits ever use through the wall fins? I realize they aren’t “necessary” in many applications but I still would prefer if given the choice. For example Apogee just announced their new MPR “Quick Draw’” kit but without a pre-slotted body tube and through the wall fins I am just not interested….

I completely agree and would love to see more rockets with slots. The first rocket I built with fin tabs was a the Quest Astra. The old blue and white model. I dumped everything on the table and remember looking at the fins and thinking how great of an idea it was to pre-slot the tube. Today there are a few rockets out there that have them.

Pretty much anything you buy from LOC all the way down to their 1" kits have slots. I think the only kit I've built from them that didn't have them was the old 1" Ultimate. Also the BMS school rocket has slots. I do wish that more kits had them but I understand that it comes at additional cost.

If you look back and some of my builds, I've replaced the stock fins and cut fin slots into the body tube for additional strength.

 

[neil_w]

1. Completely build the motor mount and attach the fins, then slit the back of the BT and push in the MMT assembly

In this case you surface-mount the fins to the motor mount while it is inserted (but not glued) into the body. Then you remove it, apply internal fillets as desired, and glue it into the body. The should generally eliminate the issue of getting the fins aligned perfectly with the slots.

Strength of the body at the rear should not be an issue; it is well-supported by the external fillets and the rear centering ring.

1. Build the MMT and attach it to the BT, then attach fins

This is the default for smaller rockets that are not targeting very high performance and needing ultimate strength.

Both methods work fine.

 

[neil_w]

As for the discontent over the built up fins: lots of reasons to do this. The main one — they are appropriately thick for an upscale of the original — so they look right — while at the same time not being terribly heavy.

I believe that the only time you really see this construction style is for upscale kits, where (as you say) the fins are made thicker than otherwise necessary to retain scale appearance. E.g., Super Big Bertha, MDRM, Mega Mosquito, etc.

Speaking only for myself, I actually prefer kits that require me to do more building and rely less on pre-manufacturing. Pre-slotted tubes are nice but for me not any kind of necessity. The one thing I definitely appreciate are laser-cut fins, especially when either (a) the shapes are complex and hard to hand-cut, or (b) when tight tolerances are necessary for good fit (such as multi-piece fins, or TTW fin tabs).

 

[BEC]

I believe that the only time you really see this construction style is for upscale kits, where (as you say) the fins are made thicker than otherwise necessary to retain scale appearance. E.g., Super Big Bertha, MDRM, Mega Mosquito, etc.

And I followed this approach for my 256% upscale Nova Payloader as well, for all the same reasons: It isn't terribly heavy and and it looks right. With the MDRM (and my big NP) of course the center ply is made from plywood, not balsa as with the SBB or Mega Mosquito. But I expect you know that....

 

[jqavins]

most of them feel that there is no need for them, especially with low power kits, and it also cost them more, so they lose profit.

Yup, that. And lest anyone think I'm grumbling about greedy profiteering rocket kit makers, that's not it at all. They exist to make a fair and reasonable profit. If the increased cost of a slotted tube does not raise the price that the market wil bear for the product by a commensurate amount then they are absolutely right not to do it.

The BMS 3" School Rocket uses TTW and had Plywood fins.

The little original one has TTW balsa fins.

Chances are if the fin is busted, the rest of the rocket is damaged beyond repair.

A TTW plywood finned rocket fin can is a strong design...

But a TTW balsa fin can easily be broken without totalling the rocket.

The Estes Super Big Bertha kit in this regard has two annoying aspects (I hesitate to use the word "flaws")

To wit: First, you have to cut the slots in your self.

Sounds painful.

But I dilligently cut them in. As if I had another choice.

Choice? You could have cut the tabs off and surface mounted the fins.

But on top of that, the fin construction beat that for added parts and extra construction steps.

Instead of four solid balsa fins, you had a balsa frame to which you had to build up with four or five fin components. So, instead of a fin parts count of four---one single piece fin/four fins--- you had to piece together fifteen or twenty pieces to get the assembled fins.

How big are those fins? It could be that cutting them as single pieces would require excessively large sheets. And with grain direction considered (or are they plywood?) there would be correspondingly more waste.

Ok, I concede This Is A Kit---Some Assembly Required and all, but those two steps seemed a bit like busy work. And, in looking at it from a novice builder's point of view...

But it's not a novice kit.

Did Estes's profit margins really require the lack of pre-slotting here? Or one piece fins? Ok, maybe.

The fins, I guess maybe. The tube slots (Remember tube slots? This is a song about tube slots.) maybe not, if they've got the machines set up to slot tubes of the same size for other kits. But even at that, yeah, a little, since it's an extra process step. Four extra process steps actually.

Strength of the body at the rear should not be an issue; it is well-supported by the external fillets and the rear centering ring.

And, if one is concerned about the three or four bits of tube aft of the aft centering ring, one could add a ring of coupler just wide enough to span the unsupported height. This is, I would guess, probably not necessary, and I would probably do it anyway.

 

[techrat]

To wit: First, you have to cut the slots in your self. I found that an unecessary task to go through---Why didn't it come pre-slotted, like the Doorknob, or the Nike Smoke, both of which I bought at the same time.? And both pre-slotted?

But I dilligently cut them in. As if I had another choice.

But on top of that, the fin construction beat that for added parts and extra construction steps.

Instead of four solid balsa fins, you had a balsa frame to which you had to build up with four or five fin components. So, instead of a fin parts count of four---one single piece fin/four fins--- you had to piece together fifteen or twenty pieces to get the assembled fins.

After building both Super Big Bertha and an Estes Star Orbiter, I think I can answer this ---- my guess is that Bertha uses two IDENTICAL BT-80 tubes you need to join together to create the body. And if you build the Star Orbiter, you'll notice it uses two BT-60 tubes you have to join together, but they are NOT identical. One is white and the other is brown. And the brown one is pre-slotted. Ready to guess what happened in SBB early prototypes?? Both tubes in the kit may have been accidentally pre-slotted. I'd be willing to bet a steak dinner that with no way to differentiate the two BT-80 tubes, some kits were getting two pre-slotted and others were getting none. So Estes nixed it, and said cut the slots yourself.

I did cut my own slots in Bertha and I did build up the fins. And I found the process enjoyable, because I got to build it my way. Rather than just assembling pre-cut pieces, it felt more like real assembly. Almost as enjoyable as when I scratch build. Overall, quite happy with how my super bertha came out.

 

[Initiator001]

After building both Super Big Bertha and an Estes Star Orbiter, I think I can answer this ---- my guess is that Bertha uses two IDENTICAL BT-80 tubes you need to join together to create the body. And if you build the Star Orbiter, you'll notice it uses two BT-60 tubes you have to join together, but they are NOT identical. One is white and the other is brown. And the brown one is pre-slotted. Ready to guess what happened in SBB early prototypes?? Both tubes in the kit may have been accidentally pre-slotted. I'd be willing to bet a steak dinner that with no way to differentiate the two BT-80 tubes, some kits were getting two pre-slotted and others were getting none. So Estes nixed it, and said cut the slots yourself.

As I mentioned in an earlier post the lack of tube slotting was a cost-saving measure.

 

[teepot]

I have to cut slots in every rocket. I use a Dremel with a cut off wheel. I can slot a heavy LOC tube for 4 fins in a minute or two.

 

[Dane Ronnow]

I've never had a problem cutting slots in my scratch builds. I use an aluminum angle and a box cutter with a new blade. I make two or three passes and get a nice, clean cut every time.