Removable Fins

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May 3, 2004
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Hi All!

I'm returning to model rocketry after a long interruption to get some schooling in, get married, and start a family. Well, now that my oldest is getting ready to start driving, I need to get back into my old hobby.

This looks like a great site, and I hope you can answer my question. I'm building a scale-like Delta II rocket (has 9 strap-on boosters for looks) and I'm wondering what to do about fins.

First off, would it be stable enough with those boosters acting as fins? If not, then I'd like to add removable fins. I've seen options for bolt-on fins, but it'll be really tight in there w/ the boosters. I've also seen where people have used fins mounted to a removable ring, but there again I have the boosters to contend with.

Am I overlooking the obvious? Please help!

An alternative is for fins that slot into the base of your rocket. The whole fins set is then aft of where your rocket would normally end. It's a good way of hiding the fixing points, particularly for scale models such as yours.

Thanks for the quick reply! I'm not quite sure I follow though. Are you talking about a complete fin/engine mount unit that gets added to the end of the rocket? How is it secured?

Thanks for responding...
sometimes it's easier to use a detachable clear fin unit. Either built from scratch or one of the old estes units Here's one of the later units I used on a Mercury Atlas. really makes for a very clear looking model. If memory sures the fin unit and mount are from an Estes "Beta launch vehicle" which is the same unit used on the OLD gemini-titan models BT-60 model.
I've seen many people use Lexan for fins who want to have a rocket with a finless appearance. The clear Lexan virtually dissappears at a distance.
Originally posted by bobt85

Thanks for the quick reply! I'm not quite sure I follow though. Are you talking about a complete fin/engine mount unit that gets added to the end of the rocket? How is it secured?

Basically, yes. Though the motors are still in the main body section. Micromister explained it better than I did!
Ya mean like this Mark;)
the fins on this crayon are epoxy rivit attatched directly to the exterior of the model (no thur the wall) than overlayed with a 1/4" epoxy fillet. many flights I've only cracked one fin with a tangled chute hard landing:)
Originally posted by MarkABrown
I've seen many people use Lexan for fins who want to have a rocket with a finless appearance. The clear Lexan virtually dissappears at a distance.

Lexan is also good if the scale prototype has undersized (for stability) fins. You can replace the undersized fins with lexan fins that have the outline of the original fins painted on. Like these:
Heres a look at a set of .032" clear lexan fins on a Vanguard TV-4 Scale model. Same basic attachment method using a pin hole rivit instead of the usuall 1/16" hole and 1/8" epoxy fillets.
Hope this helps
I did some simulation work on the Delta E (see attached RockSim version 7) file. This simualtion predicts that you should be able to make this design fly stable with only the three side pods. If you send me the details and dimensions, I will try to siumulate your Delta II design with all nine strap on boosters.

In lieu of flat fins you could make a clear conical transition shroud that fits to the base or around the base of the rocket. You can fabricate the cone from a 2 liter clear soda bottle.

Bruce S. Levison, NAR #69055

I think you did that for me.

It worked great but I flew can't take pics and fly at the same time...

Why not create 3 small clear fins and attach them to the boosters directly ?

In looking at my own DMF3 Delta 3 drawing, found below:

if these were attached to boosters 4,5 and 6, you'd have plenty of stablization, and they could still fall off at ejection, if you used Ray Dunakin's booster harware.

I'm not sure what scale your building, but as long as they protrude out 2 to 4 inches from the body, you "should" gain enough stability.

Bruce would be one of the guys to verify this, being a Rocksim gurus and all..8)

Hope that helps,
Originally posted by bobt85
Hi All!

First off, would it be stable enough with those boosters acting as fins? If not, then I'd like to add removable fins. I've seen options for bolt-on fins, but it'll be really tight in there w/ the boosters. I've also seen where people have used fins mounted to a removable ring, but there again I have the boosters to contend with.

Am I overlooking the obvious? Please help!


Is there any space at all between the strap-ons? If there is, you might be able to fabricate some fins with flanges at the root that slide down from above the strap-on noses, with the flange held in the gap between them and the main body.

This would require that the strap-ons holding the fin be attached securely enough to act as the fin root, and have something between them near the bottom to keep the fin from sliding all the way down and out. Perhaps, hidden back in behind the strap-ons, you could devise some pockets on the main tube to hold the bottom of the root in place.

The flange would also have to fit tightly in the gap or you'd get flutter. Perhaps if the flange were flexible, using spring pressure of the flexible flange to help hold it in place?

OR: threaded nuts glued into the bottom of the motor mount, threaded rod longer than the main engine shroud, with one or two fins attached to each rod. Screw them in before flight, remove after. Go for nylon so you're not adding too much rear weight.
Great ideas, all. Thanks for your input.

Here's what my brain was chewing on last night: What if I had slotted tabs on the fins that went through the wall of the rocket? The fins would be inserted in and then up. Blind nuts in the rear-most engine mount ring, with an additional movable ring below it to "sandwich" the lower tab between it and the eng. mnt ring. This would keep the fins from falling out.

When I'm not flying, the only thing that would look out of the ordinary would be the fin slots, and they are between boosters -- only 1/8" gap there, so not too visible.

I'd really like to steer away from permanently attached fins.

For those of you wanting details, Rocket length, ~22", BT55 main tube, BT-5 boosters. The whole idea came from te_groen31's paper model of the Delta II. The detail is superb! I was concerned that paper would be too flimsy, and checked into sizes for body tubes. The BT55/BT5 are almost perfect! Suddenly I was looking at building a flying model. The rest is history...

Check out for this and other great paper models.
My feeling is unless the tube walls are reinforced well your attachment may be on the weak side.
I'd suggest at least the following:
reinforce the outer body tube with CA or epoxy and or a second layer on the interior surface.
Extending the tabs to sit firmly against the motor core tube, sliding between balsa vertical strips to prevent side to side movement.
Reverse the slot direction to seat in the downward postion, while devising some type of a forward pin or locking shim.. something to keep the fin tabs in the down/locked postition druing ejection and landing.

Hope this helps
bobt, your basic design approach could work just fine, especially when combined with some of Micromister's suggestions

Here is one more idea (to thoroughly get things confused). You could create a strip to wrap around the outside of the main BT from the sidewall of a clear soda-bottle. Diameter would be just large enough to slip over the nose of the rocket and slide down into place without scratching up all your paintwork. Width would be wide enough (two-three inches?) to provide a secure base for the plastic see-through fins, which would install into the same location as you showed in your diagram. That way the flight fins could be secured to the ring as solidly as you want, you would not need to screw anything in to lock the fins (you could use a small piece of cellophane tape to stop the ring from sliding forward), and you would avoid a bunch of slots.

Don't ya love it when everybody shows up and tells you what to do?

P.S.--Where are my manners?! Welcome to TRF, bobt85! We are happy you joined in here, and decided to take up rocketry again. Hope you have lots of fun launching and posting.
Well...time for me to pipe up. First post here on this message board, but I've been a long-time flyer - 31 years - with a short break over the past 3 1/2 years due to family and job considerations.

That said, this thread caught my eye rather quickly as I was perusing the board, since I have some experience with the Delta II. I've built a BT-55-based version, like bobt is planning, a BT-80 version with a core 24mm motor (D12) and 13mm motors in the boosters (A10's), with the boosters falling off at intervals. Before I stopped building and flying, I started design and construction of a 4" core version, using a 29mm mount in the core and 24mm mounts in the boosters.

I was glad to also see Ray's photo albums for his strap-on booster setup, since I had basically the same idea. Nice to see that I may not have to build the booster mounts from scratch like on my earlier versions.:D

Okay...time for some photos. This first one is the completed 1st boilerplate version on the pad. It's not apparent in the photo, but the fins are made from 0.080" Lexan or similar, mounted TTW on the core tube. They are covered in masking tape to prevent scratches, in case I decided to re-use them for the final model.
This pic was taken during booster prep. Sorry about the quality of the pic, but if you look closely, you can see how similar my strap-on solution is to Ray's. The core engine bell was made by cutting up a Big Bertha nose cone - it's VERY close to the correct shape - and has plastic strips glued around it to simulate the cooling tubes for the real engine bell. The booster bells are simply cardstock cones, reinforced with CyA.

You may also notice that the booster motors are canted inwards 10 degrees. This was done to not only achieve a scale look, but also to help prevent looping flights in the event that one or more booster motors failed to light. The cant pointed the motor thrust lines at (or close to) the model's CG. There was an article about this in an old issue of HPR magazine, from around 1998 or so.
Another view of the booster end of the model. The angled booster motors and detail on the main engine bell is more apparent here.
Liftoff of the first boilerplate. I managed to get all 10 motors to light. The flight was good, with all boosters coming off cleanly, and staggered separation like the real thing. This was accomplished by using A10-0's to pop off the early seps, and A10-3's for the remainder. It was a very low and slow flight, reaching about 250-300 feet. The model was very heavy, weighing in at a bit over 2 pounds.

Recovery wasn't so good. I had accidentally installed a D12-5, when it should have been a D12-3, in the core. It hit the ground at about the same time as the ejection charge went off. Obviously, the boosters were fine, as was the lower half of the core, except where the force of the impact ripped the fins out of their TTW mounts, tearing the tube. The nose section was completely accordioned, looking a bit like Wile E. Coyote after a nice fall.
Version 2 of the boilerplate at liftoff. This version, as you'll notice, has small fins on each booster, as well as smaller core fins. This, combined with lighter construction techniques and a corresponding reduction in nose weight, reduced launch weight by about 8 ounces. Even though the boosters from the first flight were okay, I completely rebuilt them to accommodate the fins. All the fins on the second model were a thinner plastic, which also helped reduce weight, especially at the tail end.

As you may have already noticed, not all the motors lit on this second flight. In fact, only 5 A10's on one side lit, although it looks fairly even in the photo. The D12-3 never lit either. This was where the 10 degree cant to the booster motors really shone, as the model only achieved an altitude of about 9-10 feet, with no evidence of wanting to loop due to off-center motors firing. The model actually hovered for a bit, with about a foot of sideways "crabbing", which would be expected with the angled motors firing on one side only.

At burnout, the model dropped straight down, butt first, landing on the core motor mount and shoving it up into the body tube. The same thing happened to the boosters that did not light, since they had no way of dropping off.

While both flights were successful in terms of testing the separation mechanism, the angled booster motors, the stability of the design, and other things, I was very discouraged. I had spent the past 6 months building two very complex models, and neither one was still flight-worthy. Not to mention the countless hours spent on the computer tweaking RockSim files, drawing up templates and patterns, and other stuff. That second Delta flight was my last flight, taking place in late summer 2000, I think. I worked the next couple months on designing a BT-101-based Delta II, but then a harddrive crash lost all my CorelDraw drawings, RockSim files, and other things. This, combined with the aforementioned family and job considerations, caused me to leave rocketry until a week or two ago.

I still haven't unpacked anything, but I've been checking out a lot of the links I used to go to, and discovered this forum. So, here I am, for what it's worth...
My 18mm, BT-55-based Delta II (far right, not on the rack) at a demo launch at the Neil Armstrong Museum in Wapakoneta, Ohio in July, 1999. I believe that is Peter Alway's vintage 1/70 Saturn 1B in the van in the background. A couple of his models are on the rack, but I can't recall which ones.
First, and only flight, of the BT-55 version. The parachute didn't deploy and it landed on someone's truck, leaving a little of the model's paint on the hood. This model has three balsa fins, but I had plans drawn up for a different version using clear fins on the boosters.
Howdy Maxx,

First off welcome to the forum, and thanks for sharing your details of the Delta rocket. Looks like a lot of work went into it but it certainly looks great with the canted motors. 8)

The Delta 3 is my favorite in the Delta series, given its classic booster lines, with larger upper section.

Excellent pics though, really captured those engines well at lift-off. 8)

Wish I could take credit for the pics. The ones of the first model were taken by Larry Rice, Pres of NAR Sec #113 here in Columbus. The pic of the second model was taken by another club member whose name I can't recall.

As for the Delta III, I've got the Apogee kit, unbuilt, and have been thinking about seeing if I could do one in a similar scale to my 2.6" version...maybe someday.

Thanks for the input. I admire you for making the Delta with lightable boosters -- I'm too chicken. Anyway, I opted to mount the fins TTW, butted up against the engine mount with slotted rings to help stabilize them. I also wrapped the main tube with a thin layer of fiberglass to strengthen it. The whole assembly is very rigid...we'll see when it comes to flying. As the Dad of 4 very active boys, I'm running this way and that, and don't have much time for modelling right now. Hopefully have something soon.


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