TTW fins with clustered mounts

[Landshark]

I've got the itch to build a three fin BT-80 based cluster rocket (cloning a certain OOP "obese" model) with either 3 x 18mm or 4 x 18mm mounts.

I'd like to do through the wall fins like the original, but I'm having a hard time figuring out the best way to attach them to the motor mount inside since the motor tubes will be an "irregular" mounting surface.

In theory, I guess I could probably make templates out of cardboard and custom make each fin's TTW tab extend to a good spot on the motor mounts although each one tab would probably be a different length and I'd be worried that the joint might not be the best on some of them. I did something like this on my Super Goblin upscale, but that only had 2 x 24mm mounts and four fins and was easier to do.

The other idea would be to mount the engines inside a smaller diameter tube (BT-70?) then try to center that inside the BT-80, thereby making a regular mounting surface for the fins. This would add some weight to the motor mount but would allow for nearly identical fins and solid attachment.

Any thoughts?

 

[Lentamental]

If you have a three fin rocket, and use a three engine cluster, you can have regular fin tabs. It is your choice whether you shoot for the cracks between the engines, or go for the extremities (I would go for the cracks).

Clustering with TTW fins is easy as long as you have as many motors as you have fins.

 

[troj]

If you have a three fin rocket, and use a three engine cluster, you can have regular fin tabs. It is your choice whether you shoot for the cracks between the engines, or go for the extremities (I would go for the cracks).

Agreed, especially on going for the spots between the motors.

-Kevin

 

[powderburner]

Another way to get good access to the "innards" when doing TTW construction is to dry-fit-only the rear CR when installing the MMT. Leave the whole thing alone for long enough for the fwd CR to set completely, then remove the aft CR and go ahead with your TTW fin tabs. As a last step, apply plenty of adhesive and reinstall the aft CR.

When pulling the dry-fitted aft CR, you can use a bent wire hook to pull through the slots for the motor clips. Or, you can use a hole-punch to make a couple strategically-placed access holes around the edge of the CR and hook through those holes to pull the CR out. (The fwd CR will still give you a gas seal for the ejection charges.)

 

[JRThro]

I've got the itch to build a three fin BT-80 based cluster rocket (cloning a certain OOP "obese" model) with either 3 x 18mm or 4 x 18mm mounts.

Since the Fat Boy has/had 3 fins, you probably ought to go with a 3-motor cluster, and do as Lentamental and troj suggested.

 

[jj94]

I agree with the above comments. I'd go for three outboards simply for the ease of TTW fin installation. And powder's comment with dry fitting the aft CR is a great tip. I use this exclusively on my TTW rockets, and the great thing is that you get full access to the innards and have all the time you want to work with it. You can get much better fillets in there as the glue can dry better and you can get the glue in the spots you want it a lot easier AND you have more than one chance to do it. But...if you don't have a center tube, then there won't really be much for the TTW tabs to join to. So for this reason, I'd like to suggest adding a center motor tube whether it be 18, 24, or 29. It opens up a lot more flight configurations for you too. Happy building!

 

[powderburner]

If you are "cloning" a design, and cutting your own fins anyway (as compared to using a kit with pre-laser-cut fins), just cut the root tabs to extend all the way to the rocket's centerline. Join the three fins together at the center.

Nest each of the MMTs into a 120-degree corner formed between fins. The MMTs won't be touching, but there is no law against that. The MMTs would each have a rock-solid attachment to two fin tabs and two CRs.

You would have to cut (or order) a slightly custom set of CRs to spread the MMTs this way.

 

[Landshark]

Thanks for the advice... Seems like three 18mm might be the way to go (I've already got a single 24mm FatBoy from my first BAR episode 12-13 years ago).

I also like the idea of an "interchangeable" mount like on the Semroc SLS Laser-X. It's got an interesting system where there's a central tube and comes with a few mounts that are keyed / lock into the main tube. Like this (shameless snip from Chan's review of the product on EMRR's site):

I don't have one of those models, but essentially, if I understand correctly, you can interchange between a single 29mm or other combinations of smaller engines. I like the idea of having flexibility of running anything from one 18 all the way up to 4 x 18mm or whatever.

Anyone ever done this on a project? On this one, the idea could be to use CR's to center a central BT-70 (or whatever) tube into the BT-80 of the FB then build the mounts (for BT-70) and figure out a retention system. The fins would attach to the central tube with TTW techniques...

 

[new2hpr]

If you want 4, put one in the center and the 3 around it. Gives you the option of 1,3,or 4 motor flights. Better yet, just do it right and put 6 around 1, like nature intended. Bees had the right idea.

-Ken
(cluster junkie)