Converting kit to MD

Hey everyone.

I picked up a Scarab 54 from Rocketry Warehouse during their recent sale. My buddy has challenged me to rebuild as 54mm minimum diameter. Since I've never attempted anything like this, I thought it would be prudent to ask for some advice.

My first thought is to trim down the fin tabs to the thickness of the body tube. Then using a 54mm case, wrapped with wax paper, as a mandrel, epoxy the fins into the slots. Apply fillets, then a couple of layers of tip-to-tip glass.

Does this seem like a reasonable plan?

Kevin

That is sort of what I'm planning for my MD 54mm scratch build. I did the same with a motor case, but with mylar instead of wax paper, for my non minimum with through wall in the fin can and motor mount tube.

With good fillets using a good quality epoxy you shouldn't need tip to tip. Unless you want the practice.

Just went to the site and looked at the kit, fins are only 1/16 thick, if you went up to 3/32" thick you could get away without tip to tip. If you use the stock fins you will need tip to tip to increase the thickness of the fins.

I want to build up a little shape to the flat fins at the same time. So tip-to-tip seems like a good plan.

A preliminary design in OpenRocket shows I'm going to need 10oz or more of nose weight.

maxvelocity said:

With good fillets using a good quality epoxy you shouldn't need tip to tip. Unless you want the practice.

Just went to the site and looked at the kit, fins are only 1/16 thick, if you went up to 3/32" thick you could get away without tip to tip. If you use the stock fins you will need tip to tip to increase the thickness of the fins.

Click to expand...

I bought a Wildman Shape Shifter Jr and plan on doing the same thing. Cut down the tabs and use nice big fillets with good epoxy.

I have CTI 54mm hardware so I'm just going to use CrazyJim's method of mounting an eyebolt into the charge well to attach the shock cord.

https://www.rocketryforum.com/showthread.php?119921-Putting-an-eyebolt-into-Pro38-forward-closure

In a similar thread, somebody suggested that fins with TTW tab removed and glued in the body tube slots were not as strong as typical surface mount fins. Also, fins designed for TTW are typically thinner, like the 1/16 you have here, thus reducing the strength even more. That all makes sense to me. You probably won't have any problems since you are fiberglassing, but there are risks in converting TTW fins to MD fins.

kruland said:

Hey everyone.

I picked up a Scarab 54 from Rocketry Warehouse during their recent sale. My buddy has challenged me to rebuild as 54mm minimum diameter. Since I've never attempted anything like this, I thought it would be prudent to ask for some advice.

My first thought is to trim down the fin tabs to the thickness of the body tube. Then using a 54mm case, wrapped with wax paper, as a mandrel, epoxy the fins into the slots. Apply fillets, then a couple of layers of tip-to-tip glass.

Does this seem like a reasonable plan?

Kevin

Click to expand...

Kevin, Your plan is a good one. I have a whole stable of rockets where I threw away the MMT and assembled in the manner you describe. I usually use JB Weld for the initial tacking in of the fins. You do not want thin epoxy smooging around inside the BT and JB Weld works for me on that.

I'm not familiar with the kit you have, but be sure you're accounting properly for the distribution of the motor weight and the removed MMT parts. I hate nose weight.

BTW, those extra parts give you a good start on a motor adapter. I built one with a tall cone that is pretty slick.

--Lance.