Help with Starship Excalibur fin strength

[bcook7135]

I got parts, and I am building a Starship Excalibur (where Estes released only plans and Semroc made an actual production model that has since been discontinued).
I am very concerned after gluing the pods on the fins. The fin stock is 1/8" but very light weight. The pods are well glued but the fins wobble very easily with the pods attached with pressure on the outermost tip of the pod. Seems like they might be destroyed in a launch. Is there a way to stiffen the fins after having the pods attached?

 

[neil_w]

A bit of flex on an assembly such as that is not surprising. Which joint feels wobbly, the fin-pod attachment, or the fin root attachment?

Also note those pods are swept forward so they should normally not be absorbing much or any landing shock.

 

[bcook7135]

Well, it is not the joint that feels wobbly; the whole fin flexes, but more so at the pod-fin attachment. And, my concern is at launch - it just seems that the launch pressure might rip the pods right off.

 

[neil_w]

Ah. Well, I would tend to think (no guarantees of course) that if you stick with lower-thrust motors (i.e. Estes BP) then it's probably OK. A standard way to strengthen fins like that would be to paper them, but I'm not entirely sure how you'd do that with those fins already in place. If the pod/fin attachment is solidly glued and filleted, it *should* stay together.

I would just stay away from the high-thrust Q-jets.

 

[bcook7135]

Oh yes. I will probably stick with Estes C6.

 

[GlenP]

I would say this falls in the category of a proven design that should work if you follow the standard building techniques and stick with the recommended motors. However, there is always some variability in balsa stock, and you might have a sheet that is a bit more flexible than perhaps what feels suitable. You could possibly paper the fin, or one side of the fin, since it is already built and you can't go with a thicker balsa sheet or a more rigid sheet. Alternately you could soak some CA glue along the leading and trailing edges of the fin to increase the stiffness somewhat, and maybe along two thin diagonal stripes from the fin corners in an X pattern, just to add a bit of strength, probably don't need to soak the whole fin with CA. The paper layer on one side, or both, will help more with resistance to fin twisting, it would have been much easier before the model was built, but not totally impossible now, though the fin pod will make it a challenge.

If you are really concerned with strength, and might want to go to higher power than the stock kit is rated for, you could add an airfoiled strut from each fin mid span near the LE to the adjacent fin, or a thin ring fin that secures them all together to ensure they can't twist due to wing-tip pod loads.

 

[bcook7135]

Thanks! yes, I had considered the CA option. I still may try it. I just did not realize the low rigidity of the balsa stock when I started building.

 

[David Schwantz]

1/2 oz glass cloth on both sides. 1/64th ply on one side only. Another 1/32 balsa sheet glued on top of fin with grain crossing.

 

[kuririn]

Fins will flutter a bit with this kind of design. Same thing with the Mars Snooper.
Looks like you have the joints well filleted.
Stick with the recommended motors and you should be OK.

 

[bcook7135]

Wow! Mars Snooper... I built one pf those when I was a teenager. It was heavy and I hated the stilts under the pods - weak at landing. Didn't fly it much. But it looked good.
I have recently built another Estes Interceptor. Its pods point backward. But, the balsa stock, even being 1/8" was very strong and I have no flutter.
Thanks for the advice.

 

[bcook7135]

1/2 oz glass cloth on both sides. 1/64th ply on one side only. Another 1/32 balsa sheet glued on top of fin with grain crossing.

so, is this three different methods?

 

[David Schwantz]

Yes.

 

[Sandy H.]

Depending on your current skillset and/or your desire to learn a new technique, I was amazed at how well really thin fiberglass can be applied and how little weight is added vs. the amazing strength increase. I'm not trying to say to overbuild, I'm suggesting using techniques like the FAI modelers do, RC Sailplane guys etc. I learned from an RC glider guy and if done properly, it is amazingly light and stiff.

Having said that, it takes a while to get the hang of it and a major part of the skill is not having too much resin, so its very easy to either get bad results or to make it heavier than it should be.

The pic below might not make sense, but it was a test piece I did. I am trying to break it in half by pushing my thumbs in and the goal was to show all the blood pushed out of my thumbnails to give a concept of the force, like if you pushed your thumbs down on a table. (I don't know the SI conversion for thumb nail color from imperial units, so it might not be helpful for those outside of the US).

The total thickness of the fiberglass is 0.015" (i.e. 0.0075" per side, since both sides were laminated). I imagine there is about 20% too much resin on this test, but that is just a guess.

To be clear, I'm not advocating some of the stuff that was popular on forums (this and others) back in the early 2000's where people were asking how many layers of carbon fiber should they do on the tip-to-tip application for their L1 rocket, as some of those rocketeers grabbed on to the wrong info and made some rockets that were so heavy they had to fly them on a higher impulse class. I concede I was guilty of this 3 times total, and those rockets aren't fun for me to fly as it costs too much for what they do. I am advocating very fine fiberglass as a technique that can be worth exploring if it seems interesting and it would likely make your structure crazy strong for minimal weight. Regretfully, I don't remember the fiberglass weights, so maybe David's 1/2 oz comment is correct. I'd probably just go see what the sailplane guys are doing and get something like that or thinner. For LPR, I imagine whatever weight is the lightest you could get would still be overkill, but maybe I'm wrong.

Wow, that turned out way longer than I thought it would. . . sorry.

Sandy.

 

[ShadowSpire48]

I’d try either the light fiberglass option or papering but papering is a tad easier before building. The glass would definitely be much stronger and the better choice at this time.

 

[David Schwantz]

One nice thing about glass is it will conform to the pods and to the body tube. That way you could run a bit up past both joints. And 1/2 oz cloth is so light a frail you would need to wear latex gloves as my hands are rough enough they will snag the fibers.
As far as to much resin, when wetting out the cloth, if it is still white, there is not enough resin. If the cloth is shiny, then there is too much. I like to use card stock as a squeegee. They are very disposable.

 

[BEC]

Having built and flown this model and been a little concerned about that soft fin stock....I've not had any in-flight failures but I am not ever going to put a C or D Q-Jet into the model. It handles Estes C6s just fine.

That said, the idea of using 1/2 ounce/sq. yd. fiberglass to reinforce is intriguing and would certainly toughen her up quite a bit as long as it was done with no more than "just enough" resin.

 

[bcook7135]

I may try the glass/resin option.

 

[Sandy H.]

I may try the glass/resin option.

If you haven't done it before, watch a few tutorials, buy the right products and do a few trial pieces, IMO. It would be a shame to screw up a nice rocket. It isn't hard, but it does require some technique that may take a try or two to get right.

Sandy.

 

[bcook7135]

Good advice

 

[Stewman]

I don't have additional advice for you, but I wish you luck with your build and hope you post pics. The Excalibur was one of the first Estes plans I built as a kid, and it looked like a kid model, LOL, but I was very proud of that rocket. Always a favorite design. Are you making or buying decals for it?

 

[bcook7135]

I don't have additional advice for you, but I wish you luck with your build and hope you post pics. The Excalibur was one of the first Estes plans I built as a kid, and it looked like a kid model, LOL, but I was very proud of that rocket. Always a favorite design. Are you making or buying decals for it?

I was able to obtain a set of decals from Semroc. This model is indeed like a kid model. But I am 60 and still enjoy those futuristic designs. I obtained parts which cost me twice as much as it should have. But I have wanted to build this rocket for a while.

 

[Alan15578]

I was able to obtain a set of decals from Semroc. This model is indeed like a kid model. But I am 60 and still enjoy those futuristic designs. I obtained parts which cost me twice as much as it should have. But I have wanted to build this rocket for a while.

I built one of those 50 or so years ago. I suspect that it will survive flight, but I have no feel for your fin stiffness. I am concerned that a vendor might sell parts with unsuitable wood. I do understand that balsa is much more expensive, and that good wood is hard to find.

The easiest way to reinforce your fin, especially in torsion, is to simply dope on a layer of Jap tissue.

 

[user 35280]

Are these reproductions correct,
http://morerockets.com/Reproduction-Rockets/Reproduction-Model-Rocket-Kits/Starship-Excalibur
http://morerockets.com/upscale starship Excalibur estes

 

[bcook7135]

Those are the reintroduction models. I am building the reproduction of the original from parts. It looks like the below…

 

[bcook7135]

So, getting cloth and resin is going to be relatively expensive. I am not sure if I want to go that route.
I have put so much money into this build already… but I may use for future builds as well as this one making the relative cost minimal. So the cost and peace of mind may be worth it.

 

[lakeroadster]

So, getting cloth and resin is going to be relatively expensive. I am not sure if I want to go that route.
I have put so much money into this build already… but I may use for future builds as well as this one making the relative cost minimal. So the cost and peace of mind may be worth it.

As @ShadowSpire48 suggested in post #14, papering would be an option. And that's nearly a zero cost adder too. If you've never tried it, you'll be surprised at the stiffness it adds to the fins.

I think I'd follow the advice of the folks here though that have built this type of rocket, and just build it per the manufacturer's instructions.

 

[Stewman]

Those are the reintroduction models. I am building the reproduction of the original from parts. It looks like the below…
View attachment 511362

Still a great looking model after all these years!

 

[David Schwantz]

OP, I can put enough .5oz cloth in an envelope with just a stamp for you. You can buy glass resin in the small size at Walmart for like 9 bucks. PM me if you'd like.

 

[Marc_G]

I built one of these some years back (Semroc edition) and I believe I papered the fins and CA saturated them, more to get smooth finish than for rigidity (but, the process adds rigidity).

Even with the model glued up already papering is not hard, just make a template based on the fin outline, trim it for correct fit around the fillets, and make enough for three fins.

I use printer paper, with a light spray of spray adhesive on the back. Apply to fins, rub down to adhere, soak with CA, let cure in the garage overnight. Easy peasy.

 

[Buzz McDermott]

I built one of these some years back (Semroc edition) and I believe I papered the fins and CA saturated them, more to get smooth finish than for rigidity (but, the process adds rigidity).

Even with the model glued up already papering is not hard, just make a template based on the fin outline, trim it for correct fit around the fillets, and make enough for three fins.

I use printer paper, with a light spray of spray adhesive on the back. Apply to fins, rub down to adhere, soak with CA, let cure in the garage overnight. Easy peasy.

Marc got it right. I've built several Excaliburs over the years and I have always papered the fins. IMO soaking with thin CA is optional. The plain paper will look great after one sanded coat of filler primer (like Rustoleum 2 in 1, Kilz, ...), and they will add considerable stiffness w/o the CA. I built one of the Excaliburs with 24mm mount and the papered fins (and no CA) worked great with C11 and D12 motors.