USR Berlinetta

[BobCox]

Lance Alligood recently posted a review of the US Rockets Berlinetta on EMRR. This looks like a very interesting kit with the long exhaust vent tubes in the middle. The ventilated shroud assembly breaks new ground for complexity using the ACE Fugue shroud technique popularized by USR. Great job, Lance and Jerry!

Question for Lance: How big are those fins? What was the failure mode of the fin that broke? Did it merely detach from the body, or did the wood split?

The reason I ask is that they looks similar to the original fins on the USR Tall Tail 10 ( 3-inch chord, 9-inch semi-span, 3/32" thickness). As I mentioned in my EMRR review of the TT10, those fins were too long and thin, causing them to flutter and splinter during flight. Since then Jerry has switched to shorter thicker fins for the TT10.

The fin stock on the Berlinetta is much thicker, so it would be interesting to see how well the computer simulations predict the failure speed.

 

[lalligood]

Hey Bob. Glad you like the Berlinetta. I've never seen or built anything like it...

The fins are HUGE. They are 9 inch wide ellipses with a 6 inch root chord (so the total fin span is 9+9+4=22 inches! ) They are cut from some of the hardest 1/4" balsa stock I've ever used.

To make a long story short, I had left the Berlinetta in a poor choice of a location and my wife stumbled into it. She knocked a couple of fins loose & completely destroyed one of them. I (mistakenly) made a replacement fin with 3/16" thick balsa stock. I figured I wasn't going to fly it on any super high thrust motors & figured it wasn't worth replacing it (yet).

The flight was on a single H128W-S. (Keep in mind that it has 3 29mm MMTs + 2 24mm MMTs!) It zipped off the pad much faster than I thought that behemoth would (after all it barely tips the scales at 2 pounds). The thinner fin was ripped right off the body tube but the fin itself & the glue used to attach it held together. I suppose I could glue it back on but I'm will do it right & make another fin with 1/4" balsa.

I could have & should have flown it on a single G64-4W... One of these days I'll do a clustered launch too

Probably the coolest feature of this rocket is that it doesn't require any wadding. The way the ejection charge is routed through a baffle and then up through the 4 canted tubes is pretty sweet.

Let me know if you have any other questions--I'd be glad to help answer them!

 

[Stefan_Jones]

I'm seriously considering getting a Berlinetta on the basis of the review.

I'm not sure where I'd store the sucker, though! I think I'd have to add couplers in various places so it could be broken down.

I would use a different fin pattern myself. Something a little swept back,

 

[lalligood]

Originally posted by Stefan_Jones
I'm seriously considering getting a Berlinetta on the basis of the review.

I'm not sure where I'd store the sucker, though! I think I'd have to add couplers in various places so it could be broken down.

I can't imagine how/where you'd put them... The design just doesn't allow for that compromise.

Originally posted by Stefan_Jones
I would use a different fin pattern myself. Something a little swept back,

With the balsa stock that the kit comes with, you could do that I suppose. But I'd still mount them far enough forward so that they don't take any of the impact on landing. The design is waaaaay overstable so you have lots of directions to go in with different fin shapes...

HTH,

 

[eugenefl]

Originally posted by lalligood
I can't imagine how/where you'd put them... The design just doesn't allow for that compromise.

Maybe up in the duct area? Couplers could be used. The only risk might be an inflight separation at ejection. Maybe those sections could be taped together for flight. I dunno....just tossing out ideas.

I got to see Lance's Berlinetta in person a while back. It's truly a unique design and very impressive in size.

 

[WiK]

How about couplers in the ducts, then hold them in with plastic rivets during flight?

 

[BobCox]

Originally posted by WiK
How about couplers in the ducts, then hold them in with plastic rivets during flight?

It might get pretty tricky trying to line up four couplers at the same time.

How about this? Instead of gluing the ejection gas seal disc to the inside of the upper tube, do this:
1) Glue a TC-26-3 coupler to the top of the ejection gas seal disc and allow to dry.
2) Slide the upper tube down over the coupler until it contacts the four vent tubes (about 3 inches down).
3) Drill 1-4 holes through the upper body tube into the edge of the gas seal disc and insert screws or pins.

This allows the entire upper section to come off in one piece. The coupler on top of the disc will provide structural rigidity and limit flexing. This USR Tall Tail 10 uses a coupler like this and it works well.

Two 40-inch pieces with a 24-inch fin span is still going to take up a lot of space in the car, but it's better than one 80-inch piece.

 

[bsexton]

Lance, as far as fin attachment did you think of using the "glue rivet" technique? I've started using that method of attaching all surface mount fins and the adhesion seems to be much stronger.

 

[lalligood]

I don't think it was a glue issue so much as it was a poor choice of fin replacement material... Mathematically, the replacement fin had 25% less surface area where it attached to the body tube than the remaining 3 original fins. And the glassine had been mostly pulled off so I'm quite confident that it wasn't a question of adhesive failure...

So had I'd flown iy on that G64 it more than likely wouldn't have been a problem. But nooooo, I gotta push it with a higher thrust motor

 

[BobCox]

Originally posted by lalligood
I don't think it was a glue issue so much as it was a poor choice of fin replacement material... Mathematically, the replacement fin had 25% less surface area where it attached to the body tube than the remaining 3 original fins. And the glassine had been mostly pulled off so I'm quite confident that it wasn't a question of adhesive failure...

Surface mounted fins only adhere to the outer layers of glassine and paper. I believe that the glue rivets help the glue penetrate into the deeper layers of the paper tubing.

Surface mounting scares me on a rocket that big with such huge fins. My first inclination would be to use TTW construction. That's no longer an option for you since yours is already built.

I would recommend adding triangular balsa fillets like I put on my USR Tall Tail 10 . They can be retrofitted to an existing rocket, and they should increase the bonding surface area by 3X or more.