Optima clone...Fixable?

A few weeks back, I noticed Hobbylinc had 5-packs of BT-80 on sale for super cheap, so I picked up a couple for use on some projects.

I had always liked the Estes Optima / Shadow, but it's out of production. Thankfully the plans are all online and Excelsior even offers the decals. The build was pretty straight forward with the exception of the super long stuffer tube setup and multiple (5) centering rings. Essentially, there's one big BT-50 stuffer tube that spans all the way up until 12" or so from the top. The fins are so big that they're a two-piece design so they can be cut from 4" wide balsa.

Anyway, I was pleased with the result and had to fly it on a clear day...without paint or decals. It did great on D12's and was impressive on E9's. It somewhat reminded me of the Excecutioner (same lift-off speed and flight characteristics).

Here's a shot of it on the pad waiting...

You really don't realize the scale of this thing until you see it up close. It's pretty beefy for an Estes design. That's a normal Estes pad in the pic for comparison.

Anyway, since I had some time over the 4th weekend and I really wanted to try out my new RMS 24/40 case, I loaded it up with a D15-4 reload and let it rip. The flight went fine (first time ever putting a reload together), but on the landing, the Optima came down onto the gravel / asphault road...nozzle first.

Since the nozzle extends a bit more than the fins, the shock of the landing traveled up the motor casing, to the motor mount and ended up tweaking the stuffer tube right after the thrust block / one of the rear centering rings.

At first I thought it was the ejection charge that had blown out the tube, but then I saw that it was actually the force of the landing...the stuffer looks pinched in on either side about 1/4 of an inch. When you look down the stuffer, you see a rectangular hole since the sides are pushed in. Can't tell whether the stuffer is split. The rear CR is fine and the MMT tube appears to be still straight.

So... what could I do to I try to fix this? I'm worried that the ejection charge from any future launches will blow out the stuffer and end up popping the other centering rings / not ejecting the NC.

I thought about trying to straighten the distorted parts with a dowel through the engine end, but that may not fix it completely. I could try cutting the "fin can" off and trying to splice in a new section of stuffer along with a body tube coupler...

I know that I could just build another, but *sniff*...it's so young, I hate to see this one die without more flights (it needs an RMS E for sure)...or paint for that matter. Any thoughts?

Hey Landshark.

How about cutting off the body tube somewhere above the little fins. You'll also cut off your stuffer tube, since I imagine there is a centering ring somewhere above that point. You could make your repairs and then use a coupler to put it all back together. If I read your thread right, this should be an easy fix.

With that said, is a stuffer tube really needed?

The "cut and splice" method is something that I thought about. I need to look to see where the actual damage is compared to the "little fins". Come to think of it, the plans and wireframe model in RS might help with that as well.

Good question about the stuffer tube. The only thing I can think of is that the designers at Estes thought the volume of the BT-80 tube would have been a bit much for the D's and E's to fill / pressurize effectively (it's more than 40" in length). I guess one might also argue that the CR's and stuffer add some limited rigidity to the model as well.

You could get some 34" coupler stock from BMS (scroll down to Couple Stock). Cut it to length and slide it down from the forward end. It might take some finagling to get it past the damaged area. Once in it will seal the stuffer tube and make it stronger too.

I'm wondering how the stuffer tube can be kinked and the body tube isn't. The only way I can see that happening is if one end of the stuffer tube moved in relationship to the BT. That would tell my you have loose CRs someplace. Probably the ones below the kink. If the CR/BT joint is good, I would look at the CR/MMT joints.

Splitting the tube and fixing it that way, and adding internal fillets to the CRs and stuffer tube might be the best move.

That's an excellent question about how the damage to the stuffer could have happened without kinking the main BT (there's no damage to that section). I am sure I put glue on all the CR joints but I'll verify that section when/ if I do the "cut and splice".

I hope there's enough room in that area to work in the couplers.

Ok... I finally had the time to cut the Optima just above the large fins and examine the damage.



Looking at the rocket, I can say that the CR's on either side are both firmly in place / glued in. What's odd is that the crushed part looks almost like it's collapsed rather than kinked...like it was under vacuum. As you can see, the tube is definitely damaged over about 6".

On the Optima, it uses a 2" long sleeve to go over the engine hook / BT-50 stuffer in the engine mount end. The damage starts immediately after that sleeve which sits just in front the thrust block. Odd. I guess it could be that the shock was transferred to the spot where that long tube was "less rigid" than the rest and simply collapsed.

Anyway, I'm not sure how to go about fixing this. The BT-50 can't be straightened and short of designing a tool, I can't cut off the BT-50 tube short enough (it's way down inside the BT-80) to remove the damaged section and get a coupler / sleeve in there.

Any ideas?

Could I completely remove the stuffer from that section? The engine would fire the ejection charge into a chamber of BT-80 about 10" long, then would have to travel up the rest of the stuffer (around 20") and into the parachute chamber to eject the NC. Would that work or am I asking for trouble with the CR's?

The other idea might be to use a larger tube to fully encase the BT-50 or use a BT-20 to go inside of it.

Geez...this is starting to sound like BP is working on this thing.

Crazy thought here.

You had a very large sealed volume in the body of the rocket. It flew on a composite motors so it would have gained considerable altitude. Is it possible that the air pressure in the sealed compartment that stayed at ground pressure, or higher, if it heated in the sun during prep or while on the pad, crushed the stuffer tube when it's inner pressure was at the low pressure due to altitude?

That would explain how the tube got crushed when none of the CRs moved in relation to the BT. The tube does look like a thin wall Estes BT type tube.

Having sealed voids in rockets is generally not an issue, but most rockets use heavy wall MMT on the inside of the voids and most voids are only between the fins.

Handeman... the composite motor I flew was actually a 24mm D, so I bet it didn't get nearly as high as the E's I had launched in it the week before. I was thinking about pressures as well, though. This is an odd problem.

I've been looking at tubes (semroc) that are slightly larger to "sleeve" the damaged BT50...but I'd rather cut off the crushed parts first. Have to figure out a way to cut about 3" down inside the BT-80 tube. I feel a contraption based on grill tongs and razor blades coming on...

a dremel ought to do it in as quick as anything else .
rex

Dremel? That'd be the easy way out. I'm on to chopsticks, a broken hacksaw blade and a handful of tictacs. Where's your sense of McGyver? Just kidding... good idea. Now I just have to swipe the Dremel back from my Dad.

Landshark said:

Handeman... the composite motor I flew was actually a 24mm D, so I bet it didn't get nearly as high as the E's I had launched in it the week before. I was thinking about pressures as well, though. This is an odd problem.

Click to expand...

Were the weather conditions different this time then the previous? Did the rocket sit in the sun this time and not the previous launches, even before prep and flying? If it was pressure then something had to be different this time from the last few launches. The first thing the pops in my head is heat and increased pressure from being hot.

I'd say it was probably 10 degrees warmer outside the second time I launched it (just before the damage), but I actually didn't prep the rocket / insert the engine until just before the launch. I would think that wouldn't allow the pressures to build up in the car or sitting in the sun prior to launch.

I know that I put in the entire ejection charge when I built the motor and thinking back on it, a friend had told me that I may or may not need the entire amount depending on the size of the rocket. Not sure if that's true or not, but if the charge was too violent, could it have somehow caused the damage?

When I cut it apart, there seemed to be a lot of residue in the stuffer tube. Could be normal, but what if the charge powder "flashed" over very quickly in the tube and caused some sort of pressure issue...not sure.

I was thinking the sealed void between the stuffer tube and BT might have gotten higher pressure this time because it sat in the sun or in the hot car. The ejection charge might have cause the stuffer tube to expand and the pressure in the void snapped it back afte ejection and it kept going, collapsing the tube. Kind of like when you balance yourself on an empty beer can then reach down and tap opposite sides of the can, it instantly collapses.

Just a theory.

I've had a Shadow and despite the stuffer tube I've had quite a number of partial parachute deployments. Tempted to up the anti with an Aerotech motor. Always wondered if the increased weight in the back end might cause internal stuffer tube damage. Hmmmm...

Just to follow up on this thread...

I ended up fixing the Optima by cutting the rocket apart just before the main fins (as seen above), removing a section of damaged stuffer, then using a section of tube from semroc to connect it all back together.

The Semroc tube was *just* the right diameter to slip over the damaged stuffer all the way up to the CR and also attach to the motor mount tube stub that was left. After that, I used a BT80 coupler to put the rear fin can back on.

Going for broke, I figured the best way to test it would be to send it up with an E18 in it. Nice flight... good altitude and no damage! This thing has had a heck of a life already...and it's not even painted yet! I'll post pics of that when it's done.