Groove Tube Dilemma

[jdud]

Okay fellow rocketeers, here is my dilemma. I have been wanting to build an upscale Centuri Groove Tube for some time now. It will be 2.2" diameter, and I had originally planned to use LOC tubing and fiberglass the inside of the tube fins (for added landing strength). However, I decided to switch over to Blue Tube to make things simpler (no fiberglass), and I kinda' wanted to try out Blue Tube.

The dimensions of the parts are: 6 Tube fins - 7.4", body tube 34", nose cone - 11.12", total rocket length - 45.12".

I wanted to use a 38mm motor mount such that I could fly it on I's and set it up for dual deploy. Upon reading some other threads on this design, I'm beginning to wonder about its stability (especially since blue tube is heavier than loc tubing).

Would adding nose weight be the best option for stabilizing a tube fin design? Or, should I add the parachute bay above the 34" body tube section (which would ruin the "true" upscale factor)?

I had rather have a SAFE, stable rocket than preserve the "true" upscale factor. But, if I can have both that would be even better.

 

[Handeman]

A rocksim file of it should tell you if it's stable or not, but if you put an av-bay in it anywhere above the fins and it's not stable, I'd be shocked. The original rocket was stable and with the extra weight of the av-bay, I don't think any longer tube would be needed or any nose weight. Again, run a rocksim of it.

 

[jdud]

A rocksim file of it should tell you if it's stable or not, but if you put an av-bay in it anywhere above the fins and it's not stable, I'd be shocked. The original rocket was stable and with the extra weight of the av-bay, I don't think any longer tube would be needed or any nose weight. Again, run a rocksim of it.

I've never owned an original Groove Tube, but apparently it included a fairly heavy nose cone (made out of a wood other than balsa i suppose). Subsequent cloners have had to add nose weight to modern balsa/plastic cones. I wasn't really worried about getting it stable until I cut the blue tube into pieces and felt the weight of the tube fins. As you suggested, I figured the AV-bay would work in my favor and will also use fairly substantial eyebolts for hardware. I don't have Rocksim, but a swing test should help me figure it out. I've seen HPR rockets go unstable, and I just don't want that to be me!

 

[troj]

A sim is definitely the way to go.

FWIW, there's no need to fiberglass the inside of the tube. Your original plan, with LOC tubing, but with the fiberglass left off, is sufficient.

Blue Tube is nice stuff, though, and durable as all heck!

-Kevin

 

[Rocketjunkie]

Tube fin rockets are very stable. A conservative CP calculation is to use 6 fins the diameter and length of the tubes. My 3" Groove Tube was stable with any motor that fit, including 54/1706 K160s. If the CG is a BT diameter or more in front of the tube fins, you won't have a problem.

 

[rocket9005]

I did what you want to do, built a Grove Tube type rocket using LOC 2.26 inch tubing (which is 54mm motor tube). I cut the front (leading edge) on the tube to a 60º angle, and the trailing edge to a 45º angle. I built it like a tank. but I put a 29mm mount in it, in retrospect I should have used a 38mm mount. The main tube is a full length tube, which I think is 34 inches, and I put a 12" payload section on it, which could be adapted for electronics in the future. It's real draggy, that's one reason I cut the tubes at an angle. I've only flown on on G80's and G125's, both he shortest delays possible and it works fine: with no hint of instability.

 

[jdud]

Well, I downloaded the trial version of Rocksim and played around with it last night (too late last night due to a bit of a learning curve with the software). I'm pretty sure I got all of my mass objects correct (weighed the AV- bay w/electronics, parachute, eyebolts, etc.) and imported the Blue Tube info from their website. As it stood, the stability was marginal (.75 and lower depending on engine loaded). I forgot to look at the exact CP location, but it was at least one caliper in front of the tube fins (a little further forward than I expected).

I added about 3 inches to the main tube length (total 37") to allow a bit more weight and length for the payload/parachute bay. I also added 2 oz of nose weight. This brought CG forward and resulted in stability slightly above 1.0 (even with an AT J350 loaded). I'll try to upload my Rocksim file later tonight when I get home. Any more pointers? Does all of this seem logical?

 

[MarkH]

I have an original Centuri Groove tube, and lost the plastic nose cone decades ago. I put a balsa cone from Semroc on it a few years back and it was unstable on a C. So I added a few washers to the nose, which did the trick. I don't know if the original plastic cone was any heavier, and I think I only few it on As and Bs when I was a kid. But it's safe to say the orginal design was not overstable to begin with.

I also wouldn't say that tube fins are very stable designs. They may be stable if designed properly, but very stable to me implies over stable. The extra weight of having six tubes on the back decreases the stability margin such that the main body tube should be sufficiently long and/or nose weight added to compensate.

The Quest Totally Tubular is a similar kit and at one time Quest (secretly) shortened the body tube length slightly and the kit developed stability issues.

As Handeman said, If you're flying dual deploy then the extra weight of the av-bay is probably all you need. For larger motors, if flying without an av-bay, I would use rocksim and add nose weight as needed.