Design for G37-10?

[astronboy]

The rocket flew so fast that the launch photo turned out to be nothing but a smoke trail....

Anyway, it really flew staright for about 1000', then it started to wobble, and started spinning wildly.

Then it came apart.

The NC and the streamer seperated, and headed due east.....

The airframe came in ballistically. and formed quite a core smple in the soft mud about 50 feet from the launch pad.

When I got to airframe and pulled it out, the fins, and all of the airframe from the front of the transition up to the front of the fins was torn out. Gone.

One fin was found later in the day, about 150 feet away while recovering another rocket......

 

[astronboy]

Post Mortem:

The remaining fin showed that the fins did not shred or shatter. Th Balsa held!! The fin had bits of airframe and engine mount tube still attached.

Two theories arose: Both were probably true.

1) Yellow glue fin mounts were melted/weakened by engine heat during 3.1 second burn.

2) As the rocket approached max velocity (Rocsim says 867 fps, or mach.75) the airframe tube could not sustaing the buffeting on the fins, and they simply tore loose.

I will have a pic tomorrow, as I am just too buched to shoot a photo of the remains tonight.

Phred


Tomorrow I will also start over in Rocsim.... I will take your advice GLP, and cut down the fin span, lengthen the airframe appropriately... epoxy also appears to be a must. Of course, I will work on how to strengthen the airframe in the fin can area without adding any appreciable weight.

stay tuned of for Blizten Zwei !!

Ph

 

[GL-P]

Dude, that sucks!!! I know that an EM F got real hot in my aura but luckily my aura is well built. I agree that the tube probably crumpled and might have lost the outside layer because it couldn't stand the fluttering of the fins.

Try a little 3/4 oz cloth from tip to tip or even only half way up each fin. I found that it adds a little weight but is really strong. I haven't gotten a chance to fly it on a G but I figure it'll hold together everything even better on a BT-60 airframe. Try to keep the span length as low as possible compared to the root length but obviously still keep a stable rocket. Less leverage on the fins. Look at some of the record holders for ideas on fins and play around with Rocksim's Cd feature. I think it calculates M1 drag doesn't it???

Good luck and post .rkt files and the basic dimensions of the fins and stuff. It's a real bummer because that rocket had an awesome paint job!!! I wish my paint jobs were that good!!!

Good luck!!! Oh yeah, where'd ya get the decals? Custom made??? They are real sweet!!!

 

[astronboy]

Here is the new roc-sim filed I banged together tonight. I lopped off .5" from the fins so they are now at the exact dimensions GH Stine recommeded for minimum drag.

My thought was to double the BT in the fin can area, with a bit of epoxy or gel CA as a fixative.

Decals? They are my business!! excelsiorrocketry.com These took all of about 10 minutes...

Ph

 

[GL-P]

I think the span shortening should be okay if as you say double the BT and add some fibreglass. You won't even need the doubling of the BT if you use 3/4oz fibreglass. The light fibreglass cloth is IMO the best thing to use in this situation. It'll keep the tube or fins from flexing.Take a look at the LOC Aura. It's a similiar fin shape but the diff. is that it's 1/8" plywood. Just add some big internal epoxy fillets to keep the thing firmly together. A little glass and she'll be real strong!!!

 

[astronboy]

OK, Here are some pictures of the remains.

First, the fractured airframe and one fin:

 

[astronboy]

Next is a close-up of the fin can area. Note that there is no balsa attached....

 

[astronboy]

And finally, a close-up of the one fin found. Note that the balsa is compkletely intact. This fin was ripped out by its roots....

 

[GL-P]

Hmmmmmm.....Looks like the glassy layer on the mmt wasn't soaked through with glue so that layer was taking all the stress. Stick with balsa fins, they worked better at that speed than I thought they would. You might want to consider removing the glassy layer off the mmt then use CA to tack the fins down and then some thin long cure epoxy so that the epoxy has a chance to soak into the tube more. Then as you said double the bt in that area. More surface area for glue to stick to.

Good luck!!! Obviously the Estes tube was fine for this rocket!!! Look forward to a Blitzen Mark II!!!

 

[astronboy]

Yes, I agree. The yellow glue peeled right off of the MMT tube. I believe that part of this was the glue melting due to the heat of the motor, but I also agree that I will sand the tube lightly with 400 grit, and CA soak it before applying the fin with epoxy on Blitzen II.

I am still working on a way to add strength to the fin can without adding much weight. I have some very light fiberglass cloth from an older casting project, and I may use that.

I will fillet the fins, but the fillets cannot be huge for weight reasons.

My concern is to keep the rocket light enough to use up that 10 second delay..... My latest sims are down to 9.5 seconds.

Phred

 

[Stymye]

Phred,Is there enough room to lock the fins into small notches in the centering rings?or possibly sandwich the fin tabs betwen two rings? that way your don't have to count on just the root surface glue holding the fin from twisting,not sure if you have room tho

 

[astronboy]

That is a good idea.... I will play around with some centering rings and see what works. I think that notching the rings may just add the strength I need, without adding excess weight.

 

[GL-P]

Oh, that should work well! Just lock it into both rings and glue down with epoxy and it'll be pretty tough!!! That should work great!!! Awesome idea Styme!!!! I should try that also!!! Wouldn't need to double the BT with that fin attachment. Now maybe add that light fibreglass and it'll be super strong!!!!