Bizzar failure

[pdooley]

I built a little BT5 rocket from spare parts for the weekend launch.
It was so light I figured just a piece of Kevlar cord tied to the NC would suffice for tumble recovery.

The first flight was nice and high on an A10-3T, recovered undamaged after a little searching.
The second flight was a lawndart. I figured the engine ejected out the back, but to my suprise the NC appeared to have popped out and went back into the BT pointy side first.
I have never seen or heard of this before. Anyone else have this happen to them??
I haven't touched the NC, this is how I found it.

 

[jflis]

oh man, go buy a lottery ticket!

that has got to be a first!

 

[OKTurbo]

Yeah...

Go buy a lottery ticket. You are one lucky(unlucky) guy.

John

 

[Micromeister]

Actually guys its a fairly common occurance usually a product of a short shock line.
I'm sure you've experienced the forward end of your model body being dented, with the short rubberband type Estes shock cords. The nose on these models didn't quite get turned around, impacting on the forward edge of the model body doing the damage. I think my first nose re-insertion was on an old Estes model the Bat. I'd broken the already short rubberband shock cord, simply re-tying the now shorter band to the nose, next flight..Point insertion and crash.
I'd be willing to bet ya only have a foot or less Kevlar beyond the forward end of the body tube. Having the cord on the short side allows the nosecone to hit the end of the cord with some thurst, inertia being what it is brings the body forward as the nose recoils. Even good old "no stretch" Kevlar/body/plastic nocecone combination has enough "give" to allow this situation to occur, Presto! point insertion.
I solved this problem adding 3 to 4 times the body length of shock line to my models. For instance: all my micro models which use straight 50lb kevlar shock lines have a minimum of 30" long shock lines. Larger clustered LMR's start with 3 times the body lenth in 130lb or heavier Kevlar with another body length of 1/4 elastic attached between the kevlar and Nose. Haven't put the point in the body since changing the shock cord lengths.
Hope this helps.

 

[Fore Check]

I've never had this happen to me, but as Micromister says, it must not be *that* uncommon. I seem to remember reading another thread around here not so long ago where another rocketeer had a very similar thing happen.

 

[Stewart32]

Search under yowzah. Not sure how to reference the other link here, but it has a couple images of the same phenomenon.

 

[Karl]

This has happend to me with the Estes Wizard , and its useually called the 'Estes-Dent'. Ive never seen it happen with kevlar though , and kevlar isnt elasticated..............What I think is, the motor ejected, the nosecone came out, and just before it made contact with the earth , the nosecone was pointing inside of the tube , as as it hit the ground the force pushed the nosecone up inside the rocket ,resulting in the BT ripping as the nosecone is wider than it.
-Karl

 

[Micromeister]

Originally posted by Karl
This has happend to me with the Estes Wizard , and its useually called the 'Estes-Dent'. Ive never seen it happen with kevlar though , and kevlar isnt elasticated

Karl:
If you read my post carefully you'll see I didn't say Kevlar was"elasticated", thou it does indeed have a small, slight might be a better word stretch. its the combination of attachment to the body, the kevlar and the give in the plastic nose cone that togather create enough stretch to cause a recoil IF the Kevlar shock line isn't long enough to allow the nose cone to slow down after ejection on its own. As I mentioned All my Micro-Maxx models use only straight kevlar shock lines without any type of elastic at all. in fact most have basswood nose cones and the shock line is anchored around the motor mount or motor block. That said, I've had 2 early Micro models with short shock lines suffer what I call recoil nose insertion. These usually happen on the way up in micro-maxx models with the short motor delays. I have see it happen on a maxi big bertha also on the way down

 

[WiK]

I think what Karl is saying is that it isnt elasticated, so why should it bounce back. I dont think he was correcting you for saying it was.

 

[Micromeister]

yes Wik:
When I read Karls post I could see i hadn't explained the process very well, Hope I clarifed what I've observed in the follow up post.

 

[pdooley]

What I think is, the motor ejected, the nosecone came out, and just before it made contact with the earth , the nosecone was pointing inside of the tube , as as it hit the ground the force pushed the nosecone up inside the rocket ,resulting in the BT ripping as the nosecone is wider than it.

karl, the motor did not eject.
Also, the rocket arched over at apogee and came down like a rock, front end first.
If you have ever seen a rocket "lawndart" you know what I'm talking about.
The NC had to of stuck in the body tube at ejection. Also, no way that much BT damage would have resulted from a slow tumble back to earth followed by last second NC sticking.

I think the problem may of occured because no recovery device was attached, just NC flopping around attached by a length of kevlar to BT. The NC had no drag from streamer or chute, and wandered back to where it came from.
Still, a low probabilty occurance.

Anyway, no big deal. I trimmed the top edge of BT down a few inches. The roc is fine, just a little shorter

 

[Bob Stephenson]

Originally posted by pdooley
... but to my suprise the NC appeared to have popped out and went back into the BT pointy side first...

I've never seen this happen before, but ouch ! How hard was the surface that the rocket 'lawn darted' into ? It has really driven the nose cone way into the body tube...

Cheers

(Evil) Bob

 

[pdooley]

Originally posted by Bob Stephenson
How hard was the surface that the rocket 'lawn darted' into ?

This was in a grassy field. The same place my Big Bertha came down on via streamer recovery with no damage.