Disintigration on second launch

[NewEntity1]

Today at R.O.C. in the Lucerne Dry Lake, I fly my Estes Big Daddy again one time, and the recently finished Big Bertha Mod twice.

The Big Daddy went up flawlessly, and the modded Big Bertha went up nearly flawlessly the first time except for a little more angle to its flight than I would have liked. One the second flight however...

The rocket separated on ejection, and the main portion of the rocket cored sampled into the ground

I found on inspecting the rocket that the shock cord had burned through right above where it attached to the steel leader. I had either pushed the elastic down into the tube too far, or else I did not push the wadding down far enough.

Even more alarming, I found that the motor had try to fly the motor mount through the center of the rocket!
The BT50 tube had broken loose of the epoxy and pushed up through the centering rings, and furthermore the centering rings had partially broken loose of the BT60 body tube and tried to surge forward as well!!

This second motor had had a VERY tight fit into the motor mount, and I shoved it in pretty hard. I'm wondering if I might have cracked the grain near the top of the motor, causing it to burn much more rapidly. The reason I wonder this is that the first motor, and identicle D12, caused *absolutely NO damage* to the motor mount or any other part of the rocket.

At this point, the only parts that are salvagable are the nose cone, the parachute, and I could maybe cut the fins loose (since they were surface mounted). The rest is a crushed and melted pile of glop...more melted than cruched, actually, despite the core sample.

 

[n3tjm]

Was the shock cord that came with the kit the stock elastic from the kit? The elastic that comes with Estes kits is very cheap. I always replace that with tougher elastic from a department store.

The damage from the motor mount probabily occured when the rocket hit the deck. It hits the ground with the same force that it took off... but this time, it came to a sudden halt. The motor mount tried to keep on going.

 

[powderburner]

On any motor mount like the ones in a Bertha, as long as I have the thing apart for assembly, I go ahead and add some reinforcing webs between the motor tube and the main BT. The webs can be cut from cardboard or balsa sheet. If you use balsa, the grain direction should be inboard-outboard, NOT forward-aft. Cut the web so it extends all the way to the front and rear centering rings, and makes good contact to both the motor tube and main BT. Use enough webs to support the motor tube, like three or four (you can line up these internal webs directly under the fins for a bit of additional support).
If you glue these webs in place and add some small fillets, you will have an almost indestructible motor mount.
As to the burn-through on your recovery system anchor, I suggest that you switch to a kevlar cord for the part of the tether that attaches near the motor mount, leaving enough cord extending out the front to be able to make shock cord attachments and repairs. Personally, I use braided hardware-store string (not kevlar) for my tethers, and coat them with glue before assembly, mostly because it is cheap and I can buy it locally. If you can get some kevlar, that may be a better way to go.
When you prep your rocket, you should pull everything out the front and hold the tether taut out the front while you fill with wadding (or dog barf). That will positively get all the delicate stuff on the safe side of the wadding.

 

[Steward]

OUCH...!!!

 

[saxophone]

I would agree with n3tjm. The motor mount probably came plowing
through when the body hit the ground. The only other thing I can
think of is that when you shoved the motor in, you might have
weakened the motor mount or bent a centering ring. But n3tjm's
explanation seems more likely.

I try not to mount my shock cords the Estes way any more. I tie
kevlar string to the motor mount the way it is done in Quest kits.
I usually replace the shock cord with a longer and strong one also.

 

[eugenefl]

Ouch! Sorry to hear about the failed recovery.

One thing I've noticed that I used to do out of habit was to stuff the shockcord in first. I would then add my wadding and then lay the rolled chute on top of the wadding. Well, it turns out that all those flights I was scorching the shockcord. This may have happened to you as well. Or, the darn elastic just took a hot chunk of BP and melted.

Try pushing wadding past the shockcord mount (if paper mounted) and then lay the rest of the recovery components on top of the wadding. I know that some Estes model require that you install the shockcord through the motor mount. If that's the case, definitely do what saxaphone has suggested - a Kevlar leader a la Quest style.

 

[Steve]

snip

Originally posted by powderburner
On any motor mount like the ones in a Bertha, as long as I have the thing apart for assembly, I go ahead and add some reinforcing webs between the motor tube and the main BT...

I think I know what you are describing, but would you happen to have any pics? I can see how this would provide a lot more glue surface area than just the centering rings - is that the objective?
Thanks.

Stephen

 

[NewEntity1]

The shock cord was attached to a steel leader, as per the suggestion of membors on this board when I was building it, and not attached directly to the motor mount. The elastic part of the shock cord didint start until about seven inches above the top motor mount tube. This tells me that a piece of the elastic portion of the cord must have gotten pushed down deep, below the wadding, when I stuffed the too-large chute into the body tube. I had mentioned on another thread I had noticed that the 18" LOC chute I was using was too bulky for the BT60 body tube, and I had to really cram it in there. This could not have helped the situation.

n3tjm, saxophone: Yeah, your probably right about the motor mount being forced forward by the core sample, and not the motor burning I hadn't thought of that.

Also, I shouldn't mix epoxy when it's 45 degrees in the shed where I work ...it may not have been the strongest bonds.

Ok, looks like I'll have to make a new bertha mod after I finish the Executioner and the Deuce. Things I will do different:

1) Body tube wont be split in the center and joined by a coupler. As one person pointed out, the couple is a great place for the chute to get stuck. Expecially the THICK coupler I used (see web pics).

2) Instead, Ill use a 9" length of BT50 tubing, instead of the 4" length of tubing I used. I'll cap the end of tubing and drill baffle holes in the sides. I'll use 3 centering rings instead of two, and the 3rd ring will be mounted above the baffle holes, with additional holes of its own for the ejection gasses. This ring will serve as a bulkhead, and the steel leader would be attached to this.

3) The LOC chute is really tough! ...but a bit bulky for a BT60. I saw some TopFlight 18" chutes at the launch...I'll try one of those instead, or else switch to a 15" chute.

 

[rbeckey]

I built my Bertha fairly sturdy. What some folks think of as overbuilt, I am sure, but I like to use a 12 inch chute on her. I also have a soft field, except for the surrounding road and parking area.
I suggest nylon chutes from ThrustAero.com, our own flying_silverad. They pack well, are light weight and come with a snap swivel. Pick a color, including camo, and he can make it for you. I use them all the time. Bought a couple of each size and just switch them in and out of which ever bird needs one. He offered me a discount on a bulk order and might be convinced to do the same for you.

 

[NewEntity1]

I suggest nylon chutes from ThrustAero.com, our own flying_silverad. They pack well, are light weight and come with a snap swivel.

I'll have to check those out as well

At any rate, other than this one failure, It was a very nice day at the R.O.C. launch on Saturday

I will have a few pictures of some of the rockets at the launch available tomarrow. I'll attach the pictures in a new thread where I describe some of the rockets and events that day (a thread that is a little more upbeat ).

 

[TwoWalks]

NE, sure hated to hear this. You put a lot of thought and work into getting Bertha to the launch this weekend. On the up side, it appears you picked up some good improvements for Bertha 2.

I put my Bertha on the shelf and plan to get parts for an upgrade also and I have learned a lot of good ideas from everyones input and your experience. So I guess there is always the positive side to every action, even this one.