Needs some quick Big Bertha answers:

Long story short: An older rocketeer cleaned out his low-power rockets and trickle down left a completed Big Bertha for me.

There are no support materials, no instructions, no supplies.
The rocket is in great shape.

Upon pulling the nosecone off, I find there is no parachute tucked inside. It must be replaced.
(1) Does anyone know what size mylar chute the Big Bertha "takes"... 12", 15", 16", 18" ?
I have some replacement JonRocket chutes that I can cut to size, if I knew what this requires.

Second, the original owner had a piece of woven elastic shock cord mounted on a teabag anchor that looks to be in good shape.
The free end of the shock cord has been looped and tied back onto itself, leaving a small 1/2" loop available.
The nosecone features a short "pig-tail" portion of the same type of shock cord tied to the base of the nosecone. The "free end" is just that...
it is not looped nor tied to anything... though it appears at one time that it might have been tied in a loop or to something.
(2) What would the strategy of having two shock cords be?
(3) How would they be configured? (There is no evidence that it snapped or failed. The ends are clearly cut squarely.)

I'm thinking that perhaps the loop was designed to fasten a self-tightening fishing swivel 'hook', so that a different parachute could be used on different days, different wind conditions.
But I'm not clear how the nosecone would secure if it had an identical small loop in it's shock cord. I can't imagine that someone would simply loop the fishing swivel through both small loops:
That would make the fish swivel the weak link, and if it failed (or either of the shock cords) the rocket tube and nosecone could/would freefall without recovery air-breaking.

Or am I over-thinking this?

The last club launch is tomorrow, Saturday, and I forgot I had to deal with this until today.
Thanks.

"I can't imagine that someone would simply loop the fishing swivel through both small loops:" but that's probably what he did. Or something broke, or he had a knot he couldn't get out. Either way, it's just a shock cord, tie it up however you want. The swivel should be fine if reasonably sized, I use them all the time for chutes and shock cords on model rockets.

As for chute, I'd use at least an 18" - generally the biggest you think your field will support. Those long trailing fins are prone to breaking at high landing speeds, better safe than sorry.

Consulting the Bible (1991 Estes catalog) shows an 18" parachute.

The Big Bertha comes with an 18 inch 'chute. I routinely fly mine with a 15 inch JonRocket 'chute with a big spill hole. The Semroc Vega (very similar model) comes with a 12 incher and it's been adequate over 30+ flights. But I generally have something grass-like to land models on.

I think (and if I follow your description properly) were I faced with the same situation I'd tie the free end of the short shock cord to the loop and also attach the parachute of choice via a fishing swivel to that loop. Use coastlock type swivels for this - much more secure.



Aside to Ryan - sorry we had to cancel again....

Thanks for the quick response.
I've got both an 18" and a 16" chute made up.
Considering we're likely to have some breezes tomorrow, I may just rig up the 16" to prevent as much drift.

Berthas often assume a near- horizontal attitude while descending under chutes and exhibit a type of "backslider" or modified tumble recovery; due to their low density profile.

I usually fly on a fairly grassy field and I have had Berthas only partially eject the nose cone due to the airframe's massive interior volume, and even with no chute deployment they have landed on the grass with only minimal damage, if any.

But under most normal circumstances I'd go with an 18" chute.

I have been using 12" Estes parachutes to recover Big Berthas for years. The smaller chute works just fine. I find the 18" chutes drift too far for me.

I strapped on the 16" mylar chute and went for it on a B4-6.
I got a little nervous when the chute didn't come out at apogee,
but fortunately, it did come out and flared about 15 feet off the ground,
just barely stopping in time for a cut grass landing.
Whew!

Kirk G said:

I strapped on the 16" mylar chute and went for it on a B4-6.

Click to expand...

Not the best choice of motor for flying a Big Bertha.

astronwolf said:

Not the best choice of motor for flying a Big Bertha.

Click to expand...

Yah, you're right, but it did work. (It was the only thing I had on hand at the moment...as I was saving the C6-5s for a cluster....and, the Ds weren't going to fit...)