Super Big Bertha question.

[Vance in AK]

Actually, a couple.
First, those of you that have built them, did you use the balsa gussets they give you for the fins or just use fillets or did you do both?
Next, I'm thinking about trying an ejection baffle. Anybody tried that on this rocket, or what do you think of them in general?
I saw one that Stones(I believe) made & like the looks of it. If that was you stones, what do you think now?
Thanks, Vance

 

[Stones]

Ive done a baffle on an Executioner and on a scratch built with a BT-60 airframe. The one in the Executioner has worked without a problem. The one in the BT 60 was too much baffle and ended up blowing the motor out of the MMT.
Here is a direct link to the SBB review at EMRR. It talks about a Century style baffle and has a pic.

 

[goose_in_co]

Vance,

The link that Stones gave you was for the rocket that I built. I decided to try a baffle since the instructions called for a massive amount of wadding. the first flight went well, However the second flight had afouled parachute that bent the rocket at the baffle. I have since repaired the rocket but it still has a slight lean to it. The -5 delay seems to be too ling for this rocket, the -3 would have ejected at or near apogee, and saved the rocket. I still wonder if I reduced the pressure too much with the baffle. If I were to do it again, I would increase the diameter of the holes in the baffle, and double up on the centering ring that has the baffles. It would also be a good idea to add balsa stiffeners to the stuffer tube where the holes are in it as well since it looses a lot of stiffness with the holes in it, and tends to bend when you insert it into the upper body tube section. As for the balsa gussetts, use them. The fins are very long and will snap off if they are not sufficently supported. I thought about a through the wall mount for the fins, but that was too much work.

Good luck on your build.

Mike Goss

 

[Vance in AK]

Thanks for the info Mike .
That was a beautiful rocket, sorry it got bent.
If you were going to build another one would you go with the same baffle system with the upgrades you mentioned, or would you go with baffle plates installed above the stuffer tube?
Thanks for the heads up on the delay. You confirmed something I was wondering about.
Vance

 

[wyldbill]

Hmm... here's a thought...

By creating the baffle in the way you did, you seem to have added the volume that exists outside the emount tube and between the forward "baffled" ring and the rear ring (I'm assuming two rings here, don't know the kit) to your parachute "chamber" (in effect "undoing" the stuffer-ness of the mount). This could account for the lowered ejection pressure. In the standard config, this volume is sealed off. If you were to place a third, solid ring aft of the tube baffle holes, it might help by cutting that volume out of the picture. Alternately, you could build as stock, but add another (stuffer ish) tube (maybe BT20 w/ 2050 rings?) to the top of the emount and baffle that off with a third baffled 2080 ring. Dunno what this would do to your 'chute capacity and CG tho...

just an idea,
-bill

 

[goose_in_co]

Bill/Vance,

Interesting...

Since this is an overly stable rocket, I am not too worried about CG, and I am adding weight at the midpoint of the rocket, and the baffle keeps the parachute in the top of the rocket. That all helps the situation.

there is a ton of room for the parachute in this rocket, no worries there.

as for undoing the "stuffer-ness"... Since the actual increase to the volume of the cavity is minimal due to the baffle, I was leaning more towards the reduction of velocity of the ejection charge. Since the baffle forces it to expand before the parachute chamber, it will decrease in velocity. This could be the problem that I encountered when the 5 second delay had the SBB in a post apogee death dive, the velocity of the ejection could not push out the parachute past the external wind passing by the nose cone. the parachute came slightly out resulting in a cresent shaped hole in the parachute where the rocket impaled itself into the ground. not fun to watch.