Super Better Bertha

Super Better Bertha: Nine Motor Cluster

I've spent hours drawing cluster patterns for model and mid-power rockets, but had actually only built a few implementations when I started this one. Even though I didn't have much experience, it never stopped me from jumping in with both feet when the topic came up, as it frequently does on the various web forums.

Once when I waded in, it occurred to me that I should build one of the monsters instead of just talking about it. So I decided on a BT-80 (2.6") cluster of one 24mm motor, four 18mm motors and four 13mm motors. Then I realized I could use one of my unbuilt kits instead of scratchbuilding a new rocket.

That would reduce my unwieldy stash as well as giving me the new cluster. My Maxi Alpha III was 2.6". Could I use it? Upon further consideration, the squarish nature of the cluster called for a rocket with four fins. So the Maxi Alpha III went back into the 'round-to-it pile and the Super Big Betha was chosen instead.

Here's how it turned out (right). Needless to say, I'm quite pleased.

Building it started with the motor mount (below). The tubes were glued up, then some wood strips were glued to the outside tubes. With a little sanding, they make for a perfect fit in the BT-80. Two pieces of 2-56 all-thread were epoxied into the voids between some of the tubes and a brass ring fabricated to act as motor rentention. No motor blocks were used in any of the tubes. Instead, tape rings on the aft ends of the motors function as thrust rings.

Besides the motor mounts, other key modifications include splitting it apart in the middle and 1/4" launch lugs. The nosecone was glued in, and a LOC 2.6" coupler used because its added length keeps the halves aligned much better in the presence of lateral forces. In other words, it helps keep the rocket from folding up at the joint.

Instead of the balsa pieces provided as gussets for the fin roots, I applied fillets of Elmer's Fill 'N Finish, then applied a thin layer of glass over the fillets to keep cracks from showing through.

The fins came out kind of flimsy, and it was too late to glass them, so I added a couple layers of dope and tissue to them. But two of the fins, from softer balsa, remained a little flimsy. If I had it to do over again, I'd either glass them or use thicker balsa or some other stiffer wood such as birch ply.

On Saturday, March 25 at the DARS launch in McGregor, Texas, the Super Better Bertha flew with nine motors on its maiden flight. And all nine lit. The center motor was an E9-P. It was surrounded by four C6-7's and four A10-PT's. Woo-hoo! It was a great flight. When the LCO hit the button, it took off quickly and kept burning for a good while.

The 30" chute brought it down safely. It was a pleasant surprise to find that all nine motors had indeed lit. My previous best was four out of five on the Douger.

Here's a close up of the motors after the flight clearly showing they all lit. Suhweet!

Here's a great liftoff shot from another flight. Again, I got all nine motors lit.

Here's a liftoff shot with only five motors loaded.

At right is another shot from the same flight. All five motors can be clearly seen from below.

January 10, 2009

Motor retention for the Super Better Bertha is a custom cut brass plate held in place by two nuts screwed onto some 2-56 all-thread. The brass plate engages all 8 outboard motors without interfering with the motor exhaust. A large hole accomodates the 24mm center motor.

The all-threads are mounted in the voids between the motor tubes. These were epoxied into place taking care not to get any epoxy on the exposed threads.

9-19-2010