Here are the modifications we made for "Bad Girl".
1. On the 3 engine mount centering rings, and on the ring normally used for the nozzle bulkhead (total of 4) mark and cut 4 additional openings 18mm in diameter and equal distance apart, for the 4 additional 18mm engine mount tubes. Do this on all 4 rings. Make sure all openings are in exactly the same place on all 4 rings, as they must align with the 2nd stage engine mounts when assembled. Then cut a 24mm opening for the center engine, on the nozzle bulkhead ring for the 24mm center engine.
2. Cut 4, 18mm tubes and the core 24mm tube 10.5" long and install engine blocks in each, so that engine will protrude from the rear of tube 1/4" when installed. I used a spent C engine as a "spacer tube" to push the blocks into place. Do not use engine hooks for the 4, 18mm engines. They will be friction fitted for flight.
3. Using 2 of the engine centering rings, install all 5 engine mount tubes, making sure they all seat flush with each other and evenly at the bottom. Install the mount for the 24mm center engine, including engine hook per instructions. Note which ring is precut for the 24mm engine hook and install this centering ring as the aft mount.
4. The main body tube should be 24.25" long. Mark and cut main bt exactly in half at 12 & 1/8th". We used a large adjustable hose clamp to draw a complete circle around the tube and then as a guide to make a very clean cut with an Exacto knife. Use a pencil to number them 1st & 2nd.
5. Install the lower 1st stage wrap, inter tank wrap and all fins and fairings, to the main body tube that was marked for the 1st stage. The inter tank wrap should be installed flush with the top of the first stage bt. After all have set, cut 4 openings through main bt between the fairings.
6. Fully install engine cluster mount into 1st stage bt until it is recessed 1" from bottom, noting which end is aft. The bottom of the aft ring should be flush with the top of the cut between the fins. DO NOT GLUE this mount in place. It will be removable for easy loading of engines and combining the 1st & 2nd stages for flight.
7. Bend 2 small picture hangers into an L shape. They should be about 3/4" on each side when bent. These hangers normally come with 2 nail holes but you will use 4, # 4 x 3/8" sheet metal screws, to fasten the L shaped brackets to the fairings and bottom of aft centering ring.
8. With the 1st stage cluster mount in place, use the L bracket nail holes as a guide to mark an anchor point on opposite sides of the main bt, making a pencil mark through hole in L bracket, on the back of a fairing and a corresponding mark on the aft centering ring. When finished you will have 2 anchor points for the cluster mount on opposite sides.
9. Using one of the screws, puncture the 4 marks on the bt and centering rings and drive the screw in, to thread the holes and then remove. Note where holes come through the top side of aft centering ring and reinforce with a small scrap piece of 1" x 1 " balsa and yellow glue.
10. Screw the brackets firmly into the back of fairings and secure them with CA or wood glue and allow to set. This will hold them permanently in place on the fairing backs, while allowing the top screws of the centering rings to be installed or removed, so that the cluster mount can be installed or removed as desired.
11. Remove the cluster mount and install engine blocks in the top of all 5 tubes, 1/2" into top of tubes.This will leave 1/4" of the top of tubes to act as seats, to hold bottom of engines that will be installed in 2nd stage. For the 24mm tube, I centered and glued an 18mm engine block, into a 24mm mount and then installed it in the 24mm center tube. When you see the pictures, it will make more sense.
12. When top engine blocks are set, drill a 1/4" vent hole through both sides of all engine tubes, just below the engine blocks. This will help vent the pressure so that the stages do not separate early. Also drill 4, 5/8" vent holes through main bt, 3" below the inter stage wrap, at equal distances around the bt.
13. Install tunnels on outside of 1st stage bt per instructions.
14. As with most staged rockets, the 1st stage is tumble recovery. Having flown it, I can tell you it works fine and floats down slower than most rockets on a streamer.
NOTE: First stage completely prepped weight is 9.8oz. The first stage uses a central D12-0 and 4 C6-0 boosters. According to Thrust Finder, this provides 11.91 lbs. of thrust, equaling 66.25% of an F motor.
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