In this level-2 EZI-65 lesson, we will build the payload section and altimeter bay and wire the altimeter.

• Build Payload Section
• Build the Electronics Bay
• Wire Altimeter and Finish Bay

Read this lesson carefully all the way through before starting and make sure you understand how the altimeter bay goes together. If you're changing the design or using a different altimeter, sketch out your modifications before starting and make sure everything will fit together properly.

Build Payload Section

The payload section is built mostly as described in the EZI-65 instructions. We need to adapt to the altimeter bay, so the things we will do differently are:

• skip mounting the eyebolt
• bond the fender washers on each side of the bulkhead plate
• drill a hole for the ejection charge holder
• recess the bulkhead plate 1/2" into the coupler
• leave only 2 1/2" of the coupler sticking out of the payload section

Drill a hole through the bulkhead plate 1/2" in from the outside edge for the ejection charge holder tube. Actually, the LOC/Precision 1/2" launch lugs are just a little under 5/8" O.D. If you don't have a 19/32" bit, you can do what I did and drill a 9/16" hole and use a rat tail file to enlarge the hole so that the ejection charge tube just fits.

Because the holes in my bulkhead plate were so large, about 1/2", I went ahead and bonded the supplied fender washers to the plate before assembling it. You need the fender washers to avoid the parts just falling through the hole and I like having as few loose parts as possible when I'm trying to assemble my rockets in the desert. Try to get the hole in the washers in the center of the bulkplate, and more sure the two fender washer holes are lined up.

Mark a line 1/2" inside one end of the coupler tube and another line 2 1/2" outside the coupler from the same end. You want to bond the bulkhead plate into the coupler so the aft edge touches the line inside the coupler (1/2" from the aft end) and the coupler into the payload section up to the outside line (2 1/2" coupler exposed).

Position the bulkhead plate into the coupler tube at the mark and epoxy it in place. Put a generous fillet on both sides of the plate. If the bulkhead plate does not fit tightly into the coupler, let this part gel before proceeding.

Smear a good thick bead of epoxy around the inside of the payload section tube, right at the edge. Push the coupler into the tube slowly and rotate as you go. You want the epoxy to cover the entire joint surface. Once you've pushed the coupler in up to the mark, pull it back a bit until you can see the edges of the epoxy, rotate the coupler and push it back in. If any epoxy is on the outside of the coupler, clean it off with alcohol. (Epoxy outside the coupler will prevent the rocket from assembling properly.) Smooth the squeezed out epoxy inside the payload tube into a nice fillet at the forward end of the coupler. Remove any excess epoxy.

Build the Electronics Bay

The altimeter bay is built much like the payload section, except that we need to mount the coupler so that it leaves the appropriate room inside the bay for the altimeter. (Between the two bulkhead plates.)

The bay will fit over the payload section coupler and into the main airframe. The interior space of the bay needs to accomodate the mounting plate of the altimeter of your choice. I'm using the Missile Works RRC² which is a bit under 6" long so I'm using a 6" long mounting plate.

You want a bit more than this clear between the bulkhead plates so that the altimeter bay will slide completely onto the payload bay coupler when the altimeter is installed.

(Click on the picture to the left for a large diagram. Also, see Darrell's original design.)

Building the altimeter bay is much the same as the payload section.

• drill a hole for the ejection charge holder in the bulkhead plate
• bond the fender washers on each side of the bulkhead plate
• mark the outside of the coupler as determined by your altimeter length (mine stuck out 3 1/4")
• mount the bulkhead plate just inside the aft end of the coupler
• mount the coupler inside the altimeter bay

Be especially careful to clean off excess epoxy inside the altimeter bay tube so that the payload bay coupler will slide all the way down into the altimeter bay. I left 1/4" of space between the couplers so that a little extra epoxy would be OK.

If you don't want to calculate the dimensions in advance, stop after you bond the bulkhead plate in place and let it cure. Now you can measure your parts and figure out just how much of the altimeter bay coupler should stick out. (Refer to the diagram as necessary.) Remember that the length inside the two couplers, plus the gap between the couplers will add to, and the length the payload section coupler sticks into the altimeter bay will subtract from, the usable space. Leaving extra space is OK (you can just make the altimeter plate longer), but try to balance things so about half of the coupler extends from the tube. (If I was starting all over again, I'd make the altimeter bay tube 6 1/4" long: two times half the couplers' length plus 1/4", so that both couplers could stick out halfway.)

Now we need to build the ejection charge holders. Take your 1/2" launch lug and cut it in half, making two 3" long ejection charge holders. One end will stick out through the holes we drilled in the bulkhead plates and the other will be blocked with its own mini bulkhead. Cut two bulkheads out of 1/8" plywood and bond them onto one end of each ejection charge holder. Make sure to get plenty of epoxy on this joint since you're working with a small surface area. Once the epoxy has cured, sand the base smooth so you have a nice looking cylinder, blocked at one end. We need to drill a hole in the center of each bulkhead to pass the ejection charge wire through. Start with a 1/8" hole and clean the burrs with a hobby knife. Try feeding your ejection charge wires (flashbulb wire or electric match leads) through from the oposite end. Enlarge the hole as necessary so the process isn't too painful.

Now bond the two ejection charge holders into the holes in the bulkhead plates. The open end of one holder should stick 1/2" up into the payload section (with the majority of the tube and its bulkhead sticking out the bottom of the coupler bulkhead). The open end of the other ejection charge holder should stick 1/2" through the bottom of the altimeter bay (with the majority of the tube and its bulkhead sticking up inside the altimeter bay section). When the two sections are joined, you should see 1/2" of open ejection charge holder sticking up into the payload section and out the bottom of the altimeter bay.

Once the parts have bonded, take a moment to admire your handiwork. Slip the nose cone into the payload section, the payload section into the altimeter bay and the altimeter bay into the main aiframe. Now, that looks like a rocket and you know that it's built to last!

Wire Altimeter and Finish Bay

Start by reading the instructions for your altimeter. Make sure you understand how it works and how you will hook it up. You can use any altimeter, of course. I used the new Missile Works RRC² altimeter, and the dimensions above are designed to accomodate it. You should test your altimeter according to the instructions before proceeding.

The RRC² PC board is 1 1/2" wide and 5 3/4" long, with two mounting holes at the center of each end. I decided to use a 1 3/4" wide by 6" long mounting plate. The plate should be wider and longer than the actual altimeter PC board and provide a solid mounting point. Since the RRC² only provides two mounting holes, we need to be sure the plate is stiff. I made mine out of 1/8" plywood.

Drill the two 1/8" holes for the altimeter mounting screws by using the PC board as a template. Line up one long side of the altimeter with one side of the plate and inset the ends evenly. Drill a third 1/8" hole 1/8" from the other edge of the plate. This will be used to make a strain relief for the wiring harness.

At least a power switch must be wired to the outside of the altimeter bay, and it is also a good idea to wire a shunt across the ejection charges to avoid static electricity firing your ejection charges. There are several different ways to do this, but I have chosen a simple method as discussed in the Rocketry Online Recovery forum recently. To wire this, you need two locking slide switches, which have two positions and two separate connector paths. These are commonly called "DPDT" switches for Double Pole, Double Throw. Each pole provides a separate electrical path controlled by the switch, which is what "double pole" refers to. "Double throw" means that the switch has two positions, each of which creates a separate path. A DPDT switch has six connectors in two columns of three. Each column is one pole. When the switch is to one side, the center connectors and connectors on that side are connected and vice versa for the other side.

We want to wire the switches so that one controls the power and one "shorts" the ejection charges. For the power switch, the "down" position will connect the two wires from the altimeter's J3 connector (on/off switch). For the safe switch, the "up" position will connect the altimeter outputs. Refer to the wiring diagram on the right. Note that we could have used simpler toggle switches (SPST for power and DPST for safe), but DPDT switches are easy to find and I wanted both switches to look the same.

The RRC² provides three terminal blocks on the PC board for connecting the power switch and two ejection charges. Since I didn't want to modify the board, I decided to use the terminal blocks for the shorting circuit and the outputs. (It's nice to have the terminal blocks for connecting the ejection charges.) Use a 18" piece of 6-conductor wire (22-26 guage) as a wiring harness to avoid a rats nest of wires. Remove the outer cover from 3" of one end of the altimeter end. Twist the inner wires into three pairs. Remove 2" of the cover from the switch end and twist the same pairs together. Choose one pair for the power switch and strip 1/2" of each wire. Thread one wire through both center terminals of the power switch and the other through another row. Solder all four pins where the wires to through them. Strip 1/4" of the other four wires and connect each pair to the center an and end of each column. When the switches are positioned as in the diagram, both switches will be down for flight and up for off/safe.

On the altimeter side, connect the power switch pair to the J3 terminal block on the side. Connect each of the other pairs to an output terminal block on the bottom end. We will install the switches into the airframe after the rocket is painted, so put your altimeter and wiring harness aside for now.

The last thing to do is build the threaded rod assembly and bond it to the altimeter mounting plate. Slip the payload section and altimeter bay together and poke a long stick into the fender washers until you hit the fender washer inside the bay. Mark where the stick touches the outside of the fender washer and pull it back out. Subtract 1/4" for the bulkhead and washers and this is your inside dimension of the payload bay. (Mine came out to 6 3/8".) You want the threaded rod with the rod couplers on each end to be this long. Cut your threaded rod 3/4" shorter than this length and your 1/4" launch lug 1 1/2" shorter (my couplers were 3/4" long). Thread a coupler onto one end of the rod and slip the launch lug over the other. Thread the other coupler on until it touches the launch lug and double check the overall length.

Sand one side of the launch lug and line up two flat parts of the coupler by laying the assembly on a table. Bond this assembly to the back of the altimeter mounting plate. You will need to offset this from the center to clear the altimeter mounting holes since they are at the centerline of the PC board. Now you've built the spine of the altimeter bay which supports the altimeter and keeps the bay together in flight. When we finally assemble the rocket after painting, we'll thread an eyebolt into each rod coupler from outside the altimeter bay.