LESSON FIVE: Building the altimeter bay compartment

In our last installment, we were finishing up the airframe assemblies, getting them ready to take our high performance versions of the Honest John into the sky. In this installment, we will begin assembly of the nose cone and electronics bay unit, which houses both the altimeter and the main recovery compartment.

To begin the installment, we start by preparing the lower nose cone unit for the electronics compartment. The nose cone in the standard kit comes in two separate pieces: the lower piece, which came in the kit attached to the fin canister, and the forward piece which came separately. Both pieces are blow-molded styrene plastic, and have to be handled a little differently than most polyethylene plastic nose cones.

In the photo above are two of the three lower nose cone units from the class project. On the left, the lower nose cone unit has already been cut out with the required 2-1/8" hole to hold the electronics bay. The unit on the right is marked for that cut. To cut yours, it is imperative that you get the hole directly in the middle of the unit. Mark carefully and drill a smaller pilot hole if needed.

I cut my 2-1/8" opening using a regular hole saw, and cleaned the opening up with some 80 grit sandpaper. The opening we're cutting is to allow for a 54mm coupler tube to pass through but it should be a snug fit, so sand carefully so you don't take out too much material.

You can see in the photo above where I have highlighted a similar situation that existed in the fin canister: where the styrene was thicker around the parting lines. This has to be removed so that the electronics bay tube can seat flat against the end. I used my Dremel with a cut-off wheel to remove enough styrene to allow the 54mm tube to fit flush with the end on both sides of the opening. It only takes removal of about 1/8" of material around the hole to fit properly.

The other end of the unit should be carefully sanded to allow for a 54mm body tube to pass through for use as the electronics bay. Sand carefully, taking out material evenly around the opening, because you want the tube centered in the unit. If you take too much material out of one side, the electronics bay will be off-centered and will not align properly, so make sure you take your time and do it right.

Now we are ready to prepare and attach the exterior styrene fin thrusters to the lower nose cone unit. This will consist of cutting the fin thrusters out, shaping them to fit the nose cone unit, drilling the necessary vent holes, and then attaching them permanently. In the photo above, I have already cut out my fin thrusters, sanded them to shape and drilled the necessary vent holes to allow them to route air pressure to our altimeter.

I cut them out closely around the fin thruster shape, as indicated in the original Estes instructions. Then, I stuck some self-adhesive 80 grit sand paper to the lower nose cone unit and sanded the fin thrusters to the proper shape. To make it easier to grasp the small fin thrusters, I used two strips of masking tape back-to-back like I did on the fin shells, to allow me a handle to grab them with.

After sanding the fin thrusters to shape, I used a small 1/8" drill bit in my Dremel to drill out the two simulated nozzle holes in each fin thruster. I basically used it like a Roto-Zip, hogging out the hole to the desired shape. Be careful!

Once the fin thrusters are ready to install, we have to drill holes in the lower nose cone unit to pass air through to the inside. In the photo above, the location is marked. The lower nose cone unit has four alignment spines located every 90 degrees around it to make it easy to show where to place the spin thrusters.

To locate the correct location to drill your air holes, holding the unit as shown in the photo, mark the holes at 5/8" to the right of the fin thruster alignment spine and 1-3/4" above where the second ring is located. If you are measuring from the rear-most exposed edge, it is 2". Do this four times, every 90 degrees, and then drill a 1/4" hole in each location.

You can see the internal vent hole in the photo above, with the spin thruster in the correct location. What you are doing is building a functional air path to get air pressure to the altimeter sensor.

When you are satisfied with the fit of your spin thrusters and the location of your vent holes, you can permanently attach the spin thrusters to the lower nose cone unit. I prefer Tenax 7R Plastic Welder. It goes on thin and really holds a lot better than Testors plastic cement, but use what you feel comfortable with. If you use Testors, be prepared to wait overnight for the glue to dry.

In the photo above, you can see the diagram of the electronics bay on the right and on the left are two of the electronics bay housings under construction. Let's look at the list of parts first:

1. 54mm coupler tube 2-7/8" long
2. 1/4" eyebolt
3. 1/2" long 'ring' of 54mm body tube
4. 1/4" nut coupler (long nut to connect 1/4-20 bolts - need 2) and 1/2" long 1/4"-20 bolt with washers
5. 10-32 x 1/2" long pan head Phillips machine screws with washers (need 2)
6. 54mm body tube bulkhead plate 1/8" thick
7. 10-32 nut coupler (long nut to connect 10-32 bolts - need 2)
8. 10-32 all-thread 6-3/4" long (need 2)
9. 54mm body tube 7-3/8" long
10. 2.56"-coupler-tube-to-54mm-body-tube centering ring
11. 2.56" coupler 2-3/8" long
12. 2.56"-coupler-tube-to-54mm-body-tube centering ring
13. 54mm body tube bulkhead plate 1/8" thick
14. 2.56" body tube bulkhead plate 1/8" thick

Not shown are 8-1/2" of thin-walled tubing to use as a wiring conduit to get ejection charge leads from the altimeter compartment to the main recovery compartment. I used small carbon-fiber arrow shafts for this. This will be pointed out later in several places. Also not shown are the 1/4" eyebolt, nut and washer for the altimeter bay end cap.

In the photo above, to the left, this is the basic layout of the altimeter bays. A short section of 54mm coupler fits into 54mm body tube to hold the altimeter. A permanent bulkhead is just below where the coupler tube sits, holding the attachment hardware.

Start by getting all the parts and cutting out the sections of tubes needed. I used the 2-7/8" length for the 54mm coupler tube because it came in 6" lengths and I wanted to get two out of each one. Same with the 2-3/8" long 2.56" coupler tubes — they came in 5" lengths. Once you get the tubes cut, we can start with the bulkhead plate that will hold the hardware attachment points.

In the photo above, you can see how the forward bulkhead is supposed to look, as laid out in the diagram in photo 37.

A point about drilling holes, lining up altimeter sleds, etc. It makes things a lot easier if the holes all line up, obviously, and in the same place uniformly. Try to find some method that allows that to happen. I drill all my bulkhead plates, caps, etc. at the same time so I can get the holes all in line and then I mark my pieces so they are assembled in the same order as they were drilled in the same orientation. Considerations here are that the heads of the screws clear the 54mm coupler tubing.

Once you have your holes drilled, assemble the forward bulkhead to look like the photo above. Also, assemble items 13 and 14 from the list above to form an end cap for the altimeter bay. The open hole in the forward bulkhead is for the ejection charge conduit mentioned above, so drill a hole to correspond with that tubing size. Also, I recommend wicking CA into the nuts and threads of the various hardware to keep them in place once you have them where you want them.

Now you can make the altimeter sled shown in the photo above. You can use any flat material of your choosing, but I used .062" G-10 fiberglass sheeting. The primary dimensions are 6-1/8" long by 1-3/4" wide. There are two recesses cut on the forward end to clear the 10-32 nut couplers that are 1/4" wide by 3/8" long. A central recess on the aft end clears the end cap eyebolt nut that is 1/2" wide by 1/4" deep centered on the end. There is no need to worry about the altimeter attachment holes just yet, just get the sled material cut out.

With the sled cut out, two pieces of 1/4" OD thin-wall brass tubing 5-3/4" long should be cut to mount on each side. Next, screw the two pieces of 10-32 all-thread into the forward bulkhead, then slip the two pieces of 1/4" brass tubing "rails" over the all-thread, and then slip the end cap in place. Lay the altimeter sled over the brass tubes and tack to the tubes with medium CA.

When the CA has set, remove the end cap, slide the sled off the all-thread and put solid beads of medium CA from end-to-end on both sides of the brass tubes. Take your time, don't make a mess, and try to avoid using CA kicker. It may take several sessions to get it all done nice and clean, but it's better than making a mess. If you get CA "fuming" on the board, don't worry — a Brillo pad will remove all of it when it has set up. I recommend leaving this overnight to fully cure.

After adequate time for the CA to set, reassemble the sled to the forward bulkhead, over the all-thread, and put on the end cap. Slide this into your length of 54mm body tube and bottom it out, just like it was supposed to be used. Turn it over and the forward bulkhead plate should be about 5/8" from the forward end of the 54mm body tube. Apply epoxy into the end of the 54mm body tube, and insert the 2-7/8" length of 54mm coupler, then let dry. You should now have a tube that looks like the ones in photo 37. Allow to dry.

Next, remove your end cap and the altimeter sled. Turn your tube so the two all-threads are horizontal, and the hole for the main ejection charge conduit is pointing down. Drill a 5/16" altimeter power switch access hole 1-5/8" up from the lower end at a 90" orientation from the plane of the all-thread, as in perpendicular to the plane of the altimeter sled, to look like the tube on the left in photo 37. Drill four 1/4" holes 6" from the lower end 45 degrees off-phase from the previous hole, every 90 degrees around the tube. So if the 5/16" hole you just drilled was on orientation as "0 degrees", your first hole would be 45 degrees from 0, 6" from the end, then the remaining three holes would be every 90 degrees from that, 6" from the end. The end result will look like the left tube in photo 37.

With all the major assembly done, I installed my length of arrow shaft for the main ejection charge conduit. I then mixed epoxy and poured it on top of the bulkhead plate, to cover it slightly while allowing me to roll it before it set and form fillets up onto the tube walls. I did this from both ends, in two separate sessions, so both ends were firmly attached and not coming out.

When you have that finished, it's time to install the altimeter bay compartment in the lower nose cone unit. To do that, you insert the forward end of the altimeter bay compartment through the lower end of the lower nose cone unit and slide it as far forward as it will go. The fit should be snug and tight like shown in the photo above. The arrow points to the carbon fiber arrow shaft I use as a conduit to get power to the forward ejection charge. When you are satisfied with the fit, it is time to permanently attach it.

Locate the 5/16" altimeter power switch access hole, and position it at "0 degrees". If you look at the bottom end of the lower nose cone unit, the part that forms the original coupler portion of the styrene base, there are eight ribs around the circumference. Four line up with the spines where the fin thrusters were mounted, the other four are halfway in between those prior four.

Hold the altimeter bay and turn the lower nose cone housing until the 5/16" altimeter power switch access hole lines up with one of the four of the latter ribs, the ones in between the spin thruster alignment spines. You want your power switch facing to the front, which will place it half-way between the spin thrusters. It makes more sense when you consider that from the back, this will help your spin thrusters clear the launch rail!

Once you have the proper alignment, you can secure the altimeter bay in place permanently by epoxying the 1/2" ring of 54mm body tube over the forward end of the altimeter bay, capturing the styrene body between the inside 54mm body tube, the 54mm coupler tube and the external 54mm ring. Place this aside to dry.

Now I'll turn our attention to the other end, to the end cap and how I route my wiring through it. I use W.S. Deans External Charge Connectors connected to a regular two-pole electronics screw terminal to make this connection. So you end up with a screw connection on the outside, and a Deans micro-connector on the inside. Makes disassembly and service a snap.

This photo shows the contents of the W.S. Deans External Charge Connector and a two-pole terminal. The diagram to the right shows how I accomplish this solid connection through solid wood.

Here you can see another small jig I made to hold the two connectors in place while I solder the tiny leads. The two Deans connection leads are solderd in between two terminal pins as shown in the diagram of photo 41. The distance between the two units are 1/4", the thickness of my end cap.

And here is the end result of that soldering, two units exactly 1/4" apart. To install them, you pull the shroud off the Deans connector's pins, drill two corresponding holes through the end cap in the appropriate place to route the pins through, and then using a small amount of epoxy under the terminal block, glue it to the end cap. Do not use CA unless you want to start over. Ask me how I know.

When the epoxy is dry, slip the shroud back on the back side and attach with two #2-1/4 Phillips flat head sheet metal screws. The ones that come in the package are too long. Note: I orient the terminal block 180 degrees from the 5/16" altimeter power switch access hole, toward the back side of the altimeter bay.

All that is left is to attach the 2.56" coupler tubing to form the nose cone shoulder. I recommend you take great care with the 2.56"-coupler-tube-to-54mm-body-tube centering rings, as they are fragile. Sand them so they are an easy slip fit, but not loose.

Begin by epoxying one of the 2.56"-coupler-tube-to-54mm-body-tube centering rings flush with one end of the 2-3/8" long 2.56" coupler tube, and set aside to dry. I do mine on a flat plate covered with wax paper so I can push them down and get a nice flush end.

While that is drying, apply epoxy to the 54mm body tube where it exits the styrene lower nose cone unit, and then slide the remaining 2.56"-coupler-tube-to-54mm-body-tube centering ring all the way forward.

When both of those have dried, mark a spot on the 2.56" coupler 1-5/8" up from the end with the centering ring installed. Drill a 1/4" altimeter power switch access hole here. Then apply epoxy to the inside edge of the 2.56" coupler tube and to the end of the 54mm altimeter bay tube, then slide them together, taking care to align the two altimeter power switch access holes. If you have assembled everything correctly and made the right measurements and tube cuts, everything should line up right.