Hopefully the drawings below will help give a better understanding of how all this works. Click on any of these drawings to get a better look at it.

Here are two views of the completed altimeter module.  The left end has two (white) PVC ejection charge holders and a U-bolt. The recovery harness attaches to the U-bolt. A gray foam gasket is on the backside of the flange.  The tube is 2.98" OD phenolic tubing.  There is a hole about mid way along the tube for access to one of the altimeter arming switches.  A short piece of threaded rod protrudes from the right end.

These bulk plates were purchased from A1CenteringRings.com.

Bulkhead B3 is for the forward end of the altimeter module.  It is also made by epoxing together two wood plates of different thicknesses and different diameters and then drilling the center hole.  These were also purchased from A1CenteringRings.com.

Bulkhead B4 is the forward end of the altimeter bay mount.  It was made by epoxing together two wood plates from A1CenteringRings.com.

It was purchased from A1CenteringRings.com.

All tubing used here is flexible phenolic tubing from Giant Leap Rocketry.

Another view of bulkhead B4 and centering ring CR5 before they are epoxied to the 3" ID altimeter mount tube.

CR5 is now in position at the top and bulkhead B4 is in position at the bottom of the tube.

Altimeter mount fully assembled and ready to be installed inside the Coldfire body tube.  The altimeter module will slide into the opening in this mount.  The opening will be facing aft when this mount is installed in the body tube.

This shows how it all goes together inside the Coldfire body tube.  Click on the drawing for a closer look.

Two wood blocks were temporarily bolted to the forward end (bulkhead B3) of the altimeter mount.  This setup was adjusted so that the altimeter mount would be held in place inside the body tube at precisely the correct depth.

The inside of the body tube was coated with West System epoxy and then the altimeter mount was dropped in.  The top wood block held it in position until the epoxy cured.

After the epoxy had cured, the four 5/32" diameter altimeter bay vent holes were drilled.

These three parts are for the altimeter module. Bulkhead B3 is at the left, the tube is in the middle and bulkhead B2 is on the right.

Bulkhead B3 now has the 1/4"-20 x 9.875" threaded rod installed.

Altimeter module tube with bulkhead B2 at the top of it.

The Coldfire altimeter bay is sealed off from the parachute compartment with a fire-resistant silicone foam rubber gasket. It is McMaster-Carr part number 85925K403.  It is the gray sheet in this photo.  It is 1/4" thick and has an adhesive backing.

The foam gasket was cut to size by laying the two plywood plates on it that were used to make bulkhead B2.  The bigger plate sized the OD of the foam ring and the smaller plate sized the ID of the foam ring.

Bulkhead B2 installed on the aft end of the altimeter module tube.  The foam gasket is ready to be installed next.

To get the foam ring properly centered, it was slid over the tube on the altimeter module and then the adhesive backing was pressed against the backside of bulkhead B2.

Foam gasket fully seated and adhered to the backside of bulkhead B2.

Bulkhead B2 was then removed from the tube and as shown here, the foam ring is correctly placed and is stuck to the bulkhead using its adhesive backing.

This photo shows the main parts of the altimeter module except for the actual electronic altimeters themselves.  It is all held together by the threaded rod that runs down the center of the assembly.

The two photos below illustrate how the altimeter module slides into the altimeter mount and how the gray foam gasket will get compressed to seal the joint.  (Of course these two photos were taken before the altimeter mount was epoxied into the body tube.)

Two PVC 1/2" pipe plugs are used as black powder ejection charge holders.

The bottom of each PVC plug was drilled to accept a small wood screw.

The surface of the bulkhead has also been coated with West System epoxy in order to help make it more durable and make it easier to clean off ejection charge reside each time the altimeter bay is prepped for a new flight.

Three holes were drilled through bulkhead B2 for each of the ejection charge holders.  Two of the holes are for e-match leads.  The third is for a bolt that holds a washer over the wires to seal the altimeter module from ejection charge gasses.  Two small channels were created in the surface of the bulkhead for the e-match wire leads to pass through.  These channels were created using a small piece of aluminum bar stock that is 1/8" thick and 1/2" wide.  The end of the aluminum bar was pounded into the surface of the bulkhead to create a recessed channel.  That was easy!

The wire channels that were created are just slightly longer than the washers are wide.  This allows the wires to pass under the washer and also allows the washer to sit flat on the surface.

This photo shows how the e-match wires emerge out from under the washers and enter the top of the PVC ejection charge holders.  The washer has a small machine screw that goes through the center of it and there is a nut on the back side.  The washer seals off the holes for the e-match wires so that ejection charge gasses can not enter the altimeter bay.  Some modeling clay will also be used to make this seal airtight when Coldfire is actually prepped for flight.

Backside of bulkhead B2 showing how e-match leads will pass through the bulkhead.  The small nuts on the backside are securing the screws and washers we saw on the front side.

This block is a piece of 1x2 Poplar wood that can be purchased at places like Home Depot or Lowes. It was cut to a length of 6.5" to accommodate the length of the MissileWorks RRC2 altimeters.

A 1/4" diameter hole was drilled through the entire length of the wood block. One end of the block was also drilled with a 1/2" diameter countersink hole about 3/8" deep. 

A short metal pin was added to the end of the wood block so that the block will not rotate once it is mounted into the altimeter module.  This is important in order to keep the altimeter arming switches aligned with the vent port holes in the body tube.

The short metal pin in the wood block was created by using this type of wood screw.  It was screwed into the end of the block and then the excess length was cut off.

Here we see the washer with the small hole mounted onto bulkhead B3.

Ultimately the wood block will be mounted onto the threaded rod as shown here.  The small metal pin aligns with the hole in the washer.  The countersunk hole in the end of the wood block goes over the nut holding the washer in place.  When the wood block is flush with the surface of the washer it will be locked in place by the metal pin in the washer and by a nut on the other end of the threaded rod.

The aluminum brackets are made from bar stock that is 1/16" thick and 3/4" wide.  First the bar stock is cut to length and then the switch hole is drilled.  Next the bar is bent into the shape shown here.  The last two screw holes are then drilled. The bar stock was purchased at my local Ace Hardware store.

Two switches are used to allow each altimeter to be individually switched on and off.  They are available from MissileWorks as part number MWC-SW-2.

An additional cavity was also created on the left side of the hole for the wires to the switch to route through.

This shows how the switch mounts on the side of the wood block. It needs to be partially recessed into the cavity in the side of the wood block so that the entire assembly will fit inside the altimeter module tube.  That tube is 2.86" ID.

Both switches have now been mounted.  Recessing the back end of the switches into the wood block has an added benefit of protecting the switch terminals as well as the wire connections that will be soldered to them.

The altimeters selected for Coldfire are the Missile Works RRC2 units. Unfortunately, these have been discontinued and are no longer available. They mount on each face of the wood block shown above.

Orange wires run from the green connector on each altimeter to the back end of each switch.  Duracell batteries have also been installed.

Care is taken to make sure the altimeters are mounted with the battery at the forward end of the wood block.  This ensures that the batteries are forced INTO their connectors under high G accelerations rather than being forced OUT of them.  Besides that, it also makes sense to put the e-match connectors on the aft end of the block since that will be closer to bulkhead B2 where the ejection charges are mounted.

The wood block with the altimeters is now installed inside the altimeter module.  The switches need to be lined up with the holes in the sides of the tube.  This module then gets installed into the mount built into the Coldfire body tube.

It is undesirable to have both ejection charges go off at exactly the same time.  This runs the risk of over-pressuring the parachute compartment.   To avoid this I set each altimeter to fire its chan1 when it detects apogee and then chan2 one second later. Chan1 from each altimeter goes to the same BP charge. Chan2 from each altimeter goes to the other BP charge. If both fire chan1 or chan2 at the same time, then only one charge goes off.  The only way to get both charges to go off at the same time is when one altimeter fails to fire chan1 (or the igniter on its chan1 fails) and then when it goes to fire chan2, at that exact same moment, the other altimeter fires its chan1. Therefore, this method is not perfectly GUARANTEED, but the only time you are at risk of having two charges go off at once is when at least one failure has already occurred. I use this method on all of my rockets that deploy the main chute at apogee.  This method can not be used (with these type of altimeters) on dual deployment rockets.

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