Angelfire carries two MissileWorks RRC² altimeters as well as an ARTS recording altimeter.  Since the ARTS was new to me I wanted to have the MissileWorks altimeters do the actual drogue and main parachute deployments.  At least initially.  After I gain more experience with the ARTS, then it too can be wired into the ejection charges.  I just want to make sure I know how to use and configure it properly first.

As part of my testing I use a reference altimeter dial that I purchased at a local sporting goods store.  This is a handy little item.  It can be placed into the vacuum jar along with the electronic altimeter being tested and shows what altitude the electronic altimeter should report.  They don't necessarily match exactly, but they should be pretty close.

This is my primitive altimeter test setup.  A motorized vacuum pump is in the upper left corner.  It is connected to a 5-gallon black pressure tank using some clear plastic tubing.  There are a couple of values on the tank.  One lets in external air and one connects to the jar through more plastic tubing.  The reference dial altimeter and the electronic altimeter to be tested are both placed in the jar.  Initially both values are closed and the vacuum pump is used to pump nearly all the air out of the black tank.  A gauge on the tank shows the remaining pressure in it as it gets pumped down. The gauge is handy for knowing when enough vacuum has been achieved to proceed with the test.

Once the black tank is sufficiently pumped down, the valve leading to the jar is opened.  This simulates liftoff because a large amount of air quickly flows out of the jar and into the tank.  As the pressure comes to equilibrium, the altimeter detects it as apogee.  At that point the dial indicator shows how "high" it went.  Next,  the other value is slowly opened to allow external air to flow back in.  This simulates descent toward the ground and at some point the altimeter's "main" parachute channel will fire.  The dial indicator shows the altitude for that event too.

Two small bulbs are mounted into the altimeters in the vacuum jar so that the firing of the drogue and main parachute channels can be easily observed.  The point here is not to test whether the altimeter can deliver enough current to fire an ematch.  This is a test of the pressure sensor and the microprocessor.  A test of the current capability is done separate from the pressure test. 

These bulbs are available at Radio Shack.  The part number is 272-1154.  The nice thing about them is that they have short wire leads that are spaced apart just right to fit into the MissileWorks altimeter connector.  The bulbs are rated for 12V so they work great in this application.

These are the parts for the "sled" that will hold the altimeters inside the altimeter bay.  The large sheet of yellowish-orange G10 was supplied as part of the Air-X "Black Rock" kit.  It fits perfectly inside the altimeter bay coupler tubing. The two silver colored pieces are aluminum 1/2"x1/2" angle pieces that I cut, drilled and shaped to be end caps.  They help stiffen the G10 sheet across it's width. The two long brownish tubes on the left side of the photo are Kraft/phenolic tubes with a 1/4" inside diameter.  They allow this "sled" to slide over the two 1/4" threaded rods that run the length of the altimeter bay.  They are available from MissileWorks as 1/4" launch lug tubing.

Hysol epoxy was used to secure the tubes to the sheet of G10.   Holes were also drilled in the G10 at points where the altimeters need access. 

Three power switches are used to allow each altimeter to be individually switched on and off.  The switches are the small black cylinders in the lower right corner of the photo.  They are available from MissileWorks as part number MWC-SW-2. The three metal pieces are strips of aluminum that I drilled and bent into the necessary shapes to mount these switches onto the sled.

Here we see all three switches mounted to one end of the sled.  Each one is oriented so that the face of the switch will be positioned right behind an altimeter static port vent hole when the sled is installed into the altimeter bay.  This allows each switch to be actuated by inserting a small screw driver through the altimeter static port vent holes.  This avoids the need for any additional holes in the airframe for the switches.

Another view of the switch mountings.  The two on the top side will be wired to the MissileWorks altimeters.  The one on the bottom side will control the ARTS altimeter.

The ARTS altimeter uses a separate 9V battery.  I created a simple 9V battery holder using six threaded standoffs.  The standoffs are mounted to the sled by machine screws coming into them from the other side.

The positions of the standoffs are set to accept a 9V battery to be placed between them.

The battery is secured in place with a strip of aluminum that is held by two screws into the standoffs at each end. 

Here is a view of the sled with the ARTS altimeter mounted in place and wired to the switch and battery.  The ARTS is mounted on two 1/4" high standoffs using the two mounting holes on its centerline.  Care was taken to make sure the orientation was correct for the accelerometer!

This view shows the opposite side of the sled.  The two MissileWorks altimeters are mounted using four #4-40 machine screws and nuts.  I also used four #4-40 nylon nuts as insulating spacers to lift  the altimeters off the G10 mounting surface.

The two white nylon cable clamps at the forward end of the sled provide a convenient way to anchor all the wires from the ematches.  The wires from the four ejection charges can be tie-wrapped to these anchor points to strain relief the joint at the altimeter connector.  This ensures that the connections at the connector don't get pulled or stressed during normal handling or during flight.

Now that the altimeters are finished, it is time to turn our attention to other parts of the recovery system.  Click here to see some other parts of the recovery system.

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