Performance Rocketry 4-inch Nike Smoke (Part 3 - Fabricating a small avionics bay in the nosecone)

This kit was originally designed to use only motor ejection to deploy the recovery system.  This means that the only way to ignite the ejection charge is to place it at the upper end of the motor where it is ignited at the end of a pre-determined delay.  Now there is nothing wrong with motor ejection and in fact it is the method used in almost all "model" rocket flights since the ejection charge is built into nearly all model rocket motors.

However, with the advent of reliable onboard electronics, high power rockets are increasingly using altimeters to ignite the ejection exactly at apogee where stresses on the rocket are at their minimum.

To do this, altimeters now typically include one or more firing circuits that are connected to a small onboard battery source and a model rocket motor igniter.  The igniter is placed in contact with a small amount of black powder in some sort of containment canister.  When the altimeter detects apogee has been reached it closes the firing circuit and ignites the ejection charge.

Unfortunately the Performance Rocketry Nike Smoke does not include a separate avionics compartment in it's design.  However the shoulder of the large Nike Smoke nosecone is about 4 inches long which if modified, would be big enough for not 1 but 2 onboard altimeters (1 could be used as a back-up) and still leave plenty of room for an eye bolt to fasten the recovery system to. 

 

 

Figure 1 - Original Av Bay concept sketch

 After coming up with a preliminary design (see Figure 1), I began work on the forward bulkhead that would have to receive the threaded screw-eye.

 

Figure 2 - 1/2" Plywood bulkhead laminated to fiberglass

bulkhead from kit

Just like the centering rings, I didn't feel I would get the rigidity I was looking for by using the fiberglass by itself.  I also wanted something I could mount sled rails into on the av-bay side of the bulkhead, so I laminated a 1/2" plywood bulkhead to the fiber glass bulkhead as shown in Figure 2.

Figure 3 - T-Nut installed in bulkhead

 In order for the av-bay to function, the load bearing bulkhead is moved forward and will receive and anchor a 3/8" threaded eyelet.  However since the threaded eyelet will have to be removable to gain access to the av-bay, I installed a t-nut on the forward side of the bulkhead (see Figure 3).

Figure 4 - JB Weld used to permanently secure T-Nut

The T-Nut was then permanently secured in place with JB Weld (again due to its increased adhesion to metal).  Next up will be to add any nose weight to the the rocket since that will be sealed off when the forward bulkhead is installed.

 

 

Figure 5 - Bird shot mixed with JB Weld

After doing some computations in RockSim and determining the largest motor I would ever want to fly in this rocket ( a full K), I decided to add about 10 oz of nose weight.  To do this I mixed 8 oz of lead bird shot into 2 oz of JB Weld which made a gooey heavy sludge - PERFECT!

Figure 6 - Nose weight in tip of nose

I then pushed the sludge into the tip of the nose with a long dowel and then added a bit more JB Weld as a cap.  Hopefully this nose weight is going nowhere.  Now back to the av-bay.

 

 

 

Figure 7 - Laying out the altimeter sleds

 

The altimeters themselves are fastened to wooden sleds each of which will slide onto a pair rails that hold them secure during the flight.  This will be easier to understand in the pictures that follow.  Anyway, careful planning has to be done when laying out your altimeter board so that 1.) Everything will fit and 2.) the switch that turns that altimeter on and off is accessible through the vent hole.  The vent hole is a small 1/4" or so hole on each side of the av-bay that vents the outside air pressure into the av-bay so the pressure based sensors on the altimeter can determine the altitude.

 

 

Figure 8 - The completed Raven sled

I chose two different altimeters for this project.  A "Raven" altimeter which includes separate accelerometer and pressure based altimeters on a single board.  The 2nd altimeter is a "Stratologger" altimeter from Perfectflight which is solely pressure based.  Each altimeter will be fastened to it's own sled and wired to it's own switch and power source.  This makes for a truly redundant system.
When completed the altimeter sleds look like the pictures in Figures 8 and 9.  The push button on/off switch for the "Raven" sled can be seen in the lower left corner of figure 8.  This switch on each sled is the switch that must be reachable through the vent holes in order to turn on the altimeters prior to flight.  The ejection charge wires connect to the altimeter at the terminal blocks.

Figure 9 - Showing the battery mounted on the underside of

the sled.

 The batteries for each sled are mounted on the opposite side of the sled in order to keep the sleds under 4" long.  Other configurations you may see, where length is not an issue, will put the battery adjacent to the altimeter.  Also visible in Figure 9 are the sled "rails" which are aluminum tubes that will slide over threaded rods in the av-bay that will hold them in place.

Figure 10 - The entire av-bay assembly prior to

installation

Figure 10 shows the entire av-bay assembly prior to installation in the base of the Nike nosecone.  At the bottom is the forward bulkhead and you can see the 3/8" eye bolt that passes through the center of the av-bay and threads into the T-Nut mounted on the other side.  Also visible are the four sled "rails" upon which the altimeter sleds will slide and be locked into place by the aft bulkhead which is held on by 4 wing-nuts.  The terminal blocks receive the +/- leads from the altimeter firing circuits on one side and igniter leads on the other.  The igniter is inserted along with the black powder ejection charge into the white canisters also visible.  Once again everything is redundant.  Each altimeter is connected to it's own separate ejection charge.

Figure 11 - Both primary and backup altimeters installed in the

av-bay

Finally in Figure 11, you see the completed av-bay with both altimeters installed.  It's a pretty tight fit and it required more than a little planning and extra work but having the opportunity to use electronic  ejection on this rocket makes it well worth the effort.  The fact that there was room for redundancy was a bonus!