L3 AGM-78 Electronics
The board has been designed to fit exactly into the altimeter bay with easy access through the hatch. I've kept the layout simple and clean. All wires will be under the board zip-tied and hot glued. The battery box is new. I've created perfect fit boxes for each 9V cell that each hold the battery clip very firmly on the batter. The cover is screwed on to hold the batteries flat. The entire box will be taped yellow masking tape around the edges and over the top several times to help hold the lid on.
This is the board test fitted on the 3/8" all thread between the forward and aft altimeter bay bulkheads without the need for additional lateral all threads. I wanted to do this to be able to twist the board slightly to get my drogue wires into the Euro style terminals
and keep the board close to the hatch. See the grommets. I can adjust for tightness of the grommets against the end boards with washers and nuts. When tightened it can barely be rotated. All of this added very little weight to the rocket. The forward eye nut has Lock-Tite on those threads and is permanently tightened. The forward bulkhead and box are slid into the upper airframe with the main charge wires screwed to the terminal block. It's pushed down to the forward ledge until the thread goes through the aft bulkhead. The eye nut is tightened until the lower bulkhead is very firmly seating on the lower ledge, the board slightly rotated and the drogue wiring can be screwed into the terminal posts. With the board rotated back into place, all switches are easily accessed through the hatch. 
Primary and Secondary Altimeters Electronics
Schematics
I am using two RRC2-mini Missile Works Altimeters. The altimeters were vacuum chamber tested using LED lights (not actual e-matches). These have been flown successfully 7 times (Primary) and twice (Secondary).
Physical and Operational Features
·Small "Mini" footprint (just 1" wide x 3.15" long)... 29mm capable
·Virtually all SMD construction with silicon resin conformal coating to prevent corrosive BP residue damage
·Sea Level to 40,000 MSL Operational capability
·Individually chamber calibrated using our new "Heise" absolute pressure standard (0.001 PSI resolution)
·Full HIL/ETE (Hardware In the Loop/End-to-End) calibration to null out operational tolerances
·Dedicated "large screw" compression terminals with dedicated power switch connections
·Easy "2 hole" mounting with 4-40 hardware
Functional Features
·Interactive menu programming and operation via pushbuttons and LED's... NO DIP SWITCHES
·All User setpoints are stored in non-volatile EEPROM
·2 complete setpoint "profiles" are available by the flip of a switch for programming or flight ops
·All flight data is stored in non-volatile EEPROM for post-flight recall, even across power cycles
·Complete on-board diagnostic capability
User Setpoints
·Main AGL - adjustable from 300' to 3000' AGL in 100' increments
·Mach Inhibit - adjustable from 0 to 31 seconds in 1 second increments
·Drogue Delay - adjustable from 0 to 15 seconds in 1 second increments
·Main Delay - adjustable from 0 to 15 seconds in 1 second increments
·Deployment Mode - adjustable for Standard Dual Deploy / Apogee Only / Main Only
·Operations Mode - Enable/Disable Low-Freq chirping, Power-Up battery/setpoint chirp, and LV lockout
·2 complete "profiles" of setpoints are available... setup each profile for different flight regime
Post Flight "Log" Data
·Peak AGL elevation (feet)
·Peak Velocity (feet per second)
·Time to Apogee (in seconds)
·Launch counter (cumulative total flight count)
Diagnostic Features
·Battery voltage on demand (volts and 1/10th volts)
·Current MSL elevation (in feet)
·Continuity Input Test mode
·MosFET Output Test mode
Switches:
The circuit uses two switches, one for each altimeter. Through mount slotted switches are from Aerocon Systems and set up as SPST and used for the altimeters. They mount through a 5/8" hole. Switch is SPDT. Originally used as a 120 / 240 voltage selector switch this unit is very clean and mounts almost flush.
The Radio Shack heavy duty slide switches are rated 6VDC 0.3A DPDT and are wired to "safe" the charges by shorting the igniters. When "armed", the short is broken, and the circuit is live from the altimeters to the main and drogue charges. The are mounted horizontally so that when the rocket is vertical they slide left to right to arm. Acceleration forces cannot turn the switch on or off. When armed, the switched is covered in yellow masking tape.
Pyrotechnics
I make my own igniters (e-matches) for the deployment charges and use two in parallel for each charge. These have never failed to ignite the charges. They are made from kits supplied by Rocketflite called MagFire (MF) ematch kits. This pre-measured kit includes a water proofing and desensitizing sealer, thinner and all necessary tools and instructions. I store the completed match protected with a cardboard tube made of launch lug. The low amp match blank can be fired with as little as a 1.5V AA alkaline battery. The no fire rating is 0.92 amps and the all fire is 1.6 amps.
Ejection Charge Sizing
The Main Parachute Compartment
The size of the required ejection charge can be calculated based on the desired ejection pressure and the internal "free-volume" of the rocket airframe. (The volume of the parachute and rigging inside is neglected.) First I determined the required pressure to separate and deploy the recovery system. This depends on the area of the bulkhead, hence body diameter, and the mass of the nose section. The force from the pressure must be enough to overcome the inertia and drive the mass of the nose section the length of the coupler inside the tube to the point of separation, plus a little more for momentum to fully deploy everything. If you consider the nose having to deploy into a wind, or not near apogee, you need a little more push again. Assume that the gas expands and the pressure occurs instantly and uniformly throughout the volume. The pressure exerts an instant force on the forward bulkhead intended for extension. Neglect any change in pressure and temperature from the change in volume as the nose moves forward. I'm considering a 15psi charge pressure. Volume = Length times PI times the radius squared, or 22 x 3.14 x 3.875 x 3.875 = 1037 cubic inches.
The ejection charge equation is:
Wp = (dP x V) / (R x T)
where:
- dP is the ejection charge pressure in psi.
- R is the combustion gas constant, 22.16 (ft- lbf/lbm R) for FFFF black powder. (Multiply by 12 in/ft to get in terms of inches.)
- T is the combustion gas temperature, 3307 degrees R for black powder.
- V is the free volume in cubic inches.
- Wp is the charge mass in pounds. (Multiply by 454 gm/lb to get grams
Wp = (15 x 1037) / (22.16 x 12 x 3307)
Wp = (15,555) / (879,397.4)
WP = (.0176) x 454
Wp = 8.3 grams
WP = 128.1 grains
The Drogue Parachute Compartment:
Volume = Length times PI x the radius squared, or 17 x 3.14 x 3.875 x 3.875 = 801.5 cubic inches
The ejection charge equation is
Wp = (dP x V) / (R x T)
Wp = (15 x 801.5) / ( 879,397.4)
Wp = (12,022.5) / (879,397.4)
WP = (0.014) x 454
WP = 6.21 grams
WP = 94.14 grains
(These calculations were adapted from information posted on infoCentral. Special thanks to Ted Apke for posting it.)
Shear Pins
The ejection charge force is calculated by multiplying the cross-sectional area of the body tube by the ejection charge pressure in psi. Divide this force by 35 pounds to get the maximum number of shear pins that can be used.
The cross section area of the parachute tube is calculated with this formula. Area = pi times the radius squared, or 3.14 x 3.875 x 3.875 = 47.15 square inches. So the maximum number of 2-56 shear pins would be calculated like this: 47.15 x 15 / 35 = 20.21. Half that number, or 10 shear pins. I intend to use two 4-40 nylon screws as shear pins on the nose and two 4-40 nylon screws as shear pins at the center break point for this dual deployment.
(These calculations were adapted from information posted on infoCentral. Special thanks to Duncan McDonald for posting it.)
http://www.info-central.org/?article=303
