Recovery Operation Description
The Zephyr is designed to use several different deployment alternatives. The parachute bays are amply sized, allowing for any number of parachute configurations. For my Level Three certification I have been advised on several different occasions to "keep it S-I-M-P-L-E" and this is advice I intend to heed. So the plan for my certification fight is to empoly the Single deployment method described below. I have also included the description of dual deployment, even though this method will not be used on the certification flight.
Single-Stage Deployment Description
The control systems will sense apogee by measuring both changes in both acceleration anf barometric pressure. Upon sensing apogee, each control system will send an electrical charge to their dedicated ejection pyrotechnic device. The charge will cause the pyrotechnic device to ignite. The sudden increase in pressure from the resulting explosion will eject both the nose cone and the packed parachute from the forward compartment. As the rocket falls, wind resistance will strip the deployment bag from the parachute causing the parachute canopy to fill with air and slow the rocket's descent. Estimated descent velocity using the parachute supplied by my Level Three advisor is less than ten feet per second.
Dual-Stage Deployment Description
The control systems will sense apogee by measuring both changes in both acceleration anf barometric pressure. Upon sensing apogee, each control system will send an electrical charge to their dedicated ejection pyrotechnic device located in the after parachute compartment. The charge will cause the pyrotechnic device to ignite. The sudden increase in pressure from the resulting explosion will cause the rocket to separate at the midpoint of the airframe and also eject the packed drogue parachute from the aft compartment. As the rocket falls, wind resistance will strip the deployment bag from the drogue chute causing the parachute canopy to fill with air and slow the rocket's descent to about 40 feet ber second.
The rocket will descend to a predetermined altitude of 500 feet, at which time the control devices will send an electrical charge to their dedicated ejection pyrotechnic device. The charge will cause the pyrotechnic device to ignite. The sudden increase in pressure from the resulting explosion will eject both the nose cone and the packed main parachute from the forward compartment. As the rocket falls, wind resistance will strip the deployment bag from the parachute causing the main parachute canopy to fill with air and slow the rocket's descent to less than ten feet per second.
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The motor tube and motor mount. It is a Public Missile 4-inch phenolic tube, 48 inches long. The rings are 1/2-inch 7-ply baltic birch. The additional rings give more adhesion surface. The two front rings are epoxied in first, and the aft ring will be left off until the fins are fully mounted.
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A closer photo of the recovery aft mount. The threaded rod will distribute the recovery shock to two rings. This mount will be used only in dual deployment recovery, but I will likely tether the two halves together even when I'm using only single stage recovery just to make sure all the pieces are connected. The cable is threaded past the nitrous oxide tank and through the center coupler to be attached to the drogue recovery harness.
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This bulkhead divides the center tube in the forward airframe into two compartments, drogue and main. This bulkhead is 1-inch 14-ply baltic birch.
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Another view of the forward bulkhed. Each side of the bulkhead has a harness mount and two pyrotechnic charge holders for the redundant control systems. Separate chage holders are not required - I could have chosen to have each control system send a signal to a common charge - but I felt that separate charges improve my safety margin.
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Pyrotechnics
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. (Normally the volume of the parachute and rigging inside is neglected.) This approach is used in industry for closed bomb calculations and pulsar (pressure cartridge) applications.
First you need to determine 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.
The ejection charge equation is:
Wp = (dP * V) / (R * T )
where:
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dP is the ejection charge pressure in psi.
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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.)
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T is the combustion gas temperature, 3307 degrees R for black powder.
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V is the free volume in cubic inches.
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Wp is the charge weight (mass, actually) in pounds. (Multiply by 454 gm/lb to get grams.)
The parachute main parachute compartment is 30 inches long with a 6-inch diameter (three-inch radius). Volume = Length times PI times the radius squared, or 30 x 3.14 x 3 x 3 = 848 cubic inches. Threfore, the amount of powder needed to generate 15 pounds-per-square-inch will be:
Wp = (15 x 848 x 454) / (12 x 22.16 x 3307)
Wp = 6.57 grams
(These calculations were adapted from information posted on infoCentral. Special thanks to Ted Apke for posting it.)
There are also on-line calculators found at infoCentral that verify these calculations.
Shear Pins
Shear pins are being used more frequently these days as a reliable means of holding the nosecone (and payload bay in a dual deployment rocket) on the airframe during flight. The use of shear pins avoids the possibility of drag separation during flight. Jim Jannuzzo, ROL Construction Forum moderator, found that #2 nylon screws (2-56) make excellent shear pins, reliably shearing under 35 pounds of force. #4 nylon screws (4-40) can also be used with fiberglass tubing.
The breaking force must be generated by the ejection charge. 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. Don't use less than two shear pins because it's possible the nosecone can cock to one side and jam if you only use one.
The cross section area of the parachute tube is calculated with this formula. Area = pi times the radius squared, or 3.14 x 3 x 3 = 28.26m square inches. So the maximum number of 2-56 shear pins would be calculated like this: 28.26 x15 / 35 = 12. Half that number, or 6 shear pins, is appropriate for 4-40 nylon screws. I intend to use a quantity of three 4-40 nylon screws as shear pins on the nose, which should securely retain the nose and also break easily.
I also intend to use a quantity of three 4-40 nylon screws as shear pins at the center break point when using dual deployment.
(These calculations were adapted from information posted on infoCentral. Special thanks to Duncan McDonald for posting it.)
There are also on-line calculators found at infoCentral that verify these calculations.
Ejection Test
The ejection systems were tested in my yard on March 17, 2003. The test was conducted after dark, so the video camera captured the test using the infrared filter so the images are in black & white. I tested both the drogue ejection and the nose ejection, and both worked perfectly. The shear pins worked as planned.
Click [HERE!] for streaming video of the ejection test (0.1 meg .WMV file) (right-click to download)
Click [HERE!] for downloadable video of the ejection test (0.7 meg .MPG file)
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