Relay staging system for air starting clusters

By Burl Finkelstein

My latest project was a three-stage parallel staged high power rocket. Due to the complexity of the on board systems such as staging and firing circuits, redundant altimeter dual deploy and on board camera I chose a straight forward airframe design. The project centered on a semi scale 60% Patriot, 9-1/4 diameter and 10 foot 4 inches high.

Motor configuration was decided to be a center 98mm mount with (2) 54mm outboards, and (4) 38mm outboards. All the motors could be fit theoretically around the centering ring cut for the 98mm mount. Nonetheless, I complicated the design due to my love of the Aero-Pac retainer for 98mm motors. To make the 54mm motors clear the Aero-Pac retainer I had to cut them thru the wall of the airframe in the area of the Patriots decorative conduits.(See the aft end photo)

The plan was to light the center motor on the pad. After burn out of the first motor the two 54 mm motors would fire. After burnout of the 54mm motors the 38 mm cluster would fire. My grand plan for a good show is to fly a Red L 1300, then (2) white J415s then (4) blue I 300s. Or any other combination up to an all white lighting N2000, (2) K700s and (4) J350s.

The problem is how to light up to 4 motors with a single low current blacksky Timer 2 or similar timer?

The timers I had are rated at 1.5A for .25 second. This is fine for a single e-match and possibly even two low current matches. But it could not reliably be counted on to fire 4 since even a low current match required a minimum of .5 amp to fire. My solution was to make up a simple relay firing circuit that could furnish more current. This was the easy part; the .25-second pulse from the timer would be shortened allowing time for the relay to pull in. I need to latch the relay somehow! The answer lies in the fact that the really coils take a very low current so we can use the full 1.5A pulse to charge a capacitor and then let it bleed into the coil.

Radio Shack has relays with a 500 Ohm, 9 V coils with 12 amp rated contacts: Pn. 275-005A; Perfect for the job! I robbed a set of 330 mfd. 25 V caps from an old printer I had laying around and tested the system. A fast touch of a fresh 9V battery gave about a full second of relay on time, the timer output would do the same. I was in business!

This photo shows how it was done, a second battery was added aft of the relays after the photo. The terminals on the bottom of the photo are where the igniters are hooked up.

The arming circuit is a micro switch wire to be Normally closed. This way insertion of the pin will break the firing circuit. (See the red switch on the right side of the photo)

I was careful to mount the relays so the contact set moves perpendicular to the direction of flight. This way the G force created by acceleration upon boost (if any) will not close the relays. You would not wan the relay to close before the timer tells it to.The same goesfor the orientation of the arming switch. It should not open under acceleration.I have used this switch arrangement on many rocket deployment systems and it has been reliable.

Before mounting I examined the relays to determine the direction that the contacts move. I then placed this plane perpendicular to the fore and aft plane of the rocket. The orientation chosen to give the relay resistance to acceleration was tested. First by hooking the relay up to an ohmmeter that was set to beep on continuity. Then the relay was rapped down against a stiff foam pad after a fast swing. The impact generated a few Gs of force. If it does not trip during the test it should not give a premature closure in flight.The micro switches I use for arming switches did not need a test. I have taken than apart before and know which that the contacts move in the one-inch direction of the case. When the switchers are mounted on their side the contact are not affected by acceleration forces.

The finished product with the safety pin installed:	The parallel-staged Patriot at take-off photo by Roy Green
And the schematic: