Electronic recovery deployment is an important consideration for recovery system deployment in any rocket that doesn't have an ejection charge built into the motor.  Dual deployment™, hybrid or plugged motor configurations all need a way to eject the parachute.

Components of the Galejs kit available at Aerocon Systems.

Assembled MAD units. 9 volt powered unit on top, N-cell unit on bottom.

This rocket, which flew at NARAM49, had two magnetic apogee detectors that may have caused this premature deployment. Photo: Buzz Nau.

MORE ON THE STORY:
• Professor Robert Galejs' whitepaper on the Magnetic Apogee Detector

Traditional rocket motors use a black powder charge built into the forward end of the motor to eject the recovery system, and are ignited by a delay grain of slow-burning propellant that is positioned between the upper-end of the motor's propellant and the black powder charge. 

The delay grain is ignited at the same time the motor's propellant is ignited, and the delay grain's rate of burn is specifically determined to allow it to burn for a given amount of time before burning through and igniting the black powder ejection charge, which forces the recovery system out of the rocket.

In motors without this characteristic, such as plugged motor designs; motors without this design feature, such as hybrid motors; or rockets where a multiple recovery event chain is desired, such as dual-deployment rockets, alternative means of ejecting the recovery system usually spells the need for electronics to do the job of determining when the recovery ejection charge should be ignited. 

Typical electronics for this purpose usually include altimeters, timers and accelerometer.  A device more people are playing with these days for this purpose is a magnetic apogee detector, an electronic device that utilizes sensors to detect the "Hall effect" or magnetoresistance of the earth's magnetic fields, leveraging the horizontal field that exists.

Professor Robert Galejs of MIT pioneered the use of magnetic apogee detectors for use in hobby rocketry recovery deployment, ultimately releasing a kit for hobbyists to build their own.  Companies such as Aerocon sell the kit, or for the electronically-challenged, fully assembled units.

Occasionally, premature recovery deployment events have occurred which caused rocketeers to scratch their heads, not sure why the magnetic apogee detector unit deployed the recovery system a few feet off the launch rod.  All things considering, the unit must have sensed a magnetic field shift which it considered a horizontal orientation.

According to Professor Galejs, it was probably due to a magnetized launch rod. "You can help avoid this type of failure if you have an arming circuit which disconnects the BP charge and switches in a bright test LED," Galejs said responding to a question describing the anomaly. After you put the rocket on the launch rod, turn on the MAD and slide the rocket up the launch rod so that the MAD passes the end of the rod. If the LED stays off, you're good to go. If it lights up, you've got a magnetized launch rod and you should change rods or pads.

Galejs said, "It's also a good idea to mount the MAD as far from the launch lugs as possible to limit the effect the launch rod might have."

If you have had an occurrence of this type of anomaly, it would be advantageous to discuss the circumstances openly to determine if rod, rail or pad magnetism could have played a part in your experience.  Please post and discuss the details here so that others may benefit from the pool of experience.

Dual Deployment™ is a trademark of Adept Instruments, Inc. in the United States.

08-08-2007 05:35 PM  #1
Aphyle Hot HCl and rubber @ AM   Joined: Dec 2006 Posts: 107	I wonder what effect local geology can have on the device, such that a Magnetic Apogee Detector becomes a Magnetic Anomaly Detector. I would guess that one should not use one of these near an iron mine, but there are other rock types that have magnetic signatures that might create an anomalous magnetic field at a launch side. This is purely speculative, mind you, based on how sensitive the Hall Effect sensor actually is.

08-08-2007 07:17 PM  #2
Rocketthon TRA L3 TMT   Joined: Feb 2007 Posts: 6	MAD and Port-a-Potties Being a member of the team that launched the Port-a-Potty at LDRS22 in Kansas we had used one of the MAD devices. As most will remember the chutes came out as the rocket (?) cleared the launch pole and then the two M motors had to push against two R14 chutes to a crash landing and much laughter. I figured out what was the problem and why the chutes came out earlier. We had dug a hole in that tough Kansas field to bury a pole with a large baseplate, then even used water to anchor it more firmly into the ground. The launch pole was a fifteen foot long piece of fence rail, very steel, and very connected to the earth. The MAD oriented to the big magnetic field that was going up the center of the Port-a-Potty and when it got to the top of the pole it said "must be there" as the magnetic orientation shifted 90 degrees. If we have a rocket that is flying at our field with a MAD for any recovery it must use a non magnetic rail for flight. Most of our rails are backed up by steel poles for stiffness and not good candidates for use. Rods are also not real good for use unless you have an aluminum rod. Since we have a field rule of L2 or over 10 pounds go up on rails only we don't have a lot of rod usage. Best use in my opinon of a MAD is as a double check for any regular type of altimeter, activated at plus .5 to 1 seconds so you get a reading after leaving the rail and then the altimeter would check the status to see that the rocket is a programed atitude or greater to send out the apogee deployment. With today's very programable altimeters and higher powered processors it should not be to hard to incorporate one in a programable altimeter.

08-08-2007 07:25 PM  #3
billspad Certified Level One   Joined: Sep 2006 Posts: 15	MAD early deployment Gee, thanks for making my rocket the poster boy for deployment failure. Here's what I posted in the Hybrid Yahoo Group: The launches were in New York and Michigan. In both cases it was an aluminum 8020 rail and not a launch rod. The blast deflectors and the pads were steel. This is exactly the same type rail that I've used successfully many times at CMASS launches and the CMASS pads also have substantial steel blast deflectors and pads. In both rockets the MAD's are located at the base parallel to the motor. I don't remember exactly which resistor I used to adjust the angle they fire at but I usually set them up so the rocket is pointed close to straight down before the MAD fires. This was done in my house in MA. The only difference I can think of is the geographic location.

08-08-2007 07:54 PM  #4
UhClem Certified Level Three   Joined: Oct 2006 Posts: 109	I recall having a long conversation with Dave Schaefer about this prior to flight of Jason Wares' Colonial Viper. I had previously flown a three axis magnetic sensor on several flights that happened to have been flown off of rails that had iron pipe reinforcing them. The effect on the magnetic field was obvious. The Viper was flown off of an aluminum rail with no difficulty. For the curious my magnetic field flight data is at: http://home.earthlink.net/...erint/flightlog/log.html Any ferromagnetic material will effect the field around it.

08-08-2007 11:18 PM  #5
Aphyle Hot HCl and rubber @ AM   Joined: Dec 2006 Posts: 107	Quote: Any ferromagnetic material will effect the field around it. Which is why P-3 Orions and S-3 Vikings have the long boom out the back end - a MAD looking for submarines!