Shunting Rocketry Electronics / Ematch Safety

By Phil Stein - L3, NAR# 78514, TRA# 9311

 The information contained herein is for informational purposes only.  Because this is dangerous, the author discourages trying to do any of this and assumes no responsibility for your doing so.  All rights reserved.  Copyright 8/2003 Ariel Systems, Inc.

 The following describes the reasons and methods for shunting your ematches.  In a word, it all boils down to Safety.  Note – I will use the term ematch to refer to anything that can be used to ignite a powder charge.  Although much of this information is applicable to staging, it does not address that subject in adequate detail.  Therefore, this was written primarily for recovery electronics.

 When a powder charge is exposed to an ematch, there is the possibility that the ematch will be accidentally ignited and thereby blow the charge.  The worst case consequences include loss of your rocket, damage to property, personal injuries or even death.  

 Some causes for accidental discharge that have been reported are:

Incorrectly installed batteries – Some altimeter manufacturers specifically state that connecting a battery with reversed polarity WILL ignite the attached ematches.  Using keyed connectors that prevent reversed polarity is highly recommended.

Malfunctioning electronics - May erroneously ignite ematches.  Some typical malfunctions include shorts caused by improper mounting, ejection gunk causing a high resistance short between contacts in the electronics or, the on board computer has lost it’s mind, or cracks in components to small to see (these might be caused by a less than optimal landing).  These are a few I’m sure there are many others.

Static – Ematches are designed to ignite at low current.  That is a key difference between a plain igniter and an ematch.  Composite and PML Quantum airframes have a tendency to generate a lot of static.  Combine that with friction from a parachute and you may ignite your match.  In fact either one by itself under certain conditions may ignite it both, increase the chances.  Low humidity also creates significantly more static than high humidity.

Stray RF – As the electronics we use evolve, more people are starting to transmit telemetry data from their rockets.  People have reported this as triggering their ematches.

The electronics erroneously detected a launch event -  

·        The rocket has to be removed from the rail.  It was still armed.  When reoriented from a vertical to horizontal position.  This is especially likely to happen with electronics that use a MAD, or a G Switch but, can happen with anything.

·        The rocket had to be removed from the rail.  It was still armed.  The electronics were set to use a break wire to detect launch.  The break wire gets broken.

·        Disassembly of the rocket while armed – Pulling two sections of airframe apart or removing the electronics bay from an airframe is likely to cause an air pressure change to be detected by an altimeter.  Having your hand over your vent holes when disassembling your rocket will also do this.  This is typically interpreted by the electronics as a launch event.

·        An armed rocket is dropped on the ground or, let slam into a standoff on the launch pad.  Either of these will cause G’s to be detected.

·        A strong wind blowing past the vent holes.

Our electronics are designed to ignite ematches following the detection of a launch event.

 

 

So how do we protect ourselves? 

•     Test all electronics before attempting to use them. 
•     Read and understand the manual for your electronics from front to back.  If you don’t understand something, ask someone.  I have never found anyone unwilling to help when I’ve had a question.  Remember no question is stupid.
•     Make the ability to remove power from your armed rocket a design consideration.
•     Make the ability to shunt the ematches on your armed rocket a design consideration.
•     Test your ematches without a charge, before placing it in the rocket.  This is so that you know it’s good before going to the trouble of placing it in the rocket.
•     Test you igniter again when doing your prep before placing it in close proximity to your charge.  This should allow you to know with a high degree of confidence that you have not damaged the ematch during installation and connection.  Ematches are very fragile once the protective shield is moved away from the head.  Testing at this point is best performed by your electronics.
•     Shunt the leads of the ematch before placing it in close proximity to a charge.  (For the purposes of this article, shunting is providing a path to allow undesired electrical charges to be dissipated through the shunt without going through the igniter.)  The shunt should be easily removed once the rocket is on the pad.  It should also be easy to reinstall the shunt without disassembling the rocket.
•     Once the leads are shunted and you are ready to insert the ematch into the charge.  Wear eye protection when working with charges.  Before doing this, check the area for anything that may cause premature ignition of the charge.  Examples –smokers, transmitters or, objects that are likely to generate static.  Once this has been done, complete your prep.  Remember ematches are designed specifically for low current ignition.
•     I prefer to RSO check rockets before the charge is installed.  This isn’t always practical.  If that’s not practical make sure the RSO knows there are charges installed so he can take proper precautions.  Have your shunts installed before going to the RSO.  Do not bring an armed rocket to the RSO.  Keep the rocket pointed up.
•     You should only arm a rocket with changes at the pad or a specially designated area.
•     Once out at the pad –
  •     Do not remove shunts until the rocket is upright. 
  •     Warn everyone in the area when you are about to arm.  They should stay clear when you are about to remove the shunt and arm the electronics.
  •     Be aware of people at nearby pads before arming.  If, in your judgment, it’s warranted, ask them to clear the area when you arm and remove shunts.
  •     Wear safety goggles when removing shunts and arming.
  •     If you can not be standing on the ground when arming or removing shunts, be sure that what you are standing on is solid and will remain stable when arming electronics.  Make sure it will remain stable if you get startled by premature ignition of a charge.
  •     Immediately after arming or removing shunts, quickly get away from the rocket.  Wait a full minute before approaching the rocket again.  Often, undesired ignition of charges will not happen immediately.  Do not allow your launch crew to get close until you are satisfied that everything is safe.  After a minute, approach the rocket and check for continuity on your ematches.
  •     Once your rocket is armed, there should be no motion until liftoff.  If you must move it on a standoff or make other adjustments, shunt the ematches before doing so.

 

Other considerations for success:

•     Make and use a checklist.  There are too many details that can be omitted.
•     Avoid distractions.  Take a quick inventory or your mental state as you prep.  If you feel you are getting to many distractions, you may have to request that some of the distracters give you some room to work without distraction.  Most people will understand that it is not the time for you to be distracted.  Hunger, thirst or other physical discomfort are also distractions.  Do what is necessary to remove these distractions.
•     Label all of your switches and connections.  This will help ensure that everything you check off on your check list was done right.
•     Ematches usually utilize solid copper wire.  When stripping the insulation from the wire, be sure not to nick it.  Nicks in wires cause them to break very easily.  If you nick the wire cut it off and try again.  Also, repetitive bending in the same spot will make the wire break easily.  Be careful!!
•     Only use ematches that are designed to be used with electronics.  Most electronics do not put out an adequate amount of current to use with regular igniters. 
•     Do not use Xmas Tree Bulbs.  They may be ok for ground testing but, the filament doesn't always hold up to the stress of a regular flight.  $1.50 isn't a bad investment compared to the time and money put into building a rocket.

 

Functional schematic of a typical shunting circuit.