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Rocket Electronics 101 Rocket Electronics 101

- John Wahlquist, NAR 69974, TRA 6907


Good morning class and welcome to Rocket Electronics 101. In this class we will cover the
basics of electronics use in rocketry. We will be discussing the whys, wherefores and how-tos of
using the various electronic devices we use in our rockets. As this is an intro-level class well try to
avoid many of the intricacies involved in circuit design and keep things simple.


First, lets discuss the whys of rocket electronics. Some of you will find this a bit old hat
but hang in there and well get to the more interesting aspects in just a bit. So, why do we use
electronics on a rocket? Lets list some of the reasons:

Air-start motors either for clusters or for staging
Parachute deployment - both single and dual deployment
Timed functions such as controlling a camera shutter
Tracking functions - both radio frequency, audible or physical
Telemetry - real time and delayed
Data acquisition from onboard instrumentation



Thats quite a list. Well try to hit on each of these as we go through this class, although for
obvious reasons, we will be putting more emphasis on certain select areas. So lets get started.

First, the above list, while impressive does not actually address the question of why we use
electronics. It tells us how and not why. So why do we use electronics? We use electronics
anytime we want more control over some aspect of our flight than we can get with a conventional
delay element/ejection charge or anytime we need to capture data not readily determined from the
ground. The only control we have without electronics is by means of the motors ejection charge.
This is quite limiting, both from a timing aspect and from the range of functions that can be
controlled. If we use a timer or an altimeter, we can more closely match the action of our rocket to
an ideal behavior. We can set that ideal 11.5 second delay that we need for maximum coast time to
apogee or start a timing motor to click the shutter on our camera. We can get data and information
back from our flights: How high did it go? How fast? Or my personal nightmare - how hard did it
hit? Or we can locate a rocket that drifts out of sight on the wind or follow a rocket as it flies at
night. The downside of electronics is that while they free us up to do more things with our rockets,
they also add weight, complexity and cost to our toys. On that basis, you can characterize the need
to be familiar with and use electronics as such:

Model & Mid-powered
- no real need to use electronics
Certification I flyers
- may use electronics
Certification II flyers
- should learn to use electronics
Certification III flyers
- must use electronics


Now in our discussions, Ive mentioned several devices of electronics that we can use. To
recap, the most common types of electronics finding their way into rocketry are timers, altimeters -
both recording and non-recording, flight computers, and audio/radio frequency tracking devices. We
will discuss these devices in detail beginning with the simplest of them, the timer.

Timers

Electric Matches as Initiator Devices
Timers exist as a class of electronics that we call active
devices. They are intended to initiate an action but how do
they do this? One very common way is the ignition of a
pyrotechnic device such as an electric match (e-match) or an
igniter. What is an e-match and how does it work? An e-match
is a device commonly used in pyrotechnics (read: fireworks) to
cause the ignition of pyrotechnic materials. It is usually
composed of relatively high resistance bridgewire surrounded
by a small quantity of a heat-sensitive pyrotechnic compound.
Current passing through the wire heats it and causes the
pyrotechnic composition to ignite producing a small burst of
flame. This small flame can be used to ignite other materials
(such as ejection charges) or, with augmentation (dipping in
pyrotechnic compound, attaching a length of Thermalite, etc.),
they can be used to ignite a composite motor. Current
requirements to fire an e-match vary with type of e-match
ranging from 0.4 amps to ignite an Oxral or Daveyfire N28B,
to 1+ amps to ignite a Daveyfire N28F, to several amps for
other types of igniters. This is important information, as the
amount of current your timer can put out will severely affect
your choice of igniters. Or, conversely, your choice of an e-
match that your timer doesnt have enough current to fire will
significantly enhance your chance of failure! If you are using
the timer to control a recovery event (parachute deployment)
things could get real ugly if you use the wrong match (Trust me
on this I know!!). For more information on igniters, check
out Rob Briodys article Electrical Current Requirements of
Model Rocket Igniters at
www.gwiz-partners.com/igniters.pdf.



Timers do exactly that. They time. Some
timers have a single timer on the board, some have
two, and a few have three, four, or more timing
channels. A timer measures time from an event
(usually, but not necessarily, liftoff) and then
activates some function (usually, but not necessarily,
an electric match or deployment charge). Did you
note the usually, but not necessarily, . . . Thats because you can use these timed events for any
manner of actions - they are much more flexible than the delay/ejection charge on a typical motor.
A good example of this flexibility would be to use a timing channel to repetitively trip a camera
shutter or to monitor a g-switch for the start of deceleration after motor burnout before lighting a
second stage motor. Activation is
usually by G-switch or Break-
Wire/Pull-Pin. Break-wire or pull-
pin activation involves a wire or
plug completing an electrical
circuit - when this circuit is broken
by the wire breaking or the plug
being pulled from the socket, the
timer starts counting. The timer
counts until the pre-set time is
reached and then switches power to
an output channel. Depending on
the timer, the output channel can
be activated once or pulsed
continuously from that point on.
Most timers either use a large
capacitor to store the energy they
discharge through the output
channel or they require a large
enough battery to activate
whatever is connected to the output
channel. A typical mission for a
timer would be to monitor the
liftoff of a rocket and, once liftoff
is detected, start timing a period of
time equal to the burn time of the
motor and the amount of coasting
time desired. At the conclusion of
this time interval (which must be
predetermined and set on the
ground before flight), the timer
would allow current to flow into a pair of igniters inside the airstarted motors, lighting them and
sending the rocket onward and upward (we hope) to the Oooohs and Aaaahs of the assembled
crowd.
L
O
C
T
i
m
e
r


Now that we have some understanding of what a timer is and what it can do for us, lets talk about some typical uses for these units. Timers are most typically used to control the starting of
onboard motors separate from the ground control equipment (launch system) or to eject a parachute
at a certain time after launch is detected. Figure 1 and 2 show typical placements within the airframe
of the rocket when used in this manner. Note that in Figure 1 the timer is placed back of the forward
centering ring and that in figure 2 it is placed in a separate compartment forward of the motors. The
forward placement makes it more difficult to control airstarts but makes it possible to do a two-stage
deployment, if desired.









Auxiliary Motor Tubes

Parachute
Timer
Ejection Charge
Main Motor Tube




Figure 1 : Timer placement - Aft












Parachute
Payload Bay
Timer

Ejection Charge

Motor Tube




Figure 2 : Timer placement - Forward

In all cases, the timer must be located so that it is isolated from any black powder or motor residues.
This is very important as such residues contain carbonaceous (fancy word that means the stuff
contains carbon) matter that can settle on the circuits and contacts and cause shorts within the
Other Common Initiator Devices
If we dont have access to electric matches what else can