This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Upper stages of composite powered models may be ignited by
electrical means or thermalite fuse. North Coast Rocketry has
a Technical Report covering this subject. Excellent articles have
also appeared in Sport Rocketry/AmSpam and HPRM magazines.
You cannot use a black powder booster to ignite a composite upper
stage. The gasses from a BP booster will not properly ignite a
composite. There are composite boosters on the market. These boosters
are all 'plugged' and so cannot ignite any type of upper stage motor.
Composite motors are mostly 'core burners' with the core running the
entire length of the fuel grain. A composite core burner set up like a
BP booster would ignite a BP upper stage too soon.
There are several issues involved in igniting upper stage composite
motors. (1) A timing method must be provided to delay ignition until
the appropriate time, (2) power source for the igniter is required and (3)
the igniter itself must be provided and be capable of igniting high power
motors. Whatever method of ignition is chosen, all 3 criteria must be
Timing Methods ....
Several methods of timing have been developed and used. The earliest and
cheapest timing method is to use a length of unsheathed thermalite fuse.
The fuse is typically ignited by the exhaust from the first stage motor.
The fuse is long enough to allow for the first stage motor burn time and
any desired post-burnout coast. The last portion of the fuse is sheathed
and inserted into the upper stage motor to act as the igniter. The problem
with this method is that not all thermalite burns at the same rate. Also,
the same batch of thermalite will burn at different rates depending on the
altitude, temperature and humidity at the time and place of launch.
Mercury switches were another early method of 'timing' upper stage
ignition. A mercury switch is a small glass bulb with an enclosed drop
of mercury. Two wires run out the top of the bulb. When the switch
is tilted or decelerated the mercury rolls forward to make contact with
the two wires and close the circuit. This results in a closed circuit when
the booster motor stops firing and the rocket begins to decelerate. The
ignition circuit would be set up so that power is provided to the igniter
when the mercury switch closes. EXTREME care must be exercised when using
mercury switches. Tilting the rocket closes the switch, so provisions for
disarming the circuit must be included. After the rocket is placed on the
pad and the circuit armed, any sudden movement of the rocket could set of
the second stage.
Bob Weisbe uploaded plans for a mercury switch-based staging system that he
used in a converted Estes Terrier-Sandhawk kit. The URL for these plans
The next generation of upper stage ignition systems were based on
electronic timers of various types, both analog and digital. The timer
was set for the appropriate time (first stage burn time + inter-stage
delay, if any). A contact switch, usually kept open by the launch rod,
would often be used to initiate the timer. As the rocket leaves the
launch rod the timer is started. After the preset time interval the timer
closes the circuit allowing power to the igniter. Again, great care must
be taken with these devices. If the contact switch is allowed to close
prior to the rocket lifting off the 2nd stage could ignite while the
rocket is still on the pad and there are people around.
Another form of early timing device was based on photo-electric sensors.
A sensor would be placed in a position such that light could get through
the booster motor tube after all of the fuel was spent. When the sensor
detects light the power circuit is closed.
Remote control has been used to initiate firing sequence in mutli-stage
rockets. This method has the advantage that the 2nd stage isn't ignited
unless a human being takes positive action, while the rocket is in the air.
It also requires an R/C transmitter, receiver, etc.
Some newer devices are out based on acceleration detection. These are
sometimes combined with timers. Liftoff acceleration is detected. This
either starts a timer or enables a deceleration sensor. At the specified
time interval, or when deceleration is detected, the power circuit is
Power Sources ...
Two forms of electric power are commonly used, capacitors and batteries.
A capacitor is typically charged from an external source just before
liftoff. The timing device then closes the circuit at the proper time
and the capacitor discharges, firing the igniter. One disadvantage of
this method is that the capacitor charge slowly bleeds off, meaning that
the rocket may not sit on the pad a long time after preping and still
reliably ignite the upper stage(s).
All forms of small batteries have been used, depending on the power
requirements. Common batteries for igniting a single, low power igniter
are 9V transistor and 12V alkaline lighter batteries.
Timed thermalite fuse ignited by exhaust from the booster requires no
Multi-stage rockets generally have a limited current source for igniting
upper stages, so very low power igniters are used. Two common igeiters
are electric matches and flash bulb/thermalite fuse. Both of these
igniters are described elsewhere in this document.
Readers are encouraged to review the NCR technical reports and rocketry
magazine articles on composite multi-staging.
A document describing igniters, and how one can make ignitors using
thermalite and nichrome wire is available on the sunsite archive at:
Illustrations for this document are also available for downloading: