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12.7 What is thermalite fuse and how is it involved in igniting rocket motors?




Description

This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

12.7 What is thermalite fuse and how is it involved in igniting rocket motors?

Thermalite is a type of fuse that has been used in the pyrotechnics
industry for a number of years. It comes in three burn rates, identi-
fiable by the color of the fuse wrapping:

      Color      Type     Burn Rate       Usage
      Pink       Slow     20 sec/foot     Flashbulb ignition
      Green      Medium   10/sec/foot     Ignition enhancer
      White      Fast      5 sec/foot     Not used much in rocketry

The burn rates are approximate and vary with humidity, temperature, age
of fuse, etc. The numbers also correspond to burn rates of exposed
thermalite. When enclosed in heat-shrink or teflon tubing, all three
types burn at an equally fast rate. A typical usage for thermalite is
in a flash bulb igniter:

                         |    <  1/2 to 3/4 inch of thermalite exposed out
                         |    <  end of sheathing
                        |||
                        |||   <  thermalite fuse in teflon or heat-shrink
                        |||   <  tubing (fuse should *just* fit into tubing)
                        |||
                        |||
                       + |
                      + +|    <  1/2 to 3/4 inch thermalite exposed out end
        flash bulb >  + +|    <  of sheathing and taped to flash bub using
                      + +     <  CELLOPHANE tape (NOT masking tape).
                       +
                      / \
                     /   \    <  electrical leads to ignition system

The fuse is sheathed except for about 3/4" at each end. The sheathed fuse
is inserted into the motor and must be long enough for the exposed end to
go all the way up through the core and out the bottom of the motor.
Composite motors are ignited at the top of the core (nearest the delay
charge). The sheathing on the fuse is to keep from igniting the motor
anywhere but the correct location. The other end of the fuse is tape to
a hot-burning flash bulb. The flash is then attached to the ignition
system and ignited in the normal fashion. This lights the thermalite
fuse, which then ignites the motor.

This is the ignition method of choice for clustered composite motors (in
any number above 1) and large clusters of black powder motors.

WARNING: Flash bulbs require VERY LITTLE current to set them off. Read
the warnings in 12.5, above.

NOTE: Thermalite is clasified by the BATF as a 'Class B Low Explosive'.
Out of state purchase, interstate transport, and personal storage
of *any* amount of thermalite fuse requires a Federal Low Explosives
User Permit. Refer to the section on 'High Power Rocketry' for
more details on LEUPs.

This is a change of prior enforcement practice and this material
was widely available as a Class C item for decades. We will see how
long this will last. Several advocates of easy access have suggested
that short lengths of under 12" should be exempt from LEUP and
shipping restrictions, especially those pieces included as stock
igniters with MR and HPR motors from the factory.

Thermalite is one of those magic and critical substances to
rocketry.

 

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previous page: 12.6 The ignition of rockets by other than electrical means is banned by both the NAR and Tripoli safety codes and should not be used.
  
page up: Model Rockets FAQ
  
next page: 12.8 How do you ignite second stage composite motors? Can I use a black powder booster for the first stage to ignite the second (as I do with multi-state A-D rockets)?