| In some rocketry applications, especially when clustering multiple
motors, it is necessary to ensure that a particular motor lights before
allowing the rocket to liftoff.
For example, a given cluster may have only one of its motors set up
to eject the parachute. If that motor doesn't light, but the others
do, the rocket will surely crash.
To avoid that, a string is placed under that motor so that the string
must burn through before the rocket will go. If the motor doesn't
light, the string holds the rocket on the pad while the other motors burn.
It is important that the holding stick be securely affixed to the rod,
and that the string be looped around the stick so that the crossing point
is directly under the motor. Notice the crossing point is on top
of the stick. If the string is simply passed under the stick, the
flame must reach around the stick to burn the string. The string
must be looped and crossed so that the crossing is on top of the stick
where the flame can more easily reach it. And above all, the string
must be burnable! Kevlar is very popular in rocketry, but not at
all suitable as a burn string.
The string can be attached to the rocket many ways. One way is
to use a piece of wire glued to the motor. Loops can be easily formed
in the wire to receive the string.
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