Staging composite propellant motors is more complex than staging black
motors. Composite motors are more difficult to ignite, and do not reach
operating pressure immediately. Several factors must be considered.
Timing Sustainer Ignition
Examine the thrust curve of the booster's motor and run flight simulations through a computer program to determine the best time to ignite the sustainer's motor. The best time depends in part upon your motor choices. Different motors have different thrust curves. Some produce significant thrust until burnout. Some have a rapid decrease in thrust after ignition and may not produce a significant amount of thrust during the final part of the burn.
Assume a two stage rocket where the booster motor burns for 2.5 seconds, and its thrust curve shows a significant decrease in thrust after 2 seconds. If staging is performed after booster motor burnout (after 2.5 seconds), the rocket may coast for about 1 second before the sustainer's motor ignite. Add the 1/2 second of insignificant thrust, and during the 1.5 second coast the rocket may slow and arc away from the desired flight path by the time the sustainer's motor reaches operating pressure. For this example, sustainer ignition may best be initiated about 1 to 1.5 seconds after booster motor ignition, which will allow the sustainer's motor to reach operating pressure at about the time the booster's motor's thrust will become insignificant. This could be accomplished by using a timer that detects launch and starts its countdown when the rocket launches.
Composite propellant motors can take one second to reach operating pressure after ignition. This delay must be taken into account when determining the optimum ignition time. If we assume a one second delay between ignition and full operating pressure in the example given above, the best time to ignite the second stage's motor may be one second after booster ignition. In this particular case the second stage motor will reach operating pressure at about the time the booster's thrust become insignificant.
One problem with igniting a sustainer's motor prior to booster and sustainer separation is damage due to the sustainer's motor's blast hitting the booster. This problem can be lessened or prevented by using materials in the booster that will survive the blast from the sustainer's motor, or to separate the booster and sustainer before the sustainer's motor ignites. If the booster and sustainer are separated before the sustainer reaches operating pressure it will coast for a time. The sustainer's motor needs to quickly reach operating pressure before the rocket slows and turns away from the desired flight path toward the ground.
If the booster and sustainer are to be separated prior to ignition of the sustainer's motor (the sustainer will coast for a short time), then separation can by placing a small black powder charge in the coupler, and wiring it's igniter in parallel with the sustainer's igniter.
Electronics, like a timer that detects launch, usually through a G-switch, or an accelerometer, are often used to initiate ignition of a sustainer's motor, along with a low current igniter. Most low current igniters will not ignite a composite propellant motor alone. A low current igniter like a Daveyfire N28F can form the basis for a multiple stage igniter.
Most rocketry electronics (timers and accelerometers), will not fire medium and high current igniters. Some devices are designed to dump sufficient current to work with medium and high current igniters. However, those devices often require additional batteries which adds weight.
A medium current igniter like a Fire in the Hole (Trailing Edge Technologies) with a tungsten bridgewire can be used with a device like a timer that can dump a lot of current. A couple of high current timers designed for rocketry are available. No recommendations are made here because manufacturers and product lines change.
The right ignition tool depends upon what motor is used in the sustainer. Some propellants are easier to ignite than others. Some motors have a large diameter core while others have a small diameter core.
A motor with a large diameter core may need a multiple stage igniter. This type of ignitier utilizing a low current electrical igniter like a Daveyfire N28F is described elsewhere in INFOcentral. Briefly, one or two N28F igniters are dipped into an additional pyrogen like Firestar or Magnelite pyrogen. Then slivers of AeroTech Blue Thunder propellant are attached to the igniter. The N28F igniters the additional pyrogen, which ignites the Blue Thunder slivers, which creates a fireball in the motor that ignites the motor's propellant. For a greater probability of success, use two electrical igniters if the core is large enough.
For smaller core motors place some AeroTech Blue Thunder shaving (thin, small pieces of the propellant) ahead of and around the electrical igniter. Small pieces of propellant ignite faster than large pieces. A paper (or masking tape) "cup" can be placed on the electrical igniter below the pyrogen to keep the shavings from falling out of the motor. The cup must be paper so that it will disintegrate upon motor ignition and not block the path to the nozzle. (A blocked nozzle can cause a motor to overpressurize and burst.) The igniter may need to be assembled into the motor as the motor is assembled.
Keeping the Igniter Where it Belongs
The sustainer's igniter must be retained in the motor and at the top of the motor until it fires. This can be a problem for large motors and large igniters. The G forces during boost may cause the igniter to fall out of the motor, or to collapse to the bottom of the motor. Both situations can result in failed ignition or a failed flight if the motor is ignited at the bottom.
An igniter for a motor with a small diameter core will usually have wires stiff enough to keep the igniter positioned in the motor. However, a long igniter for a long motor like an AeroTech J570 may collapse under its weight. The igniter can be secured into the motor during motor assembly with string or string (carpet) thread. The string/thread will burn at motor ignition allowing the igniter to be expelled from the motor.
Igniters for large motors can have enough mass to compress the wire leads during boost, which can cause the igniter to fall out of the motor or fall to the bottom of the motor. If the wire leads get compressed at the nozzle the nozzle could get blocked, and if the motor ignites it may burst due to overpressurization.
An igniter can be secured in a reusable motor with string or carpet thread during motor assembly. Tie the string/thread to the igniter below the pyrogen and secure it between the top propellant slug and the top of the motor. (Between the top propellant grain and the forward insulator for AeroTech motors.) This will hold the igniter in the top of the motor. Upon ignition the string/thread will burn so that the igniter wires can exit the motor. Thermalite, if available, can be used in place of string/thread, and will assist with motor ignition.
Another method to keep an igniter positioned in a sustainer's motor during boost, provided the nozzle diameter and motor core are large enough, is to attach the igniter to a thin (1/8") wood dowel. Rest the dowel against the booster in the interstage coupler. If ignition will occur during a coast after the stages have separated then use masking tape to hold the igniter in the sustainer during the coast phase. The dowel must be free to exit the motor when the motor ignites.
Submitted by Dean A. Roth