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Burnout
This is when your motor's thrust stops. Every motor has published thrust curves that give you that
exact information, you can get them either from the manufacturer's web site, a third-party web site
like thrustcurve.org, or from the data sheet that's typically include with the motor. Note that these
figures are averages based on tests of a number of samples; they are "typical" values, not absolute.
Because of that, we strongly recommend that you add 20% to the typical figure, so you have a
"worst case" value to work with.
Separation
This is when you want the booster stage to separate from the "stack". Typically, you will want to do
this before you fire the sustainer's motor, because the smoke and fire from the sustainer's motor can
do significant damage to the coupler. However, if this is an acceptable risk (maybe you have some
kind of liner in the coupler, or it's replaceable) then you may actually want the motor blast to
separate the stack.
There are three ways of doing a separation: drag separation, separation charge (or a variant of it),
and motor separation. With drag separation, the coupler is fit relatively loosely into the sustainer,
with the idea being that when the booster motor burns out the resultant negative acceleration and the
higher drag on the booster will be enough to pull the booster apart. This works best for boosters
with large fins, for example we've done it with a Rocketry Warehouse Double Shot which uses a 4"
booster with a 54mm motor, and a 54mm sustainer with a 38mm motor. The booster is much larger
than the sustainer and has large fins, so it drag separates well.
We've already mentioned motor separation, so we won't elaborate on that further except to say that
it works well but tends to cook the inside of your interstage coupler pretty badly, depending on the
motor that you're using in your sustainer. If you decide to do that, you might want to think about
putting some kind of replaceable lining inside the coupler, and possibly fill it with some "dog barf"
wadding too.
Finally, there's the separation charge method. With a separation charge, you use a pyrotechnic
charge to forceably separate the stages. Typically, this is done shortly after motor burnout, but
before your sustainer ignition. The trick is that you don't want the separation charge to pop the
stages apart while the booster is still firing, so you need to give it a little extra time from your
booster's burnout time. You also have to wait a little bit to fire the sustainer motor if it's an AP
motor, because the pressure generated by the separation charge can snuff out the sustainer's igniter;
the pressure has to subside first before you ignite the sustainer. You must also be careful to make
sure that the separation doesn't happen while the booster motor is still burning; if it does, it can
knock the sustainer (and possibly the booster) off-axis.
Sustainer Motor Ignition
This is the paydirt, when you light your sustainer motor. As has been mentioned previously, the
ideal situation would be that it happens after the booster motor burns out, you separate the stages,
and you wait a short time for the separation pressure to subside.
The reality is that this can be a tricky value to come up with. Black powder (Estes) motors igniter
almost instantly, but high-power AP motors take awhile to light and come up to pressure. The
amount of time that this takes is highly dependent on the propellant formula and the igniter; "white"
or "blue" propellants tend to light relatively fast (maybe about one second), and "red" or "green"
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