propellants tend to take a few seconds to light. Since it’s difficult to predict the actual time that the
motor will light, it can be a little difficult to precisely model the flight. Fortunately, a delay of a
second or two usually doesn’t matter, and in fact will translate to additional altitude since the rocket
will be moving at a lower velocity when the sustainer motor ignites, with a resultant reduction in
drag. In fact, the maximum altitude for a given set of motors can usually be obtained by delaying
the sustainer motor ignition for several seconds and allowing the rocket to coast. There are some
issues with doing that… more on that later.
Locating Your Electronics
Before you start building your two-stage rocket (or in some cases, before you start planning it) you
need to figure out where the separation and airstart electronics are going to go. There are several
ways of doing it:
Sustainer ignition in the interstage, motor-ignition separation, motor deploy for the booster
chute
Sustainer ignition and electronic booster chute deployment in the interstage, motor-ignition
separation
Separation and booster chute deployment in the interstage, sustainer ignition in the
sustainer’s AV bay
Booster chute deployment in the interstage, sustainer ignition and separation in the
sustainer’s AV bay
You would also add whatever electronics are doing deployments for your sustainer, of course, and if
it’s a big project you probably want to have redundancy on all of the deployments. This can get
complicated. In general, you can usually do motor deployment for the booster, since they typically
slow down relatively quickly once they separate. It never hurts to have BOTH motor deploy and
electronic deployment for the booster.
Doing sustainer motor ignition in the interstage makes the build easy, because you don’t have to run
any wires through the sustainer and you don’t have to figure out how to make the wire disconnect
when the sustainer’s drogue fires. You just mount your Proton in the interstage (with a suitable
bulkhead, of course!), and you can use the drogue channel for the booster’s chute deployment at
nose-over if you wish. The problem is that you pretty much HAVE to do separation by motor
ignition… if it drag separates or you fire a separation charge you’ll pull your igniter out of the motor
before it lights. You’ll want a relatively snug fit between the interstage coupler and the sustainer,
you might even want to use shear pins (although we haven’t tried it…). If you do this, use a
relatively gentle motor (10:1 thrust:weight ratio or so)… in general, we don’t recommend using
high-thrust motors like a CTI VMax in two-stage rockets anyway. High thrust motors can make the
sustainer jam in the interstage coupler... if your motor fires and does NOT separate, you're going to
have one heck of a pile of fried fiberglass coming down.
From a safety point of view, having the stack come apart early isn’t a bad thing, because the
sustainer not lighting if there’s a problem is always an acceptable safety option... everything just
comes down on chutes and you try it again with another booster motor. There’s also no igniter left
in the sustainer motor, so it’s basically inert. Just make sure that you have some way of getting both
pieces down safely.
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