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We've found that it's more flexible (although more complicated) to do the separation in the booster
using a separation charge in the interstage coupler, and to place the airstart electronics in the
sustainer. This makes it somewhat easier to do parachute redundancy for the booster, you can use a
single Proton in the booster for the separation charge and chute at nose-over, and use the booster's
motor ejection as the backup for the chute. If the booster drag separates (intentionally or otherwise),
your sustainer is still going to light because the igniter isn't tied to the interstage. You can use
higher thrust motors with this configuration, maybe 15:1 or so, and your rocket is most likely going
to go straight where it's pointed. Just don't use really high thrust motors like a VMax... we'll tell
you why later.
The complication with doing this is that you need to either have the electronics somehow located
near the motor so that you can run the wire to the igniter (easy for a large build, harder for a
minimum-diameter build), or run a tunnel inside the body tube from the AV bay to the motor's
nozzle end and have some kind of break-away electrical disconnect so that when the drogue fires
the wire comes apart without fouling the drogue shock cord and parachute lines. Small Molex
connectors can be used for this... just don't expect them to last for more than one or two flights
before you have to redo them, because the pins tend to get pulled out and the little tabs that hold
them into the connector tend to break. You also have to be careful that they don't get separated
during the boost, but you don't want to tape them together either or they won't come apart when
they should. Taping each end to the inside of your body tube (or the AV bay bulkplate) ususally
fixes that.
If you also do the separation in the sustainer's AV bay, you'll generally need to run TWO pairs of
wires down the body tube, and have TWO break points when the drogue side of the AV bay
separates. Obviously, this adds more complication. If you can do the separation in the interstage, it
makes things quite a bit easier on the build.
If you put the sustainer ignition in the sustainer AV bay, you'll need to have whatever normal
deployment electronics you build would use. The Proton has six channels, so chances are pretty
good that you'll have plenty of channels to go around. It also gives you the opportunity to have some
deployment redundancy, assuming you put a second altimeter in the AV bay (like an Eggtimer
Quantum, for example). In general, we recommend that you have redundancy in at least your
drogue. If your main chute gets jammed or you have a bad ematch or for some other reason it
doesn't come out, if you land under drogue the landing will be a bit hard but it generally doesn't
cause serious damage. If your drogue doesn't come out... you're coming in ballistic. Even if your
main chute fires on cue, you're going to be going so fast that it will tear up the chute and maybe take
100 ft/sec off your velocity because there's not enough time for the rocket to slow down before it
hits the ground because you're moving at ft/sec or so. Trust us... we know. Better our rockets than
yours...
A Word About Motor Selection
As we have mentioned several times, you MUST run a simulation on your airstarted flights. That's
the only way to really get an idea of what the delay times need to be, and how high it's going to go,
and what the velocity at sustainer ignition is going to be. This is particularly important for long-burn
motors in the sustainer. We've used them, and they're very cool, but you have to make sure that the
rocket is pointed "up" before you light them.
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