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In general, we recommend that your booster motor should have at least a 10:1 thrust:weight ratio at
launch. Assuming that your rocket is stable (and a typical long and skinny two-stage rocket tends to
be a bit overstable), it's going to go straight up, and it will be be pointing up when the sustainer fires.
Long-burn motors in the booster won't get the air flowing over the fins and any disturbance (wind,
slightly misaligned fins) can make your rocket point off-axis and possibly lead to you chasing your
sustainer for a very long distance. We're talking miles for a J or bigger motor in the sustainer. If
you want to use a long-burn motor in the sustainer, go on the large side for the booster... we've used
a K805 to J94 in the Double Shot, for example.
As we have mentioned before, we do not recommend the use of high-thrust motors like CTI's VMax
or Aerotech ST's for two stage rockets, particularly for the booster. One reason is that the sustainer
tends to be fairly heavy, and the shock of high thrust motors can cause the sustainer to jam in the
coupler. As was mentioned earlier, this is not a good thing. Also, there is a much higher chance of
fins shredding off, and the sustainer's igniter being pushed away from the top of the grain by the G
forces. All good reasons to avoid these motors for the booster. If you want to try one in the
sustainer... well, it would be interesting.
If your booster motor is going to get you reasonably close to Mach 1 before separation, we
recommend that you increase the sampling rate from the default to 25 or 33 samples/sec. This will
improve the accuracy of the timer's baro SOF detection mechanism (although chances are excellent
that the accelerometer's LD will kick in first anyway). Also, if your simulation shows that the
velocity at the LDA altitude (normally 200') is above 700 ft/sec, we recommend that you lower the
LDA so that it occurs at a lower velocity. This is unlikely to happen with motors in the
recommended thrust:weight ratio range of 10:1 or so, but it's something you need to be aware of.
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