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Using the Altitude and Velocity Qualifications
One thing that the Proton can do that most airstart timers can not is to qualify an ignition using
barometric altitude or velocity. This allows you to pretty much insure that your rocket is pointing
"up", because those qualifications can only happen if your rocket doesn't have a significant
horizontal component. This is useful for motors with a lower thrust:weight ratio, or if you're using a
long burn sustainer motor and you absolutely want to make sure that it only lights near vertically.
There are basically two reasons why you want to make sure your sustainer fires while pointing near
vertical. The first one is that if your sustainer fires off—axis, it's going to go a long ways away from
you. This means that you're going to have to drive or walk for awhile, or even worse, you might
lose your tracking signal and have a very long hunt. Not good. The second, and more important
reason, is that you don't want the sustainer firing if it's not pointing "up". If you have a shred and
your sustainer is pointing down when it fires, that's a huge safety issue.
So, to prevent that from happening you want to make sure that it's firing while pointing "up". The
most basic way of doing this is to make sure that your rocket has sufficient stability, and is built
sufficiently for the motor that you're using. That's another reason why we don't recommend using
very high-thrust motors for two-stage builds... they tend to make parts fly off the rocket. If your
motor has a 10:1 thrust:weight ratio and at least 2C stability at launch, then your rocket is going to
go straight up and and you'll be fine. If your motor has a little less punch (i.e. it's a long-burn) or
the rocket's stability margin is a bit less, then you might want to think about implementing a velocity
or altitude qualification.
To use these qualifications, you need to take your simulation, using the burnout, separation, and
motor ignition figures that you found earlier. Don't forget to figure in the amount of time that the
motor takes to light into your motor ignition figure... it's usually at least one second, but it can be
longer for some motors. If you aren't sure, ask around... chances are pretty good that the members
in your TRA or NAR club will have lots of advice on this. Also, be sure to add in values for the
rod/rail angle, and your expected wind, and don't be afraid to get a few different values of each in
case the conditions change during launch day. The Proton makes it easy to change these values right
before you fly, so there's no reason not to get it right.
One you have those numbers, take a look at the predicted altitude and velocity that your rocket will
be moving when the IGNITER fires, NOT when your motor will actually light. THAT is what the
qualification will be based on. What you are going to do is to take the altitude/velocity value at the
ignition time, and figure out how much you are willing to deviate from those before you decide not
to light the motor. In general, you don't want to igniter the sustainer if it's more than 20 degrees
from vertical. That's why we recommend using a booster motor thrust:weight ratio of at least
10:1... it generally keeps your rocket pointing wherever the rod or rail is pointed. Assuming that's
the case, the difference would be 20 degrees minus whatever angle your rail is pointed at. For a
close approximation, figure 1 percent for every degree, so for a 5 degree rod/rail angle that's a 15
percent variation.
So, you simply reduce your projected sustainer ignition altitude or velocity by 15 percent, and use
that as the value for your qualification. Note that these are going to be processed in the Proton
using the filtered value, which lags behind the "real" value by about ¼ second, so you'll want to take
that into account too.
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