Setting timer

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el traveiso

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Question on timer ,if a motor burn is 3sec and then coast phase is 7sec so timer will.be set for 10 sec after launch .
 
Question on timer ,if a motor burn is 3sec and then coast phase is 7sec so timer will.be set for 10 sec after launch .
It completely depends on the timer. Some timers detect the end of thrust so time can be set without compensating for the burn time. That’s better.
Of course you already knew I was going to say this: use an altimeter for recovery rather than a timer.
 
Ok, setting timer to 10 seconds may be a bad idea since the rocket will have lost most of its air speed and may not be pointing straight up. With knowing details on the rocket and motors we can not say if this is ok or bad.

You do need to run sims (OR or RocSim or other) to determine when to ignite the 2-stage (plus allowing for the time it takes for the motor to come up to pressure and produce thrust). You want enough air speed when the 2-stage lights to have a stable flight. You also need to look at conditions for 'lock-out' of ignition (too slow, not high enough, too much tilt) that indicates an unsafe trajectory.
You can post the sim file here if you need help figuring out what to look for.

2-stage airstarts or not easy with a lot more things to go wrong.

Since this sounds like your first air start rocket go through this sub-forum and read all the threads about air starts. There are not that many and they have great info about how to do it and what can go wrong.

I have a 2-stage air start rocket now with four flights. 3 flights good to ok but on one the 2nd stage motor took over 4 seconds to start and by then the rocket was pointing down ward. It then power through a tree and luckily away from the flight line. Conditions when igniter fired were good but not enough velocity margin for a motor that was slow the start.
 
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On all of my high power two-stage rockets my focus has been getting the sustainer motor to light. I have three two-stage rockets, each with a different altimeter that lights the sustainer, but each of them has an accelerometer in it that detects motor cutoff (Proton, Raven, and Marsa). Because my object if still to get the sustainer to light, rather than get max altitude, I’ve always told the sustainer to light 2 seconds after booster burnout. That way I’m confident things are still moving the right direction when the sustainer finally comes up to pressure (assuming, of course, that none of the lockouts inhibited firing the sustainer motor). On the other hand, all three of my two-stage rockets use a simpler altimeter without an accelerometer for the separation charge (RRC3, RRC3 and Quantum). For that, I simply add a half second to the expected motor burn time and fire the separation charge. I know I’m giving up altitude, but I’m working up to that and gaining confidence that I can get the separation charge to fire and the sustainer to light every time.
 
Sounds like a good plan Joe. That is what I have been doing. 900gram rocket, G74 to D15 takes it to about 800 feet.
 
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