Staging w/ Featherweight 38mm Av Bay

Good morning,
I am just getting into High-powered staging, and I am trying to set up the electronics in my PML Terrier booster. Due to the cramped conditions, I wanted to use a Featherweight 38mm av bay w/ a Raven 3 to ignite the sustainer. I have used this setup to ignite regular ematches in other rockets-does this system put off enough current to ignite an igniter? Any advice on this would be helpful-thanks!

mccrocketman said:

Good morning,
I am just getting into High-powered staging, and I am trying to set up the electronics in my PML Terrier booster. Due to the cramped conditions, I wanted to use a Featherweight 38mm av bay w/ a Raven 3 to ignite the sustainer. I have used this setup to ignite regular ematches in other rockets-does this system put off enough current to ignite an igniter? Any advice on this would be helpful-thanks!

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It is certainly possible to provide enough current (via battery selection) to fire an igniter through a Raven. According to the Raven manual, the 3rd and 4th channels have larger FETs, but there are still limits on the amount of current that can be handled without damaging the Raven for the required current and in case the igniter leads short out after the igniter fires. A safer approach is to use an ematch or a dipped ematch for your sustainer motor if it will fit. Many CTI motors have the black powder plug, so you would just use an ematch with them, and any motor can be lit with a dipped match. Use of a Raven for staging will also let you use an "altitude check" to inhibit ignition if the flight is not nominal.

Jim

Sounds good-my concern was that the tiny 3.7 volt battery would not be able to produce enough current. As long as a ematch dipped in pyrogen would light 29mm motors (biggest that I can put in the Mini BBX sustainer), I'll be fine. How would I do the "altitude check"? Pretty much just program that if the vehicle is below a set altitude, the output will not fire?

WJM

mccrocketman said:

Sounds good-my concern was that the tiny 3.7 volt battery would not be able to produce enough current. As long as a ematch dipped in pyrogen would light 29mm motors (biggest that I can put in the Mini BBX sustainer), I'll be fine. How would I do the "altitude check"? Pretty much just program that if the vehicle is below a set altitude, the output will not fire?

WJM

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I think I gave you some bad data. The bay you're talking about has the perch with the small LiPo? Assuming that's the case, then the Raven should be safe even if the igniter leads short out. So, it's just a matter of whether that battery is capable of lighting whatever igniter you want to use. You would just have to test that. Obviously, it is capable of lighting an ematch. My favorate pyrogen for dipping is the stuff that PML sells, which I think is the same as magnalite.

Doing an altitude check with a Raven first requires a simulation. For your simulation, you need to decide how long you want to delay before lighting the sustainer. Maximum altitude would be obtained (I'm reasonably sure, and per your other thread) by letting the sustainer coast to a stop and then lighting it. That would not be a good idea, so what you need to determine is how low you want to let the velocity drop. My criteria on lower altitude flights is something like 300 ft/s minimum. If the boost doesn't get that fast to begin with, or is only a little above that, then you can't allow more than a second or so of coast (particularly if you don't separate the stages at burnout). Once you determine the point where you want the sustainer motor to light, then subtract a second to determine the point where you want the igniter to light. Hypothetically, let's assume that the simulation gives A=3000 and T=5 at at point. You want the Raven to light the igniter at 3000 feet. It's supposed to get to that point in 5 seconds, but you want to allow a little margin in case the flight or simulation isn't perfect. So, you might program the Raven as follows: T<7 seconds, A=3000 or A>3000, pressure decreasing, after burnout 1.

Jim

I see what your saying. I was planning on doing a spin test/computer test w/ my multimeter. I plan on trying that here in a little bit. Any advice where I can get some pyrogen to dip these ematches in?

JimJarvis50 said:

Maximum altitude would be obtained (I'm reasonably sure, and per your other thread) by letting the sustainer coast to a stop and then lighting it.

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Actually I studied this with my low power electronically ignited three stage. I ran a bunch of simulations to zero in on the maximum altitude and plotted them on a 3 axis graph to show the optimal delay combination. This would be even easier with 2 stages. The resulting delays for best altitude were much lower than I expected. I think the velocity at ignition were in the neighborhood of 200 ft/s on each airstart. I believe this is because the sims take into account arcing over and weather cocking. So waiting longer may give you better distance traveled but not so much altitude Obviously you can do a lot more coasting with high power though.

micro said:

Actually I studied this with my low power electronically ignited three stage. I ran a bunch of simulations to zero in on the maximum altitude and plotted them on a 3 axis graph to show the optimal delay combination. This would be even easier with 2 stages. The resulting delays for best altitude were much lower than I expected. I think the velocity at ignition were in the neighborhood of 200 ft/s on each airstart. I believe this is because the sims take into account arcing over and weather cocking. So waiting longer may give you better distance traveled but not so much altitude Obviously you can do a lot more coasting with high power though.

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I would think that if you look at this for flights well below mach, that you would get a result where zero velocity is the optimum. Perhaps you saw the effect of arcing over. However, it's possible that flights going through mach would have an optimum that is not exactly zero. I've done two-stage simulations where both the booster and sustainer go above mach and then back down again. The optimum, for what it's worth, isn't zero velocity. I use long staging delays on my two-stage flights (I think 18 seconds is my record so far). People think I'm going for altitude doing that, but actually, I'm trying to reduce the maximum velocity as much as I can to survive the flight.

Jim

On my Aerobee-Hi with a Featherweight 38mm Av Bay, I used a Raven with the stock 3.7v LiPo to light a QuickDip'ed Q2G2 igniter in the sustainer. The motor was a single-use Aerotech F23FJ-7, which isn't particularly easy to light. As you stated in the original post, I've never had a problem burning e-matches with this setup for deployment.