Some propellants are harder to ignite than others. This doesn't matter too
much for single-stage rockets, but obviously matters a lot for second
stages. Among AeroTech motors, the blues are the easiest and the smokeys
are the hardest.
(Gently) roughing up the inside of the top grain is an attempt to clear off
oxidized APCP and increase the surface area.
Pyrogen is used to use to dip home-made igniters and to augment electric
matches. People have various recipes; most likely someone has a favorite
they can share with you at the launch.
Two igniters wired in parallel is a good idea, although this can be hard
for motors with smaller cores.
If your second stage is lighting at high altitude, you might want to cap
the nozzle as well. (I've never had to do this, but in theory allowing the
pressure to build longer would allow more heat transfer from the igniter to
the grain.
What I have done is use a single igniter, taped to a thin dowel (to keep it
at the top of the motor). I bend the electric match head at an angle so
that it's pressing against the grain and augment it with a pyrogen "dip".
If you're lighting a motor with a large core, you can tape two igniters to
the dowel with the bends on opposite sides. I tend to like long-burning
sustainers so getting even one igniter all the way up to the top of a
moon-burner is a bit of a challenge. (You can build it into the motor, but
not if you have to get past an alert RSO.)
John
On Sun, May 13, 2018 at 11:08 AM, Terry McKiernan <terry@xxxxxxxxxxxxx>
wrote:
I have another question about my 2-stage rocket project that I hope some
experienced people can help me with. This is separate from the 3D coupler
/ rail buttons thing, and from the PET2+ vs. altimeter safety question, so
I figured I should start a new thread.
For the airstart of the sustainer, it seems that ignition failures are a
common problem -- after all, consider how many times we burn an igniter in
a rocket on the pad. They just don't work 100% of the time. So, what can
be done to make this more reliable or provide some redundancy?
The obvious solution seems to be to have multiple igniters, so if the
first one fails the second one might get the sustainer started. My
question is, what's the best way to wire this up?
Option 1: Use 2 separate pyro channels on the altimeter, with separate
wires connecting to the igniters. Set the channels to fire, say, 0.5
second apart so that the rocket is still coasting upwards when the second
igniter fires. Pro: complete redudancy. Con: more complex wiring &
altimeter setup, and if the first igniter burns out, it might take out the
2nd igniter with it.
Option 2: Use only 1 pyro channel. Put both igniters on the same channel,
wired in parallel, so they both ignite at the same time. We'll call this
the "big bang" approach :). Pro: simpler wiring and altimeter setup. Con:
less redundancy (igniters but not channels); single pyro channel is a
single point of failure.
Option 3: ???
Any advice?
Also I read that it's a good idea to rough up the sustainer grains to make
them more likely to ignite. Is that true?
I even saw someone saying to spread some Pyrogen on top of the sustainer
grains. I feel unsure about that (and I don't have any Pyrogen anyway).
Seems like that could cause an explosion, like black power, rather than a
controller burn. Is this is valid / recommended idea?
Thanks!
Terry McKiernan
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