"What I have done is use a single igniter, taped to a thin dowel (to keep it at
the top of the motor)."
The igniter not being in the right place to light the motor probably factors in
a lot. Unlike in a motor that's lit on the ground, the igniter in the 2nd
stage is subjected to the g's of the 1st stage motor burn.
Mike
On Sunday, May 13, 2018 11:21 AM, John Coker <john@xxxxxxxx> wrote:
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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