Model Forum / General / Rockets / October 2004

<snip>

I like your thread concept, especially letting it dry in place.   The coil
part might be kind of messy to implement, but it you just brought the thread
out straight to the side of the motor and then painted the entire surface of
the nozzle (not inside) with pyrogen, you'd have an even-better chance of
ignition from non-optimally aimed sparks.

Thinking of setting up a test for this:  Two engines in gap-staging
configuration, sustainer ignition enhancer as above, center of sustainer
nozzle blocked with something to simulate the failure of booster
blow-through reaching the sustainer grain.   Suggestions?  especially on how
to block the nozzle-center without interfered with the burn behavior of the
pyrogened thread?

I'm pretty sure the failure had less to do with temperature of gasses in the
chamber and more to do with the statistical probability of not getting
glowing "blow-through" particles from the booster actually entering the
sustainer nozzle.

My thought was:  by painting the nozzle surface with pyrogen, the chance of
some blow-through particles impacting an ignitable surface would be greatly
increasing.  Once any part of the pyrogen started, it would burn right into
the nozzle throat and ignite the sustainer grain.

The tests I've done so far have shown no problem with the booster
blow-through igniting the pyrogen, so it seems the ignition temp is not
problematic, per se.

I've been too busy to chime in, but here is what I did late last year to use up my
remaining A10-0T motors as they approached the de-certification date.

When staged A10-0T to A10-3T (or if more than 2 stages, A10-0T to A10-0T, etc.) they
worked every time as-is. Large nozzle and large exposed propellant surface are always
good. That's why Estes A8-5 and B4-6 (now long gone) motors worked great in upper stages.
Tiny nozzles do not help ignition. Estes B6 and C6 motors *love* to "pop & stop".

So, when I tried to stage my A10-0T to A3-4T motors or 1/2A3-4T motors, they did not
ignite 80% of the time. This is with direct contact and cellophane tape holding them
together AND scraping the propellant surface of the upper motor to remove surface dust.
Reason: tiny upper nozzle presents too small a target AND large A10-0T booster nozzle
allows gas out the back end.

Solution: I painted a thin layer of magnelite pyrogen onto the entire outer nozzle surface
of the tiny nozzled upper stage motors and also down the nozzle throat to the propellant.
Then they worked 90% of the time. I often had a half-second ignition delay.

I have never done this with larger motors. My success rate with staging 18mm and larger
motors is near 100%. I clean the nozzles, I cellophane tape the motors together and I try
to use larger nozzles in the upper stages. Quest motors are now my upper stage motor of
choice. A Quest C6-5 has plenty of delay time. Ditto the A6-4 for lower altitude flights.

All the teams I helped with TARC had great success with all combinations of 24 mm motors -
even the semi-tiny nozzled C11 and E9 motors. There were a few early ignition failures as
they learned how to do the gap staging (often with clustering). Lots of coupler
engagement. Vent holes.

As for the attempts to use a "wick": Back in the 1970's I attempted to use Jetex wick and
also Sure Shot wicks (when they were fresh and not 30 year old crumbling fossils). Jetex
never worked for me in upper stages. The wick would 'snuff out' as it attempted to pass
whatever was holding it in the nozzle (tape, wadding ball, balsa stick...).  The Sure Shot
would also snuff out if secured with a wadding ball, but not if held with tape. Of course,
with tape, it's not really "secured" since it can fall out as the tape is burned.
Conclusion: blockages like wadding balls and sticks will suck the heat energy out of the
burning wick/fuse and stop it from burning. They obviously don't need oxygen to burn, but
they can be snuffed.

Thermalite would probably be a lot harder (or impossible) to snuff out. But it can't be
used in a "Model Rocket".

-Fred Shecter NAR 20117

I've been too busy to chime in, but here is what I did late last year
to use up my
remaining A10-0T motors as they approached the de-certification date.

When staged A10-0T to A10-3T (or if more than 2 stages, A10-0T to
A10-0T, etc.) they
worked every time as-is. Large nozzle and large exposed propellant
surface are always
good. That's why Estes A8-5 and B4-6 (now long gone) motors worked
great in upper stages.
Tiny nozzles do not help ignition. Estes B6 and C6 motors *love* to
"pop & stop".

So, when I tried to stage my A10-0T to A3-4T motors or 1/2A3-4T
motors, they did not
ignite 80% of the time. This is with direct contact and cellophane
tape holding them
together AND scraping the propellant surface of the upper motor to
remove surface dust.
Reason: tiny upper nozzle presents too small a target AND large A10-0T
booster nozzle
allows gas out the back end.

Solution: I painted a thin layer of magnelite pyrogen onto the entire
outer nozzle surface
of the tiny nozzled upper stage motors and also down the nozzle throat
to the propellant.
Then they worked 90% of the time. I often had a half-second ignition
delay.

I have never done this with larger motors. My success rate with
staging 18mm and larger
motors is near 100%. I clean the nozzles, I cellophane tape the motors
together and I try
to use larger nozzles in the upper stages. Quest motors are now my
upper stage motor of
choice. A Quest C6-5 has plenty of delay time. Ditto the A6-4 for
lower altitude flights.

All the teams I helped with TARC had great success with all
combinations of 24 mm motors -
even the semi-tiny nozzled C11 and E9 motors. There were a few early
ignition failures as
they learned how to do the gap staging (often with clustering). Lots
of coupler
engagement. Vent holes.

As for the attempts to use a "wick": Back in the 1970's I attempted to
use Jetex wick and
also Sure Shot wicks (when they were fresh and not 30 year old
crumbling fossils). Jetex
never worked for me in upper stages. The wick would 'snuff out' as it
attempted to pass
whatever was holding it in the nozzle (tape, wadding ball, balsa
stick...).  The Sure Shot
would also snuff out if secured with a wadding ball, but not if held
with tape. Of course,
with tape, it's not really "secured" since it can fall out as the tape
is burned.
Conclusion: blockages like wadding balls and sticks will suck the heat
energy out of the
burning wick/fuse and stop it from burning. They obviously don't need
oxygen to burn, but
they can be snuffed.

Thermalite would probably be a lot harder (or impossible) to snuff
out. But it can't be
used in a "Model Rocket".

-Fred Shecter NAR 20117

I've been too busy to chime in, but here is what I did late last year
to use up my
remaining A10-0T motors as they approached the de-certification date.

When staged A10-0T to A10-3T (or if more than 2 stages, A10-0T to
A10-0T, etc.) they
worked every time as-is. Large nozzle and large exposed propellant
surface are always
good. That's why Estes A8-5 and B4-6 (now long gone) motors worked
great in upper stages.
Tiny nozzles do not help ignition. Estes B6 and C6 motors *love* to
"pop & stop".

So, when I tried to stage my A10-0T to A3-4T motors or 1/2A3-4T
motors, they did not
ignite 80% of the time. This is with direct contact and cellophane
tape holding them
together AND scraping the propellant surface of the upper motor to
remove surface dust.
Reason: tiny upper nozzle presents too small a target AND large A10-0T
booster nozzle
allows gas out the back end.

Solution: I painted a thin layer of magnelite pyrogen onto the entire
outer nozzle surface
of the tiny nozzled upper stage motors and also down the nozzle throat
to the propellant.
Then they worked 90% of the time. I often had a half-second ignition
delay.

I have never done this with larger motors. My success rate with
staging 18mm and larger
motors is near 100%. I clean the nozzles, I cellophane tape the motors
together and I try
to use larger nozzles in the upper stages. Quest motors are now my
upper stage motor of
choice. A Quest C6-5 has plenty of delay time. Ditto the A6-4 for
lower altitude flights.

All the teams I helped with TARC had great success with all
combinations of 24 mm motors -
even the semi-tiny nozzled C11 and E9 motors. There were a few early
ignition failures as
they learned how to do the gap staging (often with clustering). Lots
of coupler
engagement. Vent holes.

As for the attempts to use a "wick": Back in the 1970's I attempted to
use Jetex wick and
also Sure Shot wicks (when they were fresh and not 30 year old
crumbling fossils). Jetex
never worked for me in upper stages. The wick would 'snuff out' as it
attempted to pass
whatever was holding it in the nozzle (tape, wadding ball, balsa
stick...).  The Sure Shot
would also snuff out if secured with a wadding ball, but not if held
with tape. Of course,
with tape, it's not really "secured" since it can fall out as the tape
is burned.
Conclusion: blockages like wadding balls and sticks will suck the heat
energy out of the
burning wick/fuse and stop it from burning. They obviously don't need
oxygen to burn, but
they can be snuffed.

Thermalite would probably be a lot harder (or impossible) to snuff
out. But it can't be
used in a "Model Rocket".

-Fred Shecter NAR 20117

Those are all pretty interesting ideas, though perhaps more complicated than
that for which I was hoping.  I'm keeping your post for future
"bag-of-tricks" value. :^)

About the ignition temperature question:  I think the sustainer ignition
failure was more likely due to the unfortunate lack of blow-through
particles entering the sustainer nozzle, rather than due to low temperature
of said particles.   The nozzle, being that of a C6-5, is quite small at
about 3mm diameter.  So, while its not quite as difficult as Luke's shot at
the Death Star, it still takes some luck to get a good hit.

It seems a fair assumption that the blow-through particles are hot enough to
ignite pyrogen given that in my last test the entire sustainer nozzle,
throat, and exposed grain were covered with pyrogen, and in order for the
sustainer to ignite (which it did), the pyrogen had to burn first.

As such, it appears to me that the lowest hanging fruit is increasing the
probably of a particle impacting an ignitable surface, just as your
nitrate/BP or flash paper/BP approach would do.  Painting pyrogen, I think,
achieves that same effect, but with less effort, complexity, mess, and
hassle.

Fred mentions elsewhere in the thread that he has used this technique
successfully with A3-4Ts.  The improvement in ignitable area for an A3-4T
would be a factor of 11.  If we say that A is the probability of a sustainer
ignition failure due to no burn-through particle hitting the throat of the
sustainer, then A^11 would be the probability of no particle hitting any
part of the nozzle.  Fred says his failure rate prior to coating he nozzle
with pyrogen was about 80%, so A = 0.8 and A^11 = 0.086.   This agrees
surprizingly well with his estimate that his success rate after pyrogen
painting was about 90%.

For the C6-5 that I used the area increase is a factor of 18, so one should
expect an success-rate improvement substantially better than even Fred was
for his A3-4Ts.

PS: Jerry - I think THIS should be in the FAQ.  ;^)