Model Forum / General / Rockets / November 2004

Is that the same or similar principle that contributed to the Pepcon AP
explosion?

Fred

Only one of two nitrous tanks in the apartment burned.  The other was in a
back bedroom that didn't burn due to the door being closed.  Some of the
expelosions heard by firefighters were probably aeresol paint cans.

The tank that burned is still intact.  It appears the nitrous vented
through the relief valve.  The tenant has not been allowed in the
apartment yet due to various govenment agencies combing through it.  But,
he has been able to observe things through the broken windows and was able
to see the tank.

Brian Elfert

All well and good. But with a basement full of BP and APCP I've always said
that in the event of a house fire, by the time those motors ignited, the
house would be a total loss already. And I have a LOT more rocket motors
than the average bear :-)

BTW, my house is typical construction: poured concrete foundation, 2 steel
I-beams suported by steel posts across the basement, and wood frame from
there up. In the event of a major fire, the steel would fail before the 2x12
joists.

I've been in contact with the local Fire Department since I first moved out
here, long before the JBGTs got into the picture. They've always known what
I've got and where it was. The most dangerous thing in my basement, both in
the event of a fire, or as the SOURCE of a fire are the gas appliances,
furnace and hot water heater. Next is the metal cabinet paint locker, with
solvents, aerosol cans, etc. Then perhaps the bottle of 151 proof rum from
our honeymoon. The rocket stuff is so far down the list as to not be
significant.

    Bob Kaplow    NAR # 18L    TRA # "Impeach the TRA BoD"
        >>> To reply, remove the TRABoD! <<<
Kaplow Klips & Baffle:    http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf
   www.encompasserve.org/~kaplow_r/    www.nira-rocketry.org    www.nar.org

       We have awakened a sleeping giant and instilled in it a terrible
       resolve. -- Admiral Isoroku Yamamoto, WWII.

Sure does. See:

http://www.rocketmotorparts.com/resources/n2o.pdf

Gary

Hi Anthony, thanks for your reply.  

I selected the 5 gallon figure for illustration purposes only as it
seemed to be a reasonable amount, as opposed to 0.5 or 50 gallons.  I
have no information on the actual inventory of N2O present.  You are
certainly correct that the adiabatic flame temperature increases when
using N2O as the oxidizer.  This will have an effect similar to
enrichment of air with oxygen in addition to the actual enrichment
with oxygen due to N2O decomposition.  Bouyancy-aided flame spreading
rates are certainly affected by oxygen concentration as seen in this
report:

<http://fire.nist.gov/bfrlpubs/fire92/art066.html>

However flame spreading is a heat and mass diffusion-controlled
process that responds proportionally to increases in flame
temperature, not an exponential rate dependence (see Figure 2.17 for
instance).

I think most fires would need to progress substantially before they
would heat a stored N2O bottle enough to rupture it, in which case the
oxidizer addition would be made to an already relatively large fire,
unless the fire was actually started next to the tank.  I do not know
what the design of this particular bottle was, however it is common
for compressed gas cylinders designed to contain a large "liquid"
inventory (e.g. ethylene) to be equipped with a rupture disk or relief
valve to prevent an energetic disassembly.  A rupture disk would
normally allow an initial large puff of vapor to be vented that would
quickly drop the tank pressure and temperature, followed by a nearly
constant flow rate due to boiling of the liquid.  In this case the
venting rate is dictated by the heat transfer rate to the bottle -
similar to boiling water on a fire in a paper cup.  I therefore would
expect flame acceleration effects to be localized and the overall
flame spreading rate through the structure substantially unaffected by
the addition of N2O to the fire at that point in its progress.  If the
escaping N2O were instead dispersed in the air in the structure, the
flame temperature of the wood/carpet/plastic/paper would not be
substantially higher than with air alone, so again the overall flame
spreading rate would not be materially affected.

In the case of wholesale rupture of the N2O bottle, the time of an
enriched N2O atmosphere would also be pretty short.  Most structures
cannot withstand more than a couple psi or so of overpressure (150 mph
hurricane winds only have a 0.4 psi dynamic pressure, for instance),
which is not enough to really affect combustion reaction rates
themselves, particularly for diffusion flames - although the
shattering effect on the structure could certainly make more fuel
available to the fire.  That could occur with other compressed gases
as well (e.g. propane) and is not unique to N2O.

I (still) conclude that the presence of a tank of N2O used by hybrid
HPR enthusiasts would have little (not NO effect, but little) to do
with the final severity of fire damage (how much was burned, $$$ loss)
in most cases.

Experiment, anyone?

Brad Hitch