NSWCDD-PN-18-00104; Distribution Statement A:
Approved for Public Release; distribution is unlimited 1
An Investigation into a Proper Heating Rate for Slow Cook-off Testing
David Hubble
Naval Surface Warfare Center Dahlgren Division, Dahlgren, Virginia
Abstract
Historically, slow cook-off (SCO) testing has been performed by heating the munition under test
in an oven at a constant rate of 3.3°C/hr until a reaction occurs. Recently, however, the validity
of this heating rate has been disputed and it has been argued that it is too slow to represent a
realistic threat scenario. While many agree that the heating rate should be increased, there has
been no real consensus on what the new rate should be. This investigation was performed to
help determine what heating rates are possible for munitions and to help select a more
appropriate heating rate for future SCO testing. This was done by examining historical
accidents, reviewing existing analysis, and modelling possible threat scenarios. In the course of
this analysis, no data was found or generated which supports a rate as slow as 3.3°C/hr and it is
concluded that a heating rate faster than 10°C/hr is more appropriate and better represents real-
world threats to munitions.
Background
SCO testing is performed to simulate accident scenarios in which a munition is slowly heated
over an extended period of time. This can result when a fire occurs but is separated from the
munition by some barrier such as the walls of a magazine. This is in contrast to a fast cook-off
(FCO) where the munition is directly exposed to the fire. In a SCO scenario, the heat fluxes into
the item are much smaller than in the FCO and the resulting temperature gradients are much
lower. Therefore, if the munition cooks off, the reaction can be severe because much of the
energetic material is at an elevated temperature when the cook-off occurs. This elevated
temperature can cause normally stable energetics to detonate during slow heating. SCO testing
is therefore necessary to help developers improve the response of munitions to this type of
thermal threat and ensure that any reaction that occurs is as mild as possible.
The current SCO test procedure, as outlined in STANAG 4382, specifies that the munition be
heated in an oven wherein the air temperature is increased at a constant rate of 3.3°C/hr
(6°F/hr) until the item reacts. There is also a provision that allows a different heating rate to be
selected (procedure 2) based on a threat hazard assessment (THA), but the test generally
defaults to the 3.3°C/hr rate specified in procedure 1. In addition to the ramp rate, other
parameters such as item preconditioning and temperature gradients within the oven are also
specified in the test standard. A passing criteria is a reaction violence no more severe than
burning (type V).
The origin of the 3.3°C/hr heating rate is not known for certain. Some point to ship fires during
WWII that exploded up to 2 days after suffering below deck fires. By dividing the predicted cook-
off temperature by the fire duration a heating rate of approximately 3°C/hr can be obtained.
Others have speculated that the slowest possible heating was desired and 3.3°C/hr was simply
as slow as oven controllers could reliably function at the time. Regardless of the origins, the
SCO test has primarily been performed at a rate of 3.3°C/hr for more than 50 years. Recently,
however, there has been increasing pressure to change the document so that the rate specified
in procedure 1 better represents realistic heating scenarios. The concern is that an item that has
been designed to pass the 3.3°C/hr heating rate of the SCO test could react more violently at
the higher rates that the item is more likely to encounter while in service.
