Approved for public release: distribution unlimited.
An Approach to Predict the Slow Cook-off Response of Confined
and Vented Full-Scale Munitions Based on Small Scale Tests
N. Albert Moussa and Vijay V. Devarakonda, BlazeTech Corp., Woburn MA; Michael J. Kaneshige,
Sandia National Laboratories, Albuquerque, NM; and Lori Nock, NSWC IHEODTD N00174
NDIA Insensitive Munitions and Energetic Materials Technology Symposium, April, 2018, Portland, OR
Abstract
We have developed an approach to predict the cook-off response of confined and vented full-
scale munitions based on small scale testing and analysis. This approach has 4 steps: (1)
Measure the thermal degradation rates of confined and vented explosive versus temperature
through small scale tests (~2 g of explosive per test), (2) Measure the burn rates of pristine,
heated and thermally degraded explosive in a strand burner (~3 g of explosive/test), (3) Capture
the above processes in a fast running cook-off model that includes algorithms for thermal
degradation kinetics versus temperature and venting (from step 1), and burn rate versus
temperature, pressure, extent of thermal degradation and venting (step 2), and (4) Validate the
model by comparing its predictions with cook-off test data.
A summary of key findings from the implementation of the above approach to PBXN-111 follows:
• The rate of thermal degradation depends on temperature and confinement. For example,
the mass loss of confined PBXN-111 due to thermal degradation increases from 0.74% in
32.6 hours at 151.8°C to 13.2% in 5.8 hours at 175.7°C. The initial thermal degradation
rates of confined and vented PBXN-111 are almost identical, but at later times the rate of
reaction is higher in confined systems.
• Up to 2000 psig, the burn rate of PBXN-111 is almost independent of temperature,
pressure and time but it increases marginally (up to 7.5 times) with the extent of thermal
degradation. Above 2000 psig, the burn rate increases significantly with the extent of
thermal degradation and pressure. We observed up to ~3 orders of magnitude increase
in burn rate due to a combination of thermal degradation and pressure.
• The wall temperature required for ignition increases with heating rate.
BlazeTech’s thermal degradation, burn rate and cook-off tests with PBXN-111 are presented
along with the data analysis and model development. We find that tracking the pressure evolution
(while ignored by others) is critical to proper modelling of slow cook-off.
Introduction
Historical data (USS Oriskany 1966, USS Forestall 1967, USS Enterprise 1969 and USS Nimitz
1981) suggest that accidents involving energetic materials and munitions can lead to large scale
damage during regular military operations. This has prompted the DOD and DOE to develop
Insensitive Munitions (IM) that are safe under normal conditions but can be activated on-demand
under a narrow range of conditions. However, the explosive formulations being evaluated as IMs
can cook-off when exposed to heat. Cook-off response is commonly studied using full scale tests
that are time and resource intensive. In addition to developing safer chemistries, the research
community is examining safety methods such as latent venting to protect against various types of
hazards. We developed an innovative approach to evaluate the safety of new formulations to
cook-off and develop vent design parameters. Our approach consists of a coordinated set of small
scale tests and modeling covering thermal degradation, ignition, combustion and venting. It was
implemented on the slow cookoff of PBXN-111 through a Phase II SBIR project funded by the US
Navy and it can be applied to other munition formulations. PBXN-111 consists of 43% ammonium
perchlorate, 25% aluminum, 20% RDX, and 12% HTPB/IDP binder system. Our models can also
be used to design vents to lower the cook-off violence and are equally valid to fast cook-off. Our
approach consists of four steps described in this paper.
