Seeking a safer pyrotechnic delay
September 28, 2012
- Pyrotechnic delays have also long been used in military munitions.
- However, the problem with existing military pyrotechnic delays is that they contain toxic chemicals.
- To address this issue, Picatinny Arsenal has been developing environmentally friendly pyrotechnic delays.
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PICATINNY ARSENAL, N.J. (Sept. 28, 2012) -- Although the term "pyrotechnic delay system" may be met with blank stares, such items are actually more commonplace than most people might realize.
In fact, on the Fourth of July, they are an integral component of most commercial fireworks.
Pyrotechnic delays serve as "chemical timers." In simple terms, a pyrotechnic delay is used to produce a time delay between two events, such as the time between when a firework is lit to when it expels its payload.
For example the payload can be a colorful shower of sparks or an explosive charge producing a loud boom.
Pyrotechnic delays have also long been used in military munitions. One important example of the use of pyrotechnic delays is in Army hand-held signals for battlefield signaling and illumination.
Hand-held signals, such as the M159 white star cluster, contain a rocket motor and pyrotechnic payload inside an aluminum launch tube.
When the primer at the base of the tube is struck, an initiating charge burns and ignites the rocket propellant.
Hot propellant gases ignite the delay element that burns while the rocket is in flight.
After about five seconds, the delay element ignites an expulsion charge, which then ejects and ignites the smoke or illumination payload.
The white star clusters in the M159 produce bright white light and burn for several seconds.
However, the problem with existing military pyrotechnic delays is that they contain toxic chemicals that are facing increasing scrutiny due to environmental regulations.
For instance, after munitions are used, chemical components from the delay system, such as chromates and perchlorates, remain on the residue. These residue chemicals can seep into the ground and cause ground water contamination.
To address this issue, physical scientist Jay C. Poret and chemist Anthony P. Shaw from Picatinny Arsenal have been developing more reliable, and environmentally friendly pyrotechnic delays.
Poret and Shaw work at the Pyrotechnics Technology and Prototyping Division in the Energetics, Warheads and Manufacturing Technology Directorate.
Their work has been funded by the Army Environmental Command's Environmental Quality Technology Program.
Their project started in 2010, when Poret and Shaw wanted to develop a new system with chemicals that are safer for the environment and for the manufacturers.
One of the chemical systems they considered was the "self propagating high-temperature synthesis," or SHS, approach.
This involves mixtures of two elemental metals, such as nickel and aluminum that react to produce nickel-aluminum alloy and heat.
However, because the starting metals and the alloy product readily transmit heat, the chemical reaction would not propagate when pressed into small metal tubes.
The metal tubing would absorb the heat thus quenching, or stopping, the chemical reaction from propagating. For this reason, along with time constraints, this method was abandoned.
Instead, Poret and Shaw explored a more traditional approach based on pyrotechnics.
This method was different from the SHS approach because it uses combinations of metal fuels and oxidizers versus two elemental metals.
While the fuel is usually a metal, the oxidizer could be many different oxygen-containing compounds such as potassium nitrate.
Moreover, because pyrotechnic-based delays are primarily thermal insulators, they lose heat slowly, in contrast to SHS systems.
This allows the pyrotechnic delay to burn more slowly without extinguishing, and allows munitions designers to develop items that generate the desired effect at the appropriate time.
"It's been a very difficult project, probably one of the hardest things we've worked on," Poret noted.
"I think, because in the course of trying to develop entirely new chemical systems, there are a lot of physical and thermal factors we have to consider too."
Aside from testing different environmentally benign chemicals, another complicating factor was the "housing," said Poret.
The delay housing, or the tube that a delay mixture is placed in, can greatly affect how pyrotechnic delays function.
How fast the delay burns is affected by many different factors, such as the type of material used to construct the housing as well as its geometry.
The size of the housing is also important because the tube's diameter greatly affects heat flow in the system.
For instance, when the diameter is narrow and the tube's walls are thick, there is a higher probability that the reaction will be quenched, resulting in incomplete propagation, and item failure.
"With pyrotechnic delays, it seems to be very easy to get them to burn quickly, but it's a lot harder to get them to burn slowly and reliably," Shaw said.
The team recently had a big success testing the new compositions in actual hand-held signals.
"We knew the systems we developed worked well in the lab, but there's no substitute for the real thing," said Poret.
In recent field tests, delay compositions made by Poret and Shaw functioned well in the M159, releasing the white star clusters as intended.
"We were very excited about this successful test, since we plan to use the technology we're developing in other military items," said Shaw.
The next stage for the project includes testing to ensure long-term stability and reliability, as well as ensuring that the manufacturer can make the compositions in a reproducible way on a large scale.
Poret and Shaw expect the new delay compositions to be fielded in the hand-held signals within the next few years.