Safe & Arm Device History

One of the catalyst for the development of safe and arm devices was the fire on the USS Forrestal in 1967. After this catastrophe a great deal of effort was put into developing safe and arms to protect personnel around ordnance. The first safe and arm created was the electromechanical SAD. And shortly after, in the seventies and eighties, the first laser safe and arm system was  developed. In parallel to and starting significantly earlier, Lawrence Livermore and Los Alamos Labs had been developing high speed electronic detonators for use in simultaneous detonation of high explosives for nuclear weapon systems. That technology was called an exploding bridge wire (EBW). This was the first example of an electronic initiator which did not use or require a primary explosive.

History of Safe and Arm Devices

An EBW instead discharges several thousand volts into a very fine bridgewire element which explodes and imparts sufficient shock energy

This was the beginning of using explosives that have both a low order and a high order output type where the explosive may be able to burn and deflagrate. However, it won’t detonate high order, unless it is imparted with a detonation input.

In the nineties, yet another form of insensitive initiator was developed. It is called a slapper detonator, another name for the Exploding Foil initiator. These use even safer, more insensitive secondary explosives and were really the beginning of the list of explosives that we see today in MILSTD- 1316. The most common energetic materials used in an EFI today would be HNS, or CH6 or CL20. These secondary explosives will only produce a high order detonation output if they receive a large enough detonation energy input. Heat or friction or electro-static discharge (ESD), or sparks will not cause that type of explosive to go high order.

In order to achieve the detonation input at that detonation velocity, an exploding foil initiator generates a flyer chip by discharging high voltage into a semiconducting bridge, which is covered with the polyamide film. When the bridgewire explodes, it propels a polyamide bit flyer at detonation velocity directly into the secondary explosive. You can see this happening in the picture (a) below; Safe and Arm X-Raythat is the semiconducting bridge with the polyamide layer. When the bridge wire explodes after the energy is delivered, the flyer propels off the surface and unpacks the chip.

Unfortunately, the EFIs that were made in the nineties still require the same kind of energy input as an exploding bridge wire, so they were still greater than 3000 volts systems. They required very large capacitors, large mechanical switches, or gas discharge tubes, and because the voltage was so high, the circuit isolation requirements were extreme.- Thus, any high voltage safe and arm used for firing an early EFI in the nineties would have been large, heavy, bulky and generally difficult to manufacture.

In the nineties and early two thousands, China Lake took that technology and improved the semiconducting bridge technology itself to lower the voltage requirements and still achieve the required velocity of the flyer. They also brought the voltage requirements down to the range of a thousand volts instead of 3000 volts. When this happened, it allowed out for a reduction in the total size of the high voltage firing circuitry that allowed for the use of smaller firing capacitors and for solid state firing switches. It’s that technology we use today that we call the Low Energy Exploding Foil (LEEFI).

Common Safe and Arms

Types of Safe and Arms

To begin, a non-SAD is a system that doesn’t use a safe and arm mechanism.

Hot Wire Devices

Hot Wire DeviceA pyrotechnic system consisting of just a power source, and a simple switch or relay on a low voltage. A hot wire device, squib, or a low voltage hot wire is a non-safe and arm device system. Today, this technique, coupled with a low voltage device, is often implemented for non -safety critical events and not implemented when any of the four critical areas of safety are present.

Hot wire devices are used during payload separation of a launch vehicle, the firing of cable cutters, initiating fire suppression cartridges or any other event outside of the four main safety critical events. This type of implementation can be sensitive to
environmental stimuli, such as a stray voltage, EM, coupling to the pins or cook off fire as well as a variety of other stimuli, which is why they are not used in safety-critical events.

Electro-Mechanical Safe and Arm

The Electro-Mechanical Safe and Arm still uses a low voltage bridgewire initiator, however the initiator is integrated internally to the SAD and a mechanical device physically moves or rotates the initiator to a state where even if inadvertent initiation occurred, no pyrotechnic event transfers out of the SAD device. An internal initiator is physically rotated out of line inside the safe and arm from the output pyrotechnics, hence they are referred to as “Out-of-line” Safe and Arms. If the low voltage
initiator were to inadvertently initiate, it will not transfer any energy to the next device in the pyrotechnic chain and the electromechanical safe and arm cannot be armed in this case. To be armed, the internal initiator must be mechanically rotated  Electro-mechanical Safe and Armfrom an out of line state to an inline state prior to firing and it is only in that state that the initiator would reliably transfer energy to the next pyrotechnic in the chain.
► If a low voltage initiator is needed it can be coupled with an electromechanical SAD and still uses a low voltage bridgewire type initiator.
► Arming an electromechanical safe and arm requires mechanical rotation of the initiator from an out-of-line state to an in-line-state prior to firing.

LASER Safe and Arm

LASER SADs are extremely safe since no electronic components are used within the laser initiator. These are desirable due to their exceptional safety when it comes to the initiator itself. There’s no electrical circuitry connected to LASER Safe and Armthe ordinance in a laser safe and arm. A fiber optic cable makes the system fundamentally immune to electromagnetic environments. No electromagnetic energy can be coupled to a fiber and fibers are relatively lossless allowing for very long fibers to be used. A laser source is fed through a fiber and the light energy directly initiates the pyrotechnic material. Unlike the high voltage safe and arm there is no bridgewire or semiconducting bridge in the pyrotechnics. Instead, the LASER pulse travels through the fiber
optic cable and passes through a lens, the direct heat from this laser pulse initiates the pyrotechnic material.

A LASER SAD is like an electromechanical SAD due to the pyrotechnic material used. It is sensitive primary explosive. It is required to contain an optical block which would prevent inadvertent energy from passing out through the fiber to the
device when it’s not armed. The laser SAD utilizes a physical blocking device in-between the laser diode and the fiber path to the initiator. Only the diode can drive light through the fiber. A benefit with a laser SAD: there’s no firing energy loss over a
long fiber which does occur with the wires on a hotwire device. This is still an “out-of-line” SAD. Fundamentally a laser safe and arm is still an out-of-line safe and arm.
► Arming requires removal of optical block for the diode energy to reach the fiber
► Desirable due to the exceptional safety since there is no electrical circuit connected to the ordnance. Only a fiberoptic cable
► Fundamental immunity to all electromagnetic environments
► No firing energy loss in a long fiber
► The pyrotechnic material is a primary, and therefore a sensitive energetic

Electronic Safe and Arm

High Voltage Safe and ArmThe most recent pyro-technology type is the High Voltage Electronic Safe and Arm Device (HV SAD). Unlike all other safe and arm devices, it is an inline safe and arm, meaning, there is no physical block between the firing energy and the output pyrotechnic device. The output pyrotechnics for a HV SAD is an exploding foil initiator.

For a high voltage safe and arm system to be considered a safety system, there are few requirements it must meet. It can only utilize insensitive secondary explosives. These are listed in MIL-STD-1316. And its initiator must require greater than 500 volts DC for initiation of the output. The two requirements are achieved by using what we call an exploding foil initiator (EFI). And the EFI is the fundamental enabling technology for the high voltage, safe and arm.

Sandia developed this enabling technology in the 90s, which does not depend on heat to ignite the pyrotechnic. Instead it uses an insensitive secondary explosive which is directly initiated with a high velocity detonation impact. The high velocity flyer is generated by discharging very high current (>1000A) through a small reactive semiconductor which then explodes and propels a tiny flyer at velocities greater the 3000m/s into the explosive pellet. Due to the insensitivity of this technology and the very specific electrical impulse required to fire it, the U.S. Safety Board community allowed for the use of an all-electronic firing system without the need for mechanical/physical barriers. Arming requires 3 MOSFETs to close in order to charge the
high voltage capacitor:
► Two Static
► One Dynamic