Fuel injection System

Scope

The United States Air Force is looking for a device to allow them to test the initiation criteria for an idealized fuel spray from a Fuel Air Explosive.  In order to accomplish this several goals had to be reached.  A design which allows maximum flexibility, sturdiness, and repeatability must be obtained in order for experimentation to be successful.  Several designs were developed during the concept generation stage, with the design team choosing what was felt as the most suitable design.  This design is to use inert gas to pressurize the fuel, an electrically driven valve for volume control, and a set of commercially available nozzles to meet the spray criteria.  A housing of aluminum plate will protect the pressurization and control subsystems while an expendable clear plastic spray shield will be held in place by steel square tubing.  Spray characterization schemes will be provided with the design, with future work possibly being done at the university as well as at Eglin AFB. 

Project Scope

In an effort to reduce American casualties in warfare the United States Air Force is researching the effectiveness of disposable unmanned aerial vehicles to replace manned attack aircraft.  One design concept requires that the vehicle crash into an enemy position once it has expended its main payload.  With the current design the vehicle explodes on contact with the enemy position in a manner similar to most bombs.  The Munitions Directorate is looking to maximize the damage done to the enemy and as such is experimenting with differing ways to detonate the vehicle’s remaining on board fuel supply.  One way to do this is to treat the vehicle as a Fuel Air Explosive (FAE) which will provide incendiary damage as well as volumetric damage due to the high overpressures cause by the explosion. 

The Munitions Directorate needs a device that will help them study the initiation criteria and thresholds in fuel air mixtures of the type that are present in an FAE.  In order to carry out this research the device must be able to create and simulate an explosively dispersed fuel cloud through non-explosive means.  Since the device will be used to study initiation requirements it must create a cloud that is idealized, yet still is representative of an explosively dispersed cloud. 

Two key variables in the cloud must be adjustable in order to carry out the research.  First, the mean droplet diameter must be adjustable from 500 μm to 5000 μm.  Second, the fuel to air ratio, also called the equivalence ratio, must be adjustable from .8 to 1.2.  The equivalence ratio refers to the actual amount of fuel divided by the stoichiometrically required amount of fuel for perfect combustion.  Thus, the fuel to air ratio must be variable to between 20% fuel lean to 20% fuel rich. 

Characterization of the resultant fuel-air cloud must be done in order to carry out the required research.  Methods for this characterization must be provided to the Air Force.  While the implementation of these methods may not be done by the students, the design must allow for their use. 

The final design must incorporate components that protect the environment.  The environment must be protected from the fuel contamination in the case that the cloud does not ignite during a test.  Also, the system must be designed with the safety of the researchers in mind.  Components must have built in safety factors with the design constraints being delivered to the Air Force at the end of the project.  All applicable rules and regulations defining the use, storage, and handling of the fuel must be met.  Remote operation and appropriate safety devices must be present in the design in order to reduce the risk of injury.

Differing explosive initiations will be tested by the Munitions Directorate.  As such, the device must be able to allow for maximum structural flexibility so that differing locations for the initiation may be tried.  The final design may be built by the students, although it is not required by the Munitions Directorate, with final characterization and testing being performed by the Air Force. 

Background Information

 Fuel Air Explosive Devices

             Fuel Air Explosives have been actively used by the United States military since the mid 1960’s.  The most famous example is the Vietnam era BLU-82 “Daisy Cutter”.  In Vietnam the BLU-82 was used to clear patches of jungle for use as helicopter landing pads.  Modern FAEs are smaller, more accurate weapons designed to inflict the maximum damage to the target.  In particular the Marine Corps’ CBU-72 is a cluster-type bomb containing three 25lb FAE sub-munitions designed for use against “soft targets” such as aircraft, minefields, and personnel.  BLU-96 and BLU-97 FAEs are used by the US Navy and Marine Corps to clear maritime mines.

            In order to understand how to perfect an FAE, one must understand how one works.  In general, an FAE uses three main components in its attack.  At the central axis of the bomb is a bursting charge, whose sole job is to break open the fuel container and disperse its fuel.  The fuel used varies from application to application, but for the purposes of this design the Air Force has decided to us JP-10 rocket fuel.  When the bursting charge explodes it throws the fuel out into a distinctive “pancake” shaped cloud of mixed fuel and air which is exploded by a larger, secondary explosive charge.  This ignition is what causes the damage to the target.

            Of key importance to effectiveness of an FAE are the characteristics of the fuel-air cloud.  The cloud is ideally detonated when it has reached a diameter large enough to contain the perfect stoichiometric ratio for complete combustion of the fuel.  Once at this ideal ratio the secondary explosive ignites the fuel-air mixture through the combined effects of shockwave induced pressure waves and the heat of the rapidly moving flame front.  If, for example, the stoichiometric ratio is not correct, or the secondary explosion is not powerful enough the FAE will not provide the desired results.