By Phil Rowe
The squid is a resourceful sea creature. When seriously threatened it discharges a masking cloud of black ink that enables it to escape unseen. In some sense, that is what some expendable countermeasures systems do for aircraft in combat.
Back in WWII days we learned to toss small strips of tinfoil out of aircraft windows, hatches and even bomb bays to create false targets on enemy radar screens. The relatively imprecise ground radars of the day were unable to discern the false (masking) targets from the real ones. Soon the ground and airborne based radar systems were improved to make that ‘window’ chaff dropping technique less effective. Radar systems were able to discern between moving airplanes and the chaff decoys that simply floated earthward at slight velocities. MTI (moving target indicator) radar devices readily separated the static clutter from the echoes of higher speed moving aircraft.
Years later a new threat came along, one that employed the thermal signature of the airplane’s engines and other heat sources to reveal the presence of the target to airborne detectors and missile guidance systems. Infrared detection systems posed a new and complex threat to our aircraft from the Vietnam War forward. Enemy ground-based and air-to-air missile tracking systems were able to follow their prey by merely following the ’hot spot’ of a heat-emitting airplane from the cold air around it. Some devices employed centroid-tracking techniques, whereby the apparent center of the heat source was followed and homed in on by missiles sent to knock down the target.
Millions of dollars were spent to foil infrared tracking systems. Pyrotechnic devices called flares were designed and installed aboard aircraft to decoy and fool such trackers. But an aircraft can carry only a finite number of such infrared decoys. And knowing precisely when to deploy them for even marginal effectiveness remains an unknown.
Tactics were developed, changed, improved and often discarded. Sudden maneuvers, initiated at the instant flares are dispensed, are sometimes useful. Flares do cause tracking devices to be deceived or and least confused about which heat source in the real target.
The infrared signature of the real target is changed by the presence of multiple heat sources. But soon the airplane flies away from the protection of the quickly consumed Flare. Many flares are often needed to maintain the distortion of the target’s signature centroid, causing at worst a near miss or at best a wide miss by the incoming missiles.
The finite number of flares that an aircraft can carry requires two things. Firstly, it is important to use the limited resources wisely. Wasting flares in response to false alarms requires a reliable warning system to alert pilots when to properly deploy flares. That remains a serious concern. Secondly, deploying only the minimum number of flares to get the job done requires precise information on the specific characteristics of the missile’s tracking system, its capabilities and limitations, as well as it’s vulnerabilities to such decoys. Tactical procedures often proscribe deployment of a fixed number of flares on a set schedule or interval, to be accompanied by various maneuvers to take the target out of the field of view of the incoming threat missile.
Many years ago I proposed a low-cost, fairly simple infrared countermeasure. I called it Squid. It was conceived after I viewed a piece of 16mm movie film that depicted an
F-111 airplane dumping jet fuel. The fuel was ignited by the airplane’s engine exhaust and resulted in a trailing plume of burning jet fuel. The visual effect, especially at night, was impressive. But more importantly it got me to thinking about the potential benefits of intentional dumping and igniting of controlled amounts of jet fuel as an infrared decoy system.
Analysis of the infrared characteristics of that trailing plume of burning jet fuel revealed that there was a close similarity between the airplane’s normal signature and the burning cloud. Further, the cloud, following behind the airplane, resulted in a distortion of the centroid signature. It proved worthy of study, I thought, for distorting the centroid just might cause the incoming missile to miss its target.
Just how much jet fuel would be needed? What size flame cloud would do the job? And how would you reliably ignite the dumped fuel? Many factors had to be considered. I approached my superiors, suggesting that the concept had potential merit and deserved formal study, perhaps even testing to prove or disprove its viability.
Immediately people pooh-poohed the idea. Some suggested that intentionally dumping fuel was foolhardy. That fuel would be needed to complete the mission. They failed to appreciate that my own studies indicated only a small quantity would be needed to emulate the decoying effect of conventional flares. I was talking a matter of less than a hundred pounds of fuel for large airplanes and half that for smaller ones. A bomber, carrying tens of thousands of pounds of jet fuel would not even miss a few hundred pounds dispensed to save it from a missile threat. And besides, I argued, fuel remaining in that situation was the least of the aircrew’s problems.
I could not convince my superiors that burning a mist of jet fuel would actually produce a more effective IR decoy than high intensity flares. There are infrared trackers that can differentiate between high temperature flares and relatively low temperature jet engine hot spots and aircraft exhaust plumes. The IR signature of my Squid decoy was very close to that of the airplane itself.
To ignite the dispensed mist of jet fuel required one of two possible devices. One was a chemical that, upon exposure to air, spontaneously ignited (a pyrogolic material). That approach required carrying a supply of the chemical to be dispensed along with the fuel being dumped. But a more effective and far cheaper ignition system was a simple spark plug that lit the fuel as it departed the dump nozzle.
I did a lot of study and tried in vain to convince my superiors that the simple Squid system was a cost-effective, viable and worthwhile countermeasure to infrared homing and tracking threats. To this day I contend that the idea has merit and could save lives.
It would also give large airplanes a practically unlimited supply of decoys, not the limited number they now carry. Now, if a little fuel was wasted on false alarms, it would not deplete significantly the available supply.
Ah well. I ran smack into the "not invented here (NIH)" syndrome from the countermeasures community and higher headquarters. So what else is new?