AHEAD Munition Shoots THEN Sets Fuze
I recently learned about a particular bit of engineering behind AHEAD ("Advanced Hit Efficiency and Destruction") ammunition, and I was impressed. It came up as part of worldwide social media spectating on the currently ongoing Russian invasion of Ukraine. History books will note it as the first "drone war", with both sides using uncrewed aircraft of all sizes for both strike (bombing) and reconnaissance (spying). Militaries around the world are taking notes on how they'd use this technology for their own purposes and deny the enemy use of theirs. "Deny" in this context ranges anywhere from electronic jamming to physically shooting them down.
"Just shoot them down" is actually a lot easier said than done, especially for small cheap multirotor aircraft like the DJI Mavic line widely used across the front. They have a radio range of several kilometers carrying high resolution cameras that can see kilometers further. Shooting anti-aircraft missiles to take them down is a financial loss: the quadcopter cost a few thousand US dollars, far less than the missile. And that's if the missile even works: most missiles are designed to go against much larger targets and have difficulty against tiny drones. Every failed shot caught on camera gets turned into propaganda.
When missiles are too expensive, the classic solution is to use guns to throw chunks of metal at it. But since these are tiny drones flying several kilometers away, it's nearly impossible to hit them with a single stream of bullets. The classic solution to that problem is to use some sort of scatter-shot. A shotgun won't work over multi-kilometer distances (skeet shooting uses shotguns at a distance of a few tens of meters) so the answer is some sort of airburst ammunition: cannon shells that fly most of the way as an aerodynamic single piece then explode into tiny pieces, hoping to hit the target with some fragments.
OK great, but when should the shell burst apart? "Have it look for the drone!" is a nonstarter: even if something smart enough to detect a drone can be miniaturized to fit, it would be far more expensive than a dumb shell. The cheap solution is a timer: modern technology can make very accurate and precise timers, durable enough to be fired out of a cannon, at low cost.
It's pretty easy to set a timer before the shell is fired, but what do we set the timer to? If it detonates too early, the fragments disperse too much to take down the target. Detonating too late is... well... too late. If the shooting cannon has a radar to know the distance to target, in theory we could divide distance by speed to calculate a time. But what speed do we use in that math? Due to normal manufacturing tolerances, each cannon shell will be a tiny bit faster or slower relative to another. Narrowing the range of tolerance is possible but expensive, opposite of the desire for cheap shells. It'd be nice to have a system that can automatically compensate for the corresponding variation.
Enter AHEAD. It removes one uncertainty by measuring the velocity of each shell as it is fired then sets the timer after that. Two coils just past the end of the barrel can sense the projectile as it flies past. Its actual velocity is calculated from the time it took for the shell to go from one coil to the next. That information feeds into calculation for the desired timer countdown. (A little more sophisticated than distance divided by velocity due to aerodynamic drag and other factors.) Then a third coil wirelessly communicates with the shell (which, as a reminder, has already left the barrel) telling it when to scatter into tiny pieces.
When I read how the system worked, I thought "Hot damn, they can do that?" It felt like something from the future, even though the Wikipedia page for AHEAD said it's been under development since 1993 and first fielded in 2011. The page also included this system cross-section image (CC BY-SA 4.0):
It shows the three coils in red: two smaller velocity measurement coils followed by the larger inductive timer programming coil. Given the 35mm diameter of the shell, there seems to be roughly 100mm between the two velocity measurement coils. Wikipedia page for an AHEAD-capable weapon lists the muzzle velocity around 1050 meters per second, which calculates out to ~95 nanoseconds to cover the distance between those two coils. The length of the shell is a little over five times the diameter, and the inductive communication coil is somewhere towards the back so call it 5*35 = 175mm from tip of shell to inductive coil. The distance from second velocity coil to programming coil is roughly the diameter of the shell at 35mm. 175 + 35 = ~210mm distance. That implies in the neighborhood of 200 nanoseconds from the time the tip clears the second coil, to the time the two inductive communication coils line up. That 200ns is my rough guess as to the time window for the computer to perform its ballistic calculations and generate a timer value ready to transmit. That transmission itself must take place within some tens of nanoseconds, before the communication coils separate. That is not a lot of time, but evidently within the capability of modern electronics.
Here's a YouTube video clip from a demonstration of an AHEAD-armed system firing against a swarm of eight drones. Since it's a sales pitch, it's no surprise all eight drones fell from the sky. But for me, the most telling camera viewpoint was towards the end, when it showed the intercept from a top-down camera view. We can see the airburst pattern to the left of the screen and the target swarm to the right. From this viewpoint, the top-down variation is due to aerodynamic and other effects and the left-right variation is due to shell-to-shell variation plus aerodynamic and other effects. To my eyes, the airbursts are in a circle, which I inferred to mean the system successfully compensated for shell-to-shell variation.
I'm not very knowledgeable about military hardware so I don't know how this system measures up against other competitors for military budgets. But from a mechanical and electronics engineering perspective I was very impressed there is a way to set the fuze timer after the shell has been fired.