WASHINGTON: The Army is about to award development contracts for future artillery shells that will increasingly look like missiles, with precision guidance, fins, and even ramjets. The program – part of a much larger buildup of US missiles and cannons – aims to increase both range and accuracy far beyond anything possible with gunpowder alone. The goal: to allow the currently outdated and outdated American artillery force to compete with more advanced Russian and Chinese guns.
The US military has long rocket– boosted howitzer shells. The Cold War M549A1 has a range of around 30 kilometers (not quite 19 miles). The new XM1113 rocket-assisted projectile travels 40 km (25 miles) or more of the current M109 Paladin cannon and 70 km (44 miles) of the XM1299 Extended Range Cannon Artillery (ERCA) currently in development. But to beat triple digits – 100 kilometers (62 miles) and more – you can’t rely on rockets: you need something much more powerful, like a ramjet.
There are three main ways to extend a projectile’s range, and the Army’s Extended Range Artillery Munitions Suite is exploring them all in combination for the upcoming XM1155 round, Nick Berg, chief of staff, told me. ERAMS project in an interview:
- Increase initial speed. The faster the projectile exits the barrel of the weapon, the farther it can travel before drag and gravity bring it back to earth. Higher muzzle velocity is the focus of the XM1299 ERCA Howitzer, which has a longer barrel and more powerful propellant than the current Paladin.
- Add lifting surfaces. Basically, this means adding wings and fins to the hull – like a missile or a miniature airplane – to make it more aerodynamic. The higher the lift generated, the longer the projectile can fly. Of course, the wings, ailerons and electronics all have to survive the brutal shock of being fired from a cannon.
- Add “post-launch propulsion”. That’s where rockets and ramjets come in: they kick in after the projectile has cleared the barrel of the barrel (hence “post-launch”) to give it extra thrust. Again, the engines must first survive the shock of the launch.
“We actually investigated and looked at all of these areas,” Berg told me. “We really focused on lifting surfaces to increase your glide [distance]but we also considered solid fuel ramjets as a post-launch propulsion mechanism to propel you to extended distances.
Why ramjets rather than rockets? The fundamental difference is that rockets contain their own oxidizer to burn their fuel, while jets of all kinds – from ramjets to turbofans – draw their oxygen from the atmosphere. (Hence the name “aerobic engine”). This saves weight, since you don’t have to carry oxidizer; it improves safety, because the fuel does not ignite as easily; and it prolongs endurance, since the jet can keep pushing as long as it has air and fuel. (Longer, smoother burning is also more aerodynamically efficient, reducing drag). A rocket-propelled artillery shell could burn for only 10 seconds, Berg told me, while a jet plane could last much longer.
How is a ramjet different from other types of jet engines? All jets work by drawing air in from the front, compressing it, mixing it with fuel and igniting it, throwing it out the back. (The shortcut is “suck in, squeeze, knock, blow.”) At subsonic speeds, you have to mechanically compress the air with some sort of fan, like the one on the front of an airliner’s turbojet engine. But at supersonic speeds, around Mach 2, the air arrives at the front of the engine so quickly that it compresses, without mechanical assistance: it’s a ramjet.
A ramjet is actually mechanically simpler than a conventional jet, as it doesn’t need a compressor, which is one less thing that could break when fired from a cannon. Historically, the hardest part with ramjets has been that they don’t operate at speeds below Mach 2 – but being fired from a cannon allows you to reach those speeds.
Ramjets are therefore in some respects a natural choice for artillery propulsion. This does not make them easy to build.
“The ramjet offers a bit more of a technical challenge,” Berg told me, because you have to fit sophisticated, shock-resistant electronics into a small box with inlets to supply air to the ramjet itself.
Some of the technologies involved get quite exquisite, even exotic. The munition’s control system must detect airflow, pressure, heating, including phenomena that do not occur at lower velocities, said Bob Bakos, CEO of Innovatea small company working with the Picatinny Arsenal Armament Center on ramjet ammunition. “You’re talking about thousands of degrees” of heating due, essentially, to air friction, he told me. To steer in such extreme conditions, he continued, the projectile needs the traditional flaps and fins, but it might be possible to use tiny air ducts, electromagnets or even plasma to affect the airflow and correct the trajectory.
Once the XM1155 shell is developed, a future “cargo” variant could be used to deliver other payloads in addition to explosives, such as sensor arrays and jamming modules.
The army has already implemented phase I of the ERAMS programme, with the participation of BoeingGeneral dynamics, Northrop Grummanand Raytheon. He is now working on two contracts for Phase II, with rewards expected within two weeks. A Raytheon spokesperson confirmed they were no longer working on the program, while Boeing confirmed they were in competition.