The US Army has long had\u00a0rocket<\/em>-boosted howitzer shells<\/a>. The Cold War M549A1 has a range of roughly 30 kilometers (not quite 19 miles). The new XM1113 Rocket Assisted Projectile goes 40 km (25 miles) or more from the current M109 Paladin cannon and 70 km (44 miles) from the XM1299 Extended Range Cannon Artillery (ERCA) now in development. But to break triple digits \u2013 100 kilometers (62 miles) and more \u2013 you can\u2019t rely on rockets: You need something much more powerful, like a ramjet.<\/p>\n There are three main ways you can extend the range of a projectile, and the Army\u2019s Extended Range Artillery Munitions Suite is exploring all of them in combination for the future XM1155 shell, ERAMS project manager Nick Berg told me in an interview:<\/p>\n \u201cWe\u2019ve actually investigated and looked at all of those areas,\u201d Berg told me. \u201cWe really have focused on lifting surfaces to increase your glide [distance], but then also we\u2019ve looked at solid fuel ramjets as a post-launch propulsion mechanism to boost you out to extended ranges.\u201d<\/p>\n Why ramjets over rockets? The basic difference is that rockets contain their own oxidizer to burn their fuel, while jets of all kinds \u2013 from ramjets to turbofans \u2013 get their oxygen from the atmosphere. (Hence the name \u201cair-breathing engine\u201d). That saves weight, since you don\u2019t have to carry oxidizer; it improves safety, since the fuel doesn\u2019t ignite as easily; and it extends endurance, since the jet can keep thrusting as long as it has air and fuel. (A longer, gentler burn is also more aerodynamically efficient, reducing drag). A rocket-boosted artillery shell might burn for just 10 seconds, Berg told me, while a jet could conceivably last much longer.<\/p>\n How does a ramjet differ from other kinds of jet engines? All jets work by sucking in air at the front end, compressing it, mixing it with fuel, and igniting it, blasting thrust out the back end. (The shorthand is \u201csuck, squeeze, bang, blow.\u201d) At subsonic speeds, you need to mechanically compress the air with some kind of fan, like the one on the front of an airliner\u2019s turbofan engine. But at supersonic speeds, around Mach 2, the air is coming in the front of the engine so fast that it compresses itself, without mechanical assistance: That\u2019s a ramjet.<\/p>\n A ramjet\u2019s actually mechanically simpler than a conventional jet, since it doesn\u2019t need a compressor, which is one less thing that might break when shot out of a cannon. Historically, the hard part with ramjets has been that they don\u2019t work at speeds below Mach 2 \u2013 but being shot out of a cannon gets you to those speeds.<\/p>\n So ramjets are in some ways a natural fit for artillery propulsion. That doesn\u2019t make them easy to build.<\/p>\n \u201cThe ramjet does offer a little more technical challenge,\u201d Berg told me, because you have to fit sophisticated, shock-resistant electronics into a small package along with inlets to feed air to the ramjet itself.<\/p>\n Some of technology involved gets pretty exquisite, even exotic. The munition\u2019s control system needs to sense airflow, pressure, heating, including phenomena that don\u2019t happen at lower speeds, said Bob Bakos, CEO of\u00a0Innoveering<\/a>, a small firm working with the Picatinny Arsenal\u00a0Armaments Center<\/a>\u00a0on the ramjet ammo. \u201cYou\u2019re talking about thousands of degrees\u201d of heating from, essentially, the friction of the air, he told me. To steer in such extreme conditions, he went on, 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 course.<\/p>\n Once the XM1155 shell is developed, a future \u201ccargo\u201d variant could be used to deliver other payloads besides explosives, such as sensor packages and\u00a0jamming pods<\/a>.<\/p>\n The Army\u2019s already run Phase I of the ERAMS program, with participation by\u00a0Boeing<\/a>, General Dynamics,\u00a0Northrop Grumman<\/a>, and\u00a0Raytheon<\/a>. Now it\u2019s working on two contracts for Phase II, with awards expected within two weeks. A Raytheon spokesman confirmed they\u2019re no longer working on the program, while Boeing confirmed they\u2019re competing.<\/p>\n\n
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