February 04, 2024

RAM

I've previously discussed Standard, Sea Sparrow, ESSM and Phalanx, but there is one last air-defense weapon that deserves discussion. This is the RIM-116 Rolling Airframe Missile, better known as RAM.


A RAM is launched from USS Green Bay

Much like the other point-defense systems, RAM's origins trace back to Eilat, and the panic that it provoked within the USN. It was conceived to work in pretty much the same niche as Phalanx, providing a last-ditch defense against incoming anti-ship missiles. Effective as it was, Phalanx had a serious limitation, even while it was still in development. The use of a gun limited effective range to no more than 1500 yards, which was a serious problem in the face of supersonic missiles. The available window to engage such a weapon was short, and even if the Phalanx did shoot it down, the debris was likely to strike the defended ship. The obvious solution was to use a missile, which could engage at significantly longer range.

The initial program that led to RAM was based on the Redeye missile, the first American man-portable SAM system, although it would be fitted with a combined radar/IR seeker to allow it to engage closing targets (which were difficult to engage purely with IR seekers at the time) at reasonable range. The only real concern was the small size of the missile, 2.75" in diameter and about 18 lb, and Congress directed the Navy to study something the size of Sidewinder, 5" and about 160 lb, instead. The initial contract was signed with General Dynamics in 1976, with West Germany coming onboard as a development partner. Development didn't go particularly smoothly, with delays from testing and cost adding up to around 5 years, and both the US and Germany came close to withdrawing from the program at various points. But things were eventually worked out, and RAM entered in the early 90s.


A RAM in flight

RAM is a rather unusual missile. It gets its name because it is fired from a rifled tube and rolls in flight thanks to the tube and four fins. The roll allows it to use only two control fins in flight, instead of the usual four. The basic airframe, motor, fuze and warhead initially came from the Sidewinder, while the IR seeker was derived from the Stinger. Because RAM was expected to be fired at incoming targets, where the hot engine would not be visible, the seeker would need to rely on glint (reflected IR radiation from the sun), which sharply limited range. As a result, initial guidance would be provided by a passive RF system, which could home in on the radar seeker of a typical cruise missile.1 The rolling missile could also get away with a 2-sensor radar inferometer, instead of requiring four sensors, like a more conventional missile. The accuracy of the RF seeker was limited, hence the inclusion of the IR seeker, but it was in theory possible for the missile to use it all the way to the target if it's dark or cloudy and the IR seeker doesn't work. The wide cone of the RF sensor also allows the missile to be fired "around the corner", reducing the size of sectors blocked by the ship's structure by 10-15°.


Sailors load a RAM launcher aboard the Truman

But there were serious concerns about the effectiveness of RAM against missiles that used IR or semi-active homing, so even before the Block 0 missile entered service, work began on Block 1, with an imaging IR seeker that is capable of searching for targets on its own. It still has the RF seeker, and is capable of using the original dual-sensor mode, homing entirely on IR or switching to IR search if it loses RF track. Block 1 entered service in 1999, and achieved a 95% success rate in intercepting incoming missiles across 180 trials. A further upgrade took place in 2002, to give better performance against helicopters, slow aircraft and surface targets. This upgrade, known as HAS, was implemented entirely as a software upgrade. In the mid-2000s, a second upgrade, Block 2, was started. It was a considerably bigger overhaul than Block 1, with a new 6.25" motor2 (which increased range significantly, from 6 nm to 10 nm), a 4-fin steering system, and an improved RF seeker. Block 2 was cleared for service in 2015, and is currently in production.


SeaRAM onboard Japanese helicopter destroyer Izumo

The fire-and-forget nature of RAM meant that it imposed relatively minimal burdens on the firing ship's combat system, particularly given the ability of the missile to search for targets after launch. The combat system still mattered in terms of firing at the correct time and in the right direction, but it opened up new possibilities for smaller ships that couldn't support Sea Sparrow or the like. The most extreme version of this was SeaRAM, which was essentially a Phalanx system with the gun removed and replaced by an 11-round RAM launcher. Like Phalanx, it is capable of operating independently of the ship that carries it, automatically detecting and engaging incoming missiles. SeaRAM is primarily carried by the Independence class LCS, although a few Burkes are also fitted with the system to provide some defense against cruise missiles when the main radar is in ballistic missile defense mode.


A RAM launcher on the German missile boat Ozelot

But the more common launcher is the 21-round Mk 49, a trainable launcher integrated with the ship's combat system and fitted to a number of ships, including all American aircraft carriers and amphibious ships, as well as the Freedom class LCS. Germany has equipped all of its warships with RAM, while Egypt, Greece, Japan, Mexico, Qatar, South Korea, Saudi Arabia, Turkey and the UAE have bought the system for their ships as well. If anything, RAM seems destined for wider service in the years to come. It is the most minimal system capable of providing protection against anti-ship missiles, a threat that has been graphically demonstrated in the Red Sea in recent months, and which is only likely to continue to proliferate.


1 There are claims that it can home in on the radar altimeter for IR-homing missiles, but I can't find good support for that claim, and Norman Friedman says the opposite.

2 I believe that in launchers for Block 2, the rifling is removed and initial spin is imparted by an ablative vane behind the rocket motor.

Comments

  1. February 04, 2024redRover said...

    Because RAM was expected to be fired at incoming targets, where the hot engine would not be visible, the seeker would need to rely on glint (reflected IR radiation from the sun), which sharply limited range.

    I assume that if the sun is down even the blocked exhaust plume of a straight in missile would stand out enough in IR to be useful?

  2. February 04, 2024bean said...

    It isn't clear in my sources. That may be the case, or it may be that the RF seeker would need to be used at night. I suspect that the Block I missile is somewhat better at this.

  3. February 04, 2024friendlynokill said...

    Can't believe I only just came across this site. Just spent an entire morning reading through past articles. Love your content.

  4. February 06, 2024Tacofordogs said...

    Hoping that you can take on non USN systems when timing allows. Love the work here!

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