WORLD BLOG LOG: missile defense

By 1972, constraints placed on ballistic missiles by the SALT I treaty prompted U.S. nuclear strategists to think again about using cruise missiles. There was also concern over Soviet advances in antiship cruise missile technology, and in Vietnam remotely piloted vehicles had demonstrated considerable reliability in gathering intelligence information over previously inaccessible, highly defended areas. Improvements in electronics—in particular, microcircuits, solid-state memory, and computer processing—presented inexpensive, lightweight, and highly reliable methods of solving the persistent problems of guidance and control. Perhaps most important, terrain contour mapping, or Tercom, techniques, derived from the earlier Atran, offered excellent en route and terminal-area accuracy.
Tercom used a radar or photographic image from which a digitalized contour map was produced. At selected points in the flight known as Tercom checkpoints, the guidance system would match a radar image of the missile's current position with the programmed digital image, making corrections to the missile's flight path in order to place it on the correct course. Between Tercom checkpoints, the missile would be guided by an advanced inertial system; this would eliminate the need for constant radar emissions, which would make electronic detection extremely difficult. As the flight progressed, the size of the radar map would be reduced, improving accuracy. In practice, Tercom brought the CEP of modern cruise missiles down to less than 150 feet (see Figure 1).
Improvements in engine design also made cruise missiles more practical. In 1967 the Williams International Corporation produced a small turbofan engine (12 inches in diameter, 24 inches long) that weighed less than 70 pounds and produced more than 400 pounds of thrust. New fuel mixtures offered more than 30-percent increases in fuel energy, which translated directly into extended range.
By the end of the Vietnam War, both the U.S. Navy and Air Force had cruise missile projects under way. At 19 feet three inches, the navy's sea-launched cruise missile (SLCM; eventually designated the Tomahawk) was 30 inches shorter than the air force's air-launched cruise missile (ALCM), but system components were quite similar and often from the same manufacturer (both missiles used the Williams engine and the McDonnell Douglas Corporation's Tercom). The Boeing Company produced the ALCM, while the General Dynamics Corporation produced the SLCM as well as the ground-launched cruise missile, or GLCM. The SLCM and GLCM were essentially the same configuration, differing only in their basing mode. The GLCM was designed to be launched from wheeled transporter-erector-launchers, while the SLCM was expelled from submarine tubes to the ocean surface in steel canisters or launched directly from armoured box launchers aboard surface ships. Both the SLCM and GLCM were propelled from their launchers or canisters by a solid-rocket booster, which dropped off after the wings and tail fins flipped out and the jet engine ignited. The ALCM, being dropped from a bomb-bay dispenser or wing pylon of a flying B-52 or B-1 bomber, did not require rocket boosting.
As finally deployed, the U.S. cruise missiles were intermediate-range weapons that flew at an altitude of 100 feet to a range of 1,500 miles. The SLCM was produced in three versions: a tactical-range (275-mile) antiship missile, with a combination of inertial guidance and active radar homing and with a high-explosive warhead; and two intermediate-range land-attack versions, with combined inertial and Tercom guidance and with either a high-explosive or a 200-kiloton nuclear warhead. The ALCM carried the same nuclear warhead as the SLCM, while the GLCM carried a low-yield warhead of 10 to 50 kilotons.
The ALCM entered service in 1982 and the SLCM in 1984. The GLCM was first deployed to Europe in 1983, but all GLCMs were dismantled after the signing of the INF Treaty.
Although their small size and low flight paths made the ALCM and SLCM difficult to detect by radar (the ALCM presented a radar cross section only one one-thousandth that of the B-52 bomber), their subsonic speed of about 500 miles per hour made them vulnerable to air defenses once they were detected. For this reason, the U.S. Air Force began production of an advanced cruise missile, which would incorporate stealth technologies such as radar-absorbent materials and smooth, nonreflective surface shapes. The advanced cruise missile would have a range of over 1,800 miles.

The single most important difference between ballistic missiles and cruise missiles is that the latter operate within the atmosphere. This presents both advantages and disadvantages. One advantage of atmospheric flight is that traditional methods of flight control (e.g., airfoil wings for aerodynamic lift, rudder and elevator flaps for directional and vertical control) are readily available from the technologies of manned aircraft. Also, while strategic early-warning systems can immediately detect the launch of ballistic missiles, low-flying cruise missiles presenting small radar and infrared cross sections offer a means of slipping past these air-defense screens.
The principal disadvantage of atmospheric flight centres around the fuel requirements of a missile that must be powered continuously for strategic distances. Some tactical-range antiship cruise missiles such as the U.S. Harpoon have been powered by turbojet engines, and even some non-cruise missiles such as the Soviet SA-6 Gainful surface-to-air missile employed ramjets to reach supersonic speed, but at ranges of 1,000 miles or more these engines would require enormous amounts of fuel. This in turn would necessitate a larger missile, which would approach a manned jet aircraft in size and would thereby lose the unique ability to evade enemy defenses. This problem of maintaining balance between range, size, and fuel consumption was not solved until reliable, fuel-efficient turbofan engines were made small enough to propel a missile of radar-evading size.
As with ballistic missiles, guidance has been a long-standing problem in cruise missile development. Tactical cruise missiles generally use radio or inertial guidance to reach the general vicinity of their targets and then home onto the targets with various radar or infrared mechanisms. Radio guidance, however, is subject to line-of-sight range limitations, and inaccuracies tend to arise in inertial systems over the long flight times required of strategic cruise missiles. Radar and infrared homing devices, moreover, can be jammed or spoofed. Adequate long-range guidance for cruise missiles was not available until inertial systems were designed that could be updated periodically by self-contained electronic map-matching devices.
Beginning in the 1950s, the Soviet Union pioneered the development of tactical air- and sea-launched cruise missiles, and in 1984 a strategic cruise missile given the NATO designation AS-15 Kent became operational aboard Tu-95 bombers. But Soviet programs were so cloaked in secrecy that the following account of the development of cruise missiles focuses by necessity on U.S. programs.

The V-1

The first practical cruise missile was the German V-1 of World War II, which was powered by a pulse jet that used a cycling flutter valve to regulate the air and fuel mixture. Because the pulse jet required airflow for ignition, it could not operate below 150 miles per hour. Therefore, a ground catapult boosted the V-1 to 200 miles per hour, at which time the pulse-jet engine was ignited. Once ignited, it could attain speeds of 400 miles per hour and ranges exceeding 150 miles. Course control was accomplished by a combined air-driven gyroscope and magnetic compass, and altitude was controlled by a simple barometric altimeter; as a consequence, the V-1 was subject to heading, or azimuth, errors resulting from gyro drift, and it had to be operated at fairly high altitudes (usually above 2,000 feet) to compensate for altitude errors caused by differences in atmospheric pressure along the route of flight.
The missile was armed in flight by a small propeller that, after a specified number of turns, activated the warhead at a safe distance from the launch. As the V-1 approached its target, the control vanes were inactivated and a rear-mounted spoiler, or drag device, deployed, pitching the missile nose-down toward the target. This usually interrupted the fuel supply, causing the engine to quit, and the weapon detonated upon impact.
Because of the rather crude method of calculating the impact point by the number of revolutions of a small propeller, the Germans could not use the V-1 as a precision weapon, nor could they determine the actual impact point in order to make course corrections for subsequent flights. In fact, the British publicized inaccurate information on impact points, causing the Germans to adjust their preflight calculations erroneously. As a result, V-1s often fell well short of their intended targets.
Following the war there was considerable interest in cruise missiles. Between 1945 and 1948, the United States began approximately 50 independent cruise missile projects, but lack of funding gradually reduced that number to three by 1948. These three—Snark, Navaho, and Matador—provided the necessary technical groundwork for the first truly successful strategic cruise missiles, which entered service in the 1980s.

Snark

The Snark was an air force program begun in 1945 to produce a subsonic (600-mile-per-hour) cruise missile capable of delivering a 2,000-pound atomic or conventional warhead to a range of 5,000 miles, with a CEP of less than 1.75 miles. Initially, the Snark used a turbojet engine and an inertial navigation system, with a complementary stellar navigation monitor to provide intercontinental range. By 1950, due to the yield requirements of atomic warheads, the design payload had changed to 5,000 pounds, accuracy requirements shrank the CEP to 1,500 feet, and range increased to more than 6,200 miles. These design changes forced the military to cancel the first Snark program in favour of a “Super Snark,” or Snark II.
The Snark II incorporated a new jet engine that was later used in the B-52 bomber and KC-135A aerial tanker operated by the Strategic Air Command. Although this engine design was to prove quite reliable in manned aircraft, other problems—in particular, those associated with flight dynamics—continued to plague the missile. The Snark lacked a horizontal tail surface, it used elevons instead of ailerons and elevators for attitude and directional control, and it had an extremely small vertical tail surface. These inadequate control surfaces, and the relatively slow (or sometimes nonexistent) ignition of the jet engine, contributed significantly to the missile's difficulties in flight tests—to a point where the coastal waters off the test site at Cape Canaveral, Fla., were often referred to as “Snark-infested waters.” Flight control was not the least of the Snark's problems: unpredictable fuel consumption also resulted in embarrassing moments. One 1956 flight test appeared amazingly successful at the outset, but the engine failed to shut off and the missile was last seen “heading toward the Amazon.” (The vehicle was found in 1982 by a Brazilian farmer.)
Considering the less than dramatic successes in the test program, the Snark, as well as other cruise missile programs, probably would have been destined for cancellation had it not been for two developments. First, antiaircraft defenses had improved to a point where bombers could no longer reach their targets with the usual high-altitude flight paths. Second, thermonuclear weapons were beginning to arrive in military inventories, and these lighter, higher-yield devices allowed designers to relax CEP constraints. As a result, an improved Snark was deployed in the late 1950s at two bases in Maine and Florida.
The new missile, however, continued to exhibit the unreliabilities and inaccuracies typical of earlier models. On a series of flight tests, the Snark's CEP was estimated to average 20 miles, with the most accurate flight striking 4.2 miles left and 1,600 feet short. This “successful” flight was the only one to reach the target area at all and was one of only two to go beyond 4,400 miles. Accumulated test data showed that the Snark had a 33-percent chance of successful launch and a 10-percent chance of achieving the required distance. As a consequence, the two Snark units were deactivated in 1961.