Cruise Missiles ~ WORLD BLOG LOG

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.