MILITARY MISSILES

FLIGHT PERFORMANCE

4.8. MILITARY MISSILES

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gravitational attraction. The figure shows that a practical payload becomes too small for orbits higher than about 1200 km. To lift heavier payloads and to go to higher orbits requires a larger launch vehicle than Pegasus vehicle. Figure 4–16 is based on the assumption of a particular payload separation length (38 in.) and a specific Δu vehicle velocity reserve (220 ft./sec) for variables such as the normal changes in atmo-spheric density (which can double the drag) or the mass tolerances of the propulsion systems. Similar curves are produced by the makers of other launch vehicles.

The Space Shuttle achieved its maximum payload when launched due east into an orbit with 28.5∘ inclination from Kennedy Space Flight Center in Florida, namely about 56,000 lbm (or 25,402 kg) at a 100-nautical-mile (185-km) orbit altitude. Such payload decreased by about 100 lbm (45.4 kg) for every nautical mile increase in altitude. When the inclination is 57∘, the payload diminishes to about 42,000 lbm (or 19,051 kg). If launched in a southerly direction from Vandenberg Air Force Base on the U.S. West Coast in a 98∘ inclination into a circular, nearly polar orbit, the payload will be only about 30,600 lbm or 13,880 kg.

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4.8. MILITARY MISSILES 145 and 19–3). Cryogenic propellants are not suitable for military missiles. If altitudes are low and flight durations are long, such as with a cruise missile, an air-breathing jet engine and a vehicle that provides lift will usually be more effective than a long-duration rocket. However, a large solid propellant rocket motor may still be used as a booster to launch the cruise missile and bring it up to speed.

Liquid propellant rocket engines have recently been used for upper stages in two-stage anti-aircraft missiles and ballistic defense missiles because they can be pulsed for different durations and randomly throttled. For each application, optima can be found for total impulse, thrust and thrust-time profile, nozzle configuration (single or multiple nozzles, with or without thrust vector control, with optimal area ratios), chamber pressure, and some favored liquid or solid propellant grain configuration.

Low-exhaust plume gas radiation emissions in the visible, infrared, and/or ultravi-olet spectrum and certain safety features (making the system insensitive to energy stimuli) become very important in some of the tactical missile applications; these are discussed in Chapters 13 and 20.

Short-range, uncontrolled, unguided, single-stage rocket vehicles, such as military rocket projectiles (ground and air launched) and rescue rockets, can be quite simple in design. The applicable equations of motion are derived in Section 4.3, and a detailed analysis is given in Ref. 4–1.

Unguided military rocket-propelled missiles are currently produced in larger num-bers than any other category of rocket-propelled vehicles. In the past, 2.75-in. diam-eter, folding fin unguided solid propellant rocket missiles were produced in the U.S.

in quantities of about 250,000 per year. Guided missiles for anti-aircraft, antitank, or infantry support have been produced in annual quantities of over a thousand.

Because these rocket projectiles are essentially unguided missiles, the accuracy of hitting a target depends on the initial aiming and the dispersion induced by uneven drag, wind forces, oscillations, and misalignment of nozzles, body, or fins. Deviations from the intended trajectory are amplified if the projectile is moving at low initial velocities, because the aerodynamic stability of a projectile with fins decreases at low flight speeds. When projectiles are launched from an aircraft at a relatively high initial velocity, or when projectiles are given stability by spinning them on their axis, their accuracy of reaching a target is increased 2- to 10-fold, compared to simple fin-stabilized rockets launched from rest.

In guided air-to-air and surface-to-air rocket-propelled missiles the time of flight to a given target, usually called the time to target t_t, is an important flight performance parameter. With the aid of Fig. 4–17 it can be derived in a simplified form by consid-ering the distance traversed by the rocket (called the range) to be the integrated area underneath the velocity–time curve. Simplifications here include the assumptions of no drag, no gravity effect, horizontal flight, relatively small distances traversed dur-ing powered flight compared to total range, and linear increases in velocity durdur-ing powered flight:

t_t= S +¹

2u_pt_p

u₀+ u_p (4–37)

Here, S is the flight vehicle’s range to target corresponding to the integrated area under the velocity–time curve, and u is the velocity increase of the rocket during

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FIGURE 4 – 17. Simplified trajectory for an unguided, nonmaneuvering, air-launched rocket projectile. Solid line shows ideal flight velocity without drag or gravity and the dashed curve shows a likely actual flight. Powered flight path is assumed to be a straight line, which intro-duces only a small error.

powered flight (up to the time of burnout or propellant termination). The time of rocket operation is t_p and u₀is the initial velocity of the launching aircraft. For the same flight time, the range of the actual vehicle velocity (dashed line) is less than for the dragless vehicle. For more accuracy, the velocity increase u_pas given by Eq. 4–19 may be used. More accurate values are also calculated through step-by-step trajectory analyses including the effects of drag and gravity from Eq. 4–17.

In unguided air-to-air or air-to-surface rocket-powered projectiles, target aiming is principally done by orienting and flying the launching aircraft into the direction of the target. A relatively simple solid propellant rocket motor is the most common propul-sion choice. In guided missiles, such as air-to-air, air-to-ground, or ground-to-air, the flight path to target is controlled and can be achieved by moving aerodynamic con-trol surfaces and/or propulsion systems, which may be pulsed and/or throttled to a lower thrust. As the guidance system and the target seeker system of a guided missile senses and tracks the flight path of a flying target, a computer calculates a predicted impact point, and the missile’s flight control changes the flight path of the guided mis-sile to achieve impact with the intended target. The control system may command the propulsion system to operate or fire selected liquid propellant thrusters from an engine with multiple thrusters (or to selectively provide thrust through multiple nozzles with hot-gas shutoff valves in solid motors). A similar set of events can occur in a defen-sive ground-to-incoming-ballistic-missile scenario. This requires propulsion systems capable of pulsing or repeated starts, possibly with some throttling and side forces.

Rocket engines with such capabilities can be seen in Figs. 6–14, 12–27, and 12–28.

In both unguided projectiles and guided missiles, the hit probability increases as the time to target t_tis reduced. In any particular air-to-air combat situation, the effec-tiveness of the rocket projectile varies approximately inversely as the cube of the time

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