The majority of all rocket propulsion systems built today are for military purposes. There is a large variety of missiles and military missions and therefore many different propulsion systems. All are chemical propulsion systems. They range from simple, small, unguided, fin-stabilized single-stage rocket projectiles (used in air-to-surface missions and surface-to-surface bombardment) up to complex, sophisticated, expensive, long-range, multistage ballistic missiles, which are intended for faraway military or strategic targets. The term "surface" means either land surface (ground launch or ground target), ocean surface (ship launched), or below the ocean surface (submarine launched). A tactical missile is used for attacking or defending ground troops, nearby military or strategic installations, military aircraft, or war missiles. The armed forces also use military satellites for missions such as reconnaissance, early warning of impending attack, secure communication, or navigation.
Strategic missiles with a range of 3000 km or more have been two- or three-stage surface-to-surface rocket-propelled missiles. Early designs used liquid propellant rocket engines and some are still in service. Beginning about 30 years ago, newer strategic missiles have used solid propellant rocket motors. Both types usually also have a liquid propellant reaction control system (RCS) for accurately adjusting the final payload flight velocity (in magnitude, direction, and position in space) at the cut-off of the propulsion system of the last stage. A solid propellant RCS version also exists. The flight analysis and ballistic trajectories of the long-range missiles are similar in many ways to those described for launch vehicles in this chapter. See Fig. 4—9.
Solid propellant rocket motors are preferred for most tactical missile missions, because they allow simple logistics and can be launched quickly (Ref. 4-15). If altitudes are low and flight durations are long, such as with a cruise missile, an air-breathing jet engine and a winged vehicle, which provides lift, will usually be more effective than a long-duration rocket. However, a large solid propellant rocket motor is still needed as a booster to launch the cruise missile and bring it up to speed. There are a variety of different tactical missions, resulting in different sized vehicles with different propulsion needs, as explained later in this section and in Ref. 4—15.
For each of the tactical missile applications, there is an optimum rocket propulsion system and almost all of them use solid propellant rocket motors. For each application there is an optimum total impulse, an optimum thrust-time profile, an optimum nozzle configuration (single or multiple nozzles, with or without thrust vector control, optimum area ratio), optimum chamber pressure, and a favored solid propellant grain configuration. Low exhaust plume gas radiation emissions in the visible, infrared or ultraviolet spectrum and certain safety features (making the system insensitive to energy stimuli) can be very important in some of the tactical missile applications; these are discussed in Chapters 12 and 18.
Short-range, uncontrolled, unguided, single-stage rocket vehicles, such as military rocket projectiles (ground and air launched) and rescue rockets, are usually quite simple in design. Their general 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 today produced in larger numbers than any other category of rocket-propelled vehicles. The 2.75 in. diameter, folding fin unguided solid propellant rocket missile has recently been produced in the United States in quantities of almost 250,000 per year. Guided missiles for anti-aircraft, anti-tank, or infantry support have been produced in annual quantities of hundreds and sometimes over a thousand. Table 1-6 lists several guided missiles.
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, and fins. Deviations from the intended trajectory are amplified if the projectile is moving at a low initial velocity, because the aerodynamic stability of a projectile with fins is small 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 two- to ten-fold, compared to a simple fin-stabilized rocket 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„ is an important flight-performance parameter. With the aid of Fig. 4-17 it can be derived in a simplified form by considering the distance traversed by the rocket (called the range) to be the integrated area underneath the velocity-time curve. This simplification assumes no drag, no gravity effect, nearly horizontal flight, a relatively small distance traversed during powered flight compared to the total range, and a linear increase in velocity during powered flight.
-Probable actual curve allowing for drag and non-linear change in mass
- Time to reach target -
- at burnout of propellant
Actual velocity is decreased by drag
- launching aircraft
FIGURE 4-17. Simplified trajectory for an unguided, non-maneuvering, air-launched rocket projectile. Solid line shows flight velocity without drag or gravity and dashed curve shows likely actual flight.
Here S is the free-flight (unpowered) range, up is the velocity increase of the rocket during powered flight up to the time of burnout, t is the time of rocket burning, and w0 is the initial velocity of the launching aircraft. For more accurate values, the velocity increase w0 is the initial velocity of the launching aircraft. For more accurate values, the velocity increase up is given by Eq. 4-19. More accurate values can only be obtained through a detailed step-to-step trajectory analysis that considers the effects of drag and gravity.
In unguided air-launched air-to-air or air-to-surface projectiles the aiming is done by orienting the launching aircraft. In guided missiles (air-to-air, air-to-ground, ground-to-air, or ground-to-incoming-missile) the rocket's thrust direction, thrust magnitude, or thrust pulse timing can be commanded by an intelligent guidance and control system to chase a maneuvering moving target. The guidance system senses the flight path of the target, calculates a predicted impact point, and then controls the flight path of the guided missile to achieve an impact (or near-impact if a proximity fuse is used) with the target. It can also apply to a ground-launched or a satellite-launched antiballistic missile. In both the unguided projectile and the guided missile the hit probability increases as the time to target t, is reduced. In one particular air-to-air combat situation, the effectiveness of the rocket projectile varied approximately inversely as the cube of the time to target. The best results (e.g., best hit probability) are usually achieved when the time to target is as small as practically possible.
The analysis of the missile and propulsion configuration that gives the minimum time to target over all the likely flight scenarios can be complex. The following rocket propulsion features and parameters will help to reduce the time to target, but their effectiveness will depend on the specific mission, range, guidance and control system, and the particular flight conditions.
1. High initial thrust or high initial acceleration for the missile to quickly reach a high-initial-powered flight velocity.
2. Application of additional lower thrust to counteract drag and gravity losses and thus maintain a high flight velocity. This can be a single rocket propulsion system that has a short high initial thrust and a smaller (10 to 25%) sustaining thrust of lower duration. It can also be a system that applies discrete pulses of thrust to increase vehicle velocity after drag forces have caused it to diminish, thus maintaining a higher average flight velocity.
3. For higher supersonic flight speeds, a two-stage missile can be more effective. Here the first stage is dropped off after its propellant has been consumed, thus reducing the inert mass of the next stage, and improving its mass ratio and thus its flight velocity increase.
4. If the target is highly maneuverable and if the closing velocity between missile and target is large, it may be necessary not only to provide an axial thrust, but also to apply large side forces or side accelerations to a tactical missile. This can be accomplished either by aerodynamic forces (lifting surfaces or flying at an angle of attack) or by multiple nozzle propulsion systems with variable or pulsing thrusts; the rocket engine then has an axial thruster and several side thrusters. The thrusters have to be so located that all the thrust forces are essentially directed through the center of gravity of the vehicle. The thrusters that provide the side accelerations have also been called divert thrusters, since they divert the vehicle in a direction normal to the axis of flight direction.
5. Drag losses can be reduced if the missile has a large L/D ratio (or a small cross-sectional area) and if the propellant density is high, allowing a smaller missile volume. The drag forces can be high if the missile travels at low altitude and high speed.
A unique military application is rocket assisted gun launched projectiles for attaining longer artillery ranges. Their small rocket motors withstand very high accelerations in the gun barrel (5000 to 10,000 g0 is typical). They are in production.
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