AIR-TO-AIR MISSILES: DEVELOPMENT PROSPECTS

The effectiveness criteria used to evaluate the accomplishment of a combat mission are: probability of victory in a dogfight, mathematical expectation of losses among belligerents in an air battle, etc. To a considerable extent these criteria depend on the performance characteristics of air-to-air missiles. Since an air-to-air missile is a combination of a destructive weapon and dynamic system designed to carry this weapon to a target intercept point, its characteristics should reflect their ability to change state (phase state in general) at the required time, i.e. to make a maneuver and inflict damage to a target. Therefore, the target engagement probability and target engagement envelope are referred to as basic performance characteristics.

Fig. 1 shows various conditions of employment of aircraft, which are continually being developed and which perform as carriers of guided missile weapons, on the one hand, and present air targets to an adversary, on the other hand:
 

Aircraft operation conditions

  
(aircraft with M = 0.35 to 2.35;
helicopters, M = 0 to 0.3;
H = up to 4 km;
high-speed aircraft with M 
of up to 3 and H = 25 km;
hypersonic aircraft with M of up to 4; H = 30 km. By the year 2005 the value of M is likely to reach about 6 and H > 35 km).

Let us consider the main problems which have an effect on combat effectiveness of aircraft.

Development of a missile for low-signature aircraft. In developing missiles for low-signature aircraft, the designers focus on the following major directions:

1. Adaptation of missiles to different aircraft types.
2. Engagement of Stealth targets.
3. Reduction of the missile's signature.
4. Reduction of drag of the carrier aircraft with missiles carried on attachment points.

According to published information, the U.S.-made Xev-Dash-2 experimental missile meets all of the above requirements.

Due to the asymmetric configuration of the missile's body (the missile's mid-section is an isosceles triangle) and flat underfuselage surface, it can be arranged inside and outside its carrier.

The Xev-Dash-2 missile was originally designed to equip Stealth aircraft and replace AMRAAM missiles (USA). The missile's asymmetric configuration provides for: (1) its arrangement inside or outside the carrier (flush with its skin); (2) maneuvering at up to 50 g loads while performing a turn with a roll.

The missile's characteristics are:
Launch weight, kg 180
Drag not over 1 % 
of that of the F-15 aircraft 
carrying no weapons

Launching lugs/control surfaces fold/unfold time about 75 ms.
 

Basic missiles

 
After studying the information about the Xev-Dash-2 experimental missile, Russian engineers were given the task of making judicious analysis of new engineering solutions incorporated in the missile, namely: (1) rejection of traditional axisymmetric arrangements and round body; (2) rejection of traditional missile guidance methods using Cartesian coordinate system.

Defense of the Missile Carrier-Aircraft's Rear Hemisphere
This issue has repeatedly been discussed but no final solution has been found until now. The main problem was that fighter aircraft were lacking backward-looking radars. Today, some types of aircraft are equipped with such radars.

Generally, the rear hemisphere defense missile configurations can be as follows:
1. A forward-launch missile (angle of attack at launch is zero) describing a circle with a minimum turning radius and with the speed vector turning through an angle of about 180o (the so called missile's omnidirectivity);
2. A backward-launch missile (angle of attack at launch is 180o). The sign of the vector of the missile's speed imparted to it by its rocket motor is opposite to the carrier's speed, due to which fact the missile has to move past the absolute speed zero point.

Development of Modular-Design Missiles

Various requirements for future missiles and their considerable dependence on the conditions in which an air combat takes place call for modular designs employing different types of detachable homing seeker heads: radar (active, semiactive, and passive), IR, etc., as well as replaceable engines of different types. The modular principle of design of the R-27 missile has proved its worth. As the missile's modular design makes the weapon more flexible and adaptable to specific operating conditions, it becomes possible to vary the constituent elements of the missiles to be carried by one carrier or a group of carriers. In this case, different combat missions can be assigned to individual aircraft in a group. 

Higher Target Engagement Altitude

Modern medium-range and long-range missiles can reach a target at an altitude of 25 to 30 km, which is believed to be satisfactory. Any increase in this parameter causes problems associated with the missile's aerodynamics (overload restraints) and creates difficulties in providing the required missile guidance accuracy (radar homing head altitude restraints due to direction-finding errors).

Presently, the need arises to use missiles of this type against high-flying hypersonic targets, ballistic missiles and their warheads. To accomplish this mission, their operating altitude must exceed 100 km. The missiles capable of reaching such altitudes must have two stages. The second stage should utilize the gas-dynamic principle of generation of control forces.

Launching Range

To further increase the launching range of long-range and super long-range missiles, their ballistics and guidance and control systems have to be improved. The guidance and control systems can be improved by combining these systems with inertially corrected semiactive and active radar homing facilities.
Modeling an aerial combat fought by fighter aircraft armed with the RVV-AE (Russia) and AMRAAM missiles indicates that the overall supremacy of aeroballistic characteristics of missiles makes a fighter armed with such missiles superior to the fighter aircraft armed with other types of missiles, thus increasing the probability of victory. The level of aeroballistic characteristics is determined by a combined effect of a ballistic coefficient and missile's power-to-weight ratio.

To develop medium- and long-range missiles with high aeroballistic characteristics and stringent launch weight and size restraints, new engineering solutions have to be introduced. They are:

1. Use of rocket-ramjet propulsion on prospective missiles.

2. Development of two-stage missiles with a jettisonable launching booster.
The development work related to the first direction is carried out both in Russia and other countries.

As regards the specific impulse, the advantages of this motor over its solid-propellant counterpart make it possible to substantially increase the ballistic flight phase and the launch range.

According to estimates, the maximum range in the forward hemisphere for missiles propelled by rocket-ramjet engines is 1.5 times greater than that of the missiles equipped with solid-propellant motors.

Enhancement of the Missile's Maneuverability

Prospective missiles must be highly maneuverable. They must withstand 40 to 50 g overloads and be capable of carrying out maneuvers at angles of attack of 40 to 45o. Such an increase in the angles of attack manifests itself in an unfavorable influence of the nonlinearity of the missile's aerodynamic characteristics and interchannel crosstalk on its overall performance, making the designing of aerodynamic features of the missile of this type more complicated than for earlier generation missiles. The above-mentioned drawbacks can be eliminated by instrumentation methods, which have been developed and tested. 

Guidance System Algorithms

Guidance systems of modern missiles, beginning with the fourth generation, are based on weapon- borne digital computers which impart these systems some new qualities. 

These qualities are: a considerably increased launching range (up to a few hundred kilometers); complete or partial self-sufficiency; multitarget capability (or the ability to engage several targets at a time). However, greater launching range calls for combined control systems utilizing different guidance principles at different phases of missile flight. Currently, operational missile guidance systems are of the following three major types: inertial systems receiving correction signals from carrier-aircraft, semiactive homing systems and active homing systems.

The long inertial trajectory leg, which is 60 to 70 percent of total missile flying time, requires a precise inertial guidance facility incorporated in its inertial guidance system. A digital computer included in the inertial guidance system performs as the missile's master computer. In addition, the guidance system employs the homing seeker's and carrier-aircraft's computers. The structure of the guidance system based on three computers and used on some fourth-generation missiles is currently considered the most preferable one. As each subsystem of the guidance system is activated by a definite sensor (carrier-aircraft's radar, active and semiactive channels of the homing seeker), the need arises to develop a set of versatile algorithms. 

Assessment of the Missile's Scattering Area

The need to assess the scattering area of missiles under development has arisen because of the necessity to make them less visible to the enemy. This issue draws designers' attention when they develop traditional missiles too. For example, while developing the RVV-AE missile, its designers decided to equip it with latticed control surfaces, whose primary advantage is a small hinge moment. The hinge moment on the control surfaces of a missile weighing about 170 kg, can be brought to about 1.5 to 2.0 kgm. Moreover, latticed control surfaces easily fold and, in combination with a very low wing aspect ratio, make the transverse size of the missile small, which is good for its transportation.

Estimates made by designers indicate that when the control surfaces are folded their drag increases and so does the scattering area.

Improvement of Onboard Equipment of Prospective Pissiles

Analysis indicates that when designing missiles for future applications their onboard equipment should be upgraded. The upgrades should include:
 

1. Introduction of a radio altimeter into the missile's guidance system. The radio altimeter is particularly helpful as there is the requirement to reduce the minimum missile launching altitude.

2. Development of multimode homing seekers, operating in the centimeter- and millimeter-wave bands, and combined seekers which drop the nose cone in flight.

3. Development of antenna systems featuring low levels of side lobes and cross polarization; two-band and conformal fixed antennas.

4. Development of superwideband heat resistant low-absorption radio transparent domes causing only minor direction finding errors.

5. Development of a digital computer capable of performing up to 106 operations per second. 

6. Development of a miniaturized IR homing seeker based on a matrix photo detector using a quick-acting cooling system.

7. Development of strapdown inertial systems based on highly accurate laser gyrometers and accelerometers.

8. Development of proximity fuzes based on radar and laser modules adaptable to low-flying targets.

9. Development of power sources and electric drives that would work longer.

Due to the great number of missile-carrier interconnections, the task of making an objective assessment of the missile's capabilities and finding its optimal design was rather difficult.

To resolve this issue, a comprehensive systems approach is necessary to allow the designers to take into account at the early missile development stage the conflicting requirements imposed on its parameters by higher-level systems.