The Role of Airpower in Active Missile Defense

Summer 2010 | 57 Ballistic missile defense is a conten-tious issue. Some people consider it an essential tool for modern security; others believe that it diverts critical re- sources from more pressing needs.1 Ques- tions have continued to surface ever since the first German V-2 missile fell on Europe in 1944. During Pres. George W. Bush’s ad- ministration, the military deployed an ini- tial defensive capability against long-range missiles and increased the numbers as well as improved the quality of existing theater defenses.2 However, the theater ballistic missile (TBM) threat has also changed with the evidence of new, dangerous capabilities on the horizon.3 Given the new emphasis on capabilities against near-term regional threats, perhaps now is a good time to reex- amine the role that airpower might play in this challenging mission area.4 What is the proper role of airpower, and what does it bring to active missile defense that surface- and space-based forces do not? Should combat air forces have a primary The Role of Airpower in Active Missile Defense Col Mike Corbett, USAF, Retired Paul Zarchan 58 | Air & Space Power Journal Corbett & Zarchan role in this mission area? Finally, can we undertake a new mission area without jeop- ardizing the traditional core capabilities of the combat air forces? Air-Launched Hit-to-Kill This article describes a concept that treats ballistic missiles in the same manner as conventional air-breathing threats, using similar doctrine and many of the same technologies employed by today’s combat air forces. Known as Air-Launched Hit-to- Kill (ALHK), this concept employs small kinetic interceptors directed to targets by a staring infrared search and track system (IRSTS). Initially fighters would carry the interceptors, but unmanned combat air sys- tems would eventually assume that task as well. ALHK is not a new idea, but we and other individuals in the military, industry, and academia have worked to refine it into the concept presented here. This article ar- gues that airpower enables this distributed operational concept and could enable the engagement of most threat ballistic missiles in the boost, ascent (early midcourse), and terminal phases of their flight. Performance estimates offered here are based on unclassified threat models and time- lines from the American Physical Society’s report on boost-phase intercept systems, published in 2004.5 We used the society’s models, incorporating them in a three- degree-of-freedom, three-dimensional, end- to-end simulation of the entire intercept process to generate the results contained herein. This Monte Carlo simulation (i.e., repeated simulation trials that produce sta- tistical performance projections) includes sensor noise; realistic predicted intercept- point errors; and combat-proven guidance and filtering techniques that can be used to hit a target during its boost, ascent, or terminal phase of flight. This engagement simulation is an extension of the one originally pre- sented in a previous work.6 Our results to date indicate that the ALHK system concept could engage ballistic missiles at their most vulnerable points and, perhaps most impor- tantly, do so in a cost-effective manner. However, before we examine this concept, we need to take a closer look at the threat. Besides the number of missiles produced and the number of countries that have them, is the threat really growing? To date, con- ventional (nonnuclear) TBMs have never constituted a militarily significant capability that could hold key assets at risk or prevent the attainment of key objectives—although they could penetrate most defenses.7 A nu- clear warhead changes the story, but we could argue that deterrence works pretty well against adversaries with enough capa- bility to develop nuclear weapons. So, is the threat of TBMs really changing? Indications suggest that it is. Countries such as Iran are building ballistic missile arsenals and equipping them with precision- guidance capability.8 This is not a tremen- dous technological jump, given access to the global positioning system or an equiva- lent system. It becomes just a matter of pro- viding the warheads a means to navigate to their targets, in many ways resembling the way Joint Direct Attack Munitions work. The difference is that, instead of dropping them from an airplane, a TBM “tosses” in its warheads—but the last 15 seconds of flight would be very similar with both using aero- dynamic forces to correct navigation errors. We must also consider other guidance methods (antiradiation, laser illumination, etc.) and decide whether any of these could also work with a ballistic-missile delivery system. We believe that at some point, even mobile assets may be at risk to precision attacks delivered by ballistic missiles. Consequences of an Adversary’s Obtaining Precision-Guided !eater Ballistic Missiles To better understand the importance of precision guidance, we should consider how the German missile attacks on Ant- werp could have changed the outcome of a Summer 2010 | 59 The Role of Airpower in Active Missile Defense critical battle during World War II, had such guidance been available. From the fall of 1944 to the spring of 1945, the Allied cam- paign depended upon an adequate flow of material into Europe, and Antwerp was one of the few ports available. Thwarted by the Allies’ air superiority, the Germans turned to V-1 and V-2 weapons to attack the port and slow the flow of Allied logistics. Over 1,700 V-2s and 4,000 V-1s targeted the Antwerp area during this period although only about 30 percent reached the heart of the city.9 The attacks killed over 3,700 people, sank one ship, and constricted supply lines yet never put the port out of action. The impact might have proven decisive had the Germans been able to target individual ships, docks, or warehouses when the Battle of the Bulge hung in the balance. The Thanh Hoa Bridge in Vietnam pro- vides a historical example of the transition to weapons with precision guidance. For over six years, a total of 871 US Air Force sorties dropped unguided bombs on the bridge but failed to close it. However, the first operational application of laser-guided bombs on 13 May 1972 resulted in direct hits on the supporting piers, dropping the center span and closing the bridge.10 Al- though the US military has long understood the value of precision attack, to date we have never been threatened by such a strike. Precision-guided TBMs may change that in the near future. Finally, we should consider an adver- sary’s ability to concentrate his attack at a specific point and time. Timing multiple launches for simultaneous arrival is not dif- ficult, and a sufficient number of ballistic missile launches can overwhelm any surface- based defense. Combining this ability to mass the attack (i.e., the simultaneous ar- rival of many weapons, a capability now possessed by some potential adversaries) with precision guidance would allow an ad- versary to overwhelm any surface-based defense system and destroy its critical tracking radars. The absence of sensors eliminates a defensive system’s ability to intercept ballistic missiles, after which the adversary can deny allied forces access to ports and airfields. We believe that the threat is really chang- ing in ways that will affect how and where future battles will be fought. This growth in an adversary’s capability comes not from mating ballistic delivery systems with weapons of mass destruction but with preci- sion guidance, which, combined with an adversary’s ability to attack key locations in mass, may significantly inhibit a future al- lied force’s power projection options. A Closer Look at the !reat TBMs are difficult to locate and need not emit any exploitable signals prior to launch. They can be hidden for long periods and then rolled out, erected, and launched with- out warning. Once the engine fires, the TBM becomes very visible and easily distin- guishable from other missiles encountered on the battlefield. Surface-to-air missiles ac- celerate very quickly, their engines usually burn for less than 20 seconds, and they fol- low a somewhat erratic path as they guide toward their target.11 Ballistic missiles, on the other hand, accelerate more slowly and their engines burn much longer. Those with longer range (medium to intercontinental) rise nearly vertically at first, taking as long as a minute to climb through an altitude of 10 kilometers (km). Depending on their size and range, their engines may burn for more than four minutes, and the missiles may have more than one stage. Some reach accelera- tion levels of 8 g’s to 15 g’s or more prior to burnout or staging.12 (See fig. 1 for a simula- tion of a single-stage generic intermediate range ballistic missile’s [IRBM] altitude and acceleration profiles.) It is important to note that part of the axial acceleration of the IRBM appears as a target maneuver to a pursuing interceptor, and the amount of re- quired interceptor acceleration to engage the target is related to the magnitude of this apparent target maneuver. An interceptor capable of defeating such a threat during the boost phase must be 60 | Air & Space Power Journal Corbett & Zarchan able to accelerate similarly within the envi- ronment where the intercept will occur. Be- low 35 km, TBM acceleration levels are still relatively low, but they grow quickly as the threat consumes its fuel load. For intercepts above 50 km altitude, TBM accelerations can exceed 5 g’s (fig. 1). The required in- crease in an interceptor’s acceleration rela- tive to the threat depends upon the geometry of the engagement and the type of guidance used. Traditional proportional navigation guidance demands that the interceptor have a significant maneuver advantage over the threat (a ratio of three to one or greater). However, we believe that optimized guid- ance can significantly reduce this maneu- ver margin, possibly to a fraction of the tar- get’s acceleration capability.13 After the boost phase, the guided warhead will likely separate from the booster, and defensive countermeasures such as decoys may also deploy. Unless a postboost system applies thrust—either to correct boost-phase navigation errors or compensate for a mov- ing target—the flight path will remain bal- listic and highly predictable during this midcourse period. Depending on the range to the target, this ballistic period can last many minutes and give defending aircraft time to respond from regional ground-alert sites. In the case of our generic IRBM (fig. 2), we see that the midcourse phase of flight starts at approximately 200 seconds and ends at approximately 1,050 seconds, indi- cating that the target’s flight path is highly predictable for about 14 minutes. The terminal phase of a ballistic missile’s flight begins when the descending warhead encounters the upper atmosphere at ap- proximately 80 km altitude. Although the air is exceptionally thin at this point, it does exert a drag effect. Heating of heavy pieces begins, and light pieces such as chaff and decoy balloons fall back, each having identi- fiable signatures. As the descent continues, the atmosphere becomes progressively denser, and these effects increase. Heavy, irregular objects such as fuel tanks begin to tumble and eventually break up. By 30 km altitude, the air is dense enough for the con- trol surfaces on a cone-shaped warhead to effect small maneuvers to compensate for guidance errors or begin target homing. Everything that remains intact during this 10 8 6 4 2 0 Ac ce le ra tio n (g ) 100806040200 Altitude (km) 160 seconds 10-second time tics Figure 1. Generic IRBM acceleration as a function of altitude during the boost phase Summer 2010 | 61 The Role of Airpower in Active Missile Defense period slows and starts to get very hot. By the time a warhead passes 15 km altitude, even the fastest warhead (one that has trav- eled the longest distance) has slowed to less than five kilometers per second (km/sec) and normally approaches its target from 20 degrees above the horizon or higher. This final descent to the target from 15 km alti- tude takes about 15 seconds, during which time aerodynamic forces enable the great- est maneuvering potential.14 A simple com- puter simulation, in which the ballistic co- efficient for several items is treated as a constant, illustrates how these objects (bal- loons, tank, and reentry vehicle) traveling at 3 km/sec decelerate as they enter the atmosphere (fig. 3).15 Objects with the most drag (or smallest ballistic coefficient C) have their peak decelerations at the higher alti- tudes. The figure indicates that the decel- eration profiles of all objects are different and that quantities related to the decelera- tion may serve as useful discriminators. Although desirable, no single interceptor could engage all threats at any altitude from the surface up. Interceptors designed for engagements in the atmosphere below 35 km altitude can use aerodynamic forces for maneuvering but must cope with higher heating as velocities increase. We refer to these as lower-tier interceptors and show their performance based on a burnout ve- locity of 1.75 km/sec. Interceptors designed for higher altitudes must use lateral rocket thrust or thrust vectoring for maneuvering, and the complex interaction with missile- body aerodynamics creates adverse problems at altitudes below 50 km. These upper-tier interceptors also need much higher veloci- ties but can avoid heating problems by per- forming intercepts only above 50 km. We indicate their performance based on a burn- out velocity of 3.5 km/sec. Both upper- and lower-tier interceptors have advantages and disadvantages during the terminal phase of flight. The upper-tier 50-second time tics 200 seconds 1,000 seconds Al tit ud e (k m ) 700 600 500 400 300 200 100 0 500 1,000 1,500 2,000 2,500 3,000 Downrange (km) Figure 2. Duration and altitude of a generic IRBM’s trajectory during the midcourse phase 62 | Air & Space Power Journal Corbett & Zarchan systems would not have to cope with high deceleration levels, but having the agility needed for upper-tier boost-phase engage- ments would enable them to maneuver rap- idly and intercept warheads as atmospheric interaction revealed the countermeasures. Lower-tier interceptors might have to deal with much higher deceleration levels and might have a very narrow engagement zone, if any, against the longest-range threats. However, a very low minimum-engagement altitude can permit a second shot if the first intercept attempt misses. What Airpower Can Bring to !is Fight Airpower enables a distributed opera- tional concept that can engage the TBM threat during the boost, ascent (early mid- course), and terminal phases of flight by using common air-launched interceptors and a common aircraft-carried sensor. Air- power applied to missile defense provides more than simply a platform that can get close enough to the launch point to engage in the boost or ascent phase, or respond fast enough from ground alert to engage in the terminal phase.16 Airpower applied to mis- sile defense allows a commander to focus defensive capability with the same speed and flexibility commonly associated with attack operations. Instead of utilizing a fixed defensive deployment tied to station- ary radars, a commander could rapidly es- tablish or reinforce a defensive posture, move aircraft forward to pursue boost or ascent engagements, or cover the move- ment of surface forces with a combat air patrol providing terminal defense. In addition, launching an interceptor missile above 12 km altitude has a signifi- cant impact on its performance. Although E=1,000 kg/m2 (tank) E=10,000 kg/m2 (reentry vehicle) 3 km/second initial velocity Altitude (km) E=10 kg/m2 (balloon) Ac ce le ra tio n (g ) 20 10 10 5 0 0 20 40 60 80 Figure 3. Early peak decelerations for objects with the most drag (or lowest ballistic coefficient) Summer 2010 | 63 The Role of Airpower in Active Missile Defense a supersonic fighter may be traveling only 0.3 km/sec, launching the interceptor mis- sile at an altitude above 90 percent of the atmosphere has the effect of reducing aerodynamic drag on the missile and may add over 1 km/sec to the interceptor’s burnout velocity. For example, based on engagement- simulation results from previous works, a notional 3,000 km IRBM (figs. 4 and 5) launched from northern Iran toward Rome would impact in approximately 17 minutes.17 Strike or escort aircraft operating within Iran could autonomously detect and engage threatening ballistic missiles during their boost phase. Moreover, combat air patrols operating in eastern Turkey could autono- mously detect threats in their boost phase, engage them in their ascent phase, and sub- sequently pass precise threat-tracking data downstream for follow-on terminal engage- ments. Assuming nominal times for detect- ing the launch, issuing the warning, scram- bling, and climbing out, fighter aircraft on ground alert at Aviano Air Base, Italy, would have sufficient time to scramble, ac- quire, and track the threat, and then launch an interceptor for a terminal-phase engage- Terminal-Phase Intercept Boost-Phase Intercept Figure 4. Operational areas for aircraft using a lower-tier interceptor to defend Rome against an IRBM launched from Iran 64 | Air & Space Power Journal Corbett & Zarchan ment.18 The two figures represent opera- tional areas for an aircraft defending Rome against an IRBM launched from Iran—fig- ure 4 depicting the capability of a lower-tier interceptor and figure 5 representing the operational area of an upper-tier intercep- tor. We can see from figure 4 that the lower- tier system will not have ascent-phase capa- bility against this category of threat. Each aircraft can operate autonomously for boost- or ascent-phase engagements or as part of a network for terminal defense. Aircraft providing defense can be massed at a particular point or distributed over a large area. They can provide terminal defense for a limited time at a port or airfield during deployment of a persistent surface-based system, or they can thin the wave of attack- ing threats through boost-phase engagements during fighter-sweep operations. Finally, but perhaps most importantly, we base this concept on the development of a small in- terceptor that should cost less than the threat it will attempt to engage, a character- istic that holds the promise of making air- power-based missile defense a cost-effective concept. Terminal-Phase Intercept Boost-Phase Intercept Ascent-Phase Intercept Figure 5. Operational areas for aircraft using an upper-tier interceptor to defend Rome against an IRBM launched from Iran Summer 2010 | 65 The Role of Airpower in Active Missile Defense !e Air-Launched Weapon What would these defensive weapons look like? The size of the weapon is directly related to its maximum employment range. The air-launched interceptor must attain a high velocity so that it can quickly close the distance to the predicted intercept point, yet retain the capability to maneuver to the precise target location. It also requires suf- ficient lateral acceleration to actually hit the target. A lower-tier interceptor may use aerodynamic forces for maneuvering; how- ever, any attempts by an interceptor to en- gage at ranges greater than 150 km will re- sult in intercepts outside the atmosphere, thus requiring propulsive thrusters so that it could maneuver in response to guidance commands. Because maximum-range en-