The SHMAC (Standoff Hypersonic Missile with Attack Capability) is proposed as a weapon system which has in-theater dominance capability. This weapon system strikes quickly, accurately, and can survive enemy air defenses. The SHMAC can be fired from future hypersonic aircraft such as the SHAAFT (Supersonic/Hypersonic Attack Aircraft), from a low-speed conventional aircraft like the F-15E or the future F-22, from standard ship-based vertical launch system (VLS) tubes, or from mobile or fixed ground launch sites. The propulsion system and warheads will be varied to accommodate the launch platform and the service employing the SHMAC, be it the Army, Navy, Air Force, or Marines. In order to best exploit the range and response time of hypersonic weapons, the SHMAC will be most effective when launched from a hypersonic weapon system, such as the SHAAFT. The SHMAC concept has evolved into an in-theater dominance hypersonic missile, whose design is based upon the need to strike quickly with a high probability of success. The SHMAC will be the primary weapon delivered from the SHAAFT. Its range allows the SHAAFT to safely deliver SHMACs outside the range of air defense systems.

The United States armed forces do not have the ability to strike enemy centers of gravity quickly, decisively, and with a high degree of safety. To destroy targets such as space launch facilities, power grids, communication facilities, and command centers, a rapidly deployable, highly survivable, extremely accurate weapon is needed. Hypersonics is the key to reaching these heavily defended targets in a timely manner and attacking them with a high probability of success. The range and response time inherent in a hypersonic weapon gives the United States armed forces the ability to destroy any ground target in any theater. This is an enormous advantage for US forces as it allows complete in-theater dominance. The SHMAC is a hypersonic weapon system capable of fulfilling this mission.

Several factors drive the design of the SHMAC. These include range, time to target, survivability, guidance requirements, payload requirements, launch platform size restrictions, heating rates, acceleration loads, and maintenance requirements. Initial designs include easy-loading modular payloads. The limitation for the payload is a maximum warhead of 500 to 1,000 pounds. This restriction is driven by the weight limit and size limit of the entire vehicle. Modularity offers flexibility of application of the SHMAC in a fluid war environment.

The missile body has a conventional cruise missile configuration adapted for hypersonic speeds. The propulsion concept is a combination of a rocket for the initial acceleration and a scramjet for sustained propulsion to the design speed of mach 8. The rocket engine will not be necessary for the high-speed air-launched version (SHAAFT launched) because the missile will be deployed at or above cruise speed and altitude. The technology for the rocket/dump combustor-scramjet propulsion system has been studied in the ramjet form by engineers at the Flight Dynamics Laboratory at Wright Labs from 1977 to 1980.

The leading edges could be comprised of ablative materials or an ultrahigh temperature ceramic (UHTC) composed of a dibromide material like ZrB2/SiC.15 Ablators are an economical thermal protection system (TPS) because the SHMAC is a single use weapon. Albatross are much less expensive than more exotic reusable materials. The shape of the missile will not change during flight when high temperature regions are protected with UHTC materials. This ensures that the flight characteristics of the missile will not change during the course of the flight. The ablative technology employed in the thermal protection system is currently available, while UHTC materials are currently being developed by the Ames Research Center.16

The guidance technology takes into account the unique high-speed environment in which the missile will be operating. Possible technologies employed in the SHMAC guidance system include inertial navigation systems (INS) and global positioning system (GPS) usage for the cruise phase. Synthetic aperture radar (SAR) and infrared (IR) guidance is employed in the missile's terminal phase. The technology required to support the design of this missile should mature and become readily available within the next 10 years.

The SHMAC is the first step in developing a line of hypersonic vehicles to meet the needs of the Air Force well into the twenty-first century. These technologies will build upon each other, covering the complete spectrum of hypersonic speeds all the way to orbital velocities. The weapons systems range from in-theater dominance to global and space power projection. This hypersonics program will be an integrated effort (S3) which allows one program to build upon previous research and development and the lessons learned in the other projects.

General Mission Requirements

Range and Time to Target

There are several minimum requirements that military planners have set for a hypersonic weapon system like the SHMAC. The ultimate constraint is for the missile to have a range of 1,000 nautical miles or more. It is desirable to be able to travel this distance in approximately 20 minutes although this is not as important as the range. Figure 3-1 shows the effective ranges of the high-speed air launched, low-speed air launched, and surface launched SHMACs in the Middle Eastern and European theaters. The time-to-target requirement of 20 minutes is based upon the time from missile launch until the SHMAC reaches the intended target. This time requirement, as well as survivability considerations, drives the need for the missile to cruise at mach 8. Technologically, mach 8 is an upper limit on the speed because of the desire to use endothermic hydrocarbons as a scramjet fuel, eliminating the need for cryogenics and the associated complexities.

Figure 3-2. Standoff Hypersonic Missile with Attack Capability (SHMAC).

The missile configurations for the high-speed air launched and the low-speed air launched are virtually identical. A potential conceptual design is shown in figure 3-2. Both have the same dimensions, the difference is the additional weight associated with a rocket in the low-speed air launched version. The missile's dimensions are 180 inches long, 54 inches wide, 23 inches high, and a nose radius of 1.5 inches. There is one slanted surface on the bottom of the missile which forms the compression ramp for the air entering the engine inlet. This also provides a component of lift to complement the wings. A lift-to-drag ratio of 4.5 was determined for the SHMAC based upon calculations as well as values determined from other sources.17

In order for the system to be employed by the Navy through a VLS it must be modified. The missile is longer and more slender in this configuration than the traditional SHMAC. This was driven by the need to retain volume for the rocket fuel while fitting within the slender confines of a VLS tube. The dimensions are 250 inches long, 16 inches high, and 22 inches wide. In addition, folding control fins are utilized to allow the missile to fit within a VLS tube. These will deploy after launch and provide the required control and stability for the missile.

The SHMAC exploits modular payload designs. This missile must be flexible in the types of targets it can hit. As a result, the SHMAC has the ability to change payload depending on the intended target. Payload variations range from high explosives to smart submunitions. Based on current missile designs, we plan to target the enemy with approximately 500 to 1,000 pounds of explosive material. The entire missile design is an iterative process that must balance propulsion, aerodynamics, payload, guidance and navigation, and many other considerations.

Propulsion

The first area of consideration is propulsion. For the low-speed air launched or surface launched SHMAC configuration, we recommend concentrating on developing an integral rocket/scramjet engine. This choice of engine is driven by the desire to accomplish the mission at a low cost without sacrificing effectiveness. This type of combined propulsion cycle provides high initial acceleration without multiple air-breathing propulsion concepts. The rocket will quickly accelerate the missile to high altitude and a mach number where the scramjet takes over.

The driving force behind the entire design of this missile is the mission. However, as previously mentioned, further considerations must be made to account for the delivery platform. For example, if the SHAAFT will be the primary delivery system, the missile needs to be easily compatible with that aircraft. Further modifications need to be made if the SHMAC is to be used by today's fighter/bomber aircraft because of their unique limitations. Ship based SHMACs will be sized to fit into the Navy's VLS tubes. The largest modification for a land or sea fired SHMAC is the rocket engine. A rocket propulsion system is required to accelerate the SHMAC to cruising altitude and mach number before the scramjet engine becomes effective. The rocket/dump combustor scramjet combination is shown in figure 3-3.

Figure 3-3. Rocket/Dump Combustor Scramjet.

The SHMAC uses both a rocket and a scramjet to take advantage of the unique capabilities of each propulsion system. A rocket provides large initial acceleration at low mach numbers. Rocket fuel is more dense than scramjet fuel due to the need to carry oxidizer within the fuel. Because of the low ISPs of rockets, more fuel is required to produce the same amount of thrust. This means an all rocket concept is not desirable due to the large size and weight resulting from the rocket fuel.

On the other hand, a scramjet provides efficient high-speed cruise performance. This is due to its ability to gather oxygen from the atmosphere and its relatively high ISP as compared to rockets. The ISP for the rocket is due to the oxidizer contained within the solid propellant.18 All of these attributes keep the size of a scramjet small. The drawback of the scramjet is its inability to function at low speeds. Therefore, the optimum propulsion concept is a rocket for low-speed acceleration and a scramjet for high-speed cruise.

A low-speed airbreathing concept without rocket acceleration would include a turbojet propulsion system. This results in the need for moving mechanical parts, increased expense and complexity, as well as large size and weight. A turbojet can not provide the quick boost of acceleration to scramjet operating speed and altitude that a rocket can. This high acceleration is desirable to reduce mission time and increase SHMAC survivability. Since the SHMAC has a need for both quick response time and a long range, a combined propulsion system like the rocket and scramjet combination is a must.

The propulsion system which will be used in the SHMAC is a combined rocket and scramjet. This system incorporates an ejectable rocket case. The tolerances required for a scramjet to function in the mach 8 regime would be violated by using a scramjet chamber clogged with the remains of a burnt out solid rocket. The residue and spent fuel of the solid rocket will be removed from the scramjet chamber by ejecting the entire rocket casing after its burn is completed. The high heating rates along the combustor section of the rocket casing will be the most critical area driving the need for the engine material. Typical rocket fuels burn around 6500 oR.19 A single rocket nozzle will be used. This provides simplicity, low weight, and low cost, though multiple nozzles would reduce the overall volume.

The rocket system, with its high initial acceleration, will be used to boost the missile up to a speed and altitude where the scramjet becomes efficient. Limited trade studies have shown an optimum altitude of 100,000 feet and mach 6 for the transition from rocket propulsion to scramjet power. The scramjet system, with the advantage of highly efficient cruise capability, will keep the missile at its altitude and speed until reaching the descent point for the target.

Guidance

Another area considered is guidance. The missile will be programmable while in flight. This enables it to receive updated information permitting in-flight vectoring to a moving target. One means to accomplish this is an inertial guidance system with GPS redundancy which will be updated with the new coordinates as the target moves. A design challenge is ensuring that the guidance signals sent to the missile have enough power to penetrate the hot boundary layer and relay information to the guidance system of the missile. As the SHMAC nears the target area, it will employ self-guidance procedures.

One of these self-guidance procedures is synthetic aperture radar which will guide the SHMAC to a fixed target. This type of target may not have as distinctive a heat signature so a SAR system will have a much greater chance of success against this type of target. Todays SARs have resolutions of less than one meter at 70,000 feet, so the capability certainly exists to acquire and destroy a target using SAR.20

The most effective means to target a recently fired launcher is initial detection of the infrared signature of the launching missile coupled with radar tracking for pinpointing its location. The North American Aerospace Defense Command has the ability to use these two technologies to locate launch vehicles anywhere in the world. Sending the coordinates of the target to the SHMAC can be used to guide the missile to the target area. However, the final acquisition and tracking of the launcher must use a different form of terminal guidance such as IR or SAR because the launcher may have moved after launching.

Modular Weapons Bay

The SHMAC has so many different missions (flexibility is the key to air power), that modular design to accommodate different weapons and guidance payloads is a must. This will simplify the tasks for maintenance crews on the flight line by allowing them to reconfigure the missile easily. In general, this missile will not be harder to maintain than another single use weapon like the Sparrow or Sidewinder. The rocket booster contains solid fuel and the scramjet uses an easily maintainable endothermic hydrocarbon fuel, JP-8 or some derivative. Fueling the missile before it is placed upon the aircraft will not require excessive support personnel or time.

Special Considerations

An important consideration in propulsion system design is the engine inlet. All three SHMAC versions are characterized by an underbody engine inlet that was shown in figure 3-2. A favorable forebody compression field is created by the interaction of the shock from the nose and the inlet lip. Other oblique shocks are formed along the inlet ramp to the combustion chamber which further slow the flow. The disadvantage of an underbody inlet is that the missile needs bank to turn in order to ensure good flow into the engine during flight maneuvers.21 This increases the complexity of the flight control system.

The ideal intake scenario is one in which the flow transitions from laminar to turbulent upstream of the inlet. A laminar boundary layer is desired in front of the inlet, since it will keep the heating rates along the compression face of the missile lower than if this flow is turbulent. However, a mature turbulent boundary layer is required before the flow enters the inlet. This transition needs to occur soon enough so that the inlet shock has a turbulent boundary layer across it. The shock-boundary layer interaction works better with turbulent flow. Turbulent flow is also better suited for rapid mixing of fuel and air in the scramjet.22 The burn phase must be completed extremely quickly for the scramjet to operate effectively. At high mach numbers, the flameholders and fuel injectors must be highly advanced to successfully mix the fuel and airflow and fully combust it in the scramjet chamber for the most efficient burn.

One solution to the flameholder problem is to use highly reactive fuels (such as hydrocarbons with 20 percent ethyl decaborane).23 Reactive fuels spontaneously combust when mixed with the airflow, eliminating the need for flameholders. This would enhance the performance of the engine by reducing the drag and flow problems caused by the flameholders. One problem with reactive fuels is safely storing and maintaining them as well as their high cost when compared to conventional fuels.

The scramjet will have an on-design point of mach 8 which is the desired cruising speed. To further reduce the cost, the inlet to the scramjet will be fixed geometry designed for mach 8 freestream velocity. Since the scramjet has no moving parts, the overall cost of manufacturing it will be fairly low, an important consideration in a single-use weapon. The cost of the scramjet is mostly driven by the materials contained in the scramjet/ rocket engine. These materials will need to withstand the burning of the endothermic hydrocarbons and the oblique shocks formed on the inlet ramp.

The next area considered is the thermal protection system for the SHMAC. A great deal of thermal protection system research was conducted during the Apollo and Shuttle program. This research established many low cost thermal protection alternatives which are readily available for use on the SHMAC. The Space Shuttle program has also led to the development of new TPS. There has also been great progress in the study of ultrahigh temperature ceramics. Since the SHMAC is a single-use vehicle, the most cost-effective form of TPS seems to be ablators. However, significant research still needs to be conducted in this area as to what form of TPS is best for the SHMAC. Further considerations in this area are discussed in greater detail in chapter 5.

Mission

Flight Profiles

Spreadsheets were used to develop the high-speed and low-speed air launched and surface launched representative mission profiles. For each flight phase, values were calculated based on a simple free body diagram of the missile. The independent variables in this iteration were rocket boost end altitude, rocket boost downrange, descent downrange, acceleration cruise downrange, and unaccelerated cruise downrange. The estimated rocket fuel and scramjet fuel values were adjusted to match the iterated values produced from the calculations. All of these variables were iterated and manipulated until the mission profile could be successfully met, and the overall vehicle weight did not exceed 4,000 pounds.

Several assumptions were made to produce these profiles. One assumption was that rockets have a thrust to weight ratio of 10. Another was that a scramjet at 100,000 feet has a thrust to weight ratio equal to 0.1.24 A representative rocket fuel (polyvinyl chloride/ammonium perchlorate/aluminum) was used. This fuel has a density of 0.064 lb/in3 , burns at 6150 oF, and has an ISP of 265 sec-1. Scramjets operating at mach numbers from six to eight have ISPs between 900 and 1200 sec-1 (see fig. 2-3). A representative scramjet ISP of 1100 sec-1 was used in the spreadsheets.

These iterations produced the following mission for the SHAAFT launched SHMAC shown in figure 3-4. The SHAAFT launches the missile at 100,000 feet at mach 8. The scramjet ignites, and the missile cruises at mach 8 over the next 10 minutes. This cruise phase brings the missile 810 nautical miles down range. Finally, the scramjet shuts off and the missile pitches over into the descent phase. This phase lasts for 11 minutes and allows a target that is an additional 620 nautical miles away to be destroyed. The entire mission gives the SHAAFT launched SHMAC a range of 1,440 nautical miles in a flight time of 21 minutes

Figure 3-4. High Speed Air Launched SHMAC Mission Profile.

The second mission we considered was launching the SHMAC from a conventional fighter or bomber such as an F-15 or B-1. This low-speed air launched profile is shown in figure 3-5. The SHMAC will be launched from approximately 30,000 feet and mach 0.8. The solid rocket booster accelerates the missile at an average flight path angle of 50o to 80,000 feet at mach 6. This results in an average acceleration rate of 9 g's. The boost places the missile seven nautical miles downrange in 18 seconds. The cruise phase then accelerates the SHMAC to mach 8, 100,000 ft and an additional 460 nautical miles downrange in a little over six minutes. The glide phase carries the missile another 630 miles downrange and slows it to mach 4. This gives this variant a total range of 1,100 nautical miles in 17 minutes.

Figure 3-5. Low Speed Air Launched SHMAC Mission Profile.

The third mission considered is for the surface launched version and is shown in figure 3-6. The SHMAC will be launched from an altitude of approximately zero feet and a mach number of zero. The solid rocket booster accelerates the missile at an average flight path angle of 54o to 50,000 feet at mach 6. This results in an average acceleration rate of 9 g's. The boost places the missile six nautical miles downrange in 20 seconds. The cruise phase then accelerates the SHMAC to mach 8, 100,000 feet and an additional 410 nautical miles downrange in six minutes. The glide phase carries the missile another 630 miles downrange and slows it to mach 4. This gives this variant a total range of 1,040 nautical miles in 17 minutes.

Objective

The ability to attack key centers of gravity and strategic targets in a theater without prepositioned forces is beneficial for several reasons. This allows the United States to use its military instrument of national power immediately, at any location in the world. This ability can help avoid the development of a protracted conflict by immediately reducing the enemy's will and ability to fight a war. Of equal importance, the immense expense associated with maintaining an overseas presence during peacetime can be avoided by the development of a long-range-quick strike capability. The logistics footprint associated with a large deployment of US troops, like in Desert Storm, is a major expense and hardship on our nation. The SHAAFT/SHMAC combination avoids this footprint by providing the ability to strike anywhere in the world from a CONUS base.

The SHMAC gives the Air Force the essential capability to make a decisive strike in the first hours of a conflict. If a conflict arises, it takes a significant amount of time to mobilize a response force. The SHAAFT/SHMAC integrated weapon system gives the US the ability to strike enemy centers of gravity within hours.

Another advantage of the SHMAC is its ability to protect other war-fighting assets. The 1,000 nautical mile range of the SHMAC allows the SHAAFT to place weapons on target from a safe distance. The SHMAC is released from its host, be it a SHAAFT, F-15E, sea launcher, or ground launcher from a distance safely outside of enemy air and ground defenses. The SHMAC can attack enemy centers of gravity such as command and control centers, access to space assets, and power and communication centers without putting more valuable assets, like aircraft, ships, and, most importantly, human lives into harms way.

The speed of information gathering and distribution in warfare has matured at a phenomenal rate, but the military technology to deliver ordnance quickly enough to take advantage of this increased capability has not followed suit. The inability to attack detected targets of opportunity is a major shortcoming of our present force structure. These targets may include recently fired mobile ballistic missile launchers or military commanders whose whereabouts were recently discovered. This is where the speed and range of a hypersonic missile is a needed and crucial advantage. With SHMAC technology, the enemy will have no safe haven or freedom of movement. Anytime they are detected, they can be quickly attacked and destroyed.

There will be no escape from the oncoming SHMAC. The SHMAC will expand the Air Force's power-projection ability and increase our national security by enhancing the attack capability of all our armed forces. As mentioned earlier, the US military will now be able to stop the development of a protracted war without deploying any troops.

Possibly the greatest advantage of the SHMAC is its survivability. This weapon is highly survivable due to its mission profiles. The SHMAC will cruise at mach 8 and at an altitude of 100,000 feet. This flight regime is exceedingly difficult to reach with current air defense weapon systems. A surface-to-air missile system with 200 miles of coverage would have just over one minute to acquire and launch a missile at the SHMAC. This assumes the SHMAC is detected and classified as a threat at the limit of the missiles radar range. If they delay longer than this period, the missile will already be overhead and almost impossible to catch up to in flight. During the descent, to the target the missile never slows below mach 4 and numerous submunitions can be deployed. Therefore, there is very little chance that an enemy will be able to destroy it with a conventional surface-to-air missile in the terminal flight phase. Furthermore, the missile is not a dumb bomb but is capable of maneuvering, further increasing its survivability and success.

Possible threats to the SHMAC in the future are directed energy weapons such as lasers and microwaves. However, though these weapons may be developed, their complexity and high-power consumption will limit who is able to deploy them and how many are deployed. Only well-developed countries will be able to afford these weapons and only to protect key targets. This kind of threat is a definite possibility, but the standoff capability of the SHMAC ensures that the missiles will be targeted instead of manned aircraft, ships, or trucks.

Although the SHMAC has the potential to be used for many different types of missions, it was designed with a specific mission in mind. That mission is to strike a ground target 1,000 nm away in 20 minutes or less after release from a launch system. This mission was chosen to be the primary focus because it represents a current void in the US's ability to project military force. The ability to strike and destroy ground targets deep inside enemy territory is a mission that will continue to plague US forces in future conflicts unless this problem is solved now.

Future variants of the SHMAC may accomplish different types of missions using the same basic SHMAC technologies incorporated into the first version. These additional missions may include ballistic missile intercept, cruise missile intercept, air to air, surface to air, antiship, close air support, interdiction, and psychological operations. The speed and survivability of the SHMAC can enhance all of these missions. However, modifying the SHMAC to complete these missions will need to be accompanied by large advances in technology in other areas, especially guidance and control. This list represents the flexibility of a hypersonic missile, it is not an advertisement of the near-term capability of the SHMAC.

One of these missions, ballistic missile intercept, was a particularly plaguing problem for the US during Operation Desert Storm. The most effective way to destroy a ballistic missile is to reach it in its boost phase. Attempting to destroy the missile in the reentry phase when decoys, submunitions, and debris are present is extremely difficult. Hypersonic technology is required to reach a ballistic missile in the boost phase. The SHMAC could provide this capability.

Boost phase intercept capability will become more important in the future as more countries obtain the capability to employ weapons of mass destruction. We do not want to destroy a chemical or nuclear weapon over our own troops since the chemicals or fallout will then harm our own forces. Destroying a nuclear biological chemical weapon over our foe's territory is an extremely attractive option for a commander in the field.

When the technology required for a boost phase intercept is developed, this will still be a difficult mission for the SHMAC. One challenge in this mission is getting to the enemy missile while it is still in the boost phase. The SHMAC's speed and range is essential for completion of this mission during the enemy missile's vulnerable boost phase. The largest technological challenge is targeting another hypersonic missile in the air. Closure rates of well over 12,000 feet per second are probable when a SHMAC intercepts another missile. Not only must the SHMAC detect and track the missile, it needs to be able to physically strike the enemy missile to achieve a kinetic kill.

Component Summary

It is critical that funding be provided for the SHMAC immediately. With it the US will truly be able to dominate any theater during any future conflict. Also, the average cost of a fleet of SHMACs will still be considerably lower than the current cost required for the Navy's tomahawk land attack missile to hit a target. In addition, it will be highly survivable, fast, and lethal. In short, there will be no escaping the oncoming SHMAC.

A hypersonic attack missile should be the first step towards developing an Air Force that can truly achieve Global Reach/Global Power through hypersonics. As explained before, the SHMAC falls into the first of three major categories of hypersonic vehicles: in-theater dominance, global reach/global power (SHAAFT), and access to space (SCREMAR). An in-theater dominance weapon like the SHMAC has the simplest mission and is closest to development today; using existing hypersonic vehicle and missile technologies. The SHMAC can become a stepping stone towards developing more complex vehicles and should later be integrated into other hypersonic platforms like the SHAAFT.