There are numerous single-stage-to-orbit (SSTO) vehicle concepts under active study that should result in development of a TAV. These TAV concepts, sometimes referred to as reusable launch vehicles (RLVs), are plausible enough that McDonnell/Boeing, Lockheed/Martin, and Rockwell are all investigating proprietary concepts. Both the Rockwell and Lockheed/Martin RLV concepts are vertical take-off/horizontal landing, have longitudinal payload bays (like the shuttle), and are being designed for commercial payloads. The McDonnell Douglas/Boeing RLV concept is similar except it is a vertical take-off/vertical landing.¹⁰⁹ The US Government (USAF, NASA) is in partnership with McDonnell/Boeing, Lockheed/Martin, and Rockwell to develop a military/commercial version currently called the X-33. They expect to fly the X-33 RLV in 1999.¹¹⁰ The McDonnell Douglas/Boeing RLV is a vertical take-off/landing system with eight rocket engines. It navigates using GPS, will use 200-foot pads instead of runways, and is designed for low maintenance and infrastructure. Development costs are expected to be about the same as a new commercial airliner (Boeing 777).¹¹¹

Whichever concept the US government and industry decide to pursue, a revolution must occur in TAV engine technology before it becomes viable.¹¹² Conventional rockets have low dry weight (without fuel) and high gross take-off weight (lots of fuel and oxygen) to reach the Mach 25 speeds necessary to reach space. Rockets give up maneuverability and ease of handling in the atmosphere by being so configured. Air breathers, on the other hand, have high dry weight and low take-off weight because they use the oxygen in the atmosphere as part of their fuel. They have great maneuverability, but have considerably lower top speeds. Furthermore, the major costs of a TAV are its dry weight components. Fuel, although relatively inexpensive, consumes 80+ percent of the total gross weight, leaving only 3-6 percent for cargo. The 2025 TAV must have moderate dry weight and gross weight, which may be obtained using a combination of rocket and air-breathing technology called a rocket-based combined cycle air-augmentation.¹¹³ This will require a revolution in rocket technology. But three pieces of this revolution will very likely be available by 2025.

The first piece of the revolution deals with the advanced technology "pulse detonation wave" combustion process in a rocket instead of the conventional "constant volume combustion" process. Researchers believe this technology would increase the pressure during detonation by 20 to 40 times and significantly increase the specific impulse (Isp) of the fuel. More importantly, it will decrease (by 20 to 40 times) the pressure, heating, and wear and tear on the fuel turbine-feed pumps that are the cost, reliability, and safety concerns of rocket engines.¹¹⁴ Thus, rocket engines could be made cheaper, smaller, and more reliable by orders of magnitude. Rocketdyne and Adroit have both been working aggressively in this area.

The second piece of the revolution concerns the "air augmentation" portion of the engine. The leading concept involves wrapping sheet metal with an inlet around the base of the engine. This technique, properly applied, should "squeeze" every bit of oxygen possible out of the "sensible atmosphere" and make it available as part of the fuel. This would make the air-breathing part of the engine useful at altitudes of up to 120,000-140,000 feet and increase the thrust by 300 percent. This alone could double the payload.¹¹⁵

The final piece deals with the combuster in the air-breathing portion of the engine. The combuster creates the highest pressure in the engine as the fuel mixes with oxygen, burns, and then provides thrust. As speed increases, between one-third and one-half of the thrust comes not from the fuel, but from the heat and pressure within the engine. Under these conditions, the fuel is not efficient and energy losses increase dramatically, heat increases, and sheer stresses increase resulting in lower final speeds. However, a revolutionary premix, shock-enhanced (oblique standing detonation wave) combustion engine could increase the Isp by 30 percent. This would allow the combuster to be reduced in length by 75 percent, thus decreasing the weight of the engine significantly.¹¹⁶

The final design entails placing the rocket engine inside the air breather. At low Mach speeds, the air breather would be used alone. At Mach 15 to 20 during pull up to space, the rocket would light up and pressurize the air breather, keep burning atmospheric oxygen (would not have to carry nearly as much), and create a synergistic effect using both the rocket and the air breather. The result could be a highly efficient, viable engine for the 2025 TAV.

Lightweight structures are another must for the 2025 TAV. Dr Dennis Bushnell, Chief Scientist for NASA at Langley, Virginia, reported that the Japanese are working on a Carbon-60 (Fullerine) material that is lightweight, but is an order of magnitude stronger than the most modern of composites. He also stated that advances in static stability will help the TAV of 2025.¹¹⁷ Currently, spent uranium is placed in the nose of vehicles for ballast (keeps center of gravity forward of center of pressure to prevent tumbling); researchers, however, are investigating placing "longitudinal vortices" and using active controls to maintain this positive stability instead of weights. This would allow designers to move things around for efficiency without worrying as much about the center of gravity. Finally, Dr Bushnell reported that the Navy is aggressively researching "designer aerodynamics" and circulation control of air-breathing vehicles. With sensors and actuators, this could give the TAV "bird-like flight" characteristics. Thus, it truly could become an airplane and a space plane.¹¹⁸

In addition, the 2025 TAV should be easily upgradeable as technology improves. We must not produce a TAV that will become obsolete and difficult to maintain even as we are trying to build it. Modularity of design will be important. For example, as guidance system technology is improved and further miniaturized, maintenance workers can expect to pull out a "black guidance box" and replace it with a new, improved version (lighter, less volume).

Finally, all the systems of the TAV must be integrated with all the other systems that interface with it. On-board (guidance, maneuvering) and off-board (surveillance, some processing) systems must all work together as a distributed "system-of-systems."

Capabilities

The TAV "fleet" of 2025 will possess incredible capabilities. The TAVs will have highly efficient, reliable engines that perform equally well in the atmosphere or in space; the TAV structures will be made of strong, lightweight composites that are easily replaceable; and the payload capability (weight and volume) will be versatile and adaptable to many different types of payloads and missions. Commercial carriers will exist that are capable of lifting 20,000 to 40,000 pounds into LEO.¹¹⁹ The Black Horse TAV concept calls for a payload of approximately 5,000 pounds into a LEO (although some critics believe that due to design flaws, it cannot take any payload into orbit),¹²⁰ whereas the X-33 concept proposes a payload of 10,000 pounds (polar orbit) to 20,000 pounds (eastern LEO).¹²¹ The military requirement will probably be in the 10,000 to 20,000 pound range. A versatile TAV should certainly be able to carry payloads for at least three basic missions: 1) to deploy/retrieve small to medium satellites (large satellites are "dinosaurs") for many, although not all, missions; the trend is towards small/microsatellites; thus, all of the TAV concepts should suffice for low and medium earth orbit missions; 2) to carry a small team of special operations forces along with their operational gear to crisis spots throughout the world (TAVs would probably need a 2,000 pound capacity to carry four-man teams¹²²); and 3) to perform as a sensor and weapon platform (for short periods of time analogous to aircraft sorties).

The satellite deployment/recovery capability could be critical for fielding or reconstituting space-based components of weapon systems in 2025. In fact, using a "Pop-up" flight profile (see fig. 3-4), the TAV could potentially launch multiple satellites and grant access to all orbits (e.g., LEO, Polar, Sun Synchronous, Molniya, geosynchronous earth orbit (GEO). In an eastern LEO, the TAV would be able to deploy 15 1,000-pound small satellites and pick up four to bring back. It could deploy as many as four satellites to a GEO orbit..¹²³ This capability should make the TAV extremely flexible for space-force applications.

Source: Briefing, Phillips Laboratory PL/VT-X, "Military Spaceplane Technology and Applications," January 1996

Other necessary capabilities/requirements for a TAV-type vehicle include all-weather performance, rapid call-up time, short turn-around time, long service life, low-maintenance engines, vibration-resistant systems and structures (to survive reentry and "hypercruise" speeds near Mach 25), all azimuth earth access, global range of operation, and the ability to be upgraded easily and inexpensively.¹²⁴ The requirements for all azimuth access and global range of operation mean that the TAV will probably need refueling capability. A study by W. J. Schaeffer Associates (4 February 1994) on the feasibility of an aerially refueled "spaceplane" concluded this capability "appears feasible and practical."¹²⁵ Refueling could occur in the atmosphere or in space. Call-up times on the order of a few hours and turn-around times of from six hours (emergency) to 24 hours (routine) will probably be required.¹²⁶ More importantly, the TAV of 2025, once launched, will reach anywhere in the world within 60 minutes or less. Of the 60 minutes required, approximately 20 minutes would be from launch to space plus another 40 minutes to the target area (a TAV could be over the most likely target areas after only 20-30 minutes in space).¹²⁷ The 2025 TAV could also deliver multiple payloads (e.g., laser, KEW, reconnaissance, satellites, strike team, ASAT weapon, etc.) depending on the mission.

Source: Briefing, Phillips Laboratory PL/VT-X, "Military Spaceplane Technology and Applications," January 1996

Countermeasures

Like aircraft, TAVs are naturally vulnerable on the ground and would need protection. Moreover, TAVs emit a variety of easily detected signatures (e.g., radar, enhanced infrared due to high-speed passage through the atmosphere, and acoustic) and are also vulnerable to attack during launch and landing. TAV launch facilities could be safely located in the continental United States (CONUS) but if a team of "space marines" must be landed outside the CONUS, the TAV and its payload would be vulnerable to a variety of weapons (space-based, airborne, or terrestrial) and tactics. Fortunately, a force of three or four TAVs (analogous to a flight of modern combat aircraft in tactical formation) could be extremely difficult to attack if at least one is used as protection for the others. Another consideration is that, during conflict, any US spacecraft (particularly if it is a manned platform) would instantly become a high-value, high-priority target. Thus, ASAT-type weapons could be directed against a TAV in endo- or exo-atmospheric flight. However, the highly maneuverable TAV, if configured to carry high-speed precision weapons onboard, could itself become an "anti-ASAT" weapon-the attacking ASAT would then become the target.

Evaluation

Flexibility, provided by its ability to put human judgment at the developing crisis location rapidly, is the greatest asset of the TAV. Responsiveness and timeliness, while not in the same class as space-based light-speed weapons, are at least moderate (hours for call up followed by 60 minutes maximum flight time). Since a TAV could carry a broad range of payloads (e.g., many different types of weapons, a special forces unit, or even limited space maintenance and repair facilities), it rates high in precision, survivability, and selective lethality. Reliability as a weapon system for force-application or space-control missions (see AF 2025 counterspace white paper) could be very high, since the TAV could launch active radar or inertially- uided weapons (such as KEW devices) through weather conditions that would baffle directed-energy weapons, provided the developmental problems that have plagued spaceplanes can indeed be solved.¹²⁸ When not needed as a weapon system, the TAV could "earn its keep" through a variety of useful, nonbelligerent missions such as rapid replenishment/repair of small satellites, high-value airborne/spaceborne surveillance and reconnaissance sortie, emergency clandestine low probability of intercept (LPI) communications or command and control link, or (properly configured) even as a truly high-speed airborne warning and control system (AWACS)/joint surveillance, tracking, and radar system (JSTARS) platform. A small fleet of TAVs would be a highly flexible, adequately responsive component of an effective space-strike system in 2025.

A careful evaluation of the weapon systems discussed in this chapter leads us to eliminate most candidate systems based on the desired capabilities of timeliness, responsiveness, flexibility, precision, survivability, reliability, and selective lethality. A summary of the evaluation can be found in table 1, where the following potential weapon systems are listed: the distributed laser (DL) (e.g., earth-based laser or space-based mirror), the space-based laser (SBL), the TAV itself, the space-based HPM, the EMP weapon (as a small, conventionally triggered bomb only), the hypervelocity KEW (as a payload on a TAV), other projectile weapons (ballistic missile payloads or BM), space-based NPB, and the "illusion weapon" concept (ILL). Each potential system's "score" against a desired capability is given as high, medium, or low.

Large space-based weapon platforms are eliminated because they are not considered to be survivable in 2025 or practical in terms of weight, cost, size and in most cases power requirements (incoherent light, NPB, HPMW weapons). EMP weapons are eliminated because they lack flexibility, are not precise enough to limit collateral damage, and their selective lethality is at best questionable. Projectile weapons, while very precise, are eliminated because they are not considered to be highly flexible or capable of providing selective lethality. ILLs are ruled out because they do not provide the redundant signatures (e.g.., optical, infrared, radar, and so on) that would make them sufficiently believable and because these notional weapons, too, do not provide selective lethality.

As we carefully studied the characteristics and capabilities of the various candidate weapon systems, it became evident there was no one "super weapon system" that could do all the things the US government would require in 2025. This is not a surprising conclusion-earth-based weapon systems have always complemented each other. We call the weapon system-of-systems that best addresses the US government's likely requirements in 2025 and incorporates an optimum mix of desirable capabilities the Global Area Strike System (GLASS). GLASS consists of: 1) a directed-energy weapon (DEW) system based on the continuous wave laser described previously, and 2) a TAV system (manned or unmanned), which will be used primarily as a weapons platform. The DEW system is composed of powerful earth-based lasers that "bounce" their high-energy laser beams off of space-based mirrors to reach the target. The desired TAV is a flexible platform capable of employing compact, onboard DEWs and KEWs when the space-based mirrors are out of range, disabled, or otherwise unavailable for use. The TAV can also deliver KEWs to mobile or stationary targets; drop special operations strike teams to any hotspot in the world; carry EMP bombs, jamming devices, or a myriad of more conventional weapons; and carry small satellites into space or retrieve them from orbit. Perhaps most significantly, the TAV can also be used to sustain and maintain the GLASS constellation of space-based mirrors.

What would the GLASS system of systems look like? The DEW system would consist of a distributed complex of earth-based lasers (located in the CONUS) that direct their beams (continuously variable in output power) to a constellation of adaptive, space-based mirrors (10 - 20 meter diameter depending on the laser wavelength and spot size desired at the target). The mirrors would have moveable covers to protect their surfaces when they are not in use, solar cells and/or chemical fuels for prime power (with small, efficient batteries for backup), an advanced pointing and tracking system, an on-orbit attitude control and maneuvering system and adaptive beam-sensing and control system (a more capable version of today's Guide Star technology), and a communications package for C3 and for linking with the global information network described in appendix B. The mirrors would be placed in one or more low earth orbits (250 - 500 nm) to reduce the target range, thereby minimizing the amount of laser power required to accomplish the mission and decreasing the pointing and tracking requirements. The use of a number of different orbits and inclinations might be necessary to increase the survivability and operational availability of GLASS. According to reputable studies, at least 24 - 32 orbiting platforms would be needed to ensure reasonable global access.¹²⁹ However, the requirement for near instantaneous response could drive the size of the space-based constellation to over one 100 mirrors. Obviously, the actual size of the constellation of mirrors must be determined by a detailed technical analysis beyond the scope of this white paper.

Capabilities

The capabilities previously discussed separately for the laser, the KEW, and the TAV apply to the GLASS. It will be able to perform strategic, operational, and possibly tactical missions. All of these will involve targeting and applying force to both static and mobile targets. The effects on and types of targets for the lasers, the KEWs, and other possible weapons (using the TAV as a platform) were discussed at length earlier in chapter 3.

It is important to note that the laser and/or TAV (with KEWs/other weapons) give the GLASS a full range of lethality-from temporary denial and disruption to partial damage to complete destruction (as described in the sections on the laser and KEW weapons). The laser provides near instantaneous response time, a light-speed attack that negates all conceivable forms of active defense, and the ability to strike anywhere on the planet. The requirement for a global, all-weather strike capability might be met by using a different laser wave length to "burn" a hole through clouds, smoke, or aerosols (using the same mirror or a different one) or by employing alternative weather-control techniques before striking for effect. With a well-designed, distributed laser network based in the CONUS, there should always be several ground-based lasers with clear enough skies to fire. However, when times arise when it is impossible to use the laser (e.g., when the target itself is "weathered in"), the TAV will be able to respond to crises on short notice (2 - 6 hours depending on container package required), putting human judgment and human adaptability "on site" as needed.

In the world of 2025, the GLASS can become America's "forward presence without forward basing." This system-of-systems can be used to extend America's eyes and fists around the globe in near real-time while minimizing the need for vulnerable overseas infrastructure and forward deployment of personnel. The GLASS can be global power and global awareness all in one package, and without actually placing any weapons in orbit (TAVs carry weapons through space in a manner entirely analogous to the modern ICBM).

The GLASS is a powerful concept, but it cannot function independently. Both the DEW and TAV-mounted KEW will require real-time external handoff of precise target location (and possibly target characteristics); a credible "identification - friend or foe" (IFF) capability; and a secure command and control system. The DEW will also require real-time information on battlefield, atmospheric, and space weather conditions that could affect beam propagation and target coupling. Beam-control systems with submicroradian pointing and tracking accuracies with active satellite vibration and thermal control systems for space-based platforms will also be needed. Powerful SAT/BDA with onboard processing systems will be essential to acquire and track mobile targets. The TAV will also require real-time information on battlefield conditions (especially to avoid fratricide and "friendly" kills). When the TAV-mounted KEW is used, it will require hypervelocity flight control, high-g and high-temperature flight hardening, and smart fusing. A method for maintaining tracking and control during the terminal phase despite the sheath of hot, shocked gas surrounding the reentry projectile may also be required.

To fully exploit the global omnipresence of sensors and the proliferation of sensor types in 2025, the SAT/BDA system should be given near real-time access to the global surveillance and reconnaissance and communication systems. The ability to receive and interpret other views of the target will greatly enhance the mission success rate, and might prove to be the enabling capability for some weapon concepts.

Countermeasures

The countermeasures previously discussed in this chapter for the coherent light laser and the TAV still apply when they are employed separately to engage targets. However, when employed as a system, the enemy would have to target the ground-based lasers (virtually all of them) and the TAV launch sites (again, nearly all of them) to disable GLASS-a daunting task when you realize that most of GLASS's components are based in the CONUS. If the enemy only attacked a few space-based mirrors or a few TAVs, the remaining CONUS-based TAV fleet could quickly (within a day) reconstitute a significant portion of GLASS's constellation of orbiting mirrors. Moreover, the enemy must remember that these two components of the GLASS, the laser and the TAV, are also very robust. That is, not only can they apply force upon the enemy, they can protect each other. The laser can hit targets, in space or on earth, that threaten the TAV launch sites, the ground-based lasers, or the mirrors, and the TAV can likewise respond to these same threats, but with more flexibility (launched quickly into any orbit with a wide variety of weapons).

Weather and atmospheric conditions will always be a concern for the GLASS. As stated before, the laser can be blocked or at least degraded by cloud cover. The weather modification concepts discussed in the associated AF 2025 white paper may therefore be needed to provide all-weather, space-strike capability.¹³⁰

The cost of the GLASS is a large concern. Space systems are inherently expensive due to the high cost of space lift and the difficulty of designing and building systems to operate for long periods of time in the hostile space environment. Moreover, projecting what a system will cost 30 years in the future is quite risky (especially using technologies yet to be developed) -planners have not had great success in projecting system costs accurately even two to five years in the future.

McDonnell/Boeing claims that the cost of developing a TAV system would be the same as the development cost of the Boeing 777-about $5 billion. This estimate may be close, considering that a Boeing 777 has about 80,000 parts, whereas a TAV, although operating in space, would only have about 30,000 parts.¹³¹ The cost to produce a single TAV would probably be similar to the cost to produce a B-2 bomber-$750 million to $1 billion. However, if the government does not drastically improve its cumbersome acquisition process, these costs could rise dramatically in the coming decades. Fortunately, with the many space mirrors and the fleet of TAVs required by GLASS, and the hundreds (if not thousands) of satellites proposed both in other AF 2025 white papers and in advertised commercial space systems (Iridium, Teledesic), government and industry should be able to develop stable production lines that produce relatively inexpensive, identical satellites vice the large, hand-built, unique satellites of today. The US must certainly keep an assembly line (both commercial and military) going for production of the TAV. America's TAV must not become merely an advanced version of the Space Shuttle-available in small numbers at astronomical cost and with limited usability.

The cost of a directed-energy weapon system that includes ground-based continuous wave laser stations and a constellation of space-based mirrors is more difficult to project since there is no present space-based weapon system to use as a baseline. The USAF is currently developing an airborne laser system (ABL) as a boost-phase ballistic missile defense system. The USAF expects to spend approximately $5 billion to design, test, and field a small number of operational airborne laser systems.¹³² Development costs alone for a distributed system with a space-based element (the mirrors) would be at least as great as this.

The likely cost of some individual components of the DEW element of the GLASS can also be forecast. A high-quality, properly figured and polished laser mirror about 15 to 20 meters in diameter will cost between $20 and $30 million for the substrate alone (coatings will cost more). The total cost of the support structure and mirror will be in the range of $60 to $90 million. Provided the technological challenge of power scaling for solid-state and/or diode lasers can be met, the cost of a single ground station including a megawatt-class laser and its associated infrastructure will be on the order of approximately $100 to $200 million (USAF experts forecast the cost of a 100 megawatt chemical laser alone at $50 to $100 million).¹³³

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