In the future the USAF will operate aerospaceplanes and global-reach strike platforms
Unmanned systems, space access and future strike technologies are the major focus areas for AFRL's Air Vehicles (VA) directorate. Programmes under way range from the Unmanned Combat Air Vehicle (UCAV), which could be flying alongside the Joint Strike Fighter (JSF) by early next decade, to the Future Strike Vehicle, which could replace the US Air Force's bomber fleet around 2025.
These efforts involve VA's four divisions:
aeronautical sciences, which includes aerodynamics research, computational sciences and plasma physics; control sciences, which covers systems and swarming control theory and distributed feedback systems; structures, which encompasses combined environments, the links between loads, thermal and acoustics stress; integration, which specialises in multi-disciplinary design and demonstration.As head of the VA's UAV focus area, Dave Lanman takes note of a recent statement by Senator John Warner, head of the US Senate Armed Forces Committee, that around one-third of USAF combat aircraft should be unmanned by 2010.
Lanman says the USAF's approach to unmanned platforms has three phases. The first includes operational platforms such as the General Atomics RQ-1A Predator, Northrop Grumman RQ-4A Global Hawk and Boeing X-45 UCAV. Within phase two, Lanman places the Sensor Craft intelligence, surveillance and reconnaissance UAV (see P38), and upgrades to platforms such as the Global Hawk to make them "an adjunct" to the Airborne Warning and Control System and Joint Surveillance Target Attack Radar System. Phase three includes the integration of directed energy weapons into UAVs, space access and future strike platforms.
Lanman describes the UCAV as a revolution in tactical air power designed for high-risk/high-payoff missions. It is also a "new paradigm in affordability", he says. An operational UCAV is targeted to cost less than a third that of a JSF - which itself is designed to be affordable - with operating costs less than 25% of those of today's Lockheed Martin F-16CJ, the USAF's dedicated suppression of enemy air defences fighter. The X-45 programme is intended to demonstrate the UCAV's effectiveness in SEAD and strike roles, says Lanman. Key developments will be in line-of-sight and beyond-line-of-sight command and control that is secure and robust.
For space access and future strike technologies, VA's task is to "evolve, develop, demonstrate and validate technologies that are pervasive for Mach 3 to space insertion platforms", says focus area manager Doug Dolvin. He says there is a drive to reduce the operating and support costs of space access vehicles and that, unlike current launchers, these costs will be measured per flight, not per kilogramme of payload inserted into orbit. This is because the USAF requires a "launch on demand capability" rather than today's rigidly scheduled launches. This will provide responsiveness and enable forces to be placed on alert.
To achieve this, SOVs will have to be easier to maintain than the Space Shuttle, and highly reliable. Additional considerations for the Future Strike Vehicle (FSV) include survivability and lethality. Together, SOV and FSVreflect the USAF's desire to have global reach from the continental USA and are part of its transition to an "aerospace force", says Dolvin. Operating from US bases provides increased security and reduces the infrastructure expenditure needed to establish overseas bases.
On-demand space access
A first-generation, air-breathing SOV could be a reality by 2015, AFRL estimates, while the FSV is intended as the long-term replacement for today's strategic bombers, entering service in 2020-25. The SOV is required to provide on-demand access to space, while also reducing operating and support costs 50%, improving sortie generation rate five-fold, boosting mission reliability by a factor of 10 and increasing mission flexibility by 20%.
Bill Gillard, SOV technology office programme manager, says the unmanned military spaceplane will be a reusable booster/lifter or "truck" for carrying various stages into orbit: a Space Manoeuvre Vehicle (SMV) for space control missions; a Modular Insertion Stage for rapid elevation of equipment from the low-Earth orbit of an SOV to geostationary orbit; or a Solar Orbit Transfer Vehicle for tasks such as satellite repair.
Gillard says AFRL has not specified whether the SOV should be two-stage-to-orbit or single-stage-to-orbit design, but acknowledges the former is probably more practical in the medium-term. Although VA uses a variety of images to illustrate the SOV, including Lockheed Martin's X-33, "they are simply concepts", Gillard says. "We don't know the configuration. There is a diversity of concepts. We're trying not to focus on any one."
AFRL is leveraging NASA's reusable launch vehicle (RLV) research and development efforts, but USAF requires a light-to-medium lift capability - around 6,800kg (15,000lb) compared to NASA's 22,700kg heavylift needs - plus high sortie rates, surge capacity, significant cross-range ability to deviate from orbit and all-azimuth launch capability. Cross-range performance will allow a landing at the launch site after one orbit, the vehicle needing good manoeuvrability to land back at the take-off site, which will have "moved" because of the Earth's rotation. All-azimuth launch will remove today's requirement for launches over the sea to reduce risk if something should go wrong.
Gillard says key SOV technologies are split into five core areas: propulsion, airframe, ground systems, vehicle systems and payload integration. Propulsion technologies include reusable engines that are throttleable and can be restarted many times. Airframe technology needs include a robust, "most-weather" thermal protection system which allows rapid attachment and detachment for maintenance. Propellant tank producibility and cycling effects need to be understood, Gillard says, adding that despite problems with the composite fuel tank for the Lockheed Martin X-33 RLV technology demonstrator, "composites are not ruled out" for hydrogen tanks.
To reduce maintenance, the SOV will require integrated prognostic and health management systems and, because the platform will be autonomous, it will need a fault-tolerant guidance, navigation and control system. Payload integration will require automated alignment, mating and verification while standard launcher/payload interfaces will also be required.
Dolvin says FSV configuration and trade-off studies now under way are expected to answer the question: "Is stealth necessary [at speeds] above today's Mach numbers?" Weapons for such platforms are also being considered. Work with impact on the studies includes DARPA's new Quiet Supersonic Platform programme to develop technology for long-range, low-boom aircraft and ongoing supersonic-combustion ramjet engine development. He says the USAF needs to invest in demonstrations of aerospace propulsion and weapons. These could culminate in an X-vehicle subscale technology demonstrator in 2005-6, he continues.
Unlike the SOV, the FSV's range is not critical as the aircraft might not have to return to the continental USA after a mission. Alternatively, while the vehicle will fly to its target at maximum speed and altitude - making air-to-air refuelling undesirable, as it takes a long time to slow down and descend to a tanker's altitude - the FSV may use lower altitudes and speeds to either return home or refuel in the air on the way back. An orbital FSV could coast back to Earth.
Exoatmospheric operation is attractive as it eliminates the need to request overflight rights. This is the reason for the SOV being considered as an FSV, says Dolvin, particularly if all-azimuth launch can be achieved. As a result, the SOV and FSV could become derivations of a single aerospace type, but "essentially the same", concludes Dolvin.
Source: Flight International
