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X-54 Relaxed Isentropic Compression

It is reported that Gulfstream has been assigned an experimental aircraft designation for a very poorly attested supersonic aircraft called the X-54. The X-54A designator was assigned by DOD to Gulfstream Aerospace on May 5, 2008. Described as a NASA supersonic experimental aircraft to conduct flight research. To be capable of generating relevant ground sonic boom signatures to gather data in support of NASA and a regulatory change process.

Accordinng to one account, "The X-54 will be able to reach speeds of 2,500-mph, or nearly double what the Concorde was capable of achieving.... new design techniques are said to have reduced the sonic boom that resulted in so many restrictions for Concorde, bringing it down to more of a sonic puff or plop.... The X-54 will be the first of three supersonic models being developed by the partnership. The medium sized business jet is expected to debut in 2020, with larger commercial versions to follow in 2030 and 2035."

Another report stated that "reported collaborative effort between NASA, Boeing, Lockheed Martin and Gulfstream would see the 'X-54' fly at around 2,160 kt [Mach 3.2] at 62,000 ft. Concorde cruised at around 1,180 kt [Mach 1.77], and today's fastest business jet - the Gulfstream G650 - makes about 560 kt [Mach 0.84]. Composite materials and engine technology should allow the aircraft to fly direct from London to Sydney in around 4 hours - currently the trip takes about 23 hours.... The experimental X-54 would not fly before 2020 - probably leading to a full in-service 20-seat business jet by about 2030."

Gulfstream has released drawings of supersonic business jet designs over the years, including a patent from 2012 for a relaxed isentropic compression inlet. Gulfstream drawings released in 2007 and 2009 for a concept aircraft identified in trademark applications as the "Whisper". In July 2012, Gulfstream resubmitted an application for the Whisper trademark, describing its intended use for a supersonic aircraft featuring quite-boom technology.

The 2012 drawings depict a long slender telescoping nose - a legacy of the company's "Quiet Spike" experiments - divided into six distinct sections at full extension. Shockwaves develop around aircraft as they near Mach 1, or about 760 mph, the speed of sound at sea level. When an aircraft travels supersonically, the resulting shockwaves can produce a loud sonic boom that rattles windows and nerves on the ground under the path of the supersonic jet. Because of sonic boom intensity, the Federal Aviation Administration prohibits supersonic flight over land, except in special military flight corridors.

In 2006 Gulfstream Aerospace and NASA's Dryden Flight Research Center teamed in a project called Quiet Spike to investigate the suppression of sonic booms. The project centered around a retractable, 24-foot-long lance-like spike mounted on the nose of NASA Dryden's F-15B research testbed aircraft. The spike, made of composite materials, created three small shock waves that travel parallel to each other all the way to the ground, producing less noise than typical shock waves that build up at the front of supersonic jets.

A supersonic inlet employs relaxed isentropic compression to improve net propulsive force by shaping the compression surface of the inlet. Relaxed isentropic compression shaping of the inlet compression surface functions to reduce cowl lip surface angles, thereby improving inlet drag characteristics and interference drag characteristics. Supersonic inlets in accordance with the invention also demonstrate reductions in peak sonic boom overpressure while maintaining performance.

Many supersonic aircraft employ gas turbine engines that are capable of propelling the aircraft at supersonic speeds. These gas turbine engines, however, generally operate on subsonic flow in a range of about Mach 0.3 to 0.6 at the upstream face of the engine. The inlet decelerates the incoming airflow to a speed compatible with the requirements of the gas turbine engine. To accomplish this, a supersonic inlet is comprised of a compression surface and corresponding flow path, used to decelerate the supersonic flow into a strong terminal shock. Downstream of the terminal shock, subsonic flow is further decelerated using a subsonic diffuser to a speed corresponding with requirements of the gas turbine engine.

As is known in the art, the efficiency of the supersonic inlet and the diffusion process is a function of how much total pressure is lost in the air stream between the entrance side of the inlet and the discharge side. The total-pressure recovery of an inlet is defined by a ratio of total pressure at the discharge to total pressure at freestream.

Supersonic inlets are typically either “2D”, having a rectangular opening, or axisymmetric, having a circular opening. The supersonic inlet includes a throat positioned between a converging supersonic diffuser and a diverging subsonic diffuser. Supersonic inlets are generally also classified into three types: internal compression, mixed compression, and external compression.

Internal compression inlets accomplish supersonic and subsonic compression completely within the interior of the inlet duct. The primary theoretical advantage of this inlet type is the extremely low cowling angle that results from a completely internalized shock train. While this inlet design appears theoretically advantageous, in practice it requires a complex and performance-penalizing shock control system in order to position the shock train, to “start” the inlet, and to maintain dynamic shock stability to avoid the inlet's high sensitivity to shock train expulsion (“unstart”). The challenges associated with this type of inlet have limited its use to primarily air-breathing missile applications designed for high Mach number. Below speeds of about Mach 3.5, mixed compression and external compression inlets offer a more practical compromise between performance and complexity.

As the name implies, mixed compression inlets offer a blending of external and internal compression and seek a more practical balance between performance and complexity than that offered by fully internal compression designs in the Mach range from approximately 2.5 to 3.5. The internal portion of the shock train of a mixed compression inlet is less sensitive to flow disturbances than a fully internal design, and has lower cowling angle and drag than a fully external compression inlet designed to the same speed. But mixed compression nevertheless requires a complex control system for starting the internal shock train and for stability management to avoid inlet unstart. Two notable applications of mixed compression include the inlets on the XB-70 Valkyrie and SR-71 Blackbird aircraft.

External compression inlets are most appropriate for applications below about Mach 2.5. In this speed range, external compression offers a design simplicity that typically outweighs its generally inferior pressure recovery. Because the shock train is completely external, cowling angles, and therefore installed drag characteristics, tend to be higher when compared against internal and mixed compression designs at similar speed. However, because the shock train on an external compression inlet remains completely outside of the internal flow path, it is not subject to the sudden unstart expulsion produced by upstream or downstream flow disturbances. External compression shock stability is therefore superior to mixed or internal compression designs, requiring a significantly less complicated inlet control system. Notable examples of inlets employing external compression include those used on the Concorde, the F-14 Tomcat, and the F-15 Eagle.

Traditional inlet design methods have generally focused on improving propulsion system performance by maximizing total inlet pressure recovery and hence gross engine thrust. Complicated secondary systems and variable geometry inlets are often used to accomplish this. While high pressure recovery definitely provides certain gains, maximizing pressure recovery typically comes at the price of significant inlet drag and inlet complexity, characteristics that typically run counter to a robust and low cost-of-operation design.

For example, attempts to increase pressure recovery include bleed air-based methods, which, as is understood in the art, improve inlet pressure recovery through shock strength management and boundary layer removal. The Concorde used a method of bleed air extraction at the inlet throat that weakened the strength of the terminal shock thereby improving total pressure recovery. However, bleed air-based methods typically take a large portion of the intake flow to produce the desired results and suffer corresponding drag-related penalties once the bleed flow is eventually dumped back overboard. Additionally, extensive secondary systems are typically required, consisting of complex flow routing equipment.

Inlet ramp positioning is another method used to improve pressure recovery through more optimum placement of the compression shock system, particularly at off-design operating conditions. The Concorde, F-14, and F-15 are all examples of aircraft that employ ramp positioning for improved pressure recovery. However, ramp positioning requires electric or hydraulic actuators and an inlet control system, resulting in a large increase in inlet part count and complexity. Such systems introduce potential failure points and add significantly to development and operating costs.

The traditional supersonic inlet design process begins with the selection of compression surface geometry that best meets the performance and integration requirements of the intended application, for example aircraft design speed and/or terminal shock Mach number. For an external compression inlet, a compression surface configuration typically focuses the inlet-generated shocks, at supersonic design cruise speed, at a location immediately forward of the cowl highlight or cowl lip, generally referred to as shock-on-lip focusing. This arrangement generally provides good pressure recovery, low flow spillage drag, and a predictable post-shock subsonic flow environment that lends itself to more basic analytical techniques and explains the technique's traceability to the earliest days of supersonic inlet design.

External compression inlet design practice also uses cowl lip angle to align the cowling lip with the local supersonic flow in the vicinity of the terminal shock and the cowl lip. Aligning the lip with the local flow helps to prevent the formation of an adverse subsonic diffuser flow area profile or a complex internal shock structure in the lip region, which reduce inlet pressure recovery and flow pumping efficiency, as well as undermine diffuser flow stability.

However, as understood in the art, as supersonic design speed increases, so does the amount of compression necessary to decelerate the flow to a fixed terminal shock Mach number. Additional compression implies the need for more flow-turning off of the inlet axis, resulting in a corresponding increase in the cowl lip angle (in order to align the cowl lip angle with the local flow at the terminal shock). Any increase in cowl lip angle results in additional inlet frontal area, increasing inlet drag as speed increases. This adverse trend is a key reason why conventional external compression inlets lose viability at high supersonic Mach numbers.

Inlet compression surfaces are typically grouped as either ‘straight’ or ‘isentropic.’ An isentropic surface generally represents a continuously curved surface that produces a continuum of infinitesimally weak shocklets during the compression process. By contrast, a straight surface generally represents flat ramp or conic sections that produce discrete oblique or conic shocks. While an inlet employing an isentropic surface can have theoretically better pressure recovery than an inlet employing a straight-surface designed to the same operating conditions, real viscous effects combine to reduce the overall performance of isentropic inlets and can lead to poorer boundary layer health when compared to their equivalent straight-surface counterparts. Both straight and isentropic inlet types conventionally designed to the same terminal shock Mach number also produce similar flow turn angle at the cowl lip and, consequently, similar cowl lip angles. As such, neither the straight-surface inlet design nor the conventional isentropic inlet design provides a cowl drag benefit relative to the other.

The term “relaxed isentropic compression” surface refers to an isentropic compression surface characterized by a series of Mach lines in which at least a plurality of those Mach lines do not focus on the focus point where the initial shock and the terminal shock meet. This lack of Mach line focusing results in a total level of compression less than the level of compression generated by a conventional isentropic compression surface designed to the same criteria. The relaxed isentropic compression design approach may be applied to any external compression or mixed compression inlet concept, including axisymmetric, partial conic, and two-dimensional intakes. The cowling angles for external compression inlets designed with a relaxed isentropic compression surface may be reduced to approach those employed by traditional mixed compression inlets, merging the inherent shock stability robustness of external compression geometry with the high installed performance of mixed compression geometry.

Relaxed isentropic compression inlet shaping provides an increase in the design latitude for lofting the inlet cowling region while permitting control over other key inlet design parameters such as terminal shock Mach number, diffuser flow distortion, and total pressure recovery. The relaxed isentropic compression inlet shaping may also enable a reduction in cowling surface angles and, as a result, may be configured to improve inlet drag and interference drag characteristics. The reduced slope of the cowling may also lower the contribution of the inlet to the overall vehicle sonic boom characteristic during supersonic flight and decrease the potential for aerodynamic cross-interference between close-coupled inlets.

Embodiments of the invention may includes a supersonic inlet comprising a leading edge configured to generate an initial shock wave and a compression surface positioned downstream of the leading edge and having at least one curved section configured to generate isentropic compression. The supersonic inlet may also include a cowl lip spatially separated from the compression surface such that the cowl lip and the compression surface define an inlet opening for receiving a supersonic flow. The compression surface may be configured to generate a second shock wave that, during operation of the supersonic inlet at a predetermined cruise speed, extends from the compression surface to intersect the initial shock wave at a point substantially adjacent to the cowl lip. The isentropic compression generated by the curved section may be characterized by a series of Mach lines where, during operation of the supersonic inlet at the predetermined cruise speed, at least a plurality of the Mach lines do not focus on the point substantially adjacent to the cowl lip.

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