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Inlet cone
Inlet cone
Inlet cone
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MiG-21MF inlet cone

Inlet cones (sometimes called shock cones or inlet centerbodies[1]) are a component of some supersonic aircraft and missiles. They are primarily used on ramjets, such as the D-21 Tagboard and Lockheed X-7. Some turbojet aircraft including the Su-7, MiG-21, English Electric Lightning, and SR-71 also use an inlet cone. The inlet cone for circular/axisymmetric inlets has its equivalent in the intake ramp for 2-D/rectangular inlets.

Purpose

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Main article: Components of jet engines § Air intakes

An inlet cone, as part of an Oswatitsch-type inlet used on a supersonic aircraft or missile, is the 3D-surface on which supersonic ram compression for a gas turbine engine or ramjet combustor takes place through oblique shock waves. Slowing the air to low supersonic speeds using oblique shocks generated by a cone minimizes loss in total pressure (increases pressure recovery). Also, the cone, together with the inlet cowl lip, determine the area which regulates the flow entering the inlet. If the flow is more than that required by the engine then shock position instability (buzz) can occur. If less than that required then the pressure recovery is lower which reduces engine thrust.[2]

An inlet with cone may be used to supply high pressure air for ramjet equipment which would normally be shaft-driven on a turbine engine, eg to drive turbopumps for the fuel pump on the Bristol Thor ramjet and hydraulic power on the Bristol Bloodhound missile.

Shape

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The cone angle is chosen such that, at the design condition for the inlet (Mach 1.7 for the English Electric Lightning inlet[3]), the shock wave that forms on its apex coincides with the cowl lip. The inlet passes its maximum airflow and achieves its maximum pressure recovery.[4] A higher design speed may require two oblique shocks focussed on the lip to maintain an acceptable pressure recovery and pass maximum airflow. In this case a biconic cone is required with two angles ( the Bristol Thor ramjet has 24 and 31 degrees for a design speed of Mach 2.5).[5] For higher speeds a more smoothly contoured transition between cone angles may be used in what is known as an isentropic spike (Marquardt RJ43 ramjet).[6]

The conical body may be a complete cone centerbody in a round inlet (MiG-21), a half cone in a side-fuselage inlet (Lockheed F-104 Starfighter) or a quarter cone in a side-fuselage/underwing inlet (General Dynamics F-111 Aardvark).

The rear of the cone beyond its maximum diameter, rear-facing and unseen inside the duct, is shaped for a similar reason to the protruding front part. The visible cone is a supersonic diffuser with a requirement for low loss in total pressure, and the rear, streamlined part, together with the internal surface profile of the duct, forms the subsonic diffuser, also with a requirement for low loss in total pressure as the air slows to the compressor entry Mach number.

For Mach numbers below about 2.2 all the shock compression is done externally. For higher Mach numbers part of the supersonic diffusion has to take place inside the duct, known as external/internal or mixed compression. In this case the rear part of the forward-facing conical surface, together with the internal surface profile of the duct, continues the supersonic diffusion with reflected oblique shocks until the final normal shock. In the case of the Lockheed SR-71 Blackbird with part of the supersonic compression taking place inside the ducting the spike and internal cowl surfaces were curved for gradual isentropic compression.[7] The inlet cone also has different axial positions to control how the capture area varies with the duct internal throat area. For best intake operation this required area ratio gets bigger with increasing flight Mach number, hence the large inlet cone movement on the SR-71 which had to perform well from low speeds to Mach 3.2. On the SR-71 the cone moves back at higher speeds.[7]

Operation

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At subsonic flight speeds, the conical inlet operates much like a pitot intake or subsonic diffuser. However, as the vehicle goes supersonic a conical shock wave appears, emanating from the cone apex. The flow area through the shock wave decreases and the air is compressed. As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip.

For higher flight speeds a moving cone becomes necessary to allow the supersonic compression to occur more efficiently over a wider range of speeds. With increasing flight speed, in typical Oswatitsch-type supersonic moving-cone inlet - the cone is moved forward (MiG-21), and if it is non-Oswatitsch-type cone inlet (SR-71) it is moved to the rear, or into the intake. In both cases, due to the shape of the cone surface and the internal duct surface the internal flow area gets less as required to continue compressing the air supersonically. The compression occurring in this path is called "internal compression" (as opposed to the "external compression" on the cone). At the minimum flow area, or throat, a normal or plane shock occurs. The flow area then increases for subsonic compression, or diffusion, up to the engine face.

The position of the cone within the intake is usually controlled automatically to keep the plane shock wave correctly located just downstream of the throat. Certain circumstances such as an engine surge can cause the shock wave to be expelled from the intake. This is known as an unstart.

Alternative shapes

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Some air inlets feature a biconic centrebody (MIG-21) to form two conic shock waves, both focused on the lip of the intake. This improves pressure recovery. Some aircraft (BAC TSR-2, F-104, Mirage III) use a semi-conic centrebody. The F-111 has a quarter cone, which moves axially, followed by an expanding cone section. Because these inlets tend to be positioned behind the forward fuselage, they are spaced away (i.e. mounted on a splitter plate) in order to bypass the forebody's turbulent boundary layer to mitigate flow distortion.

Other supersonic aircraft such as the Concorde, Tu-144, F-15 Eagle, MiG-25 Foxbat, and the A-5 Vigilante use so-called 2D inlets, where the nacelle is rectangular and a flat intake ramp replaces the dual cones. The ramp is similarly adjusted in flight to ensure that the oblique shocks are properly positioned for efficient pressure recovery; such designs can also have supersonic compression be either all external, or mixed external/internal with the XB-70 Valkyrie being an example of the latter.

Some other supersonic aircraft (Eurofighter Typhoon) use a variable lower cowl lip[8] for high angle of attack operation and a bleed system (porous wall) incorporated on the intake ramp to facilitate stabilization of the shock system at supersonic Mach numbers. For the improvement of the intake flow (reduced distortion), air is dumped via an intake bleed slot on the ramp side downstream of the intake. The ramp, which is separated from the fuselage by a diverter, produces an oblique shock in order to decelerate the flow. The leading edge of the splitter plate separating the two intakes is located downstream of this oblique shock.[9]

Many supersonic aircraft (F-16 Fighting Falcon, F/A-18 Hornet) dispense with the conical centrebody or complex variable ramps and employ a simple fixed-geometry pitot intake, which is structurally lighter and more durable; a detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poorer pressure recovery especially at higher Mach numbers. This was considered an acceptable tradeoff for aircraft that mainly operate in subsonic and transonic airspeeds with only transient supersonic dashes.

In newer aircraft, advances in aerodynamics have enabled fixed-geometry inlet designs to match the performance of variable inlet cones or ramps through careful shaping of the inlet geometry and using downstream pressure to control shock position. Examples include swept caret inlet ramps and cowls (F-22 Raptor, F/A-18 Super Hornet), which has a pair of fixed oblique ramps and a downstream bleed system to control and avoid shocks. Another is the diverterless supersonic inlet (F-35 Lightning II), which has a 3-D (non-axisymmetric) compression bump that acts similarly as a fixed half-cone to avoid shocks while also diverting the forebody boundary layer.

NASA has tested an alternative to the external/internal, or mixed compression inlet, needed for speeds above about Mach 2.2 (below that speed inlets with all-external compression are used). The mixed-compression inlet is susceptible to unstarts or expulsion of the internal shock to in front of the inlet. The NASA inlet, which they call a Parametric Inlet, does all the supersonic compression externally so there is no shock inside the ducting in a potentially unstable location.[10]

Different types of inlet cone

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See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Inlet cone

View on Grokipedia
from Grokipedia
An inlet cone, also known as a shock cone or centerbody spike, is an aerodynamic device positioned at the entrance of a supersonic aircraft's air intake to decelerate high-speed incoming airflow from supersonic velocities to subsonic speeds via controlled shock waves, thereby optimizing air compression for the engine's compressor stages. This component is essential for maintaining engine efficiency and preventing performance degradation caused by unmanaged shock interactions during high-Mach flight. The inlet cone functions by generating an oblique shock wave at its leading edge, which reduces the airflow's Mach number while minimizing total pressure losses, followed by a normal shock wave positioned strategically within the inlet duct to further slow the air to subsonic conditions suitable for the turbojet or turbofan engine. In operation, the cone's geometry interacts with the freestream airflow to capture a high percentage of the available air mass, with the shock system's stability critical to avoiding "unstarts"—sudden disruptions where shocks are expelled from the inlet, leading to thrust loss and aerodynamic instability. Boundary layer management, often via bleed slots or diverterless designs, complements the cone's role by removing low-energy air near the inlet walls to sustain shock positioning. Designs of inlet cones vary based on mission requirements, with fixed cones used in aircraft operating up to moderate supersonic speeds and translating or variable cones employed for higher Mach regimes to adjust the shock train dynamically. Fixed cones, typically sharp-nosed and axisymmetric, provide simplicity and lower weight but limited adaptability, while translating cones—such as those that extend or retract by precise increments (e.g., 1.6 inches per 0.1 Mach increase)—enable optimized performance across a wide speed range by altering the inlet's effective capture and throat areas. Advanced features like pressure sensors and bypass doors further refine airflow regulation, ensuring the normal shock remains aft of the inlet throat during acceleration or maneuvering. Notable applications include the Lockheed SR-71 Blackbird, where a translating cone in its mixed-compression inlet supports sustained Mach 3+ cruise by adjusting geometry and managing bypass flows for 112% increased capture area at peak speeds. Earlier designs, such as the fixed inlet cone on the Mikoyan-Gurevich MiG-21 supersonic fighter, demonstrated the cone's effectiveness for interceptors achieving Mach 2, emphasizing its role in post-World War II aviation advancements. These implementations highlight the inlet cone's evolution from basic shock management to integral components in high-performance military and reconnaissance aircraft, influencing thrust efficiency and overall vehicle aerodynamics.

Fundamentals

Definition and Role in Aircraft Engines

The inlet cone, also known as a spike or centerbody, is a conical or spike-shaped structure positioned at the center of an aircraft's air inlet to guide and compress incoming airflow in high-speed propulsion systems. In supersonic aircraft, it serves as the primary centerbody within axisymmetric external-compression inlets, where its geometry facilitates the management of airflow entering the engine. This design is particularly suited for decelerating high-velocity air while minimizing aerodynamic losses, ensuring compatibility with downstream engine components. The primary role of the inlet cone is to act as a centerbody that generates oblique shock waves in supersonic flow, thereby decelerating the air from supersonic to subsonic speeds for efficient compression within the engine. By employing an isentropic compression surface—typically with a small half-angle such as 8°—the cone produces a series of oblique shocks that compress the flow gradually, reducing the Mach number (e.g., from approximately 1.7 to 0.65 at the engine face) without excessive drag or heat generation. This process avoids the higher entropy losses associated with a single normal shock, optimizing overall engine performance by delivering subsonic air to the compressor stages. The inlet cone is integrated within the nacelle or fuselage inlet of turbojet or turbofan engines, where it interfaces directly with the diffuser section to channel compressed air toward the compressor inlet. Positioned upstream of the engine core, it works in conjunction with the inlet's cowl and throat to capture and precondition freestream air, often incorporating bypass ducts in advanced designs to route boundary layer flow away from the core path and reduce distortion at the aerodynamic interface plane. This integration ensures stable operation across varying flight conditions, from subsonic takeoff to supersonic cruise. At its core, the inlet cone relies on principles of isentropic compression to maximize total pressure recovery, defined by the pressure recovery factor η=Pt2Pt0\eta = \frac{P_{t2}}{P_{t0}}, where Pt2P_{t2} is the total pressure at the engine face and Pt0P_{t0} is the freestream total pressure. High recovery values, such as up to 96% in optimized designs, are achieved through the cone's shock management, which preserves as much stagnation pressure as possible for subsequent compression in the engine. This metric is critical for maintaining thrust efficiency, as lower recovery leads to reduced engine output and increased fuel consumption.

Historical Development

The development of inlet cone technology originated in the 1940s amid World War II-era wind tunnel tests focused on supersonic aerodynamics, where engineers explored conical shock waves to manage high-speed airflow. German researchers laid foundational principles for later supersonic inlets. Following the war, inlet cones saw rapid adoption in U.S. and Soviet supersonic programs during the 1950s, with NASA's Glenn Research Center (then NACA's Lewis Flight Propulsion Laboratory) conducting pivotal studies on inlet flow and shock control from the early 1950s onward. The Soviet MiG-19, entering operational service in 1955, marked one of the first uses of a nose-mounted inlet cone in a production supersonic fighter, enabling level-flight speeds above Mach 1. Similarly, the U.S. Convair F-102 Delta Dagger, operational by 1956, integrated fixed inlet designs influenced by these advancements to support interceptor roles. Key innovations emerged in the 1960s with the introduction of variable geometry, exemplified by the Lockheed SR-71 Blackbird's translating spike inlet, which debuted in its first flight in 1964 and allowed adaptive shock positioning for sustained Mach 3+ cruise. By the 1970s, efficiency demands in tactical fighters prompted a shift from fixed cones, as seen in the McDonnell Douglas F-15 Eagle's adoption of variable ramp inlets for broader operational flexibility. In the modern era, inlet cone concepts evolved toward hypersonic applications, highlighted by NASA's X-43A scramjet demonstrator flights in 2004, which tested integrated inlet designs for airframe-propulsion synergy at Mach 9.6. Post-1980s developments favored mixed-compression inlets over pure cone geometries for superior off-design performance, as validated in ongoing research. Recent optimizations in the 2020s have targeted unmanned aerial vehicles (UAVs), enhancing inlet efficiency in hypersonic waverider configurations for extended endurance and reduced drag.

Design Principles

Aerodynamic Purpose

The primary aerodynamic purpose of an inlet cone in supersonic aircraft engines is to maximize total pressure recovery while minimizing losses, enabling efficient compression and diffusion of incoming airflow to meet the engine's mass flow requirements. By generating a series of oblique shock waves, the cone decelerates the supersonic freestream to subsonic speeds at the engine face, preserving as much stagnation pressure as possible through controlled shock positioning rather than a single normal shock, which would incur higher entropy increases. For instance, axisymmetric spike designs achieve pressure recoveries exceeding 0.97 at Mach numbers around 1.4 to 2.0 by optimizing external compression. This diffusion process builds on subsonic inlet principles, extending them to handle high-speed flows while ensuring the captured airflow matches engine demands for thrust generation. Inlet cones also reduce spillage drag by aligning the conical shock system with the inlet's capture area, defined as Ac=πr2A_c = \pi r^2 where rr is the effective radius at the cowl lip, thereby preventing excess airflow from spilling over the lip and generating additive drag. This positioning ensures the shock-on-lip condition at design Mach, capturing the full streamtube without significant external flow diversion, which is critical for overall propulsion efficiency. Boundary layer growth and heat buildup from repeated shock interactions are controlled through integrated features like bleed slots, which remove low-momentum fluid to mitigate separation and prevent compressor stall downstream. These measures address the thermal loads from shock heating, maintaining flow stability across varying conditions. Performance targets for inlet cones emphasize high total pressure recovery, typically exceeding 0.95 at design Mach, serving as a key metric for effective energy preservation in the diffusion process. Designs aim to balance trade-offs between on-design cruise performance, where optimal recovery and low drag are prioritized, and off-design maneuvers, where flow stability prevents unstart or buzz. To enable reliable operation, cones are engineered to approach the Kantrowitz limit for self-starting, ensuring supersonic flow establishment without auxiliary mechanisms at the inlet's contraction ratio. These objectives collectively enhance propulsion system integration, minimizing weight and drag penalties while maximizing thrust-specific fuel consumption benefits.

Geometry and Shape Characteristics

Inlet cones are typically configured as axisymmetric structures with a semi-vertex angle θc\theta_c ranging from 15° to 25° for operations in the Mach 2–3 regime, balancing compression efficiency and drag. The length-to-diameter ratio L/DL/D is generally maintained at approximately 2–4 to promote flow stability and uniform shock positioning. These dimensions ensure the cone effectively captures and compresses incoming airflow while minimizing structural loads. Profile variations adapt to specific speed regimes: sharp-nosed cones are favored for lower Mach numbers to reduce wave drag through precise shock attachment, whereas blunt-nosed designs are employed in hypersonic applications to mitigate peak heat flux at the stagnation point. Materials selection emphasizes high-temperature resistance, with titanium alloys commonly used for their strength up to 600°C and low weight, though advanced designs incorporate composites like silicone-asbestos laminates for thermal protection and reduced density. The cone diameter scales directly with the engine's throat area to match mass flow requirements, ensuring the captured streamtube aligns with engine demand. Design constraints prioritize avoidance of buzz instability, an oscillatory flow mode that can degrade performance, with geometries optimized using computational fluid dynamics (CFD) simulations tailored to the target Mach number. For weak oblique shocks, the shock angle β\beta can be approximated as β≈arcsin⁡(1/M)+kθc\beta \approx \arcsin(1/M) + k \theta_c, where k≈0.6k \approx 0.6 for air (γ=1.4\gamma = 1.4) at high Mach numbers; this guides initial sizing for shock positioning.

Operational Mechanics

Subsonic Flow Management

In subsonic flow regimes where the freestream Mach number M<1M < 1, the inlet cone functions primarily as a diffuser, decelerating the incoming airflow and converting kinetic energy into static pressure through gradual area divergence. This process follows isentropic flow relations, with the area ratio governed by the area-Mach number function AA∗=1M[2γ+1(1+γ−12M2)]γ+12(γ−1)\frac{A}{A^*} = \frac{1}{M} \left[ \frac{2}{\gamma + 1} \left( 1 + \frac{\gamma - 1}{2} M^2 \right) \right]^{\frac{\gamma + 1}{2(\gamma - 1)}} where γ=1.4\gamma = 1.4 for air, enabling efficient pressure rise without significant losses. The divergent geometry ensures the flow remains attached to the cone surface, promoting uniform velocity profiles at the engine face to support stable compressor operation. To enhance efficiency, inlet cone designs incorporate features that maintain flow attachment and minimize boundary layer effects. The cone's smooth, diverging shape helps prevent flow separation by limiting adverse pressure gradients, while a thickened lip at the inlet cowl reduces the ingestion of low-energy boundary layer air from the aircraft fuselage, ensuring cleaner, higher-momentum flow enters the diffuser. Vortex generators or subtle surface contours may also be employed to re-energize the boundary layer, further stabilizing the subsonic diffusion process. Despite these optimizations, challenges arise in low-speed operations, such as takeoff or idle, where the cone's central blockage can reduce the effective capture area, risking flow stall and boundary layer separation due to excessive deceleration. To mitigate this, auxiliary doors or bypass systems are integrated, allowing additional air ingestion to match engine demand and prevent choking or distortion at low Mach numbers. Performance metrics for subsonic inlet cones demonstrate high effectiveness, achieving pressure recovery coefficients approaching 100% (e.g., Cp>0.98C_p > 0.98) during cruise conditions at Mach numbers around 0.8, with minimal total pressure losses. tests across Mach ranges of 0.3 to 0.9 confirm this, showing optimal recovery in diffusers with length-to-diameter ratios (L/D) of 12–25 and small angles (3.5°–8°), though drops slightly at lower Reynolds numbers due to transitional boundary layers. In variable-geometry configurations, the subsonic diffuser role extends to preparing the flow for subsequent acceleration to supersonic speeds, maintaining high recovery during mode transitions without inducing shocks.

Supersonic Shock Wave Control

In supersonic flow regimes, the inlet cone generates a conical shock wave from its surface, which compresses the incoming airflow while minimizing total pressure losses compared to a single normal shock. The conical shock reduces the Mach number of the flow, culminating in a normal shock positioned at or near the inlet throat to further decelerate the flow to subsonic speeds suitable for the engine. The relationship governing the normal Mach number component downstream Mn2M_{n2} after the shock is given by Mn22=1+γ−12M12sin⁡2βγM12sin⁡2β−γ−12,M_{n2}^2 = \frac{1 + \frac{\gamma-1}{2} M_1^2 \sin^2 \beta}{\gamma M_1^2 \sin^2 \beta - \frac{\gamma-1}{2}}, where M1M_1 is the upstream Mach number, β\beta is the shock wave angle, and γ\gamma is the specific heat ratio (typically 1.4 for air); the total downstream Mach number is M2=Mn2/sin⁡(β−θ)M_2 = M_{n2} / \sin(\beta - \theta), with θ\theta the flow deflection. This equation derives from conservation laws applied across the shock (analogous to 2D oblique shocks but adapted for 3D conical geometry), ensuring efficient compression by keeping the normal component of the Mach number below the full freestream value. The cone's half-angle θc\theta_c serves as the primary control parameter, tuning the strength and position of the conical shock to optimize performance at the design Mach number. For instance, a cone half-angle of approximately 20° is optimal at Mach 2, producing a shock angle β\beta of around 35° and high pressure recovery, while larger angles like 25° suit Mach 4 conditions by accommodating stronger deflections without excessive losses. This tuning balances shock strength against total pressure recovery, with variations of ±2° typically maintaining recovery within 2% of the optimum. Unstart risks arise from perturbations such as changes in angle of attack, which can displace the normal shock forward, expelling it from the inlet and causing a sudden drop in mass flow; the inlet "starts" when the normal shock is swallowed past the throat, enabling supercritical operation with the shock stabilized internally. In advanced designs for Mach 3 and higher, multi-shock configurations such as double-cone setups incorporate additional conical shock reflections to achieve superior pressure recovery, often exceeding 0.9 in inviscid flow by distributing compression across three or more waves before the terminal normal shock. These systems reduce entropy generation compared to single-cone arrangements, particularly at off-design conditions. Losses in these setups are quantified by the entropy rise Δs=−Rln⁡(Pt2Pt1)\Delta s = -R \ln \left( \frac{P_{t2}}{P_{t1}} \right), where RR is the gas constant, and Pt1P_{t1}, Pt2P_{t2} are the upstream and downstream total pressures, respectively; this metric highlights the irreversible effects of shocks, with multi-shock designs minimizing Δs\Delta s through weaker individual waves. Experimental validation of these shock systems relies on schlieren imaging in wind tunnel tests, which visualizes the conical and normal shock positions, confirming their alignment with theoretical predictions and enabling assessment of boundary layer interactions.

Configurations and Variations

Fixed Cone Designs

Fixed cone designs feature a rigid, non-adjustable conical centerbody positioned within the inlet duct to manage supersonic airflow, primarily through external compression where oblique shock waves form ahead of the cowl to decelerate and compress incoming air. This configuration prioritizes structural simplicity, as the cone remains stationary without actuators or moving parts, making it suitable for early supersonic aircraft optimized for a narrow range of operating conditions. Unlike more complex systems, fixed cones eliminate the need for mechanical adjustments, reducing overall system complexity and enhancing reliability in high-stress environments. The primary advantages of fixed cone designs stem from their lightweight construction and optimization for a single design point, providing high total pressure recovery at the design Mach number during cruise. By avoiding actuators and variable mechanisms, these inlets contribute to lower aircraft weight and simplified maintenance, which was particularly beneficial in the resource-constrained development of 1950s-era fighters. For instance, the design allows efficient shock positioning for transonic to low-supersonic speeds, providing stable subsonic flow to the engine compressor without the added mass of movable components. However, fixed cone designs exhibit significant limitations in off-design performance, including vulnerability to inlet unstart—where shock waves detach and disrupt airflow—particularly at off-design high Mach numbers, leading to sudden thrust loss and potential control issues. Without adaptability, these inlets suffer from reduced pressure recovery and increased drag at varying speeds or angles of attack, restricting their use to aircraft with dedicated high-speed roles rather than multi-role operations. This inflexibility often necessitates auxiliary bypass doors for subsonic flight but limits overall efficiency across broad flight envelopes. In implementation, fixed cones typically employ external compression, with multiple oblique shocks generated by the cone's geometry forming outside the cowl to minimize internal flow distortion before air enters the duct. For mixed compression variants, a portion of the compression occurs internally via normal shocks within the inlet, though this increases design challenges related to boundary layer control; external types predominate in fixed configurations for their simplicity. These inlets were commonly integrated into nose inlets on early supersonic fighters, such as the Lockheed F-104 Starfighter, which used fixed half-cone ramps optimized for Mach 2 performance to achieve maximum ram recovery at high speeds, and the English Electric Lightning, which employed a fixed inlet cone for its high-speed interceptor role. Fixed cones have largely been phased out in favor of variable geometry for modern multi-role platforms requiring versatile speed ranges.

Variable Geometry Cones

Variable geometry cones, also known as translating spikes, enable supersonic inlets to adapt their internal geometry to optimize airflow across a wide range of flight speeds, addressing the limitations of fixed designs by dynamically adjusting shock wave positions and capture areas. These cones typically employ hydraulic actuators to translate the spike axially, allowing movement to vary the effective throat area and shock structure. Alternatively, some configurations incorporate pneumatic actuators or rotational mechanisms to alter the cone's angle, facilitating precise control over compression ratios. In operation, the cone retracts forward during subsonic flight to increase the capture area and prevent flow choking, ensuring efficient mass flow into the engine. At supersonic speeds, it extends rearward to position oblique shock waves optimally within the inlet duct, for instance achieving an extension ratio of approximately 1.5:1 at Mach 3 to maintain stable supersonic diffusion. This schedule coordinates with auxiliary features like bleed doors, which open to vent excess boundary layer air and coordinate with ramp positions for smooth transitions. Control systems integrate feedback from pressure sensors to monitor inlet conditions, automatically adjusting the cone and bleed mechanisms to prevent inlet buzz—a disruptive unstart phenomenon caused by shock instability. The primary advantages of variable geometry cones include sustained high total pressure recovery, such as up to 96.5% across Mach numbers from 0.9 to 3.2, which minimizes energy losses and supports consistent engine performance. This adaptability also enables reliable afterburning operation without surge risks, as the cone positioning ensures subsonic flow at the compressor face even under high-thrust conditions. However, these systems introduce significant challenges, including increased mechanical complexity that elevates maintenance requirements due to the need for precise actuation and periodic inspections of hydraulic components. Additionally, the added actuators and control hardware impose additional weight relative to fixed inlet designs, impacting overall aircraft efficiency.

Alternative Inlet Shapes

Alternative inlet shapes to the traditional axisymmetric cone have been developed to address specific aerodynamic, structural, and integration challenges in supersonic and hypersonic aircraft engines. These designs often prioritize compatibility with rectangular engine cross-sections, enhanced stealth features, or operation in extreme speed regimes, while maintaining efficient air compression and pressure recovery. Key examples include two-dimensional (2D) ramp inlets, mixed-compression configurations, diverterless supersonic inlets (DSI), and hypersonic wedge or biplane variants. Two-dimensional ramp inlets utilize planar wedges to generate a series of oblique shock waves that progressively slow and compress incoming supersonic airflow, followed by a normal shock for final diffusion to subsonic speeds suitable for the engine. This configuration is particularly advantageous for aircraft with rectangular engine inlets, as it facilitates better aerodynamic integration with the fuselage and reduces spillage drag compared to axisymmetric cones. A prominent example is the variable-geometry ramp system in the Grumman F-14 Tomcat, where adjustable ramps (up to four) optimize shock positioning across a range of Mach numbers up to 2.4, achieving total pressure recovery efficiencies around 0.94-0.96 at design conditions. Mixed-compression inlets combine external compression via ramps or cones with internal diffusion through the duct, balancing high pressure recovery with reduced boundary layer issues and shock stability. In the Lockheed Martin F-22 Raptor, a caret-shaped 2D mixed-compression inlet employs external oblique shocks from the wedge-like forebody, transitioning to internal compression, which supports efficient operation at Mach 2 cruise while the serpentine S-duct geometry minimizes radar cross-section by blocking direct line-of-sight to engine components, enhancing overall stealth. Diverterless supersonic inlets eliminate mechanical diverters and centerbodies, relying instead on a contoured fuselage bump and swept cowl to generate oblique shocks and divert boundary layer airflow without blades or protrusions. This bladeless S-duct design is evident in the Lockheed Martin F-35 Lightning II, where it operates effectively in both subsonic and supersonic regimes up to Mach 1.6, reducing weight, complexity, and radar signature by avoiding exposed moving parts and integrating seamlessly with the stealth airframe. For hypersonic applications, such as scramjet engines, alternative inlets like wedge configurations or Busemann biplane designs forego traditional cones to minimize drag and enable shock-on-lip operation, where shocks align precisely with the inlet lip for optimal capture. The Boeing X-51 Waverider, tested by NASA in 2010, employed a wedge-based inlet integrated with the waverider body to sustain scramjet combustion at Mach 5, achieving sustained hypersonic flight through efficient shock management and reduced spillage. The Busemann biplane concept, revisited in modern studies, uses dual planar surfaces to cancel shock waves between them, potentially lowering wave drag in hypersonic flows. Trade-offs among these alternatives highlight that while axisymmetric cones excel in uniform, axisymmetric flow distribution for circular engines, 2D ramp and mixed-compression designs offer superior airframe integration for rectangular layouts and stealth-optimized shapes, albeit with potentially higher sensitivity to angle-of-attack variations.

Applications and Examples

Use in Military Aircraft

Inlet cones have been integral to the design of several iconic military aircraft, particularly during the Cold War era, enabling high-speed intercepts and reconnaissance missions. The Lockheed SR-71 Blackbird featured a movable inlet cone, or spike, that adjusted position to manage airflow for sustained Mach 3+ cruises, allowing operations at altitudes exceeding 85,000 feet while evading threats. This design contributed to the aircraft's record-breaking performance, including speeds up to approximately 2,200 mph (Mach 3.2) achieved in the 1970s, by optimizing shock wave control and engine efficiency in the thin upper atmosphere. Similarly, the Mikoyan-Gurevich MiG-25 Foxbat employed a fixed inlet cone configuration optimized for Mach 2.8 intercepts, providing rapid acceleration for air defense roles against high-altitude bombers. In fighter aircraft, inlet cones enhanced supersonic capabilities without excessive afterburner use, supporting sustained dashes critical for air superiority. The Convair F-106 Delta Dart utilized a fixed cone in its inlet system to achieve Mach 2.3 speeds, serving as a primary interceptor during the Cold War by enabling quick response times and high-altitude engagements. The Eurofighter Typhoon incorporates elements of cone-like geometry within its variable ramp inlets, facilitating supercruise at Mach 1.5, which reduces fuel consumption and infrared signature during combat patrols. Stealth considerations have influenced modern inlet designs, often recessing or adapting cone features to minimize radar cross-section. In the Lockheed F-117 Nighthawk, non-conical inlets with baffled grilles were recessed to shield engine components, prioritizing low observability over traditional cone aerodynamics for nocturnal strike missions. Contemporary variants of the Lockheed Martin F-35 Lightning II employ low-observable inlet configurations, integrating diverterless supersonic intakes that eliminate protruding cones while maintaining performance across subsonic and supersonic regimes. Inlet cones played a pivotal role in Cold War air superiority, powering interceptors like the MiG-25 and F-106 that deterred strategic bomber incursions and shaped aerial doctrines. Recent upgrades in Russia's Sukhoi Su-57 Felon (as of 2025), including enhanced variable inlet ramps, support integration of hypersonic missiles exceeding Mach 9, extending the tactical reach of fifth-generation fighters in contested environments.

Civilian and Experimental Implementations

In civilian aviation, inlet cones have seen limited adoption due to the predominance of subsonic flight regimes, where simpler pitot inlets suffice for efficient air compression. However, renewed interest in supersonic commercial transport has revived cone-based designs for their ability to manage shock waves at higher Mach numbers. The Boom Supersonic Overture, a proposed airliner targeting Mach 1.7 cruise speeds in the late 2020s (as of November 2025, in design phase with first flight planned for 2027), incorporates axisymmetric supersonic intake cones as part of its Symphony turbofan propulsion system to optimize pressure recovery and minimize drag during transonic and supersonic operations. These designs draw inspiration from earlier supersonic concepts but emphasize sustainability, with the cones integrated into low-emission engines using sustainable aviation fuels to address environmental concerns. Experimental implementations have focused on validating inlet cone performance in hypersonic regimes, particularly for scramjet engines. NASA's Hyper-X program, culminating in the X-43A vehicle's 2004 flights, explored forebody-integrated inlet geometries to capture and compress air at speeds up to Mach 9.6, achieving the first air-breathing hypersonic flight and providing data on shock positioning and boundary layer control essential for future civilian hypersonic applications. Similarly, conceptual studies for high-speed civil transports, such as the EDGE configuration evaluated in the 1990s, considered conical spike inlets as an option but selected 2-D ramp inlets to balance aerodynamic efficiency with noise reduction, simulating Mach 2+ operations in wind tunnels to inform low-boom overland flight. In unmanned aerial vehicles (UAVs) and drones, fixed-geometry inlet cones enable efficient propulsion for high-speed missions, as seen in designs for reconnaissance platforms. Emerging hypersonic UAV concepts, including those under DARPA's SR-72 program (initiated in the 2010s and remaining conceptual as of 2025), propose advanced inlet designs to handle thermal loads during unmanned Mach 6+ dashes, leveraging combined-cycle propulsion for robust shock management in experimental ground tests. Future trends in civilian and experimental inlet cones emphasize advanced manufacturing and integration challenges. Additive manufacturing enables custom cone geometries with internal cooling channels, as demonstrated in modular ramjet inlet prototypes that enhance thermal resistance for supersonic applications. Integration with hybrid-electric propulsion remains exploratory, with cone designs adapted for distributed airflows in subsonic-supersonic transitions, though efficiency gains are projected at 15-20% through optimized shock diffusion. Sustainability hurdles for civil supersonic revival include post-2025 regulations on noise and emissions, requiring cones to support low-boom signatures and net-zero carbon operations, as outlined in FAA and ICAO frameworks. Material validation occurs in arc-jet tunnels, where cone models endure temperatures exceeding 2,000 K to simulate reentry-like heating, confirming durability for next-generation hypersonic inlets.

References

  1. https://www.grc.nasa.gov/www/k-12/airplane/inlet.html
  2. https://www.smithsonianmag.com/air-space-magazine/how-things-work-supersonic-inlets-35428453/
  3. https://ntrs.nasa.gov/api/citations/20120009203/downloads/20120009203.pdf
  4. https://www.grc.nasa.gov/www/k-12/airplane/oblique.html
  5. https://archive.aoe.vt.edu/mason/Mason_f/NASASP2007-4232Hypersonics.pdf
  6. https://case.edu/ech/articles/n/nasa-john-h-glenn-research-center-lewis-field
  7. https://www.airvectors.net/avmig15_3.html
  8. https://www.militaryfactory.com/aircraft/detail.php?aircraft_id=157
  9. https://ntrs.nasa.gov/api/citations/20090007797/downloads/20090007797.pdf
  10. https://www.enginehistory.org/Convention/2014/SR-71Inlts/SR-71Inlts.shtml
  11. https://aviation.stackexchange.com/questions/103053/why-was-the-cone-inlet-design-abandoned-for-supersonic-aircraft
  12. https://www.nasa.gov/reference/x-43a/
  13. https://ntrs.nasa.gov/api/citations/20050239566/downloads/20050239566.pdf
  14. https://arc.aiaa.org/doi/pdfplus/10.2514/6.1983-1164
  15. https://aia.springeropen.com/articles/10.1186/s42774-024-00181-5
  16. https://ntrs.nasa.gov/api/citations/20230006132/downloads/Slater_AIAA-2023-Aviation_Paper_v3.pdf
  17. https://ntrs.nasa.gov/api/citations/20240008586/downloads/TM-20240008586.pdf
  18. https://apps.dtic.mil/sti/trecms/pdf/AD0476422.pdf
  19. https://arc.aiaa.org/doi/pdf/10.2514/1.J057283
  20. https://arc.aiaa.org/doi/10.2514/6.2024-90590
  21. https://www.frontierspartnerships.org/journals/aerospace-research-communications/articles/10.3389/arc.2024.13149/full
  22. https://www.sciencedirect.com/science/article/abs/pii/S009457652200217X
  23. https://apps.dtic.mil/sti/tr/pdf/ADA291365.pdf
  24. https://farside.ph.utexas.edu/teaching/336L/Fluid/node207.html
  25. https://www.grc.nasa.gov/www/k-12/airplane/isentrop.html
  26. https://ntrs.nasa.gov/api/citations/19720020645/downloads/19720020645.pdf
  27. https://www.gopalancolleges.com/gcem/course-material/ae/aircraft-propulsion/module-3.pdf
  28. https://ntrs.nasa.gov/api/citations/19830019721/downloads/19830019721.pdf
  29. https://ntrs.nasa.gov/api/citations/19730022195/downloads/19730022195.pdf
  30. https://arc.aiaa.org/doi/pdfplus/10.2514/6.2016-0771
  31. https://ntrs.nasa.gov/api/citations/19680024113/downloads/19680024113.pdf
  32. https://apps.dtic.mil/sti/tr/pdf/AD0512542.pdf
  33. https://labs.engineering.asu.edu/aircraft-design/wp-content/uploads/sites/115/2023/05/AIAA-2018-3836.pdf
  34. https://repository.gatech.edu/bitstreams/c7624383-cb0a-4e9b-8724-ec77d9f36b37/download
  35. http://www.anft.net/f-14/f14-detail-airintake.htm
  36. https://arc.aiaa.org/doi/pdfplus/10.2514/1.26830
  37. https://www.researchgate.net/figure/nlets-of-fifth-generation-fighter-1-a-caret-inlet-of-F-22-b-DSI-on-F-35_fig3_326599897
  38. https://www.jsf.mil/diverterless_inlet
  39. https://www.codeonemagazine.com/f35_article.html?item_id=181
  40. https://ntrs.nasa.gov/api/citations/20110010977/downloads/20110010977.pdf
  41. https://www.af.mil/About-Us/Fact-Sheets/Display/Article/104467/x-51a-waverider/
  42. https://www.icas.org/icas_archive/ICAS2006/PAPERS/233.PDF
  43. https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/198054/lockheed-sr-71a/
  44. https://defence-blog.com/russia-arms-su-57-fighter-with-hypersonic-weapon/
  45. https://boomsupersonic.com/flyby/introducing-symphony-the-sustainable-and-cost-efficient-engine-for-overture
  46. https://boomsupersonic.com/flyby/its-about-time-for-a-bold-new-era-of-supersonic-flight
  47. https://www.grc.nasa.gov/www/k-12/BGP/inlet.html
  48. https://ntrs.nasa.gov/api/citations/19930008866/downloads/19930008866.pdf
  49. https://commons.erau.edu/cgi/viewcontent.cgi?article=1908&context=discovery-day
  50. https://env-isa.com/wp-content/uploads/2024/02/aerospace-11-00019-with-cover.pdf
  51. https://scispace.com/pdf/arc-jet-testing-on-hfb-tasi-models-effect-of-the-geometry-on-3b08svmwyf.pdf
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