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SR-71 Inlet System & Supersonic Flow

1. Description of SR-71 Inlet System

1.1. Introduction

The SR-71 inlet system operates using a mixed compression inlet geometry. The supersonic compression is done both externally, forward of the cowl lip and internally, aft of the cowl lip.

The SR-71 was designed by David H. Campbell, which is the only operational aircraft to fly sustained cruise at a high Mach number (M = 3.2). The figure below shows a military aircraft with an SR-71 inlet system.

SR-71 Inlet System & Supersonic Flow

Figure 1. A schematic diagram of SR-71’s airflow patterns and the front view of an SR-71 aircraft as it prepares for takeoff.

Before sending the air to the engine, the inlet recovers the pressure from the air at the elevated ram pressure caused by the aircraft’s forward speed.

SR-71 Inlet System & Supersonic Flow

Figure 2. An SR-71 Jet Propulsion System

SR-71 Inlet System & Supersonic Flow

Figure 3. The placement of the SR-71 inlet system on a supersonic aircraft.

1.2. Limitation of SR-71

The SR-71, which stands for “Strategic Reconnaissance,” was designed by Lockheed for the United States Air Force. This aircraft is designed to be one of the most advanced aircraft of its time, capable of outperforming other militaryaircrafts by flying at higher speeds and at a higher altitude of approximately 80,000 feet.

SR-71 Inlet System & Supersonic Flow

Figure 4. The view from the SR-71 at 83,000 feet

This aircraft, however, has not always been this advanced. This aircraft’s initial version could not withstand the high temperatures travelling at supersonic speeds, exposing its structural integrity to damage whenever it reached its limit. Another limitation of its older version is the inability to maintain altitude in order to avoid surface-to-air missiles.

As a result, Lockheed modified the SR-71 to be the fastest aircraft at the time by switching its primary component from steel to titanium. Given the changes, the US government was forced to disguise itself as a third-world country in order to purchase titanium from the Soviet Union. Following the modifications, the aircraft was much more efficient at high speeds, flying at Mach 3.2. The US government has spent a lot of money on this mission because of the high fuel consumption as it travels at high speeds, the maintenance of its titanium covering, and having a lot of fuel. The SR-71 was designed by engineers to leak and have an overflowing fuel tank that leaks while on the runway. It is used to account for material expansion at high temperatures. The only limitation left for the aircraft is the temperature of the air passing through the engine compressor, which reduces its endurance, which could be more than 90 minutes at the time.

1.3. Inlet Geometry

The axisymmetric, mixed-compression, translating spike inlets on the SR-71 are positioned behind the vehicle’s leading edge shock and in front of the wing. To align their centerlines with the incoming flow as it is impacted by the vehicle forebody at the design flying condition, the inlets are each canted down 5.6° and inboard at 3.2°.

Spike Translation

Four struts support the inlet center body and spike within the inlet cowling. These struts support and guide the translating spike by holding up the fixed center body barrel. The oscillations of contraction and spillage needed for inlet operation are provided by this spike translation, which is scheduled with flight Mach number and aircraft orientation.

From the point of the spike rearward to about the tip of the struts, a straight centerline runs through the supersonic and subsonic compression areas. For flow alignment, this inlet part is canted. A slight curve via a constant area section between the beginning of the struts and the engine face realigns the flow with the aircraft reference line. Each strut is distinctive and different because of its bent.

SR-71 Inlet System & Supersonic Flow

Figure 5. The inlet components of SR-71.

1.4. Inlet Characteristics

The SR-71 inlet system operates in an axisymmetric, mixed compression geometry that provides several advantages in terms of weight, drag, and pressure recovery. The lower weight and drag of the inlet are associated with its axisymmetric property. On the hand, the mixed compression allows high-pressure recovery above Mach 2.2 given that the normal shock is kept at the design location downstream of the inlet throat and maintained during internal or external flow perturbations. However, should the normal shock cannot be maintained at the design location above Mach 2.2, the inlet is said to be not started, making the normal shock to slip and stabilize forward of the cowl lip, reducing the pressure recovery, airflow, and thrust to low levels, and increasing the drag dramatically. Therefore, to prevent engine damage and mitigate airplane yaw transient, the inlet must be restarted as fast as possible. This higher-pressure recovery allows the satisfaction of the aircraft performance requirements, especially range and cruise. 

1.5 Inlet Control System

The control system of the SR-71 inlet is composed of several components performing different functions at different speed ranges. The spike is positioned as a function of the Mach number with an angle of attack, sideslip, and normal acceleration bias. It is at a full forward position at low Mach numbers and gradually moves at retracting position as Mach increases to 1.6. However, in an event that unstart occurs in one inlet above Mach 2.3, both spikes will be driven to their forward position, and the forward bypass doors, which are closed when the landing gear retracts, will be opened to allow restart and reduce airplane yaw transients. The aft bypass doors, on the other hand, are manually scheduled to supply cooling air into the engine bay and ejector. Below is a figure illustrating the inlet control components of SR-71.

SR-71 Inlet System & Supersonic Flow

Figure 5. The inlet control components of SR-71.

1.5. Inlet Performance Considerations

The amount of free stream flow conditions that are “recovered” is measured by the inlet total pressure recovery, which is how aerodynamicists describe the inlet’s pressure performance. The J-58D engines that propelled the SR-71 in this instance effectively functioned as a ramjet at the higher speeds because a sizable portion of the airflow 101 bypassed the combustor and turbine and was directed directly into the afterburner. Similar to the J-58D engine, the SR-71 inlet has characteristics similar to TBCC. Starting at Mach 1.7 and going up to Mach 3.2, it is a mixed compression inlet. The inlet controls the position of the terminating normal shock and the amount of airflow passing through the engine using a translating spike, a series of bleeds, and bypasses. The inlet does, in fact, satisfy the Kantrowitz limit at Mach 1.7 and maintains the self-starting capability through Mach 3.2. The same inlet configuration has the potential to be modified in order to determine whether the flight envelope could be pushed into the hypersonic flight regime.

2. How the SR-71 Inlet System Works

The SR-71 powerplant consists of the airflow inlet, the Pratt & Whitney J58 turboramjet engine, and the convergent-divergent ejector, all enclosed within the nacelle. Below speed of Mach 2.0, the engine works similar to other turbojet engines. The air flows into the nacelle through the inlet where it diffuses behind a supersonic shockwave then moves to a multi-stage axial compressor. In the compressor, the air is compressed before going to the burners where fuel is mixed to facilitate combustion. The exhaust gasses created are propelled backwards which turns the turbine and is accelerated to high speeds by the ejector which generates a large amount of forward thrust. The turbine turns the compressor keeping the engine cycle going. An afterburner is located after the turbine where more fuel is added to use the remaining oxygen out of the exhaust, providing more thrust for faster acceleration.

SR-71 Inlet System & Supersonic Flow

The SR-71 inlet design is of critical importance to allow the J58 engine to operate properly. Located in the middle of the inlet section is the inlet spike and behind it is the diffuser where compressed air is spread out before entering the engine.

At supersonic speeds, the inlet spike takes the pressure of the leading supersonic shockwave off of the engine in order for the engine to receive the best airflow. A second shockwave (normal shockwave) is formed inside the inlet, where the incoming air transitions from low-pressure supersonic speeds, to high pressure subsonic speeds.

The location of where the normal shockwave ends up inside depends on the speed of the aircraft and the inlet shape. When the aircraft reaches Mach 1.6 the normal shock wave reaches the best place inside the inlet for pressure recovery – which is the percentage of the pressure caused by the aircraft’s supersonic flight forward which is translated into useful pressure within the diffuser of the engine. 

This ratio is extremely high in SR-71 at 90% when flying at Mach 3.2. In order to keep the normal in the optimal position for pressure recovery, the spike retracts about 1.6 inches for each 0.1 increase in Mach number above Mach 1.6. This creates a change in the relative geometry of the inlet maintaining the optimal position for the normal shockwave. When the aircraft reaches its cruising speed of Mach 3.2, the external shockwave is positioned directly at the inlet’s lip (cowl), and the inlet spike retracts 26 inches. At this speed the J58 turboramjet has its maximum fuel efficiency, where the pressure recovery at the inlet does most of the compression of air for the afterburner.  


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