Design Consideration of Hypersonic Vehicles

First Draft

 

ME 601 Term–Project 

 

 

 

 

 

 

 

 

 

 

 

 

Submitted by,

Ashish Kumar

07010308.

Imagine travelling at speed greater than Mach 5 (you will reach New York in just 3 hours, at present it take around 14 hours!!), inter-continental journey will appear today’s inter-city travel! This seems possible in next 20-25 years with development of hypersonic vehicles which start their flight at 40,000 feet and go beyond 100,000 feet only to land up in Pacific Ocean. A fascinating dream indeed, but then like all dreams there are many challenges in realizing this dream, in this report an attempt has been made to briefly discuss these challenges and efforts done to overcome these challenges.

To reach such speed one needs thrust which can either be given by rocket propellers or by integrated air-breathing scramjet engines. High temperature (Temperatures on the surface of an object travelling at Mach- 5 can reach 1000° C, at Mach-8 the temperatures can reach 2700° C at the leading edge and 3000° C in the engine combustion chamber!) and at the capsule surface makes the aircraft highly short-lived and vulnerable to accidents.

The major design issues in a hypersonic vehicle:

i) Configuration for sufficient L/D ratio at hypersonic speeds. ii) Integration of propulsion mechanism. iii) High temperature at the surface.

The strong viscous effects (high skin friction drag) and strong shock waves (high wave drag) that vehicles suffer at these high speeds decrease the L/D ratio for an aircraft as its velocity increases so design must be so as sufficient L/D ratio is generated at such high speed.

In turbojets and ramjets, the velocity in the combustor is kept below the local speed of sound to enable efficient fuel injection, mixing, and combustion. As the flight velocity increases, useful energy in the incoming air is lost—converted to excessive heat that must be actively cooled—as the air flow is decelerated in the inlet and compressor to local subsonic speeds for entry into the combustor. This subsonic combustor speed limit effectively places an upper bound on aircraft’s flight velocities of around Mach 3–5 for pure turbojets and around Mach 6–7 for pure ramjets. The scramjet reduces this loss by intentionally keeping the local flow speeds through the engine supersonic. If the scramjet engine and vehicle can be designed to withstand the flight external and internal temperatures and pressures, and if fuel combustion can be maintained, then very high air-breathing flights speeds, perhaps as high as Mach 15, may be achievable.

A scramjet requires hydrogen as the fuel for flight speeds greater than about Mach 7. Hydrogen has the ability to sustain combustion in the scramjet’s supersonic combustor at these higher Mach numbers where no other fuel appears to be able to sustain combustion unaided. Liquid hydrogen, however, has the disadvantage of having a very low density, also is very cold, with a temperature close to absolute zero. This requires added tank insulation and special fuel tank pressurization management. Increased tank volume, increased tank insulation, and special fuel tank pressurization management generally yields increased tank mass per kg of propellant carried.

The friction drag at such high speed is too high to generate so much heat that temperature often goes around 1000° C, so either we need material to sustain such high temperature or some insulation mechanism along with heat transfer arrangement is needed.

Discussing the above issues, a typical case study has been presented on the next page onwards.

Configuration:

From the experiments and as well the theory, it has been found that the infinitely thin flat plate represents the most efficient hypersonic lifting surface. The lift-to-drag ratio of the flat plate is the highest that can be achieved at hypersonic speeds. However, the flat plate is obviously not practical, especially since it cannot contain any volume for payload, engines, fuel, etc. The figure below illustrates a more realistic design that is typical of hypersonic aircraft configurations.

 

Generic hypersonic transport configuration tested by NASA.

The general characteristics of these designs:

Very small frontal area and highly streamlined shape to minimize total surface area. Very little wing area, but the fuselage is often shaped to generate additional lift. Propulsion assembly highly integrated into the vehicle.

This particular design was the subject of NASA wind-tunnel testing in the early 1980s at speeds up to Mach 8 to gauge the aerodynamic properties of hypersonic vehicles.

General Aerodynamic Performance:

The wind-tunnel tests conducted on the vehicle shown above indicate that the design is capable of a maximum lift-to-drag ratio of about 5.5 at an angle of attack of about 4°. These results agree well with those derived from Newtonian theory. However, these results also make it clear that hypersonic vehicles are capable of L/D ratios far below those typical of subsonic and low supersonic aircraft. For example, maximum L/D values on the order of 12 to 15 are common for designs like the P-51 Mustang fighter and F-111 bomber. Other studies of hypersonic designs have shown similar results leading to the conclusion that maximum L/D decreases as Mach number increases. Kuchemann analyzed this trend and formulated the general empirical relationship:

 

This relationship, based on actual data from flight vehicle and wind tunnel results, has proven generally accurate across the supersonic and hypersonic regimes.

 

Maximum lift-to-drag ratios for hypersonic vehicles and the "L/D barrier".

Note that most of the open circles fall below the solid curve leading many to refer to the curve as a "L/D barrier." The conclusion to be drawn from this figure is that high L/D ratios are very difficult to achieve at high Mach numbers. This poor performance can be attributed to the strong viscous effects (high skin friction drag) and strong shock waves (high wave drag) that vehicles suffer at these high speeds. Obviously, this result is discouraging since aircraft range in steady level cruise flight is directly dependent on L/D, probably the greatest measure of "aerodynamic efficiency." The dashed curve and solid symbols shown below represent current research efforts aimed at breaking the L/D barrier.

Compression Lift:

So as high L/D ratios are difficult to achieve at hypersonic speeds, how can vehicle shape be optimized to break the L/D barrier? This problem was first approached using elementary momentum principles. First, consider that an ideal hypersonic shape might be a slender body of revolution of continuously increasing radius, or a cone shape. As with any shape, the cone generates lift by imparting a change in momentum on the fluid it passes through. In other words, the body generates a downward force on the fluid and the fluid generates an upward force on the body by Newton's third law of motion. The L/D of the body is maximized when it maximizes the downward momentum and minimizes the forward momentum imparted on the surrounding fluid. However, since a body of revolution imparts upward momentum as well as downward, the pressure forces acting on each surface cancels. This can be thought of as generating a positive lift on the lower surface and a negative lift on the upper surface. This so-called momentum principle therefore dictates that the upper half of the body of revolution should be eliminated resulting in the shape:

 

Derivation of hypersonic body shape using momentum principle.

Next, consider the shape of the wing. The figure below illustrates a top view of the body of revolution derived in above attached to some arbitrary wing shape. As the body passes through a high-speed flow field, it generates a shock wave, and the body can only impart downward momentum on the fluid between its surface and the shock wave. Thus, the wing should extend to the shock wave to capture this change in momentum but not beyond it since this extra area will only create more skin friction drag and structural weight.

 

Derivation of hypersonic wing shape using momentum principle.

Returning to the front view of the body, the body not only imparts downward momentum on the fluid but also a lateral, or sideways, component. If this lateral momentum could also be deflected downward, lift could be further increased. A method of accomplishing this is through the use of tip flaps, or wing tips that deflect downward about a hinge line parallel to the flow direction.

 

Derivation of hypersonic wing tip shape using momentum principle

Application of the elementary momentum principle produces an aircraft configuration like:

 

Complete hypersonic vehicle shape derived using momentum principle

These so-called "flat-top" wings were tested in the wind tunnel at speeds ranging from Mach 3 to 6. It was found that although the maximum L/D decreased slightly as the tip flap deflection increased (due to decreasing lift-curve slope), L/D at low angles of attack increased for a given angle of attack, as shown below.

Although the authors of this study did not describe their findings as such, they had stumbled upon the basic design criteria that would come to be applied to waveriders. The increase in lift they attributed to momentum principles would soon come to be better understood as compression lift. Compression lift is that lift that is gained because of the increase in surface pressure realized on the underside of a hypersonic vehicle due to the close proximity of the thin shock layer. The concept of matching the wing leading edge to the shock formed off the vehicle forebody is the underlying principle behind the waverider concept.

Stability and Control:

Because of the fundamental differences between subsonic and hypersonic design criteria, we might expect fundamental differences in the stability and controllability of aircraft flying in these conditions. Indeed, this is the case and much research is being conducted to determine the optimum methods of trimming and controlling a hypersonic vehicle across its speed range.

Most hypersonic vehicles have been designed with rudders and combined elevator-ailerons (or elevons) as primary control surfaces. Another feature often used is the tip flap, discussed previously, which not only improves lift but also contributes to rudder effectiveness. A

combination of ailerons and tip flaps is sometimes referred to as tiperons. In general, hypersonic vehicles incorporating these control surfaces have been found to exhibit adequate stability characteristics at both cruise and takeoff/landing conditions. However, the overall vehicle design, especially the outer portion of the wing, plays a significant roll in stability and control behavior. For example, the vehicle shown below was wind-tunnel tested with both a straight wing and a cranked wing, or a wing in which the dihedral changes towards the tip.

 

Hypersonic waverider concept designed using conical-flow techniques and using both straight and cranked wing panels to compare stability and control characteristics

Results showed that the while the ailerons on the straight wing provided much greater roll control and less adverse yaw, the cranked wing provided improved lateral-directional stability. Both designs were found to be longitudinally unstable indicating that a hypersonic vehicle will likely need a fly-by-wire control system, a fuel control system, or careful vehicle packaging to maintain trim.

Propulsion Integration:

Subsonic aircraft typically feature engine mountings that are distinct components by themselves. For example, the engine pods of the Boeing 747 are easily identifiable. Even designs intended for low-supersonic speeds, like the Concorde, feature distinct engine pods. This will almost surely not be the case in any hypersonic air-breathing design. Since thin shock layers are so common in hypersonic flight, designers must be careful to prevent the shock wave from one component of the aircraft from adversely interfering with another component. In addition, a hypersonic vehicle will likely utilize some kind of ramjet or

scramjet engine. These engines are similar to turbojets except they dispense with the compressor and turbine stages. Instead, these engines rely on the motion of the vehicle itself to compress the incoming flow before combustion occurs.

Suffice it to say that shock interactions and ideal engine operation dictate that the propulsion system be highly integrated into the overall airframe design. An example, shown below, illustrates how a scramjet engine would likely be incorporated into a hypersonic vehicle.

 

Engine/airframe integration on a hypersonic vehicle.

The engine package is placed to take advantage of the shock generated by the vehicle forebody. The flow is compressed behind the shock increasing the pressure within the engine so that it will produce greater thrust as a result. In addition, the aft portion of the vehicle is designed to promote expansion of the exhaust so that it is actually an extension of the scramjet nozzle. Thus, the entire undersurface of the vehicle can be considered to be part of the propulsion system. The X-30 and Hyper-X (X-43 A)both utilize this engine integration concept.

Heat Transfer Issues:

Any hypersonic vehicle will spend the majority of its cruise flight in very high temperature flows, so the designer must pay close attention to temperatures on various portions of the vehicle to ensure that structures and materials will not fail at these conditions. Aerodynamic issues lead the designer to very sharp leading edge surfaces to minimize drag, but heat transfer issues require blunted shapes to spread heat flux over a larger area and to provide volume for the application of heat absorbing or ablative materials. This conflict can be alleviated to some degree using cryogenic cooling systems to pump low temperature fluids through the vehicle structure, especially along leading edges. The vehicle need not carry a special, additional fluid for this system since hypersonic vehicles will likely be fueled by liquid hydrogen or some other cryogenically cooled liquid. The technique of pumping fuel through the vehicle structure was successfully used on the B-70 Mach 3 bomber and was also proposed for the X-30 research vehicle. A hypersonic vehicle will also likely require some kind of heat-resistant flexible coating, like that of the SR-71 Blackbird and Space Shuttle.

On-going project:

NASA’s X-43 A vehicle: Hypersonic air vehicle powered by an air-breathing engine, it achieved a record-breaking speed of close to Mach 10 (12,000km/hr) in a test flight in November 2004. Apart from very high speed, the type of engine installed in the vehicle will reduce costs on space programs by increasing the payload capacity as there is no requirement for the heavy fuel and oxygen tanks installed in conventional rocket propulsion systems. The oxygen used for fuel ignition is taken from the atmosphere by the configuration of the air intake on the airframe.

A B-52 aircraft was used to carry the X-43A on its wing to an altitude over 40,000ft (12,000m) and then rocket carries the X-43A up to its test altitude of 100,000ft where the X-43A vehicle separates and flies under the power and control of its own built-in engine and pre-programmed control system. The X-43A is 4.74m in length and is expected to complete flight trials at a single speed of Mach 7 and at a single speed of Mach 10.

At hypersonic speeds parts (particularly the combustion chamber and the tip) of the air vehicle reach very high temperatures at which metals can melt or vaporize. The X-43A is constructed with a tile-based thermal protection system developed by Boeing, carbon-carbon composites and high temperature resistant metals. The materials have been selected to withstand the exceptionally high temperatures and forces on the airframe resulting from the very strong shock waves that are generated in hypersonic flight.

The X-43A is powered by scramjet engine, which uses gaseous hydrogen fuel. Scramjet engines provide design advantages in smaller size, simplicity and affordability of reusable vehicles. The scramjet engines are air-breathing engines and have significantly fewer moving parts than traditional turbojet engines. Also, scramjet engines do not require an oxidizer to be carried on board for the combustion process, as does a conventional rocket engine. Scramjets only operate at speeds in the range of Mach 4.5 or higher, so rockets or other jet engines are required to initially boost scramjet-powered aircraft to this base velocity. In the case of the X-43A, the aircraft was accelerated to high speed with a Pegasus rocket launched from a converted B-52 Stratofortress bomber.

In March 2004, the Pegasus fired successfully and released the test vehicle at an altitude of about 95,000 ft. After separation, the engine's air intake was opened, the engine ignited, and the aircraft then accelerated away from the rocket reaching mach 6.83. Fuel was flowing to the engine for eleven seconds, a time in which the aircraft travelled more than 24 km. After burnout, controllers were still able to manoeuvre the vehicle and manipulate the flight controls for several minutes as the aircraft was slowed down by wind resistance and took a long dive into the Pacific. Peak speed was at burnout of the Pegasus but the scramjet engine did accelerate the vehicle in climbing flight, after a small drop in speed following separation.

NASA flew a third version of the X-43A on November 16, 2004, achieving/maintaining a speed of Mach 9.68 at about 112,000 ft altitude and further testing the ability of the vehicle to withstand the heat loads involved.

One challenge hindering scramjet development is that ground test facilities for full-scale testing of air-breathing propulsion systems are quite limited for Mach numbers greater than about Mach 3 or 4. Achieving the correct combination of airflow velocity, mass flow rate, temperature, and sustained test conditions in test sections of the needed size for large scramjets is very difficult. Because of this, more extensive use of flight testing would be presumed alternative approach of flight testing itself. As the recent X-43A test program has shown, scramjet flight testing is very expensive and time consuming. The seven-year X-43A program, using three small expendable test vehicles, cost approximately $230 million. Two of the three tests conducted were successful in demonstrating scramjet operations for a total of about 30 seconds of test data.

Further programmes like X-51A has also been successful and may be the dream of flying at hypersonic speed will one day be realized, till then scientists are engaged in their research work, industrialists are pouring money in their faith while a common man has his fingers crossed!!