Hypersonic flight is a flight through the atmosphere at speeds ranging between Mach 5-10. This level of speed causes air molecules to break apart (dissociate) and/or collect an electrical charge (ionize);both processes create a lot of heat. There is not a particular speed in which this happens, and the term refers to the point at which they start to meaningfully affect the mechanics of flight. This level is generally accepted to be Mach 5 (3,836 Mph) in conditions of 20 degrees Celsius at sea level.
The first time hypersonic flight was achieved was during a test flight at Edwards Air Force Base in California during the 1960s. During the test, a B-52 carried the experimental X-15 aircraft, which was launched into flight at 45,000 feet. The X-15 reached speeds of 522 miles per hour and landed at Rogers Dry Lake a few miles away. The plane would go on to set the stage for Mach 4, Mach 5, and Mach 6 flight.
NASA was able to develop an experimental place, the X-43A, which set a new speed record for aircraft on November 16, 2004. The unmanned test flight saw the plane reach Mach 10, or about 6,600 miles per hour, and broke the previous flight speed record of Mach 7, set in March 2004, by the same flight in a previous test flight. This experimental plane is considered to be a space plane, because it (and hypersonic aircraft in general) flies at altitudes around 180,000 feet. Which means these vehicles are designed to withstand the temperatures above 50,000 feet.
As well, the X-43A aircraft was powered by a scramjet engine, which instead of using onboard oxygen to combust hydrogen fuel, the scramjet scoops oxygen as it travels through the atmosphere and reduces the need of the aircrafts need to carry oxygen onboard.
Since the retirement of the Concorde supersonic jet, there has been a push to develop commercial passenger aircraft capable of traveling at hypersonic speeds. Generally these planes, such as the Boeing concept Stratofly MR3, intended to be a 300 passenger airliner, would fly on the low end of hypersonic flight around Mach 5. The lower-end flight, in the case of commercial flights at Mach 4.5, offers easier physics for manufacturing aircrafts. These speeds would still reduce flight times from London to Sydney to four hours and Los Angeles to Tokyo to two hours.
In the push for the development of commercial hypersonic flight, Hermeus and NASA, through a Space Act Agreement (SAA) are in a partnership to develop high-speed aircraft. Through the agreement, NASA will evaluate technological maturity and exchange subject matter expertise between the two organizations in order to collaborate on the development of concepts of operation, high-Mach thrust performance, thermal management, integrated power generation, and cabin systems.
In developing any kind of hypersonic flight platform, the power plant or rocket engine is a necessary component. The NASA X-43A used a scramjet engine to reduce the overall weight of the aircraft and enable hypersonic flight,whereas the X-15 used a rocket propulsion system, which develops thrust generated by the mass flow through an engine and the exit velocity of the gas. These most common and popular types of propulsion systems in modern aircraft consist of two main categories of rocket engines, defined by fuel type: liquid rockets and solid rockets. A liquid rocket uses propellants composed of a liquid fuel and an oxidizer mixed in the combustion chamber of the engine. And in a solid rocket, the propellants are mixed and packed into a solid cylinder that burns when exposed to a source of heat.
Others organizations are working to create different, more efficient types of rocket engines capable of hypersonic flight. UK-based Reaction Engines is developing the Synergetic Air-Breathing Rocket Engines (SABRE), intended to allow flight up to Mach 5. Early tests of the engine and its pre-cooler, developed to combat the extreme temperatures developed by hypersonic flight, have suggested the propulsion system could reach Mach 3.3 speeds.
Researchers at the University of Central Florida and the US Naval Research Laboratory have demonstrated a concept of a fixed detonation engine, a standing oblique detonation wave engine. These are a category of engines expected to be capable of powering an aircraft up to Mach 17. The engines developed by these researchers are based on producing a continuous detonation fixed in space to keep the resulting shockwave stable and remain in the same position. This detonation is created by dividing the engine into three stages. The first stage uses a mixing chamber to ignite hydrogen and air. This hot high-pressured air flows into the next section, where it enters a converging-diverging nozzle where a stream of ultrahigh-purity hydrogen is added. The nozzle is designed to accelerate the mixture to speeds of around Mach 4.5 before it enters the final chamber that includes a 30-degree turning angle ramp. The engine is capable of producing an oblique shock stabilized on the ramp that lasts for the duration of active fueling, around three seconds, and longer than a normal detonation.
One of the more commonly referred to applications of hypersonic flight is in the military realm. Different militaries have worked to develop hypersonic flight for weapon systems and aircraft platforms for use in combat scenarios, and much of the technology of hypersonic flight has come from military experiments.
For weapon systems, the possibility of a hypersonic missile has been explored as a possible "carrier-killer," especially as aircraft carriers are a large platform in modern combat. The speed of a hypersonic missile presents the advantage of being capable of outflying defensive systems. However, the heat generated around a missile can create a sheath of plasma or gaseous matter, which can block signals from external resources, such as communications satellites, and can also blind internal targeting systems. This means the fine adjustments required to guide a missile at 35 mph are tough to achieve at Mach 5, but could be made impossible with the heat-generated sheath of plasma.
Since the early 2000s, the United States has focused efforts towards developing hypersonic cruise missiles and hypersonic glide vehicles, capable of being launched from a rocket before gliding to a target, as part of a global strike program. These weapons present as enabling responsive, long-range, strike options against distant, defended, or time-critical threat. And while interest in hypersonic weapon systems has been relatively constrained in the past, starting in FY2021, the DoD was given $3.2 billion for hypersonic research and has a FY2022 budget request for $3.8 billion.
Unlike ballistic missiles, hypersonic weapons do not have to follow a ballistic trajectory and can maneuver en route to a destination. Conventional hypersonic weapons can use only kinetic energy to destroy unhardened targets. And when given a warhead, hypersonic missiles can offer increased potential. As well, these weapons can challenge detection and defense due to speed, maneuverability, and low altitude of flight, and terrestrial-based radar cannot detect hypersonic weapons until late in the weapon's flight. The United States hypersonic weapon program is aimed towards conventionally armed weapons requiring greater accuracy and more technically challenging to fire and control. Whereas programs in China and Russia have armed hypersonic weapons that are expected to be nuclear-armed and therefore require less accuracy due to nuclear blast effects.
As of February 2021, Lockheed Martin is expected to integrate hypersonic weapons developed by the US Army. The army has worked to build a hypersonic weapon glide body and has produced launchers, trucks, trailers, and battle operation centers for a ground-launched hypersonic weapon battery. And while Lockheed Martin is the weapon system integrator, Dynetics was chosen to build the hypersonic glide body for the missile. Early testing of these weapon systems at the Pacific Missile Range Facility in Kauai, Hawaii, in March 2020, saw the hypersonic missile strike a target within six inches.
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