Boost-glide trajectories
Boost-glide trajectories describe the flight paths of advanced hypersonic vehicles and weapons that are propelled into the upper atmosphere and then glide to their targets. This concept, which gained initial attention before World War II, leverages the unique characteristics of hypersonic speeds, defined as exceeding Mach 5, which allows these vehicles to travel significantly faster than traditional aircraft and evade detection effectively. Typically launched by a rocket, these vehicles can maneuver during flight and take advantage of the lift generated by the upper atmosphere, enabling them to cover vast distances, potentially reaching targets from thousands of miles away.
Historically, various attempts to develop boost-glide technology have taken place, including projects during the Cold War and more recent initiatives by the U.S. military in response to advancements by other nations, such as Russia and China. Notable examples include Russia's Avangard and the U.S. Army's Long-Range Hypersonic Weapon, both of which utilize boost-glide trajectories for enhanced strike capabilities. The technological challenges and complexities of maintaining stability and managing heat during flight remain key considerations in this field. Overall, boost-glide trajectories represent a significant evolution in military aerospace technology, blending speed, stealth, and precision in modern defense strategies.
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Boost-glide trajectories
Boost-glide trajectories refer to the flight paths of proposed hypersonic aircraft and weapons that are launched into the upper atmosphere by missile and glide to their destination. The concept was first explored prior to World War II (1939–1945), but the technological limitations of the time made such developments impossible. Hypersonic weapons and aircraft are incredibly fast and can typically avoid detection more easily as they travel to their targets. The most common modern boost-glide design involves a craft that is launched into the upper atmosphere by a rocket or missile and glides to its destination. By 2024, the US military was actively researching a possible boost-glide weapon system in response to similar systems being developed by foreign powers.
Brief History
During World War II, the Germans launched more than 1,300 powerful V-2 rockets at targets in Great Britain, France, and Belgium. The V-2 was the world’s first long-range ballistic missile, a weapon launched into space that falls to Earth under the influence of gravity along a curved path to its destination. The rockets were effective, but were only capable of reaching targets within a range of about 120 miles (193 km).
While working for the German military, Austrian scientist Eugen Sänger attempted to develop a long-range bomber that could reach targets up to 14,900 miles (24,000 km) away. The silbervogel, German for “silver bird,” was designed to reach the United States from a launch point in Europe. The craft would be launched by rocket sled and reach a sub-orbital altitude of 90 miles (144 km). It would then “skip” on the upper atmosphere to extend its range. The silbervogel had the capability of reaching a top speed of Mach 17—seventeen times the speed of sound. The craft could carry a bomb ordinance of 8,800 pounds (4,000 kg) and was considered unstoppable by military countermeasures.
However, Sänger’s idea never reached beyond the theoretical stage, because calculations showed the heat generated by friction during the skips off the atmosphere would destroy the craft. During the 1950s, at the height of the Cold War, American scientists tried to use Sänger’s concept to develop a gliding spaceplane. The project was called the Dynasoar, short for Dynamic Soarer, and was a single-pilot vehicle designed as the first reusable spacecraft. The US government spent more than $600 million on the Dynasoar before the project was cancelled in 1963. In 2003, the United States attempted to design a series of boost-glide weapons as part of the Conventional Prompt Global Strike (CPGS) program. The effort developed two boost-glide vehicles, one with a range of 10,500 miles (17,000 km) and the other with a range of 5,000 miles (8,000 km). Both vehicles were tested in the 2010s, but neither successfully completed the tests.
Overview
The hypersonic speed threshold is defined as speeds that exceed Mach 5—five times the speed of sound. Mach 5 is roughly equal to about one mile per second, or 3,836 miles per hour (6,173 km/hour). For comparison, a hypersonic aircraft could fly across the United States in fifty minutes and around the world in about seven and a half hours. At such high speeds, the air molecules around the craft can begin to break apart or pick up an electrical charge.
A glider is an aircraft that can fly without using an engine. As they fly, gliders experience two primary forces: lift force and drag force. While lift force keeps the plane in the air, drag force gradually decreases the glider’s speed and altitude. The distance a glider can travel depends on the ratio between these two forces. Typically, gliders have a range of a few hundred miles. However, at hypersonic speeds, a vehicle would be able to glide for much greater distances.
In a boost-glide trajectory, a vehicle is launched into the upper atmosphere aboard a rocket or by a similarly powered engine. The vehicle then separates from its booster engine and uses the lift force of the upper atmosphere to glide at hypersonic speeds. In some designs, the vehicles are powered to their destinations by air-breathing engines known as scramjets. Some vehicles can also gain additional range by skipping off the top of the atmosphere. Boost-glide vehicles can be maneuvered while in flight and are difficult to detect by ground-based radar. These factors give boost-glide vehicles and weapons an advantage over ballistic missiles, which cannot make in-flight adjustments and can be detected by radar long before they reach their target. Hypersonic vehicles move too fast to be processed and successfully tracked by modern radar systems and the heat signature from an air-breathing engine would be difficult to detect.
In 2018, Russia announced that it had successfully developed a hypersonic glide vehicle called the Avangard. The Avangard is designed to be launched by an intercontinental ballistic missile to a gliding altitude, where it can reach speeds from Mach 20 to 27. The vehicle is believed to be able to extend its range by using a skipping maneuver off the atmosphere. China has also been developing hypersonic glide vehicles, which are believed to be able to reach speeds of about Mach 10. Unlike Russia’s Avangard, the Chinese vehicles are not believed to be equipped to carry nuclear warheads.
In response to the Russian and Chinese efforts, the US Department of Defense began developing its own boost-glide systems. The Army, Navy, Air Force, and the Defense Advanced Research Projects Agency (DARPA) were each working on projects to create hypersonic weapons systems. Some projects focus on hypersonic glide vehicles and others on a hypersonic cruise missile that would use scramjet technology. For example, DARPA and the Air Force are working on the Tactical Boost Glide vehicle that could reach speeds of more than Mach 7. The Air Force’s Air-Launched Rapid Response Weapon (ARRW) is capable of reaching speeds up to Mach 20 and has a range of about 575 miles (925 km). In 2020, the Pentagon requested $3.2 billion in the 2021 budget to fund the various projects. By 2024, the US military had debuted its Long-Range Hypersonic Weapon (LRHW), which utilizes boost-glide trajectories to precisely strike targets at extreme ranges.
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