Boost-glide trajectories are a class of spacecraft guidance and reentry trajectories that extend the range of suborbital spaceplanes and reentry vehicles by employing aerodynamic lift in the high upper atmosphere. In most examples, boost-glide roughly doubles the range over the purely ballistic trajectory. In others, a series of skips allows range to be further extended, and leads to the alternate terms skip-glide and skip reentry.
The concept was first seriously studied as a way to extend the range of ballistic missiles, but has not been used operationally in this form. However, after many years since the concept was first explored, the Russian Armed Forces are in 2018-19 testing the Avangard glide vehicle. The underlying aerodynamic concepts have been used to produce maneuverable reentry vehicles, or MARV, to increase the accuracy of some missiles. More recently the traditional form with an extended gliding phase has been considered as a way to reach targets while flying below their radar coverage.
The concept has also been used to extend the reentry time for vehicles returning to Earth from the Moon, who would otherwise have to shed a large amount of velocity in a short time and thereby suffer very high heating rates. The Apollo Command Module used what is essentially a one-skip reentry (or partial skip), as did the Soviet Zond and Chinese Chang'e 5-T1. More complex multi-skip reentry is proposed for newer vehicles like the Orion spacecraft.
The conceptual basis for the boost-glide concept was first noticed by German artillery officers, who found that their Peenemünder Pfeilgeschosse arrow shells traveled much further when fired from higher altitudes. This was not entirely unexpected due to geometry and thinner air, but when these factors were accounted for, they still could not explain the much greater ranges being seen. Investigations at Peenemünde led them to discover that the longer trajectories in the thinner high-altitude air resulted in the shell having an angle of attack that produced aerodynamic lift at supersonic speeds. At the time this was considered highly undesirable because it made the trajectory very difficult to calculate, but its possible application for extending range was not lost on the observers.
In June 1939, Kurt Patt of Klaus Riedel's design office at Peenemünde proposed wings for converting rocket speed and altitude into aerodynamic lift and range. He calculated that this would roughly double range of the A-4 rockets from 275 kilometres (171 mi) to about 550 kilometres (340 mi). Early development was considered under the A-9 name, although little work other than wind tunnel studies at the Zeppelin-Staaken company would be carried out during the next few years. Low-level research continued until 1942 when it was cancelled.
The earliest known use of the boost-glide concept for truly long-range use dates to the 1941 Silbervogel proposal by Eugen Sänger for a rocket powered bomber able to attack New York City from bases in Germany and then fly on for landing somewhere in the Pacific Ocean held by the Empire of Japan. The idea would be to use the vehicle's wings to generate lift and pull up into a new ballistic trajectory, exiting the atmosphere again and giving the vehicle time to cool off between the skips. It was later demonstrated that the heating load during the skips was much higher than initially calculated, and would have melted the spacecraft.
In 1943, the A-9 work was dusted off again, this time under the name A-4b. It has been suggested this was either because it was now based on an otherwise unmodified A-4, or because the A-4 program had "national priority" by this time, and placing the development under the A-4 name guaranteed funding. A-4b used swept wings in order to extend the range of the V2 enough to allow attacks on UK cities in The Midlands or to reach London from areas deeper within Germany. The A-9 was originally similar, but later featured long ogive delta shaped wings instead of the more conventional swept ones. This design was adapted as a crewed upper stage for the A-9/A-10 intercontinental missile, which would glide from a point over the Atlantic with just enough range to bomb New York before the pilot bailed out.
In the immediate post-war era, Soviet rocket engineer Aleksei Isaev found a copy of an updated August 1944 report on the Silbervogel concept. He had the paper translated to Russian, and it eventually came to the attention of Joseph Stalin who was intensely interested in the concept of an antipodal bomber. In 1946, he sent his son Vasily Stalin and scientist Grigori Tokaty, who had also worked on winged rockets before the war, to visit Sänger and Irene Bredt in Paris and attempt to convince them to join a new effort in the Soviet Union. Sänger and Bredt turned down the invitation.
In November 1946, the Soviets formed the NII-1 design bureau under Mstislav Keldysh to develop their own version without Sänger and Bredt. Their early work convinced them to convert from a rocket powered hypersonic skip-glide concept to a ramjet powered supersonic cruise missile, not unlike the Navaho being developed in the United States during the same period. Development continued for a time as the Keldysh bomber, but improvements in conventional ballistic missiles ultimately rendered the project unnecessary.
In the United States, the skip-glide concept was advocated by many of the German scientists who moved there, primarily Walter Dornberger and Krafft Ehricke at Bell Aircraft. In 1952, Bell proposed a bomber concept that was essentially a vertical launch version of Silbervogel known as Bomi. This led to a number of follow-on concepts during the 1950s, including Robo, Hywards, Brass Bell, and ultimately the Boeing X-20 Dyna-Soar. Earlier designs were generally bombers, while later models were aimed at reconnaissance or other roles. Dornberger and Ehricke also collaborated on a 1955 Popular Science article pitching the idea for airliner use.
The introduction of successful intercontinental ballistic missiles (ICBMs) in the offensive role ended any interest in the skip-glide bomber concepts, as did the reconnaissance satellite for the spyplane roles. The X-20 space fighter saw continued interest through the 1960s, but was ultimately the victim of budget cuts; after another review in March 1963, Robert McNamara canceled the program in December, noting that after $400 million had been spent they still had no mission for it to fulfill.
Through the 1960s, the skip-glide concept saw interest not as a way to extend range, which was no longer a concern with modern missiles, but as the basis for maneuverable reentry vehicles for ICBMs. The first known example was the Alpha Draco tests of 1959, followed by the Boost Glide Reentry Vehicle (BGRV) test series, ASSET and PRIME.
This research was eventually put to use in the Pershing II's MARV reentry vehicle. In this case, there is no extended gliding phase; the warhead uses lift only for short periods to adjust its trajectory. This is used late in the reentry process, combining data from a Singer Kearfott inertial navigation system with a Goodyear Aerospace active radar. Similar concepts have been developed for most nuclear-armed nation's theatre ballistic missiles.
In contrast to these maneuvering warhead concepts, there has been growing interest in the traditional boost-glide concept not to extend range per se, but to allow it to reach a given range while flying at a much lower altitude. The goal, in this case, is to keep the reentry vehicle below radar coverage until it enters the terminal phase. Such a system is assumed to be used on the Chinese DF-21D anti-ship ballistic missile, which is also believed to maneuver during the terminal phase to make interception more difficult. The later DF-26, a development of the DP-21, may be armed with the WU-14, a hypersonic glide vehicle that has been successfully tested six times by the Chinese. Similar efforts by Russia led to the Kholod and GLL-8 Igla hypersonic test projects, and more recently the Yu-71 hypersonic glide vehicle which can be carried by RS-28 Sarmat.
In the early 21st century, boost-glide became the topic of some interest as a possible solution to the Prompt Global Strike (PGS), which seeks a weapon that can hit a target anywhere on the Earth within one hour of launch from the United States. PGS does not define the mode of operation, and current studies include Advanced Hypersonic Weapon boost-glide warhead, Falcon HTV-2 hypersonic aircraft, and submarine-launched missiles. The WU-14 would be similar to these weapons. Hypersonic Glide Vehicles could be used for delivering quick nuclear decapitating strikes.
Lockheed Martin is developing the hypersonic AGM-183A Air-Launched Rapid Response Weapon ARRW.
In March 2018, Russia unveiled hypersonic glide vehicle Avangard.
While flying below the operational envelope of Exoatmospheric Kill Vehicles, the tradeoff with HGVs in comparison to conventional MIRVs are many-fold, including no needle in a haystack protection from missile decoys and both less speed and altitude as they near the target, all of these characteristics result in HGVs having poorer survivability odds when placed against lower-tier interceptors. Some examples of which include the high thrust mach-10 Sprint missile, its US derivatives and the still operational mach-17 Russian 53T6, ABM-3 Gazelle. Moreover, the possible re-emergence of nuclear or hit-to-kill stratosphere reaching guns, guided and triggered by forward operating flight-path sensors (such as the 2016 Hypervelocity Projectile (HVP) in development for the M109 howitzer) also will decrease HGV survivability odds.
Other more speculative counter-hypersonic vehicle measures may involve laser or rail gun technologies.
Reentry vehicle use
The technique was used by the Soviet Zond series of circumlunar spacecraft, which used one skip before landing. In this case a true skip was required in order to allow the spacecraft to reach the higher-latitude landing areas. Zond 6, Zond 7 and Zond 8 made successful skip entries, although Zond 5 did not. The Chang'e 5-T1, which flew mission profiles similar to Zond, also used this technique.
The Apollo Command Module used a skip-like concept to lower the heating loads on the vehicle by extending the re-entry time, but the spacecraft did not leave the atmosphere again and there has been considerable debate whether this makes it a true skip profile. NASA referred to it simply as "lifting entry". A true multi-skip profile was considered as part of the Apollo Skip Guidance concept, but this was not used on any crewed flights. The concept continues to appear on more modern vehicles like the Orion spacecraft, using onboard computers.
Using simplified equations of motion and assuming that during the atmospheric flight both drag and lift forces will be much larger than the gravity force acting on the vehicle, the following analytical relations for a skip reentry flight can be derived:
Where gamma is the flight path angle relative to the local horizontal, the subscript E indicates the conditions at the start of the entry and the subscript F indicates the conditions at the end of the entry flight.
The velocity V before and after the entry can be derived to relate as follows:
Where L/D equals the lift to drag ratio of the vehicle.
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