Boost-glide

Silbervogel introduced the boost-glide concept as part of an "antipodal bomber" able to attack New York.

Boost-glide trajectories are a class of spacecraft guidance and reentry trajectories that extend the range of suborbital spaceplanes 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 term skip-glide.

The concept was introduced as a way to extend the range of ballistic missiles, but has not been used in this form. To date the system has been used primarily to produce maneuverable reentry vehicles, or MARV, but it has also been considered as a way to reach long range targets while below radar coverage.

Early concepts

The conceptual basis for the boost-glide concept was first noticed by German artillery officers, who found that their Peenemünder Pfeilgeschosse arrow shells travelled 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 resulted in the shell having an angle of attack that produced aerodynamic lift off the fins at the rear of the shells. At the time this was considered highly undesirable because it made calculating the trajectory very difficult, but its possible application for extending the range of ballistics was not lost on the observers.^[1]

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.^[2] 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 for the next few years. Low-level research continued until 1942 when it was cancelled.^[3]

The earliest known use of the boost-glide concept for truly long-range use dates to the 1941 Silbervogel concept 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".^[4] It was later demonstrated that the heating load during the skips was much higher than initially calculated, and would have melted the spacecraft.^[5]

Later, in 1943, the A-9 work was dusted off again, but 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,^[3] or because the A-4 program had "national priority" by this time, and placing the development under the A-4 name guaranteed funding.^[6] 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.^[1] The A-9 was originally similar, but later adapted as a manned 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.^{[6][lower-alpha 1]}

Post-war development

To date, the X-20 Dyna Soar is the project that has come closest to actually building a manned boost-glide vehicle. This illustration shows the Dyna Soar during reentry.

In the immediate post-war era, Soviet engineers heard of the Silbervogel when rocket engineer Alexey Isayev found a copy of their updated August 1944 report on the concept. The paper was translated to Russian, where 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. This did not come to pass, so in November 1946 the Soviets formed their own group under Mstislav Vsevolodovich Keldysh to develop their own version. Their early work convinced them to convert it to ramjet power, and development continied for a time as the Keldysh bomber.^[8]

In the United States the concept was advocated by the many German scientists who moved there, primarily Walter Dornberger and Kraft Ehricke at Bell Aircraft. In 1952 Bell proposed a concept known as Bomi, and 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.^[9] The two also collaborated on a 1955 Popular Science article pitching the idea for airliner use.^[10][11]

The introduction of successful Intercontinental Ballistic Missiles (ICBMs) in the offensive role ended any interest in the 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. In the late 1960s a number of boost-glide layouts were explored as possible maneuverable reentry vehicles for ICBMs, leading to Alpha Draco tests, the Boost Glide Reentry Vehicle (BGRV) test series, ASSET^[12] and PRIME.^[13]

Production use

Some of this work has seen use in missile systems, notably the Pershing II's MARV reentry vehicle which used aerodynamics to control the final decent under the direction of a radar-based active terminal homing guidance system. In these cases, which have been adapted by most nuclear armed nation's theatre ballistic missiles, the purpose is to improve accuracy, not extend range. However, there has been ongoing 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, keeping the RV 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.^[14]

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 boost-glide, hypersonic aircraft, and submarine launched missiles.^[15] According to reports from The Pentagon, the Chinese have started development of a similar weapon known as WU-14.^[16]

Notes

References

Citations

Bibliography

• Neufeld, Michael (1995). The Rocket and the Reich: Peenemünde and the Coming of the Ballistic Missile Era. Simon and Schuster. ISBN 9780029228951. 
• Yengst, William (April 2010). Lightning Bolts: First Maneuvering Reentry Vehicles. Tate Publishing. ISBN 9781615665471.