Boost-glide



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 operationally in this form. The 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.

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. 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 km to about 550 km. 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.

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 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.

Post-war development
In the immediate post-war era, Soviet engineers heard of the Silbervogel when rocket engineer Alexey Isayev found a copy of an 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.

Sänger and Bredt turned down the invitation, so in November 1946 the Soviets formed the NII-1 design bureau under Mstislav Keldysh to develop their own version. 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 unneeded.

In the United States the concept was advocated by the many 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. The two 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 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 and PRIME.

Production use
The hypersonic lift concept originally explored for boost-glide has seen use in missile systems, notably 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.

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 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.

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. According to reports from The Pentagon, the Chinese have started development of a similar weapon known as WU-14 that now is under test flights, and Russia having Kholod and Igla hypersonic test projects earlier but now carring the test flights of similar Yu-71 hypersonic warhead. Also, Europe and Germany with IXV and SHEFEX has similar test projects in test flights while India and Australia with Hypersonic Technology Demonstrator Vehicle and AUSROCK are under early stages properly