describing an intercontinental spaceplane trajectory, 1940s.
Early concepts The conceptual basis 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 to about . 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 proposal for the boost-glide concept for truly long-range use dates to the 1941
Silbervogel, a proposal by
Eugen Sänger for a rocket powered
bomber able to attack
New York City from bases in
Germany 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
ogival 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.
Post-war development is the project that has come closest to actually building a crewed boost-glide vehicle. This illustration shows the Dyna Soar during reentry. 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. 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.
Missile use 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 primary goal was to have the RV change its path during reentry so that
anti-ballistic missiles (ABMs) would not be able to track their movements rapidly enough for a successful interception. 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 nations'
theatre ballistic missiles. The
Soviet Union had also invested some effort in the development of MARV to avoid US ABMs, but the closure of the US defenses in the 1970s meant there was no reason to continue this program. Things changed in the 2000s with the introduction of the US's
Ground-Based Midcourse Defense, which led
Russia to reanimate this work. The vehicle, referred to as
Object 4202 in the Soviet era, was reported in October 2016 to have had a successful test. The system was revealed publicly on 1 March 2018 as the
hypersonic glide vehicle (HGV) Avangard (; ), which officially entered active service as an ICBM payload on 27 December 2019.
Vladimir Putin announced that Avangard had entered serial production, claiming that its maneuverability makes it invulnerable to all current missile defences.
China has also developed a boost-glide warhead, the
DF-ZF (known to US intelligence as "WU-14"). In contrast to the US and Russian MARV designs, the DF-ZF's primary goal is to use boost-glide to extend range while flying at lower altitudes than would be used to reach the same target using a purely ballistic path. This is intended to keep it out of the sight of the
US Navy's
Aegis Combat System radars as long as possible, and thereby decrease the time that system has to respond to an attack. DF-ZF was officially unveiled on 1 October 2019. 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. Boost-glide became the topic of some interest as a possible solution to the US
Prompt Global Strike (PGS) requirement, 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.
Lockheed Martin is developing this concept as the hypersonic
AGM-183A ARRW.
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, which made the first American skip entry in the
Artemis 1 mission, using onboard computers. == Flight mechanics ==