View source for Astronomy:Non-ballistic atmospheric entry

{{Short description|Glide and reentry mechanisms that use aerodynamic lift in the upper atmosphere}}
[[Image:Skip reentry trajectory.svg|thumb|500px|Phases of a skip reentry]]
'''Non-ballistic atmospheric entry''' is a class of [[Astronomy:Atmospheric entry|atmospheric entry]] trajectories that follow a non-ballistic trajectory by employing aerodynamic lift in the high upper atmosphere. It includes trajectories such as skip and glide.<ref name="urlFrom Sänger to Avangard – hypersonic weapons come of age, From Sänger to Avangard – hypersonic weapons come of age - Royal Aeronautical Society">{{cite web |title=From Sänger to Avangard – hypersonic weapons come of age, From Sänger to Avangard – hypersonic weapons come of age - Royal Aeronautical Society |url=https://www.aerosociety.com/news/from-saenger-to-avangard-hypersonic-weapons-come-of-age/}}</ref><ref>{{Cite web |date=6 June 2017 |title=Here's How Hypersonic Weapons Could Completely Change the Face of Warfare |url=http://www.thedrive.com/the-war-zone/11177/heres-how-hypersonic-weapons-could-completely-change-the-face-of-warfare}}</ref>  

'''Skip''' is a flight trajectory where the spacecraft goes in and out the atmosphere. '''Glide''' is a flight trajectory where the spacecraft stays in the atmosphere for a sustained flight period of time.<ref name="urlFrom Sänger to Avangard – hypersonic weapons come of age, From Sänger to Avangard – hypersonic weapons come of age - Royal Aeronautical Society" /> In most examples, a skip reentry roughly doubles the range of suborbital spaceplanes and reentry vehicles over the purely ballistic trajectory. In others, a series of ''skips'' allows the range to be further extended. 

Non-ballistic atmospheric entry was first seriously studied as a way to extend the range of [[Physics:Ballistic missile|ballistic missile]]s, but was not used operationally in this form as conventional missiles with extended range were introduced. The underlying aerodynamic concepts have been used to produce [[Engineering:Maneuverable reentry vehicle|maneuverable reentry vehicle]]s (MARV), to increase the accuracy of some missiles like the Pershing II. More recently, the concepts have been used to produce [[Engineering:Hypersonic glide vehicle|hypersonic glide vehicles]] (HGV) to avoid interception as in the case of the [[Engineering:Avangard (hypersonic glide vehicle)|Avangard]]. The range-extension is used as a way to allow flights at lower altitudes, helping avoid [[Engineering:Radar|radar]] detection for a longer time compared to a higher ballistic path.

The concept has also been used to extend the reentry time for vehicles returning to Earth from the Moon, which 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 also used what is essentially a skip re-entry , as did the Soviet Zond and Chinese [[Astronomy:Chang'e 5-T1|Chang'e 5-T1]]. 

==History==
===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 [[Physics:Angle of attack|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.{{sfn|Yengst|2010|p=29}}

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.{{sfn|Neufeld|1995|p=92}} He calculated that this would roughly double range of the [[Engineering:Aggregat (rocket family)#A4 (V-2 rocket)|A-4]] rockets from {{convert|275|km}} to about {{convert|550|km}}. Early development was considered under the A-9 name, although little work other than [[Physics:Wind tunnel|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.{{sfn|Neufeld|1995|p=93}}

The earliest known proposal for the boost-glide concept for truly long-range use dates to the 1941 ''[[Engineering:Silbervogel|Silbervogel]]'', a proposal by [[Biography:Eugen Sänger|Eugen Sänger]] for a rocket powered bomber able to attack {{wipe|New York City}} from bases in {{wipe|Germany}} then fly on for landing somewhere in the [[Earth:Pacific Ocean|Pacific Ocean]] held by the [[Place:Empire of Japan|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.<ref>{{cite book |last=Duffy |first=James |title=Target: America — Hitler's Plan to Attack the United States |publisher=Praeger |year=2004 |page=[https://archive.org/details/isbn_9780275966843/page/124 124] |isbn=0-275-96684-4 |url-access=registration |url=https://archive.org/details/isbn_9780275966843/page/124 }}</ref> It was later demonstrated that the heating load during the skips was much higher than initially calculated, and would have melted the spacecraft.<ref name=westman>{{cite book |last=Reuter |first=Claus |url=https://books.google.com/books?id=Sr6JtOoWghkC |title= The V2 and the German, Russian and American Rocket Program |publisher=German - Canadian Museum of Applied History |date= 2000 |isbn= 9781894643054 |page=99}}</ref>

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,{{sfn|Neufeld|1995|p=93}} or because the A-4 program had "national priority" by this time, and placing the development under the A-4 name guaranteed funding.{{sfn|Yengst|2010|pp=30-31}} A-4b used [[Engineering:Swept wing|swept wing]]s in order to extend the range of the V2 enough to allow attacks on UK cities in the Midlands or to reach [[Place:London|London]] from areas deeper within Germany.{{sfn|Yengst|2010|p=29}} The A-9 was originally similar, but later featured long [[Engineering:Delta wing|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 [[Engineering:Parachute|bailed out]].{{sfn|Yengst|2010|pp=30-31}}{{efn|Yengst's chronology of the A-series weapons differs considerably from most accounts. For instance, he suggests the A-9 and A-10 were two completely separate developments, as opposed to the upper and lower stages of a single ICBM design. He also states that the A-4b was the SLBM development, as opposed to the winged A-4.{{sfn|Yengst|2010|p=31}}}}

===Post-war development===
[[File:NASA Color Dyna Soar.jpg|thumb|To date, the [[Engineering:Boeing X-20 Dyna-Soar|X-20 Dyna Soar]] 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 [[Biography:Aleksei Isaev|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 {{wipe|Joseph Stalin}} who was intensely interested in the concept of an antipodal bomber. In 1946, he sent his son Vasily Stalin and scientist [[Biography:Grigori Tokaty|Grigori Tokaty]], who had also worked on winged rockets before the war, to visit Sänger and [[Biography:Irene Sänger-Bredt|Irene Bredt]] in Paris and attempt to convince them to join a new effort in the [[Place:Soviet Union|Soviet Union]]. Sänger and Bredt turned down the invitation.<ref name="West"/>

In November 1946, the Soviets formed the NII-1 [[Engineering:OKB|design bureau]] under [[Biography:Mstislav Keldysh|Mstislav Keldysh]] to develop their own version without Sänger and Bredt.<ref>{{cite web |first=Mark |last=Wade |title= Keldysh |url=http://www.astronautix.com/astros/keldysh.htm |archive-url=https://web.archive.org/web/20021025042642/http://astronautix.com/astros/keldysh.htm |url-status=dead |archive-date=October 25, 2002 |website= Encyclopedia Astronautica}}</ref> Their early work convinced them to convert from a rocket powered hypersonic skip-glide concept to a [[Engineering:Ramjet|ramjet]] powered supersonic [[Engineering:Cruise missile|cruise missile]], not unlike the [[Engineering:SM-64 Navaho|Navaho]] being developed in the United States during the same period. Development continued for a time as the [[Engineering:Keldysh bomber|Keldysh bomber]], but improvements in conventional ballistic missiles ultimately rendered the project unnecessary.<ref name="West">{{Cite web | last=Westman | first=Juhani | title=Global Bounce | website=PP.HTV.fi | year=2006 | url=http://www.pp.htv.fi/jwestman/space/sang-e.html | access-date=2008-01-17 | url-status=dead | archive-url=https://web.archive.org/web/20071009055651/http://www.pp.htv.fi/jwestman/space/sang-e.html | archive-date=2007-10-09 }}</ref>{{efn|Navaho met the same fate in 1958, when it was cancelled in favor of the [[Engineering:SM-65 Atlas|Atlas missile]].}}

In the United States, the skip-glide concept was advocated by many of the German scientists who moved there, primarily [[Biography:Walter Dornberger|Walter Dornberger]] and [[Biography:Krafft Arnold Ehricke|Krafft Ehricke]] at [[Company:Bell Aircraft|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 [[Engineering:Boeing X-20 Dyna-Soar|Boeing X-20 Dyna-Soar]].<ref>{{cite book |last=Godwin |first=Robert |title=Dyna-Soar: Hypersonic Strategic Weapons System |url=https://books.google.com/books?id=jazLM9jVTh4C |publisher=Apogee Books |year=2003 |isbn=1-896522-95-5 |page=42}}</ref> 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.<ref>{{cite journal |title=Rocket Liner Would Skirt Space to Speed Air Travel |journal=Popular Science |date=February 1955 |pages=160–161}}</ref><ref>{{cite tech report |first=Walter |last=Dornberger |title=The Rocket-Propelled Commercial Airliner |publisher=University of Minnesota Institute of Technology |year=1956}}</ref>

The introduction of successful [[Engineering:Intercontinental ballistic missile|intercontinental ballistic missile]]s (ICBMs) in the offensive role ended any interest in the skip-glide bomber concepts, as did the [[Astronomy:Reconnaissance satellite|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.<ref>{{cite journal |first=Amy Shira |last=Teitel |title=The Space Plane That Wasn't |journal=Popular Science |date=12 June 2015 |url=http://www.popsci.com/space-plane-wasnt-everything-you-never-needed-know-about-dyna-soar}}</ref>

===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 [[Engineering:Anti-ballistic missile|anti-ballistic missile]]s (ABMs) would not be able to track their movements rapidly enough for a successful interception. The first known example was the [[Engineering:Alpha Draco|Alpha Draco]] tests of 1959, followed by the Boost Glide Reentry Vehicle (BGRV) test series, [[Engineering:ASSET (spacecraft)|ASSET]]<ref>{{cite web |first=Mark |last=Wade |url=http://www.astronautix.com/craft/asset.htm |archive-url=https://web.archive.org/web/20020425122853/http://www.astronautix.com/craft/asset.htm |url-status=dead |archive-date=April 25, 2002 |title=ASSET |website=Encyclopedia Astronautica }}</ref> and [[Engineering:Martin X-23 PRIME|PRIME]].<ref>{{cite book |first1=Dennis |last1=Jenkins |first2=Tony |last2=Landis |first3=Jay |last3=Miller |url=https://web.archive.org/web/20200425225303/https://history.nasa.gov/monograph31.pdf |title=AMERICAN X-VEHICLES An Inventory—X-1 to X-50 |publisher=NASA |date=June 2003 |page=30}}</ref>

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 [[Earth:Inertial navigation system|inertial navigation system]] with a [[Company:Goodyear Aerospace|Goodyear Aerospace]] active [[Engineering:Radar|radar]].<ref>{{cite web |first=Mark |last=Wade |title= Pershing |url=http://www.astronautix.com/lvs/pershing.htm |archive-url=https://web.archive.org/web/20020305193550/http://www.astronautix.com/lvs/pershing.htm |url-status=dead |archive-date=March 5, 2002 |website=Encyclopedia Astronautica}}</ref> Similar concepts have been developed for most nuclear-armed nations' [[Engineering:Theatre ballistic missile|theatre ballistic missile]]s.

The [[Place:Soviet Union|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 {{wipe|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.<ref>{{cite web|url= https://ria.ru/radio_brief/20161028/1480246498.html|title=Эксперт об "изделии 4202": теперь США будут меньше бряцать оружием |website= Ria |date=28 October 2016 |access-date=16 September 2018}}</ref> The system was revealed publicly on 1 March 2018 as the [[Engineering:Avangard (hypersonic glide vehicle)|hypersonic glide vehicle (HGV) Avangard]] ({{lang-ru|Авангард}}; {{lang-en|Vanguard}}), which officially entered active service as an ICBM payload on 27 December 2019.<ref name="tass_combat_duty">{{cite news|url=https://tass.ru/armiya-i-opk/7436431|title=Первый ракетный полк "Авангарда" заступил на боевое дежурство |website=TASS|date=27 December 2019|access-date=27 December 2019|language=ru}}</ref> Vladimir Putin announced that Avangard had entered serial production, claiming that its maneuvrability makes it invulnerable to all current missile defences.<ref>{{cite web |url=http://tass.com/defense/992297 |title= Russia begins serial production of new cutting-edge glide vehicle|publisher= TASS}}</ref>

{{wipe|China}} has also developed a boost-glide warhead, the [[Engineering:DF-ZF|DF-ZF]] (known to US intelligence as "WU-14").<ref>{{cite web |title=Chinese Develop "Kill Weapon" to Destroy US Aircraft Carriers |url=http://www.usni.org/news-and-features/chinese-kill-weapon |website=US Naval Institute |date=21 March 2009}}</ref> 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 [[Astronomy:Kholod|Kholod]] and [[Engineering:GLL-8 (Gll-VK) Igla|GLL-8 Igla]] hypersonic test projects, and more recently the Yu-71 hypersonic glide vehicle which can be carried by RS-28 Sarmat.<ref>{{Cite web|url=http://www.express.co.uk/news/world/680167/Russia-tests-Yu74-hypersonic-nuclear-glider-capable-carrying-24-atomic-warheads|title = Russia testing hypersonic nuclear glider that holds 24 warheads and travels at 7,000mph|date = 15 June 2016}}</ref><ref>{{cite news |url=http://www.washingtontimes.com/news/2014/jan/13/hypersonic-arms-race-china-tests-high-speed-missil |title=Hypersonic arms race: China tests high-speed missile to beat U.S. defenses |newspaper= The Washington Free Beacon |first= Bill |last=Gertz |date=13 January 2014}}</ref>

Boost-glide became the topic of some interest as a possible solution to the US [[Engineering:Prompt Global Strike|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 {{wipe|United States}}. PGS does not define the mode of operation, and current studies include Advanced Hypersonic Weapon boost-glide [[Engineering:Warhead|warhead]], Falcon HTV-2 [[Physics:Hypersonic flight|hypersonic aircraft]], and submarine-launched missiles.<ref>{{cite tech report |first=Amy |last=Woolf |title=Conventional Prompt Global Strike and Long-Range Ballistic Missiles: Background and Issues |url=https://fas.org/sgp/crs/nuke/R41464.pdf |publisher=Congressional Research Service |date=6 February 2015}}</ref> [[Company:Lockheed Martin|Lockheed Martin]] is developing this concept as the hypersonic [[Engineering:AGM-183A ARRW|AGM-183A ARRW]].<ref>{{Cite web |url=https://www.janes.com/article/82420/lockheed-martin-secures-second-hypersonic-air-to-surface-weapon-contract |title=Lockheed Martin secures second hypersonic air-to-surface weapon contract &#124; Jane's 360 |access-date=2018-12-16 |archive-url=https://web.archive.org/web/20181216163741/https://www.janes.com/article/82420/lockheed-martin-secures-second-hypersonic-air-to-surface-weapon-contract |archive-date=2018-12-16 |url-status=dead }}</ref>

===Reentry vehicle use===
The technique was used by the Soviet [[Astronomy:Zond program|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. [[Astronomy:Zond 6|Zond 6]], [[Astronomy:Zond 7|Zond 7]] and [[Astronomy:Zond 8|Zond 8]] made successful skip entries, although [[Astronomy:Zond 5|Zond 5]] did not.<ref>{{Cite web|url=http://www.astronautix.com/l/lunarl1.html|archive-url=https://web.archive.org/web/20160915192930/http://www.astronautix.com/l/lunarl1.html|url-status=dead|archive-date=September 15, 2016|title = Lunar L1}}</ref><ref>The Soviet Space Race with Apollo, Asif Siddiqi, pages 655 and 656</ref> The [[Astronomy:Chang'e 5-T1|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. [[Organization:NASA|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.<ref>{{cite web |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790072398_1979072398.pdf |title= Apollo Skip Guidance |publisher=Bellcom |first=I.|last=Bogner| date=August 4, 1966}}</ref> The concept continues to appear on more modern vehicles like the Orion spacecraft, which made the first American skip entry in the [[Engineering:Artemis 1|Artemis 1]] mission, using onboard computers.<ref>{{cite thesis|last=Bairstow|first=Sarah Hendrickson|title=Reentry Guidance with Extended Range Capability for Low L/D Spacecraft|degree=M.Sc.|publisher=Massachusetts Institute of Technology|year=2006|hdl=1721.1/35295}}</ref><ref>{{cite conference|title=Skip Entry Trajectory Planning and Guidance|first1=Christopher W.|last1=Brunner|first2=Ping|last2=Lu|conference=AIAA Guidance, Navigation and Control Conference and Exhibit|date=20–23 August 2007|location=Hilton Head, South Carolina|doi=10.2514/6.2007-6777|url=https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=16811&context=rtd}}</ref><ref>{{cite conference|title=A Comparison of Two Orion Skip Entry Guidance Algorithms|first1=Jeremy R.|last1=Rea|first2=Zachary R.|last2=Putnam|conference=AIAA Guidance, Navigation and Control Conference and Exhibit|date=20–23 August 2007|location=Hilton Head, South Carolina|doi=10.2514/6.2007-6424}}</ref>

== Flight mechanics ==

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:<ref>{{cite book|last1=Mooij|first1=E|title=Re-entry Systems Lecture Notes|date=2014|publisher=Delft TU}}</ref>

<math display=block>\gamma_\mathrm{F} = -\gamma_\mathrm{E},</math> 

where <math>\gamma</math> is the flightpath 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 <math>V</math> before and after the entry can be derived to relate as follows:

<math display=block>\frac{V_\mathrm{F}}{V_\mathrm{E}} = \exp{\frac{2 \gamma_\mathrm{E}}{L/D}},</math>

where <math>L/D</math> is the [[Engineering:Lift-to-drag ratio|lift-to-drag ratio]] of the vehicle.

==See also==
*High Speed Strike Weapon (HSSW) (USA)	
*[[Engineering:Prompt Global Strike|Prompt Global Strike]] (PGS) (USA)
*[[Engineering:Alpha Draco|Alpha Draco]] (USA)
*ArcLight (missile) (USA)
*[[Engineering:DARPA Falcon Project|DARPA Falcon Project]] (USA)
*[[Engineering:Boeing X-51 Waverider|Boeing X-51 Waverider]] (USA)
*[[Engineering:North American X-15|North American X-15]] (USA)
*[[Engineering:Tupolev Tu-130|Tupolev Tu-130]] (Russia)
*[[Engineering:BrahMos-II|BrahMos-II]] (India / Russia)
*[[Engineering:Hypersonic Technology Demonstrator Vehicle|Hypersonic Technology Demonstrator Vehicle]] (India)
*[[Engineering:Multiple independently targetable reentry vehicle|Multiple independently targetable reentry vehicle]]

==Notes==
{{notelist}}

==References==

===Citations===
{{reflist|30em}}

===Bibliography===
*{{cite book 
| last =Neufeld | first =Michael 
| title =The Rocket and the Reich: Peenemünde and the Coming of the Ballistic Missile Era 
|publisher = Simon and Schuster 
|year = 1995
|url =https://books.google.com/books?id=p-CZ5iydXoUC 
|isbn = 9780029228951 
}}
*{{cite book 
|first= William |last= Yengst 
|title= Lightning Bolts: First Maneuvering Reentry Vehicles 
|url=https://books.google.com/books?id=lN0w6X0PG3QC 
|publisher= Tate Publishing 
|date=April 2010 
|isbn= 9781615665471 
}}

{{Missile types}}

[[Category:Atmospheric entry]]
[[Category:Apollo program]]
[[Category:Spaceplanes]]
[[Category:Space weapons]]
[[Category:Hypersonic aircraft]]

{{Sourceattribution|Non-ballistic atmospheric entry}}