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== Technical details == [[File:Aggregat4-Schnitt-engl.jpg|left|upright=1.35|thumb|Layout of a V2 rocket.]] The A4 used a 75% [[ethanol]]/25% water mixture ([[List of stoffs|''B-Stoff'']]) for fuel and [[liquid oxygen]] (LOX) ([[List of stoffs|''A-Stoff'']]) for [[oxidizer]].<ref>{{cite web|last=Dungan|first=T|title=The A4-V2 Rocket Site|url=http://www.v2rocket.com/start/makeup/design.html|access-date=2 June 2011|url-status=live|archive-url=https://web.archive.org/web/20110531211234/http://www.v2rocket.com/start/makeup/design.html|archive-date=31 May 2011}}</ref> The water reduced the flame temperature, acted as a coolant by turning to steam and augmented the thrust, tended to produce a smoother burn, and reduced [[thermal stress]].<ref name="sutton">{{cite book |last1=Sutton |first1=George |title=History of Liquid Propellant Rocket Engines |date=2006 |publisher=American Institute of Aeronautics and Astronautics |location=Reston |isbn=978-1-56347-649-5 |pages=740–753}}</ref> Rudolf Hermann's [[supersonic wind tunnel]] was used to measure the A4's aerodynamic characteristics and center of pressure, using a model of the A4 within a 40 square centimeter chamber. Measurements were made using a [[Mach number|Mach]] 1.86 blowdown nozzle on 8 August 1940. Tests at Mach numbers 1.56 and 2.5 were made after 24 September 1940.<ref name=hunley/>{{rp|76–78}} At launch the A4 propelled itself for up to 65 seconds on its own power, and a program motor held the inclination at the specified angle until engine shutdown, after which the rocket continued on a ballistic free-fall trajectory. The rocket reached a height of {{cvt|80|km}} or 264,000 ft after shutting off the engine.<ref>[[History (U.S. TV channel)|The History Channel]] V2 Factory: Nordhausen 070723</ref> The fuel and oxidizer pumps were driven by a [[steam turbine]], fueled by decomposition of concentrated [[hydrogen peroxide]] ([[T-Stoff]]) facilitated by a [[sodium permanganate]] ([[Z-Stoff]]) [[catalyst]]. Both the alcohol and oxygen tanks were an aluminum-magnesium alloy.<ref name=Kennedy>{{cite book |last=Kennedy |first=Gregory P. |title=Vengeance Weapon 2: The V-2 Guided Missile |year=1983 |publisher=Smithsonian Institution Press |location=Washington, DC |pages=27, 74}}</ref> The [[turbopump]], rotating at 4,000 [[Revolutions per minute|rpm]], forced the fuel mixture and oxygen into the combustion chamber at 125 liters (33 US gallons) per second, where they were ignited by a spinning electrical igniter. The engine produced 8 tons of thrust during the preliminary stage whilst the fuel was gravity-fed, before increasing to 25 tons as the turbopump pressurised the fuel, lifting the 13.5 ton rocket. Combustion gases exited the chamber at {{convert|5100|°F|°C|order=flip}}, and a speed of {{convert|2000|m|ft|abbr=on}} per second. The oxygen to fuel mixture was 1.0:0.85 at 25 tons of thrust; as [[ambient pressure]] decreased with flight altitude, thrust increased to 29 tons.<ref name=walter>{{cite book |last1=Dornberger |first1=Walter |title=V-2 |date=1954 |publisher=The Viking Press, Inc. |location=New York |pages=17–18, 120, 122–123, 132}}</ref><ref name="Zaloga2003p19">Zaloga 2003 p19</ref><ref name=redstone /> The turbopump assembly contained two centrifugal pumps, one for the fuel mixture, and one for the oxygen. The turbine was connected directly by a shaft to the alcohol pump and through a flexible joint and shaft to the oxygen pump.<ref>16:04 https://www.youtube.com/watch?v=EgiMu8A3pi0&t=2036s</ref> The turbopump delivered {{convert|55|kg|lb|abbr=on}} of alcohol and {{convert|68|kg|lb|abbr=on}} of liquid oxygen per second to a combustion chamber at {{convert|1.5|MPa|psi|0|abbr=on|lk=on}}.<ref name=hunley/> Dr. Thiel's 25 ton rocket motor design relied on pump feeding, as opposed to earlier [[Pressure-fed engine|pressure-fed]] designs. The motor used centrifugal injection, and used both [[regenerative cooling (rocket)|regenerative cooling]] and film cooling. Film cooling admitted alcohol into the combustion chamber and exhaust nozzle under slight pressure through four rings of small perforations. The mushroom-shaped injection head was removed from the combustion chamber to a mixing chamber, the combustion chamber was made more spherical while being shortened from 6 to 1-foot in length, and the connection to the nozzle was made cone shaped. The resultant 1.5 ton chamber operated at a combustion pressure of {{convert|1.52|MPa|psi|0|abbr=on}}. Thiel's 1.5 ton chamber was then scaled up to a 4.5 ton motor by arranging three injection heads above the combustion chamber. By 1939, eighteen injection heads in two concentric circles at the head of the {{convert|3|mm|in|2|abbr=on}} thick sheet-steel chamber, were used to make the 25 ton motor.<ref name=walter />{{rp|52–55}}<ref name="hunley">{{cite book |last1=Hunley |first1=J.D. |title=Preludes to U.S. Space-Launch Vehicle Technology: Goddard Rockets to Minuteman III |date=2008 |publisher=University Press of Florida |location=Gainesville |isbn=978-0-8130-3177-4 |pages=67–76}}</ref> The warhead was a source of trouble. The explosive used was [[amatol|amatol 60/40]] detonated by an electric [[contact fuze]]. Amatol had the advantage of stability, and the warhead was protected by a thick layer of [[glass wool]], but even so it could still explode during the re-entry phase. The warhead weighed {{convert|975|kg|lb}} and contained {{convert|910|kg|lb}} of explosive. The warhead's explosive percentage by weight was 93%, a very high portion compared to other types of munitions. A protective layer of glass wool was also used for the fuel tanks to prevent the A-4 from forming ice, a problem which plagued other early ballistic missiles such as the [[balloon tank]]-design [[SM-65 Atlas]] which entered US service in 1959. The tanks held {{convert|4173|kg|lb}} of ethyl alcohol and {{convert|5553|kg|lb}} of oxygen.<ref>''War machine encyclopedia'', Limited publishing, London 1983 pp. 1690–92 {{ISBN?}}</ref> [[File:Antwerp V-2.jpg|thumb|Captured V-2 on public display in Antwerp, 1945. Exhaust vanes and external rudders in tail section shown.]] The V-2 was guided by four external rudders on the tail fins, and four internal [[graphite]] vanes in the jet stream at the exit of the motor. These 8 control surfaces were controlled by [[Helmut Hölzer]]'s [[analog computer]], the {{Lang|de|[[Mischgerät (V-2 guidance computer)|Mischgerät]]}}, via electrical-hydraulic [[servomotor]]s, based on electrical signals from the gyros. The Siemens ''Vertikant'' LEV-3 guidance system consisted of two free [[gyroscope]]s (a horizontal for pitch and a vertical with two degrees of freedom for yaw and roll) for lateral stabilization, coupled with a [[PIGA accelerometer]], or the Walter Wolman radio control system, to control engine cutoff at a specified velocity. Other gyroscopic systems used in the A-4 included Kreiselgeräte's SG-66 and SG-70. The V-2 was launched from a pre-surveyed location, so the distance and [[azimuth]] to the target were known. Fin 1 of the missile was aligned to the target azimuth.<ref>Stakem, Patrick H. ''The History of Spacecraft Computers from the V-2 to the Space Station'', 2010, PRB Publishing, {{ASIN|B004L626U6}}</ref><ref name=hunley/>{{rp|81–82}} Some later V-2s used "[[Beam riding|guide beams]]", radio signals transmitted from the ground, as an added input to the Mischgerät analog computer to keep the missile on course in azimuth.<ref>[http://www.cdvandt.org/Hoelzer%20V4.pdf Helmut Hoelzer's Fully Electronic Analog Computer used in the German V2 (A4) rockets.] {{webarchive|url=https://web.archive.org/web/20160428214820/http://cdvandt.org/Hoelzer%20V4.pdf |date=28 April 2016 }} (PDF, English, German)</ref> The flying distance was controlled by the timing of the engine cut-off, ''[[Brennschluss]]'', ground-controlled by a [[Doppler effect|Doppler]] system or by different types of on-board integrating [[accelerometer]]s. Thus, range was a function of engine burn time, which ended when a specific velocity was achieved.<ref name="Zaloga2003p19" /><ref name=walter />{{rp|203–204}}<ref name=redstone>{{cite book |title=A-4/V-2 Rocket, Instruction Manual (in English) |date=2012 |publisher=Periscope Film LLC |isbn=978-1-937684-76-1 |pages=8–9, 135, 144}}</ref> Just before engine cutoff, thrust was reduced to eight tons, in an effort to avoid any [[water hammer]] problems a rapid cutoff could cause.<ref name=sutton/> Dr. Friedrich Kirchstein of [[Siemens]] of Berlin developed the V-2 [[radio control]] for motor cutoff ({{langx|de|Brennschluss}}).<ref name=Irving />{{Rp|28, 124}} For velocity measurement, Professor Wolman of Dresden created an alternative of his Doppler<ref name=Pocock />{{Rp|18}} tracking system in 1940–41, which used a ground signal transponded by the A-4 to measure the velocity of the missile.<ref name=Neufeld />{{Rp|103}} By 9 February 1942, Peenemünde engineer Gerd {{Proper name|deBeek}} had documented the radio interference area of a V-2 as {{convert|10000|m|ft|abbr=off}} around the "Firing Point",<ref name=Klee /> and the first successful A-4 flight on 3 October 1942 used radio control to command motor cutoff.<ref name=Dornberger />{{Rp|12}} Although Hitler commented on 22 September 1943 that "It is a great load off our minds that we have dispensed with the radio guiding-beam; now no opening remains for the British to interfere technically with the missile in flight",<ref name=Irving />{{Rp|138}} about 20% of the operational V-2 launches were beam-guided.<ref name=Dornberger />{{Rp|12}}<ref name=walter />{{rp|232}} The Operation Pinguin V-2 offensive began on 8 September 1944, when {{Lang|de|Lehr- und Versuchsbatterie No. 444}}<ref name=Pocock />{{Rp|51–2}} (English: 'Training and Testing Battery 444') launched a single rocket guided by a radio beam directed at Paris.<ref name=Klee />{{Rp|47}} Wreckage of combat V-2s occasionally contained the transponder for velocity and fuel cutoff.<ref name=Ordway />{{Rp|259–260}} The painting of the operational V-2s was mostly a [[Splittertarnmuster|ragged-edged pattern]] with several variations, but at the end of the war a plain olive green rocket was also used. During tests the rocket was painted in a characteristic black-and-white [[chessboard]] pattern, which aided in determining if the rocket was spinning around its longitudinal axis. [[File:Esquema de la V-2.jpg|thumb|upright=1.9|A [[United States Army|U.S. Army]] cut-away diagram of the V-2.]] The original German designation of the rocket was "V2",<ref name="Thimm" /><ref>{{cite web |url=http://www.v2werk-oberraderach.de/ |title=A4 (V2) Raketenfertigung in Friedrichshafen 1942–1945 |access-date=2019-05-09 |first=Thomas |last=Kliebenschedel |language=de |archive-url=https://web.archive.org/web/20190605130629/http://www.v2werk-oberraderach.de/ |archive-date=5 June 2019 |url-status=live }}</ref> unhyphenated – exactly as used for [[RLM aircraft designation system#Prototypes and variants|any Third Reich-era "second prototype" example]] of an [[RLM aircraft by manufacturer|RLM-registered]] German aircraft design – but U.S. publications such as ''[[Life (magazine)|Life]]'' magazine were using the hyphenated form "V-2" as early as December 1944.<ref name="V2_LIFE">{{cite magazine |date=25 December 1944 |title=V-2: Nazi Rocket Details Are Finally Revealed |url=https://books.google.com/books?id=uUEEAAAAMBAJ&pg=PA46 |magazine=[[Life (magazine)|LIFE]] |volume=17 |issue=26 |pages=46–48 |access-date=29 October 2015 |archive-url=https://web.archive.org/web/20160428115211/https://books.google.com/books?id=uUEEAAAAMBAJ&pg=PA46 |archive-date=28 April 2016 |url-status=live }}</ref> === Testing === {{See also|List of V-2 test launches}} {{For|a description of a test explosion|Test Stand VII}} The first successful test flight was on 3 October 1942, reaching an altitude of {{convert|84.5|km|mi|abbr=off}}.<ref name=Neufeld /> On that day, Walter Dornberger declared in a meeting at Peenemünde: {{blockquote|This third day of October, 1942, is the first of a new era in transportation, that of space travel...<ref name=Dornberger /><sup>17</sup>}} [[File:Rocket engine A4 V2.jpg|thumb|right|upright|A sectioned V-2 engine on display at the Deutsches Museum, Munich (2006).]] Two test launches were recovered by the Allies: [[Sweden during World War II#The Bäckebo rocket|the Bäckebo rocket]], the remnants of which landed in Sweden on 13 June 1944, and one [[Home Army and V1 and V2|recovered by the Polish resistance]] on 30 May 1944<ref># (Polish) Michał Wojewódzki, Akcja V-1, V-2, Warsaw 1984, {{ISBN|83-211-0521-1}}</ref> from the [[Blizna V-2 missile launch site]] and transported to the UK during [[Operation Most III]]. The highest altitude reached during the war was {{convert|174.6|km|mi|abbr=off}} (20 June 1944).<ref name=Neufeld /> Test launches of V-2 rockets were made at Peenemünde, Blizna and [[Tuchola Forest]],<ref name=walter />{{rp|211}} and after the war, at [[Operation Backfire (WWII)|Cuxhaven by the British]], [[White Sands Proving Grounds]] and [[Cape Canaveral]] by the U.S., and [[Kapustin Yar]] by the USSR. Various design issues were identified and solved during V-2 development and testing: * To reduce tank pressure and weight, rapid flow turbopumps were used to increase pressure.<ref name=Neufeld />{{Rp|35}} * A short and lighter [[combustion chamber]] without burn-through was developed by using centrifugal injection nozzles, a mixing compartment, and a converging nozzle to the throat for homogeneous combustion.<ref name=Dornberger />{{Rp|51}} * Film cooling was used to prevent burn-through at the nozzle throat.<ref name=Dornberger />{{Rp|52}} * Relay contacts were made more durable to withstand vibration and prevent thrust cut-off just after lift-off.<ref name=Dornberger />{{Rp|52}} * Ensuring that the fuel pipes had tension-free curves reduced the likelihood of explosions at {{cvt|4000|-|6000|ft|order=flip}}.<ref name=Dornberger />{{Rp|215, 217}} * Fins were shaped with clearance to prevent damage as the exhaust jet expanded with altitude.<ref name=Dornberger />{{Rp|56, 118}} * To control trajectory at liftoff and supersonic speeds, heat-resistant graphite vanes were used as rudders in the exhaust jet.<ref name=Dornberger />{{Rp|35, 58}} ==== Air burst problem ==== Through mid-March 1944, only four of the 26 successful Blizna launches had satisfactorily reached the [[Sarnaki]] target area<ref name=Klee>{{cite book |last=Klee |first=Ernst |author2=Merk, Otto |title=The Birth of the Missile: The Secrets of Peenemünde |orig-date=1963 |issue=English translation |year=1965 |publisher=Gerhard Stalling Verlag |location=Hamburg |page=47}}</ref>{{Rp|112, 221–222, 282}} due to in-flight breakup ({{Lang|de|Luftzerleger}}) on re-entry into the atmosphere.<ref name=Johnson>{{cite book |last=Johnson |first=David |title=V-1, V-2: Hitler's Vengeance on London|year=1982 |publisher=Stein and Day |location=New York |page=100 |isbn=978-0-8128-2858-0}}</ref>{{Rp|100}} (As mentioned above, one rocket was collected by the Polish [[Home Army]], with parts of it transported to London for tests.) Initially, the German developers suspected excessive alcohol tank pressure, but by April 1944, after five months of test firings, the cause was still not determined. Major-General Rossmann, the Army Weapons Office department chief, recommended stationing observers in the target area – {{circa}} May/June, Dornberger and von Braun set up a camp at the centre of the Poland target zone.<ref>Neufeld 1995 pp. 221–222</ref> After moving to the Heidekraut,<ref name=Ordway />{{Rp|172–173}} SS Mortar Battery 500 of the 836th Artillery Battalion (Motorized) was ordered<ref name=Klee />{{Rp|47}} on 30 August<ref name=Pocock>{{cite book |last=Pocock |first=Rowland F. |title=German Guided Missiles of the Second World War |year=1967 |publisher=Arco Publishing Company, Inc |location=New York |pages=51, 52}}</ref> to begin test launches of eighty 'sleeved' rockets.<ref name=Irving />{{Rp|281}} Testing confirmed that the so-called 'tin trousers' – a tube designed to strengthen the forward end of the rocket cladding – reduced the likelihood of air bursts.<ref name=Johnson />{{Rp|100}}<ref name=walter />{{rp|188–198}}
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