Editing Northrop B-2 Spirit (section)

===Stealth===
[[File:B2 Spirit closeup.JPEG|thumb|The B-2's engines are buried within its wing to conceal the engines' fans and minimize their exhaust signature.]]

The B-2's low-observable, or "[[stealth technology|stealth]]", characteristics enable the undetected penetration of sophisticated anti-aircraft defenses and to attack even heavily defended targets. This stealth comes from a combination of reduced [[Acoustic signature|acoustic]], [[Infrared signature|infrared]], [[Aircraft camouflage|visual]] and [[Radar cross-section|radar signature]]s ([[multi-spectral camouflage]]) to evade the various detection systems that could be used to detect and be used to direct attacks against an aircraft. The B-2's stealth enables the reduction of supporting aircraft that are required to provide air cover, [[Suppression of Enemy Air Defenses]] and [[electronic countermeasure]]s, making the bomber a "[[force multiplier]]". {{As of|2013|09}}, there have been no instances of a missile being launched at a B-2.<ref name="chiles201309" />

To reduce optical visibility during daylight flights, the B-2 is painted in an [[aircraft camouflage|anti-reflective paint]].<ref name= "tucker 177">{{harvnb|Tucker|2010|p=177}}</ref> The undersides are dark because it flies at high altitudes ({{convert|50000|ft|m|abbr=on|disp=}}), and at that altitude a dark grey painting blends well into the sky. It is speculated to have an upward-facing [[Photodetector|light sensor]] which alerts the pilot to increase or reduce altitude to match the changing illuminance of the sky.<ref>{{cite book|last=Sweetman|first=Bill|title=Inside the Stealth Bomber|year=1999|url=https://books.google.com/books?id=Q_wyJ_pKcqoC&pg=PA55|publisher=Zenith Imprint|page=55|isbn=978-1-61060-689-9|access-date=19 October 2016|archive-date=15 February 2020|archive-url=https://web.archive.org/web/20200215143146/https://books.google.com/books?id=Q_wyJ_pKcqoC&pg=PA55|url-status=live}}</ref> The original design had tanks for a [[contrail]]-inhibiting chemical, but this was replaced in production aircraft by a contrail sensor that alerts the crew when they should change altitude.<ref>Gosnell, Mariana. [http://www.airspacemag.com/flight-today/Flight_Lines.html "Why contrails hang around."] {{Webarchive|url=https://web.archive.org/web/20131112175852/http://www.airspacemag.com/flight-today/Flight_Lines.html |date=12 November 2013}} ''Air & Space magazine'', 1 July 2007. Retrieved 13 September 2009.</ref> The B-2 is vulnerable to visual interception at ranges of {{convert|20|nmi|mi km|abbr=on}} or less.<ref name="chiles201309" /> The B-2 is stored in a $5&nbsp;million specialized air-conditioned hangar to maintain its stealth coating. Every seven years, this coating is carefully washed away with crystallized wheat starch so that the B-2's surfaces can be inspected for any dents or scratches.<ref name=NI_Only_20_B-2s_Stealth_Bombers>{{Cite web|url=https://nationalinterest.org/blog/buzz/why-air-force-only-has-20-b-2-spirit-stealth-bombers-35802|title=Why the Air Force Only Has 20 B-2 Spirit Stealth Bombers|author=Sebastien Roblin|date=11 November 2018|website=National Interest|access-date=5 November 2020|archive-date=18 September 2020|archive-url=https://web.archive.org/web/20200918112631/https://nationalinterest.org/blog/buzz/why-air-force-only-has-20-b-2-spirit-stealth-bombers-35802|url-status=live}}</ref>

====Radar====
The B-2's clean, low-drag [[flying wing]] configuration not only provides exceptional range but is also beneficial to reducing its radar profile.<ref name="croddy 341-2" /><ref>{{harvnb|Siuru|1993|pages=114–115}}</ref> Reportedly, the B-2 has a [[radar cross-section]] (RCS) of about {{convert|0.1|m2|ft2|abbr=on}}.<ref>{{harvnb|Richardson|2001|p=57}}</ref> The bomber does not always fly stealthily; when nearing air defenses pilots "stealth up" the B-2, a maneuver whose details are secret. The aircraft is stealthy, except briefly when the bomb bay opens. The flying wing design most closely resembles a so-called infinite flat plate (as vertical control surfaces dramatically increase RCS), the perfect stealth shape, as it would lack angles to reflect back radar waves (initially, the shape of the Northrop ATB concept was flatter; it gradually increased in volume according to specific military requirements).<ref name="SpiritofInnovation">{{cite web |url=https://www.northropgrumman.com/wp-content/uploads/B-2-Spirit-of-Innovation.pdf |title=B-2: The Spirit of Innovation |website=Northrop Grumman Corporation |access-date=15 October 2023 |archive-date=8 April 2023 |archive-url=https://web.archive.org/web/20230408033411/https://www.northropgrumman.com/wp-content/uploads/B-2-Spirit-of-Innovation.pdf |url-status=live}}</ref> Without vertical surfaces to reflect radar laterally, side aspect radar cross section is also reduced.<ref>{{cite web|url=https://nationalinterest.org/blog/buzz/americas-new-b-21-stealth-bomber-vs-russias-s-300-or-s-400-who-wins-36137|title=America's New B-21 Stealth Bomber vs. Russia's S-300 or S-400: Who Wins?|first=Kris|last=Osborn|date=15 November 2018|website=nationalinterest.org|access-date=16 November 2018|archive-date=15 November 2018|archive-url=https://web.archive.org/web/20181115195225/https://nationalinterest.org/blog/buzz/americas-new-b-21-stealth-bomber-vs-russias-s-300-or-s-400-who-wins-36137|url-status=live}}</ref> Radars operating at a lower frequency band (S or L band) are able to detect and track certain stealth aircraft that have multiple control surfaces, like canards or vertical stabilizers, where the frequency wavelength can exceed a certain threshold and cause a resonant effect.<ref>{{cite web|url=https://nationalinterest.org/blog/buzz/how-russia-could-someday-shootdown-f-22-f-35-or-b-2-stealth-bomber-35512|title=How Russia Could Someday Shootdown an F-22, F-35 or B-2 Stealth Bomber|first=Dave|last=Majumdar|date=8 November 2018|website=nationalinterest.org|access-date=21 November 2018|archive-date=20 November 2018|archive-url=https://web.archive.org/web/20181120221212/https://nationalinterest.org/blog/buzz/how-russia-could-someday-shootdown-f-22-f-35-or-b-2-stealth-bomber-35512|url-status=live}}</ref>

[[File:B-2 radar reflection.jpg|thumb|left|upright=1.35|Illustration of the B-2's basic radar reflection angles]]
RCS reduction as a result of shape had already been observed on the [[Royal Air Force]]'s [[Avro Vulcan]] strategic bomber,<ref>Dawson 1957, p. 3.</ref> and the USAF's [[Lockheed F-117 Nighthawk|F-117 Nighthawk]]. The F-117 used flat surfaces ([[faceting]] technique) for controlling radar returns as during its development (see [[Lockheed Have Blue]]) in the early 1970s, technology only allowed for the simulation of radar reflections on simple, flat surfaces; computing advances in the 1980s made it possible to simulate radar returns on more complex curved surfaces.<ref name="rich4">{{harvnb|Rich|1994|p=21}}</ref> The B-2 is composed of many curved and rounded surfaces across its exposed airframe to deflect radar beams. This technique, known as ''continuous curvature'', was made possible by advances in [[computational fluid dynamics]], and first tested on the [[Northrop Tacit Blue]].<ref>{{cite book|author=Christopher Lavers|title=Reeds Vol 14: Stealth Warship Technology|url=https://books.google.com/books?id=wMB3Pg3vtbsC&pg=PA13|year=2012|publisher=Bloomsbury Publishing|isbn=978-1-4081-7553-8|page=13|access-date=19 October 2016|archive-date=5 February 2021|archive-url=https://web.archive.org/web/20210205102939/https://books.google.com/books?id=wMB3Pg3vtbsC&pg=PA13|url-status=live}}</ref><ref name="SpiritofInnovation"/>

====Infrared====
[[File:B-2 inlet.jpg|thumb|The gap below the air intake has the purpose of keeping the [[boundary layer]] out of the jet engine.]]

Some analysts claim [[infra-red search and track]] systems (IRSTs) can be deployed against stealth aircraft, because any aircraft surface heats up due to air friction and with a two channel IRST is a {{CO2}} (4.3&nbsp;μm absorption maxima) detection possible, through difference comparing between the low and high channel.<ref>{{cite web |last=Radar |first=Cordless |url=http://www.flightglobal.com/blogs/the-dewline/2008/10/infamous-jsf-report-precedes-a.html |title=RAND Report Page 37 |work=Flight International |access-date=16 December 2010 |archive-date=10 May 2013 |archive-url=https://web.archive.org/web/20130510192736/http://www.flightglobal.com/blogs/the-dewline/2008/10/infamous-jsf-report-precedes-a.html |url-status=live}}</ref><ref name="fas_stealth">{{Cite web |title=VI – STEALTH AIRCRAFT: EAGLES AMONG SPARROWS? |publisher=Federation of American Scientist |url=https://fas.org/spp/aircraft/part06.htm |access-date=21 February 2008 |journal= |archive-date=13 February 2008 |archive-url=https://web.archive.org/web/20080213001551/http://www.fas.org/spp/aircraft/part06.htm |url-status=dead}}</ref>

Burying engines deep inside the fuselage also minimizes the thermal visibility or [[infrared signature]] of the exhaust.<ref name= "tucker 177"/><ref>Croddy and Wirtz 2005, p. 342.</ref> At the engine intake, cold air from the [[boundary layer]] below the main inlet enters the fuselage ([[boundary layer suction]], first tested on the [[Northrop X-21]]) and is mixed with hot exhaust air just before the [[nozzle]]s (similar to the [[Ryan AQM-91 Firefly]]). According to the [[Stefan–Boltzmann law]], this results in less energy ([[thermal radiation]] in the infrared spectrum) being released and thus a reduced heat signature. The resulting cooler air is conducted over a surface composed of heat resistant [[carbon-fiber-reinforced polymer]] and [[titanium alloy]] elements, which disperse the air laterally, to accelerate its cooling.<ref name="janes">Jane's Aircraft Upgrades 2003, p. 1711f</ref> The B-2 lacks [[afterburner]]s as the hot exhaust would increase the infrared signature; breaking the [[sound barrier]] would produce an obvious [[sonic boom]] as well as [[aerodynamic heating]] of the [[aircraft skin]] which would also increase the infrared signature.

====Materials====
According to the [[Huygens–Fresnel principle]], even a very flat plate would still reflect radar waves, though much less than when a signal is bouncing at a right angle. Additional reduction in its radar signature was achieved by the use of various [[radar-absorbent material]]s (RAM) to absorb and neutralize radar beams. The majority of the B-2 is made out of a [[carbon]]-[[graphite]] [[composite material]] that is stronger than steel, lighter than aluminum, and absorbs a significant amount of radar energy.<ref name= "boyne 466"/>

The B-2 is assembled with unusually tight [[engineering tolerance]]s to avoid leaks as they could increase its radar signature.<ref name="tirpak199604" /> Innovations such as alternate [[high frequency]] material (AHFM) and automated material application methods were also incorporated to improve the aircraft's radar-absorbent properties and reduce maintenance requirements.<ref name= "tucker 177"/><ref>Lewis, Paul. [http://www.flightglobal.com/news/articles/b-2-to-receive-maintenance-boost-144086/ "B-2 to receive maintenance boost."] {{Webarchive |url=https://web.archive.org/web/20130516141406/http://www.flightglobal.com/news/articles/b-2-to-receive-maintenance-boost-144086/ |date=16 May 2013}} ''Flight International'', 5 March 2002.</ref> In early 2004, Northrop Grumman began applying a newly developed AHFM to operational B-2s.<ref>Hart, Jim. [http://www.irconnect.com/noc/press/pages/news_releases.html?d=56001 "Northrop Grumman Applies New Coating to Operational B-2."] {{Webarchive |url=https://web.archive.org/web/20111209000416/http://www.irconnect.com/noc/press/pages/news_releases.html?d=56001 |date=9 December 2011}} "Northrop Grumman Integrated Systems", 19 April 2004.</ref> To protect the operational integrity of its sophisticated radar absorbent material and coatings, each B-2 is kept inside a climate-controlled hangar (Extra Large Deployable Aircraft Hangar System) large enough to accommodate its {{convert|172|ft|m|adj=on}} wingspan.<ref name=AW_20070107_Away>Fulghum, D.A. [https://archive.today/20130117150011/http://www.aviationnow.com/search/AvnowSearchResult.do?reference=xml/awst_xml/2007/01/08/AW_01_08_2007_p50-51-01.xml "First F-22 large-scale, air combat exercise wins praise and triggers surprise" (online title), "Away Game".] ''[[Aviation Week & Space Technology]]'', 8 January 2007. Retrieved 13 September 2009.</ref>

====Shelter system====
B-2s are supported by portable, environmentally-controlled hangars called B-2 Shelter Systems (B2SS).<ref name="stripes">{{cite news |url=https://www.stripes.com/news/portable-b-2-bomber-shelters-are-built-in-parts-officially-unknown-1.1054 |title=Portable B-2 bomber shelters are built ... in parts (officially) unknown |newspaper=[[Stars and Stripes (newspaper)|Stars and Stripes]] |first=Wayne |last=Specht |date=16 January 2003 |access-date=24 June 2018 |archive-date=25 June 2018 |archive-url=https://web.archive.org/web/20180625022005/https://www.stripes.com/news/portable-b-2-bomber-shelters-are-built-in-parts-officially-unknown-1.1054 |url-status=live}}</ref> The hangars are built by American Spaceframe Fabricators Inc. and cost approximately US$5&nbsp;million apiece.<ref name="stripes"/> The need for specialized hangars arose in 1998 when it was found that B-2s passing through [[Andersen Air Force Base]] did not have the climate-controlled environment maintenance operations required.<ref name="stripes"/> In 2003, the B2SS program was managed by the Combat Support System Program Office at [[Eglin Air Force Base]].<ref name="stripes" /> B2SS hangars are known to have been deployed to [[Naval Support Facility Diego Garcia]] and [[RAF Fairford]].<ref name="stripes"/>