Editing Polar Satellite Launch Vehicle (section)

=== Fourth stage (PS4) ===
The fourth stage is powered by regeneratively cooled twin engines,<ref>{{cite web|url=http://asaco.in/aerospace/sub-systems-for-aerospace/|title=ASACO sub-systems for space.|archive-url=https://web.archive.org/web/20171211095555/http://asaco.in/aerospace/sub-systems-for-aerospace/|archive-date=11 December 2017|url-status=live|access-date=16 October 2018}}</ref> burning [[monomethylhydrazine]] (MMH) and [[mixed oxides of nitrogen]] (MON). Each pressure fed engine generates {{cvt|7.4|kN}} thrust and is gimbaled (±3°) to provide pitch, yaw and roll control during powered flight. Coast phase attitude control is provided by six 50N RCS thrusters.<ref>{{cite journal|last1=Ramakrishnan|first1=S.|last2=Somanath|first2=S.|last3=Balakrishnan|first3=S. S.|date=2002-01-01|title=Multi-Orbit Mission by PSLV-C3 and Future Launch Opportunities|journal=Iaf Abstracts|pages=936|bibcode=2002iaf..confE.936R}}</ref> The stage is pressurized by [[helium]]<ref>{{Cite web|title=Latest Volume14-Issue21 News, Photos, Latest News Headlines about Volume14-Issue21|url=https://frontline.thehindu.com/magazine/issue/vol14-21/|access-date=2021-04-07|website=Frontline|language=en|archive-date=7 May 2021|archive-url=https://web.archive.org/web/20210507005441/https://frontline.thehindu.com/magazine/issue/vol14-21/|url-status=live}}</ref> and carries {{cvt|1600|kg}} to {{cvt|2500|kg}} of propellant depending on the mission requirements. PS4 has three variants L1.6, L2.0 and L2.5 based on propellant tank capacity.<ref name="sport"/><ref>{{Cite web |title=Signatures, Newsletter of the Indian Society of Remote Sensing – Ahmedabad Chapter. Volume: 24, No.2, April- June 2012 |url=https://www.sac.gov.in/SAC_Industry_Portal/publication/newsletter/Signatures_RISAT.pdf |archive-url=https://web.archive.org/web/20211221142837/https://www.sac.gov.in/SAC_Industry_Portal/publication/newsletter/Signatures_RISAT.pdf |archive-date=21 December 2021 |access-date=31 July 2023 |quote=The fourth stage has three variants designated as L1.6, L2.0 and L2.5 based on the propellant loading capacity of 1.6t, 2t and 2.5t respectively required for a particular mission.}}</ref>

On PSLV-C29/TeLEOS-1 mission, the fourth stage demonstrated re-ignition capability for the first time which was used in many subsequent flights to deploy payloads in multiple orbits on a single campaign.<ref name=":12">{{cite web|title=ISRO Successfully Tests Multiple Burn Fuel Engine During Launch of Six Singaporean Satellites|url=http://www.newindianexpress.com/states/andhra_pradesh/ISRO-Successfully-Tests-Multiple-Burn-Fuel-Engine-During-Launch-of-Six-Singaporean-Satellites/2015/12/16/article3180961.ece|access-date=16 December 2015|archive-date=3 May 2016|archive-url=https://web.archive.org/web/20160503044453/http://www.newindianexpress.com/states/andhra_pradesh/ISRO-Successfully-Tests-Multiple-Burn-Fuel-Engine-During-Launch-of-Six-Singaporean-Satellites/2015/12/16/article3180961.ece|url-status=dead}}</ref>

As a [[space debris]] mitigation measure, PSLV fourth stage gets [[Passivation (spacecraft)|passivated]] by venting pressurant and propellant vapour after achieving main mission objectives. Such passivation prevents any unintentional fragmentation or explosion due to stored internal energy.<ref>{{Cite journal|last1=Adimurthy|first1=V.|last2=Ganeshan|first2=A.S.|date=February 2006|title=Space debris mitigation measures in India|url=https://linkinghub.elsevier.com/retrieve/pii/S0094576505002961|journal=Acta Astronautica|language=en|volume=58|issue=3|pages=168–174|doi=10.1016/j.actaastro.2005.09.002|bibcode=2006AcAau..58..168A|access-date=26 December 2021|archive-date=12 April 2022|archive-url=https://web.archive.org/web/20220412004038/https://linkinghub.elsevier.com/retrieve/pii/S0094576505002961|url-status=live}}</ref><ref>{{Cite journal|last=Bonnal|first=C|date=2007-06-01|title=Design and operational practices for the passivation of spacecraft and launchers at the end of life|url=http://journals.sagepub.com/doi/10.1243/09544100JAERO231|journal=Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering|language=en|volume=221|issue=6|pages=925–931|doi=10.1243/09544100JAERO231|s2cid=110656798|issn=0954-4100}}</ref><ref>{{Cite web|title=Indian Presentation to the 47th Session of Scientific and Technical Subcommittee of United Nations Committee on the Peaceful Uses of Outer Space Agenda 8 'Space Debris Activities in India'|url=https://www.unoosa.org/pdf/pres/stsc2010/tech-33.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.unoosa.org/pdf/pres/stsc2010/tech-33.pdf |archive-date=2022-10-09|url-status=live}}</ref>

The [[niobium alloy]] nozzle used on twin engines of fourth stage is expected to be replaced by lighter, [[silicon carbide]] coated [[Reinforced carbon–carbon|carbon–carbon]] nozzle divergent. The new nozzle was hot tested at facilities of [[ISRO Propulsion Complex|IPRC, Mahendragiri]] in March and April 2024. This substitution should increase payload capacity of PSLV by {{Convert|15|kg|lb}}.<ref>{{Cite web |title=ISRO Develops Lightweight Carbon-Carbon Nozzle for Rocket Engines, Enhancing Payload Capacity |url=https://www.isro.gov.in/ISRO_Develops_Lightweight_Carbon_Carbon_Nozzle_for_Rocket_Engines.html |archive-url=https://web.archive.org/web/20240419183918/https://www.isro.gov.in/ISRO_Develops_Lightweight_Carbon_Carbon_Nozzle_for_Rocket_Engines.html |archive-date=19 April 2024 |access-date=2024-04-21 |website=www.isro.gov.in}}</ref> ISRO also replaced imported [[Niobium|Columbium]] materials in the engine nozzle divergent  with [[Stellite]], which resulted in cost savings of 90%. The newly modified engines were tested at IPRC in April 2025.<ref>{{Cite web |title=ISRO Successfully qualifies fourth stage engine of PSLV with Satellite Nozzle Divergent |url=https://www.isro.gov.in/ISRO_Successfully_qualifies_fourth_stage_engine_of_PSLV_with_Satellite_Nozzle_Divergent.html |archive-url=http://web.archive.org/web/20250418060419/https://www.isro.gov.in/ISRO_Successfully_qualifies_fourth_stage_engine_of_PSLV_with_Satellite_Nozzle_Divergent.html |archive-date=2025-04-18 |access-date=2025-04-18 |website=www.isro.gov.in |language=en}}</ref>

ISRO successfully completed 665-second hot test of [[3D printed]] PS4 engine, produced by Wipro 3D through [[selective laser melting]]. A total of 19 weld joints were eliminated through this process while engine's 14 components were reduced to one piece. It saved 60% of the production time and drastically decreased the amount of raw materials used per engine, from 565&nbsp;kg to 13.7&nbsp;kg of metal powder.<ref>{{Cite news |date=2024-05-11 |title=With 3D-printed rocket engine, Isro adds another feather to cap |url=https://timesofindia.indiatimes.com/india/with-3d-printed-rocket-engine-isro-adds-another-feather-to-cap/articleshow/110019883.cms |access-date=2024-05-13 |work=The Times of India |issn=0971-8257}}</ref>

==== PS4 stage as orbital platform ====
{{Main|PSLV Orbital Experiment Module}}
PS4 has carried hosted payloads like AAM on PSLV-C8,<ref name=":6"/> [[Luxspace|Rubin 9.1]]/[[Rubin 9.2]] on PSLV-C14<ref>{{cite web|url=https://www.isro.gov.in/sites/default/files/flipping_book/PSLV-C14/files/assets/common/downloads/publication.pdf|url-status=dead|title=PSLV C14/Oceansat-2 brochure|access-date=23 February 2020|archive-date=6 August 2020|archive-url=https://web.archive.org/web/20200806094152/https://www.isro.gov.in/sites/default/files/flipping_book/PSLV-C14/files/assets/common/downloads/publication.pdf}}</ref> and mRESINS on PSLV-C21.<ref>{{cite web |url=https://www.dos.gov.in/sites/default/files/flipping_book/Space%20India%20July%2012-Aug%2013/files/assets/common/downloads/Space%20India%20July%2012-Aug%2013.pdf |url-status=dead |title=Space-India July 2012 to August 2013 |access-date=23 February 2020 |archive-date=6 August 2020 |archive-url=https://web.archive.org/web/20200806152719/https://www.dos.gov.in/sites/default/files/flipping_book/Space%20India%20July%2012-Aug%2013/files/assets/common/downloads/Space%20India%20July%2012-Aug%2013.pdf }}</ref> But now, PS4 is being augmented to serve as a long duration orbital platform after completion of primary mission. PS4 Orbital Platform (PS4-OP) will have its own power supply, telemetry package, data storage and attitude control for hosted payloads.<ref>{{cite web|url=http://www.unoosa.org/documents/pdf/copuos/stsc/2019/tech-55E.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.unoosa.org/documents/pdf/copuos/stsc/2019/tech-55E.pdf |archive-date=2022-10-09 |url-status=live|title=Opportunities for science experiments in the fourth stage of India's PSLV|date=21 February 2019}}</ref><ref>{{cite web|url=https://www.isro.gov.in/sites/default/files/orbital_platform-_ao.pdf|url-status=dead|title=Announcement of Opportunity (AO) for Orbital platform: an avenue for in-orbit scientific experiments|date=15 June 2019|access-date=23 February 2020|archive-date=6 August 2020|archive-url=https://web.archive.org/web/20200806072033/https://www.isro.gov.in/sites/default/files/orbital_platform-_ao.pdf}}</ref><ref>{{cite news|url=https://timesofindia.indiatimes.com/india/2-days-after-space-station-news-isro-calls-for-docking-experiments-on-pslv-stage-4/articleshow/69800354.cms|title=2 days after Space Station news, Isro calls for "docking experiments" on PSLV stage-4|first=Chethan|last=Kumar|work=The Times of India|date=15 June 2019 |access-date=23 February 2020|archive-date=24 August 2019|archive-url=https://web.archive.org/web/20190824073052/https://timesofindia.indiatimes.com/india/2-days-after-space-station-news-isro-calls-for-docking-experiments-on-pslv-stage-4/articleshow/69800354.cms|url-status=live}}</ref>

On [[PSLV-C37]] and [[PSLV-C38]] campaigns,<ref>{{Cite web |title=In-situ observations of rocket burn induced modulations of the top side ionosphere using the IDEA payload on-board the unique orbiting experimental platform (PS4) of the Indian Polar Orbiting Satellite Launch Vehicle mission - ISRO |url=https://www.isro.gov.in/situ-observations-of-rocket-burn-induced-modulations-of-top-side-ionosphere-using-idea-payload-board |access-date=2022-06-27 |website=www.isro.gov.in |language=en |archive-date=21 September 2022 |archive-url=https://web.archive.org/web/20220921000216/https://www.isro.gov.in/situ-observations-of-rocket-burn-induced-modulations-of-top-side-ionosphere-using-idea-payload-board |url-status=live }}</ref> as a demonstration PS4 was kept operational and monitored for over ten orbits after delivering spacecraft.<ref>{{cite web |url=https://www.isro.gov.in/sites/default/files/article-files/node/9805/annualreport2017-18.pdf|title=Department of Space Annual Report 2017-18|url-status=live|archive-url=https://web.archive.org/web/20180213093132/https://www.isro.gov.in/sites/default/files/article-files/node/9805/annualreport2017-18.pdf|archive-date=13 February 2018}}</ref><ref>{{cite web|url=https://timesofindia.indiatimes.com/india/in-a-first-isro-will-make-dead-rocket-stage-alive-in-space-for-experiments/articleshow/67067817.cms|title=In a first, ISRO will make dead rocket stage "alive" in space for experiments|first=Surendra|last=Singh|work=The Times of India|date=16 December 2018|access-date=23 February 2020|archive-date=8 November 2020|archive-url=https://web.archive.org/web/20201108132514/https://timesofindia.indiatimes.com/india/in-a-first-isro-will-make-dead-rocket-stage-alive-in-space-for-experiments/articleshow/67067817.cms|url-status=live}}</ref><ref>{{cite web|url=https://www.deccanchronicle.com/science/science/200617/isro-to-lower-rockets-altitude.html|title=Isro to lower rocket's altitude|last=rajasekhar|first=pathri|publisher=Deccan Chronicle|date=2017-06-20|access-date=23 February 2020|archive-date=6 August 2020|archive-url=https://web.archive.org/web/20200806095054/https://www.deccanchronicle.com/science/science/200617/isro-to-lower-rockets-altitude.html|url-status=live}}</ref>

[[PSLV-C44]] was the first campaign where PS4 functioned as independent orbital platform for short duration as there was no on-board power generation capacity.<ref>{{cite news|last=Rajwi|first=Tiki|url=https://www.thehindu.com/news/national/kerala/pslv-lift-off-with-added-features/article25981654.ece|title=PSLV lift-off with added features|date=2019-01-12|newspaper=The Hindu|issn=0971-751X|access-date=23 February 2020|archive-date=6 August 2020|archive-url=https://web.archive.org/web/20200806161530/https://www.thehindu.com/news/national/kerala/pslv-lift-off-with-added-features/article25981654.ece|url-status=live}}</ref> It carried KalamSAT-V2 as a fixed payload, a 1U CubeSat by Space Kidz India based on [[Interorbital Systems]] kit.<ref>{{cite web|title=PSLV-C44 - ISRO|url=https://www.isro.gov.in/launcher/pslv-c44|access-date=26 June 2020|website=isro.gov.in|archive-date=17 January 2019|archive-url=https://web.archive.org/web/20190117013408/https://www.isro.gov.in/launcher/pslv-c44|url-status=dead}}</ref><ref>{{cite tweet|number=1088526772109422592|user=interorbital|title=Congratulations to ISRO and SpaceKidzIndia on getting their CubeSat into orbit! The students modified their IOS CubeSat kit, complete w/ their own experiments!|author=Interorbital Systems|date=25 January 2019}}</ref>

On [[PSLV-C45]] campaign, the fourth stage had its own power generation capability as it was augmented with an array of fixed [[solar cell]]s around PS4 propellant tank.<ref>{{cite web|url=https://spaceflightnow.com/2019/04/01/indian-military-satellite-20-more-planet-imaging-cubesats-aboard-successful-pslv-launch/|title=Indian military satellite, 20 more Planet imaging CubeSats launched by PSLV|last=Clark|first=Stephen|publisher=Spaceflight Now|access-date=2020-02-23|archive-date=6 April 2019|archive-url=https://web.archive.org/web/20190406042135/https://spaceflightnow.com/2019/04/01/indian-military-satellite-20-more-planet-imaging-cubesats-aboard-successful-pslv-launch/|url-status=live}}</ref> The three payloads hosted on PS4-OP were the Advanced Retarding Potential Analyzer for Ionospheric Studies (ARIS 101F) by [[IIST]],<ref>{{cite web|url=https://www.iist.ac.in/avionics/sudharshan.kaarthik|title=Department of Avionics, R. Sudharshan Kaarthik, Ph.D (Assistant Professor)|access-date=23 February 2020|archive-date=13 February 2020|archive-url=https://web.archive.org/web/20200213005932/https://www.iist.ac.in/avionics/sudharshan.kaarthik|url-status=live}}</ref> an experimental [[Automatic identification system|AIS]] payload by ISRO, and AISAT by [[Satellize]].<ref>{{cite web|url=https://satellize.com/index.php/exseed-sat-2/|title=Exseed Sat-2|publisher=Satellize|access-date=23 February 2020|archive-date=23 February 2020|archive-url=https://web.archive.org/web/20200223011624/https://satellize.com/index.php/exseed-sat-2/|url-status=live}}</ref> To function as orbital platform, fourth stage was put in [[Spin-stabilisation|spin-stabilized]] mode using its RCS thrusters.<ref>{{Cite web |date=16 June 2021 |title=Opportunity for Scientific Experiments on PSLV Upper Stage Orbital Platform |url=https://www.unoosa.org/documents/pdf/psa/hsti/Hyper-Microgravity_Webinar2021/Hyper-Microgravity_Webinar2021/9_RegionalActivities/R._Senan_Hypermicrogravity_ISRO.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.unoosa.org/documents/pdf/psa/hsti/Hyper-Microgravity_Webinar2021/Hyper-Microgravity_Webinar2021/9_RegionalActivities/R._Senan_Hypermicrogravity_ISRO.pdf |archive-date=2022-10-09 |url-status=live}}</ref>

On the [[PSLV-C53]] campaign, the PS4-OP is referred to as the [[PSLV-C53#PSLV Orbital Experimental Module|PSLV Orbital Experimental Module (POEM)]], and it hosted six payloads. POEM was the first PSLV fourth stage based orbital platform to be actively stabilised using Helium based cold gas thrusters after the primary mission and stage passivization.<ref>{{Cite web |last=Kumar |first=Chethan |date=June 25, 2012 |title=Bengaluru's Digantara, Hyderabad startup Dhruva become first to get IN-SPACe authorisation |url=https://timesofindia.indiatimes.com/india/bengaluru-digantara-hyderabad-startup-dhruva-become-first-to-get-in-space-authorisation/articleshow/92460375.cms |url-status=live |archive-url=https://web.archive.org/web/20220625153537/https://timesofindia.indiatimes.com/india/bengaluru-digantara-hyderabad-startup-dhruva-become-first-to-get-in-space-authorisation/articleshow/92460375.cms |archive-date=2022-06-25 |access-date=2022-06-26 |website=The Times of India |language=en}}</ref><ref>{{Cite web |title=PSLV-C53/DS-EO mission |url=https://www.isro.gov.in/sites/default/files/article-files/pslv-c53-ds-eo-mission/pslv-c53-ds-eo-mission-brochure-english/brochure_pslv_c53-ds-eo_mission_english.pdf |archive-url=https://web.archive.org/web/20220705162540/https://www.isro.gov.in/sites/default/files/article-files/pslv-c53-ds-eo-mission/pslv-c53-ds-eo-mission-brochure-english/brochure_pslv_c53-ds-eo_mission_english.pdf |archive-date=5 July 2022}}</ref><ref>{{Cite web |title=పీఎస్‌ఎల్‌వీ-సి53లో నూతన సాంకేతికత |url=https://www.eenadu.net/telugu-news/related-stories/general/2505/122117906 |url-status=live |archive-url=https://web.archive.org/web/20220706195945/https://www.eenadu.net/telugu-news/related-stories/general/2505/122117906 |archive-date=2022-07-06 |access-date=2022-07-06 |website=EENADU |language=te}}</ref><ref>{{Cite web |author=News9 Staff |date=2022-09-08 |title=Har Ghar Tiranga happened in Antriksh. ISRO did hoist the Indian flag in Space! |url=https://www.news9live.com/india/har-ghar-tiranga-happened-in-antriksh-isro-did-hoist-the-indian-flag-in-space-194691 |access-date=2022-09-09 |website=NEWS9LIVE |language=en |archive-date=9 September 2022 |archive-url=https://web.archive.org/web/20220909095750/https://www.news9live.com/india/har-ghar-tiranga-happened-in-antriksh-isro-did-hoist-the-indian-flag-in-space-194691 |url-status=live }}</ref>

==== PS4 on RLV-OREX ====
The [[RLV Technology Demonstration Programme|Reusable Launch Vehicle Technology Demonstration program]] is an prototype [[spaceplane]] project currently being processed by ISRO. It is planned to use a GSLV, modified by replacing it's Cryogenic Upper Stage(CUS) with the PS-4 as the RLV would not required the excess thrust created by the CUS.<ref>{{Cite news |title=RLV re-entry mission to use GSLV with PSLV last stage; landing gear to change & more |url=https://timesofindia.indiatimes.com/india/rlv-re-entry-mission-to-use-gslv-with-pslv-last-stage-landing-gear-to-change-more/articleshow/111267553.cms |access-date=2024-06-26 |work=The Times of India |issn=0971-8257}}</ref><ref>{{Cite web |title=MSN |url=https://www.msn.com/en-in/news/other/rlv-re-entry-mission-to-use-gslv-with-pslv-last-stage-landing-gear-to-change-more/ar-BB1oSNaF |access-date=2024-06-26 |website=www.msn.com}}</ref>