View source for RS-68

{{Short description|A large hydrogen-oxygen rocket engine that powers the Delta IV rocket}}
{{Use dmy dates|date=July 2024}}
{{Infobox rocket engine
| image                      = RS-68 rocket engine test.jpg
| caption                    = An RS-68 engine undergoing hot-fire testing at NASA's [[Stennis Space Center]] during its developmental phase.
| name                       = RS-68
| purpose                    = First stage engine
| associated                 = [[Delta IV]]{{, }}[[Delta IV Heavy]]
| first_date                 = {{Start date|2002|11|20|df=y}}
| last_flight                = 
| successor                  = 
| country_of_origin          = United States
| manufacturer               = {{ubli
   | Rocketdyne (2002–2005)
   | [[Pratt & Whitney Rocketdyne]] (2005–2013)
   | [[Aerojet Rocketdyne]] (2013–2024)
   }}
| last_date                  = {{End date|2024|04|09|df=y}}
| designer                   = [[Rocketdyne]]
| type                       = liquid
| status                     = Retired
| fuel                       = {{chem2|LH2|link=liquid hydrogen}}
| oxidiser                   = [[LOX]]
| cycle                      = [[Gas-generator cycle (rocket)|Gas-generator]]
| nozzle_ratio               = 21.5:1
| description                = 
| thrust(SL)                 = '''RS-68''': {{cvt|2,950|kN}}<br />'''RS-68A''': {{cvt|3137|kN}}<ref name="ula-delta4">{{Cite web |title=DELTA IV |url=http://www.ulalaunch.com/products_deltaiv.aspx |url-status=live |archive-url=https://web.archive.org/web/20140720073154/http://www.ulalaunch.com/Products_DeltaIV.aspx |archive-date=20 July 2014 |access-date=13 July 2014 |publisher=United Launch Alliance}}</ref>
| thrust_to_weight           = <!--values derived from thrust/weight numbers-->'''RS-68''': 45.3:1<br />'''RS-68A''': 47.4:1
| specific_impulse_vacuum    = '''RS-68''': {{cvt|410|isp}}<br />'''RS-68A''': {{cvt|411.9|isp}}<ref>{{Cite web |date=June 2013 |title=Delta IV User's Guide |url=http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf |url-status=live |archive-url=https://web.archive.org/web/20140710005717/http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf |archive-date=10 July 2014 |access-date=13 July 2014 |publisher=United Launch Alliance}}</ref>
| chamber_pressure           = {{convert|1,488|psi|MPa|abbr=on}}
| burn_time                  = 
| dry_weight                 = '''RS-68''': {{cvt|14560|lb|order=flip}}<br />'''RS-68A''': {{cvt|14870|lb|order=flip}}<ref name="ula-delta4" />
| length                     = {{cvt|5.20|m}}
| diameter                   = {{cvt|2.43|m}}
}}

The '''RS-68''' (Rocket System-68) was a [[liquid-fuel rocket]] engine that used [[liquid hydrogen]] (LH<sub>2</sub>) and [[liquid oxygen]] (LOX) as propellants in a [[gas-generator cycle]]. It was the largest hydrogen-fueled rocket engine ever flown.<ref>{{Cite press release |title=ATK Propulsion and Composite Technologies Help Launch National Reconnaissance Office Satellite |date=19 January 2009 |publisher=[[Alliant Techsystems]] |url=http://atk.mediaroom.com/index.php?s=25280&item=57973 |url-status=live |archive-url=https://web.archive.org/web/20140222134317/http://atk.mediaroom.com/index.php?s=25280&item=57973 |archive-date=22 February 2014}}</ref>

Designed and manufactured in the United States by [[Rocketdyne]] (later [[Pratt & Whitney Rocketdyne]] and [[Aerojet Rocketdyne]]). Development started in the 1990s with the goal of producing a simpler, less costly, heavy-lift engine for the [[Delta IV]] launch system. Two versions of the engine have been produced: the original RS-68 and the improved RS-68A. A third version, the RS-68B, was planned for the [[National Aeronautics and Space Administration]]'s (NASA) [[Ares V]] rocket before the cancellation of the rocket and the [[Constellation program|Constellation Program]] in 2010.

== Design and development ==
One of the main goals of the RS-68 program was to produce a simple engine that would be cost-effective when used for a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch [[RS-25]] Space Shuttle Main Engine (SSME).<ref name="AIAA_RS-68">[http://www.pwrengineering.com/dataresources/PropulsionForThe21stCentury-RS-68.doc "AIAA 2002-4324, Propulsion for the 21st Century—RS-68"] {{webarchive | url=https://web.archive.org/web/20090319075127/http://www.pwrengineering.com/dataresources/PropulsionForThe21stCentury-RS-68.doc | date=19 March 2009}}. AIAA, 8–10 July 2002.</ref> The adverse consequences of this simplicity were a significantly lower thrust-to-weight ratio and a 10% lower specific impulse compared to the SSME. The benefit of this simplicity is the RS-68's reduced construction cost.<ref name="AIAA_RS-68" />

The RS-68 was developed at [[Rocketdyne]] Propulsion and Power, located in [[Canoga Park, Los Angeles]], California, where the SSME was manufactured. It was designed to power the [[Delta IV]] [[Evolved Expendable Launch Vehicle]] (EELV). The initial development engines were assembled at the nearby [[Santa Susana Field Laboratory]] where the [[Saturn V]]'s [[Rocketdyne F-1]] engines were developed and tested for the Apollo missions to the Moon. The initial testing of the RS-68 occurred at the [[Air Force Research Laboratory]] (AFRL), [[Edwards Air Force Base]], California, and later at NASA's [[Stennis Space Center]].<ref name="Boeing April 2001 press release">{{Cite web |date=23 April 2001 |title=Boeing Tests RS-68 Rocket Engine (Apr. 24) |url=https://www.defense-aerospace.com/boeing-tests-rs-68-rocket-engine-apr-24/ |access-date=26 June 2023 |website=Defense Aerospace}}</ref><ref name="Boeing May 2001 press release">{{Cite press release |title=Boeing Delta IV CBC/RS-68 Engine Successfully Completes Test Program |date=9 May 2001 |publisher=Boeing |url=https://boeing.mediaroom.com/2001-05-09-Boeing-Delta-IV-CBC-RS-68-Engine-Successfully-Completes-Test-Program |access-date=26 June 2023}}</ref> The RS-68 was certified in December 2001 for use on Delta IV rockets.<ref>{{Cite press release |title=Rocketdyne RS-68 Engine Certified for Boeing Delta IV |date=19 December 2001 |publisher=Boeing |url=http://www.boeing.com/news/releases/2001/q4/nr_011219s.html |url-status=dead |archive-url=https://web.archive.org/web/20121010153432/http://www.boeing.com/news/releases/2001/q4/nr_011219s.html |archive-date=10 October 2012}}</ref>

An RS-68 was part of each Delta IV [[Common Booster Core]]. The largest of the launch vehicles, the [[Delta IV Heavy]], used three CBCs mounted together.<ref>{{Cite web |title=Atlas V and Delta IV Technical Summary |url=https://www.ulalaunch.com/docs/default-source/rockets/atlas-v-and-delta-iv-technical-summary.pdf}}</ref>

The engine produced {{convert|758000|lbf|kN|sigfig=3|lk=on}} in a vacuum and {{convert|663000|lbf|kN|sigfig=3}} at sea level. The engine's mass was {{convert|14560|lb|kg}}. With this thrust, the engine had a thrust-to-weight ratio of 51.2 and a [[specific impulse]] of {{convert|410|isp}} in a vacuum and {{convert|365|isp}} at sea level.<ref>{{Cite web |title=United Launch Alliance Delta IV Heavy |url=https://www.ulalaunch.com/rockets/delta-iv}}</ref> The RS-68 was [[Gimbal#Rocket engines|gimbaled]] hydraulically and was capable of throttling between 58% and 102% thrust.<ref>[http://www.engineeringatboeing.com/dataresources/PropulsionForThe21stCentury-RS-68.doc Boeing white paper on RS-68 development] {{webarchive | url=https://web.archive.org/web/20070415124430/http://www.engineeringatboeing.com/dataresources/PropulsionForThe21stCentury-RS-68.doc | date=15 April 2007}}</ref>

The RS-68A is an updated version of the RS-68, with increased specific impulse and thrust (to over {{convert|700000|lbf|kN|sigfig=2}} at sea level).<ref>{{Cite press release |title=United Launch Alliance First RS-68A Hot-Fire Engine Test a Success |date=25 September 2008 |publisher=United Launch Alliance |url=https://www.ulalaunch.com/about/news/2008/09/25/ula-first-rs-68a-hot-fire-engine-test-a-success |quote=Currently, the RS-68 engine can deliver more than 660,000 pounds of sea level thrust and the upgraded RS-68A will increase this to more than 700,000 pounds. The RS-68A also improves on the specific impulse, or fuel efficiency, of the RS-68. |access-date=10 April 2023}}</ref> The first launch on 29 June 2012, from the [[Cape Canaveral Air Force Station]] used three RS-68A engines mounted in a [[Delta IV Heavy]] rocket.<ref>{{Cite press release |title=United Launch Alliance Upgraded Delta IV Heavy rocket successfully Launches Second Payload in Nine Days for the National Reconnaissance Office |date=29 June 2012 |publisher=United Launch Alliance |url=http://www.ulalaunch.com/united-launch-alliance-upgraded-delta-iv.aspx?title=United+Launch+Alliance+Upgraded+Delta+IV+Heavy+Rocket++Successfully+Launches+Second+Payload+in+Nine+Days+for+the++National+Reconnaissance+Office&archived=True&Category=all&Page=22 |url-status=live |archive-url=https://web.archive.org/web/20160820020437/http://www.ulalaunch.com/united-launch-alliance-upgraded-delta-iv.aspx?title=United+Launch+Alliance+Upgraded+Delta+IV+Heavy+Rocket++Successfully+Launches+Second+Payload+in+Nine+Days+for+the++National+Reconnaissance+Office&archived=true&category=all&page=22 |archive-date=20 August 2016}}</ref>

The RS-68 was retired as of the last Delta IV Heavy flight in April 2024.

=== Proposed uses ===
In 2006, NASA announced an intention to use five RS-68 engines instead of SSMEs on the planned [[Ares V]]. NASA chose the RS-68 because of its lower cost, about $20 million per engine including the cost of NASA's upgrades. The upgrades included a different ablative nozzle to accommodate a longer burn, a shorter start sequence, hardware changes to limit free hydrogen at ignition, and a reduction in the amount of helium used during countdown and flight. Thrust and specific impulse increases would occur under a separate upgrade program for the Delta IV rocket.<ref>{{Cite press release |title=NASA's Exploration Systems Progress Report |date=18 May 2006 |publisher=National Aeronautics and Space Administration |url=http://www.nasa.gov/home/hqnews/2006/may/HQ_06226_RS-68_ENGINE.html |access-date=30 May 2006 |url-status=live |archive-url=https://web.archive.org/web/20120405050707/http://www.nasa.gov/home/hqnews/2006/may/HQ_06226_RS-68_ENGINE.html |archive-date=5 April 2012}}</ref> Later, the Ares V was changed to use six RS-68 engines, designated the RS-68B.<ref name="CaLV">{{Cite web |title=Overview: Ares V Cargo Launch Vehicle |url=http://www.nasa.gov/mission_pages/constellation/ares/aresV/index.html |archive-url=https://web.archive.org/web/20080926214451/http://www.nasa.gov/mission_pages/constellation/ares/aresV/index.html |archive-date=26 September 2008 |access-date=30 September 2008 |publisher=National Aeronautics and Space Administration}}</ref> Ares V was dropped as part of the cancellation of the [[Constellation program]] in 2010.<ref>{{Cite news |last=Amos |first=Jonathan |date=11 October 2010 |title=Obama signs Nasa up to new future |url=https://www.bbc.com/news/science-environment-11518049 |access-date=7 June 2019 |agency=BBC news}}</ref> NASA's current successor heavy-lift vehicle, the [[Space Launch System]], uses four [[RS-25]] engines instead.<ref>{{Cite web |title=Space Launch System Factsheet |url=https://www.nasa.gov/sites/default/files/atoms/files/0080_sls_fact_sheet_10092018.pdf |access-date=7 June 2019 |publisher=National Aeronautics and Space Administration |archive-date=13 May 2019 |archive-url=https://web.archive.org/web/20190513024103/https://www.nasa.gov/sites/default/files/atoms/files/0080_sls_fact_sheet_10092018.pdf |url-status=dead }}</ref>

=== Human-rating ===
In 2008, it was reported that the RS-68 needed over 200 changes to receive a [[human-rating certification]].<ref>{{Cite web |date=27 September 2008 |title=United Launch Alliance First RS-68A Hot-Fire Engine Test a Success |url=http://forum.nasaspaceflight.com/index.php?topic=14446.msg318877#msg318877 |url-status=live |archive-url=https://web.archive.org/web/20180328171435/http://forum.nasaspaceflight.com/index.php?topic=14446.msg318877#msg318877 |archive-date=28 March 2018 |website=NASAspaceflight.com}}</ref> NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems.<ref>{{Cite web |title=Frequently Asked Questions, question 3 |url=http://www.nasa.gov/exploration/about/faq.html#3 |url-status=live |archive-url=https://web.archive.org/web/20100112151132/http://www.nasa.gov/exploration/about/faq.html#3 |archive-date=12 January 2010 |publisher=National Aeronautics and Space Administration ESMD}}</ref><ref>{{Cite web |last=Bearden |first=David A. |last2=Skratt |first2=John P. |last3=Hart |first3=Matthew J. |date=1 June 2009 |title=Human Rated Delta IV Heavy Study Constellation Impacts |url=http://www.nasa.gov/pdf/377875main_081109%20Human%20Rated%20Delta%20IV.pdf |url-status=live |archive-url=https://web.archive.org/web/20170228145012/https://www.nasa.gov/pdf/377875main_081109%20Human%20Rated%20Delta%20IV.pdf |archive-date=28 February 2017 |publisher=National Aeronautics and Space Administration |page=8}}</ref>

== Variants ==
* RS-68 is the original version. It produces {{convert|663000|lbf|kN|lk=on|sigfig=3}} thrust at sea level.<ref>{{Cite web |date=October 2005 |title=RS-68 Propulsion System |url=http://www.pw.utc.com/StaticFiles/Pratt%20&%20Whitney%20New/Media%20Center/Assets/1%20Static%20Files/Docs/pwr_RS-68.pdf |url-status=dead |archive-url=https://web.archive.org/web/20180714135306/http://www.pw.utc.com/StaticFiles/Pratt%20%26%20Whitney%20New/Media%20Center/Assets/1%20Static%20Files/Docs/pwr_RS-68.pdf |archive-date=14 July 2018 |access-date=7 April 2019 |publisher=Pratt & Whitney Rocketdyne}}</ref>
* RS-68A is an improved version. It produces {{convert|705000|lbf|kN|abbr=on|sigfig=3}} thrust at sea level and {{convert|800000|lbf|kN|abbr=on|sigfig=3}} thrust in a vacuum.<ref>{{Cite web |title=P&W Successfully Completes Hot-Fire Test on 2nd RS-68A Certi |url=http://www.asdnews.com/news/32037/P&W_Successfully_Completes_Hot-Fire_Test_on_2nd_RS-68A_Certification_Engine.htm |url-status=live |archive-url=https://web.archive.org/web/20110725014233/http://www.asdnews.com/news/32037/P%26W_Successfully_Completes_Hot-Fire_Test_on_2nd_RS-68A_Certification_Engine.htm |archive-date=25 July 2011 |access-date=25 April 2018 |website=asdnews.com}}</ref> Its specific impulse in a vacuum is {{convert|411.9|isp}}.<ref>{{Cite web |date=June 2013 |title=Delta IV User's Guide |url=http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf |url-status=live |archive-url=https://web.archive.org/web/20140710005717/http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf |archive-date=10 July 2014 |access-date=13 July 2014 |publisher=United Launch Alliance}}</ref> Certification was completed in April 2011.<ref>{{Cite web |title=RS-68A {{!}} L3Harris® Fast. Forward. |url=https://www.l3harris.com/all-capabilities/rs-68a |access-date=6 February 2024 |website=www.l3harris.com |language=en}}</ref>
* RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program.<ref name=CaLV /> The Ares V was to use six RS-68B engines on a {{convert|10|m}} diameter core stage, along with two 5.5-segment solid rocket boosters. It was later determined that the ablative nozzle of the RS-68 was poorly suited to this multi-engine environment, causing reduced engine efficiency and extreme heating at the base of the vehicle.<ref>{{Cite web |date=June 2013 |title=The engines that refused to retire – RS-25s prepare for SLS testing |url=http://www.nasaspaceflight.com/2013/06/engines-refused-retire-rs-25s-prepare-sls-testing/ |url-status=live |archive-url=https://web.archive.org/web/20170830160316/https://www.nasaspaceflight.com/2013/06/engines-refused-retire-rs-25s-prepare-sls-testing/ |archive-date=30 August 2017 |website=nasaspaceflight.com}}</ref>

== See also ==
* [[Comparison of orbital rocket engines]]
* [[M-1 (rocket engine)]]
* [[National Launch System]]
* [[Rocketdyne J-2]]
* [[RS-83]]
* [[Space Launch Initiative]]
* [[TR-106]]

== References ==
{{reflist}}

== External links ==
{{Commons category|RS-68 (rocket engine)}}
*[https://www.rocket.com/space/liquid-engines/rs-68a Aerojet Rocketdyne's RS-68 page] {{Webarchive|url=https://web.archive.org/web/20230108171304/https://www.rocket.com/space/liquid-engines/rs-68a |date=8 January 2023 }}
*[https://web.archive.org/web/20020626221732/http://www.astronautix.com/engines/rs68.htm RS-68 page on Astronautix.com]
*{{Cite conference |last=Wood |first=B.K. |year=2002 |title=Propulsion for the 21st Century—RS-68 |url=http://www.pwrengineering.com/dataresources/PropulsionForThe21stCentury-RS-68.doc |format=doc |conference=38th Joint Liquid Propulsion Conference |location=Indianapolis, Indiana |publisher=AIAA |archive-url=https://web.archive.org/web/20090319075127/http://www.pwrengineering.com/dataresources/PropulsionForThe21stCentury-RS-68.doc |archive-date=19 March 2009 |url-status=dead}}

{{Rocket engines}}
{{Project Constellation}}

[[Category:Rocket engines using hydrogen propellant]]
[[Category:Rocketdyne engines]]
[[Category:Rocket engines using the gas-generator cycle]]
[[Category:Rocket engines of the United States]]