HG-3 (rocket engine)

{{Short description|Predecessor to the RS-25 thrust machine}}

{{Infobox rocket engine

|name =HG-3

|image =MS-II-2 stage.png

|image_size =

|caption =MS-II-2 stage diagram, using five HG-3 engines

|country_of_origin=United States

|date =

|first_date =

|last_date =

|designer =MSFC/Rocketdyne

|manufacturer =Rocketdyne

|purpose =Upper stage engine

|associated =Saturn MLV
Saturn IB-B
Saturn V/4-260
Saturn INT-17

|predecessor =J-2

|successor =RS-25

|status =Canceled

|type =liquid

|oxidiser =Liquid oxygen

|fuel =Liquid hydrogen

|mixture_ratio =

|cycle =

|pumps =

|description =

|combustion_chamber=

|nozzle_ratio =

|thrust =

|thrust_at_altitude=

|thrust(Vac) ={{Convert|1400.7|kN|abbr=on}}

|thrust(SL) ={{Convert|869.6|kN|abbr=on}}

|thrust_to_weight=

|chamber_pressure=

|specific_impulse=

|specific_impulse_vacuum={{convert|451|isp}}

|specific_impulse_sea_level={{convert|280|isp}}

|total_impulse =

|burn_time =

|capacity =

|dimensions =

|length =

|diameter =

|dry_weight =

|used_in =

|references ={{cite web|url=http://www.astronautix.com/engines/hg3.htm|publisher=Encyclopedia Astronautica|title=HG-3|accessdate=18 February 2012|date=17 November 2011|author=Mark Wade|url-status=dead|archiveurl=https://web.archive.org/web/20111115163306/http://www.astronautix.com/engines/hg3.htm|archivedate=15 November 2011}}{{cite web|url=http://www.astronautix.com/engines/hg3sl.htm|publisher=Encyclopedia Astronautica|title=HG-3-SL|accessdate=18 February 2012|date=17 November 2011|author=Mark Wade|url-status=dead|archiveurl=https://web.archive.org/web/20111117134837/http://www.astronautix.com/engines/hg3sl.htm|archivedate=17 November 2011}}

|notes =

}}

The HG-3 was a liquid-fuel cryogenic rocket engine which was designed for use on the upper stages of Saturn rockets in the post-Apollo era. Designed in the United States by Rocketdyne, the HG-3 was to have burned cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing {{convert|1400.7|kN|abbr=on|sigfig=3}} of thrust during flight. The engine was designed to produce a specific impulse (I_sp) of {{convert|451|isp}} in a vacuum, or {{convert|280|isp}} at sea level.

Developed from Rocketdyne's J-2 engine used on the S-II and S-IVB stages, the engine was intended to replace the J-2 on the upgraded MS-II-2 and MS-IVB-2 stages intended for use on the Saturn MLV, Saturn IB-B and Saturn V/4-260 rockets, with a sea-level optimised version, the HG-3-SL, intended for use on the Saturn INT-17. The engine was cancelled, however, during the post-Apollo drawdown when development of the more advanced Saturn rockets ceased, and never flew, although the engine was later used as the basis for the design of the RS-25 engine.{{cite web|url=http://history.msfc.nasa.gov/saturn_apollo/propulsion_center.html|publisher=NASA|accessdate=13 December 2011|title=MSFC Propulsion Center of Excellence is Built on Solid Foundation|year=1995|archive-url=https://web.archive.org/web/20150427090253/http://history.msfc.nasa.gov/saturn_apollo/propulsion_center.html|archive-date=27 April 2015|url-status=dead}}