Editing SpaceShipTwo (section)

=== Rocket engine ===
The [[Hybrid rocket|hybrid rocket engine]] design for SpaceShipTwo was problematic and caused extensive delays to the [[flight test]] program. The original rocket engine design was based on [[hydroxyl-terminated polybutadiene]] (HTPB) fuel and [[nitrous oxide]] oxidizer, sometimes referred to as an [[Nitrous oxide|N<sub>2</sub>O]]/[[Hydroxyl-terminated polybutadiene|HTPB]] engine.<ref>{{cite web |title=Propulsion Systems: multiple-burn, green and low-cost |url=http://sncspace.com/pdfs/Propulsion%20Systems_FINAL_web.pdf |publisher=Sierra Nevada |access-date=8 March 2013 |archive-url=https://web.archive.org/web/20140323035056/http://www.sncspace.com/pdfs/Propulsion%20Systems_FINAL_web.pdf |archive-date=23 March 2014 |url-status=dead }}</ref><ref>{{cite web|title=Safe Hybrid Rocket |url=http://www.virgingalactic.com/overview/safety/|year=2013|publisher=Virgin Galactic|access-date=8 March 2013|url-status=dead|archive-url=https://web.archive.org/web/20130304220709/http://www.virgingalactic.com/overview/safety|archive-date=4 March 2013}}</ref> It was developed by Scaled Composites subcontractor [[Sierra Nevada Corporation]] (SNC) from 2009 to early 2014. In May 2014, Virgin Galactic announced a change to the hybrid engine to be used in SpaceShipTwo, and took the development effort in-house to Virgin Galactic, terminating the contract with Sierra Nevada and halting all development work on the first-generation [[rocket engine]].<ref name=sncpr20141031>{{cite web |title=SNC Statement in Response to Inquiries Regarding 31 October 2014 Virgin Galactic SpaceShipTwo Incident |url=http://www.sncspace.com/press_more_info.php?id=417 |access-date=1 November 2014 |publisher=Sierra Nevada Corporation |date=31 October 2014 |archive-url=https://web.archive.org/web/20141101025107/http://www.sncspace.com/press_more_info.php?id=417 |archive-date=1 November 2014 |url-status=dead }}</ref> Virgin then modified the engine design to include a change of the hybrid rocket fuel from a HTPB to a polyamide fuel formulation. In October 2015, Virgin announced that it was considering changing back to the original HTPB fuel.<ref>{{cite web |url=http://spacenews.com/virgin-galactic-switching-back-to-rubber-fuel-for-spaceshiptwo/ |title=SpaceShipTwo Bounces Back to Rubber Fuel |first=Jeff |last=Foust |date=14 October 2015 |access-date=16 October 2015 |publisher=SpaceNews}}</ref>

==== 2014 change of engine ====
Rather than the rubber-based HTPB-fuel engine—engines that had experienced serious engine stability issues on firings longer than approximately 20 seconds — the engine would instead be based on a solid fuel composed of a type of plastic called [[thermoplastic]] [[polyamide]]. The plastic fuel was projected to have better performance (by several unspecified measures) and was projected to allow SpaceShipTwo to make flights to a higher altitude.<ref name=nsj20140524>{{cite news |last=Foust |first=Jeff |title=Virgin Galactic changes fuels as it prepares for its next round of test flights |url=http://www.newspacejournal.com/2014/05/24/virgin-galactic-changes-fuels-as-it-prepares-for-its-next-round-of-test-flights/ |access-date=25 May 2014 |newspaper=NewSpace Journal |date=24 May 2014 }}</ref><ref name=nbc20140523>{{cite news|last=Boyle|first=Alan|title=Virgin Galactic Makes a Switch in SpaceShipTwo's Rocket Motor|url=https://www.nbcnews.com/storyline/virgin-voyage/virgin-galactic-makes-switch-spaceshiptwos-rocket-motor-n113216|access-date=24 May 2014|publisher=NBC News|date=23 May 2014}}</ref><ref name=aw20140524>{{cite news|title=New Fuel to Boost SpaceShip Two |url=http://www.aviationweek.com/space/new-fuel-boost-spaceship-two|publisher=Aviation Week|date=24 May 2014|last=Morris|first=Guy|access-date=10 December 2015}}</ref>

{{As of|2014|05}} when the version 2 engine by Virgin Galactic was publicly announced, the engine had already completed full-duration burns of over 60 seconds in ground tests on an engine test stand.<ref name=nbc20140523/> The second-generation engine design also required the modification to the SS2 [[airframe]] to fit additional tanks in the wings of SpaceShipTwo — one holding [[methane]] and the other containing [[helium]] — in order to ensure a proper burn and shut-down of the new engine.<ref name=pa20140630>{{cite news|last1=Messier|first1=Doug|title=WhiteKnightTwo in the Air Over Mojave Today |url=http://www.parabolicarc.com/2014/06/30/whiteknighttwo-air-mojave-today/|access-date=1 November 2014|publisher=Parabolic Arc|date=30 June 2014}}</ref> Additional ground tests were performed on the new engine between May and October 2014.

==== 2015: another fuel change ====
Following a series of rocket engine tests, Virgin announced in October 2015 that they would be changing the rocket motor back to [[hydroxyl-terminated polybutadiene]] (HTPB), with a similar formulation as they used earlier in the development program before switching to a nylon-based fuel grain. They would use HTPB to power the SpaceShipTwo when it would resume flight following the loss of the initial SS2 test vehicle in October 2014. Full qualification tests remained to be completed at the time.<ref>[http://spacenews.com/virgin-galactic-switching-back-to-rubber-fuel-for-spaceshiptwo/ SpaceShipTwo Bounces Back to Rubber Fuel] SpaceNews, 14 October 2015, accessed 26 November 2015</ref>

==== RocketMotorTwo ====
{{Main|RocketMotorTwo}}

The second-generation RocketMotorTwo engine was a variant of the earlier SNC basic design, but was fueled by polyamide plastic fuel, while continuing to use the same nitrous oxide oxidizer. The second-generation engine was made in-house by Virgin Galactic rather than by SNC. By December 2012, 15 full-scale tests had been successfully conducted,<ref name="TestFires09-12">[http://www.scaled.com/projects/rocketmotortwo_hot-fire_test_summaries RocketMotorTwo Hot-Fire Test Summaries] {{Webarchive|url=https://web.archive.org/web/20120426095422/http://www.scaled.com/projects/rocketmotortwo_hot-fire_test_summaries |date=26 April 2012 }} Scaled.com Updated 9 August 2012 Retrieved 16 December 2012</ref><ref name=June2012Tests/> and additional ground tests continued into March 2013.<ref>{{cite web|url=https://www.virgin.com/richard-branson/blog/this-isnt-sci-fi|title=This isn't sci-fi|publisher=Virgin.com|date=5 March 2013|access-date=5 March 2013|url-status=dead|archive-url=https://web.archive.org/web/20130307153321/http://www.virgin.com/richard-branson/blog/this-isnt-sci-fi|archive-date=7 March 2013}}</ref> In June 2012, the [[Federal Aviation Administration]] (FAA) issued a rocket testing permit to Scaled Composites, allowing it to begin SS2 test flights powered by RocketMotorTwo;<ref name=FAAPermit2012>{{cite web |url=http://www.flyingmag.com/news/spaceshiptwo-gets-thumbs-rocket-powered-flights|title=SpaceShipTwo Gets Thumbs Up for Rocket-Powered Flights|publisher=Flying Magazine|date=1 June 2012}}</ref> the first such powered flight took place on 29 April 2013.<ref name=scaledsum130429/> The engine produced {{cvt|270|kN}} of thrust.