XS-1 (spacecraft)

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The DARPA XS-1 was an experimental spaceplane/booster with the planned capability to deliver small satellites into orbit for the U.S. Military.[1] It was reported to be designed to be reusable as frequently as once a day, with a stated goal of doing so for 10 days straight.[2] The XS-1 was intended to directly replace the first stage of a multistage rocket by taking off vertically and flying to hypersonic speed and high suborbital altitude, enabling one or more expendable upper stages to separate and deploy a payload into low Earth orbit. The XS-1 would then return to Earth, where it could ostensibly be serviced fast enough to repeat the process at least once every 24 hours.[3][4]

The DARPA XS-1 program operated 2013–2020.[5] After several years of refinement and proposals, in May 2017, DARPA selected Boeing for Phase 2/3 to build and test an XS-1 spacecraft (now called the Experimental Spaceplane program).[6] At the time, test flights were scheduled to start no earlier than 2020.[6] On 22 January 2020, it was announced that Boeing was ceasing its role in the program, effectively ending it.[7]


The XS-1 program followed several previous failed attempts to develop a reusable space launch vehicle. The Rockwell X-30 in the 1980s and X-33 VentureStar in the 1990s never flew because of immature technologies. DARPA's last attempt was the Responsive Access, Small Cargo, Affordable Launch (RASCAL) program in the early 2000s with the goal of placing 300 lb (140 kg) payloads in orbit for less than $750,000.

The XS-1 program was announced in November 2013 at a DARPA industry day. DARPA stated that the XS-1 was more feasible due to better technologies, including light and low-cost composite airframe and tank structures, durable thermal protection, reusable and affordable propulsion, and aircraft-like health management systems.[5] Jess Sponable, the XS-1 program manager, spoke on February 5, 2014, at NASA's Future In-Space Operations group, stating, "The vision here is to break the cycle of escalating space system costs, enable routine space access and hypersonic vehicles."[8]

By July 2014, three companies were awarded contracts to design a demonstration vehicle. The selected companies were Boeing with Blue Origin, Masten Space Systems with XCOR Aerospace, and Northrop Grumman with Virgin Galactic. Unlike other DARPA programs that were handed off to parts of the United States military once proven successful, this initiative was designed from the start to be a direct partnership between the agency and industry. In August 2015, Boeing, Northrop Grumman, and Masten Space Systems all received additional funding from DARPA to continue their design concepts for Phase 1B of the program. As of 2015, the first XS-1 orbital mission was planned to occur as early as 2020.[9]

DARPA began Phase 2 of the XS-1 program in April 2016.[10] In July 2016, DARPA stated that they believed "the time is right for a renewed effort, one that began in 2013/14, but [in 2016 was] ramped up through a solicitation process, allowing for several industry concepts to be created. Per the [solicitation] requirements, the winged craft [requirements would continue to need to] be capable of performing 10 flights in 10 days, with a payload capacity greater than 3,000 lbs for a cost of less than $5 million USD per flight."[11]

In May 2017, DARPA selected Boeing for Phase 2/3 to build and test the XS-1 (now called the Experimental Spaceplane program).[6] The phase 2/3 contract included $146 million in DARPA funding and an unspecified contribution by the company.[12]

On 22 January 2020, DARPA announced that Boeing was pulling out of the XS-1 program "immediately" and effectively ending the program.[7][12]

Program goals[edit]

The goals of the program as of September 2013 were:[3][13] The space plane must carry a 3,000–5,000 lb (1,400–2,300 kg) payload to low Earth orbit for less than a cost of US$5 million per flight,[4] at a rate of 10 or more flights per year; currently, launching that type of payload requires using an Orbital Sciences Corporation Minotaur IV expendable booster, priced at $55 million once per year.

  • hypersonic flight to Mach 10 (12,300 km/h) or higher
  • fast one-day turnaround time, including flying 10 times in 10 days
  • a 1,800 kg (4,000 lb) payload on a trajectory to orbit
  • launch cost less than 1/10 that of current launch systems, approximately US$5 million per flight[4]
  • uncrewed vehicle
  • utilize a reusable first stage booster to fly at hypersonic speeds to a suborbital altitude, coupled with one or more expendable upper stages that would separate and deploy a satellite[14][15]

Entrants and selection[edit]

Boeing, Northrop Grumman Aerospace Systems, and Masten Space Systems have Phase 1 conceptual design contracts.

Boeing performed trade studies with Blue Origin initially. Boeing's design would allow the autonomous booster to carry the second stage and payload to high altitude and deploy them into space. The booster would then return to Earth, where it could be quickly prepared for the next flight by applying operation and maintenance principles similar to modern aircraft.[16]

Northrop Grumman used its aircraft, spacecraft, and autonomous systems experience to work with its team consisting of Scaled Composites to lead fabrication and assembly, and Virgin Galactic to head commercial spaceplane operations and transition; Virgin Galactic and Scaled Composites both worked on the SpaceShip Two, the world's only commercial spaceline. The team also leveraged technologies developed during related projects for DARPA, NASA, and the U.S. Air Force Research Laboratory to give the government "return on those investments." Their concept included a clean-pad launch[clarification needed] using a transporter erector launcher with minimal infrastructure and ground crews, highly autonomous flight operations, and horizontal landing and recovery on standard runways.[14]

Masten Space Systems has experience in rapid reusable rocket-powered vehicles, with their Xombie, Xoie, and Xaero vertical takeoff, vertical landing (VTVL) designs having already met or exceeded the 10 flights in 10 days objective set by the program. Although the company consists of approximately 30 employees and is headquartered in a small building at the Mojave Air and Space Port, they have spent years flying various small VTVL systems on short hops at the spaceport, serving as test beds for guidance, navigation, and control (GNC) systems designed to safely land spacecraft on the Moon and potentially other planets. Their concept showed a VTVL system taking off vertically from a launch pad with wings and a tail fin. Masten Space Systems was partnered with XCOR Aerospace for Phase 1A.[17]

Phase 2 & 3

In May 2017 Boeing was selected to partner with DARPA to build the XS-1.[18] Aerojet Rocketdyne was to provide AR-22 engines, derived from the RS-25 engine, for the spacecraft.[19] The phase 2/3 contract to build and fly the prototype included US$146 million of DARPA funding.[12][clarification needed]

Boeing XS-1 Phantom Express[edit]

A rendering of Boeing's XS-1 Phantom Express launch vehicle on LC-48

The Boeing design was a vertical takeoff, horizontal landing (VTHL) craft[6] called Phantom Express.[20] The planned specifications include a vehicle height of 100 feet (30 m), with a 62 foot (19 m) wingspan. The Phantom express was to use an Aerojet Rocketdyne AR-22 engine, which was originally built for the Space Shuttle program, but has been modified to be reused ten times within ten days, for less than $5 million per launch. This performance requirement was demonstrated on a test stand in July 2018.[21] On 22 January 2020, it was announced that Boeing was ceasing its role in the program, effectively[why?] ending it.[7]

See also[edit]


  1. ^ David Axe (2015-08-03). "Pentagon Preps for Orbital War With New Spaceplane". The Daily Beast. Retrieved 2015-08-03.
  2. ^ "Experimental Spaceplane (XS-1)". DARPA. Archived from the original on 2016-06-16. Retrieved 2016-06-20.
  3. ^ a b Foust, Jeff (2013-09-12). "DARPA To Start Reusable Launch Vehicle Program". Space News. Archived from the original on September 13, 2013. Retrieved 2013-09-13.
  4. ^ a b c Howell, Elizabeth (1 May 2015). "XS-1: DARPA's Experimental Spaceplane". Space.com. Retrieved 2015-05-14.
  5. ^ a b "Darpa Targets Lower Launch Costs With XS-1 Spaceplane". Aviation Week. 2 December 2013.
  6. ^ a b c d "DARPA Picks Design for Next-Generation Spaceplane" May 2017
  7. ^ a b c "Boeing drops out of DARPA Experimental Spaceplane program". Space News, 22 January 2020.
  8. ^ "US Military Space Plane aims for 2017 lift off". spacedaily.com. Retrieved 2014-03-21.
  9. ^ DARPA Awards $20 Million for Continued Development of a Military Space Plane - Defense-Update.com, 8 August 2015
  10. ^ XS-1 Program to Ease Access to Space Enters Phase 2 - DARPA.mil, 7 April 2016
  11. ^ Gebhardt, Chris (2016-07-13). "DARPA pushing new effort with Experimental Spaceplane, XS-1". NASASpaceFlight.com. Retrieved 2016-08-31.
  12. ^ a b c "Boeing’s Phantom Express Vanishes into Thin Air". parabolicarc.com, 22 Jan 2020.
  13. ^ "DARPA fires up XS-1 space plane quest | Cutting Edge". CNET News. Retrieved 2014-03-21.
  14. ^ a b Northrop Grumman Developing XS-1 Spaceplane For DARPA - Spacedaily.com, 20 August 2014
  15. ^ DARPA issues first-phase solicitation for XS-1 hypersonic space plane for deploying satellites - Militaryaerospace.com, 15 November 2013
  16. ^ Clark, Stephen (13 June 2017). "Boeing, DARPA to base XS-1 spaceplane at Cape Canaveral". Spaceflight Now. Retrieved 11 February 2020.
  17. ^ Masten Space Systems Aims High on XS-1 Military Space Plane Project - Space.com, 26 August 2014
  18. ^ "Boeing is building DARPA's new hypersonic space plane". Engadget. 24 May 2017. Retrieved 11 February 2020.
  19. ^ "Aerojet Rocketdyne Selected As Main Propulsion Provider for Boeing and DARPA Experimental Spaceplane". 24 May 2017. Retrieved 25 May 2017.
  20. ^ "Boeing: Phantom Express". www.boeing.com. Retrieved 19 July 2018.
  21. ^ Clark, Stephen (July 18, 2018). "Rapid-fire engine tests raise hopes for DARPA's planned reusable spaceplane – Spaceflight Now". spaceflightnow.com. Pole Star Publications Ltd. Retrieved 19 July 2018.

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