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Artist's rendition of the X-33 in orbit.
FunctionUncrewed re-usable spaceplane technology demonstrator
ManufacturerLockheed Martin
Country of originUnited States
Project cost$922 million NASA + $357 million Lockheed Martin[1]
Height20 m (66 ft)[2]
Mass129,000 kg (285,000 lb)[2]
Launch history
Engine details
Powered by2 XRS-2200 linear aerospikes[1]
Maximum thrust1,800 kN (410,000 lbf)[1]

The Lockheed Martin X-33 was a proposed uncrewed, sub-scale technology demonstrator suborbital spaceplane that was developed for a period in the 1990s. The X-33 was a technology demonstrator for the VentureStar orbital spaceplane, which was planned to be a next-generation, commercially operated reusable launch vehicle. The X-33 would flight-test a range of technologies that NASA believed it needed for single-stage-to-orbit reusable launch vehicles (SSTO RLVs), such as metallic thermal protection systems, composite cryogenic fuel tanks for liquid hydrogen, the aerospike engine, autonomous (uncrewed) flight control, rapid flight turn-around times through streamlined operations, and its lifting body aerodynamics.[3]

Failures of its 21-meter wingspan and multi-lobed, composite-material fuel tank during pressure testing ultimately led to the withdrawal of federal support for the program in early 2001. Lockheed Martin has conducted unrelated testing, and has had a single success after a string of failures as recently as 2009 using a 2-meter scale model.[4]


In 1994 NASA initiated the Reusable Launch Vehicle (RLV) program. After a Phase I programme developing proposals from Rockwell International, McDonnell Douglas, and Lockheed Martin, a Phase II contract to develop the X-33 as a demonstrator vehicle was awarded to Lockheed Martin in 1996.[5] At the same time Orbital Sciences was awarded a contract to develop the X-34, an air-launched hypersonic research vehicle.[5]

The goals of the RLV program were:[5]

$1 billion was spent through 1999 with about 80 percent coming from NASA and additional money contributed by the industry partners.[5] The goal was to have a first flight by March 1999, and to have the VentureStar, the operational resusable space vehicle, flying in 2006.[5]

.. to build a vehicle that takes days, not months, to turn around; dozens, not thousands, of people to operate; with launch costs that are a tenth of what they are now. Our goal is a reusable launch vehicle that will cut the cost of getting a pound of payload to orbit from $10,000 to $1,000.

— D. Goldin NASA administrator on the RLV program[5]


The program was cancelled in February 2001.[6]

Construction of the prototype was some 85% assembled with 96% of the parts and the launch facility 100%[7] complete when the program was canceled by NASA in 2001, after a long series of technical difficulties including flight instability and excess weight.

In particular, the composite liquid hydrogen fuel tank failed during testing in November 1999. The tank was constructed of honeycomb composite walls and internal structures to reduce its weight. A lighter tank was needed for the craft to demonstrate necessary technologies for single-stage-to-orbit operations. A hydrogen fueled SSTO craft's mass fraction requires that the weight of the vehicle without fuel be 10% of the fully fueled weight. This would allow a vehicle to fly to low Earth orbit without the need for the sort of external boosters and fuel tanks used by the Space Shuttle. But, after the composite tank failed on the test stand during fueling and pressure tests, NASA came to the conclusion that the technology of the time was simply not advanced enough for such a design. While the composite tank walls themselves were lighter, the hydrogen tank shape necessary to fit inside the aerodynamic mouldline resulted in complex joints increasing the total mass of the composite tank to above that of an aluminum-based tank, and too heavy for an SSTO vehicle.[8]

Microcracking problem discovered in the liquid-hydrogen (LH2) multi-lobed tank core by NASA scientists at Goddard Space Flight Center ultimately caused NASA to cancel the X-33 program

NASA had invested $922 million in the project before cancellation, and Lockheed Martin a further $357 million. Due to changes in the space launch business—including the challenges faced by companies such as Globalstar, Teledesic, and Iridium and the resulting drop in the anticipated number of commercial satellite launches per year, Lockheed Martin concluded that continuing development of the X-33 privately without government support would not be profitable.[citation needed]

In 2004 Northrop Grumman successfully built and tested a simple cylindrical composite cryogenic hydrogen tank as part of early work for the Constellation program.[9]

Design and development

X-33 model being prepared for testing in a wind-tunnel in 1997
Test of the X-33's thermal protection system, 1998
Space art of X-33 in orbit
Simulated in-flight view of the X-33.

Through the use of the lifting body shape, composite multi-lobed liquid fuel tanks, and the aerospike engine, NASA and Lockheed Martin hoped to test fly a craft that would demonstrate the viability of a single-stage-to-orbit (SSTO) design. A spacecraft capable of reaching orbit in a single stage would not require external fuel tanks or boosters to reach low Earth orbit. Doing away with the need for "staging" with launch vehicles, such as with the Shuttle and the Apollo rockets, would lead to an inherently more reliable and safer space launch vehicle. While the X-33 would not approach airplane-like safety, the X-33 would attempt to demonstrate 0.997 reliability, or 3 mishaps out of 1,000 launches, which would be an order of magnitude more reliable than the Space Shuttle. The 15 planned experimental X-33 flights could only begin this statistical evaluation.

X-33 launch facility already completed at Edwards Air Force Base

The uncrewed craft would have been launched vertically from a specially designed facility constructed on Edwards Air Force Base,[7] and landed horizontally (VTHL) on a runway at the end of its mission. Initial sub-orbital test flights were planned from Edwards AFB to Dugway Proving Grounds southwest of Salt Lake City, Utah.

Once those test flights were completed, further flight tests were to be conducted from Edwards AFB to Malmstrom AFB in Great Falls, Montana, to gather more complete data on aircraft heating and engine performance at higher speeds and altitudes.[citation needed]

On July 2, 1996, NASA selected Lockheed Martin Skunk Works of Palmdale, California, to design, build, and test the X-33 experimental vehicle for the RLV program. Lockheed Martin's design concept for the X-33 was selected over competing concepts from Rockwell International and McDonnell Douglas. Rockwell proposed a Space Shuttle-derived design, and McDonnell Douglas proposed a design based on its vertical takeoff and landing (VTVL) DC-XA test vehicle.[citation needed]

The uncrewed X-33 was slated to fly 15 suborbital hops to near 75.8 km altitude.[10] It was to be launched upright like a rocket and rather than having a straight flight path it would fly diagonally up for half the flight, reaching extremely high altitudes, and then for the rest of the flight glide back down to a runway.

The X-33 was never intended to fly higher than an altitude of 100 km, nor faster than one-half of orbital velocity. Had any successful tests occurred, extrapolation would have been necessary to apply the results to a proposed orbital vehicle.[10]

The decision to design and build the X-33 grew out of an internal NASA study titled "Access to Space".[11] Unlike other space transport studies, "Access to Space" was to result in the design and construction of a vehicle.

Commercial spaceflight

Based on the X-33 experience shared with NASA, Lockheed Martin hoped to make the business case for a full-scale SSTO RLV, called VentureStar, that would be developed and operated through commercial means. The intention was that rather than operate space transport systems as it has with the Space Shuttle, NASA would instead look to private industry to operate the reusable launch vehicle and NASA would purchase launch services from the commercial launch provider. Thus, the X-33 was not only about honing space flight technologies, but also about successfully demonstrating the technology required to make a commercial reusable launch vehicle possible.[citation needed]

The VentureStar was to be the first commercial aircraft to fly into space. The VentureStar was intended for long inter-continental flights and supposed to be in service by 2012, but this project was never funded or begun.[citation needed]

Aerospike engine test at Stennis Space Center, August 6, 2001

General characteristics


Continued research

After the cancellation in 2001, engineers were able to make a working liquid-oxygen tank from carbon-fiber composite.[12] Tests showed that composites were feasible materials for liquid-oxygen tanks[13]

On September 7, 2004, Northrop Grumman and NASA engineers unveiled a liquid-hydrogen tank made of carbon-fiber composite material that had demonstrated the ability for repeated fuelings and simulated launch cycles.[14] Northrop Grumman concluded that these successful tests have enabled the development and refinement of new manufacturing processes that will allow the company to build large composite tanks without an autoclave; and design and engineering development of conformal fuel tanks appropriate for use on a single-stage-to-orbit vehicle.[15]

The alternative proposals

Five companies expressed interest and proposed concepts. Of those five Lockheed Martin, Rockwell and McDonnell Douglas were selected for workup into more detailed proposals.[16]


Rockwell proposed a Space Shuttle-derived design.[17] It would have used one Space Shuttle Main Engine (SSME) and two RL-10-5A engines.[18]: 49 

In a subsequent full-scale system to reach orbit Rockwell planned to use six Rocketdyne RS-2100 engines.[18]: 49 

McDonnell Douglas

McDonnell Douglas featured a design using liquid oxygen/hydrogen bell engines based on its vertical takeoff and landing DC-XA test vehicle.[6] It would have used a single SSME for the main propulsion system.[18]: 47 [19]

See also


  1. ^ a b c Mark Wade. "X-33". Encyclopedia Astronautica. Archived from the original on February 15, 2017. Retrieved February 25, 2015.
  2. ^ a b Wikisource:X-33 Advanced Technology Demonstrator
  3. ^ Conner, Monroe (March 30, 2016). "Lockheed Martin X-33". NASA. Retrieved April 9, 2023.
  4. ^ David, Leonard (October 15, 2009). "Reusable rocket plane soars in test flight". NBC News. Retrieved October 27, 2009.
  5. ^ a b c d e f "Reusable Launch Vehicle". NASA Spinoff. Archived from the original on February 24, 2013.
  6. ^ a b "X-33 Proposal by McDonnell Douglas - Computer Graphic". DVIDS.
  7. ^ a b "X-33 Launch Complex (Area 1-54)" (PDF). USAF. Archived from the original (PDF) on June 5, 2011. Retrieved June 30, 2011.
  8. ^ Bergin, Chris (January 4, 2006). "X-33/VentureStar — What really happened". NASA Space Flight.
  9. ^ ""Northrop Grumman, NASA Complete Testing of Prototype Composite Cryogenic Fuel Tank"". September 7, 2004. Archived from the original on January 10, 2017.
  10. ^ a b "Environmental Impact Statement, Notice of Intent 96-118". NASA. October 7, 1996. Flight tests would involve speeds of up to Mach 15 and altitudes up to approximately 75,800 meters... The test program was baselined for a combined total of 15 flights.
  11. ^ "The Policy Origins of the X-33". NASA. September 23, 1998. Archived from the original on October 22, 2014. Retrieved February 16, 2011.
  12. ^ Graf, Neil (2001). "Subscale Composite Liquid Oxygen Tank Testing" (PDF). High-Performance Composites.
  13. ^ Graf, Neil (2001). "Oxygen Compatibility Testing of Composite Materials" (PDF). High-Performance Composites.
  14. ^ Northrop Grumman. "Northrop Grumman, NASA Complete Testing of Prototype Composite Cryogenic Fuel Tank", News Releases, September 7, 2004, accessed January 9, 2017.
  15. ^ Black, Sara (November 2005). "An update on composite tanks for cryogens". High-Performance Composites.
  16. ^ "X-33". Archived from the original on December 28, 2016.
  17. ^ Rockwell International X-33 Technology Demonstrator Proposal image
  18. ^ a b c "RAND 1995 Project Air Force Workshop on transatmospheric vehicles - chapter 3" (PDF).
  19. ^ Full 1995 RAND TAV workshop proceedings