Skunk Works's X-33 and VentureStar
Lockheed Martin's X-33 was a technology demonstrator of the VentureStar orbital spaceplane funded by NASA in the 1990s. Part of the Space Launch Initiative, the X-33's ultimate goal was to prove the feasibility of a completely reusable single-stage-to-orbit (SSTO) launch vehicle.
Artist's depiction of the VentureStar in flight (NASA)
NASA initiated the Reusable Launch Vehicle (RLV) program in 1994 under the management of the Marshall Space Flight Center in Alabama. According to NASA, a full-scale RLV would lower the cost of putting a single pound of payload into orbit from $10,000 to only $1,000. In 1996, NASA accepted proposals from Lockheed Martin, McDonnell Douglas partnered with Boeing, and Rockwell International's Space Division for a sub-scale SSTO vehicle.
Each of these uncrewed, vertical-takeoff-horizontal-landing (VTHL) half-scale vehicle concepts needed to have low-cost maintainability while also having a rapid turn-around time between flights. In 1999, Lockheed Martin's Advanced Development Programs (aka Skunk Works) was selected to design, build, and fly the X-33 Advanced Technology Demonstrator, a concept similar to its Star Clipper Spaceplane.
The X-33 had a length of 69 feet (21 m), a wingspan of 68 feet (20.7 m), and a thrust at sea level of 410,000 pounds. Lockheed planned to incorporate new technologies into the X-33 to prove their feasibility in future SSTO RLV's; lightweight composite fuel tanks, a more advanced thermal protection system, automated flight control systems, and perhaps most famously, a linear aerospike engine. The aerospike is based on the J-2 engine used by the Saturn V moon rocket and was considered for use on the Space Shuttle. The spike adjusts to changing atmospheric pressure in order to maintain a more efficient combustion process. It also allows the engine to perform better over a wide range of altitudes when compared to traditional engines.
Traditional bell-shaped nozzle engine vs the linear aerospike engine (NASA Armstrong)
The X-33 would also pioneer a new thermal protection system, metallic panels placed over insulating materials. Test panels were strapped to the bottom of a NASA F-15 aircraft and were validated to protect the X-33 from temperatures up to 1,800° F. These new metallic panels were cheaper to maintain than the ceramic heat-absorbing tiles on the Space Shuttle, eventually rendering them obsolete. The vehicle's main propulsion system consisted of two J-2S aerospike engines, one aluminum liquid oxygen tank in the front, and two composite liquid oxygen tanks in the rear that were built to conform to the vehicle's shape a. If one engine was to fail during launch, the other would provide sufficient power to make a safe landing at a runway or abort site. The X-33 was meant to make only suborbital flights and reach an altitude of 50 miles.
The launch site at Edwards Air Force Base in California was 100% complete by the time the program was cancelled and would have supported the expected fifteen launches of the X-33.
The 50-acre complex was meant to service and launch the vehicle with a rapid turnaround time and ground crew of fewer than fifty people b. Landing sites for the X-33 and subsequent VentureStar included Malmstrom Air Force Base in Montana, Michael Army Air Field (part of the Utah Test and Training Range) south of Salt Lake City, and the Silurian Dry Lake Bed near Baker, California. The X-33 would be carried back to Edwards on the back of one of NASA's Boeing 747.
The complete X-33 launch complex in California (NASA)
As well as delays in developing the thermal protection system and aerospike engine, the X-33 experienced delays and failures in its composite fuel tanks. The tank was constructed of honeycomb-shaped walls to reduce its weight, which was critical for SSTO operations. In November of 1999, one of the tanks failed on the test stand during fueling and pressure tests. After micro-cracking was discovered in the tank while undergoing structural loads testing, it was determined that composite material technology was not developed enough to build cryogenic reusable tanks. Lockheed Martin proposed replacing the composite tanks with aluminum tanks to continue the X-33 program, but NASA "decided the benefits of testing the X-33 in suborbital flight without proving the critical composite technology did not justify the cost." b
While individually these delays and failures were not mission-killers, NASA felt the X-33's performance requirements demanded too many unproven technologies that could not be developed cheaply and reliably. The vehicle was about 75% completed when NASA ceased funding in March of 2001, effectively cancelling the related Orbital Sciences X-34 spaceplane as well. NASA's investment totaled $912 million with Lockheed Martin investing $357 million. Supporters of the X-33 within Congress proposed handing off the project to the US Air Force, but this did not progress further.
VentureStar, X-33's successor, was meant to be a cheap replacement for the Space Shuttle, which was planned to retire in 2010. This vehicle would be commercially operated and carry cargo as well as crew. VentureStar was essentially a larger version of the X-33 with seven aerospike engines rather than two. It would have had a height of 38.7 feet (11.8 m), a length of 127 feet (38.75 m), a wingspan of 68 feet (20.75 m), and a wing area of 2,125 feet squared (198 m squared). Its thrust at sea level would have been just over 3 million pounds.
NASA/Lockheed Martin model of VentureStar (Smithsonian)
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Gaudreaux, Ann C. "X-33 Metallic Heat Shield Ready for Flight." Langley Research Center, https://www.nasa.gov/centers/langley/news/releases/1999/Feb99/99-008.html
Krebs, Gunter. "X-33 Venture Star." Gunter's Space Page, https://space.skyrocket.de/doc_lau/x33.htm
"Lockheed Martin X-33 Reusable Launch Vehicle Prototype." aerospaceweb.org, https://aerospaceweb.org/aircraft/research/x33/
Wade, Mark. "X-33." Encyclopedia Astronautica. http://www.astronautix.com/x/x-33.html
"X-33 Advanced Technology Demonstrator." Marshall Space Flight Center, https://www.nasa.gov/centers/marshall/news/background/facts/x33.html