X-30 , aka National Aerospace Plane (NASP); 1986 – 1993; $2+ billion (much of it from the US military rather than NASA).



A very ambitious project, meant to use advanced air-breathing engines to take off from a runway and fly directly into orbit (or fly long distances within the atmosphere at extremely high speed).



It was eventually cancelled due to persistent technical problems and skyrocketing cost estimates. There was no replacement, although some of the technology work was continued by small research programmes, including the X-43, a hypersonic aircraft project that smashed the world record for aircraft speed in 2004 when an air-breathing plane flew briefly at nearly 10 times the speed of sound. (Illustration: NASA-LaRC)

Advanced Solid Rocket Motor (ASRM); 1986 – 1993; $2.2 billion. After the Challenger shuttle accident in 1986, NASA launched a project to replace the shuttle's solid rocket boosters – which had been the cause of the failure. The improved design was intended to be safer and somewhat more powerful, and would also be built in a government-owned factory so NASA wouldn't be at the mercy of a single contractor.



With work progressing and the factory mostly completed, Congress refused further funding due to spiralling costs and a slipping schedule.



ASRM's one crucial function – increasing the shuttle's lifting capacity in order to launch heavy space-station modules – was taken over by a lighter shuttle external fuel tank made of an aluminium-lithium alloy.



In this image, a small, 1.2-metre solid rocket motor tests design features for the ASRM at a NASA facility in 1991. (Image: NASA-MSFC)

Shuttle-C ; 1987 – 1990; perhaps $200 million.



An unmanned cargo derivative of the shuttle, Shuttle-C was designed to replace the shuttle orbiter with a cargo pod atop an engine module. It was intended to carry heavier loads than the shuttle and perhaps hazardous ones (like liquid-fuel upper stages).



There had been many earlier design studies, some with the same name, for shuttle-derived cargo vehicles. For a few years after the Challenger accident, NASA pursued work on the Shuttle-C project, with the initial aim of launching heavy space-station modules. But rising cost estimates for development and dwindling hopes for launch-cost savings gradually made the project look less worthwhile. Funding was zeroed out by Congress to help pay for the massive cost overruns of the space station. (Illustration: NASA-MSFC) Advertisement

National Launch System (NLS); 1989 – 1993; perhaps $300 million.



This was a joint NASA-US Air Force project for a low-cost shuttle-derived expendable launcher to replace existing expendable rockets, such as Atlas, Delta and Titan launchers.



Congress cut its funding due to problems with cost and schedule, a hopelessly confused management structure, and the lack of a clear requirement for the system, since there was nothing obviously wrong with existing launchers and Congress was sceptical that the NLS would be significantly cheaper.



After some fumbling, the US Air Force replaced it with the Evolved Expendable Launch Vehicle (EELV) programme that produced the Delta IV and Atlas V rockets. (The RS-68 engine that powers the Delta IV is probably derived from work on the "Space Transportation Main Engine" for the National Launch System.)



In this image, a National Launch System engine is test-fired at a NASA centre. (Image: NASA-MSFC)

X-34 ; 1994 – 1996, $8 million; and 1996 – 2001, $60 million.



The X-34 was originally a joint project between NASA and the aerospace firms Orbital Sciences and Rockwell. It was designed as a two-stage launcher for small satellites, with the first stage being reusable.



Orbital and Rockwell withdrew less than a year after the contract was signed, because they decided the project could not be done for the promised amount. (A major disagreement between Rockwell and NASA over engine choice likely contributed to the decision.)



The X-34 was reborn as a programme for a suborbital reusable-rocket technology demonstrator (illustrated). But when the first flight vehicle was near completion, the programme died after NASA demanded sizable design changes without providing any new funding, and the contractor, Orbital Sciences, refused. (Illustration: NASA-MSFC)

X-38 ; 1995 – 2002; perhaps $100 million.



Meant primarily as a "lifeboat" for the space station, the X-38 was a wingless vehicle whose body shape was designed to provide some lift, giving it the ability to do controlled gliding flights and to be guided to a desired point before its parachute was deployed for landing.



It was developed in-house at NASA's Johnson Space Center, with some later European involvement, but was cancelled as part of space-station revisions provoked by station cost overruns. It was replaced by the EELV-launched Orbital Space Plane (OSP), which eventually merged into the Constellation programme currently planned to replace the space shuttle.



In this image, a prototype of the X-38 crew lifeboat drops away from a plane during a 13-minute test flight in late 2001. (Image: NASA-DFRC)

X-33 ; 1996 – 2001; $912 million (NASA share). The X-33 was a NASA-Lockheed Martin suborbital technology demonstrator for a reusable rocket that would reach orbit without using multiple stages or dropping fuel tanks. It was intended to be followed by a commercial "single-stage-to-orbit" (SSTO) vehicle, VentureStar.



But the X-33 was cancelled due to slipping schedules and performance, rising costs that Lockheed Martin was unwilling to absorb, and a major failure during a test of its composite-material tanks.



There was no replacement; NASA has taken the failure of the X-33 as proof that SSTO is not a credible concept. (Illustration: NASA-MSFC)