d back to reach only Mach 8, and then cancelled in 2001 after NASA's entire budget of more than $1 billion had been spent, leaving it several $billion short of having a flight vehicle. (Article)
A supposedly commercial follow-on to the X-33 demonstator. However, the proposer, Lockheed-Martin Corp, had stated that the satellite and government launch markets for which it was designed were not large enough to make it a viable business venture, and so it has expired with the X-33.
|[an error occurred while processing this directive]X-34[an error occurred while processing this directive]||
The [an error occurred while processing this directive]X-34[an error occurred while processing this directive] was announced as an unpiloted, reusable [an error occurred while processing this directive]HTOL[an error occurred while processing this directive] rocket test vehicle to fly first in 1999, eventually reaching Mach 8. The main objective was to provide a flight-test vehicle for the NASA developed 'Fastrac' engine and some other hardware. Having overrun the original $100 million budget by 30%, NASA's request for an addition $200 million was turned down and the project was scrapped in 2001 before any flights were performed. (Article)
By contrast, the piloted, reusable [an error occurred while processing this directive]HTOL[an error occurred while processing this directive] rocket-test-vehicle, the X-15, reached Mach 6.6, flying successfully 199 times in the 1960s.
"Spaceplanes" that take off like an aeroplane and fly to orbit have been a dream of some space engineers and science fiction writers for decades. Expendable [an error occurred while processing this directive]SSTO[an error occurred while processing this directive] [an error occurred while processing this directive]VTOL[an error occurred while processing this directive] vehicles are simple to design using existing components (see "A Single-Stage-to-Orbit Thought Experiment"). By contrast, reusable [an error occurred while processing this directive]SSTO[an error occurred while processing this directive] [an error occurred while processing this directive]HTOL[an error occurred while processing this directive] vehicles aren't technologically possible today. Today, the competition is beteen [an error occurred while processing this directive]VTOL[an error occurred while processing this directive] [an error occurred while processing this directive]SSTO[an error occurred while processing this directive] designs like [an error occurred while processing this directive]Kankoh-Maru[an error occurred while processing this directive] and New Shepard and two-stage spaceplanes like SpaceShipTwo/White Knight Two. [an error occurred while processing this directive]VTOL[an error occurred while processing this directive] designs have the appeal of a single stage which avoids the engineering issues of in-flight separation. Spaceplanes have the advantage of a large legacy of practical aviation experience to draw on, and an easier fit with existing airport operations and regulations.
Although, in future, new-technology air-breathing rocket engines may come to be used for some part of passenger launch vehicles' flight to orbit, it's difficult to make a convincing case that vehicles using technology that doesn't exist yet will be competitive with ones using known technology for at least the next few decades. Henry Ford didn't wait for the V-8 engine to be invented and developed before he started selling cars!
See Rockets versus Planes for more on this ongoing debate. [an error occurred while processing this directive] [an error occurred while processing this directive]