The HL-20 was built at Langley in October 1990 and is a full-scale non-flying mockup. This mockup was used for engineering studies of maintainability of the vehicle, as testing crew positions, pilot visibility and other human factors considerations. The HL-20 was a direct derivative of the HL-10 vehicle tested in the 1960s and bears a very close resemblance to engineering drawings produced at that time. Although evaluated as a possible 'space taxi,' the HL-20, sometimes called the 'Personnel Launch System,' was never built.