HL-20 Space Taxi. (An Artist's Impression)). Artist's concept showing an HL-20 lifting body atop a National Launch System. Developed by the Langley Research, the HL-20 was one of two concepts considered by NASA in the early 1990s as a type of Personnel Launch System (PLS) - a small space taxi system (NASA Media Archives). Buy this as a superb wall poster, a canvas print or a framed print.