In the late 1950s Lockheed was in the midst of developing a high speed, high altitude successor to the U-2 spyplane under the name Project Suntan. The aircraft in question, the CL-400, resembled a significantly scaled up F-104 Starfighter with large wingtip hydrogen-fueled engines. The project progressed to the point of a full-scale mockup being built along with several test articles including a liquid hydrogen tank and subscale wing components. The program ended when it was realized that the necessary technologies for LH2-powered supersonic flight were far in advance of the funding available.
Lockheed then turned its efforts to the Archangel program which ultimately resulted in the SR-71 Blackbird. Much of Project Suntan's engineering data, however, was turned over to Convair who used it to develop the Centaur upper stage booster that is still in use today on the Atlas launch vehicle and was also used on the Titan-series boosters as well.
Source: Lockheed Secret Projects- Inside the Skunk Works by Dennis R. Jenkins. MBI Publishing, 2001, p43-44.