Project Isinglass Project Isinglass was the code name given to two heavily classified , manned reconnaissance aircraft studied by the Central Intelligence Agency ( CIA ) as potential replacements for the Lockheed A - 12 and SR - 71 during the mid 1960s . The first proposal under the Isinglass name , a high - altitude plane to fly at Mach 4 to 5 , was considered an insufficient advancement over existing aircraft ; the second , much more advanced design , sometimes referred to as Project Rheinberry , was an air - launched , Mach 20 rocket - powered boost - glide aircraft that would use a very - high - altitude trajectory to avoid defenses . This aircraft was considered too costly for development , and the project was abandoned in 1967 . Origins . Project Isinglass was developed as a result of the vulnerability of existing manned reconnaissance aircraft , such as the Lockheed U - 2 and the Lockheed A - 12 , to Soviet air defenses in the early 1960s , catalysed by the shooting down in May 1960 of Francis Gary Powers . Although there were continuing plans to overfly the Soviet Union with the A - 12 – referred to by the CIA as Project Oxcart – these failed to come to pass , and the CIA began plans for an aircraft with superior performance to replace Oxcart . The Convair proposal . The initial aircraft proposed under the Project Isinglass name was developed by the Convair division of General Dynamics , and was developed from work done on the Super Hustler , FISH , and Kingfish programs , as well as leveraging off work done on the F - 111 tactical bomber . Convair ' s design utilised avionics and hydraulics systems that had been developed for use by the F - 111 , and was intended to be capable of cruising at speeds of Mach 4 to Mach 5 , at an altitude of . The feasibility study conducted by General Dynamics was completed in the fall of 1964 ; the aircraft was determined to be too costly , and was also still considered potentially vulnerable to projected Soviet air defense capabilities , so the project was not proceeded with . The McDonnell proposal . Design and development . An alternative design completed by McDonnell Aircraft in 1965 is usually considered part of Project Isinglass , however some documents refer to the aircraft as having been codenamed Project Rheinberry . McDonnell ' s proposed boost - glide aircraft , submitted to the CIA independently of the Convair Isinglass proposals , featured a small , manned , rocket - powered craft with a high lift - to - drag ratio that would be air - launched by a B - 52 bomber while flying over the Atlantic Ocean . The aircraft would ignite its rocket engine and adopt a trajectory that would take it over the Soviet Union at speeds of Mach 20 and at an altitude of over , before descending over the Pacific Ocean to a landing at Groom Lake , Nevada , as a glider , landing on the lake bed using a skid landing gear . The Isinglass / Rheinberry concept was considered to be superior to spy satellites in a number of ways , including rapid turnaround time and quick response capability . As there was little funding to be had from the CIA budget , McDonnell developed the aircraft using its own funds , although technical support gleaned from Oxcart was supplied by the CIA . The McDonnell Isenglass / Rheinberry proposal ' s shape remains classified , although it has been described as being similar to the Space Shuttle , albeit much reduced in size . A 1 / 3 cross - sectional model of the aircraft was constructed to illustrate the principles used in its construction . Simulations showed that the aircraft would be essentially unstoppable by existing or projected air defenses ; even surface - to - air missiles armed with nuclear warheads could do no better than force the aircraft to change course to avoid the fringe effects of the initiation s . Cancellation . Following fourteen months of work , McDonnell had developed the aircraft to the point where serious proposals were made for its construction . However , neither the CIA or the National Reconnaissance Office had an official requirement for such an aircraft ; in addition , the projected cost of the aircraft was astronomical , the cost for eight aircraft being projected to be $ 2 . 6 billion USD in 1965 dollars ( inflation adjusted US $ billion in ) , a sum considered to be far too high for the available budget . In addition , there were concerns that the aircraft ' s trajectory could be mistaken for that of an incoming ballistic missile . When the CIA failed to receive approval for funding , McDonnell approached the U . S . Air Force with the project ; however the Air Force proved uninterested in adopting the CIA project , despite the support of General Bernard Schriever , chief of the Air Force Systems Command . Therefore in late 1967 the Isinglass project was terminated , with a brief effort to revive the project in 1968 proving unsuccessful . Engine development . Although the development of the Isinglass / Rheinberry aircraft was conducted using McDonnell ' s own funding , the United States Air Force ' s Air Force Rocket Propulsion Laboratory did fund the development of the aircraft ' s intended engine , the Pratt & Whitney XLR - 129 , intended to be a reusable rocket engine . To be powered by liquid hydrogen and liquid oxygen and producing thrust at full throttle , development of the XLR - 129 continued even after the cancellation of Project Isinglass , and was considered for use by the Space Shuttle , but was cancelled in the early 1970s .