However, between the U-2 and the A-12\/SR-71 there was another Skunk Works project,\u00a0 It was the original successor to the U-2.\u00a0 Due to insurmountable issues, it was never produced.<\/p>\n
The aircraft design was the CL-400.\u00a0 The project was code-named \u201cSuntan\u201d.<\/p>\n
. . . <\/b><\/p>\n
The U-2 was designed in 1955, and began overflying the Soviet Union the following year \u2013 on July 4, 1956, to be precise.\u00a0 (smile) The overflights were a huge success.\u00a0 No less of an authority than Richard Helms (CIA director from 1966-1973, and at the time a highly-placed official for the CIA \u2013 he oversaw Operation Gold<\/a>) has stated that the U-2 overflights were \u201cthe greatest intelligence breakthrough of the 20th Century.\u201d<\/p>\n Yet on the first overflight, the US received a nasty surprise concerning the U-2.\u00a0 Our initial analysis had indicated that Soviet radars of the day \u2013 based on US radars provided via World War II\u2019s Lend-Lease program – would not be able to track the U-2 at its operational altitude of 70,000 feet.<\/p>\n The initial analysis was wrong.\u00a0 The Soviets had made significant advances in their radar technology; they tracked each early U-2 flight reasonably well.\u00a0 Only the facts that (1) their surface-to-air missiles of the day could not maneuver effectively above 55,000 feet, and (2) their existing fighter aircraft could not reach that altitude allowed the overflights\u2019 success.<\/p>\n All concerned knew it was only a matter of time before the Soviets were capable of shooting down a U-2.\u00a0 Lockheed analysts estimated 2 years.\u00a0 We were rather lucky \u2013 it actually took the Soviets nearly 4, and then we may have aided them through bad mission planning.<\/p>\n Still, all concerned knew that the U-2\u2019s days of overflying the USSR were numbered.\u00a0 So work began almost immediately on designing a successor aircraft.<\/p>\n The original successor to the U-2 was not<\/span> the A-12.\u00a0 It was a very different aircraft design \u2013 one that in some ways was as impressive, and would have performed in some ways even better.<\/p>\n Unlike the U-2 and A-12, this aircraft was not to be developed for the CIA.\u00a0 Instead, this was to be an Air Force program from day one.<\/p>\n The program was code-named \u201cSuntan\u201d.\u00a0 Its classification was \u201cabove top secret\u201d (presumably, what we would call today a \u201cspecial access program\u201d).\u00a0 The design proposal selected was Lockheed\u2019s CL-400.<\/p>\n . . . <\/b><\/p>\n Suntan\u2019s specifications at the time were stunning.\u00a0 They were developed in 1956-1957; the project wasn\u2019t declassified until 1973.\u00a0 Even today, they’re damned impressive.<\/p>\n Suntan would in effect be a huge manned flying Dewar (thermos) filled with liquid hydrogen at a temperature of around\u00a0 -400 Fahrenheit – and having a skin temperature estimated to reach at least 350 Fahrenheit at times.<\/p>\n Work began on the design.\u00a0 Lockheed produced numerous designs for the airframe.\u00a0 Here’s the original design:<\/p>\n Unfortunately, Suntan was never to fly.\u00a0 Two problems precluded it from ever being built.<\/p>\n . . . <\/b><\/p>\n The first of these problems was financial.\u00a0 From the beginning, Suntan was going to be expensive – very expensive.\u00a0 It was estimated to cost nearly $100M in 1956 dollars when the program started.\u00a0 Two years later, the costs had increased by over 60 percent.<\/p>\n Add the infrastructure required to produce\/ship\/handle liquid hydrogen to various locations worldwide \u2013 and if necessary, to develop a means of aerial refueling (which was apparently studied) \u2013 and costs quickly became unsupportable.\u00a0 The late 1950s saw a truly nasty but short recession.<\/p>\n The second problem was more fundamental.\u00a0 The bird simply didn\u2019t have the range required.<\/p>\n Although liquid hydrogen is a terrific fuel on a per-weight basis, it isn\u2019t very dense.\u00a0 It thus occupies a much higher volume than other fuels.<\/p>\n More volume means more skin area for the vehicle.\u00a0 This isn\u2019t that much of a problem for a rocket; it can be made larger in diameter; volume of a cylinder goes up with the square of the radius. So building a larger diameter rocket adds dramatically more fuel and\/or oxidizer capacity. Further, rockets only operate in regions where drag is important for a short time.\u00a0 Brute force \u2013 e.g., more thrust \u2013 can overcome drag pretty easily if a flight is to be short and the craft isn\u2019t to be re-used often (or at all).<\/p>\n However, for a manned aircraft, the operating profile is different. It’s intended to return safely.\u00a0 And more drag over a multi-hour flight means more fuel is required.\u00a0 Lots more fuel.<\/p>\n The best the Skunk Works could do was an unrefuled mission radius of slightly more than 2,000 km.\u00a0 More range was needed.\u00a0 And in the end, aerial refueling was evaluated and was deemed not practical with liquid hydrogen.<\/p>\n Lockheed informed the USAF that the plane wasn\u2019t cutting it, range wise, and that they couldn\u2019t \u201cmake it so\u201d.\u00a0 (Reputedly Kelly Johnson called the SECAF and told him, “Mr. Secretary, I’m building you a dog,” and recommended cancellation of the program.) After some additional study (and a few other proposals from other aircraft firms at the 11th hour), the USAF concurred and pulled the plug.<\/p>\n Lockheed reportedly returned approximately $90 million of the original $96 million dollar program funding to the Federal government.\u00a0 The remainder had been spent on design studies, developing the pilot liquid hydrogen production capability, beginning the main liquid hydrogen production plant, and Pratt & Whitney\u2019s engine development and testing.<\/p>\n . . . <\/b><\/p>\n Though unsuccessful, the project was not a total loss.<\/p>\n The expertise in high-speed, high-altitude aircraft design acquired by Lockheed was used on a follow-on project.\u00a0 It was called the A-12 \u2013 and later was modified to become the SR-71.\u00a0 Pratt & Whitney similarly transferred its expertise in high-performance engine development to other projects.<\/p>\n The liquid hydrogen engine research and production techniques directly transferred to the US space program.\u00a0 The R&D efforts paid for themselves there in spades.<\/p>\n Yes, as a program Suntan didn’t cut it. It should have been cancelled, and was. Pulling the plug was the correct decision. And the R&D results were transferred to where they could be put to best use.<\/p>\n And yet . . . in one way that\u2019s a pity.<\/p>\n I truly would love to have seen this one fly.\u00a0 It really would have been something to behold.<\/p>\n Sources<\/span>:<\/p>\n Images are from the site http:\/\/xplanes.free.fr<\/a>.\u00a0 This site has an excellent discussion of the Suntan program at http:\/\/xplanes.free.fr\/suntan\/cl400-1.html<\/a>.\u00a0 Unfortunately, virtually the entire presentation is in French vice English.\u00a0 It does have an excellent reference list in English.<\/p>\n NASA\u2019s official history Liquid Hydrogen as a Propulsion Fuel,1945-1959<\/span>, by John L. Sloop<\/span>, is available on-line.\u00a0 It covers Suntan beginning at http:\/\/history.nasa.gov\/SP-4404\/ch8-1.htm<\/a>.<\/p>\n Ben Rich and Leo Janos discuss Suntan in Chapter 8 of Rich\u2019s memoir, Skunk Works<\/span> (ISBN-10: 0316743003; ISBN-13: 978-0316743006).\u00a0 The chapter is titled \u201cBlowing Up Burbank\u201d \u2013 a nod to the danger involved in figuring out how to produce and handle liquid hydrogen in bulk. The Helms quotation regarding the value of U-2 overflights is from a different chapter of this source.<\/p>\n There are also numerous other articles about the CL-400 and the Suntan Program on the Internet.\u00a0 However, the NASA history seems to be the best of the lot, with the French site having the best collection of images (though they can be hard to locate on that site.)<\/p>\n","protected":false},"excerpt":{"rendered":"\n
![\"\"](\"http:\/\/xplanes.free.fr\/suntan\/p145a.jpg\")
<\/center>Later designs were<\/p>\n![\"\"](\"http:\/\/xplanes.free.fr\/suntan\/images\/sun6.jpg\")
<\/center>and<\/p>\n
\n![\"\"](\"http:\/\/xplanes.free.fr\/suntan\/images\/sun7.jpg\")
<\/center>The mission profile (presumably the same for all variants) was as follows:<\/p>\n
\n![\"\"](\"http:\/\/xplanes.free.fr\/suntan\/p145b.jpg\")
<\/center>To put things in perspective, here\u2019s a diagram comparing Suntan’s longest variant design with contemporary USAF aircraft of the day.<\/p>\n
\n![\"\"](\"http:\/\/xplanes.free.fr\/suntan\/images\/sun8.jpg\")
<\/center>Lockheed proved that liquid hydrogen could be produced in quantity, developing a pilot plant with 200 gallon-per-day capacity at the Skunk Works facility (collocated with the Burbank airport).\u00a0 Pratt & Whitney produced working prototypes of the hydrogen-fueled engine, and tested them for 25+ hours. Air Research and also began work on a larger liquid hydrogen production plant.<\/p>\n