The Suntan We Didn’t Get

Longtime TAH readers know I have a soft spot for history, and in particular that supporting the intel and military communities.  Well, here’s another blurb on a bit of less-known aviation history.

Most of us know about Lockheed’s Skunk Works.   Many know that the Skunk Works produced a number of pioneering high-performance aircraft for the US:  the F-80, the F-104, the U-2, the A-12 (the design for which later was modified to become the SR-71), and the F-117.  I’ve written a brief article about the U-2 and A-12 previously, and a somewhat longer one about the A-12.

However, between the U-2 and the A-12/SR-71 there was another Skunk Works project,  It was the original successor to the U-2.  Due to insurmountable issues, it was never produced.

The aircraft design was the CL-400.  The project was code-named “Suntan”.

. . .

The U-2 was designed in 1955, and began overflying the Soviet Union the following year – on July 4, 1956, to be precise.  (smile) The overflights were a huge success.  No less of an authority than Richard Helms (CIA director from 1966-1973, and at the time a highly-placed official for the CIA – he oversaw Operation Gold) has stated that the U-2 overflights were “the greatest intelligence breakthrough of the 20th Century.”

Yet on the first overflight, the US received a nasty surprise concerning the U-2.  Our initial analysis had indicated that Soviet radars of the day – based on US radars provided via World War II’s Lend-Lease program – would not be able to track the U-2 at its operational altitude of 70,000 feet.

The initial analysis was wrong.  The Soviets had made significant advances in their radar technology; they tracked each early U-2 flight reasonably well.  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’ success.

All concerned knew it was only a matter of time before the Soviets were capable of shooting down a U-2.  Lockheed analysts estimated 2 years.  We were rather lucky – it actually took the Soviets nearly 4, and then we may have aided them through bad mission planning.

Still, all concerned knew that the U-2’s days of overflying the USSR were numbered.  So work began almost immediately on designing a successor aircraft.

The original successor to the U-2 was not the A-12.  It was a very different aircraft design – one that in some ways was as impressive, and would have performed in some ways even better.

Unlike the U-2 and A-12, this aircraft was not to be developed for the CIA.  Instead, this was to be an Air Force program from day one.

The program was code-named “Suntan”.  Its classification was “above top secret” (presumably, what we would call today a “special access program”).  The design proposal selected was Lockheed’s CL-400.

. . .

Suntan’s specifications at the time were stunning.  They were developed in 1956-1957; the project wasn’t declassified until 1973.  Even today, they’re damned impressive.

• It was to cruise at Mach 2.5 – sustained.
• It was to fly at 30,000+ meters – or roughly 100,000 feet.
• It was to use liquid hydrogen as a fuel.
• One design (one of the final two, I believe) was almost 300 feet long.

Suntan would in effect be a huge manned flying Dewar (thermos) filled with liquid hydrogen at a temperature of around  -400 Fahrenheit – and having a skin temperature estimated to reach at least 350 Fahrenheit at times.

Work began on the design.  Lockheed produced numerous designs for the airframe.  Here’s the original design:

Later designs were

and

The mission profile (presumably the same for all variants) was as follows:

To put things in perspective, here’s a diagram comparing Suntan’s longest variant design with contemporary USAF aircraft of the day.

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).  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.

Unfortunately, Suntan was never to fly.  Two problems precluded it from ever being built.

. . .

The first of these problems was financial.  From the beginning, Suntan was going to be expensive – very expensive.  It was estimated to cost nearly $100M in 1956 dollars when the program started.  Two years later, the costs had increased by over 60 percent.

Add the infrastructure required to produce/ship/handle liquid hydrogen to various locations worldwide – and if necessary, to develop a means of aerial refueling (which was apparently studied) – and costs quickly became unsupportable.  The late 1950s saw a truly nasty but short recession.

The second problem was more fundamental.  The bird simply didn’t have the range required.

Although liquid hydrogen is a terrific fuel on a per-weight basis, it isn’t very dense.  It thus occupies a much higher volume than other fuels.

More volume means more skin area for the vehicle.  This isn’t 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.  Brute force – e.g., more thrust – can overcome drag pretty easily if a flight is to be short and the craft isn’t to be re-used often (or at all).

However, for a manned aircraft, the operating profile is different. It’s intended to return safely.  And more drag over a multi-hour flight means more fuel is required.  Lots more fuel.

The best the Skunk Works could do was an unrefuled mission radius of slightly more than 2,000 km.  More range was needed.  And in the end, aerial refueling was evaluated and was deemed not practical with liquid hydrogen.

Lockheed informed the USAF that the plane wasn’t cutting it, range wise, and that they couldn’t “make it so”.  (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.

Lockheed reportedly returned approximately $90 million of the original $96 million dollar program funding to the Federal government.  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’s engine development and testing.

. . .

Though unsuccessful, the project was not a total loss.

The expertise in high-speed, high-altitude aircraft design acquired by Lockheed was used on a follow-on project.  It was called the A-12 – and later was modified to become the SR-71.  Pratt & Whitney similarly transferred its expertise in high-performance engine development to other projects.

The liquid hydrogen engine research and production techniques directly transferred to the US space program.  The R&D efforts paid for themselves there in spades.

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.

And yet . . . in one way that’s a pity.

I truly would love to have seen this one fly.  It really would have been something to behold.

Sources:

Images are from the site http://xplanes.free.fr.  This site has an excellent discussion of the Suntan program at http://xplanes.free.fr/suntan/cl400-1.html.  Unfortunately, virtually the entire presentation is in French vice English.  It does have an excellent reference list in English.

NASA’s official history Liquid Hydrogen as a Propulsion Fuel,1945-1959, by John L. Sloop, is available on-line.  It covers Suntan beginning at http://history.nasa.gov/SP-4404/ch8-1.htm.

Ben Rich and Leo Janos discuss Suntan in Chapter 8 of Rich’s memoir, Skunk Works (ISBN-10: 0316743003; ISBN-13: 978-0316743006).  The chapter is titled “Blowing Up Burbank” – 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.

There are also numerous other articles about the CL-400 and the Suntan Program on the Internet.  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.)

Category: Air Force, Historical