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Lockheed CL-400 (Suntan)

Liquid Hydrogen-Fueled Spyplane Proposal [ 1956 ]

The Lockheed CL-400 Suntan was a short-lived, liquid hydrogen-fueled attempt for the USAF to find a suitable high-speed, high-altitude spyplane solution.

Authored By: Staff Writer | Last Edited: 09/20/2022 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

The Lockheed CL-400 "Suntan" was a projected Cold War-era supersonic high-altitude spyplane designed by famous American aerospace engineer Clarence "Kelly" Johnson to succeed the company's subsonic "U-2" (detailed elsewhere on this site) and satisfy a United States Air Force (USAF) interest in such an aircraft. One of the key driving elements of the design was its propulsion scheme which was to be entirely fueled by liquid hydrogen. The aircraft would be used in secret flights over contested Soviet airspace, strictly for reconnaissance purposes, and both its operating altitude and speed were expected to keep the vehicle out of harm's way.

While the U-2 completed its first Soviet overflight on July 4th, 1956, work on the successor - the CL-400 - was already underway.

Lockheed was contracted for a pair of prototype airframes and the first of these was set to fly within two years. A follow-up contract for six more airframes followed even before the prototypes were compelted.

The proposed aircraft was to have a slim, highly-streamlined fuselage with the twin-seat (tandem) cockpit set over and aft of the nosecone. The fuselage was appropriately tapered at both ends with the rear section given a single vertical tail fin with high-mounted horizontal planes. Under the tail was a smaller-area ventral fin for added directional stability, particularly in the supersonic flight envelope - this plane was designed to retract when no needed. The mainplanes were seated at midships and tapered from wingroot to wingtip. At each wingtip would be affixed a podded engine for the needed performance. A retractable tricycle undercarriage would round out the list of features of the aircraft - the nose leg sitting under and aft of the nose with main legs positioned at each engine nacelle. It is conceivable that a third main leg would have supported the center-section of the aircraft as well.

All fuel would be housed in tanks set across the fuselage. This would be run through the wings to reach the engine nacelles at each wingtip. The use of liquid hydrogen offered an efficient fuel source as it was lightweight and its properties understood. Engine-maker Pratt & Whitney was enlisted to develop the propulsion scheme (2 x PW Model 304) for the CL-400 of which each unit could produce upwards of 9,445lb of thrust output.

The end result (at least on paper) was a high-performance spyplane capable of Mach 2.5+ speeds in straight-level flight utilizing just two engines and achieving a service ceiling near 100,000 feet. The operational phase would involve the large aircraft taking off under conventional means and reaching its target altitude, flying to the designated target zone, and cruising out of harm's way once the mission was completed (or fuel stores ran low). Descent would then take the aircraft to a holding pattern prior to achieving a conventional landing back at home base.

As drawn up, the CL-400 was to feature a general weight of 48,515lb with a take-off gross weight of 69,955lb. Up to 21,440lb of fuel would have been carried with a mission equipment load of up to 1,500lb. The fuselage was to be 160.7 feet long and hold a diameter of 9.84 meters. With the mainplanes, the complete beam measurement of the vehicle would reach 83.9 feet. Projected height to the top of the tail unit was 30 feet.

However, even before the end of the decade, the program ran into all sorts of trouble. Beyond the volatile nature of the fuel itself, there proved technological, performance, and logistical limitations for the aircraft to the point that the program was terminated in full in 1958 at the behest of Johnson himself. Such was the secrecy surrounding the CL-400's development that it was not made public until 1973 by which point satellites were providing the much-needed "eyes in the skies" for the United States.©MilitaryFactory.com
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Service Year

United States national flag graphic
United States

Development Ended.


Lockheed Aircraft Corporation - USA
(View other Aviaton-Related Manufacturers)
National flag of the United States United States (planned, cancelled)
(OPERATORS list includes past, present, and future operators when applicable)
Intelligence-Surveillance-Reconnaissance (ISR), Scout
Surveil ground targets / target areas to assess environmental threat levels, enemy strength, or enemy movement.
X-Plane (Developmental, Prototype, Technology Demonstrator)
Aircraft developed for the role of prototyping, technology demonstration, or research / data collection.

164.0 ft
(50.00 m)
83.9 ft
(25.57 m)
30.0 ft
(9.15 m)
Empty Wgt
48,513 lb
(22,005 kg)
69,997 lb
(31,750 kg)
Wgt Diff
+21,484 lb
(+9,745 kg)
(Showcased structural values pertain to the Lockheed CL-400 production variant)
monoplane / mid-mounted / straight, tapered
Design utilizes a single primary wing mainplane; this represent the most popular mainplane arrangement.
Mainplanes are mounted along the midway point of the sides of the fuselage.
The planform involves use of basic, straight mainplane members.
Straight, Tapered
The planform uses straight mainplane members which taper towards the wing tips.
(Structural descriptors pertain to the Lockheed CL-400 production variant)
Installed: PROPOSED: 2 x Pratt & Whitney Model 304-2 liquid-hydrogen-fueled engines developing and estimated 9,445lb of thrust each unit.
Max Speed
1,920 mph
(3,090 kph | 1,668 kts)
Cruise Speed
1,243 mph
(2,000 kph | 1,080 kts)
Max. Speed Diff
+677 mph
(+1,090 kph | 589 kts)
98,425 ft
(30,000 m | 19 mi)
1,264 mi
(2,035 km | 3,769 nm)

♦ MACH Regime (Sonic)
RANGES (MPH) Subsonic: <614mph | Transonic: 614-921 | Supersonic: 921-3836 | Hypersonic: 3836-7673 | Hi-Hypersonic: 7673-19180 | Reentry: >19030

(Showcased performance specifications pertain to the Lockheed CL-400 production variant. Performance specifications showcased above are subject to environmental factors as well as aircraft configuration. Estimates are made when Real Data not available. Compare this aircraft entry against any other in our database or View aircraft by powerplant type)
None. Payload up to 1,500lb reserved for reconnaissance-minded mission equipment (held internally).

Supported Types

(Not all ordnance types may be represented in the showcase above)
CL-400 - Base Series Designation.
"Suntan" - Base USAF program name.

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Image of the Lockheed CL-400 (Suntan)
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