Lockheed's first gas turbine-powered Connie: the L-151

The Lockheed Constellation family was unquestionably one of the finest airliner dynasties to come of age after the end of the World War II, spawning evolutionary developments such as the improved L-649 and L-749 versions of the original Constellation, the L-1049 Super Constellation, the L-1649 Starliner, and the EC-121 Warning Star, but also the turboprop-powered L-1249 (R7V-2/YC-121F) and the unbuilt XB-30 heavy bomber. The L-1249 that was derived from the Super Constellation constituted the first instance of any variant of the Constellation family being built with turboprop engines, although it never advanced beyond the prototype stage. However, what has been overlooked in books on the Constellation is the fact that even as World War II was winding down, Lockheed looked at fitting the baseline Constellation with gas turbine powerplants.

Left: Three-view drawing of the Lockheed L-151-1 jet airliner
Right: Three-view drawing of the L-151-2 along with a side view of the L-151-3 (upper right)

In 1945, the development of jet engines was destined to make the Lockheed Constellation and many other large piston-powered American transport aircraft designs envisaged during the years of US involvement in World War II slowpokes compared to jet or turboprop aircraft designs being worked on by US aircraft manufacturers in the 1943-1945 timeframe. Lockheed had developed America's first jet engine design, the L-1000 (military designation XJ37), to power its L-133 jet fighter project, but when the L-133 itself was deemed too advanced for its time and the initial L-1000 design turned to be more complex than imagined by engineer Nathan Price, it ended up being redesigned as a simple jet engine comprising two sixteen-stage axial compressors connected by an intercooling stage. Capitalizing on flight testing of the P-80 (later F-80) Shooting Star jet fighter, Lockheed in early 1945 conducted design studies to equip the Constellation airliner with gas turbine engines under the company designation L-151. The first design, called L-151-1, replaced the L-049's Duplex Cyclones with six L-1000 turbojets in two underwing nacelles (three engines per nacelle), and judging from what little technical data has survived, the L-151-1 had the same length and wingspan as the commercial L-049 but would have a slightly higher gross weight of 100,000 lb (45,359 kg). Two turboprop-powered L-151 iterations were worked out, the L-151-2 with four General Electric TG-100 turboprops and the L-151-3 with four Westinghouse turboprops. These designs had same gross takeoff weight as the L-151-1 and compared to the L-049, they would have greater top speed, with the L-151-2 being designed to cruise at 335 mph (539 km/h) and the L-151-3 having a cruising speed of 370 mph (595 km/h). The L-151-2 and L-151-3 also had a higher service ceiling and rate of climb at 18,000 feet (5,486 meters) than the L-049, with L-151-2 having a climb rate of 610 ft/min (3.10 m/s) and capable of reaching a 28,000 ft (8,534 meter) altitude and the L-151-3 featuring an estimated climb rate of 1,060 ft/min (5.348 m/s) and an altitude of 31,000 feet (9,449 meters). Nevertheless, the one performance pitfall of the L-151-2 and L-151-3 designs was operating range. The L-151-2 and L-151-3 would have had operating ranges of 3,500 miles (5,633 km) and 3,100 miles (4,989 km) respectively, well below the L-049's maximum range of 4,400 miles (7,081 km). The dearth of specifications for the L-151-1 makes it unclear whether the L-151-1's range would have been equivalent to or exceeded that of the Constellation, but the marginally lower operating range of the L-151-2 and L-151-3 compared to that of the L-049 meant that Lockheed did not pitch those designs to the airlines. In the meantime, the L-1000 engine which the L-151-1 would have used ended up being cancelled by the late 1940 without leaving the development phase, and the L-151-1 also would never reach the hardware phase.

Front views of the different L-151-5 variations accompanied by side and bottom views of different auxiliary turbojet placements proposed for the L-151-5 (far-right)

This was not the end of the L-151 story, however. In 1950, Lockheed took advantage of its development of the L-1049 Super Constellation with a stretched fuselage by envisaging a spree of design studies for an L-1049C with two 4,200 lb (18.7 kN) thrust auxiliary jet engines, designated L-151-5. The proposed L-151-5 was quite akin to the B-36 Peacemaker being fitted with four auxiliary General Electric J47s in two podded pairs for extra speed beginning with the B-36D version, and a number of L-151-5 schemes were proposed as follows:

• Mounting the auxiliary turbojets below underwing pylons (as with the B-36)
• Placing the auxiliary turbojets at the rear of the outboard R-3350 Duplex Cyclone radial piston engines to provide minimum drag for jet engine out condition
• Placing the auxiliary turbojets on the wingtips to provide minimum drag when all engines were operating (with or without auxiliary fuel tanks pyloned below the outboard wing sections)

Lockheed calculated the performance of the L-151-5 design and determined that when compared with the L-1049C, the L-151-5 offered 25 percent greater block speed and a shorter takeoff distance of 3,750 feet (1,143 meters) along with a slightly lower operating cost. Still the operating range of the L-151-5 would be slightly lower than that of the L-1049C due to the drag generated by the jet engines when they were not in operating condition. Even though the L-151-5 would have been easy to create by equipping an L-1049C airframe with auxiliary turbojets because it required no substantial structural modifications compared to the L-151-2 and L-151-3, no airline expressed an interest in the L-151-5 at all.

Notwithstanding the fact that the L-151 was used by Lockheed as a catchall company designation for a variety of proposals to fit the Constellation with gas turbine engines, one good question arises: would the L-151-1 have been an outstanding and profitable airliner with respect to its impact on passenger air travel in the late 1940s and early 1950s had it been built? There's no question that US industry was developing more advanced jet engines with high power/weight ratios by the time that World War II ended, including the Allison J35 and General Electric J47, but since the L-1000 engine had offered the L-133 project far greater estimated top speed than that of the Shooting Star or Airacomet, in my own opinion, the L-151-1 would have carried out passenger flights at much shorter flight times, although it have somewhat slower the Boeing B-47, B-52, and 707 due to its straight wings.

References:

Slayton, B., 1999. "The Lockheeds That Never Were, Part II." AAHS Journal 44 (2): 102-113.