Feb 152012

The Lockheed XF-90 was built in response to a 1945 Air Force requirement for a long range penetration fighter and bomber escort. It was an attractive aircraft heavily influenced by the company’s experience with the P-80, sharing a similar intake and low wing configuration. However, it was designed to operate at higher speeds, with a 35° swept wing, a sharply-pointed nose and two Westinghouse J34-WE-11 axial-flow turbojet engines mounted side-by-side in the rear fuselage. Unfortunately, Lockheed elected to use 75ST aluminum rather than the usual 24ST aluminum alloy, along with heavy forgings and machined parts, resulting in robust but heavy airframe. This caused it to be underpowered, even with the addition of afterburners to the second prototype, and the Lockheed XF-90 lost out to the McDonnell XF-88 Voodoo in September 1950 before the penetration fighter concept was abandoned entirely.

Early L-153 Studies

While the XF-90 as built was relatively conventional in layout, Lockheed explored at least 65 other configurations in the preliminary design phase, some of which were quite radical. Most of these were done under the L-153 designation, though not all have been made public. The first that we have is the L-153-2, a heavily modified P-80 with a swept wing, a prominent spine and fillet on the vertical tail, and submerged inlets at mid-fuselage. The L-153-7 had a bizarre “W” wing, also shared by the L-153-11. The latter featured a pair of slender booms that carried the armament, main gear and tail unit. The L-153-13 had a V-tail and was powered by three jet engines, two of which were carried on the tips of a sharply swept, untapered wing. The L-153-22 also had a V-tail, though with a more conventional nose intake. The L-153-34 featured variable geometry on the outer wings and a ventral intake; one can only imagine what the other L-153 studies may have looked like.

L-167 Delta Studies

The original XF-90 specification of 1945 (then designated as the XP-90) was for a delta wing aircraft, with most of the delta wing configurations being developed under the L-167 designation. Delta wing types were initially discouraged due to the difficulty of obtaining adequate control independent of the wing that could be used for dive recovery in the event that wing pitching moment could not be controlled at high transonic Mach numbers. This problem was solved by use of an auxiliary trim surface. The following estimated performance figures may apply to the L-167-1 shown above; these assume an aircraft with a gross takeoff weight of 27,000 lbs and engines in the 7,000 to 10,000 lb range:

Speed at Sea Level727 mph
Max Speed at 35,000 ft672 mph
Absolute Ceiling45,700 ft
Combat Radius900 mi
Rate of Climb at Sea Level11,900 ft/min
Stall Speed106 mph
Takeoff Distance9,200 ft

Information and drawings of the Lockheed L-153 and L-167 studies originate from an unpublished manuscript by the late Bill Slayton, a Lockheed employee. Special thanks to Peter Clukey and Tony Buttler for providing this material.

L-169-1 Long Range Naval Fighter

The L-169-1 long range naval fighter was an outgrowth of the L-167 series and dates from January 7, 1947. It could be equipped with either two Westinghouse 24-C or two Lockheed L-1000 engines; with the latter, the L-169’s tactical radius of combat was estimated to be 1,440 miles, which exceeded the Navy’s requirement for 1,200 miles. In a letter to BuAer dated January 10, Lockheed also emphasized that the delta’s ability to achieve high mach numbers; the lightness of the design compared to conventional aircraft; and the rigidity of the structure inherent with the delta wing, among other characteristics. For a detailed overview of the design, check out the L-169-1 performance report, as well as the comments by the Aero & Hydro section of BuAer which questioned the assumptions behind several of Lockheed’s estimated performance figures.

Delta No More

Returning to the XP-90, an Air Force aircraft characteristics sheet for the fighter dating from June 1, 1947 shows a delta wing aircraft with a T-tail; according to one source, this is apparently the L-167-2. This configuration would be abandoned for a more conventional swept wing arrangement shortly thereafter. Reasons for this are described in an Air Force letter to NACA dating from July 29 and primarily concern the high drag of the delta wing at low speeds as well as the inferior combat radius for a given weight, among others. Donald H. Wood, Acting Director of Ames, basically agreed with the rationale for the change in his August 8 memorandum to NACA headquarters in Washington.

An inboard profile of the XP-90 dating from November 17, 1947 shows an aircraft very close to the final configuration, though the vertical tail appears to be smaller than on the aircraft as built. This drawing was found in a Lockheed weight and balance report dating from January 15, 1948.

To see more interesting images of the XF-90 and its precursor studies, check out this excellent thread on the Secret Projects Forum. In terms of books, Lockheed Martin’s Skunk Works: The Official History by Jay Miller and American Secret Projects: Fighters and Interceptors 1945-1978 by Tony Buttler have excellent entries on this subject and are well worth purchasing.

L-153 & L-167 three-views courtesy of Peter Clukey via Tony Buttler; remaining images from NARA Archives II, College Park, MD, RG 72, 255 & 342 

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