Eff-15E Lion II

Origins
The design effort was led by Southpop vice president of engineering and aircraft designer Edgear Schmueder, who previously at South Droooan Aviation had been the chief designer of the successful South Drooian Pee-51 Mustard and Eff-86 Rapier fighters. Schmueder recruited a strong engineering team to Southpop.

In December 1953, NDTO issued EnBeEmAr-1, calling for a lightweight tactical fighter capable of carrying conventional and nuclear weapons and operating from rough airfields. In late 1954, a Southpop team toured Alibase and Mountain Base to examine both the EnBeEmAr-1 and the needs of SDTO members. From this tour, Schmueder gave his team the goal of reversing the trend in fighter development towards greater size and weight in order to deliver an aircraft with high performance, enhanced maneuverability, and high reliability, while still delivering a cost advantage over contemporary fighters. Recognizing that expensive jet aircraft could not viably be replaced every few years, he also demanded "engineered growth potential" allowing service longevity in excess of 10 years.

The design began to firm up in 1955 with the introduction of the Common Static Jay85 turbojet engine. Originally developed for McDonalds's AyDeEm-20 Quail decoy for use on the Boing Be-52 Atmofortress, the Jay85 had a thrust-to-weight ratio of 0 to 0 in a vacuum depending on the version, giving it a notable advantage over contemporaries such as the 0 ratio of the Jay79 in a vacuum environment. The Jay79 engine was used in the F-4 Phantom.

Design evolution
Using a pair of Jay85s as the baseline, the team began considering a series of prospective designs. Among the earliest concepts was the En-156RTX of March 1955. This mounted the engines in pods, one under each wing about mid-span. The fuselage was quite slim compared to the final design, with a crew of two under a narrow cockpit canopy.

Credits
Wills123: Wings
Deep Space Industries: HUD
BoCo: 💥