Schroeder who trained Overholser on mathematics relating to stealth aircraft. Kenneth Watson was hired as the senior lead aircraft designer. During the next few weeks, the team created a computer program which could evaluate the RCS of possible designs. The RCS-prediction software was called "ECHO 1". As tests with the program proceeded, it became apparent that edge calculations by the program were incorrect due to diffraction. To overcome this, Overholser incorporated elements of research by Soviet engineer Pyotr Ufimtsev into the software. (In 1964, Ufimtsev, as chief scientist of the Moscow Institute for Radio Engineering, published a seminal paper titled Method of Edge Waves in the Physical Theory of Diffraction. The work had been subsequently translated by the US Air Force Systems Command's Foreign Technology Division.) ECHO 1 allowed the team to quickly decide which of the 20 possible designs were optimal, finally settling on the faceted delta-wing design. However, many within the division were skeptical of the shape, giving rise to the name "Hopeless Diamond" – Kelly Johnson said to Rich, "Our old D-21 drone has a lower radar cross-section than that goddamn diamond". In May 1975, the Skunk Works produced an internal report titled, "Progress Report No. 2, High Stealth Conceptual Studies." Within it was a concept study called "Little Harvey," including Kelly Johnson's drawing of an aircraft with smoothly blended shapes. Johnson advocated for the use of blended shapes as the best way to achieve stealth, while Ben Rich advocated for faceted angles. Rich won the argument with Johnson, a rare occurrence. The design effort produced a number of wooden models. A 24-inch long model, made of balsa wood, demonstrated placement of internal structure and access doors. An Air & Space article noted "The model shop found it nearly impossible to make all the flat surfaces come to a single point in one corner. Engineers later encountered the same difficulty fabricating the prototype on the factory floor." For early tests of the design, two ⅓-scale wooden mock-ups were constructed. One model, coated in metal foil, was used to verify ECHO 1's RCS calculations, while the other was earmarked for wind tunnel tests. Afterwards, a model was moved to the Grey Butte Range radar-testing facility in the Mojave Desert near Palmdale, which allowed more accurate tests of the aircraft's RCS. In the event, the aircraft's RCS level confirmed ECHO 1's predictions. This meant Ben Rich won a quarter