Although President Calvin Coolidge was not an expert on air power, his famous quote, “Nothing in this world can take the place of persistence,” should be studied and practiced today by leaders in the Office of the Secretary of Defense and the U.S. Air Force. The OSD’s decision to renege on the Air Force goal of procuring only fifth-generation F-35A fighters and, instead, budget for new fourth-generation F-15Xs in the Air Force budget shows both a lack of persistence and sound logic. It also weakens Air Force support to grow its fighter force and obtain a healthy return on its large F-35A investment, and it adds to the growing list of new programs the Air Force must fund.

In just six months the Air Force appears to have changed its modernization priorities. At the Air Force Association convention in September 2018, Secretary Heather Wilson announced the goal of expanding the force size from 312 to 386 operational squadrons that included seven new fighter squadrons. There was no mention of ever buying new fourth-generation fighters — only the F-35A.

But now we learn that OSD is the driver in the F-15X decision. At an AFA symposium on Feb. 28 in Orlando, Florida, Wilson acknowledged that the Air Force budget for 2020, when submitted to OSD, “did not include additional fourth-generation aircraft.” The F-15X was added by OSD, and apparently Air Force leadership has not been able to change minds in OSD.

Over the past 15 years, the Air Force has been both consistent and persistent in advocating only the new fifth-generation, stealthy F-35A and retiring over time its non-stealthy fourth-generation F-15s and F-16s. Now, the goal announced by the Air Force is to procure 72 new fighters per year including both F-35As and F-15Xs.

The need to deal with great power competition from Russia and China, plus the deployment of advanced air defense systems like the Russian S-400 covering eastern Europe and southwest Asia, has provided overwhelming justification to modernize the Air Force fighter fleet with only F-22s and F-35As which can penetrate these defenses effectively and destroy modern air defenses.

The F-15s and F-16s are no longer able to penetrate and survive, and their effectiveness will only decrease over time. So the OSD decision to add F-15Xs in the Air Force budget is puzzling — a step backward.

Moreover, introducing the F-15X will not allow the Air Force to increase its F-35A annual production rate above 48-60 per year, a far cry from the 84-96 per year in its plans, and thus reducing the return on its investment. The Air Force learned in the F-22 and B-2 programs that continually reducing the annual buys created an acquisition death spiral because buying fewer led to ever-increasing unit costs that further led to their becoming unaffordable, therefore unviable.

The business case for the F-35 was to generate high annual production rates for more than 20 years at low-unit flyaway costs, a huge return on investment much like the F-16 program. That goal now appears unlikely.

The Air Force is attempting to rationalize the OSD decision by criticizing the sustainment costs of the F-35A and invoking the term “increased capacity” with the F-15X. F-35A sustainment has growing pains like any new weapon system. They will come down over time. The Air Force has not defined “increased capacity,” but no amount of capacity can overcome an inability to penetrate modern defenses and destroy high-value targets — severe limitations of the F-15X.

Air Force leaders have a chance to practice Coolidge’s admonition about persistence by finding a way to increase F-35A annual production so it can reach their goal of an all-stealthy fighter force necessary to deal with the modern threats fielded by great power competitors. They cannot achieve this goal buying new, non-stealthy fighters at the expense of F-35As.

Gen. John Michael Loh (ret.) served as the U.S. Air Force vice chief of staff and the commander of Air Combat Command.