Lords of the Starship, a 1967 novel by Mark S. Geston; set on far future Earth, not another planet.
Here is the beginning of the Wikipedia plot summary:
In the far future, on an Earth devastated by millennia of war, the Caroline Republic is hostile towards its neighbors although sharing their dire economic straits. Outside the declining remains of civilization lie ruins and wastelands populated by mutants and monsters. It is generally felt that humanity lost its vitality long ago.
To a leading politician of the Caroline the aged veteran General Toriman proposes a centuries-long scheme to build the nation by taking control of an ancient shipyard hundreds of miles away which was apparently designed to build spacecraft. Ostensibly, the purpose of the project will be the construction of a spaceship seven miles long called the "Victory" to carry the population of the despairing world to a paradise planet called "Home". In fact, the ship will never be completed, but the effort will revitalize the nation's economy and perhaps restore mankind's missing quality.
The back cover blurb from the 1967 Ace paperback edition:
The ship was to be seven miles long, a third of a mile in diameter and have a wingspread of three and a half miles. It would take two and a half centuries to construct. Its announced purpose: to carry humanity away from its ruined world, from the world that had become a perpetual purgatory.
To build this vast ship would require the undivided activity of an entire nation and would mean carrying out a ruthless program of war and conquest, of annihilation and reconstruction, and of education and rediscovery.
But was this starship really what it was claimed to be? Or was there a greater secret behind its incredible cost—a secret so strange that no man dared reveal it?
One example given is a hydro-power dam being built "for the ship", with eight power lines running to the ship's construction site - but only two of them are real, the rest of the power being used elsewhere.
The second party of four men was to confirm a progress report from the Armories. According to the report, a "modification" had been performed on a power chain leading from a hydroelectric dam on the Denligh River in southern Yuma to the Yards. Three transformer stations had been established on the line before its eight cables reached the yards. All in all the line ran for several hundred miles through four new protectorate nations of the Caroline and the Badlands. That the lines had reached the Yards at all — all but two being dummies — was a bit of a miracle, for while the southern lands that the line traversed did not carry the legendary stigmata that the far north and west did, they still held the more pedestrian horrors in abundance. It was thus only slightly incredible that the Armories reported that only three men had been lost in its modification; two hundred men in the original crew had died. It had taken the Armories five months to carry out its mission; the War Office engineers had needed three years to build the real and fake power lines.