Mr. Spaceship — Philip K. Dick

In Mr. Spaceship, humans in the distant future are at war with alien life forms known as Yuks. Using outdated, mechanical machines to wage war against the life-based weapons of the advanced Yuks, a research team decides to build a spaceship powered by a human brain. Enlisting a dying professor who agrees to donate his brain to the project, the humans believe they have turned the tables on the Yuks. But the spaceship has other plans.