Spin drives are a form of propulsion utilizing Astrophage in Project Hail Mary.
How It Works[]
The Hail Mary utilizes "spin drives" (1,009 of them, according to Dmitri Komorov) to leverage Astrophage as propulsion for space travel. A spin drive uses a three-phase cycle to achieve this: first, it attracts Astrophage fuel to coat one side of a transparent triangular drive mechanism using low-intensity light with wavelengths of 4.26μm and 18.31μm (peak frequencies of the CO2 emission spectrum, called the Petrova frequency after the discoverer of the Petrova Arc); then rotates the now coated surface to be exposed to space and increases the intensity of said light to cause the Astrophage to "push" even harder against the slide, imparting momentum to the craft; and finally the slide is rotated back into the engine housing where the now "dead" Astrophage is scraped off to allow the cycle to repeat.
Usage[]
Only the Hail Mary is confirmed to use spin drives. The Blip-A might use a very similar form of propulsion or something vastly different, it is not explained in the book other than that the Blip-A does use Astrophage as fuel.
Spin drives have an extraordinary specific impulse and delta v for even a small amount of fuel, as Astrophage are very energy efficient.