Glass Rocket

Here is an image of a recovered Ariane 5 booster. After being recovered from the sea, the boosters were examined to ensure the stage performed as expected. However, refurbishing and reusing the recovered boosters was deemed non-cost-effective due to design and reliability complexities.

Single use solid rocket booster would be economical to operate compared to a multi use liquid propulsion stage like in SpaceX. The key is the size. It is easier and cheaper to scale a solid booster compared to a reusable liquid engine. Liquid engine design eat up R&D budget in no time. Instead invest on reducing the cost of producing the solid propellent mix and molding it into a rocket casing. This investment pays of while it would be utilized by military missile manufacturing as well. When you have high capacity and lower cost, you can build giant rockets or build hundreds of missiles on demand. Missiles are single use so as the solid boosters.

Ideally the empty rocket shell should completely melt down before hitting the ground. However I don't know a material which can withstand high pressure and temperature than melts down completely in several minutes (free fall time of a released stage). An option could be tempered glass rocket shell. It can withstand high temperatures and pressures with proper additives. And it can shatter into small round pieces when struck from a weak point like in Prince Rupert's drop. Even the rocket nozzle can be made out of shatter able glass (?). Given the trajectory of the rocket goes over the sea, small shattered glass would be scattered over the sea. Yielding minimum environmental hazard compared to a traditional rocket shell that has many exotic materials not good for the sea life.