We've seen 3D-printed rocket engines, but what about 3D-printed rocket fuel? Florida-based Rocket Crafters, Inc. (RCI) has been awarded a patent for a method of fuelling hybrid liquid/solid rockets using 3D printing technology. The company claims that the process ensures the fabrication of flawless, high-performance, safer-handling fuel grain. It will enter service in 2019 for launching satellites.

Used in Virgin Galactic's SpaceShipTwo, hybrid rockets are a sort of halfway design between liquid and solid rockets. Instead of incorporating a solid oxidizer in the fuel, a hybrid uses a liquid or gaseous oxidizer that flows through the combustion chamber of a solid rocket motor, which allows the pilot to throttle the motor and even cut it off without having burned all the fuel. In addition, it doesn't present the same risk of explosion as liquid rockets or solid monopropellants, like gunpowder, that incorporate their own oxidizers, and is simpler in design with lower operating costs.

The problem is that a hybrid's solid rocket component suffers from the same difficulties as traditional solid fuel rockets. Solid fuels are a common type of rocket propellant that are used in everything from sky rockets to Trident nuclear missiles. They usually consist of a polymer that looks a bit like synthetic rubber that forms an inner chamber in a geometric pattern where combustion takes place, so the fuel is also the motor. It's a reliable system that's been around for centuries, but making the solid rocket motors in which the fuel is also the motor itself is tricky and inexact.

Turning the solid fuel into a rocket motor involves either melting the fuel and pouring it into a mold, or compacting the solid grains either inside or outside the rocket. If the fuel is relatively insensitive, it can also be milled. Whatever the method, making solid rocket motors is time-consuming, dangerous and imprecise, plus conventional methods produce inconsistencies in the motor's makeup and generate vibrations in flight. Worse, for hybrid motors, the geometric pattern needs to be more complex and precise to produce the desired performance.

The new patent, granted to RCI co-founder, President, and CTO Ronald Jones, allows for the construction of safer and less expensive launch vehicles that have only two moving parts, and that can deliver small satellites to orbit at half the price of conventional launchers.

It works by using 3D printer technology to precisely place grains of fuel inside the rocket to make up a geometric pattern, which acts as the motor's combustion chamber. The fuel is laid down grain by grain in a concentric pattern of layers, with a port in the center where the oxidizer is introduced.

As the fuel burns, the precise setting ensures that the shape of the combustion chamber remains constant, and the burning even and predictable as each layer is consumed. To double-ensure consistency, the grains themselves are manufactured by 3D printing before assembly.

"The fuel grains we are able to produce using this technology provides the structural strength needed to minimize vibration build-up while still enabling the rocket engine to consume high energy solid fuel blends at an accelerated pace," says Jones.

RCI states that it is currently developing its Intrepid-1 booster, which is billed as the world's first mass-producible orbital launch vehicle using the 3D-printed fuel hybrid engines, which are scheduled to enter service in 2019.

"I have believed for years that hybrid rockets, due to the inherent safety when propellants are protected against accidental detonation by storing them in different states, could be the solution to make rocket powered flight as safe as airline travel one day," says former NASA astronaut and retired Sandia National Labs executive Sid Gutierrez, Chairman and CEO of RCI. "With our 3D-printed fuel technology, we now have the means to make this a reality."