For decades, rocket scientists have dreamed of combining the solid and liquid-fueled engines found in launch vehicles into a single “hybrid” engine. Such an engine would possess the stability of solid-fueled engines but the controllability of liquid-fueled engines.
While hybrid engines have appeared on craft such as Virgin Galactic’s SpaceShip One, they are not as common as they could be, in part because it’s difficult to get the two phases of matter within them – solid and liquid – to combine during a burn completely and consistently.
One solution just revealed in a patent issued by the U.S. Patent and Trademark Office would be to create complicated channels within which liquid oxidizer could flow in a block of solid rocket fuel by using stereolithography, also known as 3D printing. In other words, it would literally “print” a tube of rocket fuel one millimeter-thin layer at a time.
To understand the challenges the inventors of this new approach hope to overcome, it helps to understand a little bit about the current state of rocketry. On the space shuttle, there are solid fuel boosters on either side of the craft, which can’t be turned off once they have been ignited, and the liquid-fueled engines at the back of the craft, whose firing can be controlled throughout flight. Hybrid engines possess the best characteristics of both.
An earlier patent by the same inventors described using solid fuel grains in complicated shapes. Apparently, they realized that the best way to create these shapes was by using stereolithography. Their latest invention reveals further research on complicated shapes at the macro scale, depicting three radically different designs for how a liquid oxidizer could flow through a solid propellant so as to achieve the desired burn characteristics.
The authors argue that this approach could finally realize the dream of hybrid rocket enthusiasts the world over - a consistent, controlled burn that eliminates half the liquid fuel a rocket would normally carry but which also allows thrust to be controlled just like in a liquid-fueled rocket. Considering that Virgin Galactic is already using hybrid engines on their rockets, it’s not hard to imagine such engine technology becoming a staple of the nascent space tourism and, eventually, commercial space enterprises.