Riding Carbon Water Balloons to Space

The two most important technical elements of launch vehicle design are high performance engines and high propellant mass fraction structures. A dynamically efficient launch vehicle needs more than ninety percent of its liftoff mass to be rocket fuel. Ideally the propellant tanks, which also serve as the primary structure, are very lightweight thin-walled liquid containers capable of also being pressurized to expel the propellant into the rocket engines – so something akin to a water balloon….that also must be capable of remaining a perfectly rigid cylinder while getting pushed from one side by hundreds of thousands of pounds of fire-breathing rocket force and support the rest of the rocket and payload as it accelerates to hypersonic speeds on the other side. And if that wasn’t bad enough, the problem of building lightweight propellant tanks is compounded by the fact that they must contain chemically reactive oxidizers often chilled to cryogenic temperature levels, which tends to make many engineering materials brittle and/or spontaneously catch on fire.

At Firefly we have chosen to build our Alpha structures and propellant tanks using modern materials — carbon fiber composites — which are constructed using carbon fiber fabric in a hardened resin matrix (glue). Containing liquid oxygen in a thin-walled composite tank is difficult because cured resins become brittle at low temperatures (-300 F for liquid oxygen), which leads to cracking and leaks through the wall of the tank. We have solved the problem through experimentation, where the two primary variables are carbon fabric weave pattern and resin composition. 

The photos above show a recent test of our material solution fashioned into an engineered flight-like Alpha upper stage liquid oxygen tank. Our structures team, lead by Jeff Duncan, has successfully tested this tank with liquid nitrogen at 100 psi pressure, which is both colder and at significantly higher pressure than is needed for Alpha. This accomplishment supports fabrication of full Alpha flight stages. The second picture above shows Firefly composite production technicians fabricating the domes of the tanks that will be used on our first fully integrated stage tests in June 2018.