Image Credit: IEEE / Emily Cooper
In the immediate aftermath of the Virgin Galactic disaster, SpaceShipTwo’s hybrid rocket motor came under suspicion as the most likely culprit. And, although subsequent updates by the NTSB team investigating the accident have instead centered on a problem with the craft’s feathering device and likely human error, many articles have continued the drumbeat of criticism over the selection of hybrid motors in the first place.
To a great extend, much of it is understandable, particularly considering VG’s prior experience and the fact that Sierra Nevada Corporation made a last minute change from hybrid to liquid propulsion engines for its Dream Chaser space plane. Hybrid rocket motors are a dead end technology, even if Virgin Galactic doesn’t know it yet.
Or are they?
An article in Spectrum, a product of IEEE, the Institute of Electrical and Electronics Engineers, looks at a different type of hybrid; the wax or paraffin motor, where recent advances by a private company working with NASA Ames, The Space Propulsion Group are yielding performance increases which could bring this much abused technology within the range of liquid rocket engines.
From the article:
“Over the past eight years, our company has successfully conducted more than 40 test firings of a liquid-oxygen, paraffin-fueled hybrid motor that is 28 centimeters in diameter and produces 25,000 newtons of thrust. More recently, Space Propulsion Group has been testing a 56-cm diameter motor, one capable of 100,000 newtons of thrust—enough to lift more than 10 metric tons. The result of all our trials—which at one point included a motor exploding—is that our designs are now very reliable and produce very high specific impulse.”
The critical component is the selection of a hydrocarbon which is solid at room temperature, but rapidly converts to a low viscosity liquid when heated. Apparently, with the right tweaking, paraffin, or “hurricane wax” fits the role nicely, coming in at just the right molecular weight to allow ridiculously safe handling characteristics when being formed, transported, loaded, etc., but burning quickly enough when presented with an oxidizer to allow significant thrust levels to be achieved.
Although common to the point of being mundane, wax has some interesting characteristics, finding widespread use in thermostats, where the solid to liquid conversion process (and back again) has been keeping automobile engines running at the right temperature since the 1930’s. While it seems unlikely that the operational costs of the manufacturing and swapping hybrid motor cartridges could successfully compete with the economics of liquid fuels, it might turn out that for at least some applications, space tourism being one of the them, the safety factor could hold great appeal.