Soon after Titan was discovered to possess an atmosphere (confirmed in 1944, though suspected decades before) far-thinkers have pondered the options for using that atmosphere to explore the Outer Planets. In the 1950s the chief option was using liquid methane as a nuclear rocket propellant. In the 1970s Titan was suspected to have more than just methane in its atmosphere, causing Bob Parkinson to speculate on the presence of helium and the possibility of sourcing helium-3 for the “Daedalus” starprobe. While that didn’t pan out, by the early 1980s large amounts of nitrogen was confirmed, with methane only playing a minor role in the atmosphere.
Methane exposed to UV light from the Sun undergoes chemical reactions that polymerises it into make the quasi-opaque haze that obscures Titan’s surface. In the process it should crack methane and cause it to combine into ethane and longer chain hydrocarbons – alkanes like propane, butane etc. – and a slightly more exotic hydrocarbons, the alkynes. The simplest is acetylene, C2H2, featuring two carbons united by a triple bond. Large amounts have been identified on the surface of Titan, identified thanks to clever processing of data from ‘Cassini’, which leads to thoughts of using acetylene as a propellant. Oxy-Acetylene is the first reaction that springs to mind, but free oxygen is in short supply on Titan. Oxygen will need to be cracked from water or carbon dioxide, both of which are part of the ‘lithosphere’ in the cryogenic conditions on Titan.
Acetylene by itself is quite an energetic substance, happily exploding into carbon and methane thanks to the carbon triple-bond’s instability. Unfortunately for rockets and other engines the resulting gases are too hot to handle. At least two bi-propellant mixes using molecules available on Titan are options for a ‘cooler’ reaction. One reaction is hydrogen-acetylene, the other ethane-acetylene. Storing hydrogen is a pain (at least at Earth-ambient, but less so on chilly Titan), so we might prefer ethane as the additional reactant. In the patent which first discusses the mixture about 2-3% oxygen is included, to provide the initial kick. This will need to be supplied from a ‘cracker’, but the proportion is much, much less than if it were being used as the oxidiser, rather than as an initial ‘spark’.
Relevant Patent: Use of acetylene-ethane mixture as propellant and explosive