Hi Marquis and the others, thanks for the interest!
Energy input is no huge worry for rocket fuels, whose cost isn't as extremely constrained as boat or plane fuels. So yes, pyrolysis of recycled tyres is one very interesting option. More so because the cost of garbage tyres is zero and they can provide the energy and the later hydrogen too.
Though, pyrolysis produces many compounds like aromatics, unwanted in the fuel nor in the dump. Tuning the conditions improves that, catalysts maybe too. The other option I considered here is ozonolysis, which targets specifically the double bonds and produces much more selectively the desired oligomers.
This depends on implementation at the right scale and at acceptable cost, nothing granted.