Combustion engines on the Moon and Mars
Methane engines like Raptor brought a couple things to my mind. First of all, as Starship like spacecraft commercialize space access, Titan will become a giant methane resource or in other words a Moon sized gas station. Second of all, combustion engines on Moon and Mars are a difficult thing, because there is no significant amount of oxygen in the atmosphere... but if you recycle the materials generated from combustion, a combustion engine that uses methane from Titan (in the EU cars run on natural gas) could prove an interesting solution. If one were to develop a combustion engine without emissions at all, one that recycles all the exhaust, it could find applications not only on worlds with no atmosphere or a thin one devoid of oxygen, but also on Earth. Take the idea and see what you can do with it. Feel free to send some cash to my PayPal if you monetize it.