I already wrote about SUSIE which is ESA's concept of a reusable upper stage. I think SUSIE has a lot of potential as an interplanetary "space van". With its combination of heat shields and retropropulsive landing capabilities, SUSIE would be able to land on planetary bodies with or without atmosphere.
The nameless Stoke Space upper stage (which for now I will call "the Stoke") is quite similar to SUSIE, but importantly is not just a concept and is actually being built. If the Stoke is able to fly on existing boosters such as Falcon 9 then this would be a win-win by allowing Stoke to enter service earlier and reducing the wastage of upper stages that booster companies have currently have to expend. I have no clue what boosters the Stoke might be compatible with and whether it will be possible to develop adapters to increase compatibility. About the only stats I'm aware of for the Stoke prototype is it's 19 feet tall without fairing, 13 feet wide and is intended to be able to carry 1.65 metric tons to LEO.
Update 5 Feb 2023: In Everyday Astronaut's visit video, Stoke's CEO Andy Lapsa stated that:
- The rocket is 100 feet tall (~30 metres), which would mean an 81 foot (~25 metre) tall first stage.
- The interface between the first and upper stages is "non-trivial", implying it would only work with their own booster. This is understandable given the unique integration and layout of the engines and heat shield. If the Stoke upper stage becomes popular enough then it's conceivable rocket-makers will decide to build compatible boosters.
