Well all planes that haven't been developed yet are not in service, and development cycles for jet engines are always long. Investing in engine R&D is standard practice. While Boom doesn't have a fleet of aircraft at the moment, it didn't 2 years ago either, but it does have orders. While of course those orders don't mean much if Boom can't ultimately deliver and sunk cost is not a good reason to continue a venture, the fact that RR entered into the agreement in the first place suggests they believed at some point it made business sense.
Indeed, supersonic business jets are ideal for the service contract based market. Supersonic engines need much more frequent maintenance than subsonic engines, business jet owners are much less price sensitive than mass-market airlines, and its not like you can just swap in a different supersonic engine, operators are completely locked in.
Far more likely, RR wanted Boom to use one of the supersonic engines it already has developed with minimal modification but Boom needs either heavy modification or a clean slate design to reduce fuel consumption (the second biggest issue for commercial supersonic aviation).
Indeed, supersonic business jets are ideal for the service contract based market. Supersonic engines need much more frequent maintenance than subsonic engines, business jet owners are much less price sensitive than mass-market airlines, and its not like you can just swap in a different supersonic engine, operators are completely locked in.
Far more likely, RR wanted Boom to use one of the supersonic engines it already has developed with minimal modification but Boom needs either heavy modification or a clean slate design to reduce fuel consumption (the second biggest issue for commercial supersonic aviation).