Hello, i'm not adept of hyper-realism, but... i noticed that irl all services engines have very low isp, because they are using monopropellant (315 for apollo, 330 for fregat stage, 290 for soyouz). very high isp of the launcher upper stage engine are not used once in space because O2 and H2 are not storable for long time. so, i suggest the use of monopropellant for services engine (for small ship & lander). High isp for launchers, but add a power generator, producing around no power, but consumming all oxydizer in about 24h. so you have to use the fuel for launch and first orbit injection, and then use monopropellant... this does not apply to ION & nuclear engines, designed for interplanetary travel. that would lead to a very different rocket design. thanks.