A company in Norway (Ripple Aerospace) is developing a sea-launched, plug nozzle-equipped, reusable two-stage satellite launch vehicle (Sea Serpent 1) which—like Admiral Robert C. Truax’s SEALAR/Excalibur/Seahorse/Sea Dragon vehicles—can be scaled up to arbitrarily large sizes. Below are links to information on it, as well as a bunch more Project HYDRA-type (and related) rocket material: