Elegant solution, but it lead to the death spiral of the Aris I (to air start an RS-25 Aris I would have used a split second of acceleration from the single SRB to force the liquids from the much smaller upper-stage tank into the single RS-25 engine...fire in the hole!).
The Shuttle SSME (RS-25) is unusual (maybe unique) in that it uses the hydraulic head (the fluid pressure) of all that fuel and oxidizer in the huge ET to force fuel/oxidizer through the turbo pumps and get everything going.