Continuing…
In 1985 Rockwell gave thought to adding relatively small liquid propellant boosters to the undersides of the Orbiters wings. Even though the boosters were relatively small, with only a single RL-10 engine fed from low-density, narrow-diameter liquid hydrogen tanks. Even so, Rockwell projected an additional 15,000 pounds of payload. This would seem to require some interesting modifications to the underside of the wings… not just adding mounting hardpoints, but doors that could close over them after the boosters are jettisoned.
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