dietz%usc-cse%USC-ECL@MINET-NAP-EM@sri-unix.UUCP (10/11/83)
Someone's probably said this already but... The reason it saves energy to take the ET to orbit is that to put the ET into a special suborbital trajectory into the Indian ocean the orbiter has to be put (briefly!) into that same orbit. After releasing the tank the shuttle has to push on to orbit with the OMS engines. The shuttle is forced to follow a rather strange trajectory to dump the tank, actually losing altitude at one point (down to 100 km, I believe). Saving the tank lets you go to orbit on the SSME's, which are LH/LOX powered and thus have higher specific impulse than the OMS engines. Drag is the big problem. NASA doesn't want the continual Skylab-type hysteria that randomly falling ET's would provoke. Using the SSME's instead of OMS will get you to higher orbits for the same fuel mass. Ideally the tanks would immediately be dismantled or incorporated into some space structure. The tank could be placed in a higher orbit quite cheaply by raising it on a kevlar line with the shuttle on the bottom. The ET would be equiped with a winch to reel in the line after the shuttle let go.