More on the External Tanks

This is the next of a series (to view these, click on label at end of this post) on recycling the External Tanks on the Shuttle derived launch platform, which is now in NASA's budget as the heavy lifter for future space missions.

It occurred to me that a minor redesign of this system may be necessary in order to optimize it.  For instance, what will come of the rocket engines after they shut off?  It may be more trouble than it is worth to try to recycle those.  If you can preserve most of the external tank, that could be a good start in implementing this concept.  Let's propose that the rocket engines be jettisoned and a third stage be built inside of the external tank. 

This may be a preposterous idea, yet, what to do about that rocket engine?  It is in the way, and it would be helpful to jettison it so that the external tank can be put into higher orbit.  If the rocket engine and the big hydrogen tank can be jettisoned as a unit, that could be even better.  This would free up a lot of space inside of the external tank outer shell.  The remaining oxygen tank can be used for habitat and storage.

Now, in the old Shuttle system, there was an intertank module which was located between the big hydrogen tank and oxygen tanks.  If a rocket stage is put in there to replace this, it can be used to push out the big expendable rocket engines plus big external tank assembly, and then provide thrust to move the shell into a higher orbit.  The shell would be preserved along with the oxygen tank.  Perhaps this third stage can be preserved as well, or it can be recycled, which ever is more desirable.

Naturally, you want to facilitate the remaining work in order to finish converting this into a space station.  Let's start by placing bolt holes in strategic locations so as to allow multiple tanks to be bolted together.  Subsequently, up to 4 of these tanks can be bolted together in and spun at 2 rpm in order to generate artificial gravity. 

But before that is done, the tank will require some work.  For that, two of these tanks can be tethered together in order to produce .1 g of artificial gravity as mentioned in an earlier post.  The artificial gravity will facilitate the wet engineering of the tank.  (It is easier to work in gravity than having to work in zero gravity.) 

By the time two tanks are ready, they can be reeled in and bolted together.  Two more tanks can be processed in a similar fashion giving 4 tanks in the final configuration. 

Before the entire system is bolted together and/ or tethered, a docking facility can be launched and put into place between the two pairs.  This should be at the center of gravity.  The docking facilty will have to set up in such a way as not to introduce instability into the spinning station. 

That might be easier said than done.  In additon to the foregoing discussion, it may be necessary to include some small thrusters in order to keep the system stable.

Update:

What would this cost?  A shuttle mission went for about half a billion.  Multiply this by 4 and the cost of launching 4 of these would be comparable, I would suspect.  Throw in the costs of reburbing the tanks and bolting them up into a big station, and you've up to a couple billion.  Double that and you've got the cost of an aircraft carrier.  We could afford it.  The payoff could be huge.

Update:

Additional thoughts:
I.
Two options for ejecting the rocket motor and big hydrogen tank  1) using rocket exhaust from internal stage, push against a plate in the same way an internal combustion engine pushes a piston, or  2) use an electric mass driver concept to eject them both away.  In both instances, the matter ejected can be thought of as reaction mass. 
II.
Spinning the station can produce an electrical current within the magnetic field of the Earth.  By supplying one aboard the station, these two fields can repel each other thereby creating rocketless propulsion.  Or it can supply electricity for the station.
III.
Perhaps it is possible to deploy the station at EML1 and deploy a tether which would extend all the way to the lunar surface.  Cargo and maybe even crew can be sent to the surface and back.