SLS mission idea

speculation alert

There were several posts here on the subject of re-using the big external tanks that the shuttle system used.  Since the SLS will be shuttle derived, the same proposition could exist.  Nobody at NASA is thinking about this, I'm sure.

I recall that the shuttle launch system accelerated the external tank to near orbital velocity.  Now, if you were to lighten up the payload, the same amount of fuel could get to orbit.  That means you have a big external tank up there.  What could you do with it?

One idea I just came up with is to make a fuel depot out of it.  You could convert the hydrogen tank to methane, and keep the oxygen part of the tank as is.  The new methane tank will be too big, perhaps.  Maybe you could flip them.  That is, use the big hydrogen tanks for oxygen and the smaller oxygen tanks for methane.

You'd probably want to ditch the engines.  I figure that you could ditch them and replace them with a fueling interface.  The fueling interface will facilitate re-filling the big tanks and using the big tanks to re-fuel spacecraft.

After ditching the engines, you'd reconnect the plumbing to the interface.  It just occurred to me that this would probably be easier said than done.

Anyway, the original configuration routed the fuel to the engines, so the plumbing should be re-usable.  You may not want any of the original pumps.  These would have to be bypassed if they cannot be ditched.  Perhaps a little engineering before launch would allow connections to be made that would be relatively simple.  In fact, this may be necessary as a spacewalk by an astronaut for this type of task cannot be allowed to be too challenging.

So, the interface plus the tank itself would be all that is launched.  In practice, the launch would take place.  The engines and other hardware would also be ditched.  Then the fuel interface would be installed.  After installation, it could allow docking a refilling the big tanks or spacecraft.

Thorough testing should determine operational status.  After this is determined, you have to decide where to put it.  It could be towed to EML-2, where it could be used a fuel depot for trips to Deimos and Mars.  Oxygen can be brought from the Earth and the lunar surface, whichever is easier.  It may be easier to bring oxygen from the moon because of the shallower gravity well.  But bringing it from the moon would require a launch from the surface.  Perhaps a loxleo system would be easier so that it can be brought from the Earth.

Since the major part of the mass of a methane/lox system is oxygen, it may pay off not sending it very far.  Thus, for the Earth to EML-2 leg of a journey, you'd just use loxleo oxygen.  For the EML-2 leg to Deimos, you just use lunar oxygen.  For the leg to the surface of Mars from Deimos, you'd use the oxygen and methane obtained from Deimos.  To get back to Deimos from Mars, use the Martian atmosphere and methane from water and carbon dioxide on Mars.  Reversing the steps gets you back to Earth.