That is incredible!
I've been exploring the 3D Warehouse, it's like going to a digital Smithsonian!
I mostly work with the Acad/Autodesk Land Desktop Civil package, but I've always wanted to learn 3D modeling to satisfy my Tom Swift side. (Backyard spaceship design)

I created this model back in 2005. It was before Direct 232, but a lot of people were talking about a Shuttle External Tank derived heavy lift vehicle.
The buzz at the time was to modify an ET, and stick some RS-68 rocket motors under it.
I hope this doesn't offend the very real rocket scientists here, I did this purely for fun, like building a plastic model of a P-51,...but without the glue smell.

I started out by first building a shuttle external tank in autocadd, using dimensions from the LockMart/Michoud Assembly Facility web site. It's basically just an H2 tank, an Inter-tank adapter, and an O2 tank. Each component is a separate DWG, so H2.dwg, Intertank.dwg, and O2.dwg.

For no reason other than it would look cool, I decided to use 5, RS-68 rocket motors. So, I looked up the volume of H2/O2 burned by an RS-68 in a Delta IV, and multiplied by five.
Acad users will know, since we always draw 1:1, I was able to “List” the approximate volume of the basic ET's H2 & O2 tanks. I then looked up the propellant load figures from Michoud, and derived the excess tank margin I needed. From that point it was just, lengthen the H2 tank, and create a new larger O2 tank, (Orig ET O2 tank has a ballistic nose) to reflect the additional volume required by 5, RS-68's.
I confess, I did it by trial and error, until the two tanks “Listed” at the needed volumes.

From there, all I did was use three, standard ET Inter-Tank adapters, one to construct a thrust structure for the RS-68's, (With some Saturn V like nozzle flairs) a second Inter-Tank to connect the propellant tanks, and a third as an inter-stage to mate with the Upper stage/Fairing.
Never got around to building a second stage, so it's just a big ass booster with a Fairing.

Height: 295.76' (90.15 meters)
Diameter
Core: 27.6' (8.4 meters)
Shroud: 32.81' (10.0 meters)
(file no. 52 from the old board's gallery, not restored)
(file no. 53 from the old board's gallery, not restored)
(file no. 54 from the old board's gallery, not restored)
(file no. 55 from the old board's gallery, not restored)