I've been launching a lot of the big Mark Twain rockets over and over to try to figure out why they sometimes just disintegrate and explode on boost. The big Rockomax Mainsail engines just hammer the structure with lots of vibrations as they gimbal raggedly during boost. This cumulative damage is apparently enough to cause the central core stack to break apart, usually shortly after separation of the side stacks. I can get the rocket to work by throttling down the engines, but that's not an efficient use of fuel. So I've been trying changes to make it survive a full-throttle boost. Below is the current iteration:
Recently at the suggestion of George Gassaway, I replaced the central Rockomax Mainsail with a cluster of smaller engines, and this worked out well. I originally had these mounted on small fuel tank side pods, but those were prone to getting damaged during separation of the side stacks, so I used a cool new clustering method (also pointed out by George) that uses the small cubical strut framework to make attachment points for multiple engines on the bottom of the tank. Unfortunately, when mounted this way, the engines produce enough vibration to sometimes disintegrate the central core (I think the side pod mounting tanks were absorbing some of the vibration). So I changed to using four of the LV-T45 vectoring engines and one LV-T30 in the center, which is mounted on a small fuel tank (necessary to prevent it from overheating in the center of the cluster). The central engine on the small tank vibrates around like crazy, but it holds together. I tried putting on some reinforcing struts, and this did stop the vibration...but it caused the core to disintegrate again. I think the vibrations of the small tank are absorbing some of the damage and protecting the main tanks.
The latest thing I'm trying is a set of struts that I saw suggested online. These use the small cubical framework structures tied together across the tank boundary with struts. This makes the interface between the big tanks nice and firm, and so far the modified rocket has not failed on boost. But, then, I thought that before, and eventually found problems. We'll see.
As a result of all this rocket testing, I've been accumulating several Mark Twain ships in orbit. I sent one off to the Mün and left it there for later landing missions. Another I sent off to Minmus to await further exploration landings. And I sent two to Eve and Moho (see previous posts).
Because I had some shortage-of-fuel problems on the trips to the inner planets, today's project was to dock a newly launched Twain to an orbiting refueling station that I had put into orbit earlier (with a similar launch vehicle, but without the solid boosters). Maneuvering the big Mark Twain around for rendezvous and docking was harder than doing the same thing with the small landers, but below we see the Twain moving in on the Tanker station.
A successful docking!
After docking, I re-extended the Tanker station's solar panels (which I had closed before docking to prevent accidental damage). I was able to top off the Twain's main fuel tank and RCS tanks (by alt-right-clicking on the tanks involved and clicking the necessary buttons that appeared).
So then I had a fully-fueled Mark Twain ready for an interplanetary mission!
Also then, I accidentally hit the Action Grouping key that separated one of the landers from the side of the Twain. Arrgh. That was clever. NASA has little guards over important switches to prevent them from being accidentally bumped. I wish I did. So I pumped the fuel back into the tanker and landed the Twain back on Kerbin.