Since my blog is read by the readers of Goodreads.com, I find it always a good idea to include some reference to one or more of my books no matter what I'm writing about. Since I am about to explain what the situation is with a great big rocket that NASA is trying to build that may be used to go to the moon, let me just say everybody not interested in this rather esoteric commentary but still interested in fictional accounts of going to and living on the moon should just read my wonderfully literate (and often funny) novels Back to the Moon and the marvelous trilogy (literate, funny, all) that began with Crater and then continued with Crescent followed by The Lunar Rescue Company. You can read about them and my other books here: www.homerhickam.com.
Now, with my GoodReads readers taken care of, let me explain in condensed fashion the problems with the core stage of the Space Launch System, aka SLS, and why it is what we euphemistically call the "long pole" of the development of this rocket. In other words, the core stage is the most difficult of all the components of the SLS to design and construct and will take the longest amount of time to prepare.
Why is that? Well, although vastly simplified, here are some of the reasons.
The core stage, which is really the first stage, utilizes as its base the design and materials of the external tank (ET) of the old space shuttle. This is because Congress required that NASA use components from the shuttle to build the SLS which must have seemed the right thing to do at the time. After all, here was a big rocket that already worked - the space shuttle and all. Just move its parts around a little bit, tweak them some, slap a new name on the resulting rocket, and away you go.*
Using the components of the shuttle to make a new rocket might be compared with remodeling a house. For those of us who have done both, we know that a remodel of a house is filled with problems that building a house from the ground up doesn't have. There are a lot of compromises required for the remodel that is avoided by a fresh start with a clean sheet.
With orders to turn the ET into a booster stage, the engineers assigned to it first ran into the problem that the tank would be subject to forces that were outside the scope of its original design. The ET was a marvel in engineering but it was supposed to take side mount forces (i.e., be dragged upward), not be pushed from below with a heavy load on top. In terms of forces, this is the difference between throwing a beer can (very little force on it other than flying through the air) and placing it upright on the ground and squashing it with the heel of your boot. Just like the beer can, the core stage is going to be crushed from above by the heavy upper stages while being shoved really hard from the bottom by multiple engines.
Faced with the requirement to build a rocket utilizing components outside their original design and function, SLS engineers responded by making the ET shell much stronger with lots of bracing. The designers were also faced with having to stretch the ET so it could hold more propellant which meant more bracing, all of which added weight. And then there was another force, much more insidious, that the designers had to face. Vibration. The two five-segment solid rocket motors (these also adapted from the space shuttle) strapped to the core stage is going to shake it like the San Francisco earthquake. Essentially a somewhat brittle aluminum egg, it's difficult to predict what will happen no matter how much extra support is added.
And then there are the friction-stir welds which are used throughout the core stage. If you don't know what this technique is, please go here: https://en.wikipedia.org/wiki/Friction_stir_welding
Friction-stir welds are not an unknown as NASA has been doing such welds for a long time and SpaceX has, too. They've just never experienced as much force and vibration on such welds before. SpaceX is a nice, smooth ride since all its engines are liquid. As mentioned before, a 5-segment stretch SRB is going to shake the bejesus out of the core stage and those welds. Again, how much vibration the core stage will receive is not entirely understood. That's why vibration tests were done in Huntsville. Computer models are also being run. Sadly, ultimately, the only certain way to find out the real answer is to fly the blame thing.
Another problem has to do with heat around the base. The ET, while being very strong, was thermally very fragile. Witness the Challenger. When a flame played around its base, it melted right through. On the SLS, the base of the core stage is going to receive a hotfoot by the engines strapped to its bottom. To protect it, a lot of extra shielding compared to the ET is needed down there. How much is required is not entirely understood. That's the reason for the so-called green test at Stennis that will see the engines lit. Unfortunately, those lit engines are reusable shuttle engines that are going to be thrown away. When they are used up, another kind of rocket engine is going to have to be attached at the base which means all the previous tests will have to be done again.
So there it is in a nutshell, the problems Boeing and NASA have to solve on the SLS core stage, partly because they didn't start with a clean sheet. If they had, their core stage would have probably looked a whole lot like a wide-body Falcon 9 and maybe it would have been built faster and cheaper and better.
One thing I'd like to make clear: I am not criticizing SLS engineers in this piece. My purpose is only to clarify its problems which we keep hearing about and has caused many schedule slips. The Boeing and NASA SLS engineers are doing their very best to prepare this rocket to fly and they should be praised for their work.
However, because of the problems which still loom over this rocket, my recommendation from the get-go when the Vice President decided we were going back to the moon was to take the SLS out of the critical path to the moon and instead develop it, including some flights, to where it could be put in storage and brought into service if needed later. If that concept isn't viable, I think SLS should only be used for cargo flights, not for astronauts. This would allow NASA to relax its schedule a little and make sure everything is OK before proceeding while using SpaceX and Blue Origin rockets to test out the Orion capsule and send it for a loop around the moon. But, for what may be more political reasons than technical, NASA seems to want the core stage to be ready right away, no matter what.
* As an aside for you history buffs, Ford did that very thing with the Edsel building it on the chassis of the already-existing Fairlane and Galaxie.