I mean what is he going to do with the ship alone? Is it even capable of orbital flight on its own?
Now the booster can be put to use immediately, and will probably be easier to make by far (I think)? Although, seeing how much use falcon heavy has seen, I don't know about the demand about such a powerful rocket ATM, but they can at least launch big batches of StarLink satellites, I guess...
So, what are SpaceX plans?
Here are some Starship and SuperHeavy pictures, just for fun:
Starship alone is not capable of orbital flight, but it is capable of high suborbital flights sufficient for testing reentries.
The booster's only use is as a first stage for Starship. It could only be put into use immediately if a Starship has been developed and built to stack on top of it. And it is expected to be much quicker and easier to develop than Starship:
All this means that if they started Superheavy first, they'd have it sitting around idle for the entire program to develop Starship.
Developing the booster first would also more tightly constrain Starship's design, because they'd have to go back and redesign Superheavy to make any major changes. This was an issue for Orion/Ares-I, where they developed the booster first, but ran into problems getting the upper stage and capsule light enough to reach orbit on it. Another similar issue was Starliner, which was developed to launch on the existing Atlas V and Centaur upper stage. Some aerodynamics issues arose late in development that required an "aeroskirt" be added to protect the Centaur during launch.
By starting Starship first, they'll be able to run it through a suborbital test program and further iterations of design refinements while building the Superheavy booster, they'll know exactly what Superheavy has to lift, and they'll be ready to start full orbital flights when Superheavy's done.
In simplified terms, the list of minimum requirements for completing the missions Starship is set to do is much longer and more complex than that for Superheavy. For Earth Orbit or Lunar missions, the Starship absolutely needs to:
The Superheavy booster, though, only has one absolutely necessary function, launching. Reuse for both of these vehicles, while essential for long-term financial success, is unnecessary for near-term operational use. The Superheavy's boostback and landing is very similar to Falcon 9's, which further reduces schedule risk due to unanticipated developmental delays.
As for other considerations for vehicle development like materials and assembly processes, SpaceX is hashing out all its metallurgy and welding problems on the Starship prototypes, and the optimal solution from that can be copied for the Superheavy.
I mean what is he going to do with the ship alone?
Develop it. Test it. Verify it. Perform suborbital flights.
Remember that SpaceX is also toying with the idea of offering point-to-point passenger service on Earth. (Personally, I believe that will not happen, though.)
Is it even capable of orbital flight on its own?
No, but for testing launch and landing you don't need to go to orbit.
Now the booster can be put to use immediately,
The booster can be used for exactly one thing: launch Starship. If they don't have a working Starship yet, the booster is useless.
and will probably be easier to make by far (I think)?
Yes. SpaceX have always had the maxim to tackle the hard problems first, which is another reason to focus on Starship first.
Although, seeing how much use falcon heavy has seen, I don't know about the demand about such a powerful rocket ATM, but they can at least launch big batches of StarLink satellites, I guess...
The biggest commercial use of the Starship system in the beginning will indeed be Starlink. The Starship system can launch almost seven times as many satellites per launch (400 instead of 60) as Falcon 9 can. This brings the number of launches for the planned first batch of 12000 satellites down from 200 to just 30. In total, SpaceX has just filed for another addition which (if they use everything they filed for) would bring the total number of satellites up to 42000, which would require only 105 launches on the Starship system instead of 700 on Falcon 9.
However, some commercial customers that have booked launches on Falcon Heavy have already given permission to launch on the Starship system instead, and looking further out, SpaceX will retire the whole Falcon line in favor of the Starship system. It should be cheaper since it is 100% reused and will land very close to the launch pad and not require the ASDS. (The only reason to keep the Falcon 9 for a limited amount of time would be to bridge the time where Starship is operational, but not yet human-rated, so they need to keep F9 around for Crew Dragon.)
The breakthrough idea and most demanding engineering task of Starship/SH is complete re-usability of the orbital vehicle. If you tried to reach orbit with Super Heavy alone-- it's not designed for this and so I'm not sure it's even possible-- it would just be a large expendable booster, which might well be more expensive to operate than the partially-expendable F9. Amost all the difficulty, and so required development time, as well as value lie in Starship as a reusable orbital second stage. Therefore it is natual that that is where SpaceX puts their initial development work.
An important point to understand: it is not as simple as "just put this stage on that booster." Real life is more complicated than Kerbal Space Program. There is a lot of design, test, and certification work that has to be done before using a certain stage combination -- time that is better spent working on Starship -- so the SuperHeavy booster is not really useful until Starship is finished, and Starship is the harder part of the two so SpaceX chose to start with it.
One reason I remember Musk saying in an interview or something somewhere is that the starship is much more difficult to design and much higher risk, obviously since it has to do much more, while the super heavy is much more like a scaled up falcon 9 booster. So they started with the more difficult and higher risk part, which will take longer to finish. Super heavy can be designed in a shorter period of time, so presumably they can start with that at a time such that both are ready more or less at the same time. And by doing the high risk parts first, they will find out earlier if there is something that needs to be radically changed, and not have a large wasted investment in the super heavy.
A second reason is that the super heavy requires 31 raptor engines while the starship only requires 6 or so, and only 3 for atmospheric flight. Their production rate of raptor engines isn't high enough yet to have all those engines ready, so better wait with the super heavy until enough engine production capacity is available.
(And Starship has barely enough thrust to lift itself when the vacuum engines are also used, which might be useful in an emergency escape scenario but not for any regular operation. So ideas about Starship flying to places by itself are non-starters.)
