I believe that SpaceX is planning on building the BFS (Big Falcon Spacecraft) before the BFR (Big Falcon Rocket), which serves as the first stage for the second-stage BFS. Does anyone know why they are doing things this way round? I would have thought that BFR then BFS would have been more financially viable as BFS is not going to be of much use without BFR, but presumably BFR could be used without the BFS.
Neither has much financial purpose without the other. A BFR cannot perform any useful function without an upper stage, and that is the BFS.
Since the whole platform is a major investment in a new architecture, they are starting with the smaller piece - the BFS. Since it uses some of the same engines as the BFR, it can act as a testbed for both BFR and BFS hardware. Since it is smaller, it should be cheaper to construct (moldline, propulsion, fuel, attitude control, avionics -- things like ECLSS will most likely be expensive and time consuming, beginning development now and continuing past the first flights of both vehicles). Since it is more complex, it will likely take longer to get right. Both are good reasons to build first.
The BFR is a much simpler vehicle, mostly using concepts they've established in the Falcon - just updated with technologies that they will field test in the BFS. So it should need less fine tuning after construction, but takes more space and more money to build.
There's also the fact that the BFS is where they are most likely to run into issues getting their mass fractions (and payload capacity) where they want. Changes may be required of the BFS design, which will have impacts with the BFR design. BFR is dependent on the final BFS mass and size.
Elon Musk stated in a news conference after the Falcon Heavy launch that the BFS will be the focus because they think they understand designing booster rockets pretty well, and thus they decided to focus on the more difficult piece first.
He answers this in response to a question that starts at 20 minutes 52 seconds here, and speaks specifically to starting with the BFS at 22 minutes 18 seconds. The full response has been transcribed here to provide the full comprehensive context of his statement.
Q: So... the potential to go to the moon or mars, what's your timeline there?
Elon: There are a lot of uncertainties on this program, but it is going to be our focus now that we're almost done with Falcon 9 and Falcon Heavy. We're gonna level off as I said at block 5 or version 5 as there won't be any more major versions of Falcon 9 or Falcon Heavy. Dragon is also going to level off at dragon version 2. There might be point releases, like 5.1 or dragon 2.1 or something like that but most of our engineering resources will be devoted to BFR and so I think that will make things go quite quickly.
The ship part is by far the hardest, because that's going to come in from super-orbital velocities, like interplanetary, Mars transfer velocities, Earth-Moon transport velocities. These are way harder than coming in from low-earth orbit. There's some of the heating things that scale to the 8th power. I didn't realise there was anything that scaled to the 8th power, but it turns out certain elements of re-entry heating scale to the 8th. So testing that ship out is the real tricky part. The booster, I don't want to get, you know, complacent, but I think we understand reusable boosters. Reusable spaceships that can land propulsively, that's harder, so we're starting with the hard part first.
I think it's conceivable that we do our first test flight in 3 or 4 years of a full-up orbital test flight including the booster. We'd go to low-earth orbit first, but it would be capable of going to the moon shortly thereafter. It's designed to do that.
Straightforward. SpaceX is Goal-oriented.
Better to know your needs up-front and plan around them than the other way around.
You start with the intended payload requirements because if you build a Big Rocket and try and cram your payload into it, you soon find you've built a rocket that isn't big enough for what you need, or is inefficient at carrying it, then you have to break up your payload into multiple launches (so much more expensive!) or redesign your Big Rocket again and then you might as well have started with the payload in the first place.
As with all orbital rockets, you have essentially two parts - upper stage & payload (BFS) and the means to boost it to orbit (BFR).
If you finalize the design of your booster first, that will set limits on what sort of upper stage/payload you can launch with it. Finalize the design of your payload first, and you then get to design a booster that gets it to orbit with enough performance margin to be reliable and not too much margin to be wasteful.
Going about it the other way round would be putting the cart before the horse.
Just my two layman's cents.