JF Mezei wrote on Fri, 16 Mar 2018
17:31:33 -0400:

On 2018-03-16 16:15, Jeff Findley wrote:

Block 1 will almost certainly not be Mars landing capable.

If it is able to propulsively land on Earth, doesn't that more or less
imply ability to land on Mars?


Not necessarily. You get a lot of help from aerobraking on Earth.
Mars is much more difficult because the air is so thin.


How much bigger would the paddles have to be to have same aerodynamic
control in Mars atmosphere vs Earth?


Atmospheric pressure on Mars is about 0.6% that of Earth.


In making the initial design, don't they have to make sure the
architectire will allow a Mars landing? For instance, calculate how
many engines will be needed to land and take off from Mars with X
payload, verify that there is enough space for whatever aerodynamic
devices will be needed (or failing that, allow for enough side thrusters
to provide attitude control during re-entry/landing etc ?


You'd certainly like it to, but go back to the example of Space
Shuttle Enterprise. It was built for drop testing. As such, they
didn't need to shave weight as aggressively, etc. It was originally
intended to rebuild it to 'space standard' after the tests were over,
but that just proved to be much more expensive than they'd
anticipated. The same thing could wind up being true of the first
Block of BFR Spaceship.

I'd be more inclined to think of it as 'Block 0', since I doubt if it
will consist of more than a couple ships, at most.


In other words, doesn't the 1.0 design have to inherently be
conceptually able to land on Mars, with subsequent iterations just
improving on the situation? (I use "conceptually able to land" because
I realise the 1.0 version may not have the software to do it, and
aerodynamic controls designed for Earth atmosphere).


Don't blame the software. The DESIGN might be fine, but they might
not build totally to that design. Again, think about Space Shuttle
Enterprise.


If your 1.0 design is say 25% short on thrust needed to land on Mars,
can you really bank on being able to improve engine performance by 25%
within say 15 years?


I'd say they probably can. They lowered the chamber pressure of
Raptor to make them easier to build and safer to run/reuse. SpaceX
plans to increase chamber pressure by 20% on future Raptor blocks.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw