JF Mezei wrote on Tue, 13 Mar 2018
17:01:25 -0400:

On 2018-03-13 05:53, Jeff Findley wrote:

BFS. It could be that the first BFS would be similar to Grasshopper or
Enterprise in that it won't have all the systems necessary for
supporting a crew in space. Automated testing only.

Thanks. Hadn't thought of that.

Does the ability to land on Earth absolutely imply the ability to take
off from Earth? (I know BFS is to be able to take off from Mars).


It can certainly take off from Earth. Remember, it's intended to be
able to do transcontinental flight without a booster. It's also
intended to be able to take off from Mars into TEI and then do the
insertion and land on Earth.


(I realise the prototype will be empty shell and much lighter so usable
for tests, just curious about whether the ability to take off from Earth
comes automatically with ability to land on Earth).


For some definition of 'take off' it does, but in this case BFR
Spaceship is capable of flying to orbit (if it has all its engines)
without a booster. I question the 'empty shell' assumption. Musk has
stated that it probably won't have the vacuum engines installed, since
they're not needed initially.


I know that spaceX has been pushing composite tank size limits by a huge
margin for the first stage. Is the 2nd stage/BFS also beyond current
"commodity" tech for tanks?


I'm guessing, but I would say yes. BFR Spaceship carries 1,000 tonnes
of fuel and oxidizer.



You can do a partial fill for short "hops". Does your car's gas tank
need to be full to make a trip to the grocery store?


Thinking in terms of keeping the methane liquid long enough in such a
large tank. Will the tanks be pressurized to maintain the methane
liquid, or will it be a "reduce the boiling rate and vent excess
pressure" on pad like for shuttle's ET?


There will presumably be some boil off, although that's more
manageable given the liquification point of propane as compared to
hydrogen. The bulk of the fuel in the tanks will still be liquid,
right up until you're out of fuel entirely.


Note: you car may not worry about fuel sloshing around in a small car
tank, but fuel trucks worry very much about it when the tanks are not
full, which is why they have separate tanks in the big one, and each of
those smaller tanks have baffles to reduce movement as truck
accelerates/brakes/turns.


The rocket is under acceleration. Fuel won't 'slosh' much, if at all.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn