JF Mezei wrote on Sat, 17 Mar 2018
02:40:04 -0400:

On 2018-03-16 21:57, Fred J. McCall wrote:

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


Which would mean BFS needs to have more oumph! from its engines, right?
So when they make the general design, upon which, the first test flight
will be based, they need to factor in enough Raptor engines for it, right?


Yes, they have to actually design things, which seems to be what
you're wailing about.


Enterprise didn't have engines and didn't even have fake engine bells,
it had a aerodynamic cowl in the bacjk, but the dimensions of the engine
area were the "right" ones for the 3 SSMEs and 2 OMS engines. And this
was "sized" right from the size for the types of missions the Shuttle
would be able to fly.


So?



Shuttle Enterprise. It was built for drop testing. As such, they
didn't need to shave weight as aggressively, etc.


Modern design/manufacturing make this different. The prototype tanks
cannot be built by hand. The precise fibre weaving patterns have to be
developped because you will be fueling the first prototype. You can't
just put an oversize plastic coke bottle in its place until the real
tank is designed. And SpaceX has designed the real tank.


What are you gibbering about now?


Similarly, you can't put an aérodynamic cowling below the BFS in lieu of
engines. You have to have the real engines if you want the prototype
rocket to lift off and land.


You don't need the vacuum engines to lift off and land. You do need
some if you're planning on going clear to space, but even then you
probably don't need all of them until you start flying heavy cargos.


So while Enterprise only had to demonstrate its glider capabilities,
the BFS prototype will have to demonstrate a far greater proportion of
its mission profiles. It needs real engines, real tanks, and real
landing legs and software. As a vehicle, it will lack the heat shield.
(or may have it, but won't be tested) and obviously, lack a man rated cabin.


If you don't need the heat shield then you don't need the vacuum
engines. I don't know why you're so wrapped around the axle about the
damned tank, since we all know that's done.


The other difference is that while NASA was able to greatly reduce
weight after Enterprise prototype, this doesn't happen anymore because
the modern design software already optimizes structures and generally,
some parts need to be beefed up after prototype is built. (aka: 787
ended up significantly heavier than originally predicted by Boieing,
same with the A380 from Airbus).


Untrue. It certainly does still happen.


Consider Falcon Heavy which also had to be beefed up once SpaceX
realised it needed much strenghtening.


One core out of three.


What remains to be seen is whether the first flying prptotype will have
some "heavy" skin to simulate the weight of a heat shield. (more to
measure landing/guidance than to measure launch performance).


Heat shields aren't that heavy compared to other structure. And do
you not pay attention or does your memory really reset at midnight
every day. Musk has said that they will probably build the
'Grasshopper' article without either vacuum engines or the heat shield
because that gives them a bigger fuel margin during early testing and
those things can be added later.


In most cases, modern airplanes tend to have first production units
being heavier than first prototype. During flight testing, they notice
the need to strenghten a certain area etc etc. (and then over the years,
they manage to reduce weight bit by bit).

The Shuttle was different because Enterprise was designed as a proof of
concept


No, it was designed as a test article, which is not the same thing.



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.


With Modern design techniques, Enterprise would have been built
pre-optimized.


Horse****. They didn't design the thing using bearskins and stone
axes, you know.



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.


Everything from tanks to bottom should have a "dinal" design aspect to
it (one which will be tweaked, but nevertheless, planned to be
functional. Sicne the "spaceship" portion won't be done and BFS 0.1
might as well carry another car inside, the top portion may just be an
aerodynamic shell from the composite design point of view (no load
bearing capability). But the "business end" should be fully designed,
unlike Enterprise whose business end had an aérodynamic cowling instead
of the real stuff.


No vacuum engines and no heat shield. Say it with me....


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