Le Mar/18/2018 Ã* 9:56 AM, Jeff Findley a écritÂ*:
In article ,
says...

On 2018-03-17 09:33, Jeff Findley wrote:

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

Block 1 might not be able to be refueled, so it couldn't get to Mars in
the first place.

I was asking in terms of design/concept, not about early production
models.

Huh? If the vehicle built is missing key components needed for a Mars
mission, it can't really be said to be capable of such "in concept".

Earth has stronger gravity, but also denser atmosphere. So if
you have enough thrust to decererate and land on Earth, would this
imply that you have enough thrust to land on Mars? (less gravity, but
also much less atmosphere to slow you down)

Yes, you need less thrust to land on Mars.

In a "Pan Am flight 006 to Mars and back, would the mass of the vehicle
making the drop from space and land to surface be roughly the same ? Or
would landing at one planet require much more fuel?

You'd need a lot more fuel and oxidizer to land on Mars since Mars
atmosphere is so thin you won't get anywhere near as much aerodynamic
braking as you get on earth. So shedding velocity will require a much
longer burn from the engines than on earth (where the engines only need
to perform a relatively short landing burn).

So far, Falcon 9 stage 1 landed mostly enpty without payload and with
just enough fuel to land. But BFS will land with full payload of people
and 6 months worth of garbage. So onlike Falcon 9 stage 1, BFS is likely
to require a far greater percentage of its engines to land. Right ?
(hence question of whether the number of engines needed is dictated by
the eventual landing with 100 passengers on Earth or on Mars).

https://ourplnt.com/making-life-multiplanetary/bfs/

BFS will have three sea level engines for landing. But the extras are
mostly there for redundancy. Depending on the payload, I'd think one
would be sufficient in most cases, even on earth. Raptor will have
about 400,000 lbf thrust at landing. Dry mass of BFS is reported to be
about 200,000 lbs. A "typical" payload at landing is reported to be
110,000 lbs. So one Raptor is all that's needed for a "typical"
landing.

According to the numbers you give one Raptor engine isn't enough unless
you have a lot of fuel for landing. One Raptor has enough thrust for
about 1.3 g. The first g only compensates for gravitation so you have
0.3 g of deceleration. If you are coming in at say 100 m/s, you would
need a 34 second burn. That isn't a problem, but it isn't the way
the Falcon rockets land. If you have three Raptor engines, that gives
you 3.9 g - 1 g for gravitation = 2.9 g of deceleration. So now instead
of a 34 second burn you now need a 3.5 second burn. Three engines for
3.5 seconds burn less than one third what one engine burns in 34
seconds. So again, landing with one engine isn't really a problem
but you need to keep more fuel for landing if you want to do that.


Alain Fournier