It may be that return from Mars to Earth Surface imposes significant engineering challenges on the heat shield, which are not experienced by return from Low Earth Orbit.
The propellant tanks need to be cylindrical to be remotely mass efficient and they have to carry ascent load, so lowest mass solution is just to mount the heat shield plates directly to the tank wall
There won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.John
The main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.
Quote from: livingjw on 03/09/2018 10:58 amThere won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.JohnThis is what Elon said during the latest Reddit AMA:QuoteThe main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.
Quote from: jpo234 on 03/09/2018 11:28 amQuote from: livingjw on 03/09/2018 10:58 amThere won't be any propellant in the main tanks during any reentry. Only enough ullage gas to maintain pressure for structural stabilization. Propellant is in the small tanks deep inside the BFS.JohnThis is what Elon said during the latest Reddit AMA:QuoteThe main tanks will be vented to vacuum, the outside of the ship is well insulated (primarily for reentry heating) and the nose of the ship will be pointed mostly towards the sun, so very little heat is expected to reach the header tanks. That said, the propellant can be cooled either with a small amount of evaporation. Down the road, we might add a cryocooler.I took his statement to mean during the 3-4 month coast period after TMI. I doubt having zero tank pressure during reentry would be structurally stable. Tanks can be re-pressurized prior to Mars atmosphere entry.John
I’m not knowledgable on re-entry dynamics but isn’t there some half way house between a high g deep re-entry and a low g shallow re-entry? As the BFS can generate lift as well as drag can’t it “bob” (for want of a better word) in the top layer of the atmosphere? When it’s too high and in danger of leaving the atmosphere – increase the angle of attack to increase drag and decrease lift. When it’s too low and g forces are getting too high – decrease the angle of attack to reduce the drag and increase lift…
Quote from: Slarty1080 on 03/23/2018 02:04 pmI’m not knowledgable on re-entry dynamics but isn’t there some half way house between a high g deep re-entry and a low g shallow re-entry? As the BFS can generate lift as well as drag can’t it “bob” (for want of a better word) in the top layer of the atmosphere? When it’s too high and in danger of leaving the atmosphere – increase the angle of attack to increase drag and decrease lift. When it’s too low and g forces are getting too high – decrease the angle of attack to reduce the drag and increase lift… BFS is a terrible, terrible glider.Yes, you can generate lift, but only at the expense of very considerable drag.This means that 'fancy' manoevering at the edges of the atmosphere are limited to the time before you slow down enough that the effective gravity left over after you take off the effect of the orbit you would be in without the atmosphere gets high enough that you stall.It can do a very shallow reentry up until you get to perhaps 5km/s or so, at which point it's coming in nomatter what you do.
Sounds like a plan. Do a very shallow 5-10min reentry with fancy manoevering to get from 11km/s down to 5km/s or so and then just re-enter. If this is possible why do people worry about coming in at too high a speed? Is it that the "fancy manoevering" is in the too difficult/dangerous pile?