Alain Fournier wrote:

On Dec/24/2017 at 4:57 PM, Fred J. McCall wrote :
[snip]
You can still kill most of the velocity with aerobraking. BFR
Spaceship has a low enough density on return that a normal PICA-X heat
shield on the underside is sufficient. Shuttle needed the special
tiles because it was large, dense, and flying using wings and lifting
body shape (all of which add drag and increase heat loads when
reentering).

Being large and dense is the big culprit for the high heat loads
of the Shuttle. Not the added drag due to the body shape. In fact
you want to have lots of drag in a re-entry vehicle, for two reasons.
First and most importantly, if the re-entry vehicle has a blunt shape,
air doesn't flow away as much as with an aerodynamic shape. So in
effect, you have an air cushion around the vehicle and the heated shock
layer (where you have the most heat) is away from the vehicle. Second,
if you have a blunt shape, you will decelerate more during the earlier
phase of re-entry and your speed will be lower when you reach thicker
layers of the atmosphere.


Drag generates lift. Since we don't live in a perfect universe, part
of the drag does not generate lift but generates waste heat. The
'draggier' you are the more lift (and heat) you will generate. If the
Shuttle didn't try to fly heat loads would be smaller. It's why wing
leading edges needed special insulators.


--
"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