[Starlink] some post Starship launch thoughts

[David Lang]

On Tue, 25 Apr 2023, Eugene Chang wrote:

> I found this YouTube of a deluge system test.
> It doesn’t look like it uses enough water to succeed.
> My intuition is the mass of water needed is approximately equal to the rocket’s mass.

nowhere close. The pad 39a where the Saturn 5 launched has a 300,000 gal take, 
which is ~2.4M pounds, but the Saturn 5 launch weight was around 6.5M pounds

> Maybe the system doesn’t have to fully absorb the momentum of the engine exhaust. Still, 70% would be a much greater mass than what the video shows.

it doesn't, it's not absorbing the momentum of the engine exhaust, it's 
vaporizing to cool the area and disrupt the airflow so the exhaust is less of a 
blowtorch when it hits a solid surface, and absorb enough sound to prevent it 
from damaging the rocket.

distance helps with both of these, as do the materials that the exhaust finally 
hits. Regular concrete has too much moisture in it and the water flashes to 
steam and breaks the concrete (concrete is strong in compression, weak in 
tension).

Elon mentioned a few weeks ago that even steel plates would wear down quickly 
under the exhaust, and that water cooled plates were needed in the long run (and 
they started building water cooled plates to put under the launch mount)

will that be enough? only testing will tell us for sure.

not all rockets use a flame trench, and some have very little deluge

David Lang