On Sep/4/2020 at 23:23, Sylvia Else wrote :
On 04-Sep-20 3:12 pm, Sylvia Else wrote:
How does this actually help?

The aerodynamic forces on the craft are a function of its speed and
air density. There's not much one can do about the speed, given that
the craft is accelerating.

If the craft accelerates more slowly overall, for each speed it will
be traversing less dense air than it would do otherwise. But in that
case, is there a benefit from high initial acceleration and then
reducing acceleration on the approach to max-q?

Is it really about reducing vibration loads during max-q?

Sylvia.

The thing is, it's not as if the rocket thrust and aerodynamic forces
are adding together to squeeze the vehicle from both ends. The
longitudinal component of the aerodynamic force is transferred from the
exterior to the internals of the vehicle, and acts to reduce the
acceleration imparted by the rocket. This transferred force is not a
function of rocket thrust. For some parts of the internals of the
vehicle, it reduces, not increases, the loads.

I'm not suggesting that the throttle down is not required, but the
rationale for it doesn't appear as straight forward as it might seem at
first sight.

Sylvia.

You are right, the rationale is not straight forward, there are multiple
forces at play here. But once everything is taken into account, it
becomes clear that lowering thrust is in most cases the way to go.

One thing that you must also take into account is that if you keep your
engines at 100% thrust, the acceleration of the rocket increases
(positive jolt) because as fuel is burnt you lose mass but the engines
still give the same push. So if you want to have constant acceleration
until you reach max-Q, you would decrease thrust.


Alain Fournier