by Chris Peterson » Thu Dec 31, 2015 6:49 pm
JohnD wrote:Chris will correct me, neufer, but I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
Once it contacted Earth, the reaction of a solid surface provided a further acceleration that precisely counteracted gravity.
Continued rocket thrust (of the same amount) would have made gravity and the Earth's reaction less, but not enough to counteract gravity completely and launch the vehicle again.
John
You're right. I'm right. Art's right. We're all just telling part of the story.
My original comment is correct, that landing involves controlling the thrust such that gravity is almost, but not quite compensated for. But that's just the global view of the entire landing sequence. What Art's getting at is the fact that any time you have a force, you have an acceleration. If you have a constant velocity, there is no net force.
Landing involves managing a specific velocity profile. Over the entire landing, the integrated force applied by the rocket is less than the integrated force of gravity, which is why you go down. Instantaneously, the thrust force may be greater or less than the force of gravity- if you're increasing your downward velocity, it will be less; if you're decreasing your downward velocity, it will be more (you'll be accelerating upward, even though your velocity is still downward). If you're maintaining a constant downward velocity, you might think that your two forces would be equal, but actually, you would have slightly less thust because if you have a non-zero velocity, you also have an upward directed drag force. And when you get very close to the ground, things probably get very complicated because now you don't have a purely Newtonian action/reaction system, but who-knows-what nonlinear ground effects.
[quote="JohnD"]Chris will correct me, neufer, but I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
Once it contacted Earth, the reaction of a solid surface provided a further acceleration that precisely counteracted gravity.
Continued rocket thrust (of the same amount) would have made gravity and the Earth's reaction less, but not enough to counteract gravity completely and launch the vehicle again.
John[/quote]
You're right. I'm right. Art's right. We're all just telling part of the story.
My original comment is correct, that landing involves controlling the thrust such that gravity is almost, but not quite compensated for. But that's just the global view of the entire landing sequence. What Art's getting at is the fact that any time you have a force, you have an acceleration. If you have a constant velocity, there is no net force.
Landing involves managing a specific velocity profile. Over the entire landing, the integrated force applied by the rocket is less than the integrated force of gravity, which is why you go down. Instantaneously, the thrust force may be greater or less than the force of gravity- if you're increasing your downward velocity, it will be less; if you're decreasing your downward velocity, it will be more (you'll be accelerating upward, even though your velocity is still downward). If you're maintaining a constant downward velocity, you might think that your two forces would be equal, but actually, you would have slightly less thust because if you have a non-zero velocity, you also have an upward directed drag force. And when you get very close to the ground, things probably get very complicated because now you don't have a purely Newtonian action/reaction system, but who-knows-what nonlinear ground effects.