by Chris Peterson » Fri Oct 12, 2018 7:43 pm
MarkBour wrote: ↑Fri Oct 12, 2018 7:07 pm
This is a lovely, artistic view of the launch. I'm glad for the caption, I never would have sorted it out. There are a couple of things I wonder about as I gaze at this "nebula". First, why is the most central part of the exhaust a distinct brown color, highly differentiated from the outer white and blue region? It appears that the brown exhaust is higher up (farther from the camera). Second, I did not know that the Falcon return burn involved pulsed thrusting. Why do they do that? Does it happen too fast for the naked eye to see that pattern?
I'm not sure that the pulsed engines seen in this image are from the landing thruster. There are small pulsed rockets used to rotate the landing stage and to maintain its attitude.
That said, the main boosters have to be capable of something like pulsed operation, because the way the landing works is that it basically falls until the last possible moment, and then blasts at extremely high thrust to rapidly bring it to zero velocity just as it reaches the ground. That requires very fast response engines.
I don't know if this is the case with the Falcon rockets, but in many control systems it is easier, more efficient, or more compatible with the hardware involved to simulate analog behavior by some kind of time modulation of a binary output. So in the case of a rocket, you could control thrust by having the engine either on or off, going quickly between those two states as necessary. This would eliminate the need for fancy throttles that can linearly modulate fuel flow, in favor of simple on/off valves. (Again, I don't know if that's what actually is done here, but it could explain the reasoning behind pulsed thrusters.)
[quote=MarkBour post_id=286492 time=1539371236 user_id=141361]
This is a lovely, artistic view of the launch. I'm glad for the caption, I never would have sorted it out. There are a couple of things I wonder about as I gaze at this "nebula". First, why is the most central part of the exhaust a distinct brown color, highly differentiated from the outer white and blue region? It appears that the brown exhaust is higher up (farther from the camera). Second, I did not know that the Falcon return burn involved pulsed thrusting. Why do they do that? Does it happen too fast for the naked eye to see that pattern?
[/quote]
I'm not sure that the pulsed engines seen in this image are from the landing thruster. There are small pulsed rockets used to rotate the landing stage and to maintain its attitude.
That said, the main boosters have to be capable of something like pulsed operation, because the way the landing works is that it basically falls until the last possible moment, and then blasts at extremely high thrust to rapidly bring it to zero velocity just as it reaches the ground. That requires very fast response engines.
I don't know if this is the case with the Falcon rockets, but in many control systems it is easier, more efficient, or more compatible with the hardware involved to simulate analog behavior by some kind of time modulation of a binary output. So in the case of a rocket, you could control thrust by having the engine either on or off, going quickly between those two states as necessary. This would eliminate the need for fancy throttles that can linearly modulate fuel flow, in favor of simple on/off valves. (Again, I don't know if that's what actually is done here, but it could explain the reasoning behind pulsed thrusters.)