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Throttle down for max-q



 
 
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  #1  
Old September 4th 20, 06:12 AM posted to sci.space.policy
Sylvia Else[_3_]
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Posts: 87
Default Throttle down for max-q

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.
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  #2  
Old September 4th 20, 06:44 AM posted to sci.space.policy
Scott Kozel
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Posts: 62
Default Throttle down for max-q

On Friday, September 4, 2020 at 1:12:16 AM UTC-4, 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?


The vehicle is under more structural stress when it is under high
acceleration. Throttling down reduces the rate of acceleration and reduces
the structural stress.
.. . . . .

An external opinion --

https://www.quora.com/What-would-hap...-full-throttle
C Stuart Hardwick, Award-Winning Scifi Author, Analog regular
Answered February 23, 2018

If it passed Max-Q at full throttle, it would be accelerating at full bore
right up to Max-Q. This means it would encounter Max-Q sooner, at a lower
altitude, where the total dynamic force would be greater. This would increase
the risk of some mechanical failure, or force the rocket to be made sturdier
and heavier, reducing performance.

Someone else said rockets need to slow down before Max-Q — of course that’s a
misstatement. They only slow down their rate of acceleration, in order to
delay the point of Max-Q until they reach thinner air where the peak load will
be lower.

Max-Q is the total aerodynamic load on the vehicle. It rises with speed, but
decreases with the thinning air at altitude. Throttling down right before Max-
Q is an engineering trade-off, giving up a little of the utility of the
engines in exchange for being able to make the whole ship a little lighter.

  #3  
Old September 5th 20, 04:23 AM posted to sci.space.policy
Sylvia Else[_3_]
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Posts: 87
Default Throttle down for max-q

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.
  #4  
Old September 5th 20, 03:25 PM posted to sci.space.policy
Alain Fournier[_3_]
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Posts: 548
Default Throttle down for max-q

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
  #5  
Old September 7th 20, 04:43 PM posted to sci.space.policy
Scott Kozel
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Posts: 62
Default Throttle down for max-q

On Monday, September 7, 2020 at 1:36:16 AM UTC-4, JF Mezei wrote:

For conventional shaped rockets, I have no explanation. Accelerating
the light carbon fairing doesn't involve a lot of energy compared to
acceleratibg the fuel laden ET. So aerodynamic drag would by far be the
largest force acting on it so reducing acceleration would be a small
reduction on total force.


According to this, the Saturn V did not throttle down for Max Q.

'The Saturn V's first-stage engines don't throttle, so there's no "throttle bucket" and no "go at throttle-up" call. One of the first-stage engines is shut down late in the burn, but that's to limit maximum g-force for crew comfort, not for Q limiting.'

https://space.stackexchange.com/ques...at-throttle-up
  #6  
Old September 8th 20, 01:07 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Throttle down for max-q

In article , lid
says...

How does this actually help?


It reduces the overall forces on the booster.

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.


True.

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?


Forces acting on the booster are from gravity, air drag, and thrust.
During max-Q, air drag is at a maximum. To keep the overall forces on
the booster below the maximum operating limit that they've set, they
reduce thrust during the period of time that it is experiencing maximum
dynamic pressure.

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


That's one of those "devil is in the details" things. It's my
understanding that they're trying to keep the overall forces down.
Vibration might be a contributing factor though. But, that's something
only a SpaceX engineer would know.

Note that the space shuttle did exactly the same thing. You might be
able to do a bit of Googling and find more details for the space
shuttle. For example:

Mission Control Answers Your Questions
Question: What is the maximum dynamic pressure, and why does the
shuttle throttle down at max-q?
https://tinyurl.com/yydc968v

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #7  
Old September 8th 20, 01:17 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Throttle down for max-q

In article , lid
says...

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.


Actually it is. If I clamp a board in a vice, at the center of the wood
the forces from each side of the vice do cancel out to precisely zero
(i.e. it's not accelerating due to the forces). But, the wood is under
stress due to the compression caused by the vice. Tighten the vice
enough and the wood will be crushed (permanently deformed).

That's exactly what's happening during max-Q. Thrust and drag are
acting opposite each other and are placing the vehicle under more stress
than it would be if it were operating under the same thrust in vacuum.

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.


This bit is true. Throttling down early decreases acceleration so that
max-Q happens at a higher altitude, where density is lower and therefore
aerodynamic forces are lower.

As was posted earlier in this thread, Apollo/Saturn V experienced max-Q
at a much higher altitude than the space shuttle due to its slower
acceleration.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #8  
Old September 8th 20, 01:19 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Throttle down for max-q

In article ,
says...

On Monday, September 7, 2020 at 1:36:16 AM UTC-4, JF Mezei wrote:

For conventional shaped rockets, I have no explanation. Accelerating
the light carbon fairing doesn't involve a lot of energy compared to
acceleratibg the fuel laden ET. So aerodynamic drag would by far be the
largest force acting on it so reducing acceleration would be a small
reduction on total force.


According to this, the Saturn V did not throttle down for Max Q.

'The Saturn V's first-stage engines don't throttle, so there's no "throttle bucket" and no "go at throttle-up" call. One of the first-stage engines is shut down late in the burn, but that's to limit maximum g-force for crew comfort, not for Q limiting.'

https://space.stackexchange.com/ques...at-throttle-up

Saturn V also didn't need to throttle down due to its max-Q happening at
a much higher altitude due to its lower initial acceleration. A Saturn
V took quite a few number of seconds to clear the tower. The space
shuttle, by comparison, with its SRBs seemed to jump off the pad and
cleared the tower much sooner.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #9  
Old September 8th 20, 10:25 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Throttle down for max-q

In article ,
says...

On 2020-09-08 08:17, Jeff Findley wrote:

That's exactly what's happening during max-Q. Thrust and drag are
acting opposite each other and are placing the vehicle under more stress
than it would be if it were operating under the same thrust in vacuum.


At MaxQ for normal shaped rocket, what percentage of forces on structure
comes from beloow (acceleration) vs above (drag on nose pushing down)?

Are we talking 95% acceleration, 5% drag, or 50/50 ? or is drag even
higher than acceleration?


Don't know exactly as this is one of those "the devil is in the
details" sort of things and it's the end of my work day and I really
don't want to "math" anymore.

If a rocket is built to handle 5G acceleration, but only doing 3G at the
MaxQ altitude, then the structure is perfectly able to widthstand a bit
more pressure from above so continuing with normal acceleration would
be a no brainer.


Again, the devil is in the details. The stress is distributed entirely
differently during max-Q (due to aerodynamic forces) than they are
during other portions of the flight. For example, after max-Q the
rocket "throttles up" and continues to accelerate. In fact, since it's
burning fuel, its thrust to weight ratio keeps going up!

So, perhaps the stages themselves can take more acceleration during max-
Q, but the payload shroud can't handle the aerodynamic forces.

This bit is true. Throttling down early decreases acceleration so that
max-Q happens at a higher altitude, where density is lower and therefore
aerodynamic forces are lower.


Throttling down at MaxQ doesn't raise maxQ altitude since you've already
reached it. If the goal is to raise MaxQ altitude, then the flight
profile from launch to MaxQ would have slower acceleration. (which is
likely what happens with modern rockets without SRBs).


Some launch vehicles throttle down sooner than others before they reach
max-Q. It all depends on the details.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
 




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