What is "Maximum Dynamic Pressure?"

[Brainlord Mesomorph]

I hear this a lot in RL launches. ("we have passed the point of Maximum Dynamic Pressure")

My guess is that is combination max speed vs max air pressure. (i.e. higher up faster becomes easier). I'm also guessing that (in KSP) its about 3.5 km up and about 400 m/s, You know, that spot where the rocket usually breaks in half!  

Am I right? can someone be more specific?

thanks

[KerbonautInTraining]

You were pretty much spot on with your explanation of what Max-Q is, but in KSP it varies wildly between rockets.

Edited May 19, 2016 by KerbonautInTraining

[RizzoTheRat]

Dynamic pressure is what you feel when you stick your hand out of the car window and it's pushed back.

q = 0.5 roe v²

where roe is density and v is velocity.  So as you launch density decreases but velocity increases so while q initially rises, it will start to drop again with altitude.

Max q is the point where there's the biggest force on the vessel as it's directly proportional to drag for a given vehicle (ish, there's some other funny things that go on with compressability effects) and so it's most likely to break

Edited May 19, 2016 by RizzoTheRat

[Brainlord Mesomorph]

13 minutes ago, RizzoTheRat said:

Max q is the point where there's the biggest force on the vessel as it's directly proportional to drag for a given vehicle (ish, there's some other funny things that go on with compressability effects) and so it's most likely to break

Wait, how does drag have anything to do with it? (there's no drag in that equation)

Isn't "max q" (my new word for the day) for a brick going to be same as max q for a needle? (given the same acceleration and course?)

Granted the amount of pressure is greater on the brick, but its still going to reach max for each of them at the same speed vs air pressure point, right?

[RizzoTheRat]

Correct, but in your needle vs brick example the dynamic pressure on both with will be same, but the drag on the brick will be greater.

drag = 0.5 roe v² Cd A

We know q = 0.5 roe v²

A is the reference area which is constant, and while the drag coefficient (Cd) does change with Reynolds number, it's not going to change as much as q does (and the variation in Cd is less for a streamlined object like a rocket than for a bluff object), so change in q is a major component of change in drag.

If you're rocket's going to flip out and break I assume it's the drag that causes it rather than the pressure alone, and max drag will be somewhere close (in terms of speed and altitude) to max q.

Edited May 19, 2016 by RizzoTheRat

[Brainlord Mesomorph]

the only part of that I got was " change in q is a major component of change in drag." and not the other way around. makes sense.

but now I have another question:

I get the concepts of physics. but it always comes apart for me when it comes to measurements and unit conversion. so given:

q = 0.5 roe v²

I have this table with Kerbin air pressure in ATM or Pascals, and I have velocity in m/s. can you please turn that into math I can actually use?

(My goal is an excel chart that show Max Q at various speeds)

Edited May 19, 2016 by Brainlord Mesomorph

[Dib]

The problem I think you're trying to get at is you want to be able to calculate the value of Max Q on a given rocket flight upfront, as "what is the Max Q at various speeds" is a question that makes no sense (the answer is to that specific question is always set the altitude to 0 for maximum atmospheric pressure, resulting in the highest Q for every speed. Not very useful). 

This is a problem that has more variables than just pressure and velocity though, as pressure and velocity themselves in a rocket flight are directly dependent on the rocket's TWR, atmospheric drag coefficient and flight profile. For instance, if you take a rocket straight up on a suborbital trajectory, you will exit the thicker portion of the atmosphere at a relatively low velocity. This will inevitably result in a lower Max Q on that flight than if you take the same rocket and fly it on an orbital trajectory using a gravity turn, where you'll see significantly more velocity at the same altitudes throughout the flight, resulting in a higher Max Q at some point during that flight than the highest Q you saw during the suborbital flight. 

I don't have the time right now to try and figure out the math behind that calculation. The simplest way I can imagine it happening is that you'd have to determine the overall flight profile you want to fly in advance, then discretely integrate the TWR (determined by engines, staging and time), drag component (determined by drag coefficient, current speed and altitude) and gravity loss (based on angle of thrust versus ground, determined by current altitude and flight profile) over time to determine the speed of the rocket throughout the flight, creating a speed vs altitude graph in the process. Then you would apply the dynamic pressure calculation on the speed vs altitude graph to create a dynamic pressure versus altitude graph, and finally search for the peak value of dynamic pressure on said graph. And you would have to do this for every flight profile of every rocket design if you wanted to know them all.

[Archgeek]

Oi, what's with these fish eggs in the atmosphere?  I'm pretty sure that's supposed to be Q = .5 rho v², that is the greek lowercase letter Rho, which looks kinda like a lower-case 'p'.

[suicidejunkie]

Multiply Speed squared and pressure.  The exact units don't matter, its just a question of higher or lower

 

An excel chart won't be of much use:

 - At any particular speed, max Q will be where the air pressure is maximum (0 altitude).

 - At any particular altitude/pressure, max Q will be at the highest speed.

 

The trick is that both speed and pressure are varying at the same time, in a different way for every launch and rocket.

If you were to pause frequently and jot down your altitude (to calculate pressure) and velocity during your launch, you could then make a plot of your Q, and later on figure out where your max Q was.  Perhaps adjust your launch profile and thrust for next time.

Or if you've got FAR installed, open the details window and watch for when the live readout starts decreasing.

[Gaarst]

In real launches, max-Q is usually around 10-15km and some rockets throttle down at that moment to reduce stress on the structure.

Since joints and parts are made of (molten) diamond in KSP you don't really need to care about max-Q unless your rocket has had issues with flipping.

Edited May 19, 2016 by Gaarst

[Brainlord Mesomorph]

4 minutes ago, Gaarst said:

unless your rocket has had issues with flipping.

[JonathanPerregaux]

If you have MechJeb, there is a very useful flight recorder that can graph various things, including Max Q. You would see this as a rapidly increasing curve with a sudden turnover and drop-off. The exact curve varies per spacecraft design, velocity, etc.

Adding "moar boosters" might get you up there quicker but creates a critical problem during the moment of Max Q. MechJeb can automatically throttle down during this moment using Ascent Guidance. Unless, of course, your biggest source of propulsion is coming from solid rocket boosters. This is why the space shuttle's SRBs were specifically designed to burn slower during the time when Max Q would be affecting the spacecraft. Otherwise it would cave in from the pressure. Once you're past this point, then it's "Go for throttle-up!"

Edited May 19, 2016 by JonathanPerregaux

[OhioBob]

There is a display is stock KSP that gives dynamic pressure along with a bunch of other aerodynamic data (no mod required).  Just do the following:

Press ALT+F12 to open Debug Toolbar
Click "Physics" button
Check "Display Aero Data GUI"
Press ALT+F12 to close Debug Toolbar
 

[RizzoTheRat]

40 minutes ago, Archgeek said:

Oi, what's with these fish eggs in the atmosphere?  I'm pretty sure that's supposed to be Q = .5 rho v², that is the greek lowercase letter Rho, which looks kinda like a lower-case 'p'.

Couldn't find the key combination for ρ (google suggests Alt-961 but that gives me ┴) and as it's a letter in it's own right I've never sure how to spell it

Edited May 19, 2016 by RizzoTheRat

[OhioBob]

rho = ρ

[Lukaszenko]

I can't understand why acceleration is not taken into account when calculating the greatest stress on a vessel. A rocket accelerating at 100 gs in vacuum will be experiencing more stress than one going Mach 3 through thin atmosphere, I imagine.

Edited May 20, 2016 by Lukaszenko

[Gaarst]

1 hour ago, Lukaszenko said:

I can't understand why acceleration is not taken into account when calculating the greatest stress on a vessel. A rocket accelerating at 100 gs in vacuum will be experiencing more stress than one going Mach 3 through thin atmosphere, I imagine.

Max-Q is just the point of highest aerodynamic stress on the rocket. Aerodynamic stress only accounts for "exterior" forces acting on the rocket (airflow). The internal stress generated by acceleration, vibrations... are accounted for when building the rocket or engines but you can't account for the stress on a rocket going off course at Max-Q since the forces are extremely high.

A well designed rocket will not tear itself apart if its TWR is a bit high. The best rocket you can imagine will still disintegrate if not perfectly lined up in the airflow. (Take Challenger for example: the explosion of the 700t of fuel stored in the main tank did only small damage to the orbiter, but the aerodynamic stress is what destroyed it)

[PB666]

18 hours ago, RizzoTheRat said:

Couldn't find the key combination for ρ (google suggests Alt-961 but that gives me ┴) and as it's a letter in it's own right I've never sure how to spell it

Its the undercase greek r. 

Newtons law basically states the have tonbe oppossing forces at maxq you have acceleration and aero dynamic pressure. if you nose piece is reasonable well designed, the the stress moves down the craft. You have g-force, ground relative acceleration pushing upward and drag force pushing down. The most stressed part is the first stage second stage connection, then the second stage payload connection, then the payload nosecone connection. The other force at MaxQ is the boundary separation, it separates from the nose cone and hits the rocket further down the lengthbwith speed. This is a side crushing force, below max q for the falcon it travels down the rocket, above maxQ it travels down the plume. 

[RCgothic]

1 hour ago, PB666 said:

Newtons law basically states the have tonbe oppossing forces at maxq you have acceleration and aero dynamic pressure. if you nose piece is reasonable well designed, the the stress moves down the craft. You have g-force, ground relative acceleration pushing upward and drag force pushing down. The most stressed part is the first stage second stage connection, then the second stage payload connection, then the payload nosecone connection. The other force at MaxQ is the boundary separation, it separates from the nose cone and hits the rocket further down the lengthbwith speed. This is a side crushing force, below max q for the falcon it travels down the rocket, above maxQ it travels down the plume. 

I think this is quite confusing, and a bit garbled, particularly bringing in Newton's laws which aren't really relevant to max q.

At any instance there are three basic forces acting on a rocket: Thrust, Drag and Gravity.

The Newton's first law merely states that an object in motion remains in motion unless acted on by an outside force. This is, these days, a statement of the obvious and not specifically relevant to max q.

Newton's second law can be written to state that the rocket's acceleration is the residual force on the rocket divided by the rocket's mass. This is only slightly relevant to max q, allowing us to divine something about the magnitude of each force from the rocket's acceleration over its flight. The force due to thrust is a designed and understood value. The force due to gravity is easily derived. The force due to drag is tricky to calculate. But because you can watch the rocket to determine its acceleration, you can use the second law to work back and determine what the drag is. That's where the second law is useful.

Newton's third law states that each force has an equal and opposite reaction. This merely means, e.g., that for all the force drag is exerting on the rocket, the rocket exerts an opposite force on the air. It does NOT mean that the forces acting on a body have to be balanced - per the second law there could be no acceleration if this were the case. In considering a rocket we only care about the force acting on the rocket and don't really care what forces act on the air, or the exhaust, or the Earth so the third law is not specifically relevant.

 

[PB666]

11 minutes ago, RCgothic said:

I think this is quite confusing, and a bit garbled, particularly bringing in Newton's laws which aren't really relevant to max q.

At any instance there are three basic forces acting on a rocket: Thrust, Drag and Gravity.

The Newton's first law merely states that an object in motion remains in motion unless acted on by an outside force. This is, these days, a statement of the obvious and not specifically relevant to max q.

Newton's second law can be written to state that the rocket's acceleration is the residual force on the rocket divided by the rocket's mass. This is only slightly relevant to max q, allowing us to divine something about the magnitude of each force from the rocket's acceleration over its flight. The force due to thrust is a designed and understood value. The force due to gravity is easily derived. The force due to drag is tricky to calculate. But because you can watch the rocket to determine its acceleration, you can use the second law to work back and determine what the drag is. That's where the second law is useful.

Newton's third law states that each force has an equal and opposite reaction. This merely means, e.g., that for all the force drag is exerting on the rocket, the rocket exerts an opposite force on the air. It does NOT mean that the forces acting on a body have to be balanced - per the second law there could be no acceleration if this were the case. In considering a rocket we only care about the force acting on the rocket and don't really care what forces act on the air, or the exhaust, or the Earth so the third law is not specifically relevant.

 

 

It means that drag force and side drag cannot be separated form each other completely, acceleration increases velocity, the craft is accekerating through max q. Therefore all forces matter and are complex. 

Since acceleration is being applied at the engine and it has the weight of the entire craft bering down on it plus the force of drag plus side drag, at the first stage/ second stage you do not have the weight of the first stage nor side drag because the boundary layer collapses below it. At the second stage payload connection you can subtract the weight of the first and second stage and side drag from the boundary collapse. And finally at the nose cone/payload inteface you have pretty much the drag force plus a small amount of force cotributed by its weight

In each case weight is equal to mass * acceleration. 

[kerbiloid]

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WordPad accepts "Alt+961" codes, then they can be copypasted here.
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[wumpus]

On 5/19/2016 at 10:59 AM, JonathanPerregaux said:

If you have MechJeb, there is a very useful flight recorder that can graph various things, including Max Q. You would see this as a rapidly increasing curve with a sudden turnover and drop-off. The exact curve varies per spacecraft design, velocity, etc.

Adding "moar boosters" might get you up there quicker but creates a critical problem during the moment of Max Q. MechJeb can automatically throttle down during this moment using Ascent Guidance. Unless, of course, your biggest source of propulsion is coming from solid rocket boosters. This is why the space shuttle's SRBs were specifically designed to burn slower during the time when Max Q would be affecting the spacecraft. Otherwise it would cave in from the pressure. Once you're past this point, then it's "Go for throttle-up!"

I'm pretty sure the reason that real rockets throttle down is to reduce the stress on the spacecraft (to probably something such that the force the rocket is supporting is limited, ie

TWR(maxQ)+force exerted by aero(maxQ)=TWR(vacuum) [assuming TWR is maximum at vacuum, this also ignores that the mass of the rocket in vacuum is certainly less than the mass of the rocket at maxQ, but it still has to support the upper stage with the same mass.

This part is unlikely to be important in KSP.

Edited May 20, 2016 by wumpus
weird strikethrough effect removed.