Author Topic: Max Q  (Read 3686 times)

Offline Danderman

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Max Q
« on: 06/03/2023 03:46 am »
This seems to be a bit of a hurdle for new rockets, as there is usually some trepidation about passing through this flight regime.

Often the rocket is shown as flying around 1000+ kph at 10000+ meters altitude dufing Max Q

My question is more of an observation: jetliners routinely fly in this regime, and its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

Offline tyrred

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Re: Max Q
« Reply #1 on: 06/03/2023 06:10 am »
This seems to be a bit of a hurdle for new rockets, as there is usually some trepidation about passing through this flight regime.

Often the rocket is shown as flying around 1000+ kph at 10000+ meters altitude dufing Max Q

My question is more of an observation: jetliners routinely fly in this regime, and its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

Totally different types of vehicles and flight regimes.

Airliners get to cruise altitude slowly, yet their takeoffs and landings are still the riskiest regimes.

Orbital class rockets must optimize escaping the drag of thick lower atmosphere as efficiently as possible, while making a turn to minimize gravity losses on the way to going fast enough, high enough, sideways to obtain orbit.

Most rockets are not built as robustly as say an F-15, which may be able to fly upwards completely vertical for a while, yet, also needs to be able to sustain much greater g-loads than an orbital rocket due to design and necessity of quick turns and dogfighting maneuvers.

Just a layman's explanation, and probably messed up on some details.  ::)
« Last Edit: 06/03/2023 06:11 am by tyrred »

Offline laszlo

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Re: Max Q
« Reply #2 on: 06/03/2023 11:33 am »
There's nothing special about max q per se. It's not a wall in the sky or a speed limit. What the rocket engineers are worried about is total pressure on the rocket. The structure must be designed and built to withstand the maximum total pressure that the rocket will experience on its way to space. It just happens that for a rocket the maximum total pressure occurs at max q which is why it's such a worrisome time on a first flight.

A jetliner's version of the max q danger area is VMO, the maximum operating speed. This is the indicated airspeed which above which structural damage is likely to occur, or to put it another way, dynamic pressure exceeds the structural capabilities of the airframe. It can also be expressed as a Mach number, in which case it's called MMO.

Jetliners experience a slower onset of dynamic pressure and a much longer phase of max q than experienced by a rocket. Because they never climb out of the atmosphere, the dynamic pressure never goes back to zero while flying. So they are definitely operating in different regimes, as tyrred says.

When the OP's jetliner was operating in the same speed/altitude conditions as a rocket at max q, the airliner was not exceeding its  VMO, just as the rocket was not exceeding its design criteria.

Edit[typo fixes]
« Last Edit: 06/03/2023 11:36 am by laszlo »

Online EspenU

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Re: Max Q
« Reply #3 on: 06/03/2023 12:21 pm »
As laszlo also mentioned, it comes down to design limits. Rockets are designed to survive maxQ, and no more (besides some margins of course).
Making it withstand more would just add more mass and reduce the payload capability.
So, maxQ is a "scary" point because it touches the designed load limit of the structure.

Offline Jim

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Re: Max Q
« Reply #4 on: 06/03/2023 03:07 pm »
This seems to be a bit of a hurdle for new rockets, as there is usually some trepidation about passing through this flight regime.

Often the rocket is shown as flying around 1000+ kph at 10000+ meters altitude dufing Max Q

My question is more of an observation: jetliners routinely fly in this regime, and its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

It can't be tied to a specific altitude or speed. 

q is 1/2 rho * V^2, where rho is the air density and V is the rocket velocity.

But it isn't that simple as the equation since as the vehicle accelerates and climb, the air density is decreasing.  So dynamic pressure is zero on the pad and increases as the rocket accelerates since velocity squared dominates the equation.  But at a certain point, the decreasing density starts to take over and q also starts decreasing.   The peak is Max q.  See diagram.
Rockets also hit wind shear and are arcing over.    Rockets are more slender than airliners but are heavier and more dense.   They are more susceptible to bending loads. 

« Last Edit: 06/03/2023 03:30 pm by Jim »

Offline Jim

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Re: Max Q
« Reply #5 on: 06/03/2023 03:26 pm »

its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

What altitude?

Offline Hobbes-22

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Re: Max Q
« Reply #6 on: 06/03/2023 04:33 pm »
Aircraft have much larger structural margins than rockets.
- On one hand, structural margins can be larger because aircraft operate in a regime where performance is much less critical: adding 5% to the structural weight reduces the aircraft range a bit, instead of threatening to make the whole design unviable.
- On the other hand, structural margins have to be larger in an aircraft because the peak loads are less predictable - flying in bad weather creates a huge extra load compared to flying in benign weather. Maneuvering, takeoff and landing produce a huge extra load compared to flying straight and level. On a rocket, peak loads are encountered on every flight, at max Q, and the rocket is designed with a small margin above that peak load.

Another factor is that rocket launch commentators have time to fill and max Q is an excuse to inject drama into a routine launch.

Offline Danderman

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Re: Max Q
« Reply #7 on: 06/03/2023 05:27 pm »
13,000 meters or so.

Online EspenU

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Re: Max Q
« Reply #8 on: 06/03/2023 05:38 pm »
13,000 meters or so.
On a Starlink flight for example, Falcon 9 hits that speed at 8km. So, much higher loads, on a much "flimsier" structure.

Offline Jim

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Re: Max Q
« Reply #9 on: 06/03/2023 05:40 pm »
13,000 meters or so.

Max q typical happened lower in the atmosphere.

Offline Jim

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Re: Max Q
« Reply #10 on: 06/03/2023 06:23 pm »

its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

What altitude?

13,000 meters or so.

1,200+ kph at 13,000 meters or so is going to be a q of 575 lb/ft3 or so.  Less than max q (700)
 of the shuttle.
« Last Edit: 06/03/2023 06:24 pm by Jim »

Offline Redclaws

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Re: Max Q
« Reply #11 on: 06/03/2023 06:42 pm »
This seems to be a bit of a hurdle for new rockets, as there is usually some trepidation about passing through this flight regime.

Often the rocket is shown as flying around 1000+ kph at 10000+ meters altitude dufing Max Q

My question is more of an observation: jetliners routinely fly in this regime, and its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

A key point that no one has really emphasized yet (though it was mentioned and I'm sure everyone knows):
The structure of an airliner is *much* more robust than that of a rocket.  They handle many more different kinds of load than most rockets (lots of turning of different types), are intended to last for far longer than any rocket (the F9 booster stage is working hard to reach a few 10s of flights, so, it's a *little* different than an airliner), and their penalty for extra weight is far lower.  Plus the safety expectations are vastly higher than for any rocket flying today.  So they're just a lot more heavily built than rockets.

And yes - it's also a fairly different regime for lots of regions.  But weight is at an absurd premium on a rocket compared to an airplane, and it shows.

Offline whitelancer64

Re: Max Q
« Reply #12 on: 06/22/2023 11:09 pm »
This seems to be a bit of a hurdle for new rockets, as there is usually some trepidation about passing through this flight regime.

Often the rocket is shown as flying around 1000+ kph at 10000+ meters altitude dufing Max Q

My question is more of an observation: jetliners routinely fly in this regime, and its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

I just want to add a point that I've made before, both on NSF and elsewhere:

Max Q is very important historically. Early on in the V2 flight test program, ground tests were performing well but rockets were failing in flight, breaking apart and exploding, and nobody understood why. Werner Von Braun and a few others went downrange with binoculars to watch a launch, and they saw that the skin of the rocket was buckling in flight, much like a crushed soda can. Something nobody had thought of, dynamic pressure was the culprit. They had designed the rocket for thrust loads but not the dynamic pressure loads of the atmosphere while in flight. They strengthened the skin of the rocket and their flight success rate went way up. When Von Braun and his other engineers went to the US after the war, their learned concern about Max Q seeped into the US rocketry program and lasts to this day.

The points raised earlier in this thread, that you want to design your rocket to be able to withstand Max Q, but not to add too much structural mass, is another reason why this is still a callout, although I am pretty sure that no rocket has been lost at Max Q due to dynamic pressure loads since the 60s.
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Offline Hop_David

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Re: Max Q
« Reply #13 on: 08/03/2023 04:14 pm »
This seems to be a bit of a hurdle for new rockets, as there is usually some trepidation about passing through this flight regime.

Often the rocket is shown as flying around 1000+ kph at 10000+ meters altitude dufing Max Q

My question is more of an observation: jetliners routinely fly in this regime, and its never a worry (I have experienced 1,200+ kph on a commercial airliner). So, why the worry for a rocket doing the same?

A jetliner has a much smaller delta V budget than the typical rocket. Delta V drives the exponent in the rocket equation. A big delta V budget mandates that a big majority of the rocket's mass is propellent. See my piece MF is a MoFo.

When most of the vehicle is propellant that leaves less mass to make a strong and durable structure.

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