How do you know about Max Q?

[katateochi]

So there is this thing called Max Q. I know it exists from watching RL launches and from discussions on here, but how do you know about it from within the game? What clues are there to suggest it's there and what impact does it have? Is it even something worth considering?  

The only way that I have any sense of Max Q is from watching what the MechJeb Ascent Guidance autopilot does if you select the option to Limit Q. But I don't really have a feel for what is right, so with over powered rockets I throttle back a bit, usually after reaching around 150m/s and then throttle up again.....a bit later....but it's all very hand wavy and I don't know if doing that makes any difference.

Is Max Q worth considering in KSP? Does it impact on ascent efficiency? And if so, how do you know when your craft is experiencing max dynamic pressure?

[Xd the great]

4 minutes ago, katateochi said:

So there is this thing called Max Q. I know it exists from watching RL launches and from discussions on here, but how do you know about it from within the game? What clues are there to suggest it's there and what impact does it have? Is it even something worth considering?  

The only way that I have any sense of Max Q is from watching what the MechJeb Ascent Guidance autopilot does if you select the option to Limit Q. But I don't really have a feel for what is right, so with over powered rockets I throttle back a bit, usually after reaching around 150m/s and then throttle up again.....a bit later....but it's all very hand wavy and I don't know if doing that makes any difference.

Is Max Q worth considering in KSP? Does it impact on ascent efficiency? And if so, how do you know when your craft is experiencing max dynamic pressure?

Short answer, no. The stress from aerodynamics on a conventional rocket is low, so , as long as you dont flip your rocket and use struts properly, who cares about maxQ. You wouldve blown up from overheating before stress testing your rocket.

[bewing]

It's just the maximum total drag on the vessel. If you really want to see it, then open up your AeroGUI and watch the drag number go up and down.

But as said above, trying to limit it by throttling back does not save anything.

 

Edited August 9, 2018 by bewing

[Snark]

11 hours ago, katateochi said:

So there is this thing called Max Q. I know it exists from watching RL launches and from discussions on here, but how do you know about it from within the game? What clues are there to suggest it's there and what impact does it have? Is it even something worth considering?

Yes, there's something called "max Q"-- it's the point of maximum aerodynamic forces on the rocket.  Discussion of what it is IRL in spoiler.

Spoiler

Aerodynamic forces on a rocket are the result of two things:

• How fast is the rocket going
• How thick is the air

When the rocket is sitting on the pad, the aero force is zero, because even though the air's really thick, the rocket's not moving.

When the rocket is orbiting up in space, the aero force is zero, because even though the rocket's speed is super duper fast, there's no air.  (Yes, technically there's a microscopic quantity of atmosphere, which results in teeny-weeny drag forces, which is why orbits decay.  But that's not what we're concerned with here, so never mind.)

Here's what happens when a rocket takes off:  It starts to go faster and faster, which means the aero forces get bigger and bigger.  However, as it goes faster... the air's getting thinner as the rocket climbs.  Eventually, the "air getting thinner" factor overwhelms the "rocket going faster" factor, and that's the point at which aero forces reach their maximum and start to go down again.

That's what max Q is.

Real-life rockets have to care about max Q, a lot, for a few reasons.  KSP rockets, however, don't have to care.  Technically max Q is there-- I mean, there really is some point at which aero forces reach a maximum-- but that's basically irrelevant and nobody ever bothers to keep track of it because it's not important in the game.

To understand the reason why KSP rockets generally don't care about max Q, it helps to understand why real-life rockets do, so that we can see why those reasons don't apply to KSP.

Reason #1:  Structural integrity.  Slamming a vessel through the air at several times the speed of sound is harsh.  It generates tremendous pressures and shockwaves.  It can rip a rocket to shreds.  This becomes especially important when you consider that rockets have to be built as light as humanly possible to maximize their dV, which means they tend to be made of thin, fragile materials-- you can't armor-plate 'em.  When engineers are building spacecraft components that are going to be slamming through air-- fairings, for example-- they have to decide "how thick or thin do we make the material".  Thinner is better for saving weight... but it also makes it less strong.  If you make it too thin, it'll buckle under the pressure.  So they want to make it "as thin as possible without being too thin".  To know just how thin they can get away with, they need to know "what's the maximum aerodynamic stress that this will be subjected to".  That's exactly what Max Q is all about, which is why people care.

Note that this applies not just at design time, but also at flight time.  For example, after running the numbers, they may decide "okay, we'll throttle down for a bit in early-mid-ascent, so that we wait until the air's thinner before we really hit the gas, so that we lower max Q."  That means more gravity losses, which can hurt dV... but if you've lowered the max Q, maybe you can get away with using thinner / lighter materials, and the weight savings may offset the increased gravity losses.

Why KSP rockets don't have to care about that:  They're made of magical, infinitely strong material.  Well, okay, not quite infinite, but it's pretty darn hard to cause a structural failure due to aero stress during ascent, unless you've built some weirdly fragile thing.  And in any case, KSP rockets are made out of immutable Lego-style parts, so it's not as if you have any choice of "use thinner or thicker materials".  Everything is armor plated.  Therefore, you might as well slam the throttle to maximum to conserve dV.  You don't have to worry about max Q because your rocket will never shred itself during ascent.

Reason #2:  Fuel efficiency.  A rocket needs a certain amount of dV to get to orbit.  There are two ways to lose dV:  gravity losses, and aerodynamic drag.  Gravity losses get worse if you're climbing slowly, so to minimize them, you want to accelerate as hard as possible as early as possible, and keep burning at the absolute maximum until you're finished with your ascent burn.  Aerodynamic drag, however, is the opposite:  it gets worse when you're going faster, and to minimize it you want to be careful not to go too fast when you're still down where the air is thick.  So, "what acceleration profile is the most efficient" will be some balance between the two.  It may be worthwhile for a rocket to throttle down a bit in the early part of a flight (so as to lower max Q and try to avoid too many aero losses too soon), then throttle up when the aero losses peter out.

Why KSP rockets don't have to care about that:  Kerbals live in an alternate universe with unrealistically tiny planets, which means orbital speeds are much lower.  Circular orbital speed in low Kerbin orbit is less than a third of Earth's.  This means that for most kerbal rockets, gravity losses are overwhelmingly higher than aero losses, since the rocket's not going so fast.  At any given altitude, there's some "ideal speed" for maximum efficiency, and that speed increases with increasing altitude.  Unless you've overbuilt your rocket with a ridiculously high TWR, a KSP rocket never "catches up" to that ideal speed during ascent-- the ideally-efficient speed is always faster than you're currently going.  That means you don't give a wet slap about max Q, because the answer to "when should I throttle down" is essentially "never".

 

I'm not an IRL rocket guy, so I don't know the degree to which IRL rockets care about reason 1 versus reason 2... but the point is, they have to care.  And KSP rockets don't.

So that's why you generally don't see much talk about max Q here in the forums.

[katateochi]

wow, very detailed answer @Snark, Thank you!! Looks like I won't be throttling back anymore then! 

[Snark]

9 minutes ago, katateochi said:

Looks like I won't be throttling back anymore then!

One caveat:  There is one circumstance in which throttling back can actually make sense in KSP, and that's if you've built some horribly un-streamlined, draggy contraption.  In such cases, it may actually be worthwhile to take it easy for the first few thousand meters to lower the aero losses.

Most folks design most of their rockets reasonably streamlined, so it usually doesn't come up.  However, once in a while, some odd mission might require a particularly awkward rocket design that's hard to make aerodynamic.  For example, mining ships tend to be odd contraptions with all sorts of gewgaws hanging off them (at least, mine do).  Also ships that are launching big rovers.

So, the answer is usually "don't throttle down" ... but for sufficiently awkward ship designs, it may be worth considering. 

(That's an example of a thing that KSP rockets have to worry about that IRL ones don't... since IRL nobody tries to shove a giant cylinder with multiple awkward exposed attachment sideways through atmosphere at Mach <lots>.  Heck, even Elon put his Tesla in an aerodynamic fairing while he lofted it.)

[klesh]

Woah woah, wait a minute here.  I thought the preferred method was to stay below 250m/s while deep down in the atmosphere to avoid fighting too much against the soup down there.

[katateochi]

14 minutes ago, Snark said:

For example, mining ships tend to be odd contraptions with all sorts of gewgaws hanging off them (at least, mine do).  Also ships that are launching big rovers.

I recently launched such a monstrosity, and even with attempts to take it carefully at the start, the drag at the front was too much.....so I wrapped it in a ridiculous fairing and it just worked.  I'm not proud of this rocket:

 

[Lisias]

9 minutes ago, Snark said:

However, once in a while, some odd mission might require a particularly awkward rocket design that's hard to make aerodynamic.  For example, mining ships tend to be odd contraptions with all sorts of gewgaws hanging off them (at least, mine do).  Also ships that are launching big rovers.

So, the answer is usually "don't throttle down" ... but for sufficiently awkward ship designs, it may be worth considering. 

Something like this? :-)



This is my contraption  to send tourists into orbit. 100% recoverable, but wastes a bit of fuel to be kicked into the ground and miss it. I realized that a tri-pointed star design is the best compromise between acceptable drag going up, and desired drag going down. 

And yeah, this thing had to be throttled down on the ascent due that ridiculous quantity of engines needed to takeoff!

[Snark]

3 minutes ago, klesh said:

Woah woah, wait a minute here.  I thought the preferred method was to stay below 250m/s while deep down in the atmosphere to avoid fighting too much against the soup down there.

Nope.  A reasonably streamlined and not-unreasonably-overpowered rocket in KSP loses far more to gravity loss than it does to aerodynamic drag.  You wanna go fast.

Over Mach 1 is draggier, yes, but not enough draggier to be worth the extra gravity loss.  As long as your TWR isn't any higher than 2 in the lower reaches of the atmosphere, you want to throttle as high as you can.

The only way for a reasonably-streamlined rocket to get into a situation where aero losses would actually be bigger than gravity loss would be if it launches with some ludicrous TWR, as in "much higher than TWR 2 off the pad".  But in that case, the right solution is not "throttle down", it's "put fewer engines on the rocket" because you're losing more dV to the dead weight of too-much-engine than you are to gravity losses, and you're making way too much drag.

Note that the answer used to be different, back in the dim mists of ancient time before KSP 1.0 arrived with its new aerodynamics model.  Back in the old souposphere days, then aero losses were king, so the answer would have been different.  But ever since April 2015, in the post-1.0 era, gravity losses are by far the predominant one for most rockets.

[Lisias]

2 minutes ago, katateochi said:

I recently launched such a monstrosity, and even with attempts to take it carefully at the start, the drag at the front was too much.....so I wrapped it in a ridiculous fairing and it just worked.  

The Russians would be proud of you!
 

[Superfluous J]

12 minutes ago, klesh said:

Woah woah, wait a minute here.  I thought the preferred method was to stay below 250m/s while deep down in the atmosphere to avoid fighting too much against the soup down there.

That advice hasn't been viable for over 3 years. You should also not go straight up to 10km and then crank it 45 degrees.

Edited August 10, 2018 by 5thHorseman

[Snark]

19 minutes ago, klesh said:

I thought the preferred method was to stay below 250m/s while deep down in the atmosphere to avoid fighting too much against the soup down there.

Just to make it really clear, here's the math:

For a rocket on vertical ascent, the optimally efficient speed (i.e. where the sum of "gravity loss plus aerodynamic drag" is lowest) happens when the rocket is climbing exactly at terminal velocity, where "terminal velocity" is defined as the speed at which drag equals the craft's weight.

Suppose you actually wanted to achieve that.  What would you need to do?  Answer:  It would need to be climbing with a TWR > 2.  That's because ascending at terminal velocity requires the engine to put out twice the rocket's weight-- one rocket-weight to actually bear the rocket's weight (this is the gravity loss), and one rocket-weight's worth of drag (this is the aero loss).  But that assumes that you're already going at terminal velocity-- and your rocket starts at zero velocity on the launchpad.  Which means to reach terminal velocity in the first place, the rocket will have to accelerate substantially harder than TWR 2.0 for the very start of the ascent, in order to get up to the optimum speed.

So you'd need a rocket with a launchpad TWR of way higher than 2, which then goes down to a TWR of 2 as soon as it hits terminal velocity.

Now:  A reasonably streamlined rocket is gonna have a terminal velocity at sea level that's way higher than 250 m/s.  I've built 'em as high as 1000 m/s, though more commonly it's somewhere in the 400-500 m/s range, I'd guess.  Certainly well above 300 m/s.

That means for it to be going "too fast"-- i.e. fast enough that it would actually save fuel to slow down-- it would have to be going 400+ meters per second right off the pad.

"Well, okay," I hear you cry, "obviously it can't do that right on the pad, but with a high TWR it does get up over 400 m/s pretty soon and then I should throttle down, right?"

Sure, it gets to sea-level terminal velocity fairly soon.  But by the time it gets that fast, it's not at sea level anymore.  It's up in the sky, going through much thinner air, and the terminal velocity is much higher.  (By the time you get to 10 km, you're already past about 90% of air pressure.  The air gets thin pretty quick with altitude.)  The terminal velocity has risen faster than the rocket's velocity.  So you never catch up to it.

Moral of the story:  If you make your rocket streamlined, and you don't lift off the pad with a TWR much higher than 2, then the answer to "how fast should I go" is "as fast as possible."

[klesh]

10 minutes ago, Snark said:

Nope.  A reasonably streamlined and not-unreasonably-overpowered rocket in KSP loses far more to gravity loss than it does to aerodynamic drag.  You wanna go fast. 

Over Mach 1 is draggier, yes, but not enough draggier to be worth the extra gravity loss.  As long as your TWR isn't any higher than 2 in the lower reaches of the atmosphere, you want to throttle as high as you can.

The only way for a reasonably-streamlined rocket to get into a situation where aero losses would actually be bigger than gravity loss would be if it launches with some ludicrous TWR, as in "much higher than TWR 2 off the pad".  But in that case, the right solution is not "throttle down", it's "put fewer engines on the rocket" because you're losing more dV to the dead weight of too-much-engine than you are to gravity losses, and you're making way too much drag.

Note that the answer used to be different, back in the dim mists of ancient time before KSP 1.0 arrived with its new aerodynamics model.  Back in the old souposphere days, then aero losses were king, so the answer would have been different.  But ever since April 2015, in the post-1.0 era, gravity losses are by far the predominant one for most rockets.

 

 

Jeeze, where have I been?  I knew things changed dramatically post 1.0, but ~2,000 hours in this game and I'm always staying below 250m/sec till out of the first blue color in the atmosphere.   Looks like I'll have to see what some of my designs do in full blast mode. 

12 minutes ago, 5thHorseman said:

That advice hasn't been viable for over 3 years. You should also not go straight up to 10km and then crank it 45 degrees.

 

 

Yes, of course.  I'm not that bad.   I just thought <250m/s was the new way to do it. 

[Snark]

7 minutes ago, klesh said:

Looks like I'll have to see what some of my designs do in full blast mode.

And also consider your choice of TWR.  KSP rockets can work well at low TWR or high TWR, but only if they're designed that way for the right reasons.

For example, low TWR can be a valid design choice if the reason is to conserve engines and you'd rather throw away more fuel to gravity losses than spend more money on engines.

But if your reason for low TWR is "because I don't want to go too fast"... then you may want to consider beefing up your TWR and see how that goes.

Myself, I like to keep my rockets smaller on the pad (no real reason, it's just what I like), and I like trying to optimize for "most dV in the smallest package", so I like to launch at TWR 2.0 (with good streamlining) and do a really aggressive gravity turn-- I immediately pitch nearly 10 degrees over right off the pad, and follow until my Ap's as high as I want it.  Works great. 

[Lisias]

8 minutes ago, 5thHorseman said:

That advice hasn't been viable for over 3 years. You should also not go straight up to 10km and then crank it 45 degrees.

I usually go straight trough the first atmosphere layer (light blue, up to 7500 m). At least for me, rotating earlier creates some body lifting, and more than once I lost the control of the rocket. Not viable with boosters - unless you consider carpet bombing KSC part of the mission.  

I once flew a rocket "sideways" and I found that the thing got enough lifting to keep going straight at Mach 1.5  or a bit less.  

[mystifeid]

Usually my goal is to maximize fuel remaining when orbit is reached and if there is one thing MechJeb is good for, it's testing ascent profiles for a particular craft.

[Xd the great]

Well, the last time my rockets go kaboomy due to launching too fast, I had a twr of 6.

[blakemw]

There's actually a marginally good reason to throttle back with certain engines in certain contexts.

First is core-throttling on Falcon Heavy style rockets, that is you don't throttle everything, just the core. This give you more deltav and would even do so on a vacuum world because by saving fuel in the core for after the boosters are decoupled, that fuel goes only to accelerating the mass of the core+payload, and not boosters+core+payload. If you're using a rocket with boosters and aren't using crossfeed (which isn't even a possibility if the boosters are SRBs), then it is usually an improvement to keep the TWR at about 2.0, of course you could also bring more fuel but that costs more and you could technically probably use smaller engines, but there aren't a ton of engine choices available. (like if you could custom tailor every aspect of the engines you're using, this strategy probably wouldn't be employed. But it's like SpaceX, they want to make a Falcon Heavy using "off-the-shelf" Falcon 9 boosters, without designing entirely new boosters and core, the way to maximize performance is to heavily throttle the core soon after liftoff, until the boosters have detached)

The second reason you might throttle is ISP: some launch engines, especially Swivel, get a lot more thrust per unit of fuel once out of the lower atmosphere (practically, 10000m is as good as a vacuum for ISP purposes), this often has a much more significant impact on efficiency than aerodynamic drag. Now, of course, you do need to climb and maintain a good enough TWR, most likely this will only be useful when used in conjunction with the first factor I mentioned: If you're using something like a Swivel core with Thumber SRBs then throttling back the swivel means not only are you saving fuel for when it no longer needs to accelerate the mass of the SRBs, you also get more thrust out of it thanks to a higher ISP due to altitude.

[The Aziz]

On 8/10/2018 at 2:24 AM, 5thHorseman said:

That advice hasn't been viable for over 3 years. You should also not go straight up to 10km and then crank it 45 degrees.

Yet I've seen people giving this advice very recently, so...

The 10km/45° thing is obvious, but throttling down, well.. (I've never done that unless I saw that my TWR was too high)

[Norcalplanner]

I did a study on TWR, aerodynamic drag, gravity losses, and ascent profiles two years ago, which may shed some light:

 

Edited August 20, 2018 by Norcalplanner

[sturmhauke]

On 8/9/2018 at 5:16 PM, katateochi said:

I recently launched such a monstrosity, and even with attempts to take it carefully at the start, the drag at the front was too much.....so I wrapped it in a ridiculous fairing and it just worked.  I'm not proud of this rocket:

 

Were you building St. Basil's Space Cathedral?

[sturmhauke]

On 8/9/2018 at 4:27 PM, Snark said:

Reason #1:  Structural integrity.  Slamming a vessel through the air at several times the speed of sound is harsh.  It generates tremendous pressures and shockwaves.  It can rip a rocket to shreds...

Why KSP rockets don't have to care about that:  They're made of magical, infinitely strong material.  Well, okay, not quite infinite, but it's pretty darn hard to cause a structural failure due to aero stress during ascent, unless you've built some weirdly fragile thing.  And in any case, KSP rockets are made out of immutable Lego-style parts, so it's not as if you have any choice of "use thinner or thicker materials".  Everything is armor plated.  Therefore, you might as well slam the throttle to maximum to conserve dV.  You don't have to worry about max Q because your rocket will never shred itself during ascent.

It's a bit different with spaceplanes. They are kind of "weirdly fragile things" by nature, and can more easily disintegrate in midflight from aero forces. But this isn't an argument for throttling down. Rather, they need to be designed with low drag, low frontal cross section, as little wing surface as you can get away with, and lots of control surface, especially for pitch control. A large vertical stabilizer is an underappreciated but useful thing to have too. Autostruts are useful if done correctly, but can also induce phantom forces that bend the whole craft, especially for larger ones. I try to limit the number of autostruts going to one part (use visualize autostruts for this), and use grandfather more than root. Heaviest should usually be avoided, because the current heaviest part can change over time.

Now, I have a tendency to ignore what I just said and build giant, draggy spaceplanes. Those tend to require hard struts and a crapton of engines, and a higher part count than is perhaps healthy. But they look cool.