Optimal TWR with Stock vs. FAR Aerodynamics

[wasmic]

But it always seems to lift off so SLOWLY!

That's just because it is so large - rockets in KSP are generally quite small.

On topic, the best TWR with FAR is about 1.4 (based on personal experience). A higher TWR will reduce the gravity drag that you experience (and atmo drag is negligible anyways), so a TWR of 3 at liftoff wil significantly lower the required delta-v to reach orbit. However, it will also make the rocket much harder to fly and much more likely to be damaged by aerodynamic pressure.

[Bryce Ring]

Oh, havent a clue what BC is Ask Mr. Ferram

Oh i see. I dont have that information, but terminal velocity is always optimum.

Perhaps i should try again with mechjeb...

I found this (not your question or on topic exactly) but...

http://en.wikipedia.org/wiki/Ballistic_coefficient

Edited December 14, 2014 by Bryce Ring

[GoSlash27]

Yes, for stock, achieving terminal velocity is easy and doable, since at sea level, its only 100 m/s. Thus, maintaining terminal velocity as you get high is what matters, and that is relatively easy to do even with low TWR until you get to 10 km or so. I actually once made a spreadsheet that calculates required acceleration needed to maintain terminal velocity vs. altitude:

http://i.imgur.com/RPpBAeK.png

.

Arkie,

Was this made with the assumption of a vertical launch throughout? 'Cuz the closer you get to horizontal, the less rapidly you're rising, and thus the less rapidly the terminal velocity is increasing. Upshot is (at least in my experience) the thrust requirement actually goes down with altitude, not up.

Best,

-Slashy

[arkie87]

That's just because it is so large - rockets in KSP are generally quite small.

TWR is dimensionless. A TWR of 1.4 will behave the same for a large rocket or a small rocket... (assuming drag is negligible). Your explanation does not make sense.

On topic, the best TWR with FAR is about 1.4 (based on personal experience). A higher TWR will reduce the gravity drag that you experience (and atmo drag is negligible anyways), so a TWR of 3 at liftoff wil significantly lower the required delta-v to reach orbit. However, it will also make the rocket much harder to fly and much more likely to be damaged by aerodynamic pressure.

So that makes sense. I, however, have disabled aerodynamic disassebly in FAR so i dont have to worry about falling apart

How exactly does it make rocket harder to fly though?

[arkie87]

Arkie,

Was this made with the assumption of a vertical launch throughout? 'Cuz the closer you get to horizontal, the less rapidly you're rising, and thus the less rapidly the terminal velocity is increasing. Upshot is (at least in my experience) the thrust requirement actually goes down with altitude, not up.

Best,

-Slashy

Yes, it assume vertical launch. If you are horizontal, terminal velocity increases more slowly (as you pointed out); however, the fraction of velocity in the vertical direction also decreases. Nevertheless, for a gravity-turn launch, you dont need to be climbing at terminal velocity the whole time, so the results become less applicable...

The main point for the graph is that with stock aerodynamics, TWR of 2 can get you up to terminal velocity fast, and keep you there (until 10 km or so) and even require you to throttle back to not exceed it.

[GoSlash27]

Yes, it assume vertical launch. If you are horizontal, terminal velocity increases more slowly (as you pointed out); however, the fraction of velocity in the vertical direction also decreases. Nevertheless, for a gravity-turn launch, you dont need to be climbing at terminal velocity the whole time, so the results become less applicable...

The main point for the graph is that with stock aerodynamics, TWR of 2 can get you up to terminal velocity fast, and keep you there (until 10 km or so) and even require you to throttle back to not exceed it.

Okay,

So in FAR, how much is that G limit increased? What acceleration is required in FAR to get you to/keep you at terminal velocity early in the launch?

My lightbulb just went on...

-Slashy

[GoSlash27]

Just reviewed the video, and I think I've got it... sorta.

What would really be helpful would be to drop that same rocket from space, straight down and record the speed in free-fall vs. altitude.

Does anybody have the scale height and surface pressure for the FAR atmosphere?

The numbers for stock are 101.3 kPa at the surface and scale height of 5kM

Best,

-Slashy

Edited December 14, 2014 by GoSlash27

[arkie87]

Okay,

So in FAR, how much is that G limit increased? What acceleration is required in FAR to get you to/keep you at terminal velocity early in the launch?

My lightbulb just went on...

-Slashy

G's required to get to terminal velocity would be huge, enough to make kerbals require an exoskeleton it's not really feasible to achieve terminal velocity with FAR, nor is it more efficient (in KSP) given the extra required mass of the engines (though it might be cheaper in terms of kerbucks, if you just strap loads of SRB's rather than one mainsail).

Also, with FAR, going too fast can result in high dynamic pressure and aerodynamic disassebly from too much drag (though i personally have disabled this feature).

Just reviewed the video, and I think I've got it... sorta.

What would really be helpful would be to drop that same rocket from space, straight down and record the speed in free-fall vs. altitude.

Does anybody have the scale height and surface pressure for the FAR atmosphere?

The numbers for stock are 101.3 kPa at the surface and scale height of 5kM

Best,

-Slashy

Why? Are you trying to get terminal velocity? FAR displays terminal velocity... Also, terminal velocity is a function of attitude, mach number, and weight with FAR...

[Starman4308]

Okay,

So in FAR, how much is that G limit increased? What acceleration is required in FAR to get you to/keep you at terminal velocity early in the launch?

Dependent on the vehicle in question. A small, poorly designed rocket is going to have lower terminal velocity than a large, aerodynamic rocket*. In any event, the answer is "way more than is practical". About the only place I can think of where you might run into terminal velocity is an Eve/Venus ascent, because you've got a lot more atmosphere, and a lot more time lingering in high-density atmosphere.

*Scale up a rocket by 5x, and while cross-sectional area goes up by 25x, mass (and therefore inertia) go up by 125x. It's probably the biggest reason why no orbital launch vehicle I know of has been less than ~10 tonnes.

Edited December 14, 2014 by Starman4308

[GoSlash27]

G's required to get to terminal velocity would be huge, enough to make kerbals require an exoskeleton it's not really feasible to achieve terminal velocity with FAR, nor is it more efficient (in KSP) given the extra required mass of the engines (though it might be cheaper in terms of kerbucks, if you just strap loads of SRB's rather than one mainsail).

Also, with FAR, going too fast can result in high dynamic pressure and aerodynamic disassebly from too much drag (though i personally have disabled this feature).

Why? Are you trying to get terminal velocity? FAR displays terminal velocity... Also, terminal velocity is a function of attitude, mach number, and weight with FAR...

Because the Vt readout is giving you info about your ship, not the atmosphere itself.

If my hunch is correct, I think you've misinterpreted something and it's led you astray. I need that info before I can say for sure.

Best,

-Slashy

[arkie87]

Because the Vt readout is giving you info about your ship, not the atmosphere itself.

If my hunch is correct, I think you've misinterpreted something and it's led you astray. I need that info before I can say for sure.

Best,

-Slashy

I'm not sure if FAR changes atmosphere parameters such as scale height and sea level density.

What is your hunch?

[GoSlash27]

I'm not sure if FAR changes atmosphere parameters such as scale height and sea level density.

What is your hunch?

I'll need some indulgence here. I'm working to get that pinned down, and don't want to post potentially misleading info based on an assumption.

Please bear with me,

-Slashy

[OhioBob]

Does anybody have the scale height and surface pressure for the FAR atmosphere?

If it's based on the real world atmosphere, scale height is not constant. I don't know what model FAR uses, but the following tells you about all you need to know about the real world model:

http://www.braeunig.us/space/atmmodel.htm

Table 4 sums it up pretty good.

[arkie87]

If it's based on the real world atmosphere, scale height is not constant. I don't know what model FAR uses, but the following tells you about all you need to know about the real world model:

http://www.braeunig.us/space/atmmodel.htm

Table 4 sums it up pretty good.

Wow. Much more complicated than simple scale height

[Jouni]

In discussions like this, you should first define what 'optimal' means. Do you want to maximize the payload fraction? Do you want to minimize the launch costs with or without recovery? Do you want to minimize the fuel usage relative to the payload, or are you minimizing the delta-v usage to reach orbit? Each of these definitions leads to very different answers.

My personal definition of 'optimal' is minimizing the sum of (thrust * peak Isp) over all engines of the rocket, assuming that interstage fuel lines don't exist. With this definition, I basically keep adding fuel to the rocket, as long as it increases the payload capacity. My typical launch TWRs are between 1.15 and 1.30, regardless of whether I'm playing with stock aerodynamics or FAR.

[arkie87]

In discussions like this, you should first define what 'optimal' means. Do you want to maximize the payload fraction? Do you want to minimize the launch costs with or without recovery? Do you want to minimize the fuel usage relative to the payload, or are you minimizing the delta-v usage to reach orbit? Each of these definitions leads to very different answers.

My personal definition of 'optimal' is minimizing the sum of (thrust * peak Isp) over all engines of the rocket, assuming that interstage fuel lines don't exist. With this definition, I basically keep adding fuel to the rocket, as long as it increases the payload capacity. My typical launch TWRs are between 1.15 and 1.30, regardless of whether I'm playing with stock aerodynamics or FAR.

In career mode, I think optimizing cost/deltaV ratio for a given payload is what matters, unless you care about pollution on Kerbin and Kerbin global warming

If not in career mode, then any of those other definitions can make sense, depending on what you care about...

[GoSlash27]

Okay, I got official confirmation:

FAR doesn't change anything about the atmosphere itself.

We speak of the stock KSP "souposphere", but in reality the atmosphere is the same and the *vehicles* are draggy.

So in FAR, you're still getting off of the same rock. Same radius, rotation, gravity, air pressure, gradient, and boundary.

It's just that your rocket is more aerodynamic for most of the trip and draggier in the transsonic region.

Now... attempting to read into terminal velocity in stock is fine, because every vehicle behaves the same AFA drag coefficient is concerned. It's a constant .2. But in your case, your drag coefficient changes radically and thus skews your terminal velocity reading.

Were you to drop an object with a constant .2 drag coefficient from altitude, you would find that it falls at the same terminal velocity as it would in the stock game.

So what does this mean?

Simply that your vehicle is cleaner off the pad, dirtier around Mach 1, and cleaner the rest of the way. It does not, therefore radically alter the ideal t/w or launch profile from stock. It certainly doesn't mean that 5 or 6G acceleration is more efficient.

All it really means is that you should do your gravity turn sooner than in stock and that you need more thrust in the transsonic region and can get by with less in the supersonic region.

Best,

-Slashy

[Starman4308]

Okay, I got official confirmation:

FAR doesn't change anything about the atmosphere itself.

We speak of the stock KSP "souposphere", but in reality the atmosphere is the same and the *vehicles* are draggy.

So in FAR, you're still getting off of the same rock. Same radius, rotation, gravity, air pressure, gradient, and boundary.

It's just that your rocket is more aerodynamic for most of the trip and draggier in the transsonic region.

Now... attempting to read into terminal velocity in stock is fine, because every vehicle behaves the same AFA drag coefficient is concerned. It's a constant .2. But in your case, your drag coefficient changes radically and thus skews your terminal velocity reading.

Were you to drop an object with a constant .2 drag coefficient from altitude, you would find that it falls at the same terminal velocity as it would in the stock game.

So what does this mean?

Simply that your vehicle is cleaner off the pad, dirtier around Mach 1, and cleaner the rest of the way. It does not, therefore radically alter the ideal t/w or launch profile from stock. It certainly doesn't mean that 5 or 6G acceleration is more efficient.

All it really means is that you should do your gravity turn sooner than in stock and that you need more thrust in the transsonic region and can get by with less in the supersonic region.

Best,

-Slashy

You are misunderstanding a few things. FAR changes not only drag coefficient, but also cross-sectional area. Cd is now a function of rocket shape and Mach effects, while cross-sectional area is a function of rocket shape. In stock, Cd is 0.2 for most parts, while cross-sectional area is 0.008 * mass. As such, an object with 0.2 Cd in FAR (which is only true for a certain orientation) will only fall at the same rate if it's cross-sectional area happens to be 0.008 * mass.

What this means in practice is that atmospheric drag almost disappears, because Cd * A will almost always be vastly less in FAR than stock, even if you're in the worst of the trans-sonic region. This means that TWR and launch profile are vastly changed, particularly because FAR can now model aerodynamic stability and instability: try to pull that 45 degree gravity turn at 10 km altitude, and your rocket will flip out and destroy itself.

Really, just try it. There's a reason stock aerodynamics is called the "souposphere": when you move to a realistic aerodynamic model, atmospheric drag plummets radically, particularly for streamlined designs.

EDIT: I may or may not slightly misunderstand the details of the FAR calculation (my experiences have had some wonkiness about how cross-sectional area and Cd are calculated), but from experience, I can tell you this: FAR is vastly removed from stock aerodynamics, FAR drag is vastly less than stock for any reasonable design, and FAR ascents have to be much different from stock.

Edited December 14, 2014 by Starman4308

[GoSlash27]

You are misunderstanding a few things. FAR changes not only drag coefficient, but also cross-sectional area. Cd is now a function of rocket shape and Mach effects, while cross-sectional area is a function of rocket shape. In stock, Cd is 0.2 for most parts, while cross-sectional area is 0.008 * mass. As such, an object with 0.2 Cd in FAR (which is only true for a certain orientation) will only fall at the same rate if it's cross-sectional area happens to be 0.008 * mass.

What this means in practice is that atmospheric drag almost disappears, because Cd * A will almost always be vastly less in FAR than stock, even if you're in the worst of the trans-sonic region. This means that TWR and launch profile are vastly changed, particularly because FAR can now model aerodynamic stability and instability: try to pull that 45 degree gravity turn at 10 km altitude, and your rocket will flip out and destroy itself.

Really, just try it. There's a reason stock aerodynamics is called the "souposphere": when you move to a realistic aerodynamic model, atmospheric drag plummets radically, particularly for streamlined designs.

EDIT: I may or may not slightly misunderstand the details of the FAR calculation (my experiences have had some wonkiness about how cross-sectional area and Cd are calculated), but from experience, I can tell you this: FAR is vastly removed from stock aerodynamics, FAR drag is vastly less than stock for any reasonable design, and FAR ascents have to be much different from stock.

All respect, but I'm not misunderstanding it.

FAR doesn't change the atmosphere, it changes the parts. The air is not thinner, the parts are less-draggy.

The items you mention ( Area and Cd ) are properties of the *parts*, not the atmosphere itself.

You hit on it in the last paragraph; "FAR is vastly removed from stock aerodynamics". But FAR's *atmosphere* is exactly the same as stock, so the term "souposphere" is highly misleading. It would be more accurate to say that stock KSP has lumberwagon aerodynamics.

Best,

-Slashy

Edited December 14, 2014 by GoSlash27

[Starman4308]

All respect, but I'm not misunderstanding it.

Far doesn't change the atmosphere, it changes the parts. The air is not thinner, the parts are less-draggy.

The items you mention ( Area and Cd ) are properties of the *parts*, not the atmosphere itself.

Best,

-Slashy

No: the changes are more comprehensive than that. Drag in FAR is a function of parts, how they are arranged, and the orientation/speed of the vessel. You seem to be hung up on the nickname of "souposphere" and assume I mean to say FAR affects atmosphere.

You also seem to be vastly downplaying the differences between FAR and stock: FAR plays a lot different.

[rkman]

It does not, therefore radically alter the ideal t/w or launch profile from stock. It certainly doesn't mean that 5 or 6G acceleration is more efficient.

I'd think it is the contrary because given the same vehicle with same twr, a less draggy vehicle will get up to terminal velocity more quickly than a more draggy vehicle. So with FAR you'd need lower twr to get up to terminal velocity at the same altitude as in stock.

[Starman4308]

I'd think it is the contrary because given the same vehicle with same twr, a less draggy vehicle will get up to terminal velocity more quickly than a more draggy vehicle. So with FAR you'd need lower twr to get up to terminal velocity at the same altitude as in stock.

Incorrect. Sure, you get less drag, but terminal velocity is also a lot higher. Unlike in stock, where you can get to terminal velocity relatively quickly, FAR would require an absolutely thunderous acceleration to get to terminal velocity*.

*Keep in mind that, the farther up you go, the thinner atmosphere is, and the higher terminal velocity is. If you don't hit terminal velocity in the first 10km, you're unlikely to ever hit it.

[GoSlash27]

No: the changes are more comprehensive than that. Drag in FAR is a function of parts, how they are arranged, and the orientation/speed of the vessel. You seem to be hung up on the nickname of "souposphere" and assume I mean to say FAR affects atmosphere.

You also seem to be vastly downplaying the differences between FAR and stock: FAR plays a lot different.

I assure you I'm not.

FAR's aerodynamics do change things fairly radically. All the things you mentioned and more. But it does not change Kerbin.

This is a critical point in what I'm trying to explain to Arkie; all of the changes that FAR makes are to properties of the parts, not to the atmosphere itself.

The gravity is still the same as it's always been and the pressure gradient is still the same as it's always been. It used to be semi-valid to talk about the "perfect" t/w as being 2 because in that sense an aircraft would maintain perfectly uniform drag. When they spoke of "terminal velocity", they were talking about the atmosphere, not the vehicle.

The terminal velocity displayed in KER is of the *ship*, not the atmosphere, and (especially in FAR) it has absolutely no relation to figuring out an ideal t/w on launch.

The most efficient launch profile in FAR is not going to be much different than it was all along because the most efficient path off of a planet is determined by the planet itself, not the ship making the trip. You're seeking the absolute minimum drag losses and the absolute minimum gravity losses and neither the air pressure nor the gravity have changed.

So there will be differences due to the behavior of the craft, but they won't be huge differences as far as the profile goes. Get into the gravity turn sooner and you might need a little more thrust to get you through the sound barrier. Other than that, it's still the same as it's always been; a prograde gravity turn at 2G (local) initial with the thrust reducing as pitch decreases. It's the same game with every planet with an atmosphere, just as it's the same gameplan with every planet without an atmosphere.

Best,

-Slashy

Edited December 14, 2014 by GoSlash27

[Starman4308]

The most efficient launch profile in FAR is not going to be much different than it was all along because the most efficient path off of a planet is determined by the planet itself, not the ship making the trip. You're seeking the absolute minimum drag losses and the absolute minimum gravity losses and neither the air pressure nor the gravity have changed.

I seriously suggest you play FAR a bit before making such statements. Ascent path is determined by a different, only partially overlapping set of factors in FAR.

Aerodynamics plays a huge role in ascent path. If it were "mostly determined by the planet", you'd never see the 1 km/s dV savings out of FAR that you do. Stock requires you to stay vertical for ~6-10 km because drag is so horrendous. In FAR, you're usually already several degrees off vertical before you ever crack 1000m, and your path from there to around 30 km is determined mostly by aerodynamics and TWR, because your rocket orientation is constrained to a narrow cone around prograde. This is contrasted with stock, where your pitch profile is determined solely by gravity, TWR, and atmospheric density, because aerodynamic stability/instability is just not a thing in stock.

Your "advice" is shown false by the large number of people who have actually played FAR, and the fact that few real-world rockets have first-stage TWR above 1.6 (and those which do generally lose some SRBs in less than a minute).

In stock, ideal TWR is very much a factor of terminal velocity, because you want to minimize losses in the 0-10 km zone, which means an efficient terminal-velocity ascent. In FAR, atmosphere drag is almost a negligible concern, and ascent path is more determined by other factors, such as aerodynamic stress, aerodynamic stability/instability, and balancing minimization of gravity losses against maximizing dV by minimizing engine mass.

Edited December 14, 2014 by Starman4308

[GoSlash27]

Incorrect. Sure, you get less drag, but terminal velocity is also a lot higher. Unlike in stock, where you can get to terminal velocity relatively quickly, FAR would require an absolutely thunderous acceleration to get to terminal velocity*.

*Keep in mind that, the farther up you go, the thinner atmosphere is, and the higher terminal velocity is. If you don't hit terminal velocity in the first 10km, you're unlikely to ever hit it.

This is what I'm talking about; you've conflated the terminal velocity of a powered rocket with the terminal velocity of a falling body of fixed area and drag coefficient.

They're not the same thing. The terminal velocity of your ship doesn't mean anything at all in determining the ideal launch profile. What matters is the rate at which atmospheric pressure is dropping.

Besides, you can reach terminal velocity in FAR very easily, even in an aircraft that has nearly zero thrust. Just don't climb, and eventually your speed will top out.

Best,

-Slashy