Great New Physics Thread!

[AmpsterMan]

1.0 is finally upon us! With this new update brings much to our tables. More parts, more power, but most of all, a different aerodynamics system. With new aerodynamics comes new responsibility. I would like for this to be a small section describing the more obscure and less obvious things with new aero. Furthermore, if there are any questions you would like answered, please use this thread to post your questions. It will help moderators, QA , and Experimentals testers focus their responses. Please note, you are under no obligation to post your questions in here and we are under no obligation to respond xD.

Primary Changes

• Mass no longer affects drag

• Drag is dependent on shape, orientation to the velocity vector, part occlusion, mach effects, and air density.

• Atmosphere actually exist now. Temperature and pressure vary with altitude, latitude, and time of day.

• Aero Engines have peak thrust efficiency at different Mach levels as well as altitude

• Wings have a special aero all to themselves that behaves differently from part aero

Derived Changes

• Parts heat up faster in the lower atmosphere because of denser air (hence, easier to conduct heat) as well as more friction with the air around it.

• Night time air is more dense than day time air. However, it is more dense because it is cooler, so the speed of sound is lower too.

• Air is more dense the farther from the equator one is.

• Less dense air means less air in the air-intakes at higher altitudes. Air hogging got massive nerf to the point of being impractical. The diminishing returns of each next air intake are too high.

• Since drag is no longer based around the center of Mass, the center of pressure of a craft can change depending on Angle of Attack, leading to behavior the Center of Lift indicator would deem otherwise.

• Wing aero is not dependent wing shape and design. To compensate for this, the Lift and Drag values of the wings vary with the Mach of the wings. At low Mach, wings tend to act like they had a high aspect ratio, giving substantial lift but substantial drag. At high Mach they tend to act like low aspect ratio wings, with less drag but less lift. We get the best of both worlds!

• Planes can stall now if thrust isn’t sufficient enough to produce lift

Edited May 1, 2015 by NathanKell
fixed reversing of less dense / more dense air based on temperature.

[barfing_skull]

Atmospheric re-entry seems like it's going to need a new approach. Previously, capsules, with minimal parts around them, would slow down fairly quickly below maybe 20000m or so. Now they plunge like a rock.

I managed to survive using some left-over delta-v. But say I wanted to re-enter without an engine? I never used drogue chutes before, and I suppose this is where they might come in handy, but they're higher in the tech tree. It's possible I'm just used to opening the regular parachutes too low for the new aerodynamic model. Makes hitting a target a little more challenging.

I haven't tried the heat shielding yet, but I'm guessing that only preserves components (perhaps if they're in the service bays, rather than clinging to dear life on the outside of the vessel, but I haven't tried those yet, either), but doesn't help slow things down.

Perhaps more pontification than anything, but it would be interesting to hear other experiences or advice.

Cheers,

-BS

[MiniMatt]

Good thread idea

Many of us are used to dealing with velocity in metres per second. Mach speeds may not be immediately grasped. So, to aid:

Mach 1 = ~340m/s*

Ergo:

Peak atmospheric thrust of the Basic Jet Engine is at Mach 1.7 = ~540m/s

Peak atmospheric thrust of the Turbo RamJet Engine is at Mach 3 = ~1020m/s

Peak atmospheric thrust of the Rapier Engine is at Mach 3.7 = ~1258m/s

*caveat: unsure if mach effects are calculated according to air density & temperature within game as in real life, but suspect close enough approximations remain valid such that sea-level m/s conversion will work.

[Poynting]

Warm air is less dense than cool air so I think yo have some of the statements backwards. (Unless ksp has decided otherwise )

[Angezerus]

I have a few problems with the new aerodynamics. Sorry if I post in the wrong thread, but they are pretty big issues:

-Small aircrafts are waay too maneuverable when CoM and CoL is close. I can pull 15+Gs at 300+m/s. And I can do 180turn in a 50 meters radius..

-Both jet engines are ridiculously powerful. With a small simple aircraft I could reach 800+ m/s before the end of the runway...

-Time warp in atmosphere disintegrates many aircrafts. As I already mentioned a few months ago, time warping actually makes everything go faster, instead of the time itself accelerating, so aerodynamic stress is greatly increased when warping...

So is it possible that these will be fixed or are these working as intended?

[barfing_skull]

I have a few problems with the new aerodynamics. Sorry if I post in the wrong thread, but they are pretty big issues:

-Small aircrafts are waay too maneuverable when CoM and CoL is close. I can pull 15+Gs at 300+m/s. And I can do 180turn in a 50 meters radius..

-Both jet engines are ridiculously powerful. With a small simple aircraft I could reach 800+ m/s before the end of the runway...

-Time warp in atmosphere disintegrates many aircrafts. As I already mentioned a few months ago, time warping actually makes everything go faster, instead of the time itself accelerating, so aerodynamic stress is greatly increased when warping...

So is it possible that these will be fixed or are these working as intended?

What does your aircraft look like? I put together a medium-sized craft (Mk2 cockpit, a couple fuel tanks, aerodynamic surfaces, and 2 turbofan engines with the supersonic intakes) and it would barely go 250m/s at 9000m. A smaller craft I had with 1 engine maybe got to 350m/s at most. If anything I'm seeing the opposite.

Cheers,

-BS

[EtherDragon]

Gravity Turns are much more accurately modeled! If you take any rocket with tail fins, you can do a control free gravity turn.

Step 1 - launch as normal

Step 2 - pitch over to 10 degrees off horizon at about 1000m

Step 3 - Disable SAS, let the fins and gravity do the work - no control input needed, other than throttle adjustments!

[Angezerus]

Try the following:

1 shock cone on the nose

1 cockpit

1 lf tank behind cockpit

1 turbofan behind lf tank

Add type D wings as tail and wings an the Little square steering surface

Add gears, and take off.

The thing just burns up itself.. From start till the end of runway, I usually reach 400-500 sometimes even 800 m/s. If I go straight up in a 90 degree climb, I can reach a 160km peak altitude in the zoom.

[barfing_skull]

Didn't have the shock cone yet, so I used a circular intake with the basic jet engine. Built something pretty similar to what you describe. I got up to maybe 250/275 by the end of the runway. Speed over land was 345m/s, highest speed in a very steep dive was 472m/s. Max G's was 16.0. It was very manoeuvrable.

You'll be pleased to know Valentina landed safely, but it took nearly twice the runway distance to do so. Thing is hard to slow down.

I tried the same thing with the turbojet and ram air intake. It was maybe at 350m/s at the end of the runway, and its wings melted off at roughly 1000m/s at 4000m altitude. Guess I should climb first. Obdred didn't fare so well.

Cheers,

-BS

[Wintersdark]

Fuel tank, inline cockpit, circular intake, jet engine, basic wings and rear fins. Flew extremely well, mach effects at end of runway.

I'm really early in my career save, however - thing needs brakes, badly. Touched down at the water side of the runway, stopped moving in the hills on the other side of the grasslands. That was touching down at 100m/s, with engine off and brakes on. Clearly, need to experiment with landing gear brake settings.

I'm not a good aircraft pilot, mind you, but it was really easy to fly. Definitely a lot easier to build and pilot than in 0.90, particularly at low altitudes. No flipping issues, highly responsive at all speeds, naturally recentered to prograde no matter crudely I abused the controls.

Awesome.

[rentaspoon]

So far I'm flying on the dirt runway and have unlocked only the basics but I'm really enjoying the aero's and the fins on rockets now feel like they are doing a job where as before they felt like they where doing nothing to help.

My rockets now need the fins, otherwise I have to incorporate a 360° spin into the flight.

Planes take off and fly nice and easy, however don't fall to pieces like in ferram and I could be fooled into believing that I'm flying a flight sim.

Stalling is also a enjoyable feeling too, the feedback from the planes makes it very easy to tell what you have done wrong and how to compensate design wise.

Finally you don't need SAS to fly anymore.

[panzer1b]

• Drag is dependent on shape, orientation to the velocity vector, part occlusion, mach effects, and air density.

Yup, occlusion works just as intended:

Javascript is disabled. View full album

The pictures should speak for themselves......

[Emilio]

Aircraft finally feel like they're flying, and not swimming in the souposphere.

The new thermal dynamics are also quite good, I remember that on one flight, while coasting towards the Mun, my ascent stage that was on a roughly similar trajectory got exposed to sunlight for too long and exploded - that scared the hell out of me, but when I understood what had happened I thought, good job Squad !

@panzer1b Why the negativity ? Just report the bug in the appropriate section.

[Valerian]

I didn't use planes much before 1.0, but the survey missions forced me to use them more, and I'm really enjoying them now

I'm a bit beyond the beginning in the tech tree (unlocked all nodes up to the ones costing 90 science, included).

My plane can usually go at more than 600m/s and maintain a speed of 500m/s to 600m/s easily around 8000m to 11000m.

The trick is to avoid drag as much as possible, one air intake and one engine are enough, adding more will only add too much drag.

Small wings are good to prevent drag and lift is never an issue.

I found that often the tail wing was useless, and that I could also fly some planes without canards or without elevons, removing those really helps with the drag too.

For landing I usually try to do air brakes with my wings: I do it when near the ground (500m to 1000m), by diving down and then steering up several times, then when I'm at around 80 m/s it's slow enough to be able to use the brakes successfully.

If you need/want to use science experiments, batteries, or other stuff, you should put them inside a service bay, it will prevent them to generate any drag.

A few screenshots of a plane I just made, many other combinations of wings will work fine with it:

Javascript is disabled. View full album

[Endersmens]

Yeah, I see a few things that appear backwards. From my understanding, on a warmer day, air is less dense, and a colder day means the air is more dense.

So shouldn't the equator be less dense and poles be more dense? Or is this a misconception?

[Valerian]

(...)

A few screenshots of a plane I just made, many other combinations of wings will work fine with it:

http://imgur.com/a/issk6

Ok ... I just tried the same plane but without wings at all: taking of is a bit hard but it flies just fine, a little bit hard to steer ...

MK1 cockpit reaction wheel is overpowered apparently, wings are not necessary for a plane in KSP

[Starwhip]

I do love the new aerodynamics model...

[Orange_Slime_]

Yep, I went and stuck a load of reaction wheels to a tiny little engine, it flew higher than every other spaceplane I built on my own to date. 'Nuff said!

[NathanKell]

So shouldn't the equator be less dense and poles be more dense? Or is this a misconception?

It's true. It's also true in KSP. Where have you seen that not be true?

[OhioBob]

It's true. It's also true in KSP. Where have you seen that not be true?

As described in the opening post.

[NathanKell]

...Amps...

Fixed. Thanks.

[AmpsterMan]

heh sorry about that; I mixed them up xD.

[OhioBob]

I was going to start a new thread but this one looks like it might be a good place to have this discussion. I've always enjoyed performing simulations outside the game in order to test/optimize my designs and to predict performance. Even though the old aero model was flawed, it was well understood and easy to simulate. I'm now trying to get a handle on the new physics model so I can update my simulations. I've performed several experiments to try to reconstruct what's going on, but I'm seeing some things that don't quite add up.

First off, the old model was internally inconsistent and couldn't have existed in real life. For example, at constant temperature we have the following relationships:

Hg = RT = P/ρ

where H is scale height, g is gravitational acceleration, R is specific gas constant, T is temperature, ρ is density, and P is pressure.

In the old model we were given the following sea level values: H = 5000 m, g = 9.81 m/s², T = 293.15 K, ρ = 1.223095 kg/m³, and P = 101325 Pa. With these numbers we see that Hg does not equal P/ρ.

One of the first things I did was to test the conditions in the new model for internal consistency. Taking pressure, temperature and gravity measurements at different heights at the launch pad I was able to reconstruct the following sea level conditions: H = 6749 m, g = 9.81 m/s², T = 309.75 K, and P = 101325 Pa. (I can show the work if anyone wants to see it.)

Assuming that everything is internally consistent, I computed R and ρ as follows:

R = (Hg)/T = (6749*9.81)/309.75 = 213.75 J/kg-K

ρ = P/(RT) = 101325/(213.75*309.75) = 1.5304 kg/m²

And from R we can compute the gas molecular weight, M = 38.9 kg/kmol.

Right away we see some issues. Squad has always made Kerbin very Earthlike in terms of its gravity and atmosphere, yet these values of R, ρ and M are very un-Earthlike. I do not trust that these are the numbers actually being used in the game. In fact, I've conducted some other experiments that suggest that the density of Kerbin's atmosphere is similar to the old model. I am therefore left to conclude that we again have some internal inconsistency in the way the atmosphere is modeled.

This leaves me with a problem. It is difficult to reconstruct an atmospheric model if the atmosphere doesn't follow real-world laws. I'm sure I can devise a model that can reasonably replicate the results that I'm seeing in the game, but my task would be much easier if I could understand exactly what Squad has done and use their modeling method and equations.

In additional to the above, I've performed other experiments and measurements that have provided useful information, but I keep getting to a point where the computations stop making sense. For example, I dropped a barometer through Kerbin's atmosphere and recorded a series of pressure-height measurements. From this I was able to compute the scale height at various altitudes. From H, T can be computed if R is known. Although I don't trust the R value I computed earlier, using it does produce temperatures that are in line with what I expect. However, I also performed tests in which I experimentally derived the drag coefficients for some difference nose cones. In this case, using R = 213.75 produced wonky results. I got much more realistic and believable results using an Earthlike value of R = 287.

I looked through most of the game .config files hoping that I'd find something that reveals how the game computes some of this stuff, but I found nothing.

If anybody here has any additional data or advice, it would be extremely helpful.

(EDIT) After making my original post I discovered that the flight stats in the debug menu includes "external temperature". This a better means for measuring air temperature than the 2HOT thermometer. I've already edited the above post to include some revised data. I've also been able to collect some temperature-height data that should prove quite useful. I'll post the results latter.
 

Edited September 19, 2020 by OhioBob

[OhioBob]

Kerbin Atmospheric Temperatures

(Moderators, feel free to split my posts off into a separate thread if you don't think this is the appropriate place for them.)

I'm starting to get a handle on the atmospheric model. I've obtained some good temperature measurements and a logical pattern is starting to emerge.

I obtained atmospheric temperatures using two methods. First, I sent up a probe and took direct temperature measures. To do this I used the debug tool's "external temperature", which is a better tool than the 2HOT thermometer for obtaining instantaneous ambient air temperatures. This method only worked below 25,000 meters when I could suspend the probe on a parachute. Above that altitude the probe was experiencing aerodynamic heating, either from powered flight or reentry, which elevated the temperature reading and made it useless for my purposes.

For the second method I sent up a probe with a barometer and took over 100 pressure measurements as the probe descended through the atmosphere. By analyzing the pressure rate of change I could calculate the scale height. From scale height, temperature could be calculated using T = Hg/R. For R the value 213.75 J/kg-K was used, which I computed from temperature and pressure measurements obtained at the KSC launch pad (described in my previous post).

From the direct measurements it became obvious that the temperatures are based on the U.S. Standard Atmosphere (USSA). Major changes in temperature gradient were observed at 8.8 km and 16 km. The USSA has the same gradient changes occurring at 11 km and 20 km. This tells me that the vertical height scale of the USSA has been compressed by a factor of 0.8 to make it better fit the smaller scale of Kerbin. Between the altitudes of 16 km and 25 km, the observed temperatures correspond exactly to the USSA (accounting for the compressed height scale). Below 16 km the measured temperatures were greater than the USSA, though this is likely to be due to a latitudinal modifier. The USSA is based on cooler mid-latitudes while KCS is located at the equator. More measurements are necessary to know for sure, but my guess is that Squad applies a temperature modifier based on latitude and time of day. This modifier is added to the USSA temperature. The adder is greatest at sea level , follows a curved function, and eventually tappers off to zero at 16 km.

My second method of temperature determination is less accurate and displays considerable fluctuation. However, the results clearly show that the Kerbin temperature profile follows the general shape of the USSA temperature profile. Once above an altitude of about 35 km, the pressure-derived temperatures are consistently lower than the USSA. Because of my inability to take direct temperature measurements at these altitudes, I don't know if this is a real observation or an artifact resulting from some inaccuracy in my method. I would not be surprised if Squad has also applied a latitudinal-diurnal modifier to the upper atmosphere temperatures as well. As a general rule, when lower atmosphere temperatures are greater than the norm, upper atmosphere temperatures are lower than the norm, and vice versa. This pattern seems to hold true in my pressure-derived temperatures.

All of the above is summarized in the graph below. The USSA temperature profile includes the 20% reduction in vertical scale.

 

 

I'm encouraged enough by the pressure-derived temperatures that I think this method should work reasonably well to obtain temperature-height data for other planets. So far I've focused entirely on Kerbin, though I hope to also reconstruct other atmosphere models.

One final note regarding the specific gas constant. Although R = 213.75 J/kg-K produces some unrealistic values for molecular weight and air density, it seems to be the value that you want to use for anything involving pressure and scale height calculations. A more Earth-like value of 287 J/kg-K seems to be what you want to use for any calculation involving density, speed of sound, drag, etc. This is an internal inconsistency that exists in KSP that wouldn't in real life. In KSP, air pressure decreases with increasing height more rapidly than it should according to the values of temperature and density.

Next up, a discussion of drag coefficients. I'll post some experimental results latter today.
 

Edited September 19, 2020 by OhioBob

[barfing_skull]

Okay, managed to get a craft to the surface of Eve. I'm trying to see if I can return to orbit.

Now, somebody riddle me this - what's an engine that has a reasonable enough ISP to get off the surface at those pressures? The LV-T's were running at about an ISP of 64s on the surface. Great if you want your craft to fall over. I haven't tried the aerospike, but it's listed at 290s ASL, whereas the LV-T's were 280s and 270s. So I'm not sure the Aerospike would fare much better.

Or is getting off Eve's surface just not possible any more with stock parts, unless you have a nearly weightless craft?

Cheers,

-BS