which parts of KSP1 aero do we want to keep?

[paul23]

Let's just solve the full navier stokes equation each frame!

 

But more realistically: currently lift feels really "cheating" in ksp. And without proper "wind tunnel" simulation/tests of subassemblies it's magic too. You cannot have an object that has drag but cannot generate lift (except when it's shaped as a sphere). Yet due to bad stock aero model things like a pendulum rocket is stable. Or you have rockets that are nearly as wide as they are tall, because drag is calculated "per part".

[OHara]

After playing KSP two more years, with and without the mod FAR, I have change my mind slightly on what would be good to keep from KSP1 aerodynamics.  Calculating drag and body lift per part and combining parts with simple rules, creates strange behaviour when players make craft in natural ways. 

Today there appeared a video doing a recreation, that (1) places a narrow decoupler between two wide parts, and (2) uses Mk2 parts at the front of the rocket.  Experienced KSP players, including the video-maker, know that this creates very large drag on the top of the rocket, especially if it turns even slightly off prograde.

There have been a few forum threads about what people want from aerodynamics ( link link link ) and there was also a thread about when part-clipping makes KSP a better or worse game:

Given that, I now hope that KSP 2 uses the shape of the assembled craft to figure aerodynamic drag and body lift, rather than shapes of the individual parts (aspects 0,1, and 2 in my list in the top post).

That would make "clipping for performance" more important in KSP 2 challenges, but I think players understand that intuitively and  would allow/disallow that as they like for their own play or challenges (as opposed the less-obvious node-attaching for performance in KSP1). 

 Probably too late to have any influence on KSP 2 development but you never know.   Has anyone else changed their mind on this topic?

[swjr-swis]

34 minutes ago, OHara said:

Has anyone else changed their mind on this topic?

Having missed this the first time, I had to go through the whole thread to see what had been said before.

I think I still find myself in agreement with your very first summary, with your current amendment of having drag calculated and applied not to individual parts, but to the whole ship outline, as long as such parts are still connected to the ship. Apart from a slight computational delay at the moment of calculating what that outline would be, that has to be a marked improvement in physics calculation while in flight, and should render a more accurate representation of aerodynamics as we intuitively 'know' from every day.

Also, please kill 'cannot deploy while stowed' entirely, with prejudice. Please.

[herbal space program]

On 9/22/2019 at 12:24 AM, OHara said:

StarTheory plan for KSP2 to have aerodynamics very similar to that in KSP1

2) Offsetting a part does not change the size of the aerodynamic forces on the part  (but the point where those forces are applied to the whole craft does move with the part).
 Clipping, offsetting parts inside other parts, is purely aesthetic

I believe that's only true so long as you don't then enclose all those clipped-together parts in a fairing or cargo bay. I seem to recall some craft that did otherwise totally impossible things that way, so for that reason I hope that KSP2 cracks down on part-clipping in general

On 9/22/2019 at 12:24 AM, OHara said:

7) Skin drag, from faces of a part parallel to the airflow, use the same shape factor Cd as they would have when facing the airflow.
  This gives Mk2 cargo bays slightly lower drag when open, at high speed.

Obviously this is just silly, so they need to totally rethink that.

On 9/22/2019 at 12:24 AM, OHara said:

8) As the side-face of a (non-wing) part is tilted into the airflow, the force proportional to tilt angle is all counted as drag,  where we expect drag proportional to sine-squared of the tilt angle from standard aerodynamics.
  An Mk2 spaceplane in KSP1 at 3° angle of attack feels sin3° = 5.2% (where one would expect only sin²3° = 0.3%) of the of bellyflop drag of its bottom surface.

They totally need to rethink this too. Lifting bodies ought to generate lift, not disproportionate amounts of drag!

On 9/22/2019 at 12:24 AM, OHara said:

9) The lift on wings as a function of Mach number, is higher at low speed, lower lift at high speed, compared to expectations from standard aerodynamics.
 A space-plane can easily have enough low-speed lift to glide near touchdown-speed the entire length of the runway.

I generally find this much more annoying than helpful.  Between that and the soft stall behavior, when I do what seems like a proper flare, I often end up sailing right back into the air and then gliding along on what feels like some kind of exaggerated ground effect.

[OHara]

17 hours ago, herbal space program said:

I seem to recall some craft that did otherwise totally impossible things that way [hiding engines in fairings], so for that reason I hope that KSP2 cracks down on part-clipping in general

Well, do you see any good set of rules to disallow clipping?   
I think KSP version <0.90 had parts on the same craft collide with each other, but allowing clipping seems to have been a net benefit, especially for people who try to make replicas.  KSP1 tries to prevents some parts from functioning (the "cannot deploy while stowed") if they are enclosed in cargo bays, and players were often frustrated and surprised by how exactly KSP1 decided when parts were 'enclosed'.   FAR since its 2015 'Euler' release, on the other hand, lets me clip an engine inside a fuel tank for drag-free thrust, and I see nobody complaining.
After reading the "Rethinking clipping" thread, I think it is better to let the players enforce their own clipping policy.

This reminds me of another complication from computing drag body-lift for the whole craft : the game then needs to divide the aerodynamic force among the parts, and apply the correct force to each part, assuming KSP continues to treat parts as independent physical bodies.  (I cannot figure out FAR's rules for dividing the force, just from reading its code.)

17 hours ago, herbal space program said:

I generally find this [boosted lift at low speed] much more annoying than helpful.  Between that and the soft stall behavior, when I do what seems like a proper flare, I often end up sailing right back into the air and then gliding along on what feels like some kind of exaggerated ground effect.

The boosted lift, however, does allow the fun flying by Cupcake that Laie pointed out a few posts up, and also lets new players get off the ground more easily.  KSP1 also boosts drag at low speeds, resulting in a L/D (a.k.a. glide ratio) a bit smaller than similar-looking real aircraft.  Players using FAR also complain that their planes float the length of the runway.

I did not mean to imply that it was bad that planes can float down the runway, because players add airbrakes and chutes  to stop in the available distance.

[herbal space program]

1 hour ago, OHara said:

Well, do you see any good set of rules to disallow clipping?   
I think KSP version <0.90 had parts on the same craft collide with each other, but allowing clipping seems to have been a net benefit, especially for people who try to make replicas.

I guess I kind of overstated what I meant there. It's really just  stuff like clipping together 15 fuel tanks under a 1.25m fairing to nullify all their drag that I think should be disallowed  somehow, at least in normal career/campaign modes.  Extreme part-clipping and offsetting can can of course also abuse the aero model by allowing you to build ridiculously long and thin ships that are far more aerodynamically stable than they ought to be, and I'm not a big fan of that either. OTOH I totally get the aesthetic problems of not allowing any clipping at all.  I mean, with the sorry assortment of wing parts we have, it's more or less impossible to build a decent-looking  or even ideally functional plane without it. Perhaps this could be addressed by having a limit of 10%-15% of the volume of any non-hollow part that can be occupied by any other non-hollow parts.  Hollow parts OTOH could have up to 90% of their volume occupied by other parts, and for design purposes these would include dry wings. Wet wings could also have a higher clipping limit, but at some point you'd have to pay a penalty in terms of their fuel capacity.  I think something like that would represent a reasonable compromise  between aesthetics and realism, especially since we seem to be getting at least procedural wings/control surfaces, and I hope maybe some other procedural airplane parts as well. And of course I really don't care if it's included in some kind of cheat menu or as an option in sandbox mode. It only really matters to me wrt vanilla career modes, because I don't think it's good game design for a physics simulator to have these exploits available by default. In the context of sandbox challenges, as you say it can be policed by the participants just fine.