Youen

[old thread] Trajectories : atmospheric predictions

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IRL ballistic reentries aren't very accurate either, so I think it's normal to have to do a bit of maneuvering

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7 hours ago, tg626 said:

I came here to share my experience with inaccurate landings, but it seems I'm not alone.  I don't have measurements handy, but I started with a predicted landing out in the water almost a "KSC Contient width" to the east of KSC, and wound up just west of KSC on land...  I think that's more then 90km...

That sound like a bit more than what I saw. Were your AoA settings correct, and did you follow your programmed flight path?

 

On 31.1.2017 at 11:30 PM, dlrk said:

It looks like the inaccuracies start to accumulate in the upper atmosphere, and decrease as the atmosphere thickens, more or less becoming accurate around 45km.

Thanks for testing, I saw the exact same thing with Stock Aero! Intersting that it's similar in FAR.

 

On 31.1.2017 at 11:30 PM, dlrk said:

Pitching up with the 'dot', and monitoring the deviation allows an accurate reentry to be flown, so it's not a huge problem, IMO. I was using the FAR dev build, so if you also are, it might be an FAR issue. To me it definitely seems like a minor bug, since it can be resolved by pitching to control reentry,

 

12 hours ago, Youen said:

The prediction used to be less than 10km wrong (more around 5km) for simple crafts and "classical" reentries (like 1/4 orbit inside the atmosphere, starting from LKO), so I guess something changed in the game and the mod needs to be updated (again...)

It seems so. I tried to use the TrajectoriesDebug module to read out predicted vs actual lift/drag, but the lift/drag was staying at 0.

 

12 hours ago, Youen said:

Unless you see this deviation using FAR ? In which case I don't know what would be the problem because FAR exposes a method that should remain accurate even when FAR internal model is modified.

I was using Stock Aero, but as noted by @dlrk, FAR has similar problems.

 

7 hours ago, dlrk said:

IRL ballistic reentries aren't very accurate either, so I think it's normal to have to do a bit of maneuvering

Well, IRL we have the problem of wind, varying atmospheric densities/temperatures, inaccurate pitch/yaw control, and the significantly more complicated fluid dynamics :wink:

So I think in the game we can hope for more precise predictions :wink:

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what is the frame used by trajectories for its prediction please? orbital or surface?

thank you

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57 minutes ago, Skalou said:

what is the frame used by trajectories for its prediction please? orbital or surface?

You can choose. Click "body-fixed mode" for surface, and uncheck it for orbital.

If you mean internally in the code, then it's "body-relative world position".

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55 minutes ago, Kobymaru said:

You can choose. Click "body-fixed mode" for surface, and uncheck it for orbital.

If you mean internally in the code, then it's "body-relative world position".

thank you :wink:, yes internaly probably ( when i select retrograde prediction, what should i use in game: surface mode from what i understand with your answer)

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Just now, Skalou said:

thank you :wink:, yes internaly probably ( when i select retrograde prediction, what should i use in game: surface mode from what i understand with your answer)

Depends what you want. If you want to just land, body-fixed is probably best.

If you want to do aerobraking, orbit-relative is best.

If you want to draw pretty, artsy pictures, try body-fixed with an inclined, high-eccentricity-orbit :wink:

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5 minutes ago, Kobymaru said:

Depends what you want. If you want to just land, body-fixed is probably best.

If you want to do aerobraking, orbit-relative is best.

If you want to draw pretty, artsy pictures, try body-fixed with an inclined, high-eccentricity-orbit :wink:

what i would like, is to be the most accurate as possible from the simulation: if i hit the retrograde (or whatever angle) in trajectories, i want to know which frame (orbital or surface relative) i should use in flight on my navball to be the closest from the prediction, i think it as nothing related to how is displayed the trajectory (i can follow the surface retrograde while displaying the trajectory in the orbital reference frame). there is a small difference in direction between the surface and orbital directions ( specially at low speed).

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1 minute ago, Skalou said:

what i would like, is to be the most accurate as possible from the simulation: if i hit the retrograde (or whatever angle) in trajectories, i want to know which frame (orbital or surface relative) i should use in flight on my navball to be the closest from the prediction, i think it as nothing related to how is displayed the trajectory (i can follow the surface retrograde while displaying the trajectory in the orbital reference frame). there is a small difference in direction between the surface and orbital directions ( specially at low speed).

Surface.

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On 02/02/2017 at 10:14 AM, Kobymaru said:

Well, IRL we have the problem of wind, varying atmospheric densities/temperatures, inaccurate pitch/yaw control, and the significantly more complicated fluid dynamics :wink:

So I think in the game we can hope for more precise predictions :wink:

And also, IRL the planet is ten times bigger :) which means one should multiply in-game imprecision by ten before comparing with real life.

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@Youen, I just wanted to say thanks for the continued support of this mod. :)

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Does the mod account for lifting surfaces if I wanted to land, say, a space shuttle?

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2 hours ago, Cdw2468 said:

Does the mod account for lifting surfaces if I wanted to land, say, a space shuttle?

Yes / No

In orbit it will show you where it will land like a brick. As you enter the atmosphere it will update and continue updating the lading prediction. As a plane has wings there is no way of knowing where it will land based on what angle the plane is at or when you are maneuvering. But it will give you a very good idea. I couple of tries and you will see what I mean. Trajectories still helps a ton with landing anything closer to your intended target.

Edited by V8jester

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9 minutes ago, V8jester said:

Yes / No

In orbit it will show you where it will land like a brick. As you enter the atmosphere it will update and continue updating the lading prediction. As a plane has wings there is no way of knowing where it will land based on what angle the plane is at or when you are maneuvering. But it will give you a very good idea. I couple of tries and you will see what I mean. Trajectories still helps a ton with landing anything closer to your intended target.

Ok, I'm a half decent pilot, but the trouble is getting close, so this should help. Thanks!

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@Cdw2468 It's supposed to work and predict lift of your wings (you have a settings window to indicate how you intend to fly, either by setting the angle of attack (angle relatively to your velocity vector), or the angle above/under the horizon. Since you can adjust with a plane during your descent, it's usually possible to make it to the runway. Actually, the mod was designed exactly for this purpose (I couldn't land my shuttles without it), though it's useful for other kind of manoeuvers (aerobraking, etc.)

However, recent player reports (read above) indicate that there are bugs with the latest version of the game/mod, and that the prediction does not work as expected. I haven't the time to look into this issue at the moment though :(

1 hour ago, V8jester said:

In orbit it will show you where it will land like a brick. As you enter the atmosphere it will update and continue updating the lading prediction.

I don't know why you say that ? Unless there is a bug, you should get a prediction as soon as you enter the SoI of the planet. So when you are orbiting Kerbin (for example), it should predict the trajectory with drag and lift.

As stated above, however, it appears there is a bug with the current version of the mod and KSP.

6 hours ago, Raptor9 said:

@Youen, I just wanted to say thanks for the continued support of this mod. :)

Thanks, my pleasure :) Even though I don't have time to work on the last reported bug :-/

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Warning: Warranty void if used in atmosphere

Warning: Warranty void if used to land

Warning: Warranty void if installed

EDIT: :wink: Here you go, happy? 

Edited by TDplay

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8 minutes ago, TDplay said:

Warning: Warranty void if used in atmosphere

Warning: Warranty void if used to land

Warning: Warranty void if installed

Well, this is Kerbal Space Program....

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2 hours ago, TDplay said:

Warning: Warranty void if used in atmosphere

Warning: Warranty void if used to land

Warning: Warranty void if installed

You're not being very helpful

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1 minute ago, Kobymaru said:

You're not being very helpful

Yes, it's a joke! I assume you know what a joke is.

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11 minutes ago, TDplay said:

Yes, it's a joke! I assume you know what a joke is.

I do know what a joke is, but have you heard of Poe's Law?

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OK, I edited my comment accordingly. It now has a :wink: in it. Although that joke is pretty obvious, isn't it?

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Is ModuleManager required?

EDIT: NVM. Tried it myself afterward. Just don't put .dll in there.

Edited by Jestersage

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8 hours ago, Jestersage said:

Is ModuleManager required?

EDIT: NVM. Tried it myself afterward. Just don't put .dll in there.

Module manager is used to inject a module in all command parts that will store your descent settings (angle of attack, etc.) so that you don't loose them when switching vessel or saving/loading a game. I have never tested the behavior without it, so you could expect issues. As a side note, I don't see why you would want to remove it ; it's just a tool to standardize how mods add or remove modules to parts, so that they remain compatible with each other (most of the time).

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Hi there, fantastic mod.

Just a minor bug - when launching ksp the trajectory will not display on the first flight. The mod loads ok, I can set a target and the green cross appears, but the red line / cross doesn't show (the "display trajectory" option is definitely enabled). The trajectory always displays fine on subsequent flights or by doing a quick save/load.

Easiest to reproduce by launching ksp and loading the dynawing re-entry or powered landing scenario.

KSP 1.2.2.1622, Trajectories 1.6.6

Edited by stagj

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