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4x4cheesecake

Inconsistent SSTO ascent

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Hey there :)

A week ago, I build a MK3 SSTO to do some mining & tourists contracts and everything worked quiet well. Just after the liftoff I pitched up ~10 degree, the SSTO accelerate pretty easy up to the "magical" 400m/s, the RAPIER engines got their boost to accelerate further and after reaching LKO I had ~3000m/s DeltaV left.

Today, I tried to start the same SSTO to do another contract but somehow it got pretty hard to accelerate up to 400m/s, instead im always stuck at ~340m/s or even less, depending on the altitude. The only way I found to accelerate further was to raise my altitude up to ~10000m and pitch down until the RAPIERs got their boost. But this maneuver is very fuel inefficient...after reaching LKO, I had just ~1000m/s DeltaV left :/

How is that possible? Is there a trick I've used unwittingly the first times?

KSP version 1.4.2, no physics/aerodynamic mod installed.

Screenshots of the SSTO:

https://www.dropbox.com/sh/m2g8id6xnkvmfqi/AADgwk7xz7EfE6FbYSE6ztTGa?dl=0
edit: added a .craft file if someone wants to try on his/her own. I'm using ActionGroupsExtendet so i guess the actiongroups must be reconfigured. Just stock parts are used, not even DLC parts.
Edited by 4x4cheesecake

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For the sake of completeness: Deleting/renaming the existing PartsDatabase.cfg and creating a new one by starting the game will fix most of this problem (at least for the modded tanks), but it looks like there are still some minor differences in drag when using different models.

Tests and solution can be read here:

 

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2 hours ago, bewing said:

1.4.3, yup. And that's what I'm testing to see if it's involved. But it's supposed to be necessary to specifically activate that per part. It's not just for every part with a variant.

I tried to reproduce the "different drag for different paint jobs" effect in a couple of different ways (see notes here), but couldn't reproduce it.  Wondering whether there might be some interplay of variants, ModuleManager, etc. that's causing PartDatabase.cfg to potentially get stale?  Gave some suggested steps for the folks who can reproduce this to see whether that might be at play.

I note that the FL-T800 is one of the old-style parts that uses the old "mesh = ..." syntax rather than the newer "MODEL { ... }" syntax.  Normally, using MM to add a MODEL{} section to a "mesh = " part causes the "mesh = " to get ignored.  Was wondering whether that might be subtly borked for variants, somehow (e.g. if the B/W variant is somehow getting double-dipped and actually has two copies of the model present, or something).  But I couldn't test the hypothesis because, again, I couldn't reproduce the problem yet.

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Update: lots of further testing, but while the debug tools are still showing 2% drag differences consistently, a max-altitude flight test is showing no difference at all. So it's not panning out as a real issue. The conclusion is more that the numeric aero data can be off by a couple percent.

 

 

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Are you sure you had all the engines on,  and in the same mode (jet vs rocket) both times?

I can't see your stats on the image,  but 8 rapiers seems like it shouldbe enough to accelerate a plane that size pretty well. 

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Well, the resolution shrinks a little bit too much after uploading the screens to imgur...is it possible to post pictures there in better quality?

I put them in a dropbox folder and edit the OP, that should do the job.

I've checked the engines (and air intakes), all of them are in their correcte state and running.

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I cant see for sure but if you have fairings they are bugged and create extreme amounts of drag to the part they are attached to in 1.4.2.

It will be fixed this week.

Umm if not then you should do some checks like Aegolius said and try to recall if you did something different

(fuel amount, which speed you pitched up, which altitude, if you had rocket mode inside atmo for few seconds). 

Edited by Boyster

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Nope, there are no fairings and the ascent was so simple, I couldn't make anything different. Just like I said: Right after liftoff pitch up to 10 degree and i had nothing else to do until the SSTO reached a speed of ~1300m/s at 20000m altitude. The desired speed of 400m/s should be reached at ~3500m (is that too low? Thrust got his peak there too (~1500Kn) and drops when the SSTO gains altitude)

Edited by 4x4cheesecake

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Well i hope someone helps you figure it out, if the ascent profile was simple then i can only think of mass.

How many passengers you had or how much ore/fuel, after that i cant think of anything else.

Edited by Boyster

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You get maximum thrust (with maximum fuel use) at sea level. Also, pitching up causes large amounts of drag, and it's drag that keeps you below sonic velocities.

So, only pitch up enough to maintain level flight after you launch, go to full thrust, wait patiently for your craft to go supersonic, and then let it pitch up and climb by itself (they do this automatically because of planetary curvature). The more you steer, and the more games you play on ascent -- the more problems you will have.

But if that doesn't do it, then look at your design for little things to remove. This is one of those cases where "every little bit helps" really is the truth.

Oh, and one tip: if your craft is balanced properly then hitting F will momentarily disable SAS. Doing that should cause your nose to drop slightly and dramatically reduce drag. If you are close to that sonic wall and having a hard time getting through it, then tapping F once every five seconds can get you through.

Edited by bewing

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16 minutes ago, bewing said:

If you are close to that sonic wall and having a hard time getting through it, then tapping F once every five seconds can get you through.

So SkiFree's source code was incorporated into KSP 1.4 somehow.

/me ducks

 

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3 hours ago, bewing said:

You get maximum thrust (with maximum fuel use) at sea level. Also, pitching up causes large amounts of drag, and it's drag that keeps you below sonic velocities.

So, only pitch up enough to maintain level flight after you launch, go to full thrust, wait patiently for your craft to go supersonic, and then let it pitch up and climb by itself (they do this automatically because of planetary curvature). The more you steer, and the more games you play on ascent -- the more problems you will have.

But if that doesn't do it, then look at your design for little things to remove. This is one of those cases where "every little bit helps" really is the truth.

Oh, and one tip: if your craft is balanced properly then hitting F will momentarily disable SAS. Doing that should cause your nose to drop slightly and dramatically reduce drag. If you are close to that sonic wall and having a hard time getting through it, then tapping F once every five seconds can get you through.

These are some nice tips, thank you :) I managed to start the SSTO by flying very low but my remaining DeltaV was still lower (2200m/s instead of 3000m/s).

I did some further testing and on a fresh install without any mods, the SSTO accelerated as usual :o And some tests more brought me to the amazing result: it is a bug in the missing history mod. The FL-T800 Fuel Tank got a new skin by this mod and somehow, this skin is the reason for my SSTO being stucked in the sonic barrier. Changing the skin to "orange/grey" DLC version was actually the solution to get into space, who'd have thunk? :D

I dont know anything about how (modded) parts are build but hopefully, the mod creator does and can fix it.

Thank all of you for your help, ideas and many useful tips :)

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1 hour ago, 4x4cheesecake said:

I did some further testing and on a fresh install without any mods, the SSTO accelerated as usual :o And some tests more brought me to the amazing result: it is a bug in the making history mod. The FL-T800 Fuel Tank got a new skin by this mod and somehow, this skin is the reason for my SSTO being stucked in the sonic barrier. Changing the skin to "orange/grey" DLC version was actually the solution to get into space, who'd have thunk? :D

Since you did such good research on this, I'm looking into it. So far, some of it makes sense and some doesn't.

I look at your craft, and it does indeed have FL-T800 fuel tanks. However, in the Making History DLC, there are no reskins ("variants") available for this tank. So the question is "which mod added the skins to that tank" -- because it sounds like they messed it up. In the debug menu, you can turn on info about drag on individual parts -- you might want to use it to find out what the drag values are for those tanks with the different paintjobs.

On the other hand, the DLC did indeed create quite a few tanks that do have reskins. And I'm checking through them -- and their drag values do vary by up to 7%. We were under the impression that they were all precisely identical, so thank you for pointing this out.

 

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Oh, there is a typo in my last post, the reskined variant of this tank is in the missing history mod, not making history ;)

But im quiet happy that I was able to point something out which does also affect the stock game :)

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1 hour ago, bewing said:

On the other hand, the DLC did indeed create quite a few tanks that do have reskins. And I'm checking through them -- and their drag values do vary by up to 7%. We were under the impression that they were all precisely identical, so thank you for pointing this out.

So ummm... in other words, you guys assumed instead of ensured? Sounds as if another check needs to be added to your workflow procedures. :wink:

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Can't test every single thing. This game is too large for that. We did a lot of testing. The new ME parts are all functional. The art team likes how they look. Did we test the impact and thermal resistance of every variant of every part to make sure it matches the spec? Nope.

 

Edited by bewing

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Fair enough.. however, that being said...

56 minutes ago, bewing said:

Did we test the impact and thermal resistance of every variant of every part to make sure it matches the spec? Nope.

I've done enough coding myself to know that if I wanted to make a cosmetically different variant of a part, I'd copy/paste the specs from the original, then only change those things that actually needed changing. So the question remains, how on earth did parts that should have identical specs come to diverge from baseline by up to 7% in the first place?

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59 minutes ago, JAFO said:

Fair enough.. however, that being said...

I've done enough coding myself to know that if I wanted to make a cosmetically different variant of a part, I'd copy/paste the specs from the original, then only change those things that actually needed changing. So the question remains, how on earth did parts that should have identical specs come to diverge from baseline by up to 7% in the first place?

It's worse than that. The reskins are just paint jobs all on the same model. There is no copy/pasting involved. It's only one config file. Having the paint job modify the functionality of the part is 100% completely impossible. So yeah, how could parts that have the same model and config file diverge by 7%? I don't envy the dev who gets to try to debug this one.

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4 hours ago, bewing said:

 Did we test the impact and thermal resistance of every variant of every part to make sure it matches the spec? Nope.

 

 

Ah. So that's why the Wolfhound can withstand modest impacts of 80m/s or so?;)

Edited by blakemw

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2 hours ago, bewing said:

It's worse than that. The reskins are just paint jobs all on the same model. There is no copy/pasting involved. It's only one config file. Having the paint job modify the functionality of the part is 100% completely impossible. So yeah, how could parts that have the same model and config file diverge by 7%? I don't envy the dev who gets to try to debug this one.

:0.0:  Oh god.. that's.. gahh!

(I am so tempted to drop this tale in the 1.4.3 patch thread with all the complainers whining about how debugging isn't that hard, and tell 'em to put this in their pipe and smoke it!)

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2 hours ago, bewing said:

So yeah, how could parts that have the same model and config file diverge by 7%? I don't envy the dev who gets to try to debug this one.

Wasn't there something put into 1.4.2 to change the drag cubes of different variants, or make sure they were consistent, or something like that? Or am I misremembering and that's going into 1.4.3 (just trying to give a place to start)

Edited by severedsolo

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1 hour ago, severedsolo said:

Wasn't there something put into 1.4.2 to change the drag cubes of different variants, or make sure they were consistent, or something like that? Or am I misremembering and that's going into 1.4.3 (just trying to give a place to start)

1.4.3, yup. And that's what I'm testing to see if it's involved. But it's supposed to be necessary to specifically activate that per part. It's not just for every part with a variant.

 

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20 hours ago, 4x4cheesecake said:

Well, the resolution shrinks a little bit too much after uploading the screens to imgur.

No it doesn't.  Not even when it would be a really, really good idea to shrink it.

 

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On 4/24/2018 at 9:50 PM, bewing said:

You get maximum thrust (with maximum fuel use) at sea level. Also, pitching up causes large amounts of drag, and it's drag that keeps you below sonic velocities.

So, only pitch up enough to maintain level flight after you launch, go to full thrust, wait patiently for your craft to go supersonic, and then let it pitch up and climb by itself (they do this automatically because of planetary curvature). The more you steer, and the more games you play on ascent -- the more problems you will have.

But if that doesn't do it, then look at your design for little things to remove. This is one of those cases where "every little bit helps" really is the truth.

Oh, and one tip: if your craft is balanced properly then hitting F will momentarily disable SAS. Doing that should cause your nose to drop slightly and dramatically reduce drag. If you are close to that sonic wall and having a hard time getting through it, then tapping F once every five seconds can get you through.

This technique works ok with  very high thrust aircraft and avoids having to make pitch corrections.   In the stock game, without a good joystick or autopilot mod, or with an aircraft with poor handling characteristics,   you are stuck with making "all or nothing" inputs that slam the control surfaces to max deflection   or just leaving the nose locked on SAS

The above technique gets all the pitch adjustments done just after takeoff before speed has got high enough that you can loose a lot of velocity or cause structural breakup from it.

But even within the stock aerodynamic model it's a gross oversimplification to call it "optimal".

The problem with this technique is that if you climb too shallow,  you will waste fuel fighting drag going too fast too low in the atmosphere.  Of course, in jet mode you're still getting 4-10 times the ISP of rocket mode which lessens the impact, and fact  if your SSTO doesn't have any nukes you want to use up all your excess jet fuel before switching to rocket mode or that leftover liquid fuel just becomes dead weight !

If you climb too steep you might never get supersonic,  or climb out of the atmosphere and switch to close cycle mode prematurely before the air breathers have hit their top speed.

 

For a  version that's closer to "the truth"

  •  Keep your nose no more than 5 degrees above prograde.   Drag increases very rapidly as angle of attack increases.       Without autopilot mods, that probably means right clicking on the pitch control surfaces and lowering the authority slider for the flight to orbit, if you want to make small pitch adjustments that don't kick the nose far off prograde and cost velocity.     Before landing, go right ahead and set them to full authority again so you can flare for touchdown properly.

 

  • Whether you should skim just above the water after takeoff and go supersonic right on the deck or climb  a bit first, depends on the design of your craft and how much wing it has.  Climbing a bit, then levelling off to go supersonic in thinner air can work better if your craft can get enough lift to do that without the nose rising more than 5 degrees above prograde.   If your plane is able to climb after takeoff without exceeding 240 m/s or letting the nose rise too far above prograde, let it!  The thinner air will eventually rob it of lift and cause the flight path to level off, at this point, rather than fighting it by raising the nose further, let it level off or even fall slightly to penetrate the sound barrier. 

 

  • After 440 m/s, you are properly supersonic and can climb again,  but it's best not to let her get too steep.     If the nose goes much above 10 degrees above the horizon I set prograde hold on SAS to kill lift and minimise drag.   Your aim is to level off about 20km for a speedrun to get as much velocity out of the air breathing engines as possible.     I find it's a good idea to set prograde hold to begin the process of levelling off halfway between your supersonic acceleration altitude and your speedrun altitude.    For example, it you went supersonic at 7km and want to speedrun at 21km, then set prograde hold at 14km to start levelling off at the latest.

 

  • After engaging rocket engines, pitch the nose to 5 degrees above prograde and throttle back when AP goes over 70km.

 

Unless your flying is absolutely perfect,  it is normal to get slight porpoising  or phugoids in the ascent profile (the sudden increase in thrust as you get supersonic is mostly to blame).

Phugoid8.png

Don't sweat these too much.   They waste some time but only very little fuel, provided you don't make the mistake of making harsh corrections trying to straighten out your flight path or correct an overshoot - remember drag builds up severely when you get more than a few degrees off prograde,  going way off prograde to make a harsh correction is counterproductive !   Zooming over 20km prematurely is also not a problem provided the RAPIERs aren't set to switch mode automatically - just let the plane come back down and the engines will relight.   

 

Piloting only really has two ways to hurt delta V 

 

  1. By switching to rocket mode prematurely, before air breathing top speed has been reached
  2. By Climbing too steeply in rocket mode, causing cosine/gravity losses.     You should be thrusting at (or close to the horizon) with a a climb angle less than 5 degrees.    Let the wings do lift and engines do thrust.

The OP got done over by the 1.4.1 fairing drag which no amount of piloting can overcome.

 

Stock methods of "fine pitch control"

Option One - Use Pitch Trim.

https://kerbalx.com/AeroGav/Voodoo-Ray

Craft file here with instructions.   This video has annotations (turn on subtitles) that explains how to fly this to orbit using pitch trim and no SAS of any kind below the Karman line.    If you're efficient , you can take this little plane surprisingly far.

 

Method Two - Trim flaps bound to action groups

 

Basically, fly the plane with SAS set to Prograde hold but have trim flaps bound to action groups that give you an amount of nose up trim  that's good for an efficient climb as well as for the speedrun/supersonic acceleration.   Example craft file here , with instructions

https://kerbalx.com/AeroGav/MK1-Griffon-Deep-Space-Crew-Shuttle

 

Note , this plane also has wings angled up as they attach to fuselage.  So even when airplane itself is on prograde,  the wings are still angled at 5 degrees to the airflow, making lift.  You can use trim flaps to raise or lower the nose a couple degrees though when on prograde hold

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