Delta V Calculation Error When Decoupler Crossfeed is Enabled

[tcdonald]

Hi, I just encountered a bug when trying to create a moon lander. The delta V calculation is not correct.

KSP version: 1.7.0.2483 (WindowsPlayer x64) en-us

What Happens: Delta V calculation shows wrong result when decoupler crossfeed is enabled

Mods/Add-ons: All stock

Steps to replicate:

1. Add parts in following order: Mk1-3 pod, decoupler, Rockomax X200-8 Fuel Tank, 2 Thud engine radially on the tank, decoupler, service bay, Rockomax X200-8 Fuel Tank

2. Enable crossfeed on the secound decoupler

Result: Delta V calculation does not include the second tank.

Screen capture: https://imgur.com/a/6fe7AjG, https://imgur.com/a/NEWOj13

Craft file: https://www.filehosting.org/file/details/800116/Large Moon Lander.craft

Log file: https://www.filehosting.org/file/details/800120/output_log.txt

Edited May 13, 2019 by tcdonald

[Serenity]

Can you check if the Fuel Priorities have the same number?I find that changing them completely destroys DV calculations.

When i get home i am gonna try to replicate this bug, although i had the same problems already in similirar crafts.

I hope it gets fixed soon  .

Edited May 14, 2019 by Boyster

[HebaruSan]

Images (you can "Copy image address" from the page and paste it here to embed):

Edited May 14, 2019 by HebaruSan

[tcdonald]

6 hours ago, Boyster said:

Can you check if the Fuel Priorities have the same number?I find that changing them completely destroys DV calculations.

When i get home i am gonna try to replicate this bug, although i had the same problems already in similirar crafts.

I hope it gets fixed soon  .

I tried changing fuel priority, higher, lower, equal. None gave correct delta V. I found this happen on Thud engine. Using Terrier has no such issue.

[tcdonald]

5 hours ago, HebaruSan said:

Images (you can "Copy image address" from the page and paste it here to embed):

Thanks for the tip. I just forgot how to use a forum efficiently. It's been a long time since posting something in a forum to me

[MechBFP]

As a workaround you can put a fuel line from the top to the bottom tank instead and disable crossfeed. That should make it work. 

Edited May 15, 2019 by MechBFP

[Serenity]

Hmm i wonder happens if you put the engines in the upper tank.If DV is correct then you can just Shift move them wherever you want them to be.

[tcdonald]

On 5/16/2019 at 10:13 PM, Boyster said:

Hmm i wonder happens if you put the engines in the upper tank.If DV is correct then you can just Shift move them wherever you want them to be.

The delta V only displays the value based initial position of the engines. Moving them around the ship later does not change it.

[Serenity]

43 minutes ago, tcdonald said:

The delta V only displays the value based initial position of the engines. Moving them around the ship later does not change it.

I would suggest trying KER or Mechjeb to compare the DV values(Although i suspect they might be off as well).

If ofcourse you are open to mod usage.Otherwise uhhhh i hope they improve the DV calculations.

If you try these mods put them with this, it removes the need to place the Mechjeb and Ker Part in your craft.(If you want to keep your craft completely stock)

 

 

 

Edited May 20, 2019 by Boyster

[tcdonald]

On 5/16/2019 at 1:02 AM, MechBFP said:

As a workaround you can put a fuel line from the top to the bottom tank instead and disable crossfeed. That should make it work.  

I tried this method. Didn't help with it. I think I will stay with Terrier for now.

[tcdonald]

2 hours ago, Boyster said:

I would suggest trying KER or Mechjeb to compare the DV values(Although i suspect they might be off as well).

If ofcourse you are open to mod usage.Otherwise uhhhh i hope they improve the DV calculations.

If you try these mods put them with this, it removes the need to place the Mechjeb and Ker Part in your craft.(If you want to keep your craft completely stock)

Thanks for the suggestion. I think I will spend more time with stock version first. Will try some mods later.

 

 

[vitalijbeam]

On 5/14/2019 at 3:44 PM, Boyster said:

Can you check if the Fuel Priorities have the same number?I find that changing them completely destroys DV calculations.

When i get home i am gonna try to replicate this bug, although i had the same problems already in similirar crafts.

I hope it gets fixed soon  .

 

The calculator dV in the game is also wrong if you change the priority of using tanks.

[tcdonald]

On 5/16/2019 at 1:02 AM, MechBFP said:

As a workaround you can put a fuel line from the top to the bottom tank instead and disable crossfeed. That should make it work. 

I tried this. The fuel line did not go across the decoupler.

[klesh]

Hmm.  So I recreated this, and ran into some peculiarities:

 

1) Somehow you've got 2 parachutes on your craft but did not describe adding any.  You can obviously see the Mk16-XL parachute on top, so I added that.

2) My recreation comes out to have 781m/sec, whereas yours shows 784m/sec with the extra parachute present.  If I pull all the monopropellant out of the capsule, I get 788m/sec. This is not a huge deal, as the point is the inability to include the fuel from the top tank in the calculation.

3)

On 5/15/2019 at 1:02 PM, MechBFP said:

As a workaround you can put a fuel line from the top to the bottom tank instead and disable crossfeed. That should make it work. 

 

16 hours ago, tcdonald said:

I tried this. The fuel line did not go across the decoupler.

Correct, and even working around the workaround using cubic octagonal struts to raise the fuel lines above the ship's surface (which usually works) doesn't work either:

In this shot, the deltaV has gone down to 779m/sec because of the 2 cubic octagonals and the fuel pipe.

 

Disabling crossfeed on the decouplers to force fuel down the pipe makes it almost confusingly worse (note the bizarre arrow directions):

 

That bizarre arrowing looks similar to this bug I posted over a year ago with the Engine Plates. 

https://bugs.kerbalspaceprogram.com/issues/18776

This got a stealthfix somewhere within the last year without an update to the tracker asking me and others to test the changes; I just happened to be checking old bugs I had posted to see that its now working.  I wonder if whatever is making the arrows reverse with the cubic octagonals is the same thing that made the engine plates reverse the arrows as well?  Thats sort of an aside, because clearly the calculation should be including the upper tank without having to perform workarounds.

 

 

Either setup, fuel pipe or crossfeeding decouplers allows the fuel from both tanks to be burned in flight, but neither arrangement will show up on the deltaV calculator.