UI/UX Delta V calculations inaccurate

[Opus]

Delta V calculations seem dramatically inaccurate, and or stated requirements to go somewhere appear to be drastically overexaggerated. I don't know if this is intended to provide a buffer for less experienced players or in error.  

[Rubenio]

I can confirm that the issue noted above is still an issue in the latest update.

 

KSP Version =  0.1.1.0.21572

Operating System and version  =  Windows 11 Pro - Build 22621.1413

CPU and GPU :    Intel(R) Core(TM) i7-10700F CPU @ 2.90GHz   2.90 GHz - NVIDIA GeForce RTX 3060

Description of the bug:  Kerbals do not get reinstated in the VAB after successfully landing and recovering vessel near base.

Expected Behavior

- delta V calculated in the VAB should remain the same when vessel is launched and hasn't used any fuel.

  - also, I think the dV calculation is incorrect or inconsistent.  When I remove one of the terrier engines (0.5 t), the dV available to the ship doubles.  While I realize that 2 terriers will spend fuel more quickly than one, the total dV change should reflect just the change in mass no?                                        

Observed Behavior - 

- deltaV once launched is half that of the value in the VAB.  I have replicated this with several engines

Image of 'LilShuttle' in VAB note the dV at the bottom = 1450:    https://imgur.com/w0Jn7no

Image of 'LilShuttle' once launched onto the runway, note dV = 748 (roughly 1450/2):   https://imgur.com/ImDrZqm

-if you remove one engine while in the VAB, the dV changes dramatically (and I don't think the weight difference should warrant that)

Image of the same 'LilShuttle' in VAB with one of the terriers temporarily removed, note dV = 2698:  https://imgur.com/k5cQ7cH

Edited April 10, 2023 by Rubenio

[Rubenio]

Also, I thought I should add another unexpected deltaV (dV) behavior:

Expected Behavior:

dV should not change when an engine is not in use.  The thinking is that independent of altitude, the remaining acceleration that a ship is capable of should depend on its remaining fuel, its engine and its mass.  

Observed Behavior:  When flying around Kerbin, with engines off, dV changes when you lose altitude and increases when you gain altitude.  

I've made a video of this here:  https://imgur.com/wVA7EZh, though imgur shrinks the video size which makes it difficult to see.  but, as a point the nose downwards, you can see the dV drop off and when I point back upwards (again with throttle at zero), the dV starts going back up.

I've been able to repeat this flying on Kerbin a few times now.

 

[Starhawk]

1 hour ago, Rubenio said:

dV changes when you lose altitude and increases when you gain altitude.

@RubenioThe ISP of a rocket engine varies with the atmospheric pressure surrounding the rocket.  That's why you see two ISP numbers in the VAB for a rocket engine.  One for vacuum and one for one atmosphere of pressure.  ISP increases with lower pressure and decreases with higher pressure.

The behaviour you are seeing is expected.

Happy landings!

[Rubenio]

@Starhawk, thanks for the clarification on the dV changes in atmosphere.   I think that explains the dV halving when you launch the space plane on the runway - atmosphere vs vacuum.  But, I think my second message should have been clearer -  would a small change in altitude change the dV? 

I would imagine the pressure doesn't change much +/- 100 m in the atmosphere. (but i am a chemist, not a physicist!)  these were all observable changes around 3000m altitude

Also, would the dV really be half if you add another engine?  I was thinking that dV would be the sum of the overall of possible accelerations and mass would affect that more than whether there was 1 engine or 2.  

thanks for the help!

[Wetzelrad]

2 hours ago, Rubenio said:

would a small change in altitude change the dV? 

I would imagine the pressure doesn't change much +/- 100 m in the atmosphere. (but i am a chemist, not a physicist!)  these were all observable changes around 3000m altitude

At least in Kerbal physics, that is intended behavior. Take note of the barometer to the right of the navwheel. By 3000 meters, the pressure has dropped by approximately 1/6 of the meter. You can even see it moving as you snake up and down.

3 hours ago, Rubenio said:

Also, would the dV really be half if you add another engine?

No, something else is going on there...

I was able to recreate your design and several others that show bad math on the dV calculator. Two Terriers should always have better dV than one Dart, but they simply don't. Someone who understands the math will have to check that out.

[darthgently]

2 minutes ago, Wetzelrad said:

At least in Kerbal physics, that is intended behavior. Take note of the barometer to the right of the navwheel. By 3000 meters, the pressure has dropped by approximately 1/6 of the meter. You can even see it moving as you snake up and down.

No, something else is going on there...

I was able to recreate your design and several others that show bad math on the dV calculator. Two Terriers should always have better dV than one Dart, but they simply don't. Someone who understands the math will have to check that out.

Depending on the ratio of engine mass to overall mass, an extra Terrier could definitely lower available DV, right?

[Wetzelrad]

Just now, darthgently said:

Depending on the ratio of engine mass to overall mass, an extra Terrier could definitely lower available DV, right?

Yes, however. His plane is around 9tons, so the 0.5ton Terrier won't have such a dramatic impact.

Try it for yourself. My impression is that any scenario in which more than one engine is clustered breaks the displayed dV estimates.

[darthgently]

11 minutes ago, Wetzelrad said:

Yes, however. His plane is around 9tons, so the 0.5ton Terrier won't have such a dramatic impact.

Try it for yourself. My impression is that any scenario in which more than one engine is clustered breaks the displayed dV estimates.

I see now.  That glares

[Wetzelrad]

Actually I don't know how it works. But here's a couple more screenshots of the same phenomena. If you split the engines into two stages, you get what appears to be an accurate dV estimation.

[Rubenio]

I just checked with the same design and the dV calculations are corrected in patch 2 for the ship in question:

try it out, here are the values for this build:  Mk2 cockpit, cargo bay and 2 short Mk2 rocket fuel, MK2 bicoupler and 1 or 2 terriers

1 terrier dV = 3059

2 terriers dV = 2825

There should only be a small effect on dV reflecting the extra mass of the second terrier (0.5t).

HOWEVER,  when I replace the 2 short MK2 rocket fuel tanks with 1 LONG MK2 tank, the weight should stay the and hence the dV should stay the same, but we're back to the original error where the 2nd terrier halves the dV!!  

Now, same build except that the two short MK2 rocket fuel tanks are replaced by 1 long tank:

1 terrier dV = 3059 (NOTE:  this is the same as with 2 short tanks as they weigh the same :-)

However, when I add the 2nd terrier things go awry:

dV is now 1595 which is wrong, it should still be 2825

I really don't know why. 

I did find a way to figure out the math - have a look here (if you're interested):  https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

 

Edited April 12, 2023 by Rubenio