Why does VAB show more Δv than i have in space? + Possible Bug that will double your Δv

[BechMeister]

I have a rocket - when i am in the VAB it says i have this ammount of Δv:

But when i stage to it and start burning i have this much:

 

Edited May 5, 2023 by BechMeister
Changed title - think i found a bug/exploit

[BechMeister]

This is really weird to me.. Even in the VAB it gives me two numbers:

 

[BechMeister]

This may be a bug.

Okay - I was making a refueling vehicle for my space station - to make sure i didnt use the fuel i needed to refuel i placed decouplers around the fuel stack - as seen here:

The grey fuel stack is in between the two decouplers - and i would transfer fuel from the top to the button while burning - I then connected the top to the buttom with fuel lines.
Stats were the follwing:

Vehicle A:
24t  total 
0.3 thrust to weight.
2.295 Δv

I then wanted to use the same design for a Hydrogen version. So i removed the decouplers and added a hydrogen tank.

Vehicle B:
22t
0.4 thrust to wight
1041 Δv.

I was puzzled by this - the vehicle is lighter, but has a 1000 + less Δv - So i wondered if it was a bug/exploit the other had 2k Δv



I added a decoupler and a fuel line to test it:



And i double my Δv - So... i guess i found a bug

[Streetwind]

Oh yes, there are numerous bugs with the dV display in the editor. For example, the mass of Kerbals inside cockpits is counted incorrectly (added only to wet mass but not to dry mass). Then, using multiple different engines on the same stage appears to cause the editor to report significantly less dV than there actually is (I had this effect with a Dart on the bottom of a couple tanks, to which I then added a pair of Thuds. Which halved the reported dV in the editor, but the vessel still made orbit just fine).

You've just found another. And there are more besides.

If you're ever unsure about whether the editor is showing you the truth, you can always sanity-check it by solving the rocket equation yourself. For a single stage, it's not even complicated.

• Detach everything below the stage you want to check, but leave everything above in place
• Note down the entire vessel's mass, let's call it m0
• Empty all the fuel tanks that this stage is supposed to empty during its operation
• Note down the entire vessel's mass once again, let's call it m1
• Bring up the Windows calculator (or equivalent tool on your OS of choice). You may need to set it to scientific, if it isn't already.
• Divide m0 by m1
• Press the "ln" key once (that's the natural logarithm, if you're wondering)
• Multiply by 9.80665
• Multiply by the vacuum Isp of the engine(s) on this stage
• The result is this stage's true vacuum dV

The only part where this can get complicated is if you're mixing engines with different Isp values on the same stage. In which case you first need to calculate the average Isp, which unfortunately needs to be weighted by thrust contribution. So, for each individual engine, you would multiply its Isp by its thrust, and write that number down. After doing that for all engines, you sum up all these numbers, and then divide the result by the total thrust of all engines combined.

If your own math is one or two m/s off, don't sweat it. That's an acceptable error. If your result is a two-digit number or more off, then something's fishy. Either with the editor, or with the math you used.

 

Edited May 5, 2023 by Streetwind

[BechMeister]

Okay.

Model A:

Full mass 24.230 kg
"Empty" Mass: 14.230 kg
ISP 335s

Result: 1.749 Δv - I belive this - i had around 100 Δv left last time i used the rocket, and I usually need around 1500 Δv to get the 2nd stage to orbit with my skill level atm.

Game repots: 2.813 Δv 

Model B:

Full Mass 22.090 kg
"empty" mass: 12.090 kg
ISP 335S

Result: 1981Δv (rounded up) 
Game: 1041 Δv (2609 Δv If i do the decoupler and fuel line trick) 

Im going to test the rocket with the fuel line trick to see if I have a massively more fuel.  

[BechMeister]

Finished test - 1 - The craft said i had 1041 Δv total - but i manged to get to orbit, rendezvous, dock, fuel the station (craft got lighter = more Δv) and get down.

At one point i did a test. The craft said it had 111Δv left, and the manouver costed 83Δv - Even though i should only have had 28 Δv left - I had 63 or something. Any way long story short.. I made it back Thanks @Streetwind for walking me throught he rocket equation. It really was rather simple. Also just typing it into google works x)