Reported Version: v0.1.4 (latest) | Mods: none | Can replicate without mods? Yes
OS: Windows 11 | CPU: i7-11800H | GPU: RTX 3050 Laptop | RAM: 16GB
Severity: Mid
Frequency: Low
Introduction:
What wiki says and what most people think is that Kerbin has 9.81m/s2 of surface gravity (which we can use it as standard gravity, g0, in calculations). I was a bit skeptical about this because in my Δv calculations for different crafts using a variety of engines have resulted in sometimes wildly different results compared to what the game calculates as Δv, most were pretty accurate within +-10 range. Even though they were close, the game obviously calculated the Δv in a slightly different way, so I dug deeper.
The Experiment:
My Assumptions:
Isp in vacuum and Masses of engines and parts are arbitrary set values.
At the end of a Δv calculation, the game rounds the number to the nearest integer rather than taking its floor nor ceiling.
The game calculates Δv using the rocket equation without the pressure term (Δv = Isp*g0*ln(mi/mf)).
In order to find the in game standard gravity I divided the in game Δv with pseudo Δv which is just the Δv without the g0 multiplier (Δvpseudo = Isp*ln(mi/mf) , g0 = Δvingame/Δvpseudo). Because I assumed that Δvingame is a rounded quantity, I took the average of the results of g0 from several different massed crafts using different engines so that it would get closer to the actual value. But what I found was interesting to say the least.
The Setup:
Δvingame data is the value what VAB showed me at the bottom right corner since it considers the vacuum conditions. The crafts consisted of the selected engines (just one), selected fuel tanks (extra battery in the case of Dawn) and added masses. I've summed up the masses of each part in every craft one by one since it gives you a rounded number in the engineers report section. I used these engines because they all use different propellant types and I figured the same type of propellants would result in similar ways.
Engine
Isp in vacuum (s)
Propellant mass (t)
Dawn
4200
2.8
Swivel
320
2
Swerv
1450
50
Calculations:
Here are three screenshots taken from https://www.desmos.com/calculator, averaging the resulting g0 from different masses using the same engine therefore different Δvingame.
Dawn:
Swivel:
Swerv:
M is the total mass (wet mass) of the craft of interest.
Vingame is Δvingame of the craft of interest.
Vpseudo is and gavg are self explanatory.
The Problem:
The defined value of standard gravity on Earth is g0 = 9.80665m/s2 which is probably the actual value used in game because Kerbin is the analog of Earth (crazy, I know). This value is approximately what I got from Swivel and Swerv calculations. The problem lies in the xenon engine, Dawn. It has resulted in a wildly different value of 9.728...m/s2 which is not even close to 9.8m/s2 which is intriguing.
Long burn time case:
I asked this question to the Intercept Games discord server and some people have mentioned that maybe the burn time of Dawn had some effects on this. I didn't really think it would cause any issues but I've tried it with a longer burn time methalox configuration nonetheless. The Dawn setup had a burn time of ~168h, my configuration was with Ant (Isp=330s) and 384t of methalox had ~171h of burn time. (I am aware that the burn time is dependent of Isp which is kind of ironic in this case)
Ant:
In the calculations, Ant have resulted in gavg=9.8029...m/s2 which is still considerably lower than 9.80665m/s2 but that is probably the result of unlucky coincidences due to Δvingame roundings. My reasoning behind this is that in M[5] calculation, g0=9.8098...m/s2 which is a lot higher than the rest of the data, raising the average. All the data were in between the +-0.5 range.
Conclusion:
The in game standard gravity is most likely g0 = 9.80665m/s2 and the game probably uses a different value of Isp for Dawn. That, or maybe I've just messed things up and am wrong ¯\_(ツ)_/¯.
As a bonus report, the game sometimes showed the twice the amount of Δv that the craft would've had.
Reported Version: v0.2.0 (latest) | Mods: none | Can replicate without mods? Yes OS: Windows 11 | CPU: i7-11800H | GPU: RTX 3050 Laptop | RAM: 16GB Frequency: High Severity: Medium (misleading) Δv calculation in a stage when there's only one engine is fine, along with TWR, Burn time and Start mass. Things get incorrect when there are multiple engines in a stage. More engines make Δv tool make incorrect calculations to burn time and total Δv, TWR and Start mass are not effected. Here are two screenshots of the same craft with 4 nervs and 1 nerv. 4 nervs must result in less Δv than 1 nerv but it is just wrong. Included Attachments: .ipsImage { width: 900px !important; }
