Bug #787
Higher physically correct specific impulse than calculated
100%
Description
Hello, somebody noticed that the conversion factor from Isp in seconds to Isp in ms?¹ isn't g0 (=9.81 ms?²) and instead something like 9.82 ms?².
Apart from using seconds for the Isp which already confuses many people because the g0 pop now up and it appears that values like ?v depends on the gravitational acceleration at your craft's position it also can't be used in formulas directly. Especially because we don't know the actual conversion factor (and assumed it is 9.81 ms?²) we get wrong values.
An example from the wiki: http://wiki.kerbalspaceprogram.com/wiki/PB-Ion_Electric_Propulsion_System
Now the solution would be to change the conversion factor and to show the specific impulse also in ms?¹ in the VAB. Maybe even define then in the part file that way (although this is a little harder without breaking compability with mods).
| FC_0171.png (418 KB) FC_0171.png | Fuel consumption in 0.17. If g0 were 9.82, the consumption would be 92.6 L/s | ||
| FC_0180.png (322 KB) FC_0180.png | Fuel consumption in 0.18. g0 can be shown to be 9.82. | ||
| screenshot1.png (663 KB) screenshot1.png | [email protected] |
History
#1
Updated by umbralraptor over 11 years ago
Updated by - File FC_0171.png FC_0171.png added
- File FC_0180.png FC_0180.png added
It appears to have been changed between 0.17.1 and 0.18. Here are fuel consumption examples from each version using a Mainsail in vacuum (1500 kN thrust, 330 s Isp).
#2
Updated by TriggerAu over 8 years ago
Updated by - Status changed from New to Needs Clarification
#3
Updated by xZise over 8 years ago
Updated by - File screenshot1.png screenshot1.png added
- Status changed from Needs Clarification to Updated
- % Done changed from 0 to 10
Okay I retested it on 1.1.3 and it seems to be below 9.81 m/s² now.
As seen on the screenshot, the specific impulse is 345s, the thrust 60kN and the fuel flow 3.54684 units per second. As far as I can tell the unit is the volume, so it needs to converted to kilograms per second. Because it is using liquid fuel and oxidizer with a density of 5 kilogram per unit, the fuel flow is 17.7342kg/s. This leads to a conversion factor of: 9.80665 m / s^2
Now the value mentioned in the Wikipedia is exactly that ( https://en.wikipedia.org/wiki/Gravitational_acceleration#Gravity_model_for_Earth ) so I assume that this is correct and could be marked as fixed.
But for now I just mark it as updated, as you developers should decide if the current value is correct or not.
#4
Updated by hermes47 over 8 years ago
Updated by 
Just checked my handy "Handbook of Chemistry and Physics" which has g0 = 9.80665 ms^-2 as the exact value. So this seems all fixed properly.
#5
Updated by TriggerAu over 8 years ago
- Status changed from Updated to Closed
- % Done changed from 10 to 100
nice Hermes :P Thanks