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Bug #787

Higher physically correct specific impulse than calculated

Added by xZise about 6 years ago. Updated about 3 years ago.

Status:
Closed
Priority:
Very Low
Assignee:
-
Category:
Physics
Target version:
-
Start date:
06/19/2013
% Done:

100%

Version:
Platform:
Any
Expansion:
Language:
English (US)
Votes:
Arrow u r green
Arrow d r red

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 umbralraptor, 06/23/2013 01:02 AM
FC_0180.png (322 KB) FC_0180.png Fuel consumption in 0.18. g0 can be shown to be 9.82. umbralraptor, 06/23/2013 01:02 AM
screenshot1.png (663 KB) screenshot1.png 555-555-0199@example.com xZise, 07/17/2016 04:28 PM
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History

#1 Updated by umbralraptor about 6 years ago

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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 about 3 years ago

  • Status changed from New to Needs Clarification

#3 Updated by xZise about 3 years ago

19139

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 about 3 years ago

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 about 3 years ago

  • Status changed from Updated to Closed
  • % Done changed from 10 to 100

nice Hermes :P Thanks

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