Engine efficiency in CFG?

[Deadpangod3]

Where does it show in a parts CFG file the efficiency or how much fuel it uses each second to produce thrust?

So like if I wanted to make an engine that could reach Jool on a single flt 400 tank or something.

I've looked through part cfgs and I can't find anything that even hints at it

[Taki117]

I think you are looking for Isp.

[Supernovy]

I_sp. Fuel flow rate is equal to F_thrust/(I_sp*9.82). That's mass flow (tonnes per second if F_thrust is in kilonewtons), and each fuel or oxidiser unit masses 0.005 tonnes. 400*0.005 = 2 tonnes.

You'd want Δv, Δv=(I_sp*9.82)*ln(m_w/m_d) where m_w and m_d are the wet and dry masses respectively. To get the Δv you want, find how much you need from a Δv map and solve the above equation for I_sp. This obviously depends on the design of the vehicle, so you may instead want to just increase the I_sp by a large amount (say, ten or a hundred times), which will give you enough Δv for most craft designs.

EDIT: Whoops, forgot it was called atmospherecurve in the configs. The posts below this explain it.

Edited February 3, 2014 by Supernovy

[tavert]

What you're looking for in the part.cfg is the atmosphereCurve:

	atmosphereCurve
 	{
   	 key = 0 370
  	 key = 1 320
 	}

This example is from the LV-T30. It means that at atmospheric pressure = 0 atmospheres (vacuum), the specific impulse is 370 seconds. At atmospheric pressure = 1 atmosphere (sea level on Kerbin) or higher, the specific impulse is 320 seconds. When there are only two points in the curve, the interpolation is linear versus atmospheric pressure. Atmospheric pressure is an exponential function of altitude.

[G'th]

Look for AtmosphereCurve in the .cfg

Those two keys change the two ISP's of the engine. Normally the higher one will be Vacuum ISP. Certain mod engines, such as a couple in FASA, do the opposite with atmosphere ISP being higher (Due to those engines being for missiles and not for space rockets), so if you intend to edit one of those engines be sure to note what values you're changing.

EDIT: Gah, ninja'd

Edited February 3, 2014 by G'th
ninja'd

[Deadpangod3]

Ah I was wondering what that was for.

Thanks!