How can I make intake air massless?

[cosekantphi]

Or just a lot less heavy?

Neither the stock engineering info nor KER count intake air toward the total mass of the craft while in the editor. For your typical KSP aircraft, this can be treated as a rounding error. But when you're building a decently accurate Cessna 150 with an empty weight of 520kg, it starts to become a big problem.

The prop engines I'm using come from Airplane Plus and they all seem to store 15 units of intake air. Intake air has a mass of 5kg/unit, so this means every craft I build has an extra 75kg of unaccounted for mass in the nose. It doesn't move the CoM in the editor, it doesn't show up in KER until after I launch, and FAR bases the in-editor data on the airless mass.

I tried simply reducing the amount of intake air stored in each engine by changing their configs, but for some reason it didn't actually change anything once I loaded up the game. The engines still had an extra 75kg of mass even when I changed the air intake storage to 4 units. I know this has to be a result of the intake air because it doesn't have this extra mass if I launch with the intake closed.

I was tempted to just change its density in ResourcesGeneric, but I don't know if that'll alter engine performance. Is there a way to reduce intake air mass without messing anything up?

Edited January 24, 2022 by cosekantphi

[AtomicTech]

Just now, cosekantphi said:

Or just a lot less heavy?

Neither the stock engineering info nor KER count intake air toward the total mass of the craft while in the editor. For your typical KSP aircraft, this can be treated as a rounding error. But when you're building a decently accurate Cessna 150 with an empty weight of 520kg, it starts to become a big problem.

The prop engines I'm using come from Airplane Plus and they all seem to store 15 units of intake air. Intake air has a mass of 5kg/unit, so this means every craft I build has an extra 75kg of unaccounted for mass in the nose. It doesn't move the CoM in the editor, it doesn't show up in KER until after I launch, and FAR bases the in-editor data on the airless mass.

I tried simply reducing the amount of intake air stored in each engine by changing their configs, but for some reason it didn't actually change anything once I loaded up the game. The engines still had an extra 75kg of mass even when I changed the air intake storage to 4 units. I know this has to be a result of the intake air because it doesn't have this extra mass if I launch with the intake closed.

I was tempted to just change its density in ResourcesGeneric, but I don't know if that'll alter engine performance. Is there a way to reduce intake air mass without messing anything up?

I think it's possible.

I'll look into some .cfg files to change this.

Here's the .cfg file:

RESOURCE_DEFINITION
{
  name = IntakeAir
  displayName = #autoLOC_501005 //#autoLOC_501005 = Intake Air
  abbreviation = #autoLOC_6002101 //#autoLOC_6002101 = Air
  density = 0.005
  unitCost = 0
  hsp = 10
  flowMode = ALL_VESSEL
  transfer = PUMP
  isTweakable = false
  isVisible = false
  RESOURCE_DRAIN_DEFINITION
  {
    isDrainable = false
    showDrainFX = false
  }
}

Perhaps with MM you can do this:

@RESOURCE_DEFINITION[IntakeAir]

{

  @density = 0.001

}

[cosekantphi]

3 minutes ago, AtomicTech said:

I think it's possible.

I'll look into some .cfg files to change this.

Here's the .cfg file:

RESOURCE_DEFINITION
{
  name = IntakeAir
  displayName = #autoLOC_501005 //#autoLOC_501005 = Intake Air
  abbreviation = #autoLOC_6002101 //#autoLOC_6002101 = Air
  density = 0.005
  unitCost = 0
  hsp = 10
  flowMode = ALL_VESSEL
  transfer = PUMP
  isTweakable = false
  isVisible = false
  RESOURCE_DRAIN_DEFINITION
  {
    isDrainable = false
    showDrainFX = false
  }
}

Perhaps with MM you can do this:

@RESOURCE_DEFINITION[IntakeAir]

{

  @density = 0.001

}

I was thinking of doing something like that, but I was unsure if that'd alter engine performance. I don't know enough about how the engines work to know if that'll  have a side effect like changing ISP or maybe their fuel consumption rates. Thanks for the config, I'll try it in game to see whether or not that's the case.

[BigFatStupidHead]

Changing the mass of intake air would be changing the mass of one of your propellants, which may have surprising effects on atmospheric engines.  It's probably better to change the amount of intakeair storage in the Airplane Plus engines down to something like 1. (Alternately, it should be possible to remove that storage all together as long as you have something, somewhere on your aircraft with some intakeair storage, like an intake.)

[cosekantphi]

Alright, I did AtomicTech's suggestion and reduced the density of intake air from 5kg/unit to 1kg/unit.

I loaded up a variety of aircraft in the SPH with both modded and stock air breathing engines, and from a cursory look at the KER window it would appear everything is the same. Mass, ISP, thrust,  burn time, and delta-V  all remain the same in both jet and prop powered aircraft.

However, when I launch a craft with an intake, the mass only increases by 15kg instead of 75kg. I may reduce it further depending on whether this still causes big differences in the FAR stability derivatives. For now, I'll consider this a success! Looks like it worked unless there are some more obscure but relevant parameters not listed in the KER window. If I'm missing something please let me know.

1 hour ago, BigFatStupidHead said:

Changing the mass of intake air would be changing the mass of one of your propellants, which may have surprising effects on atmospheric engines.  It's probably better to change the amount of intakeair storage in the Airplane Plus engines down to something like 1. (Alternately, it should be possible to remove that storage all together as long as you have something, somewhere on your aircraft with some intakeair storage, like an intake.)

I noticed while looking through engine configs that all the air breathing engines in both stock and Airplane Plus have:

    PROPELLANT
    {
        name = IntakeAir
        ignoreForIsp = True        
 

Would this account for any changes in intake air mass? Or would the mass affect things beyond just the ISP

Edited January 24, 2022 by cosekantphi

[JadeOfMaar]

@cosekantphi You may want to use this:

@RESOURCE_DEFINITION[IntakeAir]
{
	@isTweakable = true // so you can control its fill amount (and lock it) in the editors
	@isVisible = true // makes it appear in the resource panel in flight
}

The parameter ignoreForIsp = True causes mass flow and dV calculation to ignore the resource and its ratio to be treated as a simple multiplier of the units/sec of the propellant that has mass. The Whiplash engine's IntakeAir ratio is 8, so whatever LiquidFuel/sec is required, 8x that number in IntakeAir is required.

If not for this parameter, dV calculators would only report a few m/s and burn time of a few sec for an air-breathing vessel. The resource is still required in flight but is seen as irrelevant to the calculators. The Whiplash would then require 8/9 of the total mass flow to be IntakeAir, and 1/9 of that mass to be LiquidFuel. (The 9 is the denominator of the mass fraction and the sum of all propellant ratios in the engine) I haven't checked but, for example, it would require 1/9 its normal amount of LiquidFuel AND nearly 1/10 IntakeAir.