Engine ISP, thrust, specific impulse and everything else

[Tomato29]

Hey,

I'm currently trying to fix B9 heavy weight engines because it feels like that engine is underpowered; but I'm quite a noob at editing cfg files and I don't have a clue how to modify atmospherecurve to get the fuel consumption I want. I've already tried to understand how these math calculations work but I don't know how could I know with the calculations what to write in cfg files for the engine to work how I'd like it to work :/

please, someone here knows how to do that?

[KerikBalm]

Is this a jet engine or a rocket engine?

For rockets:

Atmosphere curve does not determine the fuel consumption... well only the vacuum value adjusts the fuel consumption.

Since 1.0, fuel consumption does not change with altitude/atmospheric pressure.

Fuel consumption is simply determined by the engine vacuum Isp and the engine vacuum thrust. The atmosphere curve modifies the thrust but not the fuel consumption.

 

For Jets:

Fuel consumption is determine by the atmcurve, and the mach curve. The atmcurve is different from the atmospherecurve for rockets... it adjusts thrust and fuel consumption. As does the velcurve.

Examples:

LV-T30:

    MODULE
    {
        name = ModuleEngines
        thrustVectorTransformName = thrustTransform
        exhaustDamage = True
        ignitionThreshold = 0.1
        minThrust = 0
        maxThrust = 215
        heatProduction = 192
        fxOffset = 0, 0, 0.8
        EngineType = LiquidFuel
        exhaustDamageDistanceOffset = 1.05
        PROPELLANT
        {
            name = LiquidFuel
            ratio = 0.9
            DrawGauge = True
        }
        PROPELLANT
        {
            name = Oxidizer
            ratio = 1.1
        }
        atmosphereCurve
        {
            key = 0 300
            key = 1 280
            key = 7 0.001

        }

The values in bold determine its fuel consumption. It consumes the amount of fuel needed to produce 215 kN of thrust at 300 Isp. At 1 atmosphere, it gets 280 Isp, produces less thrust, but has the same fuel consumption as in a vacuum

 

Whiplash:

    MODULE
    {
        name = ModuleEnginesFX
        thrustVectorTransformName = thrustTransform
        exhaustDamage = True
        ignitionThreshold = 0.1
        minThrust = 0
        maxThrust = 130
        heatProduction = 75
        useEngineResponseTime = True
        engineAccelerationSpeed = 0.2
        engineDecelerationSpeed = 0.4
        useVelocityCurve = False
        flameoutEffectName = flameout
        powerEffectName = running_thrust
        runningEffectName = shockDiamond
        engageEffectName = engage
        disengageEffectName = disengage
        spoolEffectName = running_turbine
        engineSpoolIdle = 0.05
        engineSpoolTime = 2.0
        EngineType = Turbine
        exhaustDamageMultiplier = 40
        PROPELLANT
        {
            name = LiquidFuel
            resourceFlowMode = STAGE_STACK_FLOW_BALANCE
            ratio = 1
            DrawGauge = True
        }
        PROPELLANT
        {
            name = IntakeAir
            ignoreForIsp = True
            ratio = 8
        }
        atmosphereCurve
        {
            key = 0 4000 0 0
        }
        // Jet params
        atmChangeFlow = True
        useVelCurve = True
        useAtmCurve = True
        flowMultCap = 2.0
        machLimit = 2.5
        machHeatMult = 6.0
        velCurve
        {
            key = 0 1 0 0
            key = 0.2 0.98 0 0
            key = 0.72 1.716 2.433527 2.433527
            key = 1.36 3.2 1.986082 1.986082
            key = 2.15 4.9 1.452677 1.452677
            key = 3 5.8 0.0005786046 0.0005786046
            key = 4.5 3 -4.279616 -4.279616
            key = 5.5 0 -0.02420209 0
        }
        atmCurve
        {
            key = 0 0 0 0
            key = 0.045 0.166 4.304647 4.304647
            key = 0.16 0.5 0.5779132 0.5779132
            key = 0.5 0.6 0.4809403 0.4809403
            key = 1 1 1.013946 0
        }
    }

Now here, the bolded lines are used in determining fuel consumption.

At 1 atmosphere, stationary, the engine consumes the amount of fuel needed to produce 130 kN of thrust at 4000 Isp.

At 0.16 atmospheres, there is a 0.5x thrust modifier, so if the craft was stationary, it would consume amount of fuel needed to produce 85 kN of thrust at 4000 Isp. (note 130*0.5 = 85)

If it was at mach 2.15. then the thrust is multiplied by 4.9. Mach 2.15 at 0.16 atmospheres should haev a thrust of 0.5* 4.9 * 130kN = 318.5 kN... and as Isp is always 4000, the engine of course consumes the amount of fuel needed to produce 318.5 kN of thrust at 4000 Isp

As Isp is constant for jets, more thrust -> more fuel consumption. Just tweak the multipliers for various altitudes and speeds to get the thrust you want. Also increasing the Isp decreases fuel consumption, and vice versa, across all speed and altitude ranges.

There is a flow multiplier cap... I think a flow multiplier cap of 2 actually means that fuel flow is limited to (1+2), as I know whiplashes can exceed 260kN

For rockets, fuel consumption is constant, tweak the vacuum Isp values and the thrust values to get the consumption that you want.

[Van Disaster]

Ramjets don't necessarily have fixed ISP. Anyway if you're tweaking jets - especially B9 SABRE types - don't forget they don't generate a significant amount of their thrust until they're moving quite fast.

[Tomato29]

19 minutes ago, KerikBalm said:

For rockets, fuel consumption is constant, tweak the vacuum Isp values and the thrust values to get the consumption that you want.

Thanks for replies guys! that's a 2-mode engine like the stock rapier, but without air intakes. one mode does just takes a longer time to burn.

I was actually thinking about a correlation between 1 and 0 pressure atmosphere that would determine fuel consumption and thrust variations :/

but you didn't wrote the entire jets part because i'm planning on making a big jet engine for b9 parts

[KerikBalm]

Well, if you wanted, you could make rockets use the atmcurve as well to scale their thrust/consumption with atmospheric density.

What I said was just applicable to stock engines. Stock jets have constant Isp... but they don't have to. You can easily add more values to their Isp curve to make jet Isp change with altitude as well.

I'm not sure if the old velocity curve works, the code might still be there, but that may be a way to scale Isp with velocity.