Why are my RAPIERS stuck at 95 kN thrust?

[eempc]

Why can't my RAPIER engines provide more than 95 thrust when they are rated at 175?

The other air breathing engines provide their correct thrust at sea level so why not RAPIERS?

[Streetwind]

The other air breathing engines provide their correct thrust at sea level so why not RAPIERS?

You will find this to be simply false if you go and systematically test it.

All air-breathing engines are sensitive to altitude (or rather, to atmosphere density) with respect to how much thrust they put out. They have different optimal heights. Maybe the RAPIER just has a very high optimal altitude and so its sea level thrust is low? I don't know exactly, I haven't used RAPIERs before. I have used jets and turbojets before though. Neither of them gives you full thrust at sea level either.

[KerbMav]

All air-breathing engines are sensitive to altitude (or rather, to atmosphere density) with respect to how much thrust they put out.

Huh? I thought the only thing scaling with atmospheric density in the stock game was fuel consumption?

Scaling thrust would actually be realistic, but is not done without modding (for rocket engines at least).

[eempc]

Okay, I did test it and you are right about the turbojet, but not the basic jet engine which I had incorrectly based my initial statement on. My two turbojets are capped at 105.7 kN with the brakes on.

The 3 basic jet engines here are outputting the full 150 kN thrust though.

Looks like my dreams of a RAPIER VTOL have been shattered.

[KerikBalm]

For all air breathing engines, they are flagged to use a velocity curve:

" useVelocityCurve = True "

Then look at the velocity curves:

Basic jet engine:

velocityCurve

{

key = 1000 0 0 0

key = 850 0.2 0 0

key = 0 1 0 0

}

This means at 1,000 m/s it produces 0 thrust

at 850 m/s, it produces 20% thrust

At 0 m/s, it produces 100% thrust

(and fits a curve between those points)

The turbojet:

velocityCurve

{

key = 0 0.5 0 0

key = 1000 1 0 0

key = 2000 0.5 0 0

key = 2400 0 0 0

}

and the rapier:

velocityCurve

{

key = 0 0.5 0 0

key = 1000 1 0 0

key = 2000 0.5 0 0

key = 2200 0 0 0

}

This means that both the rapier and the turbojet produce only 50% of maximum thrust while stationary

They produce maximum thrust (100% of rated thrust) at 1,000 m/s

This thrust drops off to only 50% again at 2,000 m/s

and they stop producing thrust completely at 2,400 (turbojet) and 2,200 (rapier).

Your problem is simply that your plane is stationary. Let it get faster, and its thrust will increase (propelling it even faster)

Often if it can takeoff, that means it will be able to exceed 1,000 m/s without airhogging (for rapiers/turbos)

Specific impulse varies with altitude, but thrust varies with speed.

Good luck getting a turbojet to 1,000 m/s at sea level though...

At any rate, for turbojets and rapiers, sea level is the most inefficient altitude as far as ISP is concerned, even before considering drag:

atmosphereCurve

{

key = 0 1200

key = 0.3 2500

key = 1 800

}

(this is for both rapiers and turbojets)

Thus they have the maximum ISP at .3 atmospheres.

At sea level (1 atmosphere) they have 800 ISP... ughh... when airhogging in a near vacuum, they are getting about 1200 ISP (less than half that of 0.3 atmosphere, but still 50% better than sea level)

You might think thrust scales with altitude, but its really that drag scales with altitude, and less drag means higher velocity, and higher velocity means more thrust.

In contrast, the basic jet thrust generally decreases with altitude because less drag leads to higher speeds at altitude.

For your reference, the basic jets ISP curve is:

atmosphereCurve

{

key = 0 1000

key = 0.3 1800

key = 1 2000

}

In this case, its ISP is greatest at sea level, decreasing slowly to 1800 at 0.3 atmospheres, and then decreasing ever more towards 1000 at the edge of the atmosphere...

but in practice, at those altitudes, you're travelling over 1,000 m/s, and the basic jet is producing 0 thrust and consuming 0 fuel, so the ISP doesn't really matter.

Edited June 5, 2014 by KerikBalm

[Padishar]

Huh? I thought the only thing scaling with atmospheric density in the stock game was fuel consumption?

Yes, this is indeed true. The ISP varies with atmospheric pressure.

Scaling thrust would actually be realistic, but is not done without modding (for rocket engines at least).

For jets, the thrust is scaled by the velocityCurve (when useVelocityCurve is set as described by KerikBalm).

My latest development version of KER allows you to see both these effects in the VAB by providing tweakable sliders that let you set the atmospheric pressure (as a percentage of sea level) and the velocity (0-2500m/s) used to calculate the thrust and deltaV stats.

[Streetwind]

Oooh, so they scale with speed, interesting!

It's easy to make the mistake though, as speed scales with altitude...