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# Comparison of the jet engines

## Question

I recently looked in the .cfg files for the pressure and mach curves for the jet engines, and graphed them.

I thought this might be usefull to others.

First we have % of maximum thrust relative to mach number:

as if you didn't already know, the rapier is the best high speed engine.

Also of note is that the afterburner on the panther gets you about another 0.5 to 1.0 mach...

Even without the afterburner, the panther is better at higher speed than the basic jet and other engines called "turbofans"

The Juno does a bit better at high speed than the non-afterburning turbofans, but not by much.

however, that is only one variable that affects thrust, the other is atmospheric pressure.

As the air gets thinner, thrust decreases - but the decrease is not 1:1 for any engine (though some are close).

So here I graphed (% of maximum thrust)/ (air pressure as a percent of air pressure at Kerbin's sea level)

As you can see, the whiplash and rapier don't suffer as much from decreased air pressure, enabling them to get a craft much faster because drag at a given altitude decreases more than their thrust does.

Likewise, the panther in afterburner mode doesn't suffer as much at higher altitudes like the other turbofans... in non afterburning mode, it behaves more or less like the other two turbofans.

The Juno also seems to do respectably at higher altitude.

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At least they give us the jist of the curves. Besides, I currently know of no way to plot multiple hermite splines on top of each other and be able to manipulate the data (zoom, scale, run a lookup at a specific point, divide or multiple one curve by another variable). Sadly, I have to use excel or something like that to do that crap.

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[quote name='NathanKell']You really need to use the tangents. Your graphs are shaped all wrong from not including them, you can't just assume it's linear...[/QUOTE]

I don't assume its linear, its just a presentation of the fixed points specified in the .cfg file, its easier to read with lines connecting the data points, than to just look for similar color and shape symbols on a scatterplot.

Also I wasn't sure how to use the tangent, and for the graphing program I have that can graph equations rather than just data points... I don't know how to have it only graph specific intervals using different equations.
I suppose I could have looked up how to use the tangents, used the graphing program to generate data points, and then graphed those in excell... but I was too lazy

as noted by Right, its enough to get the jist of it.

If someone wants to put more work in, and do the graphs including the tangents, I'd be interested in seeing the results... because they would be more accurate of course.
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Thanks for the graphs! These seem to go along with my own test flight observations; of course each engine has its uses. For SSTOs, the Rapier and Whiplash combo works well; I've not tried a Rapier only version in a long time. Also, another combination that's worked well for me is the Aerospike and Whiplash.

One thing that was fun to do was making one jet plane using all jet engines (except for the Goliath due to size). That allows you to see and hear the effects for each engine type during the same flight. If you fly at a constant 90 degrees (Navball reading) after leaving the runway, you'll see when they flame out. If you assign action groups, you can try the Panther wet mode, Wheezly thrust reverse, that sort of thing.
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Thanks for this chart, I'm new to the jet engines and can't seem to get above 10,000m (I have a contract that requires over 17,000).

btw does "wet" mean with the intake closed?
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[quote name='Waxing_Kibbous']Thanks for this chart, I'm new to the jet engines and can't seem to get above 10,000m (I have a contract that requires over 17,000).

btw does "wet" mean with the intake closed?[/QUOTE]

Wet means afterburning mode; keep the intakes open.

For an easy way to reach altitude in the early game: build a basic Juno-powered jet, but strap a Swivel or a few Thuds/Twitches onto it. Fly as high as you can on the Junos, then light the fireworks and lift the nose. Just make sure you have enough pitch authority to pull out of the dive afterwards.
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KerikBalm,
Thanks for posting these graphs. I'm sure they'll prove useful.

If you get the chance, could you multiply the % sea level thrust by the sea level thrust, convert the density to altitude, and flip the X axis? I bet that'd make it easier for newbies to read.

Best,
-Slashy
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About converting density to altitude, I'd like to do that, but its not as simple as before with the simple scale height.

And for the thrust, I thought it was more useful to convert to TWR instead of just pure thrust.

[img]http://i.imgur.com/1SP1bqP.png[/img]

[img]http://i.imgur.com/50F3QDm.png[/img]

This one is more about the relative thrust loss of various engines as they get higher, I don't know how useful it is to weight the values against their maximum sea level thrust.
[img]http://i.imgur.com/THsiqdx.png[/img]

I'm also thinking it would be useful to have various datapoints, like mach 0.7 at 3km, mach 2 at 10km, mach 3 at 18km, mach 4 at 20km.... etc...
But again, its not clear to me how to determine air pressure at various altitudes post 1.0, nor how to use the tangents to get proper values inbetween the defined points of their thrust curves.
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[quote name='KerikBalm']I don't assume its linear, its just a presentation of the fixed points specified in the .cfg file, its easier to read with lines connecting the data points, than to just look for similar color and shape symbols on a scatterplot.

Also I wasn't sure how to use the tangent, and for the graphing program I have that can graph equations rather than just data points... I don't know how to have it only graph specific intervals using different equations.
I suppose I could have looked up how to use the tangents, used the graphing program to generate data points, and then graphed those in excell... but I was too lazy

as noted by Right, its enough to get the jist of it.

If someone wants to put more work in, and do the graphs including the tangents, I'd be interested in seeing the results... because they would be more accurate of course.[/QUOTE]

I would definitely suggest evaluating a few more points. indeed it's very possible to write the evaluator as an excel formula.

Here's the Panther in Wet mode, for an example. You can definitely see how the linear interpretation can severely over- or underestimate peformance.
[img]http://i.imgur.com/P9nobGC.png[/img]
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For the thrust versus pressure graph, should all the engines but the rapier drop to 0 thrust instead of 1 in vacuum?
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No... I don't consider the vacuum case... thats dividing by zero.
They all are 0/0 at that point. 0 thrust, 0% of max thrust. 0 atmospheric pressure, 0% of atmospheric pressure.
Obviously, I'm only considering airbreathing engines, the closed cycle mode of the rapier doesn't count.

Imagine if at 0.5 atmospheres, one engine produced 50% of its maximum thrust, while another produces 75% of its maximum thrust.
On my graph, the first engine would have a datapoint with a Y value of 1, while the other would have a datapoint with a Y value of 1.5.

As the atmospheric pressure gets lower and lower, its still possible to have a value above 1 - however they all have precisel zero thrust at 0 atmospheres.
If I used the tangents like nathan suggests (I don't have the time or motivation to go into that much detail), I could find the limit as atmospheric pressure approaches zero for each engine... but I don't want to, and no engine will be producing thrust anyway at 0.00001 atmospheres.
Basically, I could find the atmospheric pressure at which the rapier cuts out, and end ethe graph there with every engine but the rapier dropping to a ratio of zero... extending the graph past that point would have the rapier at zero as well.
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[quote name='NathanKell']indeed it's very possible to write the evaluator as an excel formula.[/QUOTE]

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You need to set up the keys on either side of your desired value, and your desired value. Then add some columns to calculate the in-between values, and then calculating the final value should be fairly simple.
Note that for a given
key = x y z w
that's time, value, inTangent, outTangent respecitvely.

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