Jets are not weak

[Pds314]

I heard someone complaining that "our turbofans are now less efficient than IRL turbojets on full afterburner with less thrust than a piston engine prop."

No. Actually, KSP's jets would be considered state of the art, in fact, considerably better.

For example, the SR-71's J-58 turboramjet weighed 2.7 tonnes a piece, were 5.44 meters long, and developed 150 kN on afterburner at about 1900 s ISP. Our turboramjets produce 87% the thrust at 67% the weight, 41% the fuel consumption, 74% the frontal area and 160% the top speed of their real-life competitors.

 

Or the F-35's F135 afterburning turbofan engine. While it is similar in diameter to our Panther, it develops 125 kN (dry) and 190 kN (wet), is 5.6 meters long, and weighs considerably more at 1.7 tonnes. However, unlike our engines, it also has an ISP of 4000 seconds... dry. Probably more like 1600 wet. This all means that our afterburning engine is easily twice as efficient as it's real-life analog, even if it develops considerably lower thrust for the same diameter. Another comparison would be the F-100-PW-229 on the F-15 and F-16, which develops 79 kN dry and 130 wet, and achieves more like 4600 s ISP dry and 1600 wet.

 

Even the high-bypass jets are optimistic. Our J-90 has about the same specific impulse as the real life GE90. The primary difference is that the GE90 loses nearly half that efficiency under cruise conditions, whereas our J-90 retains its full static sea-level efficiency at all speeds and altitudes.

 

So to sum up:

Our jets have about the same TWR, similar size, and twice the fuel efficiency of their real counterparts. They are considerably OP, with our afterburners and turboramjets being comparably efficient to dry turbofans.

By comparison, our rockets are massively underpowered. The KS-25 has a TWR of 25.5. A real SSME has a TWR of 67 and weighs less while developing over double the thrust, and while comparing ISP between our kerosene rockets and real H2 rockets isn't fair, the real SSME is much closer to holding the record for vacuum engine efficiency in its category than our engine is in its own.

It has gotten better as KSP has developed, but overall, jets are still optimistic bordering on totally impossible ever, and liquid fuel rockets and tanks are still pessimistic bordering on laughable. (and have in fact become moreso with 1.0 lowering their ISP to normal kerosene levels).

Jets are not underpowered. They are just somewhat less overpowered than they once were. If our jets were based on the same meta as our rockets, they'd develop 30% their current thrust at half their current ISP, making them almost unuseable... Like our rockets are in real solar system.

If our rockets followed the same meta as our jets, they'd have in the 600s of ISP and produce triple their current thrust, or have the same thrust, but be 1/3rd the mass.

Edited December 21, 2015 by Pds314

[Red Iron Crown]

It's funny, KSP jets are overpowered compared to real life ones while KSP rocket engines are underpowered compared to real ones. *shrug* Guess we need useful SSTOs to work somehow.

Most people complaining that KSP jets are underpowered are comparing them to the old, pre-1.0 turbojets, which were frankly foolishly overpowered, so much so that I felt like I was cheating* when using them. 

*Cheating is in the eye of the player. Play how you like and stuff people who don't like it.

[cantab]

Working on my spaceplane recently, I felt at times it was a bit underpowered, but then realised that two jets on a 50+ ton plane maintained a climb rate of 100 m/s or so much of the time. That's fighter-jet performance from a transport aircraft, albeit test flying without a payload. So the jets are fine compared to real life, it's just KSP's a game and we're liable to be impatient and expect excessive performance.

Also, caveat, I'm using FAR. Newstock aero IIRC is draggier than is realistic. That makes some aspects of flying easier, but it means you need more powerful engines to counter the drag which then means you can climb more steeply.

Edited December 21, 2015 by cantab

[wumpus]

Therein lies the problem.  I've tried throttling a jet down where it "should" be (about 1/10 power), and it will barely get off the ground.  I'd assume that turbojets "ought" to flame out around 10000m or run out of thrust around 600m/s (the thrust is the key.  On Earth we can't get much past 3/10s of orbital velocity.  Doing so on Kerbin makes spaceplanes viable).

If you have a problem with kerbal jets, you will probably need to install realism overhaul.  Just "fixing" any one issue leads a deep rabbit hole of other issues.  Fix the size of Kerbal and then you can "resize" the rockets for the real thing and the jets work "right" as well.

[NathanKell]

My practice has generally been to give jets 1.4x the TWR their real-life counterparts have, and 2x the static Isp. All jet curves are based on real counterparts with the exception of the turboram (which is an extrapolated super-J58) and the RAPIER (which is a full-on super-duperjet with slightly-better-than-ramjet performance above Mach 1 and a decent turbine below).

[cantab]

5 hours ago, wumpus said:

 I'd assume that turbojets "ought" to flame out around 10000m or run out of thrust around 600m/s (the thrust is the key.

Well I thought of what's now called the turboramjet as an SR-71 like engine. So the altitude ceiling is more like 20 km* and the max speed Mach 3.3 or about 1100 m/s. That's actually not far off what the game has. The real SABRE is hoped to be airbreathing up to Mach 5 or so; I think if anything the RAPIER is a bit weaker but game balance is a reasonable factor.

* I'm factoring in that Kerbin's atmosphere has a smaller scale height than Earth's, though not by much.

[KerikBalm]

I disagree that the rockets are underpowered "bordering on laughable".

The thing is that in the Kerbal's system, the dV to get pretty much anywhere (except from the surface of Eve) is approximately 1/3 that of their real life counterparts. 300 Isp in the kerbal universe is like 900 Isp in ours, as far as utlility for interplanetary travel. RL solid core nerva rockets would be just a bit better than the O-10 monopropellant engines are in this game. The TWR, depending on the design we talk about could be much better, similar too, or worse than the LV-N. The tankage and other stuff would at least be lighter, giving us lower dry weights and better mass ratios, and thus the ability to squeeze more dV out of a stage.

Basically, the kerbal system is 1/10th the size of ours, if 1 kerbal meter = 1 human meter. If we instead assume 1 kerbal meter = 10 human meters... then all of a sudden the system size is about right... but now the atmospheres are too high, and more importantly, we then must assume that the surface gravity is 10x higher.... 98 m/s/s... the Isps are then 10x higher... the LV-N gets 8,000 Isp, and has an earth relative TWR of 20.4... but in this kerbal universe where 1 kerbal meter = 10 human meters... gravity is 10x stronger than it should be (and also, kerbals are giants).

So another way to think about it is that the KS-25 actually has a 254:1 TWR, and gets 3,150 Isp... but the kerbals are stuck on an an Earth size planet with a surace gravity of 10 Gs... although there is no helping the bad tank mass ratios.

My point is, for the space aspect of it... the differences are so great between the systems that you can't really compare these engines to real engines. I think the engines are far from underpowered for the Kerbal system.

The jets though... when only considering atmospheric flight, we can pretty much ignore kerbin's small radius (though it does become quite relevant for high speed flight with afterburning panthers, turboramjets, and rapiers... where they get going a significant % of orbital velocity)... and thus the jets are much easier to compare to real life jets... and they are indeed OP. But this is a space game, and as soon as you leave kerbin, they are useless (except if going to laythe). I guess they wanted to make airbreathing SSTOs more attractive than they are in real life (note that none have been built in real life, though there are some proposal that maybe could work).

Also, their Isp stats aren't so out there if you assume they run on something other than kerosene.... a jet running on liquid hyrogen will get a significantly better Isp than one running on hydrocarbons.

Although, they are only borderline impossible if one consideres the theoretical maximum... but I suspect there are a few things missing in that theory that explain why IRL we only get to about half the theoretical maximum

yea... they run on the same fuel as a LV-N... so why not say that it's lH2, and thus their Isps are completely reasonable... (storing enough H2 in such a small tank... is not reasonable though)

 

4 hours ago, wumpus said:

Therein lies the problem.  I've tried throttling a jet down where it "should" be (about 1/10 power), and it will barely get off the ground.

Ummm.... they aren't that OP'd. See the comparison of the panther to the F-100 in the OP.

[A_name]

I don't care much for comparisons to real world jets, unless we're talking about RSS.

In regular KSP terms, yes jest were nerfed but they don't feel weak at all. I mean, a RAPIER can easily accelerate 20 tons of craft to near-orbital speeds in jet mode (if heat weren't an issue). I really don't see how you would need more power than that.

[KerikBalm]

7 hours ago, A_name said:

I don't care much for comparisons to real world jets, unless we're talking about RSS.

Exactly my point above, but for low altitude subsonic speeds, the comparison isn't so bad. Its when orbital mechanics starts becoming a major factor that the comparisons break.

I still maintain that rocket engines are OP'd as well in KSP due to the 1/3rd dV requirements.

[Sandworm]

14 hours ago, KerikBalm said:

Exactly my point above, but for low altitude subsonic speeds, the comparison isn't so bad. Its when orbital mechanics starts becoming a major factor that the comparisons break.

I still maintain that rocket engines are OP'd as well in KSP due to the 1/3rd dV requirements.

The rockets, even the jets, aren't the issue.  It is all down to how tiny kerbin is.  That's ancient decision (made before time warp was a thing) is a constant issue.  It's why the atmosphere is still so thick (planes could not fly in any atmo around such a small body), requiring OP jet engines to push through, and it is why the rockets have to be so week (realworld rockets could probably achieve orbit without staging).  Nearly every apparently-crazy physics decision in KSP can be traced back to the fact kerbin is so small.

To take it to the next logical stage, another reason kerbin is so small is to do with KSP's memory management, or lack thereof.  Kerbin has to be small enough that a single texture can be wrapped around without everything being blurry.  Much bigger, and those running win32 (on a win32 system) start having issues.  (Squad seems totally unwilling to unload textures, its something that large games normally do but would be a totally new direction/skillset for them.)  So the jets are OP and the rocket UP because of the ram limit.  Every patch in KSP is a plaster, a new layer of complexity.

Edited December 22, 2015 by Sandworm

[KerikBalm]

yes, its all related, 1/10th scale kerbin/system with 1:1 scalle gravity = ~1/3rd scale dV.

As I mentioned above, if we simply say 1 kerbal meter = 10 human meters, then its not that kerbin is small, but that the gravity is ludicrously strong, the atmosphere is too thick, and the engine Isp's too high.

If one modded/hacked gravity to be 1/10th of what it was, vertically compressed the atmosphere by a factor of 3-4x, and then divided the Isps by 10, it would essentially be RSS. Kerbin would be the right size, the rockets would just be cartoonisly big...

[cantab]

I'm not sure that works. So much in the game is on the basis that 1 metre is 1 metre. If you suddenly think it's actually 10 metres you get all sorts of weird results - orbital speeds become several times too fast, accelerations become extreme, and more.

I don't believe the stock aerodynamics was strongly influences by the small size of Kerbin either. When the soup was replaced by a vague semblance of reality, the reduced delta-V to orbit was compensated for by heavily nerfing the rocket Isp - in fact I'd previously run a similar nerf myself in old FAR. If I had to blame the excessive drag on anything, I'd say it's that having played FAR, realistic drag is a nuisance because aeroplanes take ages to slow down for landing unless you use airbrakes.

[problemecium]

Amen to the OP. Jets are one of many examples of things in KSP that actually greatly outperform their real-life equivalents despite popular opinion.

For example, as I mentioned in another thread, the fuel tanks are actually very durable. 6 m/s is actually a pretty high speed for an abrupt sidelong impact, and the vast majority of human rockets couldn't withstand that.