Should jet engines be fixed or not,ever?

[Red Iron Crown]

Kegereneku, I think I read you wrong, thanks for clarifying.

[Scoundrel]

IMO? I only stated facts. I have zero interest in your opinion or how you or anybody play. I, as a customer, have problem with how KSP is designed.

Heh.

Scoundrel, I fail to see how your idea would solve the problem (pre-aero-rework) or improve the gameplay.

I was under the impression that the "problem" was such things as people getting 200km apoapsis with jet engines alone, which, as I understand it, is done through velocity (of delta v fame). Since engines themselves have an upper limit on velocity IRL it seems to me that limiting their velocity solves the "problem" as it has been put forth. Am I mistaken?

It put an upper limit to their speed (through heat) but in a way that favor minmaxing (lifting to orbit as fast as possible) rather than their expected use (slowly increasing the speed)

I don't know how to say this without sounding condescending, so please don't take it as such... generally speaking, the upper limit to any turbine engine's performance is heat, be it from heat causing excessive thermal expansion of the parts (resulting in catastrophic failure) to heat causing the blades to weaken (resulting in catastrophic failure) to the fact that no jet turbine can handle stoichiometric temperatures as a whole (resulting in cata- oh, you get my point). Heat is why there are no Mach 10 jet turbines.

As for the "slowly increasing the speed" thing... cruise missiles use turbojets and turbofans. Also FADEC allows high performance aircraft engines to go from idle to afterburner back to idle in an almost violent fashion... just look for jet engine testing for the PW-F119 or the EJ200 on youtube.

Unless you're talking about high-bypass turbofans. Then yeah, they take a little bit to spool up.

And lastly, about minmaxing... isn't that what you're supposed to do to get to orbit?

Also, inlet/intake that explode is also hardly intuitive (or realistic actually), engine however...

Well, with all the people complaining about the inlets (why does Squad call them intakes?), I thought suggesting exploding inlets was appropriate... and if you want some RL examples, I believe there are some pics floating around the internet from Purdue's hypersonic wind tunnel experiments where shockwaves and skin friction at high mach have done some serious damage to the test rigs... but you're right, it's not that realistic so consider it publicly retracted if it pleases you that I do so.

I guess the thing is that I don't see Squad creating "realistic" jet engines, as simply making them air-sucking slow-to-spool-up pseudo-rocket-engines optimized for either low-altitude or high-altitude flight still allows us to make functional aircraft. Proposing a simple solution such as engine overheating seems to me to be the best compromise between realism and KSPism, as it makes things like IntakeAir moderately pointless: it's needed to keep the engine running, but at high speed it becomes irrelevant because your engine will melt.

Gotta love thermodynamics!

Otherwise, imagine if Squad did implement realistic jet engines… someone, somewhere would make a thread arguing over whether or not the pressure recovery for the radial intake reflects the correct number of shockwaves in the inlet, or that the basic jet engine's datum blade tip Mach number on the rear compressor stage feels too high (which then deteriorates into an argument about whether or not the high pressure shaft speed needs to be reduced, blah blah blah).

Thus, in the end, it won't matter how realistic Squad goes, there will always be someone with a bone to pick with the devs because they can never do anything right (patched conics vs. n-body, aerodynamics, resources, engines, parts, etc.), and they know this. That's why realism suggestions are generally relegated to mods (ala HarvesteR's blog post in response to the aerodynamics thread), and why I prefer to approach topics in a way that increases the likelihood of Squad actually considering them.

Otherwise, Hell yeah! Bring on the turbine maps and compressor stalls!

In my opinion the fuel consumption of the jets should be corrected, they're way too fuel efficient.

A typical high bypass turbofan engine has a cruise SFC around 15-16mg/Ns, and takeoff SFC of 7-8mg/Ns, which translates to an Isp of about 7000s at cruise and about 14000s at takeoff... looking at KSP's config files, they have an ISP of 1800-2500... but that's a lie because they consider IntakeAir fuel at 15:1 (oooh, stoichiometric!), which means they have an actual Isp of over 30000s!

That said, I don't think that is necessarily a bad thing considering this is a game about exploration.

[Jouni]

That said, I don't think that is necessarily a bad thing considering this is a game about exploration.

This is a game about rockets and spaceflight, where exploration is an optional feature. If Magic Star Wars Engines (there's no reason to call them jet engines) are so good that they make rocket engines obsolete, it's a bug that needs to be fixed.

[Kegereneku]

Scoundrel, you are making a fuzz out of so little. Also you are being condescending just by saying "I don't want to be condescending".

I feel like you are taking this personally.

So, supposing you can read between my lines without requiring me to go in more lengthy explanation... (and being more condescending in return)

...thanks' for explaining your logic, However I still find "jet-engine overheat at high-speed", unintuitive, barely realist (only in the sense that it limit more than before) and since we are trying to solve the rocket-like climb this "open parameters" of yours still seem to risk being minmaxed/bypassed just like the pressure hog was.

You don't need that much speed to use the jet engine as a first stage.

I can't deny I preaching to my choir (see sign) but the problem might IMO be better cornered through definite limitation in parts specs (thrust and else...) in order to make it virtually impossible to outsmart the system by varying the number of parts/speed/climb rate...etc.

I don't think it's a question of realism since there will ALWAYS be a infinite scale toward realism. Plus as you say we don't IRL have MHD-cramjet-with-cryogenic-precooling (1 reputation point to whoever spot the absurdity in this tech) Engine capable of reaching Mach 25. Yet we want to go FAST in KSP, making this is more a question of finding a game-logic that is balanced THEN seem credible enough.

About the "slowly increasing the speed" thing..

Know that cruise missile are initially accelerated by a rocket stage/bomber and -as their name say- CRUISE using their jet-to-scramjet engines, jet engine DO throttle more slowly than rocket.

Then, my point was that unlike a rocket which must burst out from the denser part of the atmosphere, an air-breathing SSTO plane must necessarily climb with a lower TWR. Hence the comparator to "slowing increasing the speed".

So pardon me but "Why the hell" did you bicker on this ? We BOTH know spaceplane ascent take longer per their own nature !

Anyway, now seem a good time to remind that since the aerodynamic model will change, with I quote "Wing that give lift", so the basic/turbojet might in fact get a thrust/weight nerf and the relation between heat and speed might change as a result.

It seem to me we aren't just talking of their fuel-efficiency but their entire game-logic.

[GoSlash27]

This is a game about rockets and spaceflight, where exploration is an optional feature. If Magic Star Wars Engines (there's no reason to call them jet engines) are so good that they make rocket engines obsolete, it's a bug that needs to be fixed.

Agreed.

Even setting aside the "realism" angle, there's the "balance" angle to consider also.

The turbojet is so good that in most cases it's silly to use anything else, even rockets.

Best,

-Slashy

[basic.syntax]

KSP is "realism lite" - I support this discussion's request for a more realistic implementation of engines and intakes, but I think we are stuck with back-end design decisions made at the start, (IMO, can't really call the issues "bugs,") any future official changes are likely to be incremental tweaks. In HarvestR's Aero post, he writes "...nose cones properly reducing the drag of parts stacked behind it." That sounds to me like an adjustment of assigned drag values for parts behind a nose cone, not a dynamic airflow simulation. (I wonder how much of a drag discount I will get for a small nose cone attached to the middle of a 2x2 panel, flying flat-face forward.) But I'm OK with what we have, most of the game play-space is in a vacuum, and that's where most of the focus is / has been.

I'm with Red Iron Crown, on the stupid grin I got after hours of trial and error on an SSTO design, that let me put a 40t payload on a path to Duna, and then deorbit and parachute-land the now horribly off-balanced airframe (career mode, no mods: refund!)

[Kegereneku]

HarvesteR also said they would solve things like "Occlusion" which it the very code supposed to make possible for a part to "hide" behind/inside another.

So to me this address specifically what you are seeking.

But something I keep saying : KSP being unrealistic isn't always/just because "the devs don't know better", they do in fact know that if they made X or Y "minor feature" as realistic as some ask, it would not actually be as fun to play than what is said.

"You can have your cake and eat it, but do you really want to learn to cook for month to have it ?"

[Scoundrel]

Scoundrel, you are making a fuzz out of so little. Also you are being condescending just by saying "I don't want to be condescending".

I feel like you are taking this personally.

Nah, this is a game and I really don't take anything seriously. I was more concerned about the language barrier and that my words would be misconstrued as some sort of personal thing. I guess it backfired.

<snip>I still find "jet-engine overheat at high-speed", unintuitive, barely realist (only in the sense that it limit more than before) and since we are trying to solve the rocket-like climb this "open parameters" of yours still seem to risk being minmaxed/bypassed just like the pressure hog was.

It's exactly like rocket engines overheating, and players know that when an engine overheats you either throttle down or wait to see if your engine blows up. I'm literally suggesting using a mechanic that already exists in the game, that's all. A simple mod implementing heat at velocity would verify if my suggestion has any validity and would expose any potential exploits. Unfortunately I'm a writer not a coder, so I am naturally lazy and untalented, and thus obviously incapable of writing it myself.

You don't need that much speed to use the jet engine as a first stage.

You don't need any speed to use it as a first stage... the difference is that your ascent profile is different to maximize the amount of time you spend using jets, which lets you do amusing things in Kerbin's atmosphere, but beyond that they're dead weight if you spend any time outside of Kerbin's SOI. I actually find the aerospikes useless performance-wise (though I like that they're short), and that the 3.75m engines (either one) make using jet engines as a first stage pointless. The only reason I don't rant about those engines is because they're practically the end of the tech tree, so they should be good. Also NASA had input on those engines, so they'll likely be spared being nerfed.

MHD-cramjet-with-cryogenic-precooling (1 reputation point to whoever spot the absurdity in this tech)

When you say MHD scramjet I assume you're referring to an MHD bypass scramjet (as per Sheikin and Kuranov)? The absurdity then is that the MHD generator is upstream of the combustion chamber and the MHD accelerator is downstream... and since the specific heat for the flow has to remain constant for the Isp boost, precooling the flow completely negates the MHD. Actually, I don't think the MHD would function at all precooled... but I'm not a physicist.

So pardon me but "Why the hell" did you bicker on this ? We BOTH know spaceplane ascent take longer per their own nature !

Ah, okay now I understand what you meant. You were talking about ascent profiles, when I thought you meant that jet engines should spool up slower. My bad.

Anyway, now seem a good time to remind that since the aerodynamic model will change, with I quote "Wing that give lift", so the basic/turbojet might in fact get a thrust/weight nerf and the relation between heat and speed might change as a result.

Oh I suspect they'll be getting a nerf... the question is by how much, and if they fix the Isp issues (like actually making thrust vary with altitude rather than fuel consumption).

[camlost]

But something I keep saying : KSP being unrealistic isn't always/just because "the devs don't know better", they do in fact know that if they made X or Y "minor feature" as realistic as some ask, it would not actually be as fun to play than what is said.

Just in this specific case, how can we justify the mysterious "IntakeAir"? And who's dev really? I think the whole system was made by C7 as a mod.

[NathanKell]

And Chad (C7) asked for permission to fix it many a time, so let's not blame him.

As said earlier, rocket engine don't necessarily become better with altitudes. Hourglass shaped nozzle are typically shaped to work best (more thrust) at a given pressure (from Ground to Zero). So you have rocket engine that are best for lift-off and other best for space-travel.

You're confusing the design altitude of a nozzle with the optimal altitude of a nozzle. A rocket engine will *always* have its highest Isp in vacuum, because the reason for Isp ever being lower than that is because the air "pushes back" against the exhaust; when there's no pushback, it's most efficient.

For any given atmospheric altitude, and rocket engines identical except for nozzle bell, the one which is nearest its design altitude will be most efficient, but all will be more efficient in vacuum than they are at that altitude.

[NERVAfan]

The real problem is that stock turbojets are currently the best rocket engines for launching payloads less than 100 tonnes to orbit. Actual rocket engines can't compete, because turbojets have a high enough TWR and Isp around 40000 s. They should either be renamed to Magic Star Wars Engines, or their TWR should be nerfed to below any rocket engine and their fuel usage increased by 16x.

Yeah, exactly.

- - - Updated - - -

But something I keep saying : KSP being unrealistic isn't always/just because "the devs don't know better", they do in fact know that if they made X or Y "minor feature" as realistic as some ask, it would not actually be as fun to play than what is said.

In some things, I can see that, but I don't see why more realistic physics would make it less fun.

[Kegereneku]

Well, I am myself subject to the barrier language but your post is as understandable than any other. More than enough to actually discuss the subject.

Anyway, going with your latest precision, that's true that it IS just like rocket engine overheat. But if you put a limit to strict you deprive jet engine from the ability to perform the "normal" SSTO mission (slow climb, aerodynamic rocket, ...etc) we are trying to encourage.

I have nothing to add on this except my own suggestion in my sign.

And you did understood the MHD pre-cooling paradox right.

You can't really turn the airflow into frictionless ionized plasma if you are precooling it at -150°C

Just in this specific case, how can we justify the mysterious "IntakeAir"? And who's dev really? I think the whole system was made by C7 as a mod.

* GASP ! *

It would make the whole aerodynamic model the BIGGEST mods made stock !

Just joking,

though I do seem to remember that airplane were not initially planned by the devs. (I'm not sure on this)

About the "IntakeAir", using it as a pseudo resources isn't a bad thing, it is just one of numerous

Yeah, exactly.

- - - Updated - - -

In some things, I can see that, but I don't see why more realistic physics would make it less fun.

I'm afraid I can only say your view on this is biased.

Realism is an infinite scale, so more realistic physics will gradually but inevitably lead to a point where the game is "too complex" to be playable.

Because to give a few example :

- You can't actually pilot a plane manually at supersonic speed, that said not unless you turn the plane into a aerodynamically stable dart and by stable I mean "you can't steer"

- There's many different way of stalling and some of those called "deep stall" don't let you a chance to get out of it.

- Design-wise, a shape that is good to fly around, and a shape that can reach hypersonic speed is as far we know 2 different things (or you get forced into building something you didn't want to). "Realistically" the shape given by the game are not real NASA design nor tested in real hypersonic wind-tunnel.

Oh sure, you can in theory make fun and playable any starting level of realism. However you will necessarily have to do choice between fun and realism.

Imagine trying to fly the Skylon manually, the X-15 or Virgin Galactic Spaceship 2/3. (you can consider a given that if the aero model become too realistic people will need variable wing... and aerobrake to begin with)

I could continue for hours on this.

Unlike some EA game, SQUAD developers aren't lazy .... who use simplistic model that have already been tested by hundred other game (with actual Lab Test), KSP only seem "obvious" because they've done their job right.

Edited January 17, 2015 by Kegereneku

[NonWonderDog]

That's not much of an argument. I mean, the X-15 fly-by-wire system was literally called SAS. It did exactly what SAS does in KSP (although better tuned).

More generally, just try it! Install FAR and try it. It's not hard to fly a supersonic plane. It's not hard to make a plane that's flyable while supersonic and can pull 10 g turns. Or even pull 10 g turns while supersonic.

[camlost]

And Chad (C7) asked for permission to fix it many a time, so let's not blame him.

That's interesting, did he get any response?

[Scoundrel]

The spaceplane parts were originally a mod that Chad (and IIRC WinterOwl contributed a few parts as well) created back around the .13 era of KSP. Prior to the Aerospace Pack (which was the second mod to be made stock), I don't believe there were any form of aerodynamics at all (let alone orbital mechanics or an actual requirement for oxidizer).

Based on posts and podcast comments made by HarvesteR and C7 here and on other forums, Squad pushed aerodynamics back as it wasn't an urgent priority (likely since since FAR was available) compared to post-kerbin features that they needed to implement, and both HarvesteR and C7 assured us that once the game was scope complete that they would focus on fixing the aerodynamics and rebalancing the parts.

[juanml82]

Well, I am myself subject to the barrier language but your post is as understandable than any other. More than enough to actually discuss the subject.

Anyway, going with your latest precision, that's true that it IS just like rocket engine overheat. But if you put a limit to strict you deprive jet engine from the ability to perform the "normal" SSTO mission (slow climb, aerodynamic rocket, ...etc) we are trying to encourage.

I have nothing to add on this except my own suggestion in my sign.

The thing is, IRL you can't make an SSTO including jet engines.

And here's the deal with all the "realism" discussions. IMHO, there are people whose idea of "realism" ends up being "mods I like", be it FAR, DRE, TAC-LS or something else. But if we want to get into realistic air breathing engines, players shouldn't be able to make an SSTO with them, period. What we could see is to allow to change ships while in the atmosphere so carrier aircrafts can be done: a larger, air breathing aircraft carries a smaller ship to, say, 20,000 engines and release it. The larger aircraft continues flying to a secondary airfield halfway around Kerbin to land and recover. The smaller spaceship released from it ignites, say, an aerospike, and continues to orbit.

Or you have a Skylon type of design, composed of very short wings (so they have little weight) and Rapiers. And maybe, if you push it, something with aerospikes, ala the failed VentureStar.

The turbojet, realistically, could be replaced by a ramjet: an air breathing engine which doesn't work at less than match 0.5 and allows speeds around match 5. But a previous stage, say rocket assisted take off, is required to put such a spaceship into the air in the first place.

And yes, overheating the engines is a realistic approach to the problem, as that's what happens to real turbojets (and conventional airframes) if pushed to orbital speeds.

Now, this would change the game for SSTO lovers. I'm not even sure if large SSTOs would/should be possible.

[NonWonderDog]

The spaceplane parts were originally a mod that Chad (and IIRC WinterOwl contributed a few parts as well) created back around the .13 era of KSP. Prior to the Aerospace Pack (which was the second mod to be made stock), I don't believe there were any form of aerodynamics at all (let alone orbital mechanics or an actual requirement for oxidizer).

Based on posts and podcast comments made by HarvesteR and C7 here and on other forums, Squad pushed aerodynamics back as it wasn't an urgent priority (likely since since FAR was available) compared to post-kerbin features that they needed to implement, and both HarvesteR and C7 assured us that once the game was scope complete that they would focus on fixing the aerodynamics and rebalancing the parts.

We had orbital mechanics of a sort before we had a map screen, and well before we had time warp (if you consider "Kerbin as the center of the solar system" to be "orbital mechanics"). Of course, back then the rockets were a lot wobblier than today, could barely stand more than eight or ten tanks tall, and barely had the power to push themselves through the atmosphere. Making it to orbit was difficult until struts came along. Really the only thing the game had going for it was potential and Jeb's smiling face.

We're actually on Souposphere Mk 2, which I think I remember being driven by C7's mod. Souposphere Mk 1 was MUCH thicker.

Edited January 18, 2015 by NonWonderDog

[SaturnV]

I don't know how well real turbojet engine works, but in KSP currently jet engines are one-liter-fuel-to-anywhere-magic to me. I don't think that make sense.

[Scoundrel]

Of course, back then <nostalgia> the only thing the game had going for it was potential and Jeb's smiling face.

Ahhh yes, the good old days! Back when there were 5 quarters to the dollar and all the rocket parts were steam powered and KSP cost just 7 bananas. I remember trying to carefully position those fins (before symmetry!) on my lander so I wouldn't destroy the engine when I landed on the Mun... and I remember having to wait until the mun was on the horizon to make my burn just so I could get there. Makes me think they should have a map mode toggle for difficulty and you have to set maneuver nodes at KSC.

Yeah, my memory of which version did what isn't exactly the greatest as they all sort of blur together, so thanks for the clarification.

We're actually on Souposphere Mk 2, which I think I remember being driven by C7's mod. Souposphere Mk 1 was MUCH thicker.

I don't remember it that way, but I guess I never found getting to orbit as challenging as others did, and with all the time I spent on the Mun (and Duna when it came out... ohhh the heady days of discovery!) it likely didn't stick in my mind.

[NonWonderDog]

I don't remember it that way, but I guess I never found getting to orbit as challenging as others did, and with all the time I spent on the Mun (and Duna when it came out... ohhh the heady days of discovery!) it likely didn't stick in my mind.

Yeah, that was nostalgia from 0.7.3. I did manage to make it to orbit, once. It took a *lot* of boosters, and after that I basically considered the game beaten. (According to the wiki on 0.7.3: "It is nearly impossible to achieve orbit." Bah!)

The great atmosphere thinnenning happened well before the Mun, but my memory's hazy on when C7 aerospace started. A quick jump to the wiki says the atmosphere changed in 0.10.1. Before that it was basically 1 bar right up to 34500m, where it just ended.

[camlost]

The thing is, IRL you can't make an SSTO including jet engines.

And here's the deal with all the "realism" discussions. IMHO, there are people whose idea of "realism" ends up being "mods I like", be it FAR, DRE, TAC-LS or something else. But if we want to get into realistic air breathing engines, players shouldn't be able to make an SSTO with them, period. What we could see is to allow to change ships while in the atmosphere so carrier aircrafts can be done: a larger, air breathing aircraft carries a smaller ship to, say, 20,000 engines and release it. The larger aircraft continues flying to a secondary airfield halfway around Kerbin to land and recover. The smaller spaceship released from it ignites, say, an aerospike, and continues to orbit.

Or you have a Skylon type of design, composed of very short wings (so they have little weight) and Rapiers. And maybe, if you push it, something with aerospikes, ala the failed VentureStar.

The turbojet, realistically, could be replaced by a ramjet: an air breathing engine which doesn't work at less than match 0.5 and allows speeds around match 5. But a previous stage, say rocket assisted take off, is required to put such a spaceship into the air in the first place.

And yes, overheating the engines is a realistic approach to the problem, as that's what happens to real turbojets (and conventional airframes) if pushed to orbital speeds.

Now, this would change the game for SSTO lovers. I'm not even sure if large SSTOs would/should be possible.

What you described is basically AJE: the over heating ,ramjet, etc. SSTO is still easy on Kerbin, and something like Skylon is possible on Earth.

[DaMichel]

I like the realism of AJE. I recently managed to build a 4.5t lifter spaceplane using AJE and RF for stock Kerbin using mostly stock parts. But it is very tough to pull it off. The fuel consumption in air breathing mode is hard to predict. It needs lot of trial and error. Stock engines let you basically neglect this.

If i were Squad i would rescale Kerbin so 6.4 size and add some future tech stuff with fusion powered hybrid turbojets/rockets to make making orbit easy again. And of course make engines and atmosphere realistic.

Edited January 20, 2015 by DaMichel

[rynak]

I really don't think an entirely different model is neccessary to make jet and ramjets engines behave more intuitively. Would a diffferent model be neccessary to make it realistic? Yes. Do i care about making something realistic? Pretty low priority to me. Does it bother me when something behaves unintuitively, and doesn't actually do what the description says? Yes it does.

To put it simple, all that is missing, is an atmospheric pressure vs thrust curve. There is already a curve for speed vs thrust, and altitude vs ISP. Add the third mentioned curve, and almost everything can be approximated. (I.e. for ramjets, you could reduce the thrust at high atmospheric pressure. Et voila, there you have ramjets that don't work well at low altitude, but good at high alt and speed (the later can already be done via the speed vs thrust curve)).

P.S.: The reason i post in this thread so late, is because the topic title is retarded. It alone put me off from even reading the thread for days.

Edited January 20, 2015 by rynak

[Red Iron Crown]

To put it simple, all that is missing, is an atmospheric pressure vs thrust curve. There is already a curve for speed vs thrust, and altitude vs ISP. Add the third mentioned curve, and almost everything can be approximated.

If thrust rather than fuel flow scaled with Isp that would happen automatically without another curve.

[rynak]

If thrust rather than fuel flow scaled with Isp that would happen automatically without another curve.

But then you would lack a curve to scale ISP for designs that need ISP scaling - hence eventually you again would end up with a 3rd curve.