NathanKell Posted May 16, 2014 Share Posted May 16, 2014 After giving the Twin Wasp its real stats (RPMs were way off, e.g.) I had almost perfectly correct performance for the Wildcat. I'm therefore hopeful that by correcting engines' stats we'll get (close-to-real) prop-plane performance. Link to comment Share on other sites More sharing options...
camlost Posted May 16, 2014 Author Share Posted May 16, 2014 After giving the Twin Wasp its real stats (RPMs were way off, e.g.) I had almost perfectly correct performance for the Wildcat. I'm therefore hopeful that by correcting engines' stats we'll get (close-to-real) prop-plane performance.In v1.1.1 I added a default speed buff of 1.3x, meaning at 130m/s you get thrust of 100m/s. What did you change? The omega0 and omega are RPM of propellers, the engine RPM are actually omega/gearratio. Is it way off? I did not have time to test every part for FS Link to comment Share on other sites More sharing options...
Tidus Klein Posted May 18, 2014 Share Posted May 18, 2014 Hay Camlost quick question does this make props have higher thrust in worlds with a higher air density? as in if i send a prob plane to eve using the KAX electric prop will it produce more power due to the atmosphere being thicker?PS i got version 1.1 to work for me, it was something in the B9 MM cfgs that caused the game not to load parts. Link to comment Share on other sites More sharing options...
camlost Posted May 18, 2014 Author Share Posted May 18, 2014 Hay Camlost quick question does this make props have higher thrust in worlds with a higher air density? as in if i send a prob plane to eve using the KAX electric prop will it produce more power due to the atmosphere being thicker?PS i got version 1.1 to work for me, it was something in the B9 MM cfgs that caused the game not to load parts. Of course they will have higher thrust, but the relationship is not simply linear. I just realised that there's a problem with the density calculation where I ignored the difference of MW of different planets. The best way to do is to use FAR's code directly but this will make AJE dependent on FAR. Expect a hotfix soon. Link to comment Share on other sites More sharing options...
NathanKell Posted May 18, 2014 Share Posted May 18, 2014 In v1.1.1 I added a default speed buff of 1.3x, meaning at 130m/s you get thrust of 100m/s. What did you change? The omega0 and omega are RPM of propellers, the engine RPM are actually omega/gearratio. Is it way off? I did not have time to test every part for FSYeah, at first I goofed about RPM, but that's corrected now. But the prop size usually needed work (given in the file often in meters rather than the required feet) and the RPMs were still pretty low, especially since omega should be set off RPM at WEP rather than normal full throttle. (Same for HP, but you do seem to be setting HP off WEP rather than normal max for some of the engines).Also, is it really safe to do that multiplication in OnStart? I would be worried that it might be called twice on the same part... Link to comment Share on other sites More sharing options...
camlost Posted May 18, 2014 Author Share Posted May 18, 2014 Also, is it really safe to do that multiplication in OnStart? I would be worried that it might be called twice on the same part...Hasn't noticed anything wrong yet, but good to know. Link to comment Share on other sites More sharing options...
NathanKell Posted May 19, 2014 Share Posted May 19, 2014 I haven't noticed any issues either, but I'm paranoid about KSP's inconsistencies in calling that sort of method. Link to comment Share on other sites More sharing options...
MAKC Posted May 21, 2014 Share Posted May 21, 2014 (edited) Is it possible to simulate SCRamjets in EngineSim? Edited May 21, 2014 by MAKC Link to comment Share on other sites More sharing options...
camlost Posted May 21, 2014 Author Share Posted May 21, 2014 No. NASA's program assumes subsonic combustion, and nozzle exit = mach 1 Link to comment Share on other sites More sharing options...
Taverius Posted May 21, 2014 Share Posted May 21, 2014 More importantly, you can't simulate without data, and all SCRamjet hard data is top secret.They're also ... well, not very good. Very High weight, Very High fuel consumption, Very Low thrust, and you have to build the plane around the engine to make it work at all. Link to comment Share on other sites More sharing options...
camlost Posted May 21, 2014 Author Share Posted May 21, 2014 More importantly, you can't simulate without data, and all SCRamjet hard data is top secret.They're also ... well, not very good. Very High weight, Very High fuel consumption, Very Low thrust, and you have to build the plane around the engine to make it work at all.Exactly. But I think a numerical model might serve the purpose. Based on data from the papers, postulate a thrust-Mach curve and make thrust also proportional to dynamic pressure. I think that's the best way to do SABRE and LACE too.@Nathankell and others, Seems there's a point we've overlooked in jet engines. Public data often cite "max thrust", which corresponds to a designed optimal speed at sea level. For high bypass turbofans that is 0, so we are good. But for low bypass turbofans and turbojets that speed is usually at high subsonic or supersonic region. That explains why we cannot match dry/wet thrust and SFC, because they are not what's measured on the ground.Also that means all the military jets are 20-40% overpowered. I've re-calibrated most of them, but no actual test on all of them yet Link to comment Share on other sites More sharing options...
NathanKell Posted May 21, 2014 Share Posted May 21, 2014 Um, pretty sure I've been using max *static* thrust, dry/wet. Link to comment Share on other sites More sharing options...
camlost Posted May 21, 2014 Author Share Posted May 21, 2014 Then how to calibrate sfc? They change with mach number. Link to comment Share on other sites More sharing options...
NathanKell Posted May 21, 2014 Share Posted May 21, 2014 SFC is given for static thrust, usually. Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 Are you accounting for the fact that in real life, Isp is based on the mass flow of fuel (and only fuel), but in KSP, Isp is based on the mass flow of all resources being used by the engine module? In stock KSP since the engines count the mass flow of air into the Isp calculations they end up being 16 times as efficient as they claim.You're right that in real-life, ISP only is based on fuel-flow, but for jet engines you need to look at the more relevant parameter of "Effective Exhaust Velocity".Cutting to the chase, because thrust *IS* produced by accelerating the air that passes through a jet engine, as well as by the expulsion of the fuel itself, and it requires exponentially less energy to produce the same thrust with an increasing mass of propellant, what you get is an Effective Exhaust Velocity of 29,000 for a turbofan engine in real-life.http://en.wikipedia.org/wiki/Specific_impulse#ExamplesWhat that means is, a turbofan engine produces thrust consuming fuel at the same rate as a rocket engine with an ISP of 29,000 would in vacuum. This is due to the increased efficiency from propelling inert air (the 80% that isn't oxygen) out the turbofan engine.The stock game has it pretty close to correct. The stock "turbojet" engine (which is actually a turbofan engine, as internals clearly indicate) has a listed ISP of 2500, and an Effective Exhaust Velocity of 40,000 after the "bug" with using IntakeAir in the thrust calculation.Real-world turbojets have an ISP of 3000 (just check the Wikipedia link above), and an Effective Exhaust Velocity of 29,000. So, the stock turbojets are only 37.9% more efficient than a basic turbofan- and the difference can easily be accounted for if they are high-bypass...I always chuckle a little inside when KSP players, in the long, arduous pursuit of "realism" actually manage to create something less powerful or useful than the real-world version, simply because they feel something in the stock game is "overpowered".TL;DRThe effective ISP of stock turbojets is actually correct. Real world turbofans draw on their fuel reserves as if they had an ISP of 29,000 or more- so the effective value of 40,000 in stock gameplay isn't at all out of the realm of real-world high-bypass turbofans...Regards,NorthstarP.S. The mod creator might also want to check his sources for thrust values. As was suggested earlier, the dry thrust values are based on older engine designs- newer engines (the ones that aren't still classified) outperform the thrust values he nerfed the various engines down to... Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 (edited) Before you go off an a grandstand about how the stock engines last too long, however (they do, that's true), and can travel too far (that's a distinct possibility- though see my note on scale below), and therefore MUST have unrealistically-high ISP (they don't), consider these three factors:(1) You might be comparing performance on stock Kerbin, or some other Earth-analog that is at a miniature scale (anything less than a 100% scale Real Solar System install, basically). On such a small planet, orbital velocity near sea level is a measly Mach 7 or so... So any supersonic aircraft you build is going to reach a *SIGNIFICANT* fraction of orbital velocity- which means it's effectively in a partial state of free-fall, and experiencing less gravity than a comparable-speed aircraft would in real life.(2) Stock parts are 64% scale of those in real life. Which means that, if an engine has precisely the same thrust as the real-world engine it is imitating, it has 144.14% more thrust than it ought to (I am making the gross assumption thrust should be proportional to cross-sectional area. A real-world engine has 244.14% the cross-sectional area of a 64% scale miniature). Forget what I said a second ago about the mod creator using the wrong thrust values- because the thrust values he should be using should be less than half of the real-world versions due to the smaller scale.(3) Stock parts *DO NOT* imitate real-world engines. They often exceed the thrust of real-world versions, despite being 64% scale miniatures...I think it becomes rather obvious the reason stock engines out-perform their real-world analogs is due to a combination of insanely-overpowered Thrust-Weight-Ratio ratings, and lower orbital velocity on Kerbin than on Earth. Don't also forget that if a jet plane can circumnavigate Kerbin, this is only the same distance as flying about 10% of the way around Earth... (So when you hear people complaining they can circumnavigate Kerbin 3 times with their long-range aircraft at high altitudes, and that is "overpowered", remember that's still only the equivalent of flying 30% of the way around Earth: with space-grade materials and engineering...)Regards,Northstar Edited May 21, 2014 by Northstar1989 Link to comment Share on other sites More sharing options...
NathanKell Posted May 21, 2014 Share Posted May 21, 2014 Or you could, y'know, read the thread, and find out that they "last too long" because of an issue in KSP's code where the mass of air is counted in the fuel flow equation for Isp, therefore an engine with a 1:16 fuel:air ratio (as stock KSP jets have) will have effectively 16x the Isp it should, since jet Isp in real life does not include airflow (since, y'know, it's not fuel). Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 (edited) Yes but the fundamental problem in KSP is that we are using specific impulse to define a mass flow rate rather than the reverse. We also have the problem that while there are different definitions for exhaust velocity (effective and actual exhaust velocity) there is no such distinction for specific impulse - KSP hasn't really got its definitions right. I would say the solution is to simply divide the atmosphere curve values by the intake air:fuel ratio then perform a corrected isp calculation and write that to the realIsp field so that the player sees the correct value.Actually, none of this is necessary. The stock rating for ISP are correct. A "turbojet" (turbofan according to game internals) engine has a listed ISP of 2500, for instance. This is not at all unrealistic- the example table for ISP's on Wikipedia lists a turbofan's ISP as being 3000:http://en.wikipedia.org/wiki/Specific_impulse#ExamplesIt gets better though, because the inclusion of IntakeAir in generating propulsion (thrust) is actually quite accurate. This is everything I was saying before about "Effective Exhaust Velocity" being the term you really need to look at...In real life, a turbofan engine propels the entire mass of both fuel and intake air, just like it does in the stock game. Because it takes exponentially less energy to produce the same thrust with a greater mass of propellant, you actually get a term known as "Effective Exhaust Velocity"- which is defined as the exhaust velocity that would be required to produce the same thrust in a vacuum.Wikipedia sums it up nicely as follow- capitalization added:"For air-breathing jet engines, particularly turbofans, the actual exhaust velocity and the effective exhaust velocity are different BY ORDERS OF MAGNITUDE. This is because a good deal of additional momentum is obtained by using air as reaction mass. This allows for a better match between the airspeed and the exhaust speed which saves energy/propellant and enormously increases the effective exhaust velocity while reducing the actual exhaust velocity."We know from our previous testing that the turbojets are also seriously overpowered in terms of thrust as well as having dodgy velocity curves that stealthily affect the isp and that both need to be corrected before we have a realistic fix but I presume this mod can handle both of those changes.You are absolutely correct about the thrust being overpowered (by at least 3-4 times what it should be), and the velocity curves being dodgy though.No change is necessary to the stock ISP calculations, or inclusion of IntakeAir in the thrust system. It approximates real-world Effective Exhaust Velocity closely enough (once again: 40,000 m/s effective exhaust velocity is entirely within the realm of possibility for real-world engines...) The main problems with jet engines in this game, is that they produce more thrust than their real-world equivalents, while actually being much smaller and lighter; and the planets they operate on in the first place are much smaller than their real-life analogs.If you want to pursue realism, cut the thrust of the jet engines to about 1/4th their current values. The ISP calculations should be left alone though- they're quite accurate...Regards,NorthstarP.S. The stock fuel-density (too low), fuel-capacities (much too high for their size), and fuselage weight (much too low for their strength) are also screwy. This may also be responsible for the perception that stock jet engines have too much endurance...P.P.S. Although I can tell you the ISP values and calcuation system in the stock game are more-or-less correct, it's been a *LONG* time since I performed an endurance-test with jet fuel-burning engines, having long ago switched almost exclusively over to Thermal Turbojets for their much greater endurance and ability to operate on Duna. So I couldn't tell you how the range/endurance of approximate replicas of real-life planes would match up given all the differences in TWR, mass values, orbital velocity, and even drag ratios (64% scale craft necessarily have more drag compared to volume relative to their full-sized versions- as they aren't as long, and have a worse surface area:volume ratio) Edited May 21, 2014 by Northstar1989 Link to comment Share on other sites More sharing options...
Taverius Posted May 21, 2014 Share Posted May 21, 2014 (edited) The thrust is overpowered for a good reason - it has to fight against the stock drag model. Edit: oh and the runway being much shorter than whats actually used for for airliners and will be used for SSTOs if we ever make some.Taking off in KSP is much harder than it should be, if you give engines realistic thrust values with stock drag you never lift off before the end of the strip. Edited May 21, 2014 by Taverius Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 (edited) The thrust is overpowered for a good reason - it has to fight against the stock drag model. Edit: oh and the runway being much shorter than whats actually used for for airliners and will be used for SSTOs if we ever make some.Taking off in KSP is much harder than it should be, if you give engines realistic thrust values with stock drag you never lift off before the end of the strip.You may just have a point- but I'd rather see an (accurate) nerf to the thrust values than an (inaccurate) nerf to the ISP values...Or rather, to the "Effective Exhaust Velocity". Though this term is never used in-game, when you start including the IntakeAir as propellant like the stock EngineModule does, you are effectively (and reasonably accurately) re-creating this essentially important real-world mathematical quirk of jet engines that makes them so effective...Once again:Real-world (example) turbofan Specific Impulse: 3000 sReal-world (example) Effective Exhaust Velocity: 29,000 m/sKSP turbofan Specific Impulse: 2500 sKSP Effective Exhaust Velocity: 40,000 m/sSource: http://en.wikipedia.org/wiki/Specific_impulse#ExamplesThe Effective Exhaust Velocity is the value that actually determines fuel-efficiency, and can be increased by increasing the bypass ratio (at the expense of increased drag) of a turbofan engine in real-life; and as a consequence, a value of 40,000 m/s is well within the range of what can actually be achieved...Regards,Northstar Edited May 21, 2014 by Northstar1989 Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 (edited) Forget everything I just wrote. MAJOR math fail on my part. I forgot to factor in G0 in my calculations (this is why you should always double-check your math folks!)The stock turbojet/turbofan engine actually has an Effective Exhaust Velocity of 392,000 m/s- far beyond the realm of what's possible with real turbofan engines...A 16:1 downgrade to fuel-efficiency (by fixing the IntakeAir "bug") would still be inappropriate though. An appropriate downgrade would only be 13:1 or 14:1, otherwise you significantly under-perform real-world turbofan engines...Regards,Northstar Edited May 21, 2014 by Northstar1989 Link to comment Share on other sites More sharing options...
Guest Posted May 21, 2014 Share Posted May 21, 2014 I'd say, fix the IntakeAir bug and overhaul jets to real stats... which is exactly what AJE does. Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 Or you could, y'know, read the thread, and find out that they "last too long" because of an issue in KSP's code where the mass of air is counted in the fuel flow equation for Isp, therefore an engine with a 1:16 fuel:air ratio (as stock KSP jets have) will have effectively 16x the Isp it should, since jet Isp in real life does not include airflow (since, y'know, it's not fuel).I specifically addressed that "issue with the code" several times... Stop accusing me of not reading things- I've probably spent more time reading through this thread today than you have.As I stated, the most relevant term is "Effective Exhaust Velocity", not ISP. And airflow *DOES* count for that.However, I missed in my conversions that Specific Impulse in-game was given in units of time (seconds), whereas I was treating it as being in terms of units of velocity (meters/second).Both units are equally valid measures of ISP, and indeed both are often used (and interconverted as necessary), but I made the mistaken of not noticing which was being used in the game, hence statements like:"a turbofan engine produces thrust consuming fuel at the same rate as a rocket engine with an ISP of 29,000 would in vacuum"That statement is 100% true, if ISP is given in terms of units of velocity (m/s). However it is not true if comparing Effective Exhaust Velocity (in meters/second) and ISP given in terms of units of time (seconds) as I was...It looks like the use of IntakeAir in the code was a problem after all, since the numbers really don't work out... I acknowledge that- though I must point out you guys may have over-nerfed the engines by cutting out the airflow factor from fuel flow calculations entirely... Their Effective Exhaust Velocity was 13.52 times what it should be, not 16 (when comparing the value for the stock turbofan to the value listed on Wikipedia...)Regards,Northstar Link to comment Share on other sites More sharing options...
Northstar1989 Posted May 21, 2014 Share Posted May 21, 2014 I'd say, fix the IntakeAir bug and overhaul jets to real stats... which is exactly what AJE does.No, that's NOT what AJE does, that's precisely the problem.It fixes the IntakeAir bug, but doesn't upgrade the ISP values to match their real-world counterparts, and doesn't adjust the thrust values appropriately...First of all, the thrust values are being adjusted to match outdated specs that haven't been accurate for over 30 years in some cases- newer turbofans have higher thrust values than the ones being matched to.Second, there may be some issues related to the 64% scaling. Either the mass values are also scaled down (as I initially thought), or the stock game is using real-world mass values with a 64% volume scale- leading to rocket/plane parts, like the planet Kerbin itself, being excessively dense (the oceans are denser that Neutronium!)If the mass values are scaled-down, then the thrust values need to be as well. If not, then they still need to match to the newest available values for thrust (and perhaps exceed them by a bit- the performance capabilities of cutting-edge military and space-grade hardware are often kept secret, or understated...)Regards,Northstar Link to comment Share on other sites More sharing options...
Guest Posted May 21, 2014 Share Posted May 21, 2014 Remember it's meant for "realism-suite". Without FAR, it's barely usable, and since the engine configs are taken right off real ones, it only makes sense to compare it with RSS. The engines are, plain and simple, modeled after real ones, there's no "kerbalizing" going on, which is nice. Also, AJE does adjust the values to real engine stats. It's just that those engines it simulates are old (at least, stock ones are converted to old engines). It's best used with jet engine mods like B9 that add newer engines. Link to comment Share on other sites More sharing options...
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