Jet engines are way overpowered

[rkman]

A craft that uses jet engines for (most of) the ascent from Kerbin can do a manned land-and-return mission to the Mun using a total amount of fuel roughly equal to the mass and volume of a single jet engine.

Along similar lines, a manned land-and-return mission to Duna and Ike is possible with a craft that weighs only a few tonnes.

http://imgur.com/a/Ra6H7#23

I think that means once jet engines are balanced, the entire ksp spaceplane business will be rather different than it is now.

[rtxoff]

I do not think the jet engines are the core problem with this. It's more like intakes can be mounted anywhere, you can have more intakes then 1 per engine and so on.

If something has to be corrected then this.

Also seriously, people contructing absurd flying machines abusing the game mechanics and then complaining about OP engines, no no and NO.

[coolitic]

Try doing this with FAR, it would be VERY hard.

This is because KSP's aerodynamics are a bit flawed, not because of the jet engines.

[rkman]

It's more like intakes can be mounted anywhere,

That certainly is a factor, but it sure helps that one can fly around the planet on just a few drops of fuel.

Also seriously, people contructing absurd flying machines abusing the game mechanics and then complaining about OP engines, no no and NO.

Yeah seriously, i did not construct those craft. And thanks or making my point for me: the game mechanic being abused is the OP jet engines.

Try doing this with FAR, it would be VERY hard.

With FAR it takes less delta-v to get to orbit. Makes it easier, not harder.

[regex]

Does the person who made those know that intakes don't need to point at the airflow? S/he could save some part count...

[CaptRobau]

While the craft above use every exploit in the book to do what they do, realistic looking SSTO planes are rather ridiculous are indeed very easy though compared to real-life. They have thrust-to-weight ratios similar to real rockets, which is very unrealistic (this allows them to be first stage lifters for medium-sized rockets, which is just ridiculous). The Turbojet also acts as a mix of a ramjet and a turbojet. It can lift off under its own power like a turbojet, but produce thrust up to a ridiculous Mach 7. Separating the two would solve this problem. If you want to go fast with an airbreathing engine, you'll first have to get it up to quite a decent speed before it starts producing meaningful thrust. Also jets are modeled as if they were rocket engines, even though they work quite differently. In reality all jet engines lose thrust as the air gets thinner in high altitude, while Isp remains the same. However in KSP they lose fuel efficiency not thrust. As a result you can easily reach Mach 3 with any jet engine if you climb high enough.

[Jouni]

There are many things wrong with the stock jet engines. One of the biggest problems is a bug in fuel usage calculations: jet engines use about 16x less fuel than they should, based on their specific impulses. Right now, jet engines are about 100x as fuel efficient as rockets, while they should be only 5-10x more efficient.

[Tex]

The result of the engines' supposed overpoweredness comes from a multitude of factors, the biggest of which is the builder's design. I would NEVER have attempted to use jet engines like they're used in the picture because my engineering mind doesn't work that way

I've always used jet engines in actual jet planes, not light landers like this. If used properly, then the engines are fine right now, but when exploited like this (and quite ingeniously, don't get me wrong) then that's where I start being a skeptic when the designer begins complaining.

[allmhuran]

I'm curious how the two on the left manage to have a girder attached to the top of a parachute.

It's probably also worth pointing out that none of these have command pods. Can you store science in a command chair?

[rtxoff]

That certainly is a factor, but it sure helps that one can fly around the planet on just a few drops of fuel.

Yeah seriously, i did not construct those craft. And thanks or making my point for me: the game mechanic being abused is the OP jet engines.

With FAR it takes less delta-v to get to orbit. Makes it easier, not harder.

Anyone wanting to abuse game mechanics go on and abuse it. It is the same as going to the debug menu turning on infinite fuel. Why is no body complaining about the debug menu then?

There where already discussions about this. It will be fixed at some point. Even if not it is not like that KSP has a competitive multiplayer mode or the like.

And again NO, the jet engines are not OP. And also all the landers in that picture must have clipped in rocket engines. Part clipping is clearly cheating even without all this OP babble.

For what should this crafts be good anyway? There are no pods, no science parts nothing on them only a command chair. This is sure not OP.

[TeeGee]

Try using Advanced Jet Engines mod to destroy the turbojet overpowered stats and make it function faaaaaar more realistically. Getting a spacecraft/plane into orbit via SSTO using turbojets is impossible, not without switching to a rocket at lower (much lower) altitudes regardless of intake number.

Turbojets can only max out at mach 4.5 at 13000 meters (with FAR), and faster than that the compressor and fans burn up and the engine explodes unless you turn it off.

I LOVE AJE as it forces me to use rockets as opposed to using jets to get all my spacecraft into orbit.

[BudgetHedgehog]

it sure helps that one can fly around the planet on just a few drops of fuel.

I'd like to see some evidence, please. Granted, the turbojets ISP of 2500s is a little bit higher than most jets which are around 2000 but it's still not unreasonable (EDIT: looking at modern day turbojets, the upper limits of the Isp seem to be around 3000s or so, so I'd say 2500s is entirely acceptable). But yes, I'd like to see you fly around the planet on a few drops of fuel. And you are aware that long-haul real life non-stop flights from Sydney to Dallas (a journey of 13,804 km) are not only possible but frequent, right? And that Kerbin has a radius of 600km? So to travel completely around Kerbin is 1885 km, which is, you'll notice, a lot less than 13,804. Even considering stocks soupy atmosphere, it's completely believable.

Yeah seriously, i did not construct those craft. And thanks or making my point for me: the game mechanic being abused is the OP jet engines.

No, the game mechanic being abused is the unrealistic atmosphere. The turbojets, as I just pointed out, are fine. The fact that the pictured craft can fly at all points to the atmosphere being unrealistic. Try flying those with FAR, you won't make it past 10km. But that aside, the jets are still fine. The only thing wrong in the game is the fact that air and fuel can flow through parts whose mass and drag aren't factored into any equation and that aerodynamics don't affect the crafts - only the mass and available IntakeAir do.

As a though experiment, consider a turbojet with one intake. Put a chair on it with some fuel. Guess how far and fast you'll be able to go. Hint: It's pretty far. This is my biggest pet peeve when people complain about OP parts - they intentionally misuse them. You can make a 3 part SSTO rocket with a probe core, Jumbo-64 fuel tank and a Skipper. Is the Skipper OP? You can easily escape the Suns gravity using the biggest SRB and a chair. What on earth did you expect to happen..? You have 650kn of thrust pushing about 200kg. It's like putting a Formula 1 engine in a Mini and then saying it's too powerful and should be nerfed.

With FAR it takes less delta-v to get to orbit. Makes it easier, not harder.

You're correct, but that's only if your craft is aerodynamic. As I said, the pictured craft wouldn't reach 10km with FAR because they're extremely unaerodymanic. FAR is not a magic 'less-dV-to-orbit' mod, it rewards you for making believable rockets and punishes you severely if you don't. FAR would, for the pictured crafts, make getting to orbit extremely difficult. Hell, I would give £50 to anyone who could do it. Swear down, right now.

Edited June 1, 2014 by ObsessedWithKSP
Since I asked for links, it's only fair I provide them too

[Dodgey]

Seeing as there are only two bodies on which to use aircraft I think they are fine. As has been said the atmosphere is at fault. Not the engines.

[Jouni]

I'd like to see some evidence, please. Granted, the turbojets ISP of 2500s is a little bit higher than most jets which are around 2000 but it's still not unreasonable. But yes, I'd like to see you fly around the planet on a few drops of fuel. And you are aware that long-haul real life flights from Dallas to Sydney (a journey of 13,804 km) are not only possible but frequent, right? And that Kerbin has a radius of 600km? So to travel completely around Kerbin is 1885 km, which is, you'll notice, a lot less than 13,804. Even considering stocks soupy atmosphere, it's completely believable.

Turbojets would be mostly fine, if their Isp were 2500 s. In practice, their Isp is around 40000 s, because there's a bug in KSP fuel calculations that causes airbreathing engines to burn 16x less fuel than they should.

There are three reasons why passenger jets can fly over 15 hours without refueling:

1. Long-haul jets carry a lot of fuel, often over 40% of their takeoff mass.
   
2. Subsonic jets don't need that much thrust. The TWR of a fully loaded passenger jet is usually less than 0.3, and 100% thrust isn't usually used even for takeoff, as it shortens engine life. During cruise, actual TWR falls well below 0.1.
   
3. Subsonic turbofans are fuel-efficient, with typical Isp figures reaching 6000 s during cruise.
   

[BudgetHedgehog]

Turbojets would be mostly fine, if their Isp were 2500 s. In practice, their Isp is around 40000 s, because there's a bug in KSP fuel calculations that causes airbreathing engines to burn 16x less fuel than they should.

I've seen others mention this before but I've yet to see the source of it. Could you provide it? EDIT: And if that statement is true, the jets still aren't OP when their given values are taken into account. The fault lies in a bug in KSP code, not the jets efficiency.

There are three reasons why passenger jets can fly over 15 hours without refueling:

Long-haul jets carry a lot of fuel, often over 40% of their takeoff mass.

And my SSTO plane has a fuel fraction of 50%. What point are you making here? Yeah, you need a boat-load of fuel to go far, I'm not denying that (as that screenshot will show you).

Subsonic jets don't need that much thrust. The TWR of a fully loaded passenger jet is usually less than 0.3, and 100% thrust isn't usually used even for takeoff, as it shortens engine life. During cruise, actual TWR falls well below 0.1.

I'm not doubting that. Yes, my plane has a takeoff TWR of 1.1 which is entirely unrealistic/unnecessary, but if it was heavy enough (24 tonnes vs 251 tons, hmmm....), then yeah, the take off TWR would go down. What I'm trying to say is that with 4 turbojets on a 250t plane with >40% fuel fraction is possible in KSP.

Subsonic turbofans are fuel-efficient, with typical Isp figures reaching 6000 s during cruise.

Yes, turbofans are ridiculously efficient. However, I'm talking about turbojets and KSP doesn't have turbofans. Why bring them up? I could counter argue that ion drives are even more efficient, but that has nothing to do with the facts at hand. We're talking about turbojets, not turbofans, here.

Edited June 1, 2014 by ObsessedWithKSP

[Jouni]

I've seen others mention this before but I've yet to see the source of it. Could you provide it? EDIT: And if that statement is true, the jets still aren't OP when their given values are taken into account. The fault lies in a bug in KSP code, not the jets efficiency.

Try it yourself. The basic jet engine has 150 kN thrust and Isp 2000 s at sea level, and it quickly accelerates to almost full thrust. Build a test vehicle with a probe core, two Mk1 fuselages, a basic jet engine, some intakes, and a launch clamp. The weight of the 300 units of fuel in the fuselages is about 10% of the thrust, so the engine should spend around 200 s (10% of its Isp) to burn it. In practice, the fuel lasts almost for an hour.

With this bug fixed, we would see quite different spaceplanes. A single turbojet might use 1000-1500 units of fuel during the ascent, requiring much bigger fuel tanks and reducing payload fraction significantly.

Yes, turbofans are ridiculously efficient. However, I'm talking about turbojets and KSP doesn't have turbofans. Why bring them up? I could counter argue that ion drives are even more efficient, but that has nothing to do with the facts at hand. We're talking about turbojets, not turbofans, here.

You compared the operating range of KSP jets to real-world long-haul jets. I mentioned the main reasons why they can achieve it, while supersonic KSP jets should not. Basically, the operating time is Isp * fuel fraction / TWR.

[Moss]

I think there's a bunch of things to consider.

Firstly, the engines need to be powerful enough so that newer players can build functioning aircraft.

Secondly, the stats don't seem unreasonable to me. I've heard that the rocket parts are underpowered compared to their real life counterparts to compensate for the fact that the Kerbol system is really small, and I think the jet engines aren't scaled down as much.

Thirdly, and most importantly, the reason players can construct Mach 7 upper atmosphere super-planes is because KSP doesn't make engines overheat despite the fact that the intake air would be travelling at supersonic speeds. This allows them to travel at the speeds they do. Personally, I think they should implement jet overheating that would need to be addressed with a new Pre-Cooler part. (Maybe find a use for the Radial Engine Body?)

[BudgetHedgehog]

Try it yourself. The basic jet engine has 150 kN thrust and Isp 2000 s at sea level, and it quickly accelerates to almost full thrust. Build a test vehicle with a probe core, two Mk1 fuselages, a basic jet engine, some intakes, and a launch clamp. The weight of the 300 units of fuel in the fuselages is about 10% of the thrust, so the engine should spend around 200 s (10% of its Isp) to burn it. In practice, the fuel lasts almost for an hour.

With this bug fixed, we would see quite different spaceplanes. A single turbojet might use 1000-1500 units of fuel during the ascent, requiring much bigger fuel tanks and reducing payload fraction significantly.

As I said, in this case, the bug lies with a fault in the KSP code, not the values given by the turbojets. A turbojet with an Isp of 2500s is comparable to a real life turbojet. The fact that they may be OP isn't down to that, but it is down to the fact the game has a bug concerning their fuel usage. It is not the fault of the turbojets and their given stats.

[Jouni]

As I said, in this case, the bug lies with a fault in the KSP code, not the values given by the turbojets. A turbojet with an Isp of 2500s is comparable to a real life turbojet. The fact that they may be OP isn't down to that, but it is down to the fact the game has a bug concerning their fuel usage. It is not the fault of the turbojets and their given stats.

Let's compare the KSP turbojet to real-world turbojets.

The KSP turbojet produces 225 kN of thrust with Isp 2500 s. It weights 1.2 tonnes and operates at over 2000 m/s at altitudes over 30 km.

The Olympus 593 used in the Concorde produced 140 kN of thrust (169 kN with afterburner) with Isp 3000 s. It weights 3175 kg, and it was designed to operate at 600 m/s at 18.3 km (10.8 km in KSP).

The J58 used in the SR-71 produced 150 kN of thrust with Isp 1900 s. It weights 2.7 tonnes, and it was used at 980 m/s at 25.9 km (15.2 km in KSP).

As a comparison, KSP rocket engines are generally heavier, weaker, and less efficient than real rocket engines of the same size.

[piro]

Try it yourself. The basic jet engine has 150 kN thrust and Isp 2000 s at sea level, and it quickly accelerates to almost full thrust. Build a test vehicle with a probe core, two Mk1 fuselages, a basic jet engine, some intakes, and a launch clamp. The weight of the 300 units of fuel in the fuselages is about 10% of the thrust, so the engine should spend around 200 s (10% of its Isp) to burn it. In practice, the fuel lasts almost for an hour.

Your calculation doesn't account for the units of intake air from the atmosphere. Which adds to the fuel flow?

[Jouni]

Your calculation doesn't account for the units of intake air from the atmosphere. Which adds to the fuel flow?

That's precisely the bug I was talking about. KSP takes the air flow into account, when it shouldn't. Passenger jets with Isp 6000 s would be able to fly for a week without refueling, if that 6000 s included both fuel and air.

[rkman]

Anyone wanting to abuse game mechanics go on and abuse it. It is the same as going to the debug menu turning on infinite fuel.

Why is no body complaining about the debug menu then?

Because the debug menu is a dev tool, not a game mechanic.

Which why the debug menu will not be fixed, but the jet engines presumably will.

Even if not it is not like that KSP has a competitive multiplayer mode or the like.

Game balance is relevant not only to competitive multiplayer.

It will be fixed at some point. And again NO, the jet engines are not OP.

So then why will it be fixed?

[Red Iron Crown]

That's precisely the bug I was talking about. KSP takes the air flow into account, when it shouldn't. Passenger jets with Isp 6000 s would be able to fly for a week without refueling, if that 6000 s included both fuel and air.

I think you may have it backwards. Including intake air is realistic, as it is reaction mass used to propel the vehicle. It's the absurdly high Isp that's unrealistic. An engine with an Isp of 6000s would have an effective exhaust velocity of about 60,000m/s or over 200,000 km/h, which I suspect is a bit on the high side.

[Random Tank]

So then why will it be fixed?

He probably means the bug with their ISP being higher than it should be. But, to be honest, all the aircraft parts need changing...

The RAM intake is better than any other, when it should be different (no suction); the precoolers do nothing when they really should; intakes can be placed anywhere, so airflow restrictions should be added; there is only one size of landing gear, there should be more and they should be powered; wings are too small and provide weird amounts of lift, they should be bigger or have varying sizes (or just procedural would make sense); most of the part models look very un-aerodynamic and crap (and low res), they should be changed; and don't even get me started on the flight physics and drag model...

I know mods can fix most of these, but honestly they shouldn't have to; this is basic stuff of making a plane. And I'm not asking for super realism, as I think that can make things too hard or boring, but I just want planes to be.... not dumb! At the moment jet engines are just a rocket where you don't have to bring LOX, and then a plane is 'let's give my rocket wings so it uses less fuel'

Though OP: try and re-create those little 'rockets' on your post with FAR installed and without using Alt + F12... You'll find it quite impossible!

[Jouni]

I think you may have it backwards. Including intake air is realistic, as it is reaction mass used to propel the vehicle. It's the absurdly high Isp that's unrealistic. An engine with an Isp of 6000s would have an effective exhaust velocity of about 60,000m/s or over 200,000 km/h, which I suspect is a bit on the high side.

Specific impulse is defined in a way that only takes the change of vehicle mass into account. The numbers actually go higher than 6000 s: typical turbofans have Isp 12000 s at sea level. Of course, people usually talk about specific fuel usage (grams of fuel per kilonewton-second) instead of specific impulse, because it makes more sense with jet engines.