A new paradigm for air-breathing engines

[pincushionman]

I find it difficult to build and fly believable jet powered aircraft in KSP, for a couple of reasons. And both of them are in how jet engines are built in the game.

This is about the simplest jet engine nacelle one can build:

This is the shock cone intake (the heaviest intake), a full Jet Fuel Fuselage, and a Basic Jet engine (the lightest engine). Notice how far aft the center of mass of this nacelle is, even when the tank is full. It moves even further aft, obviously, as fuel is drained. Engines are the heaviest single component on KSP jets, and this is not dissimilar to the real world. But because the mass is concentrated in the "engine" component here, this causes problems - the weight of the entire aircraft is unrealistically shifted aft. In the real world, the center of mass of such a nacelle would be close to the center of this assembly, regardless of fuel load.

As we all know, aircraft are very sensitive to the fore-aft location of the center of gravity relative to the center of lift on the main wing. As a result, it is far easier to build canard-style aircraft (main wing behind the horizontal stabilizer) than it is to make traditional aircraft, with the main wing ahead of the horizontal stabilizer. If it's a single-engine plane, the jet must be at the very tail end, and there are no good massive components to move the center of mass ahead of the main wing, resulting in planes where the main wing is at the back. If it's a double-engined plane with the engines on the main wing, the engine weight will still be aft of the main wing's center of lift. We still have the limitation of no good massive components to move the CoM forward, so we still end up with planes with wings at the back. I've seen pictures of airplanes that fight this tendency to a greater or lesser degree (usually large ones), but I've yet to come up with a satisfactory solution myself.

Also, we have no propellers. All we have at our disposal are high-power jets. Which may be fine for always making spaceplanes, but for actual aircraft to explore Kerbin, it's a real constraint.

So I propose a new paradigm for designing and building aircraft.

Right now, you need three components for your jet: (1) engines, (2) fuel tanks, and (3) air intakes.

In my proposed system, you instead need (1) engine cores, (2) fuel tanks, (3) outputs, and (4) air intakes. This is a little more complicated, but may or may not require any extra parts, as I discuss below.

Engine cores: This is the component that engines are built around. They take LiquidFuel and IntakeAir, and produce a resource called EnginePower. These are now the heaviest components (usually) in the engine chain, but since they're in the center of the engine chain instead of the business end, we now have much greater flexibility in fore-aft placement, resulting in greater control of our aircraft designs. These usually look no fancier than the fuel tank in the picture. They may or may not also contain fuel, depending on model. They come in various types:

. Piston engines: Smaller, lighter, cheaper cores found earlier in the tech tree, but are less powerful then their turbine cousins. Do not suffer from turbo lag.

. Gas turbines: More advanced cores, offer more power for their weight and size, and can operate at faster speeds and higher altitudes.

. Electric motors: Exactly what it says on the tin. Uses ElectricCharge instead of LiquidFuel/IntakeAir, but eats it like it's going out of style.

Fuel tanks: No reason to discuss this, other than we need more sizes available.

Outputs: The business end of the engine. These take EnginePower and convert it into Thrust. The basic models are lightweight, but more advanced Nozzles (see below) may offer other options like vectored thrust, afterburners, or reverse thrust. Various types of this as well:

. Propellers: We sorely lack this. These are simple and relatively cheap, but more advanced models can be quite powerful.

. . . . . . . . . .Limited to low speeds, less than 1/2 Mach. Allows for "puller" designs, meaning wings can be moved further forward.

. . . . . . . . . .Usually paired with piston engines, but there's no reason you can't use a turbine to make it a turboprop.

. . . . . . . . . .May or may not come with an integrated Intake.

. Nozzles: Thrust comes out. These are the models that might offer the new options noted above.

. . . . . . . .Usually placed in back of turbine cores, but piston and electric cores could be used to make ducted fans.

. Generators: Produce ElectricCharge instead of Thrust. Why not? Not useful in space, but your part recovery rover will love you.

Intakes: Are pretty much what we have now: Supply IntakeAir. However, more advanced intakes may also have a ThrustMultiplier, converting a jet engine from a turbojet to a turbofan and increasing thrust of any attached Nozzle for the same IntakeAir and Fuel (better efficiency), at the cost of weight, bulk, and, well, cost.

Now, I recognize this would be a huge change from the system we have now, so a full swapout would be a save-breaker. Unless they're a new set of parts, making the existing ones (somewhat) redundant. But I think the flexibility we'd gain in terms of fore-aft placement of engines would be a huge step forward for the flexibility or our airplane designs. Having an integrated tank/piston engine and an integrated propeller/intake early in the tech tree might make it easier for players to design small starter planes early on in Career for survey contracts while limited to the bumpy, crappy Tier 1 runway.

Pick apart! Go!

[Justin Kerbice]

As I wrote here,

C7's engines (jet engines) masses are completely insane !

basic jet engine mass = 1 t ==> weight > 9 t (9.8 t)

turbo jet engine mass = 1.2 t ==> w ~ 11.7 t

for comparison: a complete F-16 plane have an empty weight of 8.570t (less than the KSP engine alone !) and a loaded weight of 12t, another comparison: one single big Airbus A-380 engine weight ~5t.

For a 10 times smaller world... it's weird .

--

Changing paradigm could be good, but first, some "easy" work could be done to start.

There is a lot which could be done on KSP to make it better, but it's probably a lot less cheap as "rip mods off" as Squad have chosen to do for now (AFAIK not a single 1.0 "feature" is brand new, all comes from modders in a way or another).

[Crzyrndm]

I would like to see something like this, but I think it lies fair and square in mod territory at this point

C7's engines (jet engines) masses are completely insane !

basic jet engine mass = 1 t ==> weight > 9 t (9.8 t)

turbo jet engine mass = 1.2 t ==> w ~ 11.7 t

Just maybe, you're doing something wierd here. Mass is measured in kg and is an inherent property of an object (and is the number we use in KSP and in everyday earth conversation). Weight is the force exerted on that object by gravity and is measured in Newtons (eg. you need 10kN of thrust to lift 1t off the surface of earth/kerbin). You're multiplying the mass of an object by Kerbin/Earth surface gravity and getting it's surface weight in Newtons and then trying to compare that with the mass of the earth equivalent. An unloaded F-16 weighs 85.8kN on the Earth's surface by your numbers (whether the engine weighing 9.8kN (1t mass) is appropriate I have no idea)

Edited April 11, 2015 by Crzyrndm

[Van Disaster]

@OP - I actually sat down and worked out something very similar that'd deal with oddness like ramjets too; basically an engine is a system of intake-core-output where sometimes the core doesn't actually do anything, but the intake and output devices can modify the whole system. And certain constraints like a turbofan intake has to be attached to the core - it can look like a turbofan all it likes but all it actually is is an intake and a modifier. I only really added seperate outputs because then you can easily model turboshafts. Things like afterburners would be an optional module for a standard tailpipe rather than a seperate device. The other advantage of separating out parts like that is it's easily installed in a tech tree.

The biggest difference I had to yours was my engine cores didn't produce anything at all - all they do are provide either modifiers or base stats and are parts which take up the sort of space a turbine would actually take up. You need all three parts of an engine installed in the craft to make it work, but the actual computation of work is done by the output device which effectively makes it the part. Given all we actually need is output, how it goes about that doesn't have to flow like an actual turbine. If you want to model a spool that is just an internal number to the core which tweaks it's modifier a little rather than going all the way through the resource system.

[Whirligig Girl]

Yes. Very very yes. I've always felt so ingenuine about jets in KSP. But how do we account for RAPIERS? Also, we should be able to have 2.5m engines for Mk3 craft.

[Vaporo]

As I wrote here,

C7's engines (jet engines) masses are completely insane !

basic jet engine mass = 1 t ==> weight > 9 t (9.8 t)

turbo jet engine mass = 1.2 t ==> w ~ 11.7 t

for comparison: a complete F-16 plane have an empty weight of 8.570t (less than the KSP engine alone !) and a loaded weight of 12t, another comparison: one single big Airbus A-380 engine weight ~5t.

For a 10 times smaller world... it's weird .

Could you please explain what weird math is going on here? 1 ton is 1 ton. How are you turning 1 ton into 9 tons?

[Van Disaster]

Yes. Very very yes. I've always felt so ingenuine about jets in KSP. But how do we account for RAPIERS? Also, we should be able to have 2.5m engines for Mk3 craft.

Intake - core - output. Core is a two state modifier, output uses LF + either a resource from an intake or one from a tank. You could even just have a null core & do it all in the output - or have the core just handle heating or something - however one thing I really dislike about the Rapier is that it is way too small a part so you must have all the parts to use one. B9 Sabres at least attempt to be large. SABRE type engines are pretty simple, they're just slightly more complicated rockets. A thrust vector turbofan like a Pegasus is a bit more awkward.

Edited April 11, 2015 by Van Disaster

[Justin Kerbice]

I would like to see something like this, but I think it lies fair and square in mod territory at this point

Just maybe, you're doing something wierd here. Mass is measured in kg and is an inherent property of an object (and is the number we use in KSP and in everyday earth conversation). Weight is the force exerted on that object by gravity and is measured in Newtons (eg. you need 10kN of thrust to lift 1t off the surface of earth/kerbin). You're multiplying the mass of an object by Kerbin/Earth surface gravity and getting it's surface weight in Newtons and then trying to compare that with the mass of the earth equivalent. An unloaded F-16 weighs 85.8kN on the Earth's surface by your numbers (whether the engine weighing 9.8kN (1t mass) is appropriate I have no idea)

You forgot one point: we usually express weight as the absolute value of force vector, so my computer weight X kgs, it's not the force itself, but the force value (discarding the way, which is obviously "down"). Which is the units use for the plane I use in my post.

No one use mass in everyday conversation at all ! Unless you only speak to physicists ! What the matter using mass at all ? it's too much abstract for our everyday life.

A scale didn't give you the mass, it gives you the weight, and what's the unit used ?

Could you please explain what weird math is going on here? 1 ton is 1 ton. How are you turning 1 ton into 9 tons?

Not weird at all, Kerbin gravitation constant is the same as our Earth one (9.81 m/s-2), so 1t in KSP on Kerbin weight as much as in our universe on Earth (at ground level to be thorough), which is 9.81 t in weight (again absolute value of force vector).

[Damien_The_Unbeliever]

Not weird at all, Kerbin gravitation constant is the same as our Earth one (9.81 m/s-2), so 1t in KSP on Kerbin weight as much as in our universe on Earth (at ground level to be thorough), which is 9.81 t in weight (again absolute value of force vector).

Yes, and what others are pointing out is that if you have tonnes on both side of the conversion, something's wrong. You might want to look at dimensional analysis. You yourself acknowledge above the g has units of m/s-2.

Or, to put things another way around:

A scale didn't give you the mass, it gives you the weight, and what's the unit used ?

Most scales measure weight, but give results in units of mass. This means that they've already adjusted the result by using g, on the assumption (which I think is fair) that the scale will only ever be used on the surface of the Earth.

Edited April 11, 2015 by Damien_The_Unbeliever

[Red Iron Crown]

Justin Kerbice, I'm sorry but you're applying the math wrongly. The mass units, force of gravity, and resultant weight are all calculated correctly.

Weight is a force and changes depending on gravity, mass is...well...mass and never changes.

On Earth's surface we often treat them as equivalent, because the force of gravity is constant per unit mass (~9.8N/kg). So 1t of mass has 9.8kN of weight, just as it does on Kerbin's surface. On the Mun, with its lower gravity (~1.6N/kg), that same ton of mass would have 1.6kN of weight.

Note that N/kg and m/s^2 are the same units expressed differently, it's just easier to understand N/kg when calculating forces.

[pincushionman]

@OP - I actually sat down and worked out something very similar that'd deal with oddness like ramjets too; basically an engine is a system of intake-core-output where sometimes the core doesn't actually do anything, but the intake and output devices can modify the whole system. And certain constraints like a turbofan intake has to be attached to the core - it can look like a turbofan all it likes but all it actually is is an intake and a modifier. I only really added seperate outputs because then you can easily model turboshafts. Things like afterburners would be an optional module for a standard tailpipe rather than a seperate device. The other advantage of separating out parts like that is it's easily installed in a tech tree.

The biggest difference I had to yours was my engine cores didn't produce anything at all - all they do are provide either modifiers or base stats and are parts which take up the sort of space a turbine would actually take up. You need all three parts of an engine installed in the craft to make it work, but the actual computation of work is done by the output device which effectively makes it the part. Given all we actually need is output, how it goes about that doesn't have to flow like an actual turbine. If you want to model a spool that is just an internal number to the core which tweaks it's modifier a little rather than going all the way through the resource system.

I suggested having the core create a virtual resource for a few reasons:

One, it seems an obvious way to force the user to insert a core into the assembly. With a null part, it would be just a flag "does a core exist?" which may be opaque to the user. All other useful components list the type of resource they require and the rate they need it; if the part lists EnginePower, the user should be able to assume that they need a component that provides that resource. Without it, it may not be obvious that a core component is needed, or what it is.

Two, it acts as a way to differentiate between various engine cores of various capability. A piston core would produce power at a certain efficiency and effectiveness, and a jet could have different performance curves; a RAPIER core could switch from air-breathing to LO use. All with the same nozzle output. That way you could let the outputs handle what effects you get (thrust only, or add reverse thrust, add vector thrust, etc.). You'd get more flexibility by mixing and matching, while keeping the number of parts to track more reasonable (you don't need to compare between ten different nozzles for efficiency and features; you only need three or four based on features).

Three, it allows more flexible setups. Need more engine power? Add cores. This would make a lot more sense for piston cores than jets - a twelve-cyclinder engine is three fours stacked in series. An afterburner would essentially be a turbojet core with another core attached that's staged later or toggled via action groups. You could also run power from a single core to multiple outputs - with null parts it's not clear whether that would be or should be allowed (based on how the flags are checked), but a virtual resource would simply be split between all the outputs.

But yeah, ramjets wold be pretty awkward under such a system.

Now, you said you've "worked something out" similar to this - did you intend to make a mod to implement this behavior? Because from what I understand, RoverDude's work on resources is going beyond just getting resources from the ground, but rewriting the entire resource-use system, including to be more moddable. This may be something we can approach once we know more details about that.

[Feradose]

This must happen. I want to place cores on my wings. Could this please be a mod?

[Rath]

This is what makes this part of the fourm motivational to read; I makes me want to learn how to mod more.

[Van Disaster]

The big problem I have with just using resources is an issue we have already: gaming the system by part spamming. Currently you can just keep adding intakes to get more IntakeAir without any thought of design, if engines produce EnginePower you can just pick any core option regardless of if it'd actually work - and presumably regardless of where the output device is. From what I can make out from your notes in the OP you could attach an afterburning jet pipe to a piston core and it'd be a viable combination?

If it's just an internal mechanism to transfer work between parts then sure, given it's an existing system that's a neat way to use it, but there need to be extra constraints somewhere. At that point when I was poking around with my idea I wondered why I was bothering with internal resources. I once made a jet engine that produced my own compressed air resource & fed it to air nozzles to make a Harrier-type system, & it struck me back then that there were a number of ways you could abuse that.

To be honest I don't actually think there's a really comfortable one-size-fits-all system; using resource generators does have a lot of merit but perhaps there would need to be more resource types; you could instead use Thrust and Torque, for instance, but resources do need to be flagged as not-storeable somehow so you can't make a tank full of Torque(!) and possibly some constraints on resource transfer ( which we need anyway really just for intakes ) so you can't transfer huge amounts of Thrust through a tiny pipe etc.

And no I wasn't about to embark on writing a plugin, I didn't have a concrete enough idea to go that far.

Edited April 13, 2015 by Van Disaster

[DundraL]

Looks like we both want to solve the problem of Jet engine weaight distribution, though we came up with different solutions:

http://forum.kerbalspaceprogram.com/threads/92819-An-arguement-for-a-simple-intake-jet-engine-tweak

Feel free to give it a bump if you want since it still relevant.

One problem I see with both our solutions is that there is no continuity of the actualy jet propulsion system: you can still place the intake on top of a tail fin for example, and have them magically transfer intake air to the jet engines at the wing tips. While I understand the difficulty in implementing an interally mounted jet engine in KSP, it sure does make things wierd... though the weight distribution can definitely be fixed.

Edited April 15, 2015 by DundraL

[Renegrade]

I support this concept - it would definitely help bring the CoM forward, and the idea of having different engine cores is especially cool. Would make plane design a bit less crazy, AND add a degree of realism (jet engines aren't just little tiny nozzles attached to things). Win/win!

You forgot one point: we usually express weight as the absolute value of force vector, so my computer weight X kgs, it's not the force itself, but the force value (discarding the way, which is obviously "down"). Which is the units use for the plane I use in my post.

Yes, and the force vector of 1 tonne mass at rest on a surface in a 1g field is 1000 kgf, or 9806.65 newtons. Or 1 tonne weight. Also roughly a long ton of weight.

I'm a little perplexed at the confusion here. Back when I went to school, fire hadn't been invented yet, the Earth was flat, the stars were dots on a black shell about 50% farther away than the tallest mountain, you needed constant force for constant motion, and light propagated as waves through the phlogiston, but even I know mass/weight conversion. It's really simple. X mass {unit} is X weight {unit} in 1g.

[Azimech]

I agree, but I want to be able to place "input" and "output" at any location I like and use piping to achieve the goal. With the associated drag due to length, corners and bends.

Let's get used to a proper way of building, one day fluid dynamics will come our way.

[DundraL]

I doubt KSP will ever simulated internal fluid dynamics. I also dubt Squad will make players go though the trouble of running internal ducting for air intake because.... there is no "internal" anything in ksp. Sure you can clip things, but there really isnt any free internal empty space in ksp. It would all have to be external ducting. They did it via the "intake air" resouce just to avoid that exact nightmare for a reason imo.

[skepticon]

1. So is there anything stopping me from putting my engine cores out on the wingtips, or directly behind the cockpit, while my actual thrust is generated elsewhere? Is this more realistic than the current system?

2. If thrust generation outlets are very light, won't I just use them like RCS nozzles to make silly-maneuverable craft? Why would I limit myself to only generating thrust along one axis?

[pincushionman]

1. So is there anything stopping me from putting my engine cores out on the wingtips, or directly behind the cockpit, while my actual thrust is generated elsewhere? Is this more realistic than the current system?

There is nothing, but that isn't a problem with my proposal, it's a problem with the resource distribution systems in KSP. I can currently put an intake, a fuel tank, a...girder? and a jet engine in line, and it will work. I actually think "transmissions" parts to simulate, well, transmissions and thrust ducts would be a fine idea, with an EnginePower loss per meter to discourage liberal use of the concept. Anything should be possible, but at a cost. If we want to eliminate resource spamming, this suggestion ain't it, the resource system needs looked at instead.

2. If thrust generation outlets are very light, won't I just use them like RCS nozzles to make silly-maneuverable craft? Why would I limit myself to only generating thrust along one axis?

Because they're not like RCS and don't act like it. The "normal" jet outputs would look an awful lot like the "jet engine" parts we have now. But that doesn't mean there couldn't be supplemental side-mounted small low-thrust nozzles available. It would be more like placing a bunch of RS-77 engines around a set of tanks to make a VTOL thingy...just like Squad did with one of the example crafts. If you want to make something that lifts off like a Harrier, and the available options allow it, why should this proposal stop you?

Now, keep in mind, I'm trying to propose something here that acts "more like reality" in certain respects only. Specifically weight distribution. In reality, jet engines are finely tuned pieces of equipment, and you wouldn't ever grab this compressor, that combustion chamber, this other turbine, and that nozzle into an airplane and expect it to work, like you can drop a new piston engine or carburetor into a car. Engines are a Single. Unit. Even putting a different entire engine onto an existing plane is a gigantic engineering undertaking. This is not conducive to KSP play. In KSP, we need to Lego stuff together, and that means that you are going to be able to game any system to make some wacky stuff that would never be buildable in reality. I do not wish to stop that. I do wish, however, to have a system where if a user with not KSP experience wants to make an airplane that looks like a "normal" everyday airplane, it acts like a normal, everyday airplane well enough.

[marach]

A long long time ago I was working on a mod where the nacelles produced bypass-air and exhaust-air the existing jets then used the exhaust air to convert to thrust with a reheat capability available as an extra version of each that relied on the bypass-air for an additional amount of thrust... unfortunately it never got passed initial tests since AJE launched...

I also had it set to only allow inline designs but had a flag to allow things like the Pegasus to be possible.

Edited April 20, 2015 by marach

[GarrettKajmowicz]

FWIW, this model would allow the use of a liquid fuel to turn a turbine to generate torque to power a propeller ... on Eve! It would be *far* more efficient than using a straight rocket as is done now.

[Scoundrel]

Back when I went to school, fire hadn't been invented yet, the Earth was flat, the stars were dots on a black shell about 50% farther away than the tallest mountain, you needed constant force for constant motion, and light propagated as waves through the phlogiston, but even I know mass/weight conversion. It's really simple. X mass {unit} is X weight {unit} in 1g.

Ahhh, the good old days, when everything was steam powered and we got five quarters to the dollar. I remember walking to summer school in -100° weather, in the middle of tornadoes, and through twelve feet of snow uphill in my bare feet... both ways! And that was after spending 25 hours a day toiling in the factories for tuppence a month and having to sleep in a hole in the ground on a bed of broken glass for five minutes (I know, utter luxury!) before being woken up and having a bowl of gravel for breakfast (and not that delicious premium gravel, but that disgusting bitter expired gravel that was rejected from the gravel factory) before having to go back to work!

That aside, no offence to the OP (who put some reasonable though into the suggestion, which in turn made me seriously consider it before rejecting it), I'm simply not really vibing with this suggestion because the end result is exactly the same as it is now. Game design-wise we put tanks on a rocket then an engine, and jet engines follow the same game principle, but with the added complexity of adding inlets (yes, I went there and wrote inlet instead of intake! Jets are aircraft, not vacuum cleaners!) and little else.

The fact is that the rationalizations aren't particularly compelling, and I honestly don't see it being a must have in the stock game. But that's just my opinion.

[Van Disaster]

FWIW, this model would allow the use of a liquid fuel to turn a turbine to generate torque to power a propeller ... on Eve! It would be *far* more efficient than using a straight rocket as is done now.

There's a simpler way you can do that already; set up an existing intake with one of the resource converters in it ( from Kethane/Regolith/I guess stock soon ) to convert IntakeAtm+Oxidizer to IntakeAir. IntakeAtm is a resource defined in the community resource pack which a bunch of things use, or you can do it yourself, which you can set the intake module up to gather from atmospheres; you could call it "InertAtmosphere" if you want, the name isn't important. You could make it sufficiently overpowered to run jets on but more sensibly it's a quite practical way of running already existing piston engines, like the RetroFuture ones. It does consume quite large quantities of oxidizer if you want an earth mix of inert gas + oxygen though...

Simple example would be something like this:

+PART[airScoop]:NEEDS[Regolith]
{
@name = DLTLoXmixerIntakeScoop
@title = Disaster Ltd LoX mixer intake

@MODULE[ModuleResourceIntake]
{
@resourcename = IntakeAtm
@checkForOxygen = false
}

@RESOURCE[IntakeAir]
{
@name = IntakeAtm
}

MODULE
{
name = REGO_ModuleResourceConverter
Label = LoX mixer
conversionRate = 0.35
inputResources = IntakeAtm, 0.7, Oxidizer, 0.3
outputResources = IntakeAir,1,False
}
}

Bit OT. Back on topic a bit more:

@Scoundrel: you're missing the point of jet cores and the fact that stock jet engines are ridiculously too small for what they do - aside from large craft you should never be able to tack a jet engine as powerful as a stock turbojet randomly onto a vehicle, it needs to be designed into it with some thought. Currently they're even smaller than most rocket nozzles which is just silly. Maybe if they were all centrifugal flow, but not at 110kN thrust...

There's also a huge list of useful turbine powered devices which the discussion is trying to find a sensible and coherent method of integrating rather than recreating everything from scratch for every device.

[Solarspaceprobe]

I love this! Make it happen, Squad!