Gordon Fecyk

[1.3 through 1.7] Explodium Breathing Engines v1.4.0: "Jet" engines for use on Eve [28-APR-2019]

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Important notice: If you're trying Alien Space Programs, grab the latest release of this add-on.

Explodium Breathing Engines

"Jet" engines that (actually!) use Eve's atmosphere and Oxidizer.

License: MIT License except for re-coloured textures, which were derived from art belonging to Squad / Deported B.V.

Download from SpaceDock or from GitHub. Supports KSP 1.3 through 1.7 and supports RealPlume Stock.

It had to happen. After good success with @GregroxMun's Alien Space Programs and @OhioBob's Eve Optimized Engines, along with an old discussion about atmospheric engines using oxidizer instead of liquid fuel, I just had to ask how hard it would be to make jet engines that would breathe "explodium vapour" likely found in Eve's atmosphere and burn it with oxidizer to produce efficient thrust. @JadeOfMaar and @wasml pointed me in the right direction, @DStaal filled in a few blanks, and I hacked together some parts.

Introducing Explodium Breathing Engines. A set of cloned configuration files from stock jet engine parts that use oxidizer only, and a new resource configuration found hypothetically in Eve's atmosphere: "Explodium Vapour" evaporated from Eve's Explodium Sea. This vapour may also be present on other planets in popular planet packs.

Further rebalancing has happened based on resource abundance and a pure ethane (C2H6) reaction with oxygen. Resource abundance now has a direct impact on engine performance. This is turning from proof-of-concept into something realistic, while still being fun to use. There really is some science behind this that I explain at the project's Wiki, and hammered out in a more recent add-on discussion thread.

Modified parts include explodium versions of all intakes, oxidizer + explodium versions of nacelles, oxidizer only tanks and wet wings, and underworld-themed engines:

  • J-21 "Hades"
  • J-34 "Zephyrus"
  • J-91 "Atlas"
  • J-405 "Sphinx"
  • J-X5 "Beelzebub"
  • and let's not forget our favourite: CR-8 L.U.C.I.F.E.R. (Large Unkerbal Cycle-Interchanging Fueled Explodium Rocket)

All parts should have a distinct appearance to separate them from stock parts.

ExV2-1.png

ExV2-2.png

I put all of the modified parts and ExpVapour resource configurations in a GitHub repository. MIT license to match OhioBob's choice for the Eve Optimized Engines. I'd be glad to let others into the project who know better than I do.

The to-do list includes: (The to-do list is finished!)

  • Rewrite parts as Module Manager patches to remove dependency on Squad art (Done!)
  • Fix the intakes to work on Eve only, somehow (Done! Intakes are now zero EC harvesters.)
  • Fix the resource config if needed, integrate with Community Resource Pack (Not necessary as it turns out.)
  • Rebalance engine performance based on tester feedback (we might be almost done!)
  • Produce different artwork to identify the explodium-based parts (I'm just missing some tricky parts.)
  • Create RealPlume configurations; I'd like to do a different colour plume for these like the blue from an ethane flame (This was quite easy, as it turned out.)
  • Maybe get some help with CKAN, publish to Space Dock (SpaceDock done, CKAN should be done)

Known problems, should not impact play:

  • Harvesters need manual activation because of how ModuleResourceHarvester behaves. Workaround: Assign "Toggle Duct" and "Start ExV Filter" to action groups.
  • Ore mining contracts will appear in Career mode when you unlock tech nodes with harvester parts. This might be a game design problem with contracts lumping all resource harvesters together. Pay attention to contract requirements, because there might be a "Gather 2000 units of Explodium Vapour" contract that won't make sense because there are no containers for it.
  • No colourized textures are available for the Atlas engine or Big-OX wings. The wings don't take well to colourization, and the Atlas causes null reference exceptions if using an alternate texture.
  • Harvesters have a small amount of ExpVapour loaded by default. This means engines will work for a few seconds at launch from Kerbin, but will quickly run out of vapour. This may be needed in KSP 1.3 though.
  • Slow frame rates may happen if using the RealPlume configurations. Update SmokeScreen to 2.7.4 or later to address this.
  • (KSP 1.3) Stationary harvesters will harvest zero ExV, such as craft on launch clamps or with wheel brakes engaged. Once motion starts, harvesters will start harvesting.
  • (KSP 1.3) Engine resource shows up as "Ex V" instead of "Explodium Vapour" in VAB or SPH. Other resource name strings not affected for some reason.

To use this part pack, grab a release from GitHub or SpaceDock, and put the "GameData\ExplodiumBreathingEngines" folder into your GameData folder. To use an ExpVapour intake / harvester, first deploy the harvester's filter (Open Duct), then start it (Start ExV Filter). You might want to use the Stage action group to start the filters, but you can pre-open the filters in construction.

To use a RealPlume configuration, install RealPlume-Stock and update its SmokeScreen installation to 2.7.4 or later. If you don't install RealPlume, you'll still get a different engine plume for some engines but most will use the stock jet plumes.

Now with more lift!

Change log:

Spoiler
  • v1.4.0
    • Moved RealPlume configs to a pull request for RealPlume Stock to avoid Module Manager warnings.
    • Adjust Eve's and Tekto's resource percentages, but pre-1.4 ExV craft should still work unchanged.
    • Tested on KSP 1.7.0.
  • v1.3.0
    • Converted all parts to full parts and removed Module Manager dependency.
    • Fixed text labels for harvesters to avoid confusing Open / Close with Start / Stop.
    • Verified compatibility with KSP 1.4.5.
  • v1.2.0
    • Fifth general release. Includes previously missing patches for wet wings and harvesters to support Ferram Aerospace Research. Works on the official FAR v0.15.9.1 on KSP 1.3.1 and on an unofficial v0.15.9.2 on KSP 1.4.3.
  • v1.1.0
    • Fourth general release. Extended the atmCurve float curves so these engines produce more thrust in denser air. Based on stock jets with Advanced Jet Engines, and Galileo's Planet Pack world Tellumo and its ten+ atmospheres of oxygenated atmosphere.
  • v1.0.0
    • Third general release. Includes adjustments for GregroxMun's Alien Space Programs 1.0.
  • v0.3.1
    • Second general release. Now CKAN-indexed. I think.
    • Spanish dictionary provided by Fitiales and the KSP-In-Spanish project.
    • Tech tree tuning: Explodium engines become available once the tech to get to Eve is available.
    • Tech tree swapping: Swaps stock jet engine parts with these parts if Eve Space Program is present.
    • Harvester tuning: Harvesters behave more like intakes thanks to KSP 1.3's changes in ModuleResourceHarvester, and are a closer match to the stock air intakes.
  • v0.3.0
    • Began localization of text, created en-us dictionary.
  • v0.2.1 beta 1
    • First general release. Removed unneeded comments, corrected minor mistakes.
  • v0.2.0
    • Added colourized textures for the majority of these parts. Just used blue bands near connection points or along lines similar to the LFO tanks.
    • Added copyright notice for modified textures; these belong to Squad / Deported B.V. and are used with permission per add-on posting rules part 8.
    • (Not included in release) Added Module Manager FOR statements to prepare for RealPlume configurations.
    • (Not included in release) Added RealPlume configurations using pre-existing models and textures. Cloned two jet engine plumes, then applied them to all engines.
  • v0.1.6
    • Further adjustments to harvesters and engines so resource abundance has an impact on engine performance.
    • Changed engine plumes for Sphinx in wet mode, Beelzebub and L.U.C.I.F.E.R. in open-cycle mode to use Stock blue shock diamond. This matches an ethane-oxygen flame.
    • Changed Eve's resource abundance to reflect KSP Wiki suggestion of 10% ethane.
  • v0.1.5
    • @JadeOfMaar added planetary resource definitions for the Outer Planets Pack, Galileo's Planet Pack and Gameslinx's Planet Pack.
    • Engine ISPs changed to 45% of original values to reflect energy difference between RP-1 + O2 and C2H6 + O2 reactions.
    • Rebalanced engine performance. Baseline resource ratio is 3.75 ExV to 1 OX, compared to 133 IntakeAir to 1 LF.
  • v0.1.4
    • Rebalanced engine performance based on ethane chemistry. Baseline resource ratio is 3.75 ExV to 1 OX, compared to 38.1 IntakeAir to 1 LF.
    • Engine ISPs reduced to 16% of original values to reflect energy difference between H2 + O2 and C2H6 + O2 reactions.
  • v0.1.2:
    • Converted all intakes into harvesters. Harvesters now only work on Eve! (Credit to @JadeOfMaar: You're a lifesaver!)
    • Added FAT-451 Airplane Wing with oxidizer tanks.
    • Modified resource to have 1000% chance of appearing in Eve's atmosphere; should work with any difficulty setting. (Credit to @ShotgunNinja for preventing a potential snipe hunt.)
  • v0.1.1:
    • Redid all parts as Module Manager patches.
    • Added Oxidizer versions of all Liquid Fuel-only tanks, adapters, wings and nacelles.
    • Renamed L.U.C.I.F.E.R.'s open-cycle mode to "ExplodiumBreathing."

 

Edited by Gordon Fecyk
Updates for KSP 1.7

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I'm sure you don't have the rights to include Squad art in the repository.

I'm also sure you don't need to.  :wink:

This file - placed pretty much *anywhere* inside GameData - should exactly replicate your air intake:

@PART[airScoop]
{
    @name = airScoopExV
    @title = XV-G51 Radial Air Intake
    @description = An explodium intake duct that can be mounted at the sides of a fuselage. Warranty does not cover engine flame-outs or any objects, inanimate or otherwise, sucked in by the intake. Optimized for subsonic flight.
    @tags ^= :oxygen:explodium:
    
    @MODULE[ModuleResourceIntake]
    {
		@resourceName = ExpVapour
		@checkForOxygen = false
    }
}

(I'll admit I've not tested it, and the regex replacement I'm using to modify the tags is something I haven't used before.)

Note that it doesn't include any references to anything you haven't actually changed - which includes the Squad art.  It just uses the original values for those.  With a bit of work, you can even do things like apply your own textures to Squad models, and ship only your own textures.

Here's the reference to the syntax for this: https://github.com/sarbian/ModuleManager/wiki/Module Manager Syntax

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Congratulations on your mod!

One suggestion - include a version number and date last modified in your title - would make it easier for anyone following your mod to spot when an update comes out.

And thanks for sharing!

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OK thanks folks so far. Looks like I'm learning Module Manager as it would make art management simpler too. I'll do that first to remove the dependency on Squad art, then see about making this thing Eve-only.

Any suggestions on making the intakes work on Eve only? I looked at ModuleResourceHarvester instead of ModuleResourceIntake, but it was a very rough first attempt and didn't harvest anything. The next step might be writing a plugin to mimic ModuleResourceIntake but for ExpVapour.

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58 minutes ago, Gordon Fecyk said:

OK thanks folks so far. Looks like I'm learning Module Manager as it would make art management simpler too. I'll do that first to remove the dependency on Squad art, then see about making this thing Eve-only.

Any suggestions on making the intakes work on Eve only? I looked at ModuleResourceHarvester instead of ModuleResourceIntake, but it was a very rough first attempt and didn't harvest anything. The next step might be writing a plugin to mimic ModuleResourceIntake but for ExpVapour.

I know from watching the CRP thread that it should be possible to make an atmospheric resource that you can harvest that's only on Eve using just stock.  I've never really done much with resources myself, however.  I'd suggest going to that thread and asking for help - those would be the people who know how to do what you're trying.

(Oh, and I realized I have a slight error in my MM patch above: The first character on the first line should be '+' not '@'.  The above edits the part, corrected it would duplicate it and edit the duplicate.)

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v0.1.1 release now available from GitHub repository. Requires a current Module Manager, as all parts were redone per @DStaal's suggestion. No Squad art copying needed and, thus, this is something I'm OK with distributing.

Still need to somehow make the intakes work on Eve only. I'm pretty sure the resource definition is correct, but I think the stock ModuleResourceIntake wasn't written for anything besides IntakeAir, and ModuleResourceHarvester only seems supported with ground resources, even though definitions exist for atmospheric ones. I'm going to the Community Resource Pack threads for help.

 

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v0.1.2 release now available from GitHub repository. @JadeOfMaar gets two internets for important ModuleResourceHarvester information and examples. @ShotgunNinja gets one internet for sparing me a potential resource snipe hunt. This is no longer The Part Cheat From Helltm.

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Engine balancing

Now that I have a handle on part texturing I'll try re-skinning these parts to stand out against the stock jet parts. The next step seems to be engine balancing as JadeOfMaar pointed out earlier.

I'm trying to figure out how much explodium plus oxidizer would produce how much thrust in a given engine. The KSP Wiki's article on Eve suggests an atmosphere of 90% CO2 and 10% C2H6 (Ethane) could match the molecular weight and heat capacity found on Eve. Sounds like as good a starting point as any. Compare to the 78% N2 and 21% O2 found on Earth or supposedly Kerbin. Plus we have over five times of it at Eve's sea level compared to Kerbin's 21% O2.

My first thought is how much of Intake Air is made of oxidizer. Is it 21%, or all of it? Does it use all of whatever percentage it is, or is a portion of it ejected with the rest of the hot gasses?

Assuming all of the O2 is burned, an engine is using 21% of the Intake Air then. The miniJet (Juno) uses 22 Intake Air per 1 Liquid Fuel to get 20 thrust, of which 4.4 Intake Air is O2, so just dumb math: 4.4IAO2 * 1LF = 20thrust (all invented units, not real life units). Or 4.4IAO2 = 20thrust/1LF. To get 20thrust out of a miniJetOx I'd need 'x'ExVC2H6 * 1.2OX = 20thrust, or xExVC2H6 = 20thrust/1.2OX, or 16ExVC2H6. If only 10% of Eve's air is ExVC2H6, I'd need to intake ten times that. My resulting miniJetOx would need 160 ExV per 1.2 Ox to generate 20 thrust, or 7.2 times the IntakeAir the miniJet would need on Kerbin.

Of course this isn't valid chemistry; I have no clue what the oxidizer in KSP is made of and it's only wildly guessing that Ethane is in Eve's air. I do remember that all hydrocarbons reacting with oxygen produce CO2 and H2O vapour though, so it's just a matter of how much. Burning one Ethane should produce 2 x CO2 and 3 x H2O I think, requiring 2.5 x O2 which doesn't make sense unless I double everything: 4 x CO2 and 6 x H2O from 5 x O2 and 2 C2H6. I think. What's jet fuel made of in KSP again?

Molar masses appear to be 30 for C2H6, 32 for O2, 18 for water, and 44 for CO2. So if I need 2 C2H6 and 5 O2, that's 60 kg C2H6 per 160 kg O2. Or 12 units pure ExVC2H6 per 32 units OX, or 120 units ExV (Remember 90% CO2) per 32 units OX, assuming 1 unit = 5 kg as the Wiki suggests. That's 4.5 units ExV per 1.2 units OX, much less than the 22 Intake Air per one Liquid Fuel normally needed.

So I could go one way with 160 ExV / 1.2 OX, or the other way with what looks like 4.5 ExV / 1.2 OX, or keep it at the Intake Air-like ratio of 22 ExV / 1.2 OX. And this is just for the Juno / Hades. Or I need to re-check my math and chemistry. Or something.

On the plus side, Ethane and Kerosene have similar explosive properties. Assuming Liquid Fuel is like Kerosene for jet engine use, this would put the energy per mass on par; just need to figure out how much oxidizer would be needed to get 20 maxThrust. Maybe 22 ExV / 5.8 OX? I don't know if reducing the thrust would be good for game play.

[Add on 22 APR] The Intake Air vs Liquid Fuel units make more sense if Hydrogen (H2) is the liquid fuel. With that, I get a closer match to the values in-game: 2 x H2 + 1 x O2 -> 2 x H2O, with molar masses 2 and 32 respectively. This gives me 20 kg H2 per 160 kg O2, or 20 kg H2 per 762 kg Intake Air, or 4 LF per 152 IA, reduced to 1 LF per 38.1 IA; this is a close match for the Panther in Dry mode. The Wheesley is slightly more efficient and the Goliath is tremendously more efficient. So I'll need to redo my chemistry using Hydrogen as the baseline for the others, and then I'll only adjust the ExpVapour value, leaving the Oxidizer value untouched.

Edited by Gordon Fecyk
Did even more chemistry

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So this is new: Oxidizer use by a jet engine seems to be ignored. I tried this:

+PART[miniJetEngine]
{
	@name = miniJetEngineOxBeta
	@title = J-22 "Hades" Basic Jet Engine (Beta)
	@description = A small explodium breathing turbojet. Not very efficient, anemic thrust, but hey--it's cheap! Requires Oxidizer.
	@tags = aircraft (hades plane propuls turb
	@MODULE[ModuleEnginesFX]
	{
		@PROPELLANT[IntakeAir]
		{
			@name = ExpVapour
			@ratio = 22
			%IgnoreForISP = False
			%DrawGauge = True
		}
		@PROPELLANT[LiquidFuel]
		{
			@name = Oxidizer
			@ratio = 5.8
			%IgnoreForISP = False
			%DrawGauge = True
		}
	}
}

The ratio for oxidizer is over five times the original value of 1.2. I thought this was usage per second per the KSP Wiki entry on ModuleEnginesFX, but the oxidizer use per second seems to remain fixed, where the ExpVapour usage per second will actually decrease. And the ISP value doesn't change at all.

The original miniJetEngine consumes 0.064 LF/sec. I want the miniJetEngineOxBeta here to consume 0.374 OX/sec while also consuming 1.462 ExV/sec to try this high OX usage scenario... what have I missed? Never mind then; after a lot of chemistry and editing atmosphereCurve, I got a pig of a Hades engine that guzzles 0.4 OX/sec.

Edited by Gordon Fecyk
Figured out resource usage rates

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Very interesting work Gordon! It must have taken a lot of time to write a mod like this! It's weird to see the KSC surrounded by purple.

How's flying a plane in the dense atmosphere of Eve?

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@xendelaar I only did the engines, just to be clear. The KSC on Eve is GregroxMun's work. And if I'm good, it's only because I stand on the shoulders of giants.

Otherwise, thanks.

What you really have to watch for on Eve is the dynamic pressure you'll make pushing against that thick air. If you build light and streamlined it'll work for you against the high gravity. Speed is a bad idea until you get up to, say, 15 km up where the air is as dense as on Kerbin.

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v0.1.4 now available from GitHub repository. Engines tuned to use ethane-based chemistry, and as a consequence engines are much less efficient.

My head is spinning from the science right now, so please read this post for the logic behind it.

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Nice one! I myself started a thread about this topic a while ago. See here:

My thread

 

It's heartwarming not to be the only one to have this Idea. Unfortunately, I'm not (yet) a modder.

 

I know, it's not particularly useful, but your engines should work on Jool too, as Jools atmosphere would also be able to burn if added oxidizer..

 

Edited by Physics Student

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Hey Gordon Fecyk,

Nice mod, good work! :wink:

 

Will it be available from CKAN, too?

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5 hours ago, Physics Student said:

I know, it's not particularly useful, but your engines should work on Jool too, as Jools atmosphere would also be able to burn if added oxidizer.

If Jool's atmosphere is typical of gas giants, the things would work better there than on Eve. Hydrogen releases a lot more energy per mass than any hydrocarbon, apparently. No place to land of course, and without more than one biome it would get pretty boring pretty quickly.

And this brings up a question for folks here. Most hydrocarbons have half or worse of the high heat value that hydrogen does, and that doesn't include the sheer amount of the stuff you'd need to burn to match the total energy per in-game unit. Have I picked the wrong baselines in my little chemistry session?

Somehow I have to raise the specific impulse of these engines, either by reducing thrust or by making different assumptions about Eve's air. Given the thickness (5.7 atmospheres) you don't need that much thrust to get a lot of lift, and I'm finding I use 1/3 the throttle to take off than I would with the same craft on Kerbin. Things would get closer to 'normal' at 15 km altitude and the Kerbin-like lift would then have to counter Eve's 1.7g.

As for CKAN @N3N, one thing at a time, eh? Let's get this balanced out first. Plus I want to do different artwork. I can leave RealPlume to someone else if needed.

Edited by Gordon Fecyk

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Hey Gordon Fecyk,

 

No problem, i just wanted to know. :wink:

Keep the good work going. :wink:

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3 hours ago, Gordon Fecyk said:

If Jool's atmosphere is typical of gas giants, the things would work better there than on Eve. Hydrogen releases a lot more energy per mass than any hydrocarbon, apparently. No place to land of course, and without more than one biome it would get pretty boring pretty quickly.

And this brings up a question for folks here. Most hydrocarbons have half or worse of the high heat value that hydrogen does, and that doesn't include the sheer amount of the stuff you'd need to burn to match the total energy per in-game unit. Have I picked the wrong baselines in my little chemistry session?

Somehow I have to raise the specific impulse of these engines, either by reducing thrust or by making different assumptions about Eve's air. Given the thickness (5.7 atmospheres) you don't need that much thrust to get a lot of lift, and I'm finding I use 1/3 the throttle to take off than I would with the same craft on Kerbin. Things would get closer to 'normal' at 15 km altitude and the Kerbin-like lift would then have to counter Eve's 1.7g.

As for CKAN @N3N, one thing at a time, eh? Let's get this balanced out first. Plus I want to do different artwork. I can leave RealPlume to someone else if needed.

On the specific impulse - how you approach it probably really depends a bit on what type of use you think these parts are being designed for: Are they for flying around Eve, or are they for spaceplanes?  If they are for just flying around on Eve, just reduce the thrust.  No use designing an engine for throttle settings that will never be used, after all.  (And you never have to fly over 9km, so you don't really have to worry about higher altitude.)

For spaceplanes I'd think there would be some complex tuning - probably a designed-in thrust curve that allows for higher thrust output at speed at high altitude, probably costing ISP in that range.  So you would have an engine that gradually gained thrust at higher speeds, up to whatever it's limit was, by pumping in more fuel.  (Well, oxidizer.)  I have no idea whether this is possible, of course. :wink:

You could also have different engines that are optimized for different altitude regions/speeds, so you have your normal jet that gets you to 10-12km, then switch over to a higher speed/thrust ramjet or something, etc.

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5 hours ago, Gordon Fecyk said:

And this brings up a question for folks here. Most hydrocarbons have half or worse of the high heat value that hydrogen does, and that doesn't include the sheer amount of the stuff you'd need to burn to match the total energy per in-game unit. Have I picked the wrong baselines in my little chemistry session?

The problem with hydrogen is its low density. In comparison to other burnable gasses its heat value is high relative to its mass, but pretty poor relative to its volume at a given pressure. The right workaround for this would be that "Jool engines" need a much higher atmospheric pressure to work properly, but they would still have a good ISP

 

 

Edited by Physics Student

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1 hour ago, DStaal said:

For spaceplanes I'd think there would be some complex tuning - probably a designed-in thrust curve that allows for higher thrust output at speed at high altitude, probably costing ISP in that range.  So you would have an engine that gradually gained thrust at higher speeds, up to whatever it's limit was, by pumping in more fuel.  (Well, oxidizer.)  I have no idea whether this is possible, of course. :wink:

You could also have different engines that are optimized for different altitude regions/speeds, so you have your normal jet that gets you to 10-12km, then switch over to a higher speed/thrust ramjet or something, etc.

I know of 4 curves that go into an engine so it's very possible to have it lose efficiency while gaining thrust and/or altitude and to setup complementary engines as you suggest, or have a single engine that changes between both performance envelopes in the same engine mode or two modes like the Panther.

28 minutes ago, Physics Student said:

The problem with hydrogen is its low density. In comparison to other burnable gasses its heat value is high relative to its mass, but pretty poor relative to its volume at a given pressure. The right workaround for this would be that "Jool engines" need a much higher atmospheric pressure to work properly.

 

high in the atmosphere, they should have very low thrust but also a very high ISP.

lower in the atmosphere, ISP drops but thrust increases

Typically, in game, atmospheric resources are accessible in the lower half of the atmosphere by altitude. I don't know if this applies to gas planets but if it does it actually makes any form of gas planet chemical engines pointless. Also, can hydrogen jet engines even be done? It would be better in this case to only enter a gas planet's atmosphere to harvest that H2 to process into LH2.

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10 minutes ago, JadeOfMaar said:

makes any form of gas planet chemical engines pointless

Pointless? I wouldn't say it's pointless to make a return from jools lower atmosphere possible.

I'm not a big fan of stock electric propellers. Badum-tzzz

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Nice, @DStaal. I like the idea of changing the performance characteristics of purely atmospheric engines (Juno / Hades, Wheesley / Zephyrus, Goliath / Atlas, maybe Panther / Sphinx in dry mode) for low altitude flight and lower thrust, where lift would be greater and ISP could be greater as well. Then re-tuning the characteristics for the rest (Panther / Sphinx in wet mode, Whiplash / Beelzebub, Rapier / Lucifer) for higher altitude, more thrust and less lift, where ISP might start to approach that of rocket engines, but still only need oxidizer.

Tuning the low altitude engines should be easy based on the chemistry already done; just cut the thrust in half (or to a third) and double (or triple) the ISP. Maybe the high altitude engines could stay as they are, or I could just slightly increase the ISP if I can get away with reducing the thrust.

[A few minutes later] This seems to work with one of my reference planes and the Hades at least. I found a happy medium of 40% thrust for 2.5 x higher ISP (2475 s), and the test plane (The MiG-9 in the screen shots) can maintain about 95 m/s cruise speed at 2 km up without pushing too hard. Dynamic pressure remains below 40 kPa and I get decent lift. For low atmosphere engines it'll be about finding a thrust amount that avoids exceeding 40 kPa dynamic pressure.

I can already see a two (or three) stage space plane using Zephyrus or Atlases for takeoff, Beelzebubs or Lucifers in open-cycle for hypersonic cruise and climb, then rockets or Lucifers in closed cycle. Something similar to AeroGav's Kondor that uses Wheesleys on its ascent stage.

Edited by Gordon Fecyk
Added some initial test results

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45 minutes ago, JadeOfMaar said:

I know of 4 curves that go into an engine so it's very possible to have it lose efficiency while gaining thrust and/or altitude and to setup complementary engines as you suggest, or have a single engine that changes between both performance envelopes in the same engine mode or two modes like the Panther.

This similar to the Whiplash that needs thick air to get started, but then needs thin air to pick up thrust?

The atmosphereCurve is just one line that specifies ISP in most of these. velCurve is probably OK to leave alone. atmCurve is the one I think I need to adjust. Guess if I could reduce thrust in the lower atmosphere and then increase it the higher I go, it'll do the job of throttling the engine for me and saving fuel in the lower atmosphere, and avoid pushing too hard against it (dynamic pressure).

I only just started reading about float curves and how Unity and KSP use them. There's an in-game tool for playing with them but they're just numbers without units.

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@Gordon Fecyk I'd like to know what this in-game tool is that you use. I know one but for me, it seems, it's quite troublesome and counter-intuitive. Unfortunately I don't think the units are ever shown along with the numbers. :/ 

Look at the atmosphereCurve within rocket engines as they all have varying Isp between sea level and ocean, unlike most/all jet engines. Note that Mainsail has a value for down to 9 atmosphere pressure. Squad had Eve and even harder planets in mind here.

//The Mainsail
atmosphereCurve
{
	key = 0 310
	key = 1 285
	key = 9 0.001
}

atmCurve will do what you want, yes.

Concerning the Whiplash and like engines, it's a turbo ramjet, which means it's sufficiently fed by air being rammed in and compressed, slowed via scoop or shock cone than being fed via any fan intake, and that only happens at high speed and high altitude. Low pressure and low drag at that point also theoretically help its thrust a lot.

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2 hours ago, JadeOfMaar said:

I'd like to know what this in-game tool is that you use. I know one but for me, it seems, it's quite troublesome and counter-intuitive.

I tried the in-game "Amazing Curve Editor" from Sarbian. It does work, in that it'll show you a unit-less curve if you plug in a bunch of numbers. The Windows clipboard doesn't work with it so I can't just copy a few lines from a part config and paste them in here. But I could plug in the numbers by hand, observe the resulting curve, then fiddle with the numbers to see changes to the curve. I suppose I'd then hand-copy any changes back to the part config.

The other one is the Unity plugin, the FloatCurve editor. I don't currently have the Unity SDK installed though.

Edited by Gordon Fecyk
Grammar

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