DYI Engine Balance

[spikeyhat09]

[B][U]Preface[/U][/B]
By no means is this thread meant to be interpreted as a complaint about the game's core mechanics. I have no expectation that Squad will change the game based on this thread or those relating to it, or even read this thread at all. The goal of this thread is to provide a means of discussion for people to help each other tailor their own KSP installs to meet their personal tastes. My goal is to help everyone have more fun with KSP, regardless of their opinion on current engine balance. I am not calling upon the community to mod their own versions of KSP unless they want to.

[B][U]Purpose[/U][/B]
This is about the subject of TWR (Thrust to Weight Ratio) balance between multiple engines in the game. As it stands currently, many players (or at least, I) consider some engines' performances to vastly outweigh others in terms of TWR, efficiency, and thrust vectoring ("Gimbal"). My perception is that a common gaming phenomenon known as "Power Creep" ([URL="https://en.wikipedia.org/wiki/Power_creep"]the gradual unbalancing of a game due to successive releases of new content[/URL]) is present within KSP. The most notable examples of this were the 0.16 and 0.23.5 updates, which added to the game 2.5m and 3.5m parts respectively. Prior to these updates, when the primary rocket motors were the predecessors to the 1.25m "Swivel" and "Reliant" engines, all was well. However, with the two updates mentioned and others that added new motors to the game, I allegate that the "quality" of each new part has naturally increased. As it stands now, the 1.25m engines, once considered the bread and butter of Kerbal Space Program, are now considered to be significantly worse than the rest by much of the community. Meanwhile, if you'll recall, upon the release of 0.23.5, many threads appeared on the forums claiming that the new 3.5m engines were overpowered, and I do not think they were on the wrong train of thought.

[B][U]So, What do we do about it?[/U][/B]
The easiest thing to do would be buff the old engines to the newer standard. However, upon attempting this, one will quickly discover that the resulting specifications of these upgraded parts will appear rather absurd. Additionally, this is not a permanent or elegant solution, as it does not solve the issue of power creep with further updates. The best thing to do, in my humble opinion, is to find a middle ground of engine performance between the new and old engines that balances and combines the specifications of ISP (Specific Impulse, or basically efficiency), thrust vectoring, and engine mass into a thrust output. I have spent an hour or two experimenting with various equations to come up with a good thrust function, and the one I've found to work relatively well is shown below:

[IMG]http://imgur.com/t8c1QS3.png[/IMG]

Where:
m = mass of the engine, in tons
60,000 = an arbitrary constant chosen to appropriately scale the thrust according to engine mass
ISP[SUB]vac[/SUB] = specific impulse in vacuum, in seconds
g° = thrust vectoring range, in degrees
F = force (thrust) in vacuum, in kilonewtons

[B][U]Applicable Engines[/U][/B]

[table="width: 1000, class: grid"]
[tr]
[td]Engine[/td]
[td]Old Thrust (KN)[/td]
[td]New Thrust (KN)[/td]
[td]ΔThrust (KN)[/td]
[td]Δ%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/9/9b/LV-T909_LFE.png/60px-LV-T909_LFE.png[/IMG] LV-909 "Terrier" Liquid Fuel Engine[/td]
[td]60[/td]
[td]80[/td]
[td]+20[/td]
[td]+33.3%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/9/98/LV-T30_Liquid_Fuel_Engine_recent.png/60px-LV-T30_Liquid_Fuel_Engine_recent.png[/IMG] LV-T30 "Reliant" Liquid Fuel Engine[/td]
[td]215[/td]
[td]250[/td]
[td]+35[/td]
[td]+16.3%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/6/67/LV-T45_LFE.png/60px-LV-T45_LFE.png[/IMG] LV-T45 "Swivel" Liquid Fuel Engine[/td]
[td]200[/td]
[td]264.375[/td]
[td]+64.375[/td]
[td]+32.2%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/6/6e/ToroidalAerospikeRocket.png/60px-ToroidalAerospikeRocket.png[/IMG] T-1 Toroidal "Aerospike" Liquid Fuel Engine[/td]
[td]180[/td]
[td]176.471[/td]
[td]-3.529[/td]
[td]-2.0%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/7/7f/LV-N_Atomic.png/25px-LV-N_Atomic.png[/IMG] LV-N "Nerv" Atomic Rocket Motor[/td]
[td]60[/td]
[td]225[/td]
[td]+165[/td]
[td]+275%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/4/4d/KS-25_LFE.png/37px-KS-25_LFE.png[/IMG] S3 KS-25 "Vector" Liquid Fuel Engine[/td]
[td]1000[/td]
[td]609.524[/td]
[td]-390.476[/td]
[td]-39.0%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/8/86/RockomaxPoodle.png/60px-RockomaxPoodle.png[/IMG] Rockomax "Poodle" Liquid Engine[/td]
[td]250[/td]
[td]273[/td]
[td]+23[/td]
[td]+9.2%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/9/9c/Skipper.png/55px-Skipper.png[/IMG] Rockomax "Skipper" Liquid Engine[/td]
[td]650[/td]
[td]540[/td]
[td]-110[/td]
[td]-16.9%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/1/17/Rockomax_Mainsail_transparent.png/44px-Rockomax_Mainsail_transparent.png[/IMG] Rockomax "Mainsail" Liquid Engine[/td]
[td]1500[/td]
[td]1114.84[/td]
[td]-385.161[/td]
[td]-25.7%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/1/1f/Big1.png/53px-Big1.png[/IMG] Kerbodyne KR-2L+ "Rhino" Liquid Fuel Engine[/td]
[td]2000[/td]
[td]1461.18[/td]
[td]-538.824[/td]
[td]-26.9%[/td]
[/tr]
[tr]
[td][IMG]http://wiki.kerbalspaceprogram.com/images/thumb/6/69/Quad.png/53px-Quad.png[/IMG] S3 KS-25x4 "Mammoth" Liquid Fuel Engine[/td]
[td]4000[/td]
[td]2742.86[/td]
[td]-1257.14[/td]
[td]-31.4%[/td]
[/tr]
[/table]

[B][U]Some Takeaways[/U][/B]

[LIST]
[*]The general trend seems to be that the older engines receive mild to markedly favorable changes, while the newer more powerful engines frequently see significant slashes to their maximum thrust
[*]The TWR of gimballed and high ISP motors are somewhat lowered due to their increased versatility and functions
[*]The toroidal aerospike engine, despite generally being considered inferior by most of the community, actually received a nerf to its maximum thrust in this new scheme. I believe this is due to the fact that the aerospike is actually a very good engine for its weight, but is simply too light and weak to be viable for normal sized spacecraft. I would very much like to see a larger ([URL="https://en.wikipedia.org/wiki/Aerospike_engine"]perhaps linear[/URL]) aerospike added to the game to serve as an efficient atmospheric engine for medium sized spacecraft.
[*]The nuclear engine appears to be an outlier in this new balancing scheme, receiving a huge boost to its maximum thrust. This is indicative that the model does not accurately balance this particular engine well due to its very large ISP and weight, as well as its very unique function within the game. However, it is probably indicative that this part does deserve a boost to its effectiveness in our edits.
[/LIST]

[B][U]Limitations of the Model[/U][/B]

[LIST]
[*]The model only uses default ISP, mass, and vectoring specifications to generate a maximum thrust specification, instead of deriving balanced versions of these values on its own. I do not personally feel competent enough to determine an equation for these values, but perhaps one of you will.
[*]The model is only applicable to liquid fueled main and interstage motors, and is not extendable to SRBs, radial motors, monopropellant engines, or the "Twin Boar".
[*]The model describes force as a linear function of mass when a function of a different power may be more appropriate.
[*]The model may be more effective with a coefficient higher or lower than 60,000, and may punish thrust vectoring too much or not enough.
[*]The model only describes the thrust of a given motor using its ISP in vacuum, not taking into account the ISP at sea level or saying anything about what the thrust should be at sea level. My initial thought is to use the same ratio of thrusts at sea level vs vacuum that the engines had before. (edit - typo. also, upon implementing these new part values it appears that the game automatically adjusts sea level thrust based on max thrust)
[/LIST]

[B][U]Feedback[/U][/B]

I would very much like to hear what others have to say about this model, and would love to hear ideas for improvement. The end goal is for everyone who also feels the need to change the specifications of certain parts to be able to do so either with their own model or the one provided here. Thank you for reading.

An awesome user by the name of Norpo has made a ModuleManager [URL="https://www.dropbox.com/sh/7gudaxmdi0dl3w5/AABSKxA9HNOkW6dJiU6ul550a?dl=1"]config[/URL] that applies the changes to the applicable engines shown above, with an exception being the nuclear engine (thanks Norpo!)


Feedback findings/critiques:
- The changes seem to be oriented towards sandbox mode, where every part is competing with every other part. In career mode, where many/most players spend their time, the consensus is that new motors may ought to have clear benefits over more primitive ones in the tech tree to create a sense of progression
- The example model presented grossly maladjusts "unique" engines such as the nerve and aerospike, as previously mentioned
- Engines could be further classified into groups such as low air pressure/vacuum and high air pressure/sea level parts to be evaluated separately Edited November 20, 2015 by spikeyhat09

[Streetwind]

This is pretty interesting. As someone who's more addicted to tweaking the game than to actually playing it, I've often wondered if there's a mathematical relationship between different engine performance indicators - how much lost Isp makes up for how much gained TWR and so on.

However, even if the assume for a moment that the equation you chose is the best possible mathematical model for the purpose, subjecting every single engine to the same equation has some issues. For starters, it would appear to me like you're a sandbox player, operating from a sandbox point of view where every part should be roughly equal. But with the introduction and expansion of career mode, Squad has actually [I]intentionally[/I] moved away from that model. For the last 2-3 major releases, we've had a performance progression introduced into the tech tree, which causes 1.25m, 2.5m and 3.75m engines to operate on markedly different performance baselines. This is not a result of power creep, but rather one of intention. At the points of their introduction, new engines may have had power creep, true; but for the 1.0 update, Squad redid the specs of every single engine in the game, weighted against each other. Any power creep that would have existed has been eliminated there. Instead, we now have a very rough three-tier model of engine power progression.

- Engine tier 1 has a TWR of about 15, and an average Isp around 310. Members of this tier are for example the Reliant, the Swivel, and the Terrier.
- Engine tier 2 has a TWR of about 25, and an average Isp around 300. Members of this tier are for example the Skipper, the Mainsail, and the Poodle.
- Engine tier 3 has a TWR of about 25, and an average Isp around 315. Members of this tier are for example the Mammoth, the Rhino, and the Vector.

Note, these tiers are [I]very[/I] handwavy. Actual stats are skewed widely around these rough averages; some engines trade Isp for extra TWR, while others do it the other way around, and they are all smattered around the balance curve instead of hitting it exactly. Overall, you also have engines assigned (very roughly) into three roles of "booster", "sustainer" and "vacuum engine", which dictates their stats to some extent.

Noteworthy is that steps up in engine tier largely coincide with steps up in engine size. This is probably done intentionally to avoid rendering too many parts obsolete. It would be easy to introduce a tier 3 2.5m engine; but if you never get one, then the Mainsail will always stay relevant when building 2.5m rockets, because you cannot really substitute it with something of a different size. This is also why so many people went "wait wut" when the Vector came out. The Vector clearly showed that these engine tiers aren't tied to size, but rather to tech progression; it is a tier 3 engine, regardless of its 1.25m mount. Since almost all other 1.25m engines are tier 1, this made people feel like the Vector is grossly overpowered, when it's actually not. It's perfectly balanced against the Mammoth and the Rhino, which are also tier 3 engines. It's just really unfortunately positioned because there is no true 1.25m tier 2 engine to bridge that gaping performance hole between the Vector and other engines of its size. And we also now, for the first time ever, have a situation where a higher tier engine [I]can and does[/I] substitute for older engines, even of different sizes, and thereby renders them obsolete.

And then there are the outliers, the specialty engines. The toroidal aerospike, which is pretty much positioned as a spaceplane engine that has only niche applications in rocketry; the LV-N nuclear thermal rocket, which is balanced completely on its own, with stats wide apart from the curve in both directions; the Twin Boar, which includes a fuel tank, and is secretly one of the most mis-statted and overpowered engine in the entire game; the Thud, which is underpowered but loved for its specific design applications that wouldn't be possible otherwise; and the size 0 engines, which aren't really competing with anything except each other and are overpriced and underpowered compared to anything else unlocking at the same tech levels.


I'm currently, on and off, working on a setup for a 1.1 career mode playthrough where I am adding additional engines from mods in order to have much more filled out tiers. I'll have 1.25m engines at tiers 1, 2 and 3; 2.5m engines at tiers 2 and 3; 3.75m engines at tiers 2 and 3; and a single set of 5m engines at tier 3. I'm also rebalancing almost all stats across the board, including costs, and re-sorting engine positions in the tech tree (using CTT) to achieve a progression markedly different from what I'm used to. It's not yet done, but then again, 1.1 isn't done yet either... ;) Edited November 17, 2015 by Streetwind

[AndyMt]

@Streetwind:
I fully agree with your arguments about the differences in the tech tiers. We might also consider the costs of the engines, compared to their other stats. I can't check now, but I think the Vector engine is considerable more expensive than the ones it renders obsolete.

A way to solve this could be to add more parts during progression with better stats, but otherwise using the same models/parts. This already happens with one of the probe cores if I remember correctly?

[GoSlash27]

More power to you and all... but I personally don't see the point of making all the engines balanced. So long as there are no engines that outclass everything else and no engines that are utterly worthless, I consider the differences to be part of the game progression.

Streetwind,
[quote]the Twin Boar, which includes a fuel tank, and is secretly one of the most mis-statted and overpowered engine in the entire game[/quote]
Sorta. It's only really suited to use as a lower stage because it's not stackable, but it's not nearly as cheap as a cluster of Kickbacks.

Consider:
A single Twin Boar can handle a load of 60 tonnes as a booster (min. 1.4 t/w and 1800 m/sec at 1/2 atm) and costs $26,500.
A cluster of 7 Kickbacks can do the same thing for $18,900.

Best,
-Slashy Edited November 17, 2015 by GoSlash27

[Red Iron Crown]

I like to keep my parts with the stock stats for a variety of reasons, but props to you for putting so much thought into a potential rebalance. Now you just need to make a ModuleManager config to easily implement it in stock. :)

[Johnny Wishbone]

[quote name='spikeyhat09'][B][U]Preface[/U][/B]
By no means is this thread meant to be interpreted as a complaint about the game's core mechanics. I have no expectation that Squad will change the game based on this thread or those relating to it, or even read this thread at all. The goal of this thread is to provide a means of discussion for people to help each other tailor their own KSP installs to meet their personal tastes. My goal is to help everyone have more fun with KSP, regardless of their opinion on current engine balance. I am not calling upon the community to mod their own versions of KSP unless they want to.
[/QUOTE]

Personally, I find it sad and pathetic that things in this forum have devolved to the point where this statement is even needed.

The whole point of a discussion board is to have a discussion. Unfortunately, around here, it seems like only "positive" feedback is wanted. Any sort of criticism or suggestion that something could be done better is immediately labeled as "complaining". It didn't used to be like that; I miss the old days.

[Padishar]

[quote name='Johnny Wishbone']Personally, I find it sad and pathetic that things in this forum have devolved to the point where this statement is even needed.

The whole point of a discussion board is to have a discussion. Unfortunately, around here, it seems like only "positive" feedback is wanted. Any sort of criticism or suggestion that something could be done better is immediately labeled as "complaining". It didn't used to be like that; I miss the old days.[/QUOTE]

He complained... ;)

[Wallygator]

Commendations on this. I would also suggest there are likely additional multiple related dimensions that contribute to engine assessments such as Thrust, ISP, cost, weight - all these factors likely need to be considered in kind and not to the exclusion of each other.

My opinion is that unless we (which only includes me as a consumer) start using the calculus, a proper solution shall not arise. Sadly, I forgot how to do it about 30 years ago - hence my consumer role. ;-)

Till then, I shall remain stock. Rep on its way.

[sal_vager]

Personally, I find it sad that people still try to hijack other peoples thread to complain about complaining, while failing to recognize that criticism is acceptable [I]if it is constructive and supported by reasoning[/I], simply stating that X is bad and expecting KSP's development to bend to the whims of the complainant is neither helpful or welcome :)

So stay on topic.

Also I agree with Streetwind, parts are balanced with career in mind.

[marach]

So let me see you've basically decided that all engines must be equal... so why have any engine except the reliant and swivel? In your quest for equality what you've ended up with is homogeneity which lets be honest is dull. You've fallen into the trap of overbalancing it's fairly common in people working on their first game just repeat the mantra "overbalanced is not fun. Imbalance adds fun and challenge" think how dull boss fights would be if you were equal in power.

[Red Iron Crown]

[quote name='marach']So let me see you've basically decided that all engines must be equal... so why have any engine except the reliant and swivel? In your quest for equality what you've ended up with is homogeneity which lets be honest is dull. You've fallen into the trap of overbalancing it's fairly common in people working on their first game just repeat the mantra "overbalanced is not fun. Imbalance adds fun and challenge" think how dull boss fights would be if you were equal in power.[/QUOTE]
Except that all engines are not equal in the provisional changes. They still have varying Isps, TWRs, gimballing, etc. They're not just a single engine scaled to different sizes.

[Streetwind]

[quote name='GoSlash27']Sorta. It's only really suited to use as a lower stage because it's not stackable, but it's not nearly as cheap as a cluster of Kickbacks.

Consider:
A single Twin Boar can handle a load of 60 tonnes as a booster (min. 1.4 t/w and 1800 m/sec at 1/2 atm) and costs $26,500.
A cluster of 7 Kickbacks can do the same thing for $18,900.[/QUOTE]

That's fair. The point of solid boosters is their cost efficiency, after all. But the kickback stage has multiple downsides to make up for that advantage, including fixed throttle, no thrust vectoring, physical dimensions, and the fact that you need a tier 3 launchpad for that configuration, while the Twin Boar can lift off from a tier 2 one.

Also, the Twin Boar needs to be compared more against the Mainsail than against SRBs. It's essentially an engine plus a jumbo-64, so you can subtract the jumbo's cost and mass from the Twin Boar's stats. What you get is an engine with significantly more total thrust than the Mainsail but almost the same weight, leading to the highest TWR of all engines in the entire game (~31.4, IIRC). It's also cheaper than the Mainsail despite the higher power. The only difference is that it has slightly worse Isp. In fact the relationship between the two engines reminds me a lot of that between the Reliant and the Swivel. Except one key difference: both the Reliant and the Swivel have an alternator, but only the Swivel has thrust vectoring. Meanwhile, both the Mainsail and the Twin Boar have thrust vectoring, but only the Mainsail has an alternator.

The Twin Boar would be appropriate if it dropped the thrust vectoring in return for its other advantages, like the Reliant does. It would still make a great side strapon launch booster, which is exactly where the inspiration from the engine comes from anyway. But it drops the alternator instead, which doesn't work out, because alternators are incredibly pointless things in the first place. While I have had, very rarely, the case where an orbital engine with an alternator has helped me a little bit in a mission, a launch stage is never hurt in any conceivable way by a lack of one.

[Norpo]

I could make an ModuleManager config that does the changes here in the "applicable engines" table if anyone's interested, I like the look of them and it would be nice for Sandbox players to have some balance back as "it's more expensive/in a higher tier" doesn't apply to them, I think it's a nice guideline for balance (with a few exceptions, like the Nerv, but to be fair that should probably have it's thrust bumped up a bit as well, maybe in between?)

...One thing I noticed about these changes, though, is that the Swivel now has a higher thrust than the Reliant, and had a much higher total change in thrust compared to the Reliant, whereas the old values had the Swivel have a [I]lower[/I] thrust than the Reliant. That's probably due to the Reliant having higher Isp. It'd probably be better if they had equal thrust; or possibly back to the Reliant having slightly greater thrust, as the difference between no gimbal and some gimbal is huge, at least in my opinion. I personally [I]never[/I] make a launcher with a lower stage that uses the Reliant rather than the Swivel, and when I do by accident (curse them looking almost exactly the same!) then the ship almost always leans and crashes.

[Pecan]

In addition to considering cost, etc. along with these stats there is also the issue of [I]purpose[/I].
Your figures include only vacuum Isp but for a launch engine that might not be a consideration, especially if it's intended only/mainly to be used as an first stage.
There are therefore at least two categories into which engines should be classified; launch/low atmosphere and space/high atmosphere.
Obviously there are a few specialist engines still; Ion, nuke and aerospike, with the latter being distinct in having a good Isp across a wider pressure-range than most.

[justidutch]

I am curious as to how the progression of TWR, ISP and thrust in the engines - the progression in the career tech tree - relate to the progression of our real life engines. Any basis in reality?

[Red Iron Crown]

[quote name='justidutch']I am curious as to how the progression of TWR, ISP and thrust in the engines - the progression in the career tech tree - relate to the progression of our real life engines. Any basis in reality?[/QUOTE]
Not really. Thrust and Isp are limited by the chemistry of the propellants more than anything else, today's rocket engines are only marginally more efficient than those of the '60s. TWR has improved a bit more as we've gotten more sophisticated metallurgy and a broader set of empirical data about where rockets need to be strong.

Other spacecraft systems have seen bigger improvements from more computing power and lightweight composites, as well as more advanced solar panels and electric propulsion systems.

[spikeyhat09]

[quote name='Wallygator']My opinion is that unless we (which only includes me as a consumer) start using the calculus, a proper solution shall not arise. [/QUOTE]

I am taking a college level calculus course atm, so if we were to come up with something, I could probably keep up as long as it doesn't get too dicey

[quote name='Norpo']I could make an ModuleManager config that does the changes here in the "applicable engines" table if anyone's interested...

...One thing I noticed about these changes, though, is that the Swivel now has a higher thrust than the Reliant, and had a much higher total change in thrust compared to the Reliant, whereas the old values had the Swivel have a [I]lower[/I] thrust than the Reliant...[/QUOTE]

If you're willing to dedicate the time and effort to create such a program, it would not go unappreciated.

I noticed this as well when trying out different solutions, and couldn't come up with one that keeps the Reliant above the Swivel without the rest of the engines becoming ridiculous. The main problem is that the Swivel weighs quite a bit more despite having inferior thrust in the base game. The result is that the Swivel actually has pretty terrible TWR in the base game, even in comparison to the Reliant. A Swivel with the same mass as the Reliant would have a max thrust of 220.313 KN according to the example model.

@others
I do understand the argument that engines further up the tech tree should be superior to those below. However, It seems to me like the best situation would be where the main incentive to advance the tech tree is to get parts that may be the best part to use for a [I]particular[/I] situation, but are not [I]objectively[/I] better. i.e., even once the player has unlocked every part, there should still be some reason to use parts unlocked earlier if the situation calls for it. Those familiar with certain shooters such as Battlefield (probably not the right analogy for this crowd, I know) will know what I mean. Weapons unlocked later in your progression are not necessarily better, but just allow you to have more variety. As I am primarily a sandbox player, however, one should take this view with more than a few grains of salt. Edited November 18, 2015 by spikeyhat09

[Korizan]

Some things to think about while you are playing around.
Note that engine size tends be inverse to efficiency.
Larger engines required more mass and the mass does not double on a nice linear curve as you are dealing with volume and the materials require a lot more reinforcement as the engine gets larger.
You also have to think about engines that have a high gimbal will also weigh a lot more as that motion comes at a price of weight for support.
And last but not least there are VERY few modern engines.
Technology as increased over the 30 years but most engines in use today where designed 30 years ago.
So how old are the engines in KSP are they new updated engines or just bigger versions of a 30 year old engine. Edited November 18, 2015 by Korizan

[MaxL_1023]

The RP-0 institutes "tech levels" to RCS thrusters and the Procedural SRBs. Essentially, if you have certain tech tree nodes researched, you can buy better performance using the same part. The upgraded part costs more (and has an unlock cost) but lets older engines stay competitive for quite a while. With actual engines, it is modeled by unlocking later real-life models.

An example applied to stock:

Terrier Vacuum Engine at unlock:

60 Thrust
350 ISP

Terrier Mk II, unlocked at the Mainsail Tech Level

70 Thrust
355 ISP

Terrier Mk III, unlocked at the Rhino Tech Level

70 Thrust
365 ISP
-10% Engine Weight

Sustainer like the Swivel and Skipper could gain a wider vectoring range and lighter weight, while launch stage types such as the Reliant and Mainsail would gain thrust and gain sea-level ISP.

Basically, you keep the same picture models (for simplicity, maybe a decal or something gets added on) but you get your competitive high-tech engine.

It would also give squad more chances for humorous part descriptions.

" After a 3 day snack bender, Korolev Kerman discovered a way to stick multiple combustion stages into the same engine. He was immediately committed to the insane asylum, until tests using a reliant engine showed a significant improvement in both sea level thrust and efficiency. He was then released and hired as Kebrodyne's director of rocketry research."

(References staged combustion, a real-life early improvement to rocket engines)

[Streetwind]

There's actually a standalone mod that allows you to do this with any part you like: [url]http://forum.kerbalspaceprogram.com/threads/123323[/url] :)

I considered using it for my planned career save, though I ultimately decided against because I like to have the older parts remain as an option for where they are 'good enough', since they are cheaper.

[KerikBalm]

Regarding power creep....
I don't think that's a major problem, they just did a major rebalance.
Old parts got buffed in many aspects, new parts got some nerfs (although almost everything got nerfs to Isp due to the atmo changes)

Consider:
in 0.24, after the new large parts added in 0.23.5:
mainsail: "specific impulse increased (280-330 → 320-360)"
twin boar: "reduced Isp from 320-360 s to 290-340 s"
Skipper: "Mass decreased from 4 to 3, cost increased from 850 to 2850, specific impulse increased from 300-350 to 320-370"
Poodle: "mass reduced (2.5 → 2)"


This may sound like power creep in the 2.5m line, but IRL, the skipper and poodle were useless before because a quad of the 1.25m parts were better in TWR/Isp/every metric except part count

Then in 1.0, we had a major overhaul, with pretty much all Isps being nerfed.
Noteably, the Lv-909 got a TWR increase, the KR-2L got a TWR decrease
I'll not in your rebalance, you take one of the best engines in the game, the LV-N, and massively boost its stats. To me that sounds like power creep.
They had just taken the balance in the opposite direction and nerfed the LV-N to have a mass of 3 tons instead of 2.25... that is not power creep.
They added the vector, sure its powerful, but its TWR isn't as good as the mammoth...

I'll agree that the 3.75m engines do seem to be pretty potent in terms of their TWR stats... but that's about it.

The aerospike seems to have a nice TWR... but I don't like the direction they took it... it went from 1.5 tons to 1.0 ton to get that big TWR boost, I still have only 1 engine per stack, and to use it for its aerospike properties basically requires it being a lower stage... its thrust isn't very good for a lower stage engine. I'd rather have be 1.5 tons, and have increased thrust.
Aerospikes should be best used as a core stage engine that burns from low in the atmosphere, until way up high in the atmosphere/a vacuum. If their thrust is too low to be of much use low in the atmosphere, then I might as well just have a vacuum engine that I don' activate until higher up.

IMO, the engines that need the most improvement are the LV-T45 (more TWR), the LV-T30, and the ant and spider engines.
Everything else is fine I think, and has its place (even if I'd like the LV-N TWR to be higher, and the absolute thrust of the aerospike to be higher)... but the LV-T45 and LV-T30, and those LV-1(R) LFO engines that have a TWR of only 10 (the worst LFO engine TWR in the game, excluding the rapier which is combined with a jet engine, while having poor Isp as well)... those seem to be useless.
The swivel and reliant seem like just early game engines that you use until you have something better.
I keep using aerospikes and LV-909s later in career, but from the 1.25m line, I basically retire these engines in favor of the 909 or aerospike, or another engine size.

Then again... I use SSTOs so much, that I pretty much retire all engines except the rapier for getting to orbit... then choose only vacuum engines on the payloads.
Duna gets vacuum engines (especially now in 1.05 with a mere 0.667 surface pressure), laythe gets airbreathers like Kerbin (though not just rapiers), Eve gets aerospikes or mammoths (now considering swivels).
Everything else gets KR-2Ls/poodles/aerospikes/LV-Ns/Sparks/ions, and in a few cases some radial 24-77s engines

[Streetwind]

[quote name='KerikBalm']Duna gets vacuum engines (especially now in 1.05 with a mere 0.667 surface pressure)[/QUOTE]

Typo warning - I'm sure you meant 0.067 ;)

Duna does indeed beg for vacuum engines everywhere now. The 0.3g surface gravity helps with TWR, too.

[More Boosters]

You do realize that your formula greatly favors vacuum engines right? For calculating thrust? Something that means the most to lifter engines? It's counterintuitive and unnecessarily homogenizing. It should be obvious that things with low TWR got better TWR and those with high TWR got worse TWR. Things with low TWR have low TWR for a reason and the opposite is also true.

Terrier and Poodle are just fine. The same can be said for the 3.75m engines as their TWRs are similar to 2.5m engines and only Rhino is considerably more efficient AND with better TWR than Skipper while having a similar role. The Mammoth is just fine really, it has slightly better TWR and Isp and that's it. The LFB still beats it which in turn comes with its own downsides.

I think a good indicator that this formula needs more work is that you're proposing a bigger buff to Swivel than you are to Reliant. The former is already a much superior engine in early game scenarios where you either don't have controllable canards or adding them would skyrocket the price of the booster to begin with.

I think you're putting too much stock in gimbal degrees. Not having gimbal at all is a major downside and it should count for something, but having any gimbal at all is generally good enough for control. You're also ignoring the cost aspect here, as relatively minor perks like high gimbal range and surface-attachability can be justified by the high price of Vector; as long as its not stuff like substantially increased TWR or Isp.

And well, the bit about NERVA... You really need to think this over man. Tweak the formula to make gimbal range matter less and having a gimbal at all matter more (log it up!), and try to make Swivel/Reliant better. They're really the only ones that need the help.

[theend3r]

LV-T45 having more thrust than LV-T30 doesn't make any sense when the only difference is the added gimbal.

[KerikBalm]

[quote name='Streetwind']Typo warning - I'm sure you meant 0.067 ;)

Duna does indeed beg for vacuum engines everywhere now. The 0.3g surface gravity helps with TWR, too.[/QUOTE]

Yea... that is what I meant...
I was already using vacuum engines already, my 1.04 lander was poodle equipped, and landing in the lowalnds at ~500 meters it was getting ~300 Isp... considering at 500 meters on kerbin it gets roughly 90 Isp... that is darn good.

Now I think it gets 320 instead of 300... so better than the swivel can ever hope to get.
I hadn't played around with LV-Ns on Duna since 1.0, I don't know how deverely they were affected by Duna's atmosphere at the lowest points, but now, I'm sure they'll do fine.