Fixing the methalox engines

[MirageNL]

KSP2 is still in active development and there are several engines and fuel types yet to be added to the game. However, the methalox department seems to be pretty complete and appears to be largely the same as it was in KSP1. I’ve been comparing the engine specifications for KSP 2 and sadly found that they are just as imbalanced as before, if not worse. I think there are a couple of possible fixes, which I would like to discuss in this topic.

In Developer Insights #17, Nertea wrote a neat article about engine design, in which the new approach to engine archetypes was explained:

• Deep Space for optimal fuel efficiency, when thrust is not that important;
• Orbital when you need sufficient thrust for some maneuvering.
• Sustainer for optimal atmospheric performance with good thrust.
• Launcher when you need maximum thrust.

I think this is a great setup, which already provides a much better guideline for choosing the right engine. The same article also stated 3 design principles:

1. Don't deviate from KSP1 for the sake of it. A methalox rocket in KSP2 should perform similarly to a similar looking Liquid Fuel/Oxidizer rocket from KSP1.
2. Engines of an archetype have similar characteristics.
3. Engines within a fuel type exist in a similar band of power, so newer or larger engines should not make older engines obsolete.

Because these principles are not always compatible with each other, some choices have to be made. For example, the Rhino had way too much thrust for an orbital engine and it looks more like a sustainer anyway, so its Isp stats were changed to fit the sustainer role. For the orbital role, the Labradoodle was introduced.  This makes for better gameplay and therefore the deviation is justified. In most other cases though, engines have been kept pretty much the same as in KSP1, even where change is definitely in order.

In my opinion, the methalox category should just consist of a simple, balanced set of 12 basic engines. One per archetype of each size and nothing more . This is already true for the LG and MD sizes, but not so much for the SM and XS sizes:

• First of all: the Reliant is terrible. While the Mammoth II and the Mainsail put out about 2.5x the thrust of their sustainer counterparts, the Reliant only provides 20% more thrust than the Swivel. It also still lacks any thrust vectoring, which already was enough reason to never use it in KSP1. However, like with all launchers, its sea level Isp has now also been nerfed to below that of the Swivel, making it utterly useless. The second design principle states that the Reliant should be proportional to other launchers, which means that it needs thrust vectoring and its maximum thrust should be increased dramatically to about 550 kN.
• While the TWRs of orbital engines are consistent across all size categories, the Swivel and Reliant TWRs are a bit low compared to their larger cousins. Their masses should be adjusted to about 1100 kg for the Swivel and 2100 kg for the 550 kN Reliant.
• It’s unclear to me what the roles of the Thud, Twitch and Spider are. Being radial engines, it’s easy to attach a lot of them to a rocket , providing plenty of thrust. However, their TWRs are too low and their sea-level Isps too high to be considered proper launchers., while their vacuum Isps are too low to be considered good sustainers. I think instead of having dedicated radial engines, radial engine plates would provide a lot more versatility.
• In the XS size there are no launcher or orbital engines. One option would be to refit the radial engines: the Twitch could easily be repurposed as a launcher by nerfing its sea-level Isp to 255 and increasing its TWR to around 25. With 3- or 4-symmetry it already provides sufficient thrust, so its mass would be lowered to about 65 kg. Changing the Spider into an orbital engine requires a bigger overhaul: Isp needs to be 330/165 and TWR about 12. Assuming 8-symmetry (because Spider), its thrust should be cut down to 0.75 kN and its mass to 6.5 kg.
• As a deep space engine, the Ant's Isp should be adjusted to 365/55 and its TWR lowered to under 8. To keep engine ratios similar to larger sizes, its thrust should be doubled to 4 kN and its mass increased to 55 kg.
• Last but not least: the Vector is spectacularly overpowered. It outperforms the Swivel, the Reliant and even the larger Skipper in both thrust and atmospheric efficiency, effectively making all of them obsolete. Even though it’s unlocked somewhat later in the tech tree, it brutally violates the third design principle and therefore has no business in the methalox category. Advanced and specialized engines such as the Vector and the Dart need a different fuel and since hydrogen is already an existing fuel type, a hydrolox category would be an excellent solution. This would also be realistic considering the real-world analogs of the Vector and the Dart.

Edited January 25 by MirageNL

[MirageNL]

I also made a pretty TWR vs. Isp graph to support my argument, showing all engine performances from sea-level to vacuum. Note the low TWR of the Reliant and Swivel, the poor performances of the radial engines, the high performance of the Vector and the low Isp / high TWR of the Ant:

Rebalancing the engines gives us a much more consistent line-up:

Note that I also nerfed the Hammer a bit to line up with the other SM SRBs: isp to 190/157 Isp and full mass to 3600 kg. Note that I used proper TWR for SRB engines by substracting dry fuselage weight, assuming it to be 1/8 of the fuel weight (similar to methalox tanks). The Clydesdale also seems overpowered with its high thrust, TWR and Isp (for an SRB). At least it doesn't make anything else obsolete.

Edited January 26 by MirageNL

[byaafacehead]

I sort of justified having the reliant and swivel be worse than the others, as starting engines they are a little more primitive. After the very start, you get to unlock other engines which are all a bit better and balanced among each other. Similarly, in my mind side boosters are nice as supplemental engines but perform a little worse than an optimal stack design, as you can just use them as a lazy slap-on fix for improper twr.

Though having them as more useful primary options does sound nice.

 

XS class I aslo think should be different, as you primarily are using them for small probes, landers and satellites. I don't know of it makes sense to have XS launchers.

Not necessarily disagreeing with your suggestions but just some thoughts there.

I do agree the vector is kinda crazy for it's size, hydrolox is really interesting idea.

Thoughts on the larger nuclear engine? I find its insane isp makes the smaller one very niche as even with a single large Hydrolox tank, the isp gain makes dV more using the larger engine, just because it is so efficient.

[Periple]

I started to write a reply but it started to get really long and boiled down to “I disagree with everything you’re suggesting” so I’ll just leave it at that.

In short: leave Reliant alone (or at most, give it a slight buff), it’s OK for later-game engines to be big improvements over earlier-game ones, and the fuels are just fine. Also the tiny engines work great for their intended missions, and I like the design flexibility of the radial ones.

 

 

[MirageNL]

My main point is that engines should be useful, unique, consistent and complete.

I actually have no big issues with the Swivel: its thrust and Isp values are fine. It's just a bit heavy, which won't even make that much of a difference in total rocket mass. So I guess consistency is my main argument for the adjustment. My problem with the Reliant is that it's already terrible as soon as it's unlocked. As a launcher, it should be much more powerful and it's hard to put to good use if it's unsteerable. Put it further in the tech tree if that makes it too useful early on. Also, tech progression is about expanding your options, not just replacing small engines with larger ones. The SM size remains relevant, so you shouldn't get stuck with low performance. 'Fixing' it by adding an overpowered Vector also isn't the right solution.

I also had my doubts about the usefulness of an XS launcher, but there's probably a niche for everything. Maybe for a small Tylo lander? It doesn't necessarily have to be the Twitch. There should definitely be both an XS orbital  and an XS deep space engine, but the Ant is now somewhere in between.

[Periple]

I use the Reliant a lot in the early game. Small rockets don’t need the gimbal and it’s brilliant for LF boosters.

[MirageNL]

A launcher is not meant for small rockets though; the Swivel can already manage those just fine. For taller rockets, I find that thrust and stability do become an issue. Also, when some side mission requires a ballistic flight to another part of Kerbin, some steerability is very useful for finding the right trajectory.

When it comes to boosters, that's a whole other discussion. SRB's tend to be better at increasing launch TWR, while methalox boosters tend to be better for adding momentum. However, the Flea and the Hammer are worse at both, so their only advantage is their small size. The Thumper adds more thrust, but less momentum, so that actually depends on what's needed. The Kickback adds more thrust AND momentum than the Reliant, making it the better choice for that size once unlocked. In the MD size, the Clydesdale is also better than the Mainsail in adding both thrust and momentum.

Compared to the Swivel, the Reliant is about a 10-20% better booster, but for reference: the Mainsail and the Mammoth are both about 2.5x better than the Skipper and Rhino respectively. Compared to the proposed 550 kN Reliant, the Kickback would still provide more thrust; just less momentum. The Thumper, Hammer and Flea now all have less thrust and momentum. I do like the idea of having to use SRB's in the early game though, so that would mean the Reliant should be unlocked later in the tech tree.

[Jeq]

Thuds are good when you want just a little bit more TWR for your skipper but don't want to add another skipper or tanks which would ruin shape of your rocket.. I use smaller sideral engines when there is no enought space  to put them on bottom, like landing a rover. Sideral attached engines are good for building your own RCS engines too.

 

Reliant and poodle seems to be most useless engines currently, i haven't found any use for them. Flea and hammer too doesn't need to exists. Kickback does most of jobs, clydesdale saves you when your rocket would be too big otherwise.

I might change my mind if money will be in game, because those engines would probably be cheap-end like in KSP 1 which made them save lot of money at certain missions.

Edited January 29 by Jeq

[Bej Kerman]

2 hours ago, Jeq said:

I might change my mind if money will be in game

It won't be, per the devs' plans.

[Periple]

2 hours ago, Jeq said:

Reliant and poodle seems to be most useless engines currently, i haven't found any use for them. Flea and hammer too doesn't need to exists. Kickback does most of jobs, clydesdale saves you when your rocket would be too big otherwise.

I use Reliant and Poodle a lot! Hammer too sometimes. Flea is kind of niche though!

In any case I don’t think the methalox engines are broken at all, I find uses for just about all of them!

[steveman0]

The poodle is one of my most used engines! Good vacuum performance in a great form factor for small orbiters and landers. Even in T4 tech I find myself using it as it's just a good fit for the size of tasks it's suited for.

[Jeq]

18 hours ago, steveman0 said:

The poodle is one of my most used engines! Good vacuum performance in a great form factor for small orbiters and landers. Even in T4 tech I find myself using it as it's just a good fit for the size of tasks it's suited for.

It was my favourite engine in KSP 1 too, but it got slightly nerfed as did other engines. It is more weighty now. I actually made calculator app for ksp 2 and it rarely suggests poodle anymore, it is heavily replaced with sparks, terriers, nerv, swerv and new labradoodle series.

Lets play? We can actually test this now, you give how much you want dV, cargo weight, orbiting body and twr for that body. I tell you most lightweight setup for that stage

 

Edited January 30 by Jeq

[steveman0]

I'm sure I'm not using the 100% most optimal engine for all scenarios. For many problems "close enough" is all I need. With many small transport craft in the medium size category, the poddle is the natural fit. For more optimized landers when I need it, I've gone with engine plates with arrays or by adding an extra radial where it makes sense, but many situations don't require that degree of effort. I also do a lot of overbuilding that allows this. I put in much more effort when I'm working on a tightly designed mission.

I am really looking forward to resources in part as I expect it will encourage much more efficient mission plans and optimizing craft design such as engine selection. I'm sure I'll diversify my selection more as I refine those builds. I'm still relatively inexperienced compared to many KSP veterans with only ~250h total between both games.

[MirageNL]

I tend to use the Hammer a lot, as it is intuitive to slap on a few SRB's for a bit of extra launch TWR. Because that's what they're meant for, right? Now it and the Flea turn out to be inferior to Reliant boosters. That doesn't feel right. Cost and reliability are real-life reasons to use SRB's, but those aren't a factor in KSP2. I expect future resources probably won't be an issue on Kerbin either.

Picking the right engine should be intuitive for most situations; we shouldn't need external calculators for that. That's why I propose the one engine per archetype per size rule. (Radials t.b.d.)

Usefulness is somewhat personal, but on paper the Poodle is great. It's short and for that size only the Trumpet has higher potential DeltaV. I also used it a lot in KSP1.

21 hours ago, Jeq said:

Lets play? We can actually test this now, you give how much you want dV, cargo weight, orbiting body and twr for that body. I tell you most lightweight setup for that stage

How about a 10t payload, >0.5 TWR (@1g) and>3000 dV? Those are pretty reasonable numbers I think.

[Jeq]

3 hours ago, MirageNL said:

How about a 10t payload, >0.5 TWR (@1g) and>3000 dV? Those are pretty reasonable numbers I think.

Without tier 3+ engines it would be:

2 nerv: 23,67t

3 terriers: 35,22t

1 poodle: 35,32t

 

From 3100 to 3500dv poodle is slightly better than terriers but after that terriers are again better up to 4300dv, terriers also are better than poodles from 1dv to 3000dv, ofcourse there is other engines at this range which comes better, like sparks. Optimal dv for terriers are 2504 and 2541 for poodles, meaning after this dV they gains percentually more weight than dV. Terriers are lot shorter than poodles.

What i get from this test is thought:

Shouldn't bigger engines be allways most lightweight setup within their optimal dV range when twr is reasonable ? This could be done by adjusting their mass. As i said, poodle was nerfed too much with its added weight.

Edited January 31 by Jeq

[MirageNL]

I hadn't really considerd how a multi-engine setup would compare, but yeah, that seems like a pretty small range for the Poodle. Have you also considered multiple Poodles? They do have better TWR and Isp, so at some point 2 Poodles should be better than Terriers again.

Also having to pick between a larger or multiple smaller engines adds to the problem. The TWR differences between orbital engines are pretty tiny, so I agree the differences should be larger. Maybe the Poodle was nerfed because of the deep space engines, but increasing thrust should fix it. The Labradoodle should be buffed accordingly; its thrust is now barely higher than that of the Tuba.

[steveman0]

I think colonies and resources will bring some more value to the labradoodle.  The terrier and poodle get a lot of use for me in part because their low profile makes them great engine choices for landers.  I anticipate the same from the labradoodle.  We just don't have much reason to build landers of such a size that uses a large engine (although I did for Tylo).  The tuba and other deep space engines make sense for interplanetary transfers, but wouldn't be very sensible to try to land. 

This is where math can only optimize so far.  Mission design is more than just TWR and dV.  How you will be using the stages of your craft are a big factor.  The poodle is great because it can serve the purpose of an orbiter stage all the way to a landing if needed.  One engine for two jobs shifts favor in a way that the TWR and dV alone can't account for.  It's the same with engine bulk.  The NERV and SWERV offer great dV, but you can build a very easy to fly and low drag (slim) rocket that uses the deep space engines.  These advantages are harder to quantify and are very dependent on your mission requirements.

[Jeq]

2 hours ago, MirageNL said:

I hadn't really considerd how a multi-engine setup would compare, but yeah, that seems like a pretty small range for the Poodle. Have you also considered multiple Poodles? They do have better TWR and Isp, so at some point 2 Poodles should be better than Terriers again.

Also having to pick between a larger or multiple smaller engines adds to the problem. The TWR differences between orbital engines are pretty tiny, so I agree the differences should be larger. Maybe the Poodle was nerfed because of the deep space engines, but increasing thrust should fix it. The Labradoodle should be buffed accordingly; its thrust is now barely higher than that of the Tuba.

If add more poodles then it just adds weight very much. it would be same 3100-3500dv range if twr is same and doubled cargo, just twice as many engines.

[kdaviper]

Do terriers have gimble?

[Periple]

14 hours ago, kdaviper said:

Do terriers have gimble?

Yes.

[MirageNL]

16 hours ago, kdaviper said:

Do terriers have gimble?

Most methalox engines do. Only the Ant, Dart and Reliant don't have it as far as I know.