I just can't get to like how the engines are knobbled now

[FlowerChild]

Whether or not gimping the performance of engines outside their designated roles, just for the sake of realism, is an interesting constraint is mostly what's in contention.

What I would debate in that is whether it is just for the sake of realism, which I think is an invalid assertion.

I really don't care about the realism of having different roles for different engines, but what I do care about is that it provides me with additional rocket design constraints that keep me engaged while playing. I like having a wide range of variables to consider while putting together a rocket beyond just how it looks. To me, the justification for that lays with gameplay, not realism, whereas the argument for free-form creativity lays with aesthetics, and not gameplay.

To use a chess analogy, the knights aren't restricted to a L-pattern move because of realism. It's in no way realistic. However, it does make for an interesting tactical, and thus gameplay, consideration. Depending on the set, whether the knight looks like a medieval knight on horseback, or Chewbacca, is also entirely irrelevant to the gameplay itself.

Edited May 24, 2015 by FlowerChild

[mikegarrison]

Honestly, I find a lot of the engines almost interchangeable in a lot of situations. At the very least, I generally have at least two or three choices for most situations. Poodle, Terrier, 48-7S cluster, or radial 24-77s for a medium lander? Twin-Boar, Mainsail with boosters, or T30/T45 cluster for a mid-game lifter? Picking the most efficient option often gains me only a few dozen m/sec of delta-v on a ship with several hundred or thousand, so I often end up picking an engine based on other considerations--looks, TWR, configuration, cost, landing-gear clearance, whatever.

That said, I do think the current balancing could do with some tweaking for consistency. Why, for instance, does the Ant have 73% less ISP in atmo while the Spider--basically a radial Ant in other regards--has only 10% less?

Out of curiosity, what actually IS realistic in this situation? I've always thought of the Terrier as an analog for the Apollo LM engines--would those really have been 75% less efficient in atmo? I know some engines are optimized for atmo and some for vacuum, but is the difference really that big? I'd have guessed more like 25%.

The difference between Isp at sea level and vacuum depends on the nozzle (the ratio between the throat and the exit) and the combustion pressure.

With all else the same, an engine that has a high combustion pressure (like 3000 psi) might only lose 20% of its Isp at sea level, while one that has a lower combustion pressure (like 1000 psi) might lose more than half of its Isp at sea level. The Lunar Module descent engine had a combustion pressure of only 100 psi. It probably would have had an Isp of essentially zero at sea level.

[KerikBalm]

Welcome to vacuum-specific engines. On manufacturer sites you often don't even get a sea level isp listed for an engine designed for space use.

I completely understand vacuum specific engines should perform badly at sea level. As you improve the vacuum Isp, you make the sea level Isp worse.

Vacuum Isp goes higher, sea level Isp goes lower. Raise the sea level Isp, lowerthe vacuum Isp (exception: aerospikes) - which is why the sea level focused engines have less variation in Isp.

I am completely on board with having low Isps at sea level.

I'm just wondering "how low?" it should be.

How bad should they be?

Would it really be so bad if the Poodle's sea level Isp was 160 instead of 85?

It would still be losing over 50% of its thrust at sea level.

TBH, I'm not clear on the math to determine thrust loss on these engines. Are we assuming certain expansion ratios and chamber pressures for these engines?

Maybe in a break from absolute realism, but for gameplay purposes, we could set a "baseline Isp" for liquid engines.

Lets call it 300.

Want 10 more vacuum Isp? you get an engine with 290/310 sea level/vacuum Isp

Want 50 more vacuum Isp? you get an engine with 250/350 sea level/vacuum Isp

Although... that may make vacuum engines too potent...

A 2x multiplier to lost atmo Isp?

300 baseline:

Want 10 more vacuum Isp? you get an engine with 280/310 sea level/vacuum Isp

Want 50 more vacuum Isp? you get an engine with 200/350 sea level/vacuum Isp

As it is now, it seems a bit inconsistent with the sea level losses vs vacuum gains.

Consider the LV-T30 vs LV-T45:

280/300 vs 270/320

The LV-T45 gains 20 vacuum Isp, for the loss of only 10 sea level Isp- gains 2x as much vacuum Isp as lost sea level Isp

Then we see the LV-909... it gains 45 vacuum Isp, and loses..... 195 sea level Isp. It gain 0.23x as much vacuum Isp as lost sea level Isp.

I never used them as an ascent engine pre 1.0, so it doesn't really affect me... I'm completely fine with lower thrust at sea level, and this getting more severe as the engine gets optimized for vacuum operations.

I just wonder if the specific numbers should be tweaked.

[FlowerChild]

Would it really be so bad if the Poodle's sea level Isp was 160 instead of 85?

It would still be losing over 50% of its thrust at sea level.

I think it would, and this largely comes down to an issue with the size of Kerbin. Yes, it loses 50% of its thrust at sea level, but atmospheric pressure drops off so quickly due to the lack of depth to the atmosphere relative to earth, that it would rapidly become quite efficient.

Already, with the thin shallow atmosphere in 1.0, I find that mid-stage engines like the Skipper and LV-T45 have pretty much lost all relevance, because you just don't spend enough time in atmosphere thick enough to be significant for them to really come into play. If in-vacuum engines were better in atmosphere, this problem would become even worse and you could get away with extremely short first stages followed immediately by in-vacuum engines.

I've been doing a lot of tweaking of the engine values for my mod, and while I initially balked at the extremely low in-atmo ISP of vacuum engines, the more I worked with the numbers, the more that aspect made sense to me.

Edited May 24, 2015 by FlowerChild

[regex]

I'm just wondering "how low?" it should be.

IMO the isps should reflect reality, given that we're playing a rocket simulator and reality is the only reference we have. LiquidFuel/Oxidizer is roughly analogous to Aerozine 50 (not Kerosene as I have previously claimed) and dinitrogen tetroxide, and the isps line up with that pretty well. About the only place where this breaks down is with the LV-N (and be glad we're not modelling cryogenic hydrogen, SSTOs would look like a Pregnant Guppy) and with the jet engines, but we're talking stock KSP here.

Would it really be so bad if the Poodle's sea level Isp was 160 instead of 85?

Would you use it in atmosphere if its isp were 160 instead of 85? I mean, it probably wouldn't be a bad thing at all but why does it matter if you've got so little to gain, the engine ends up being just as useless for atmosphere work as before. You go to Laythe and you bring jet engines (well, most people do), you go to Duna and the atmosphere is already pretty thin, you go to Eve and you're bringing an enormous lander anyway, and on Kerbin you're mainly just trying to get into space. So what would raising the isp for so little gain do?

Consider the LV-T30 vs LV-T45:

280/300 vs 270/320

The LV-T30 is quite clearly a low atmosphere booster engine with a nozzle optimized for getting your rocket off the pad. The LV-T45 sports a much wider isp range which places it more solidly in a general lifter category; you could just as easily use it as a lifting engine or as an upper stage engine. If you're going for efficiency, the 30s are great for sub 13km work while the 45s get you out of the second atmosphere band into thin air where your upper-stage and vacuum engines can take over.

Already, with the thin shallow atmosphere in 1.0, I find that mid-stage engines like the Skipper and LV-T45 have pretty much lost all relevance, because you just don't spend enough time in atmosphere thick enough to be significant for them to really come into play.

I find the Skipper and LV-T45 highly relevant in conjunction with SRMs to help them into the second atmosphere band where they really shine. Since they're quite efficient on their own you can use them all the way to the edge of space and then let your payload do the circularizing.

[FlowerChild]

I find the Skipper and LV-T45 highly relevant in conjunction with SRMs to help them into the second atmosphere band where they really shine. Since they're quite efficient on their own you can use them all the way to the edge of space and then let your payload do the circularizing.

Hmmm...that's a fair point. I've mainly been focusing on serially staged designs early in my mod's tech tree, but I can see them working much better as a central core with radially staged boosters or asparagus designs.

Still though, I'd say they have nowhere near the same level of relevance as previously, as you just don't spend as much time in-atmo. Don't get me wrong, I'm certainly not pining for old aero or what have you, but I do think the new has brought a number of other issues to the surface that essentially boil down to Kerbin being tiny.

Edited May 24, 2015 by FlowerChild

[Foxster]

No, that's not how it works. When you optimize an engine for space you cripple it for use within an atmosphere due to bell design....

In our real world, possibly. This game is not the real world and is not even a simulation. It is an alien game world with flexible physics and a changing atmosphere.

Anyway, if it was down to something like bell design then this would affect all the engines, not just an arbitrary few e.g. the Mainsail is 8% worse in the atmosphere, the Poodle 76%.

I see zero point in making any comparisons to real-world engines. The decisions on these game engines was made entirely as a game design decision, with no basis in reality. I think to claim otherwise is giving credit to a few graphics pixels that is not deserved. Any of these engines can have any properties and some vague justification can be made for them.

I think a slightly rushed change to the engines' thrust was made. It just needs a balancing pass to restore some engines to at least some semblance of usefulness in the atmosphere, just bringing them all into line of a similar drop off in atmosphere performance of around 10-25%. No one would suffer from this change, if someone wants to role-play something more then they can just set the thrust limiter on an engine in the VAB.

Edited May 24, 2015 by Foxster

[regex]

In our real world, possibly.

In our real world most definitely.

Anyway, if it was down to something like bell design then this would affect all the engines, not just an arbitrary few e.g. the Mainsail is 8% worse in the atmosphere, the Poodle 76%.

Yes because the Mainsail is optimized for atmospheric use, which results in a tigher but lower isp range, while the Poodle is optimized for vacuum use, which results is a very loose but higher isp range. The bell design has a large influence on this, although the look of KSP's engines is totally wrong.

I see zero point in making any comparisons to real-world engines.

I do.

The decisions on these game engines was made entirely as a game design decision, with no basis in reality. I think to claim otherwise is giving credit to a few graphics pixels that is not deserved. Any of these engines can have any properties and some vague justification can be made for them.

Sure, and giving them more realistic stats makes them more interesting to play with, gives them actual game mechanics and considerations, rather than homogenizing them. Without variation there are no interesting design decisions to be made.

It just need a balancing pass to restore some engines to at least some semblance of usefulness in the atmosphere, just bringing them all into line of a similar drop off in atmosphere performance of around 10-25%. No one would suffer from this change

Debatable since the limitations of engines is one area that affects design efficiency. That's gameplay. Homogenizing the engines removes all gameplay considerations and consequences from engine choice.

[Foxster]

Homogenizing the engines removes all gameplay considerations and consequences from engine choice.

It is not about making the engines inter-changeable or homogenizing them. Engine diversity would remain even if some engines were not completely useless in the atmosphere thanks to differences in size, overall thrust, etc.

Look, I get what people are saying. There should be a trade-off between an atmosphere efficient engine and a vacuum efficient engine. I am not advocating that every engine should be capable of doing everything. The problem is the small number of engines we have to design with. If there were a few more good atmosphere engines that were less effective in vacuum then it would open up design options, but we actually have just a couple of engines with good vacuum Isp and a lot with poor atmosphere performance.

If we aren't going to get more engines anytime soon then the next best thing is make what we have at least a little more useful.

[r4pt0r]

In our real world, possibly. This game is not the real world and is not even a simulation. It is an alien game world with flexible physics and a changing atmosphere. ~snip

Oh dear Foxster. Whether you know it or not, with that sentence you have doomed this thread to be a battlefield for the realism crowd.

Already now, I can hear their great army assembling.

[Foxster]

Oh dear Foxster. Whether you know it or not, with that sentence you have doomed this thread to be a battlefield for the realism crowd.

Already now, I can hear their great army assembling.

Yes, I too can hear the distant rumble of keyboards and the smell of bile wafting across the battlefield.

I'm actually cool with the whole realism bit if it's consistently applied and isn't just used to justify a point and is then conveniently ignored elsewhere. Case in point: all this bell-shape stuff. None of the engines in KSP model accurately a real engine, they are just a bunch of surface pixels that could have any properties but people want to find something about that arrangement of pixels to justify their position. Ignoring that another engine has a similar pixel arrangement but does not suffer from the same drawbacks implied.

[selfish_meme]

I think it would be a good thing to have more engine choice. I also think the game balance on engines right now is good for the number of engines we do have, they have their niches. We have a few more niches to fill though, like small jets and large nukes etc.

[Wanderfound]

It is not about making the engines inter-changeable or homogenizing them. Engine diversity would remain even if some engines were not completely useless in the atmosphere thanks to differences in size, overall thrust, etc.

Look, I get what people are saying. There should be a trade-off between an atmosphere efficient engine and a vacuum efficient engine. I am not advocating that every engine should be capable of doing everything. The problem is the small number of engines we have to design with. If there were a few more good atmosphere engines that were less effective in vacuum then it would open up design options, but we actually have just a couple of engines with good vacuum Isp and a lot with poor atmosphere performance.

If we aren't going to get more engines anytime soon then the next best thing is make what we have at least a little more useful.

In each size category, there is a core engine, booster engine, vacuum engine and vernier engine. Three of those four are already fully or partially optimised for low-altitude use.

Vacuum engines suck at sea level; that's part of what makes them vacuum engines.

[Starhawk]

I really don't care about the realism of having different roles for different engines, but what I do care about is that it provides me with additional rocket design constraints that keep me engaged while playing. I like having a wide range of variables to consider while putting together a rocket beyond just how it looks. To me, the justification for that lays with gameplay, not realism, whereas the argument for free-form creativity lays with aesthetics, and not gameplay.

I think that this is a very important point in regards to this discussion.

The impression I get is that there are (at least) two distinct concepts of creativity being discussed.

Creativity in an aesthetic sense seems to be what Foxster is talking about. The ability to choose from a broader 'palette' of engines for any condition in order to create a specific configuration or appearance. The desire here is that there should be less variability in engine performance, thus allowing fewer constraints in design choice. An alternative desire here is more engines to choose from.

Creativity in an engineering sense seems to be what regex is pointing at. The variation in engine performance faces the engineer with choices that matter in the final performance of the design. Creativity in this sense is about finding a way to make something work within design constraints created by greater variability in stats. The desire here is to keep the significant variability that we now have in engine performance.

My playstyle places an extremely low value on aesthetics. I want to build stuff that works. The design process is a very important and interesting part of the gameplay to me, and the improvement in variability of engine stats is very welcome from my point of view.

It is very interesting that the conditions that some find challenging and engaging, others find constraining and limiting. That is probably due to both human nature and the fascinating diversity of players and playstyles found in this community.

Happy landings!

[Roderik]

Now you don't just slap a 48-7S on something and call it good.

that engine has a name now you know.

[NonWonderDog]

Out of curiosity, what actually IS realistic in this situation? I've always thought of the Terrier as an analog for the Apollo LM engines--would those really have been 75% less efficient in atmo? I know some engines are optimized for atmo and some for vacuum, but is the difference really that big? I'd have guessed more like 25%.

The Apollo Descent Propulsion System had a chamber pressure of 100 psia, had an expansion ratio of 47.5:1, and ran on a 1.594 mixture ratio of N2O4 to Aerozine 50 (50% UDMH, 50% Hydrazine). Pop those numbers into Rocket Propulsion Analysis Lite using the default efficiency coefficients and you get a vacuum Isp of 316 s. That's only a couple percent better than real life, so so far so good. The predicted sea level Isp is... -925 s.

What's going on here? Well, there are two components to thrust. First is the momentum thrust, massflow * exhaust velocity. Equal and opposite reaction, etc. The second is the pressure thrust, (exit pressure - ambient pressure) * exit area. (These terms are actually inversely proportional, and max thrust (greatest Isp) is when the pressure thrust is zero.)

In a vacuum the flow expands to fill the entire nozzle, and the nozzle exit pressure of the Descent Propulsion System is only 0.01 bar. If the flow expanded to fill the whole nozzle at sea level, the pressure thrust would overwhelm the momentum thrust term, leading to negative thrust. Literally the naive prediction is that the rocket engine becomes a gigantic vacuum cleaner that sucks itself backwards... by blowing in the same direction.

Obviously, this doesn't actually happen. The flow separates from the nozzle, like the flow over a wing separates near stall. The result is the effective exit area is much lower, but a lot of the energy of the propellent goes into turbulence in the nozzle. This is bad, by the way. The flow is unstable and the thrust goes off in random directions. It might even shake the nozzle to pieces and cause an explosion, or burn a hole in the nozzle wall and cause an explosion. RPA Lite includes a simple flow separation predictor, but this case is a bit too extreme. It still predicts a sea level Isp of -95 s.

So how much thrust would the Descent Propulsion System produce at sea level? Very little. It might not even light off. At 5 km it might have an Isp of 60 s or thereabouts. Compare that to 308-311 s in vacuum.

I really like the new engines, by the way. My only real complaint is that the Isp values on the LVT-45 and LVT-30 are swapped. They make a lot more sense realistically, visually, and gameplay-wise the other way around.

Edited May 24, 2015 by NonWonderDog

[Rhomphaia]

that engine has a name now you know.

Not so fussed on some of the names, I am happy that the Rockomax engines now have more formal designations though.

As for the balance tweaks, I like them. the "Spark" was overpowered when it was introduced, then got Buffed in the next update, to the point where the only reason to use anything else other than the LV-N was partcount or aesthetics. Good to see some balance being considered.

[regex]

that engine has a name now you know.

vOv "48-7S" is still part of the title field in the game.

[FlowerChild]

I really like the new engines, by the way. My only real complaint is that the Isp values on the LVT-45 and LVT-30 are swapped. They make a lot more sense realistically, visually, and gameplay-wise the other way around.

I'm not sure I understand that bit, at least not the gameplay part: the LV-T45 seems to be balanced as a mid-stage engine with a lower thrust to weight ratio, but with thrust vectoring (which you need higher up as you can just use fins lower down). If the ISPs were reversed, you'd end up with a mid stage engine with no thrust vectoring, and a lifting engine with crappier thrust to weight.

[NonWonderDog]

I'm not sure I understand that bit, at least not the gameplay part: the LV-T45 seems to be balanced as a mid-stage engine with a lower thrust to weight ratio, but with thrust vectoring (which you need higher up as you can just use fins lower down). If the ISPs were reversed, you'd end up with a mid stage engine with no thrust vectoring, and a lifting engine with crappier thrust to weight.

At least building big pointy rockets, thrust vectoring is most important in the first stage. Especially strap-on boosters need to gimbal, since otherwise you don't have any roll control. In the second stage the rocket's just following prograde, and rarely needs to steer at all.

If you're using just 1.25 m parts you can do things pseudo-Soyuz style with an LVT-30 core surrounded by LVT-45 strap-ons. An LVT-45 sustainer surrounded by LVT-30 boosters looks worse, has no roll control, and doesn't have much directional control.

It's hard for me to find any real use for a powerful non-gimballing rocket engine at all, but it's worst as a first-stage lifter engine. With all the magic torque in the game, though, it sort of works on lighter upper stages that can turn themselves.

That said, asparagus staging and moving fins on launch vehicles seem so weird and silly-looking to me that I usually don't even consider them if I can solve a problem traditionally. Those change the picture a lot. In any case, I think "better vac Isp" vs "gimballing" is a more interesting trade-off than "imperceptibly-greater thrust and marginally lower weight" vs "gimballing AND better vac Isp".

[KerikBalm]

IMO the isps should reflect reality, given that we're playing a rocket simulator and reality is the only reference we have.

Well, lets consider a real engine: The space shuttle main engines.

They run the whole ascent, but only supply ~25% of the thrust on lift off. With a vacuum Isp of 452 seconds, it is definitely close to the limit for a vacuum engine.

Its sea level Isp: 366.

If you can fine a vacuum engine with such a severe Isp difference as the poodle, I'd like to know about it.

Even the AJ10 (apollo service module engine, operated onl in a vacuum) has a chamber pressure of ~9 atmospheres, I can't see it suffering so badly from the presence of 1 atmosphere.

Would you use it in atmosphere if its isp were 160 instead of 85? I mean, it probably wouldn't be a bad thing at all but why does it matter if you've got so little to gain, the engine ends up being just as useless for atmosphere work as before.

Nope, I didn't even use it when its Isp was 270, and its thrust was constant. As I said "I never used them as an ascent engine pre 1.0, so it doesn't really affect me"

However, it may placate some people that are complaining right now (let them try to be creative and use it with its better atmospheric performance), and if it doesn't make things ridiculously unrealistic, why not?

It was *just* changed, and its reasonable to question if their change was appropriate. What was their logic in making it *so* bad in the denser atmosphere?

[Hotaru]

It probably would have had an Isp of essentially zero at sea level.

So how much thrust would the Descent Propulsion System produce at sea level? Very little. It might not even light off. At 5 km it might have an Isp of 60 s or thereabouts. Compare that to 308-311 s in vacuum.

Very interesting, thanks! So it really is realistic to have some engines be practically useless in atmo.

Of course whether it makes sense from a gameplay point of view is an unrelated question--personally I think it does, but obviously plenty of others disagree!--but it's good to know the real-world baseline at least.

PS.

that engine has a name now you know.

Funny how some of the names have kind of "caught" and others haven't. I almost immediately got used to calling the LV909 the "Terrier," but I still call a lot of them by their numbers. I like some of the new names but not all, some of them lack the nice sense of progression in size of, for instance, Poodle, Skipper, Mainsail, that helps remember which is which. The old SRB's are now "Hammer," "Kickback," and "Thumper," but I sure can't remember which name goes with which booster--there's no indication which is bigger, no connection between the names and the character of the particular rocket. I'll probably keep calling them Trashcan, BACC, and NASA SRB.

Edited May 24, 2015 by Hotaru

[Laie]

My only real complaint is that the Isp values on the LVT-45 and LVT-30 are swapped. They make a lot more sense realistically, visually, and gameplay-wise the other way around.

IMO they only make sense in the context of career, where they are the first, presumably low-tech, engines. They're bad, both of them. Economies of scale are nice and fine, but is it really necessary that the entire 1.25m lineup is so inferior?

[FlowerChild]

That said, asparagus staging and moving fins on launch vehicles seem so weird and silly-looking to me that I usually don't even consider them if I can solve a problem traditionally. Those change the picture a lot. In any case, I think "better vac Isp" vs "gimballing" is a more interesting trade-off than "imperceptibly-greater thrust and marginally lower weight" vs "gimballing AND better vac Isp".

Imperceptibly better thrust? It's like 200 vs 170 thrust at sea level for the LV-T30 vs T45 with ISP factored in.

Not sure what kind of rockets you're building where a 15+% increase in thrust on liftoff (when you need it most) isn't noticeable, but I'll take it any day.

Edited May 25, 2015 by FlowerChild

[regex]

Well, lets consider a real engine: The space shuttle main engines. They run the whole ascent, but only supply ~25% of the thrust on lift off. With a vacuum Isp of 452 seconds, it is definitely close to the limit for a vacuum engine. Its sea level Isp: 366.

The SSME is probably optimized for use at all altitudes, its isp indicates a pretty generic usage. The SSME is also a pretty special case in rocket engines considering the vessel it was designed to be used with. It's telling that it required two very large SRMs to help it to an altitude where the fuel mass and thrust were finally adequate to get the craft to orbit.

If you can fine a vacuum engine with such a severe Isp difference as the poodle, I'd like to know about it.

Even the AJ10 (apollo service module engine, operated onl in a vacuum) has a chamber pressure of ~9 atmospheres, I can't see it suffering so badly from the presence of 1 atmosphere.

Most vacuum engines don't bother listing sea level isp because it's almost always non-existent. The AJ10 doesn't get an SL isp listed here, and virtually every dedicated vacuum engine listing will have no sea level isp listed. Going from what NonWonderDog posted earlier:

The Apollo Descent Propulsion System had a chamber pressure of 100 psia, had an expansion ratio of 47.5:1, and ran on a 1.594 mixture ratio of N2O4 to Aerozine 50 (50% UDMH, 50% Hydrazine). Pop those numbers into Rocket Propulsion Analysis Lite using the default efficiency coefficients and you get a vacuum Isp of 316 s. That's only a couple percent better than real life, so so far so good. The predicted sea level Isp is... -925 s.

we can safely assume that its isp at sea level was pretty much non-existent or nothing to write home to mom about.