Thrust scaling with atmosphereic pressure, instead of Fuel Flow rate. Yay!

[Scoundrel]

The whole Isp thing isn't a big deal, IMHO. As the 5thHorseman pointed out, ship building is an iterative process of adding fuel tanks and boosters, and since acceleration already always improves from launch (because we're making smoke from fuel!), it won't affect the process most of us go through when building and flying a rocket... the new aerodynamics and engine rebalancing will make a much larger impact, IMHO. Though I will confess I'm curious about whether or not they're implementing Pe/Pa for optimal Isp, or if it's going to be some weird illogical linear thing so we get negative thrust on Eve.

The other thing is that since TWR will be variable we may end up having to use specific power to compare engine (im)balance. Just for fun, here are the engines as they are now:

[TABLE=class: grid, width: 600, align: center]

[TR]

[TD]Tier[/TD]

[TD]Engine[/TD]

[TD]Specific Power[/TD]

[/TR]

[TR]

[TD]1[/TD]

[TD]LV-T30 Liquid Fuel Engine[/TD]

[TD]312[/TD]

[/TR]

[TR]

[TD]3[/TD]

[TD]LV-909 Liquid Fuel Engine[/TD]

[TD]191[/TD]

[/TR]

[TR]

[TD]3[/TD]

[TD]LV-T45 Liquid Fuel Engine[/TD]

[TD]242[/TD]

[/TR]

[TR]

[TD]4[/TD]

[TD]Rockomax Mark 55 Radial Mount Liquid Engine[/TD]

[TD]235[/TD]

[/TR]

[TR]

[TD]5[/TD]

[TD]Rockomax "Poodle" Liquid Engine[/TD]

[TD]210[/TD]

[/TR]

[TR]

[TD]5[/TD]

[TD]Rockomax "Skipper" Liquid Engine[/TD]

[TD]393[/TD]

[/TR]

[TR]

[TD]5[/TD]

[TD]Rockomax 48-7S[/TD]

[TD]515[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]LFB KR-1x2*[/TD]

[TD]556[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]LV-1 Liquid Fuel Engine[/TD]

[TD]190[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]LV-1R Liquid Fuel Engine[/TD]

[TD]190[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]O-10 MonoPropellant Engine[/TD]

[TD]316[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]Rockomax "Mainsail" Liquid Engine[/TD]

[TD]441[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD]Rockomax 24-77[/TD]

[TD]327[/TD]

[/TR]

[TR]

[TD]7[/TD]

[TD]LV-N Atomic Rocket Motor[/TD]

[TD]105[/TD]

[/TR]

[TR]

[TD]8[/TD]

[TD]Kerbodyne KR-2L Advanced Engine[/TD]

[TD]717[/TD]

[/TR]

[TR]

[TD]8[/TD]

[TD]R.A.P.I.E.R. Engine[/TD]

[TD]258[/TD]

[/TR]

[TR]

[TD]8[/TD]

[TD]S3 KS-25x4 Engine Cluster[/TD]

[TD]580[/TD]

[/TR]

[TR]

[TD]8[/TD]

[TD]Toroidal Aerospike Rocket[/TD]

[TD]223[/TD]

[/TR]

[/TABLE]

*minus the integral Jumbo-64 fuel tank

Generally speaking, an engine with a specific power of 300+ is typically SSTO capable (I've actually built an SSTO that used the O-10! It was as useless as you suspect...) in 0.90.

It also shows just how crappy the Aerospike is despite being an end-of-the-tech-tree engine. Am I the only one who finds it ironic that the hybrid rocket is better as a rocket than the one engine that should essentially be the pinnacle of chemical rocket design?

Fun fact: if the Aerospike was on par with the Mainsail or 48-7S it would have thrust in the 350-400KN class!

So yeah, in terms of gameplay, new aerodynamics coupled with engines being rebalanced means that I doubt we'll even notice the Isp fix.

[kujuman]

Huh, I wonder if the atmo density is taken from the vessel or from the part. Not that'd most users would ever notice the change, but it would be pretty fun seeing different thrust on the top of a very tall rocket as from the bottom

[Van Disaster]

I've been using FAR and KIDS in ISP-fix only mode for some time - you don't really notice the difference much, it's mostly pad TWR and the way none of the dV calculators work - I have to bump indicated pad TWR up to 1.7ish and learn to treat anything KER or MJ tell me as something of a guideline for dV, but actual rocket sizes haven't changed much.

[cicatrix]

Well, as glad I am to hear KSP will start making sense, I'm getting quite nervous. I mean, with all these huge changes such as the ISP and aerodynamics I'm sure we'll all have to relearn flying one way or another.

That alone is worth their efforts. I'm getting slightly bored with the current version anyway.

And an ascent from Eve... I'm trying not to think about it right now.

[CaptRobau]

True, besides RO, it hasn't been done.

There was another one that did only that, but it was only supported by the dev for a single update and never got in the spotlight.

[Renegrade]

Fun fact: if the Aerospike was on par with the Mainsail or 48-7S it would have thrust in the 350-400KN class!

I always knew the Aerospike was junk... by the way, how does one calculate such figures? I've never seen specific power used on rockets before (I have seen it used on other things, such as piston engines)...

I've been using FAR and KIDS in ISP-fix only mode for some time - you don't really notice the difference much, it's mostly pad TWR and the way none of the dV calculators work - I have to bump indicated pad TWR up to 1.7ish and learn to treat anything KER or MJ tell me as something of a guideline for dV, but actual rocket sizes haven't changed much.

I use FAR+KIDS with the thrust correction and zero-extension enabled for most of my 'stock-ish' games (ie mostly stock but with some physics mods), and I generally aim for a pad TWR of 1.5 - it's usually around 1.3-1.4 when I launch, but it rises very quickly as fuel is expended and such.

There was another one that did only that, but it was only supported by the dev for a single update and never got in the spotlight.

No, it's been done many, many times. KIDS (the first two buttons in the option screen enables it), BTSM, RealFuels, plus a few others. And if 1.0 hadn't had thrust correction, I would have added my own on top of that.

[Northstar1989]

I'm not sure I understand what ramifications this will have. Will it make it harder, easier, or just change the way it works?

Depends on how the devs re-balance the engine Thrust and ISP ratings...

It means your engines will gain Thrust as you ascend. Which technically, in many ways will make the game easier. It will allow the devs to give orbital engines better Thrust in vacuum without having to worry about players using them for launch-stages, for instance (as their low sea-level ISP will make them EVEN MORE worthless for this than they are now...) Although that would be contingent on stock KSP having any real orbital engines besides the NERVA...

It will make anything that gives you a boost in starting altitude- like landing on a mountain rather than in a valley on Eve- more useful. And, when multiplayer comes around, I can imagine players cooperating for some sweet air-launched rocket missions (which you can already do with the mod Flight Manager for Reusable Stages installed, by the way) with one player flying the plane back down to the ground while another flies the rocket to orbit.

It will make Space-X style missions (if you have the guts to try and pull them off in stock- it requires circularizing your upper stage before your launch stage disappears by going too deep into the atmosphere while on rails- and then switching back to the launch stage and landing it...) much more useful, as these missions generally rely on more vertical initial-ascent profiles anyways (so the launch stage doesn't have to boost-back as much to return to the Launchpad) and thus spend less time inside the atmosphere.

In general, if the devs balance the engines right, it has the potential to just make the game easier and more fun (AND more realistic) overall,

Regards,

Northstar

[KerbMav]

Balance-wise, this won't affect much, but it will make engines kick a bit harder in vacuum, so landers of the same design may have higher TWR than they used to.

I dearly hope that the advertised thrust of the engines is max thrust in vacuum.

The sea-level ISP on Laythe, Duna and Eve would be different from Kerbin anyway, so the only gameplay constant is thrust in vacuum.

And now will Eve's surface atmospheric pressure be a real killer?

Rightfully so!

if you've ever used ModuleRCSFX to fix stock bugs, well, this is one of the stock bugs it fixes. So you've been having corrected thrust without knowing it. :]

Are you saying we should not be using KIDS and ModuleRCSFX at the same time?

[Sean Mirrsen]

I read through as much of the thread as I could, and I never saw it pointed out that not all bell thrusters are made to perform most efficiently in vacuum.

Source: Wikipedia article on rocket engine Isp

I wonder if any of the engines will be made into dedicated "lifter" engines that have more efficiency/power at higher pressure than in space?

Edited February 11, 2015 by Sean Mirrsen

[Esme]

Yay! Now can we pleeeeeasssee have some proper aerospike engines in the game instead of the plug-nozzle job that's been masquerading as an aerospike? (essentially an aerospike is a more pratical version of the plug-nozzle design (uses gases to form the spike instead of a hardware spike), whch is kind of an inside-out bell nozzle design)?

I am sooo looking forward to playing the v1.0 version of the game and start my campaign game to set up a self-sustaining colony on Duna!

[Renegrade]

I read through as much of the thread as I could, and I never saw it pointed out that not all bell thrusters are made to perform most efficiently in vacuum.

Yes, but those engines still perform better in a vacuum than a pressure environment. They add some to their sea level Isp in exchange for losing some vacuum Isp, but the vacuum Isp is still higher than the sea level Isp.

An example would be the Merlin 1C and 1C Vacuum engines -> the difference in vacuum performance is 304 vs 342, even though they're both burning kero/lox. The atmospheric 1C is still only 275s in the air (and the 1C-vacuum is probably .. very bad in air).

Basically, a lifter engine would have a narrower range and lower Isp overall than the equivalent vacuum-oriented engine. They could bring back the old 330s mainsail, but give it a minimum Isp of about 299 (or 300) to make it a similar gap to the 1C.

[Red Iron Crown]

I'm so very pleased this is finally being added/fixed. The list of long-awaited features that are coming in the next update keeps getting longer, this may be the one that finally unseats 0.18 as the biggest update ever.

[Alshain]

I have absolutely no idea what this means, but everyone seems happy about it so.. yay, I guess!

[NecroBones]

I have absolutely no idea what this means, but everyone seems happy about it so.. yay, I guess!

Basically it's a more realistic way of handling how the performance of rocket engines change with atmospheric pressure.

Rocket engines are more efficient in a vacuum than they are in the atmosphere, because the air is "in the way" and has to be pushed aside by the engine exhaust, reducing the effective thrust of the engine.

In the real world, setting the throttle on an engine controls the fuel-flow rate. At 100% throttle, the engine is running as hard as it can, by pumping fuel into the combustion chamber at its maximum rated speed. So if you're sitting on the launch pad, the overall thrust is weakened compared to when you're using the engine in space, but the fuel consumption rate is constant.

In KSP, in 0.90 and earlier, it "cheats" with a simpler calculation, but results in a similar loss of delta-V. Instead of reducing the thrust, KSP leaves the thrust alone. To compensate, it draws on the fuel faster at lower altitudes. In effect, this is simulating an engine that is capable of throttling up well beyond 100% to automatically compensate for the loss of thrust, so that the effective thrust remains the same, but never allows you to do so manually.

So gameplay-wise, the impact is that your TWR on the pad will be lower, but overall dV is about the same. However, TWR recovers very quickly with altitude. It will just mean slightly different planning when building a rocket, or being more reliant on SRBs as a cheap way to get an extra kick off the pad.

[Alshain]

Basically it's a more realistic way of handling how the performance of rocket engines change with atmospheric pressure.

Rocket engines are more efficient in a vacuum than they are in the atmosphere, because the air is "in the way" and has to be pushed aside by the engine exhaust, reducing the effective thrust of the engine.

In the real world, setting the throttle on an engine controls the fuel-flow rate. At 100% throttle, the engine is running as hard as it can, by pumping fuel into the combustion chamber at its maximum rated speed. So if you're sitting on the launch pad, the overall thrust is weakened compared to when you're using the engine in space, but the fuel consumption rate is constant.

In KSP, in 0.90 and earlier, it "cheats" with a simpler calculation, but results in a similar loss of delta-V. Instead of reducing the thrust, KSP leaves the thrust alone. To compensate, it draws on the fuel faster at lower altitudes. In effect, this is simulating an engine that is capable of throttling up well beyond 100% to automatically compensate for the loss of thrust, so that the effective thrust remains the same, but never allows you to do so manually.

So gameplay-wise, the impact is that your TWR on the pad will be lower, but overall dV is about the same. However, TWR recovers very quickly with altitude. It will just mean slightly different planning when building a rocket, or being more reliant on SRBs as a cheap way to get an extra kick off the pad.

Well I guess that makes sense. It sounds like that will make it a bit easier to ascend if you design your craft with a low start TWR (still above 1) and then stage off some of your thrust. Right now it does seem at times if you design with a low starting TWR you won't have a high enough thrust after staging.

Does this effect affect solid fuel too?

[klgraham1013]

Seems like Squad is backing down on a number of their highly defended positions. This can only be a good thing. I'd like to thank the Reddit users for changing their minds.

[PDCWolf]

YES!, been playing with this option enabled on KIDS for so long I already forgot the stock way of doing it (by changing flow rate you know). Also, by using KIDS and my stock patch (look at my signature) you could give yourself some time to spend on engines with actual roles instead of the homogenized ones in stock. This'll probably make it easier to adapt early if they make that kind of change.

[NecroBones]

Does this effect affect solid fuel too?

It will, but the SRBs have a narrower gap between their sea-level and vacuum ISPs, so they are less affected by it than liquid fuel engines are.

[cantab]

Regarding the aerospike, I just think it should be made stackable. It would make a competent lander or chemical interplanetary engine that way, for cases when you want more thrust than a 909 and less bulk than a 30/45 or Poodle. But it's just such a nuisance to use a single one.

[Sean Mirrsen]

What I like more is that Squad is not just appropriating mods, but making their own spin on the features, making a game rather than a simulator. Like the delta-V thing. Of course if would make sense to have an onboard computer tell you the delta-V, but it's what a simulator would do - if you have to have an Engineer on board to tell you the delta-V reserve of your ship, you have to make gameplay-relevant decisions, especially with one-man pods.

But mostly, yeah, it's nice to see KSP adding more interesting mechanics that weren't in the plans before.

[ratchet freak]

What I like more is that Squad is not just appropriating mods, but making their own spin on the features, making a game rather than a simulator. Like the delta-V thing. Of course if would make sense to have an onboard computer tell you the delta-V, but it's what a simulator would do - if you have to have an Engineer on board to tell you the delta-V reserve of your ship, you have to make gameplay-relevant decisions, especially with one-man pods.

But mostly, yeah, it's nice to see KSP adding more interesting mechanics that weren't in the plans before.

half the time they can't just copy paste the mod due to licensing issues, They don't want to get DMCAed by a disgruntled modder (there is a precedent for it)

[Scoundrel]

I always knew the Aerospike was junk... by the way, how does one calculate such figures? I've never seen specific power used on rockets before (I have seen it used on other things, such as piston engines)...

Specific power for a rocket engine is calculated as (F*Ve/2)/mass in watts (I've listed specific power as KW/Ton), where F and Ve are calculated at Pa/Pe=1... which, since there isn't a Pe listed for the parts:mad:, I've just used vacuum stats as that is the peak F and Ve for the engine. That means specific power takes into account both TWR and Isp, and combines it into a handy number.

I read through as much of the thread as I could, and I never saw it pointed out that not all bell thrusters are made to perform most efficiently in vacuum.

Yes, but those engines still perform better in a vacuum than a pressure environment. They add some to their sea level Isp in exchange for losing some vacuum Isp, but the vacuum Isp is still higher than the sea level Isp.

An engine will have its peak Isp (and hence F) at Pa/Pe=1 (that is to say, ambient pressure and exhaust pressure are equal), so most engines IRL actually have a point in atmosphere where they produce the most thrust, as they will over-expand once in vacuum. The amount of thrust lost on a well designed vacuum engine is actually not that much but it is noticeable (this is because the optimal bell would be big and heavy, so it is shortened to save weight, which results in slight over-expansion in vacuum), and I don't think anyone anywhere has designed an engine where its Isp is actually lower in vacuum than at sea level... though it I guess it might be possible since the slope for over-expansion is much steeper than under-expansion. Interestingly enough, that engine would have a pretty high TWR at launch. And would suck.

That said, you could have a lift engine whose Pe=1/2atm, which would have lots of thrust in the first 1/4 or so of atmosphere (ballpark guess: F would peak at around 10-12km), which would then taper off as it raced towards space as it would have a noticeably reduced Isp in vacuum. It's Isp in vacuum would still be higher than Isp at sea level though.

Anyways I don't think we'll get that though as that would require Squad to assign a Pe for each engine and recalculate all the stats, or fudge* it from current stats... which is theoretically doable with a quadratic equation and a healthy appetite for derivatives.

*I say fudge because the true curve isn't a parabola

Edited February 11, 2015 by Scoundrel
Actually read what I wrote and didn't like it.

[FleetAdmiralJ]

This wasn't even part of the realism crowd. I wanted this change. I don't think anyone really wanted the Isp to work like it does now.

Yeah, I'm not sure this is necessarily realism vs. not. The practical in-game effect is probably borderline. (engine efficiency will stay the same. HOW it achieves that efficiency will be different).

I think it was just a matter of some people just wanting it to calculate "properly"

[Ikaneko]

This is great news!! It seems that this is the 'get Regex to shut the hell up by giving him what he wants' edition!

I'm not complaining!

[FleetAdmiralJ]

More importantly though, I think that ALL existing rocket designs for stock are going to be scrapped with this release given they're implementing a new aero model and are rebalancing the entire game

I think this is the one release where people should accept that they'll essentially be starting over from scratch, and not sweat the details of it. The more "game breaking" changes Squad bundles into this release, the less chance they'll have to do that again in the future, and given they'll no longer be able to swing the "Early Access" flag whenever they make save-breaking changes post 1.0, and people will have a legitimate right to get ticked off about that kind of thing, that's rather important right now.

That could very well be why they're doing it now. Since people are probably going to have to scrap everything anyway, then just as well throw this change in there as well.