LV-909 Maximum Thrust on Launch Pad = 15.1 kN with TL set at 100% WTH?

[Jaffa Cookie]

The thrust is diffrent in a vacum (VAC) and in atmosphere (ASL) since 1.0

[Landge]

I'm not mad. What gives you that idea? Surprised definitely. COULD it be a typo based on the coincidences and discrepancies of similar use engines? Why not? Anyone could have made a mistake. Did you look at the Ant engines vs the Rockomax? I didn't at first because I don't use them. My mistake. BUT after looking at their iSP settings it really raises a red flag to me of the validity of the other engines that got nerfed IMO.

Ant Engines as of 1.0:

LV-1 (Inline) VAC = 315, ASL = 85

LV-1R (Radial) VAC = 290, ASL = 260

Compared to similar

Rockomax Engines as of 1.0:

48-7S (Inline) VAC = 300, ASL = 270

24-77 (Radial) VAC = 290, ASL = 250

Can you not see the huge discrepancy between the similar engines even in the same family? Is it not logical to think that inline vs radial would be at least be close to each other, performance wise, like the Rockomax are? The Ant engines are anything but close in performance.

I'm just raising the question because of it caught my attention and I though it needed to be brought to the attention here on the forums. I would have posted it as a bug report otherwise.

Can someone on the development team answer this here or do I need to submit a bug report?

[waterlubber]

Wow, yeah, I agree with the ant engines. Something is completely wrong with that...

What we really need is a lightweight atmospheric ascent engine-for taking off from Duna, and possible Laythe.

[Foxster]

This change would have been OK if we had more engines - but we don't.

There's now just a couple of useful sea-level engines and a couple of useful vac engines. What if you don't happen to want to build a 'usual' size rocket but instead something that is small, large, lifting off in a non-Kerbin atmos or just otherwise different? Then you are stuck with trying to make the small choice of engines work for you.

This change limits what you can do in the game a lot.

We either need this change backed out (there were no players I remember asking for it) or we need a much bigger range of engines.

[NathanKell]

I think I can safely not breach NDA by saying that nope, no 1s were forgotten. Vacuum engines have crappy Isp at sea level, that's what you get for having high Vacuum Isp (unless you have an altitude-compensating nozzle, but you pay for that in low TWR and no gimbal and omgcost).

Each tier of engines (except the 3.75s and the 0.625s) has four engine types:

1. Lifter. High sea level Isp, high TWR, low vacuum Isp. Use this as your main first stage engine.

2. Vernier. Slightly worse performance than lifter, 8 degree gimbal. Use this to add more control on your first stage.

3. Sustainer/second stage. Decent to poor sea level Isp, better vacuum Isp, lower TWR. Use this as your second stage engine, or use it on the core and have groundlit boosters.

4. Vacuum engine. Very poor sea level Isp (in real life it'd be near 0 because flow separation), best vacuum Isp, poor TWR. Use this for upper stages lit high in the air, or in space. Do not expect to use it in (much) atmosphere.

There are some extras, like the LV-N (a vacuum engine on steroids), the Aerospike, etc.

[Sean Mirrsen]

I think you'll easily notice that the numbers aren't a mistake if you look at the vacuum thrust of the engines the ISP of which you've modified. Currently, the thrust value in the .cfg is given for the sea-level pressure. If you change the sea-level ISP without changing the thrust or the vacuum ISP, the engines' thrust in vacuum will drop dramatically, and will no longer be those neat round numbers you get currently.

[Landge]

OK you convinced me. I'll change it back to the new original values when realistic gravity, atmospheric and water density, physics based values are added to the game.

Until then I'll play with suspension of disbelief. lol

Edited May 3, 2015 by Landge

[Sebek]

Hello everyone,

I just registered to post this here.(I have recently got back into KSP) I see all of you talking about the ISP of this engine. But the thrust seems to me to be the issue. And I wanted to check with you guys to ask if this is just a bug on my version (running 1.0.2 at the moment) or that's how it's supposed to be :

LV-909

Max Engine Thrust (ASL): 3.64

Max Engine Thrust (Vac): 14.78

This seems ABSURD to me. Even the wiki page values are different:

[TABLE=width: 200]

[TR]

[TD]Maximum thrust[/TD]

[TD](1 atm)[/TD]

[TD]14.78 kN[/TD]

[/TR]

[TR]

[TD](vacuum)[/TD]

[TD]60.00 kN[/TD]

[/TR]

[/TABLE]

So can anyone tell me what the hell is going on?

[Red Iron Crown]

Welcome aboard, Sebek!

Are you running any mods? Those numbers are much lower than they should be, the wiki numbers are correct.

[Sebek]

Hello

Yes. I am running some:

MechJeb2

KerbalEngineer

KerbalAlarmClock

Chatterer

Kerbal Inventory

Kerbal Attachement System.

I just did a full reinstall of the game. It fixed it. I think I copied the GameData folder from the previous version 1.0 (to save my mods) but I forgot that Squad also has a folder in GameData. Somehow overwriting that messed up the max thrust for this engine. No idea how that happened. You should have seen my face when trying to land on the Mun and smashing into the ground.

But thank you for your quick reply!

[Red Iron Crown]

Overwriting the Squad folder is likely what did it. The early 1.0 releases defined sea level thrust in their configs, later versions defined vacuum thrust in the configs. I bet all your engines have stats that differ from the wiki's values, some more than others.

Replace the Squad folder with the one from the newer release and I'd imagine your issue will correct itself.

[Snark]

The fact is, it used to be the case that pretty much any engine could be used pretty much anywhere. They've made a deliberate design decision to split engines into two main classes: those which are primarily intended as atmospheric use, and those which are primarily intended for vacuum use. It doesn't make launching impossible or even hard; it's just that you have to pay attention to it as a design constraint. Use atmospheric engines for your booster stage, and vacuum engines for your upper stages.

The general nerfage to all engines' Isp, relative to pre-1.0, I expect is to compensate for the changed aerodynamics in 1.0. It takes less significantly less dV to get to LKO than it used to; they compensated for that by nerfing engines' Isp.

I don't think it's a typo, it's clearly a very deliberate choice. And I think it's a great change, it really opens up design challenges and gives a reason for just about anything, in the right place. For example, I never used to use the Poodle at all, for anything; there just wasn't any point. Sure, it had a pretty good Isp, but not noticeably better than higher-thrust engines, so what would be the point? Now I use it all the time-- it has the best non-nuke, non-ion Isp in the game, and is markedly ahead of other engines. However, it has crappy Isp in atmosphere, so it's not for atmospheric ascent. It's just a design challenge to deal with.

Similarly: I've always loved the LV-909, and continue to love it. It's just that it's not for atmospheric ascent anymore. At a vac Isp of 345, it's only barely behind the Poodle, and it's significantly better than anything else; other than the Poodle, it's the go-to engine of choice for operations in space.

Different engines for different situations means there's a place for everything. Heck, I even use TV-45's now, which I never used to; it has a better Isp than the TV-30 (more so than it used to) in vacuum, and its gimbaling ability is more important now that aerodynamic stability is a thing.

The primary argument I'm hearing against all of this seems to boil down to "The ship that I've always used, which used to work just great, doesn't work anymore in 1.0 and I don't like that." Well, that's true, but that's also true of the new aero in a big way. The rules are different, so designs need to be different. Adapt the designs to the new rules, and they work just fine.

[Sharpy]

Also: most vacuum engines reach 90-95% their efficiency at 10km. So one good solid booster will get your old design running.

[Solar71]

Best used above 10,000m

Someone said in another thread that you'll get about 98% of full power above 10,000m

[Captain Sierra]

Best used above 10,000m

Someone said in another thread that you'll get about 98% of full power above 10,000m

Doesnt change the fact that a rocket engine, by dint of being a freaking rocket engine is gonna put out more than a warm breeze even ASL. A Merlin 1D vacuum engine certainly does. I sure as heck ain't standing behind one to prove that wrong.

EDIT: And please nobody bring up the disaster that is the Aerospike.

[Lukaszenko]

I also like the change, as it's more realistic. "Grandpa been using it at the farm since '23 and it worked darn fine" is no reason to hate the change imo, especially since the old way was just wrong.

However, as someone pointed out, the change would make more sense if there were more choices.

My question is, would it make sense to not have more engines, but simply to have multiple variants of existing engines (perhaps even make them tweakable) so that you can choose where it will function best? My understanding is that the Merlin atmo and vacuum engines are the same, except with a piece of the bell chopped off. Would this same concept work with all, or at least most engines?

[Foxster]

That would be the sensible way to go. A couple of engines in each size that work at sea level and the same for vacuum.

[Snark]

That would be the sensible way to go. A couple of engines in each size that work at sea level and the same for vacuum.

But isn't that pretty much what we already have?

For 1.25m, there's Reliant/Swivel (atmosphere) versus Terrier (vacuum).

For 2.5m, there's Mainsail/Skipper (atmosphere) versus Poodle (vacuum).

For 3.75m, there's Mammoth (atmosphere) versus Rhino (vacuum).

For .625m, there's not a lot of distinction between atmosphere/vacuum, but that seems fine to me... I rarely want a .625m booster stage.

One can argue about whether Squad made the right design choice-- there are different possible approaches. IMHO, they did the right thing; I think this is a case where "less is more." I think they did a brilliant job of giving the maximum possible freedom and flexibility to build just about anything one wants, while having the minimum possible number of different pieces. It's a very Lego-like approach.

The other approach would be to have lots and lots of different parts, or to make the parts very highly tweakable so that you can have exactly the right part for any job. That's a valid approach too, but would result in quite a different game experience.

I happen to like the former approach. I like the challenge of designing a ship that does exactly what I want it to, given a constrained set of parts... and I don't find that constraint to be, well, constraining. I can build whatever I want, with a little design thought. It's what keeps me so addicted to KSP.

Edited June 14, 2015 by Snark

[MarvinKitFox]

Doesnt change the fact that a rocket engine, by dint of being a freaking rocket engine is gonna put out more than a warm breeze even ASL. A Merlin 1D vacuum engine certainly does..

Yes, it does.

In vacuum, the Merlin 1D-vac delivers an ISP of about 340

At sea level, it tests out at about 230.

This despite the fact that the Merlin1D is actually a sea-level engine, optimised for sea level operation, with only a few incidental changes made to the Merlin 1D-vac to make it a bit more efficient in space. The 1D-vac is *not* a designed-for-space engine.

for comparison, a Merlin 1D gets 311 in space, and 282 at sealevel.

Note the pattern?

In KSP the same pattern persists.

An engine designed for sealevel operation has decent ISP at pressure, and only slightly better in space.

An engine designed for space has *abysmal* sealevel ISP, and good space ISP.

On the KSP scale, the Merlin 1D is something like a smaller Mainsail in performance and role. I.E. decent launcher, tolerable boost-to-orbit, sucky but usable for space)

(Actually, take a Mainsail and divide by 9. SpaceX is only really using a smaller engine because they are *way* easier to manufacture than one monster)

Edited June 14, 2015 by MarvinKitFox

[Foxster]

But isn't that pretty much what we already have?

For 1.25m, there's Reliant/Swivel (atmosphere) versus Terrier (vacuum).

For 2.5m, there's Mainsail/Skipper (atmosphere) versus Poodle (vacuum).

For 3.75m, there's Mammoth (atmosphere) versus Rhino (vacuum).

For .625m, there's not a lot of distinction between atmosphere/vacuum, but that seems fine to me... I rarely want a .625m booster stage.

I don't see any comparison between those engines. None of them have comparable thrust or weight.

Even when you look at something that is supposedly a low-altitude engine, like the Reliant, you find the thrust at sea level on Eve is 1/3 it is in space, making it useless to launch with.

If you potter about filling contracts in the early/mid part of career and such then, yes, there are plenty of engines. If you are building things like a single-launch return mission to Eve then the choice becomes chronically poor.

Edited June 14, 2015 by Foxster

[Dispatcher]

I graphed launch/ engine tests for various KSP versions, including the latest version. These are "load" tests, with about 60% of the craft as "dry" mass. Being that these are Kerbin surface launches and flights begin low in the gravity well and with atmosphere, one may readily see which engines are intended for vacuum/ low gee. Engines not included are also intended for vacuum/ low gee, but failed to lift off from the surface. Of course some engines do well in intermediate stages, but my tests are single stage/ single engine (3 engines max for radials). The only mod used is KER. See version 1.0-ish:

https://c2.staticflickr.com/6/5330/17586916398_fe69d2d033_o.png

Edit: In these tests the Rapier Air mode reached higher than the Rapier Mixed mode because the Mixed mode flight ran out of fuel just before the cycle was to automatically change from air to closed. Each mode allows the same engine to launch with somewhat different starting masses for the craft.

Other tests (efficiency, "utility", "both") were charted but are not shown in this graph; only mentioned in connection to their "color codes".

Edited June 14, 2015 by Dispatcher

[NathanKell]

The thing that makes engines perform better at sea level is chamber pressure.

The thing that makes engines perform better in vacuum is a larger area ratio (not a larger nozzle per se, but a larger change in area from throat to exit).

If you take an engine with high chamber pressure, even if you give it a high area ratio nozzle, it'll still perform decently at sea level. That's Merlin 1D. If you take an engine actually designed for vacuum use, like RL10, it will perform quite horribly at sea level if it even works at all.

[John FX]

Also: most vacuum engines reach 90-95% their efficiency at 10km. So one good solid booster will get your old design running.

Came to post this. Fit a solid booster to get you up a bit and they will work fine.

[Foxster]

Came to post this. Fit a solid booster to get you up a bit and they will work fine.

Really?

[KSK]

Really?

Depends on the payload mass of course but yes. My stock satellite launcher for anything in the Kerbin system is still a BACC first stage and a Terrier second stage. Likewise, most if not all of my crewed ships based on 1.25m parts, use Terriers as their orbital insertion stage (which is usually the second stage). Get them high enough and they work just fine in atmosphere.