How to rank the different engines according to efficiency.

[Augustulus91]

After my first successful Mün landing, i am starting to build bigger rockets, (with lander cans),

I was just wondering, is there a way to rank the different engines according to efficiency? Is a poodle engine more fuel efficient (more thrust to fuel consumed) than two or three, or four (i don't know), of the smaller engines (the one with the small diameter).

Until now i always used a combination of the smaller engine, but recently i was astonished by the range and efficiency of the poodle engine.

Any suggestions, or support would be appreciated!

Thank you for your attention.

[Fuzzy Dunlop]

All the engines have a value called "ISP" or Specific Impulse. This is a measure of the amount of thrust produced per unit of fuel. Acctually the ISP changes depending on how thick the atmosphere is, so you have ISP - vacuum and ISP - sea level.

Often the highest ISP engine is the best, but sometimes thrust to weight can matter more.

[EndlessWaves]

The poodle is one of the most inefficient engines in the game.

ISP is what determines the fuel consumption with more being better. It does scale linearly - twice the ISP means half the fuel consumption.

In vacuum you generally want to use the LV-N.

[Ben Kerman]

The poodle is one of the most inefficient engines in the game.

390 vacuum ISP aren't that bad imo.

[Augustulus91]

So with an vacuum isp of 390, the 'poodle' is the second best engine to use?

I thought isp was the thrust factor at full fuel consumption, and didn't account for the amount of fuel used.

PS: how can i change to prefix to answered?

Edited May 29, 2013 by Augustulus91

[Ben Kerman]

So with an vacuum isp of 390, the 'poodle' is the second best engine to use?

I thought isp was the thrust factor at full fuel consumption, and didn't account for the amount of fuel used.

PS: how can i change to prefix to answered?

It's pretty efficient, but for orbital maneuvers always use the LV-N, as said above.

The thrust at full throttle is always the same as in the part description. Iirc ISP is modeled differently in KSP than in real life, so that might be what you mean.

To change to answered, just edit your post, does that work?

[Fuzzy Dunlop]

So with an vacuum isp of 390, the 'poodle' is the second best engine to use?

I thought isp was the thrust factor at full fuel consumption, and didn't account for the amount of fuel used.

PS: how can i change to prefix to answered?

Depends on the situation, it has a pretty poor thrust to weight ratio, so not great for launching/landing in high gravity enviroments. For very low gravity the nuclear engine is generally a lot better. I often use the poodle in upper stages (to get from about 1000 m/s to LKO). It also has the best gimbal range of any engine.

[EndlessWaves]

So with an vacuum isp of 390, the 'poodle' is the second best engine to use?

Other factors come into play like the mass of the engine, so the poodle is behind the other 390 engines like the LV-909 and Aerospike because you carry more mass for the same thrust. Generally it suffers from always being third or fourth choice in every situation.

I thought isp was the thrust factor at full fuel consumption, and didn't account for the amount of fuel used.

ISP is essentially fuel consumption per unit of thrust, plus a couple of conversion factors.

[Uberick]

^^^ Yup, pretty much what everyone else says.

I wish we could open a wormhole or something to the Kerbal universe and give them designs for good engines. . . the SSME has an ISP of like 450, and there are small engines that get almost 470 ISP.

[rhoark]

Kerbal's engines have realistic ISP for kerosene rockets. SSMEs are hydrogen rockets, which do have higher ISPs, but then the fuel is much less dense, so you'd have a lot less dV in the same volume of fuel tank.

For 2.5m engines you have increasing ISP for decreasing thrust going mainsail -> skipper -> poodle

For 1.25m there's T45 -> LV-909 (replace T45 with T30 if you really need the extra thrust and are sure you don't need the control authority)

You use the highest ISP one that gets sufficient TWR

4x LV-909s have the same thrust and ISP as one poodle, while weighing slightly less overall. The tradeoff is in part count and increased complexity in physical layout.

LV-Ns have higher ISP, but weigh more. They are usually the best choice in space, but not on very small crafts, or things that only have sub-munar missions. Many people also like to roleplay the implications of the nuclear technology.

[Xannari Ferrows]

Engine efficiency ranks by a number of things:

Where you want to use it, your scenario, and your time table.

To lift your rocket, use the highest amount of thrust you can, and save the better ISP engines for upper stages. To get the most efficiency out of your lifting engines, try launching asparagus style.

For space travel, I highly recommend using the nuclear rocket. The poodle is, of course, more powerful, but how far does that go? Of course you'll lose more power in a 390 ISP engine than an 800 ISP engine, so using the nuclear rocket get's you more Delta-V. If your going somewhere nearby, though, it doesn't matter much, but make sure to maximize fuel efficiency either way, just so you don't lose any precious Delta-V.

If you need to get somewhere shortly, it's almost always the case that you have to dock a separate propulsion system. If you need to go immediately, though, use the more powerful engines for the travelling stage rather than the high ISP, and save those for the last stage. As inefficient as those engines might be, at least try to find the one with the best thrust-to-weight as well as ISP.

Edited October 23, 2014 by Xannari Ferrows
I completely forgot a whole section.

[KevinW42]

These charts should be helpful: http://forum.kerbalspaceprogram.com/threads/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR

[Brainlord Mesomorph]

Kerbal's engines have realistic ISP for kerosene rockets. SSMEs are hydrogen rockets, which do have higher ISPs, but then the fuel is much less dense, so you'd have a lot less dV in the same volume of fuel tank.

4x LV-909s have the same thrust and ISP as one poodle, while weighing slightly less overall. The tradeoff is in part count and increased complexity in physical layout.

The 4 LV-909s cost twice what one poodle costs.

[Brainlord Mesomorph]

I divide the Total Thrust (in kiloNewtons) by the fuel consumption (in tons per second, vacc or asl).

The resulting number is good for a comparison.

And I think means : kiloNewton-seconds per ton of fuel.

[Starman4308]

I divide the Total Thrust (in kiloNewtons) by the fuel consumption (in tons per second, vacc or asl).

The resulting number is good for a comparison.

And I think means : kiloNewton-seconds per ton of fuel.

Congratulations: you've re-invented Isp*.

*In order to simplify things between metric and Imperial units, rocket scientists threw up their hands and divided by gravitational acceleration, getting a measure independent of metric/Imperial units. Isp as listed in the game is related to exhaust velocity in this fashion: "it would take Earth/Kerbin gravity this many seconds to slow the exhaust back down to 0 relative to where it started".

Anyways, for the OP's question: efficiency is a combination of mass and Isp. For the rocket equation, Isp is literally there in the equation as a component of exhaust velocity, and mass of the engine is part of your dry mass. If you're just trying to scoot a little 50 kg Stayputnik probe around, a Poodle is a bad choice, because the Poodle engine is 40x heavier than your payload, so you spend most of your fuel moving the Poodle. Unless you have an absurdly enormous fuel tank to go with it, the lesser mass of an engine like the Rockomax 48-7S or LV-1 will result in more overall efficiency*.

*Plus, you need to lift the engine up there in the first place, which is a second factor strongly favoring lightweight engines. The only two theoretical reasons to go bigger are to get higher Isp engines (such as the LV-N) or to improve your TWR, to better exploit the Oberth effect, not have to spend 30 minutes of real time on your burns, improve landing/ascent speed and efficiency, etc. There are practical reasons, such as "I find clusters of tiny engines ugly" and "Sure, the 48-7S is perfect, but I haven't unlocked it yet" as well.

Edited October 23, 2014 by Starman4308

[KerikBalm]

Ignoring funds, what you want to looke at is Thrust to weight ratio, and ISP.

The poodle pre-0.24 was pretty bad despite being tied for 2nd best ISP, because it was simply too heavy -> it was always better to use aerospikes or LV-909s (if you needed thrust vectoring).

The 48-7s, despite having relatively poor ISP, are one of the best engines in the game, because they produce so much thrust for so little weight.

You can come out ahead using more thrust per unit fuel, if your mass is low enough that you need less thrust (and thus less fuel).

The new NASA parts in .23.5 did manage to break the 48-7s stranglehold (which meant before you almost always wanted to use either a 48-7s cluster, of a LV-N, and nothing inbetween)

It comes down to how much the engine weighs relative to the rest of your craft (if its pushing around a 100ton payload in orbit, a 2.25 ton vs 0.1 ton engine doesn't make a big difference, but 800 vs 350 ISP does)

Basically, it coms down to a lot of math, and you should simply look at tavert's charts for now (I've made some crude graphs of my own to pick the fuel optimal engine for a reusable lander, tavert's charts are always for a mass optimal set up, which isn't what I want for reusable landers+ orbiting fuel depots)

[Brainlord Mesomorph]

Congratulations: you've re-invented Isp*.

Not a physicist.

I didn't know what Isp was (till now) but I did realize that thrust divided by fuel consumption would be a thing. So, thank you, I'll accept your congratulations.

(definitely not the first time that this has happened to me. I am apparently more creative and intelligent, than I am educated.)

Edited October 23, 2014 by Brainlord Mesomorph

[Tassyr]

[Deleted] I uh. I posted in the wrong window. Major apology. x.x

Edited January 12, 2015 by Tassyr

[LethalDose]

The linked charts ('optimal' engine for max dv and TWR) don't indicate how much better those engines are than other engines, and ignore a lot of other factors (form factor, part count, cost, etc.). It's really an over-simplification of a more nuanced decision...

To address the OP's question, I'd say that I_sp is the primary measure of efficiency, but other factors in regards to the rocket, it's payload, and it's mission come into play when deciding the correct engine for the job.

[Empiro]

The Poodle has been improved in the past couple of versions. It now has better TWR than the 909. I still rarely use it though.

[Dispatcher]

The chart and test results linked in my sig line attempt to compare all the stock KSP engines.