Whats more efficient?

[Hunony]

What is better in Efficiency; 2 ion engines or more or only a single ion engine?

[kerbonara]

The simple answer is that one single engine is more efficient. Roughly, the engine ISP is the measure of efficiency of the engine. It doesn’t matter how many engines you have, the ISP will be the same (assuming all engines are the same). But adding extra engines will cause you to carry extra weight, reducing your ship “range” (delta v). The advantage of having more engines is the increased thrust, which allows you to perform maneuvers faster or even launch from the surface of certain bodies. 

Deciding between the number of engines really depends on the purpose of your ship.

[KerikBalm]

That depends entirely on the rest of the craft and what you want to do with them.

If you're using them to land on minmus, and your craft has a 1.01:1 TWR on minmus with a single ion engine, then 2 ion engines (and enough electric charge to supply both) is going to be more efficient due to gravity losses.

If you are in orbit around the sun and want to do a plane change, then a single ion engine is going to be more efficient.

[blakemw]

1 hour ago, KerikBalm said:

 

If you're using them to land on minmus, and your craft has a 1.01:1 TWR on minmus with a single ion engine, then 2 ion engines (and enough electric charge to supply both) is going to be more efficient due to gravity losses.

While true in general this may not be true when extreme deltaV is involved. For example, if it takes 160m/s to get into orbit, but the craft has 5000m/s of dV, an extra 300m/s of gravity losses may be inconsequential compared with the total dV losses from the added weight of engine. Even with a TWR of 1.01, if you do a constant altitude burn ~15% of the thrust is available for building horizontal velocity.

Dawns are especially special in this regard because they are horrifically heavy in relation to their thrust, being for example 12.5x heavier than an Ant. In general when using LF/Ox engines you want to aim for a TWR of around 2.0 to reduce gravity losses, but for Dawn and LV-N it's acceptable to go lower because their fuel is very light in relation to their weight (for example 0.25t of Xenon is nearly a third of the largest Xenon tank so if you're not wasting more than a third of a large xenon tank to gravity losses - a burn time of about 1 hour for 1 Dawn - you don't need to add another Dawn), but it might in some cases be even smarter to add a small LF/Ox stage to provide high quality thrust - for example an OscarB + Ant has the same wet mass (actually a little less) as a Dawn has dry mass, and the Ant would have a burn time suitable for like, ascent from Minmus - once the engine has burned out the dry mass is only 1/5th that of a Dawn so regardless of when the higher TWR is needed, a Dawn + Ant+OscarB, will have more deltaV than 2x Dawn. (alternatively, if the high TWR is needed very early in a mission, a Spark + FL-T200 has slightly less dry mass than the dry mass of a Dawn, so hauling around an empty Spark+FL-T200 has about the same impact on total dV as a second Dawn engine, but with a lot of high quality thrust early in the mission)

In general my philosophy is the correct number of LV-N's or Dawns is almost always "1", and if higher TWR is needed at some point during a mission it's better to include a high thrust engine providing that much high-thrust deltaV - this works both for the 0.25t Dawn and the 3t LV-N, both engines are heavy enough in relation to their thrust that you can alternatively invest in a meaningful amount of LF/Ox.

Edited May 18, 2018 by blakemw

[KerikBalm]

2 hours ago, blakemw said:

While true in general this may not be true when extreme deltaV is involved. For example, if it takes 160m/s to get into orbit, but the craft has 5000m/s of dV, an extra 300m/s of gravity losses may be inconsequential compared with the total dV losses from the added weight of engine. Even with a TWR of 1.01, if you do a constant altitude burn ~15% of the thrust is available for building horizontal velocity.

Well, again, that comes down to what you want to do. If you wanted an ion powered lander, and all it does is go back and forth from the surface of minmus, then you want the 2nd engine.

What it comes down to is how much dV you lose due to the the extra dry mass (which depends on the craft), compared to the difference of how much dV you lose to gravity drag with 1 engine or two.

So as with many things, there is no single simple answer, unless the scenario is specific. A vague scenario like in the question leads to the answer "it depends"

[Pthigrivi]

Ive also wondered how oberth factors into this. Like if youre doing a low TWR long burn (and not breaking it into 20 passes) how much does it matter that most of your burn isnt happening at Pe?

[Dman979]

This question about the game has been moved to Gameplay Questions.

[blakemw]

3 hours ago, Pthigrivi said:

Ive also wondered how oberth factors into this. Like if youre doing a low TWR long burn (and not breaking it into 20 passes) how much does it matter that most of your burn isnt happening at Pe?

The Oberth effect has diminishing returns for larger burns. Say for instance that a ship has just reached escape velocity from Kerbin (3400m/s) and is at periapsis. An additional 50m/s, thanks to oberth effect, becomes 585m/s, 500m/s becomes 1910m/s, 5000m/s becomes 7680m/s and 50000m/s becomes 53300m/s. It can be seen that the Oberth effect is much more effective at increasing the potency of a small burn than a large one. 

If you're doing a low TWR burn then hopefully you're also using a high ISP engine (if not you must hate yourself, there's really nothing to be gained from having a TWR of less than 0.5 with LF/Ox engines because at that point the engine mass becomes negligible compared with the fuel tank dry mass). With the high ISP engines Dawn and LV-N it's really easy to pile on lots of deltaV, you actually do lose a fair bit from not fully exploiting the oberth effect, I'd estimate that it can easily amount to 500-1000m/s (i.e. that's how much more the burn ends up costing). But since an LV-N stage can trivially have 4000m/s more than a comparable LF/Ox stage, and a Dawn stage can easily have 10000m/s more these "losses" are not a big deal, just have 2000m/s more than you anticipate needing and it'll be fine.

[kerbonara]

@blakemw that is really interesting. Eventually there will be cases where using multiple Dawn engines would optimize the use of the Oberth Effect, compensating the increased dry mass.

[Pthigrivi]

46 minutes ago, blakemw said:

The Oberth effect has diminishing returns for larger burns. Say for instance that a ship has just reached escape velocity from Kerbin (3400m/s) and is at periapsis. An additional 50m/s, thanks to oberth effect, becomes 585m/s, 500m/s becomes 1910m/s, 5000m/s becomes 7680m/s and 50000m/s becomes 53300m/s. It can be seen that the Oberth effect is much more effective at increasing the potency of a small burn than a large one. 

If you're doing a low TWR burn then hopefully you're also using a high ISP engine (if not you must hate yourself, there's really nothing to be gained from having a TWR of less than 0.5 with LF/Ox engines because at that point the engine mass becomes negligible compared with the fuel tank dry mass). With the high ISP engines Dawn and LV-N it's really easy to pile on lots of deltaV, you actually do lose a fair bit from not fully exploiting the oberth effect, I'd estimate that it can easily amount to 500-1000m/s (i.e. that's how much more the burn ends up costing). But since an LV-N stage can trivially have 4000m/s more than a comparable LF/Ox stage, and a Dawn stage can easily have 10000m/s more these "losses" are not a big deal, just have 2000m/s more than you anticipate needing and it'll be fine.

Yeah this is what I've been feeling intuitively. Its been figuring out the right number of LV-N's to use on big motherships and moho/eeloo missions that has me headscratching. I've been calling my TWR bottom at .25. Part of that is just not wanting to wait through a 30m+ burn though.

Edited May 18, 2018 by Pthigrivi

[blakemw]

On 5/18/2018 at 7:02 PM, kerbonara said:

@blakemw that is really interesting. Eventually there will be cases where using multiple Dawn engines would optimize the use of the Oberth Effect, compensating the increased dry mass.

Not really in practise. Unless you're doing something really dumb you'd be better off investing the mass/cost of the dawn + power system into chemical or nuclear engines for the portion of the mission where TWR actually matters. Particularly if you take cost into account Dawns, Xenon and their power systems are very, very expensive and are easily out-performed on a cost basis by the LV-N for any remotely heavy payload. If taking only mass into consideration, Xenon is pretty mass-efficient, and technically if you want to get maximum range out of like, a 50t ship, you could do so with multiple Dawns, once the payload is heavy enough the weight of additional engines becomes trifling.

But bear in mind that high ISP is better for getting large amounts of deltaV (for Dawn, 10000m/s+) than for getting small amounts of deltaV more efficiently, that's just how the rocket equation works. Once you're in LKO anywhere in the Kerbol system is within around 2000m/s from LKO which is very comfortably done with LF/Ox. Once leaving Kerbin, most destinations offer very little oberth effect relative to the insertion/ejection burn, or gravity assists (Jool system). Eve and Duna do offer meaningful oberth effect, but both are so close you don't need high ISP fuels and both offer atmospheres for aerobraking too.

Edited May 23, 2018 by blakemw

[XtraChrisP]

Also keep in mind that for any high dV burn you'll have some horrifically long burn times on account of the low thrust provided by the ion engine.  I remember I once placed a comm-sat in a polar kerbolar orbit.  I placed the SAS to Normal, fired up my ion engine, set physics time compression to 4x, grabbed a sandwich, watched some Netflix, came back and... hmm... still 15 minutes of burn left...

[grimlock14]

On 5/25/2018 at 9:57 AM, XtraChrisP said:

Also keep in mind that for any high dV burn you'll have some horrifically long burn times on account of the low thrust provided by the ion engine.  I remember I once placed a comm-sat in a polar kerbolar orbit.  I placed the SAS to Normal, fired up my ion engine, set physics time compression to 4x, grabbed a sandwich, watched some Netflix, came back and... hmm... still 15 minutes of burn left...

I'm trying to do something similar but setting up a deep space relay network and I'm looking at a 15 day burn!!!  Granted its only 1/6 of my delta V but wow.  The only snag is that I can't seem to time accelerate  while under thrust....  That 15 day burn just went from a neat little moment to a showstopper.

[4x4cheesecake]

18 minutes ago, grimlock14 said:

I'm trying to do something similar but setting up a deep space relay network and I'm looking at a 15 day burn!!!  Granted its only 1/6 of my delta V but wow.  The only snag is that I can't seem to time accelerate  while under thrust....  That 15 day burn just went from a neat little moment to a showstopper.

You cannot use regular (on rails) timewarp while accelerating but you can use physical time warp. Physical timewarp will be activated automatically when you try to timewarp while accelerating within an atmosphere and you can enable it manually with your 'mod + timewarp' keys. On a windows system (probably linux as well), 'mod' = 'alt'.

Since 15 days is still too long, even with 4x timewarp... if you are able to use and open minded for a mod, you might want to take a look at this one:

It allows you to use a customizable timewarp, i.e. 20x physical timewarp. Such a high timewarp speed can be risky to use but you should be fine with a backup savegame

Edited July 30, 2018 by 4x4cheesecake

[Gargamel]

1 hour ago, 4x4cheesecake said:

Since 15 days is still too long, even with 4x timewarp...

I mean... he could possibly just let KSP run on it's own for 4 days... 

1 hour ago, grimlock14 said:

15 day burn!!!  Granted its only 1/6 of my delta V but wow. 

This brings up some questions in my mind.   Either your ship is way over built, or way under powered, probably both actually.    While it's probably a moot point on this particular mission, if you continue to build "under powered" ships in the future, you might want to start adding some gravity assists to your mission plan to help reduce the amount burn time you have to endure.  

While granted an "under powered" ship may be way more efficient, in game and IRL, this is a game we are talking about, and you have now run into the fact that it exists on a different time line that real life.    I have always tried to build my ships so that and big burns won't take up that much time, obviously at the cost of efficiency.    I still have some weakly powered vessels, but they are all designed so that any burn I have to do can be accomplished within a game play session, even if that means setting phys warp and going to make dinner  

[grimlock14]

21 hours ago, Gargamel said:

I mean... he could possibly just let KSP run on it's own for 4 days... 

This brings up some questions in my mind.   Either your ship is way over built, or way under powered, probably both actually.    While it's probably a moot point on this particular mission, if you continue to build "under powered" ships in the future, you might want to start adding some gravity assists to your mission plan to help reduce the amount burn time you have to endure.  

While granted an "under powered" ship may be way more efficient, in game and IRL, this is a game we are talking about, and you have now run into the fact that it exists on a different time line that real life.    I have always tried to build my ships so that and big burns won't take up that much time, obviously at the cost of efficiency.    I still have some weakly powered vessels, but they are all designed so that any burn I have to do can be accomplished within a game play session, even if that means setting phys warp and going to make dinner  

It's a relay sat with 2 Gigantors, 4 Dawns and enough reaction mass for 9km/s. TWR is 0.25

I was starting from Kerbin orbit and trying to get into a lower, Kerbol orbit by braking free of kerbin and lowering my Kerbol periapis.  Don't recall my target altitude. 

Is the "2-3km/s will get you anywhere in the system" anything like the grognard at work who thinks all a coder should ever need is emacs and a compiler, and anyone who wants an IDE is a poser who doesn't deserve to call themselves a programer?

[StrandedonEarth]

4 minutes ago, grimlock14 said:

Is the "2-3km/s will get you anywhere in the system" anything like the grognard at work who thinks all a coder should ever need is emacs and a compiler, and anyone who wants an IDE is a poser who doesn't deserve to call themselves a programer?

Well, with the right window/trajectory it will get you pretty much anywhere (not so sure about Moho) but it probably won't be enough to get you back, and may not be enough to enter orbit there, depending on your destination.

[Gargamel]

4 hours ago, grimlock14 said:

9km/s. TWR is 0.25

That doesn't add up in my head.  I don't do a lot with the math of the game, but that's roughly 2.5 m/ss, over 9,000 m/s, which works out to 3600 secs, 1 hour worth of burn.   How are you even getting a 15 day burn from anything? A 15 day burn at full thrust would get you about 3,240,000 m/s of dV.    Which.... is more than luminal.   Course, my math may be off a bit...

I'm guessing you haven't actually fired up these engines yet, and are relying on the Manuever node estimate of the burn time? 

[grimlock14]

15 hours ago, Gargamel said:

That doesn't add up in my head.  I don't do a lot with the math of the game, but that's roughly 2.5 m/ss, over 9,000 m/s, which works out to 3600 secs, 1 hour worth of burn.   How are you even getting a 15 day burn from anything? A 15 day burn at full thrust would get you about 3,240,000 m/s of dV.    Which.... is more than luminal.   Course, my math may be off a bit...

I'm guessing you haven't actually fired up these engines yet, and are relying on the Manuever node estimate of the burn time? 

I kinda rage quit and deorbited that sat....

You're right in that my burn time came off the maneuver node indicator on the nav ball.  And the more I think about it, I wonder if I didn't miss read something. I know I had 9,m/s of delta V and about an hour or so of burn time sounds familiar.  The burn was intended to put me in Kerbol orbit with a apoapsis around 13.6 Gm and a periapsis between 10 and 12 Gm. 

[Gargamel]

8 hours ago, grimlock14 said:

You're right in that my burn time came off the maneuver node indicator on the nav ball.

If you have just staged to a new engine, and haven't fired it up yet, the estimated burn time is really really really off.   You need to 'burp' the engine to get it to read properly.  Just a quick Shift and then Ctrl to get it to burp.  

[blakemw]

2 hours ago, Gargamel said:

If you have just staged to a new engine, and haven't fired it up yet, the estimated burn time is really really really off.   You need to 'burp' the engine to get it to read properly.  Just a quick Shift and then Ctrl to get it to burp.  

Better yet install the Mod "Better Burn Time", fixes that issue and adds some other minor utility.