what is the most efficient engine?

[minerbat]

i have unlocked all stuff in science mode and i want to know what of the engines (except ion engine) is the most efficient and what other engines are best for what other situations (most power, best in atmosphere, etc...)

[VoidSquid]

Depends on what you want want to use the engine for.

Simplified, you either want an engine for vacuum operations which has low thrust but high fuel efficiency. Or you need an engine which provides good thrust to leave the atmosphere.

For the latter: Reliant - Kodiak - Bobcat - Vector, Skipper - Mastodon - Mainsail - Twin Boar, Mammoth.

For the former: Terrier - Cheetah, Poodle - Wolfhound, Rhino. All nuclear engines go here, too.

Edited January 28, 2020 by VoidSquid

[Superfluous J]

Also the size of your ship matters. A NERV on a small probe is actually less fuel efficient than a Spark because the NERV engines are so heavy.

Edited January 28, 2020 by 5thHorseman

[minerbat]

22 hours ago, VoidSquid said:

Depends on what you want want to use the engine for.

Simplified, you either want an engine for vacuum operations which has low thrust but high fuel efficiency. Or you need an engine which provides good thrust to leave the atmosphere.

For the latter: Reliant - Kodiak - Bobcat - Vector, Skipper - Mastodon - Mainsail - Twin Boar, Mammoth.

For the former: Terrier - Cheetah, Poodle - Wolfhound, Rhino. All nuclear engines go here, too.

is there a way to see or calculate the fuel efficiency?

[VoidSquid]

Yepp, it's called ISP

[AHHans]

21 minutes ago, VoidSquid said:

Yepp, it's called ISP

Well, in real life I'd be less interested how efficient a certain engine is in converting fuel to mechanical work but more interested how fuel efficient a certain engine is in e.g. my car. The most efficient (in converting the energy in the fuel into mechanical work) Diesel engines that are currently built are the engines of big cargo ships. While you might be able to construct a truck than can carry - and be powered by - one of these monsters it wouldn't be an efficient truck.

So in order to answer the question "which one is more efficient" one needs to know how you define efficiency:

• Isp is the value that gives you how efficient an engine is in converting fuel into kilo-Newtons of thrust (or kN-seconds for the physicists).
• If you want to know what is the most efficient engine for a certain craft, then I'd say: stick a selection of engines onto it, and see with which of them it gets the most dV - while having an acceptable TWR.

 

Edited January 29, 2020 by AHHans
wasn't done

[VoidSquid]

22 minutes ago, AHHans said:

Well, in real life I'd be less interested how efficient a certain engine is in converting fuel to mechanical work but more interested how fuel efficient a certain engine is in e.g. my car.

I don't understand, what your point here? Provided said engine fulfills the necessary requirements, I'd always look for the most efficient one (within the obvious restrains, ofc, weight, size, etc.), who wouldn't? So, what's your point here?

22 minutes ago, AHHans said:

If you want to know what is the most efficient engine for a certain craft, then I'd say: stick a selection of engines onto it, and see with which of them it gets the most dV - while having an acceptable TWR.

Absolutely, yes. These are kind of the minor factors one (read: I) tend to ignore, i.e.: mass, costs. Classical example: Swivel vs. Reliant vs. Terrier. Turned out (at least for me), going with a two instead of of a three-stage approach (say for a Mun mission), using Reliants only, gives the best dv for the least costs (I still stick to my three-stage approach though - who cares for money anyways?  )

Edited January 29, 2020 by VoidSquid

[AHHans]

8 minutes ago, VoidSquid said:

I don't understand, what your point here? Provided said engine fulfills the necessary requirements, I'd always looks for the most efficient one (within the obvious restrains, ofc, weight, etc.) , who wouldn't? So, what's your point here?

I wanted to illustrate the difference between just looking at the Isp of an engine and looking at what engine is best for a given craft.

[VoidSquid]

Just now, AHHans said:

I wanted to illustrate the difference between just looking at the Isp of an engine and looking at what engine is best for a given craft.

I see, yes, the other factors then, from weight to TWR to costs.

[BeanThruster]

5 hours ago, minerbat said:

is there a way to see or calculate the fuel efficiency?

Yes, you can use the rocket equation. The total deltav you can get is

deltav = g* Isp*natural log(wet mass/dry mass) 

Isp is a characteristic of the engine and you can find it in the part description, while the wet and the dry mass are the mass of your vessel before and after the burn, being their difference the total fuel consumed during the burn.

So, as you can see, it depends on your payload and on how far you want to go.

you can manipulate the above xpression, obtaining that the fuel mass you need is given by

fuel mass = dry mass * (exp(deltav/(g*Isp) - 1)

you can see that a high deltav would require a high Isp, to limit that exponential. However, the mass of the engine negatively affects the dry mass of your ship. So, it depends...

You may also check: https://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

Edited January 29, 2020 by BeanThruster

[Norcalplanner]

There was a nifty chart for older versions of KSP which would plug in various types of engines for a given payload mass, min twr, and delta V required to let you know what is the most efficient.  Pretty outdated at this point, though...

https://meithan.net/KSP/engines/

 

[CBase]

Actually you might even save the page locally and just edit the javascript configuration, if you miss sooome engine or see outdated stuff. Nothing is serverside calculated.

12 hours ago, Norcalplanner said:

https://meithan.net/KSP/engines/

 

 

[Lt_Duckweed]

2 hours ago, CBase said:

Actually you might even save the page locally and just edit the javascript configuration, if you miss sooome engine or see outdated stuff. Nothing is serverside calculated.

 

I actually pulled down from the guys repo last night and updated it for 1.81 + added making history.

[Fierce Wolf]

I want to give a special mention for the Vector. When in doubt, i just throw in a Vector and call it a day, I trust it will behalve as ought to.

[VoidSquid]

29 minutes ago, Fierce Wolf said:

a special mention for the Vector

Makes me think of my Laythe/Thylo reusable lander: Three Wolfhounds for travel within Jool system, plus three additional Vectors for landing/escaping Laythe and Thylo. Perfect  

The Vector reminds me of the old Kawasaki ad for my ZX-12R: "No questions. Just answers"  

Edited January 30, 2020 by VoidSquid

[CBase]

On 1/30/2020 at 5:39 PM, Lt_Duckweed said:

I actually pulled down from the guys repo last night and updated it for 1.81 + added making history.

Would you mind to share it ? At best generate a fork on github and push your changes.

I added some SpaceY engines for instances, the nice think about github is you don't need to host it somewhere, but can use a free html preview:

http://htmlpreview.github.io/?https://github.com/cbase2/engine_charts/blob/master/engine_charts.html