Best Liquid Fuel Engine?

[KerikBalm]

8 hours ago, Mastikator said:

The Mammoth has the highest TWR and Isp at sea level. It's the best by those metrics.

Its best at kerbin sea level, but that is far from being "best". Look at @Yakuzi 's post. Its certainly not what I want to use on my return stage from moho, or a minmus lander.

At Eve sea level, its Isp is worse than an Aerospike, in space and the upper atmosphere, its Isp is worse than an aerospike.

It also lacks a bottom node, which is worth consideration.

It performs better than any other rocket engine in a narrow atmospheric pressure range. If one were to use airbreathing SSTOs a lot, then by the time one needs rocket engines, the atmosphere is too thin to matter for Isp purposes, and the Mammoth's advantageous regime is gone.

I use large cargo carrying SSTOs a lot, so often I never consider the Mammoth. I do consider the mammoth for my disposable launchers, but that isn't necessarily a majority of my flights.
And that was done on a 3x rescale game , where one still has to spend about 4000 m/s to get to orbit after the airbreathing phase (one needs to get to >4,000 m/s orbital velocity, but due to the thicker atmosphere and drag being proportional to v^2, the drag losses are much higher). In stock I can launch even more payloads by SSTO

[Mastikator]

The Aerospike lacks a bottom node too so that's hardly a point in favor of the Aerospike.

The Mammoth has a TWR of 27 in vacuum, the Aerospike has a TWR of 18 in vacuum. Higher TWR means shorter burns which means less waste, it is significant.

I can't find any metrics of the two on Eve sea level so I can't comment on that. But it is worth mentioning that the Mammoth is a good candidate for a SSTO rocket, if you're up to some vertical suicide burn landings then Mammoth is a great choice and the Aerospike is not.

[peteletroll]

The aerospike does have a bottom node since the 1.0.5 remodeling.

[GoSlash27]

For simple LFO 2 stagers to orbit, here is a list of "best" combinations

Pyld Upper/ Lower

<.14t Ant/ Spark
.14-1.4t Spark/Reliant
1.5-2.5t Terrier/ Reliant
2.6-4.3t Terrier/ Skipper
4.4-7.9t Aerospike*/ Skipper
8.0-13t Aerospike*/Twin-Boar
14-18t Poodle/ Twin-Boar
19-25t Skipper/ Twin-Boar
26-47t Skipper/ Mammoth

* The Aerospike was selected for these cases because it makes a lighter upper stage, but it's an expensive engine. You'd really be better- off with a Poodle here for total cost.

Best,
-Slashy

[xendelaar]

On Sat Jun 10 2017 at 2:24 AM, Mastikator said:

The Mammoth has the highest TWR and Isp at sea level. It's the best by those metrics.

but it also weighs like a mammoth so putting it under a payload of .1 tonnes seems a bit... excessive 

like @GoSlash27 said.. there is no best engine.  it all depends on what your payload is and where you want to go. 

Goslash made an awesome article in the past about the reversed rocket equation. if you put the maths from his article into an excel sheet you can calculate which engine is best for your current goal. that's what i did.

if you like i can try to find the article for you. i could even send you the excel file i made but from experience it's usally difficult to understand somebody's else's spreadsheet.. send me a PM of you're interested. 

[ARS]

The best engine is the one that's suitable for your needs, putting large engine on small rocket is overkill, while putting small engine on large rocket is underpowered. For small sized rocket, I'm usually using skipper for first stage, medium sized rocket uses mainsail, while the large sized ones uses mammoth or 4 vectors. Strapping a couple of SRB might help to get some extra thrust, I'm usually using kickback SRB instead of thumper SRB. On large sized rocket, I'm usually replacing SRBs with wild boar boosters

[NSEP]

And engine with no gimbal is an engine me no likey.

[KerbalNetwork]

Thanks a lot... got my first mun probe into orbit.

[suicidejunkie]

Having puttered around the low tech tree for quite some time, I find swivels unbearably posh.

The Reliant on the other hand will be your best friend and will always stand by you.

Reliant + Whiplash = SSTO spaceplane.

[Aegolius13]

On 6/11/2017 at 6:09 AM, GoSlash27 said:

For simple LFO 2 stagers to orbit, here is a list of "best" combinations

Pyld Upper/ Lower

<.14t Ant/ Spark
.14-1.4t Spark/Reliant
1.5-2.5t Terrier/ Reliant
2.6-4.3t Terrier/ Skipper
4.4-7.9t Aerospike*/ Skipper
8.0-13t Aerospike*/Twin-Boar
14-18t Poodle/ Twin-Boar
19-25t Skipper/ Twin-Boar
26-47t Skipper/ Mammoth

* The Aerospike was selected for these cases because it makes a lighter upper stage, but it's an expensive engine. You'd really be better- off with a Poodle here for total cost.

Best,
-Slashy

That is a really helpful list! I should tape this to my monitor or something.  

As far as the aerospike vs Poodle issue, the Poodle has another advantage besides cost - it maintains the 2.5m form factor.  With the aerospike, if you want to use 2.5m parts above, you'll have to deal with the drag and wobble issues that come from changing back and forth between form factors. Oh, for an effective stock 2.5m shroud for the Terrier and Dart.  

On the other hand, if you only want 1.25m parts on the upper stage, that's a factor in favor of the Dart. 

 

9 hours ago, NSEP said:

And engine with no gimbal is an engine me no likey.

I used to avoid the Reliant like the plague due to this issue.  But I've found that putting a couple of Spiders on the bottom is surprisingly effective to allow a little steering.  They're lighter and cheaper than control surfaces, and while the ISP is not great, the overall fuel consumption is low enough it does not make a big difference.   The drag is also pretty low, and a little drag at the back of your rocket is not a bad thing anyway.   Twitches are also a an option if you want a little boost in overall thrust.  

 

 

As far as "best" overall engine, as said a million times above, it depends on the task and there is no single best.  But as far as most generally useful overall, I'd say in Career mode, the Skipper is a very strong contender.  It's a great launch engine for all sorts of medium sized loads, has some use as a sustainer in heavy lift rockets, and can hold its own in some SSTO designs.  In Science/Sandbox, the Vector probably has it beat, but that cost is pretty prohibitive for Career.  The Spark is also fantastically versatile; kind of a tiny-scale Skipper. 

[wumpus]

On 6/9/2017 at 11:46 PM, GoSlash27 said:

The "best" engine is whichever one makes the lightest/ cheapest stage. It depends on your payload, environment, and DV/TWR requirement and varies far too much to point at a single engine. 

Best,
-Slashy

Nope.  The "best" is clearly the LV-N.  It's also the "only" as the others are "liquid fuel + oxidizer engines", while only the LV-N is liquid fuel only (jet engines obviously need oxidizer, even if they don't have to carry any).

Of course if you insist that oxidizers are implied or simply allowed, that does open things up a bit. 
/s (I'm quite smitten by the LV-909.  Lots of Isp, and cheap/low tech.  But I obviously can't claim its the best for many things).

[Falkenherz]

As many have already mentioned, it is all about what payload you want to move. Here is some of my experience:

I personally avoid LV-N like plague, due to low thrust to weight ratio (TWR); KSP is not made for calculating efficient continuous burns, and any ejection burn from Kerbin, which takes longer than ~40 seconds, becomes very inefficient, eating up a lot of the ISP advantage which those low-thrust engines do provide. While you usually can get by a TWR of 0.5, by now I prefer to have more, especially for landers. The less time the engine burns for a given maneuver versus a gravity well, the more efficient it is, often outweighting the advantage of higher ISP engines with a weak TWR.

Also, I often do have a symmetry or shape problem. E.g., where do I put the rover or other payload which is intended to be decoupled later? It is usually the central axis. Therefore, the engines get outsourced to dual- or quad-nascelles. This in turn requires me to "split-up" and downsize to smaller engines. This causes only a a slight loss in fuel efficiency, because bigger engines tend to be more fuel efficient for a given TWR. However, I rarely use Rhinos or even Poodles these days (they have best efficiency apart from the LV-N). Instead, I often refer to Terrier or Aerospikes in quad-nascelle arrangements and safe that central axis for a flexible payload arrangement.

Concerning launch-stages, most engines have a lesser thrust and ISP in atmosphere and thus require high thrust, which is mostly overpowerdly wasted after the initial 20km of altitude. That is why Skipper plus any number of boosters or even triple Skippers plus boosters are my favourite engine. I rarely launch payloads which require more thrust than that.

A very successful design was an SSTO with four aerospikes plus four boosters which helped to get through the initial 20km altitudes. I found staging beyond that to be necligible, as an empty fuel tank adds only very little weight and the Aerospikes serve very well in and out of the atmosphere (i.e. less initial dead weight from engines which will be fired only in later stages).

Edited June 13, 2017 by Falkenherz

[Jouni]

16 minutes ago, Falkenherz said:

I personally avoid LV-N like plague, due to low thrust to weight ratio (TWR); KSP is not made for calculating efficient continuous burns, and any ejection burn from Kerbin, which takes longer than ~40 seconds, becomes very inefficient, eating up a lot of the ISP advantage which those low-thrust engines do provide. While you usually can get by a TWR of 0.5, by now I prefer to have more, especially for landers. The less time the engine burns for a given maneuver versus a gravity well, the more efficient it is, often outweighting the advantage of higher ISP engines with a weak TWR.

The loss of efficiency is marginal. If you do a single burn from LKO to Jool with (Kerbin) TWR 0.2, you spend maybe 2200-2400 m/s of delta-v instead of the optimal 2000 m/s. On the other hand, a typical nuclear stage produces 6000-7000 m/s, while chemical rockets only produce around 3500 m/s for the same payload and mass.

For landers, the sweet spot is usually around (local) TWR 1.5. Beyond that, you spend more fuel for carrying the additional engines than you save from the higher TWR. On Tylo, the optimal TWR might be as low as 1.0.

[icantmakemodels]

I keep these subassemblies on hand:

• Vector-III: Literally a 2.5m tri-coupler and three vector engines.  
• Vector-V: S3-3600 tank with five "Vector" engines: Four (Gimbal-locked) outboard, one inboard
• Vector-VII: S3-3600 tank with seven "Vector" engines: six (gimbal-half) outboard, one inboard.  

I also sometimes use an unnamed cluster with an inboard gimbal-locked Mainsail and four (Full-Gimbal) Vectors

Edited June 13, 2017 by icantmakemodels

[Norcalplanner]

As others have said, it's like asking which car is best - it all depends on how much cargo and passengers you're bringing, the characteristics of where you're going, and your own driving (piloting) preferences. This is a game, after all, and nobody should feel forced to do something that they don't find fun.

[Cpt Kerbalkrunch]

All these engines are yours except the Nerv. Attempt no launches with it.

[Falkenherz]

10 hours ago, Jouni said:

The loss of efficiency is marginal. If you do a single burn from LKO to Jool with (Kerbin) TWR 0.2, you spend maybe 2200-2400 m/s of delta-v instead of the optimal 2000 m/s. On the other hand, a typical nuclear stage produces 6000-7000 m/s, while chemical rockets only produce around 3500 m/s for the same payload and mass.

For landers, the sweet spot is usually around (local) TWR 1.5. Beyond that, you spend more fuel for carrying the additional engines than you save from the higher TWR. On Tylo, the optimal TWR might be as low as 1.0.

Thx, sound advice! I am not very experienced with interplanetary and low-thrust burning. It was always annoying to do already within the Kerbal system, and since the arrival of ISRU harvesting, there hardly seems any need anymore for building so much deltaV.

[GoSlash27]

On 6/13/2017 at 8:10 AM, Jouni said:

a typical nuclear stage produces 6000-7000 m/s, while chemical rockets only produce around 3500 m/s for the same payload and mass.

 While I agree with the sentiment about cosine losses and low t/w, I always try to discourage this line of reasoning. An ideal stage design doesn't have more engine than it needs or more fuel than it needs. It's mission has been defined and it's the lightest, cheapest assembly that will accomplish the task.

 Looked at that way, the ability to generate more DV isn't an advantage in and of itself if you don't actually *need* the excess DV. For missions below 2 km/sec DV and 0.5g minimum acceleration, the LF&O stages are not only way cheaper than the NERV, but also lighter. Most travel in KSP can be accomplished with less than 2km/sec DV per stage, so it's a rare instance where the NERV is actually worth the cost in terms of dollars per tonne.
 Analyzing it in terms of "which can generate more DV" is getting the design process backwards.

Best,
-Slashy

 

[Jouni]

45 minutes ago, GoSlash27 said:

Looked at that way, the ability to generate more DV isn't an advantage in and of itself if you don't actually *need* the excess DV. For missions below 2 km/sec DV and 0.5g minimum acceleration, the LF&O stages are not only way cheaper than the NERV, but also lighter. Most travel in KSP can be accomplished with less than 2km/sec DV per stage, so it's a rare instance where the NERV is actually worth the cost in terms of dollars per tonne.

That 0.5g minimum acceleration is also something you rarely need outside landers and launch vehicles.

One thing I noticed after my 1.5-year break is that I could do almost any maneuver reliably under 4x physics warp. I have a faster computer, KSP is better optimized, and SAS behaves better than before the break. There are no longer 15-minute burns, as any burn can be done in less than 4 minutes of real time with 0.2g acceleration. The temptation to add more engines because of my impatience is no longer there.

Chemical rockets are good, if your destination is no further than Eve or Duna. Beyond them, even an one-way trip takes more than 3 km/s, and you start requiring an excessive number of stages for the mission. Ships get complicated and fragile, and you spend more and more time on tweaking the routine stuff.

[Red Iron Crown]

On 6/9/2017 at 8:53 AM, TheEpicSquared said:

For 3.75m rockets, a cluster of 4 Vectors works well

I am trying and failing to imagine a scenario in which 4 Vectors are better than a single Mammoth. The Mammoth is lighter and cheaper than the cluster while having the same thrust, Isp and tech level. Maybe if you need that extra gimbal range?

[Gaarst]

20 minutes ago, Red Iron Crown said:

I am trying and failing to imagine a scenario in which 4 Vectors are better than a single Mammoth. The Mammoth is lighter and cheaper than the cluster while having the same thrust, Isp and tech level. Maybe if you need that extra gimbal range?

Gimbal or compactness. Since the Vector is the only well-made engine in the game (no tank-butt + surface-attachable) you can use it however you want. I don't know of any other way to put 4MN under a 2.5m core, useful for large liquid boosters.

[Red Iron Crown]

1 hour ago, Gaarst said:

Gimbal or compactness. Since the Vector is the only well-made engine in the game (no tank-butt + surface-attachable) you can use it however you want. I don't know of any other way to put 4MN under a 2.5m core, useful for large liquid boosters.

Didn't consider large liquid boosters, my rockets these days tend to use solids around a sustainer liquid core. Compactness is definitely handy, and can help with rocket aesthetics too.

[wumpus]

19 hours ago, GoSlash27 said:

 While I agree with the sentiment about cosine losses and low t/w, I always try to discourage this line of reasoning. An ideal stage design doesn't have more engine than it needs or more fuel than it needs. It's mission has been defined and it's the lightest, cheapest assembly that will accomplish the task.

While this is mostly true, not everything has to be "one rocket: one mission".  LV-Ns are often ideal tugs to carry things around a system.  High delta-v helps, although a smaller tank might also help (with some constraints about the lifter having to refuel the tug).  To be honest, my favorite tugs were never designed as such, there just were more useful "up" than returning them for scrap.

[septemberWaves]

If you're looking for a best general-case engine, I'd definitely have to say the Swivel. It's great for small rockets, and also is good in clusters for larger rockets. It's practical for use with very tall 2.5m rockets and even in some cases for 3.75m rockets with enough engines (generally best used on 3.75m upper stages).

[GoSlash27]

4 hours ago, wumpus said:

While this is mostly true, not everything has to be "one rocket: one mission".  LV-Ns are often ideal tugs to carry things around a system.  High delta-v helps, although a smaller tank might also help (with some constraints about the lifter having to refuel the tug).  To be honest, my favorite tugs were never designed as such, there just were more useful "up" than returning them for scrap.

wumpus,

 This is true. I personally use the LV-N for exactly that job because I'm into the whole Von Braun "STS" style of play. The reusability of the the space tugs helps to defray the buy- in cost and makes it worth doing.
 But even in that case, the mission of the stage is defined. High DV and a single stage. The LV-N is ideal here simply because the other engines can't perform that job.
 My point is that when comparing 2 engines for the same role you shouldn't decide based on which can generate more DV, but rather which makes a lighter and cheaper stage.

Best,
-Slashy

Edited June 15, 2017 by GoSlash27