Which engine has the best thrust to weight ratio in vacuum (boosters, monopropellant, DLC excluded)?

[Sushi_Rocks]

Hey, I wanna build a big spacecraft that is supposed to stay in space and only operate there. Since it is supposed to fly very far distances, I wanted to ask which engine is the most efficient one for space/vacuum in terms of thrust to weight.

Monopropellant thrusters, DLC engines and boosters NOT included!

(...but feel free to include the info if you like to.)

I can only find threads about this topic that are 3 years old and they are not up to date anymore, unfortunately.

[Shelecktra]

I mod too much.... I cant tell the base game parts from the modded ones any more! xD

I think the Spark and the Bobcat are base game parts aren't they? I use those a heck of a lot myself, although there's probably better - Those are just my personal faves :)

[Azunai]

the thrust to weight ratio is almost irrelevant for an actual spaceship.

for long range missions you're looking for engines with the best specific impulse. that determines the actual efficiency (ie. how much deltaV you get from your fuel).

highest Isp in the stock game is probably the ion engine, but they have super low thrust and are really only useful for lightweight stuff. also their fuel can't be created on the fly with drills and ISRU.

next best is the nuclear engine (NERV) which is about twice more efficient compared to the most fuel efficient LF/O engines.

i think the highest Isp for LF/O engines was on one of the DLC engines (wolfhound?). something close to 400s if memory serves.

anyway, the nerv has 800s so there's really no competition at all.

[Sushi_Rocks]

Originally posted by Azunai:

the thrust to weight ratio is almost irrelevant for an actual spaceship.

for long range missions you're looking for engines with the best specific impulse. that determines the actual efficiency (ie. how much deltaV you get from your fuel).

highest Isp in the stock game is probably the ion engine, but they have super low thrust and are really only useful for lightweight stuff. also their fuel can't be created on the fly with drills and ISRU.

next best is the nuclear engine (NERV) which is about twice more efficient compared to the most fuel efficient LF/O engines.

i think the highest Isp for LF/O engines was on one of the DLC engines (wolfhound?). something close to 400s if memory serves.

anyway, the nerv has 800s so there's really no competition at all.

How is it irrelevant? I mean the engine should be more efficient the less weight/mass it has compared to its thrust, doesn't it (because it has to move less mass)?

I don't have the DLC's so they are out of the race, too...

[GunsForBucks]

I am a fan of the poodle engines and use them often for my extra big rovers going to minimus.


The nerv engine that uses only liquid fuel is heavy but since it uses only liquid fuel and not oxidizer you can get a lot more DV with them for the weight of the fuel even though the engine itself is heavy.

Both are bad in atmosphere though.


The best is probably the ion engines but I wouldn't use them on anything large unless they were used in combination with other things. They are also a pain to manage the electric usage unless you are heading toward the sun so you have to add the weight of solar panels, nukes and enough battery capacity for them to function.

[Rhomphaia]

Originally posted by Sushi_Rocks GERMANY:

How is it irrelevant? I mean the engine should be more efficient the less weight/mass it has compared to its thrust, doesn't it (because it has to move less mass)?

I don't have the DLC's so they are out of the race, too...

Does a car that can 0-60 in 3s get better mileage than one that takes 10s?

Specific impulse determines how much fuel mass you have to burn to accelerate to a certain speed. thrust only determines how quickly you can get to that speed.

[GunsForBucks]

Kodiak and Cheeta are also good but I think you need the DLC for those

Darts aren't terrible and small enough to mount in clusters. I've used them before I got the DLC for times when I do enter atmosphere instead of the Poodles

I guess when it comes down to it I just swap around in the VAB until I find something I am happy with. Ship design will play a large part in what you select.

Just make sure you set the DV tool tab at the bottom to Vacuum and you can check it out yourself.

[lizrdfishr]

Azunai and you are looking at two different aspects of the game. I gotta concur with Azunai.

Thrust to weight tells you how fast you can go per unit of fuel.

Specific impulse will tell you how far you can go per unit of fuel.

Speed only really matters to reach orbit. Once you reach orbit, you no longer need speed to get you beyond the gravity well of an object in space.

If you go faster, you need more DV to slow down into orbit when you get where you are going. Burning fuel to go fast means you have less fuel AND need more fuel when you get wherever you are going. Speed isn't the consideration. Your ability to have enough fuel to stop and do what you want when you get there is.

The further away from fuel sources you go, the more efficient you need to be. The slower you burn the fuel, the more efficient the engine tends to be. Speed is a tremendous waster of fuel.

Mind the ION engines. Yes they are VERY efficient, but the further from the sun you get, the less efficient your solar panels will be and the longer it will take to charge batteries. You may end up needing fuel cells to burn fuel to charge your batteries when you are way out there by Sarnus (OPM).

[Wobbly Av8r]

The Parts Wiki[wiki.kerbalspaceprogram.com] is pretty easy to use for these kinds of things; going to the engine section you can use any of the specifications to filter ascending or descending.

[lizrdfishr]

Originally posted by Wobbly Av8r:

The Parts Wiki[wiki.kerbalspaceprogram.com] is pretty easy to use for these kinds of things; going to the engine section you can use any of the specifications to filter ascending or descending.

But but but... reading the instructions is cheating in KSP! ;)

[desrtfox071]

Originally posted by lizrdfishr:

Azunai and you are looking at two different aspects of the game. I gotta concur with Azunai.

Thrust to weight tells you how fast you can go per unit of fuel.

Specific impulse will tell you how far you can go per unit of fuel.

Speed only really matters to reach orbit. Once you reach orbit, you no longer need speed to get you beyond the gravity well of an object in space.

If you go faster, you need more DV to slow down into orbit when you get where you are going. Burning fuel to go fast means you have less fuel AND need more fuel when you get wherever you are going. Speed isn't the consideration. Your ability to have enough fuel to stop and do what you want when you get there is.

The further away from fuel sources you go, the more efficient you need to be. The slower you burn the fuel, the more efficient the engine tends to be. Speed is a tremendous waster of fuel.

Mind the ION engines. Yes they are VERY efficient, but the further from the sun you get, the less efficient your solar panels will be and the longer it will take to charge batteries. You may end up needing fuel cells to burn fuel to charge your batteries when you are way out there by Sarnus (OPM).

I wouldn't say thrust to weight ratio determines how fast you can go with a unit of fuel. What I think you mean is that TTW (Thrust to Weight) determines your acceleration, but this even is only half true. What should be considered for TTW is the overall TTW of a rocket, not the TTW of an engine. Unlike a car or something like that, fuel makes up the vast majority of the mass of a rocket, and thus the weight. So an engine with a great TTW on it's own, but poor ISP is still going to have a lower total TTW than an engine with lower per engine TTW, but better ISP.

I will agree with what most others are saying though - ISP is the number you should be looking at. TTW means almost nothing to a rocket. Only TTW has any importance at all really, and only then when you are suborbital. Beyond that TTW, is next to useless. SO long as your rocket is above 1 for any suborbital maneuvering (landing, launching) you're OK - a better number would be about 2 to 3 total TTW. In orbit excessive TTW is actually a negative.

[poruatokin]

Originally posted by Sushi_Rocks GERMANY:

Hey, I wanna build a big spacecraft that is supposed to stay in space and only operate there. Since it is supposed to fly very far distances, I wanted to ask which engine is the most efficient one for space/vacuum in terms of thrust to weight.

Monopropellant thrusters, DLC engines and boosters NOT included!

(...but feel free to include the info if you like to.)

I can only find threads about this topic that are 3 years old and they are not up to date anymore, unfortunately.

You could do what I did - make an Excel table of the data in the game.

[Sushi_Rocks]

Can anyone explain to me why specific impulse is more important than the TTW-ratio?

My initial thoughts would be that an engine is more efficient when you can get more thrust with less weight. I have never heard of that impulse stuff before.

Also: Isn't acceleration important as well?

The tutorials taught me that the best point to adapt your flight path from an orbit is to accelerate at the apoapsis or periapsis. Aren't you less efficient when your burns need longer so that you have to accelerate before and after periapsis and apoapsis?

Is specific impulse the thrust generated per unit of burnt fuel?

[Manwith Noname]

The reality is, it takes a combination of factors to build a decent craft that does what you want.

Isp, is Specific Impulse which in really simple terms means how efficiently the engine uses fuel. You could have the best Isp engine but if the TWR (Thrust to Weight Ratio) is abysmally low, burns take forever and in the case of launches, you might not even go anywhere.

[Azunai]

the engine is only a small fraction of the total mass of the whole vessel, so the TWR of the engine itself is only relevant in fairly extreme cases. the most important thing to understand about rocketry in general is that anything that goes up into space or moves around in space requires LOTS of fuel. most of the mass of a rocket is fuel, not hardware.

raw thrust is important for launching from a planet since you have to overcome gravity (and the atmosphere) to get to space. once you are in orbit, thrust is far less important. of course it's not completly irrelevant, as was pointed out already. you still want to have a decent amount to keep the burns within a reasonable timeframe.

but in practice the decision is usually something like - use a nuclear engine that gives me 5km/s of deltaV but requires an 8 minute burn to get the ship on its transfer orbit vs. use an LF/O engine that only has 3km/s of deltaV with similar amounts of fuel but only requires a 3 minute burn. i guess most players would go with the higher deltaV value and ignore the few minutes of extra burn time.

[Wobbly Av8r]

Originally posted by Sushi_Rocks GERMANY:

Can anyone explain to me why specific impulse is more important than the TTW-ratio?
...
Is specific impulse the thrust generated per unit of burnt fuel?

Essentially, yes.

Impulse as a physics term is change in acceleration over time. "Specific" Impulse (Isp) is the term used to describe the efficiency of an engine in converting chemical energy into an accelerating force, i.e. how many pounds of thrust can be achieved with one pound of fuel? [see actual definition below, but in practice is less useful...]

The difference between Isp and TWR is primarily time. If time is critical, it may be necessary to think in terms of TWR. If resources are critical (as they usually are) then think in terms of Isp.

[Edit: FWIW, Specific Impulse (Isp) is actually a time value: it is the amount of time 1 lb of fuel can create 1 lb of thrust.]