Why is fuel so heavy?

[kspacc]

I have recently noticed that 300 units of liquid fuel weighs 1.5 tons.

So surely then it can't be in litres or gallons?

1 litre of aviation fuel weighs about 800g (0.8kg).

So 1 ton of fuel would be 1250 Litres.

Why is this?

And is there any chance of the weight being reduced in the game?

[Kerbart]

A rocket engine works on the principle of expelling mass. Reducing the weight would lower either thrust, or delta-v, and probably both.

[kspacc]

1 minute ago, Kerbart said:

A rocket engine works on the principle of expelling mass. Reducing the weight would lower either thrust, or delta-v, and probably both.

Delta V is the potential velocity calculated using amount of fuel, weight, and thrust, reducing the weight of the fuel would in no way reduce thrust and would increase Delta V.

And My point is that in reality you get far more Delta V from a ton of fuel than you do in the game.

[OHara]

The 'units' must be larger than litres.

The T400 tank, for example, holds 400 'units' and is 1.25 meters in diameter and about 2 meters long.  That is 2×(1.25)²×π/4 = 2.5 m³  outside dimensions.  Inside dimensions might be only 2m³ = 2000 litres.   So from the size of the tanks it looks like a 'unit' is about 5 litres.  5kg/unit is then a more reasonable 1kg/L.

[Kerbart]

3 hours ago, kspacc said:

Delta V is the potential velocity calculated using amount of fuel, weight, and thrust, reducing the weight of the fuel would in no way reduce thrust and would increase Delta V.

And My point is that in reality you get far more Delta V from a ton of fuel than you do in the game.

If you only carry 100 kg of fuel instead of 500 kg I'm pretty sure that your dV is going to be reduced.

I'm not so sure about getting more dV in reality. I_sp of most engines seems pretty balanced. H₂/LOX will get you better numbers, but then again we have the NERV to compensate for that.

[kspacc]

3 hours ago, OHara said:

The 'units' must be larger than litres.

The T400 tank, for example, holds 400 'units' and is 1.25 meters in diameter and about 2 meters long.  That is 2×(1.25)²×π/4 = 2.5 m³  outside dimensions.  Inside dimensions might be only 2m³ = 2000 litres.   So from the size of the tanks it looks like a 'unit' is about 5 litres.  5kg/unit is then a more reasonable 1kg/L.

That is right I guess, 2,000 litres would weigh close to 2 tons, and 400 units does weigh 2 tons in the game, so fair play.

But then why do the engines burn so much fuel?

[kspacc]

56 minutes ago, Kerbart said:

If you only carry 100 kg of fuel instead of 500 kg I'm pretty sure that your dV is going to be reduced.

I'm not so sure about getting more dV in reality. I_sp of most engines seems pretty balanced. H₂/LOX will get you better numbers, but then again we have the NERV to compensate for that.

I'm talking about reducing the weight of the fuel, not the amount of fuel.

ANY reduction in weight INCREASES Delta V.

You are talking about reducing the amount of fuel, NOT the weight of the fuel.

This page is about working out what is going on with the units of fuel in the game, because they don't correspond logically to litres or gallons.

OHara has explained how the numbers match up roughly, but there is no explaination of why they made a unit 5 litres, 4 litres would at least be an American gallon, which would at least have made some sense.

This explanation does also mean that the engines burn a lot of fuel, which still suggests to me that they might have been a bit conservative with the Delta V.

[kspacc]

Yes, 5 litres of fuel per unit makes sense.

But its still a bit heavy, because aviation fuel weighs 800kg per 1m3.

So it should only weigh 1.6 tons.

And it does mean that most of the jet engines burn over 4 litres a second, which cannot be realistic, surely?

[Guest]

I think some “suspension of disbelief” is required. It's referred to as liquid fuel but it may not be the same as our liquid fuels and burns less efficiently or is heavier. I think the fact they used “units” rather than recognised measurents we have was to separate the 2

[Kerbart]

8 hours ago, kspacc said:

I'm talking about reducing the weight of the fuel, not the amount of fuel.

ANY reduction in weight INCREASES Delta V.

You are talking about reducing the amount of fuel, NOT the weight of the fuel.

The rocket equation is all about the mass of the propellant. The volume has nothing to do with it. In fact, denser fuels have the advantage of requiring smaller tanks. This is why H_²/LOX is popular for upper stages; it performs very good and you don't need a massive tank anymore.

8 hours ago, kspacc said:

This page is about working out what is going on with the units of fuel in the game, because they don't correspond logically to litres or gallons.

OHara has explained how the numbers match up roughly, but there is no explaination of why they made a unit 5 litres, 4 litres would at least be an American gallon, which would at least have made some sense.

This explanation does also mean that the engines burn a lot of fuel, which still suggests to me that they might have been a bit conservative with the Delta V.

It's not a secret that a fuel unit supposedly is equal to roughly 5L. That's why they're called units and not "liters" or "gallons." And it doesn't really matter; what matters is the mass of the fuel. Even for jets (as opposed to rockets), fuel is measured in mass units, not volume units. By calling them "units" there's no preference over gallons or liters, so the game is serving metric and non-metric players equally, and it also makes the fuel more abstract so there's less discussion on what the liquid fuel and oxidizer really are.

[Hotel26]

1 hour ago, Kerbart said:

That's why they're called units and not "liters" or "gallons."

^ this.

a.k.a "kallons".

Edited March 29, 2021 by Hotel26

[kspacc]

I wrote a huge long explanation, why could they not just have let me submit it?

Ridiculous website.

ANY reduction in weight INCREASES Delta V.

We are NOT talking about changes the AMOUNT of fuel, it is very simple.

This page is NOT about assuming we don't change the game details, it is about assuming we SHOULD to make it more realistic.

I am suggetsing they use aviation fuel as model for the weight and efficiency, as that is what jet engines burn.

Aviation fuel weighs 800kg per m3.

You can't take both weight and volume out of the equation, or you are not calculating anything.

A ton of aviation fuel is far more than a small plane could ever carry, so it CANNOT be only enough to travel a few miles.

They have been syetmatically reducing the efficiency of all the engines in the game over the years, you used to be able to make  ion olanes that flew, now they are reduced so low you can't get any thrust, so it is not fixed or based on anything, and if it is arbitrary then there is no reason not to make it more realistic.

[kspacc]

15 minutes ago, Hotel26 said:

^ this.

a.k.a gallons.  Even if you then say they are too heavy, go study the literature about how the Kerbol system (and Kerbin itself) must be unusually dense to be as small as it is with the orbital speeds it has.  So if things are dense, Q.E.D.

On top of that, asking for an incompatible change now (after 9-10 years) is NOT going to fly.  It would upset everything and every body.

(Whereas my recent plea to lower the drag on ladders (so they are worth using) just makes good sense.  Obviously.  Because...  if nobody uses ladders...  because, ehm, they are too draggy to bother...  then you can lower the drag to 'realistic' (i.e. usable levels)...  and then people could start using them...  so...  NOT, actually, a request for an incompatible change.  Yeah?

They change THIS area of the game in almost every update, it would effect NOTHING more than usual.

And we KNOW what the grav rating is on kerbin, it is the same as Earth.

I have been playing this game since it was an unreleased piece of software with virtually NONE of the same parts it has now and NONE of the same physics.

If I updated to the latest version right now there would be changes from the one I did last year.

Can't see how making it more efficient to be more realistic would upset anyone, they make these kind of changes all the time.

I have never noticed the ladders being draggy, is that when you use more than one or something?

 

 

[Kerbal Productions]

In intensively simple terms, fuel has weight.
If you have your fuel tanks full, it'll be heavy.
If your fuel tanks are empty, they're basically dead weight.
 

Additionally, IF you chose a weak engine over a big fuel tank and a heavy payload it might just barely do any effects (except if you are in vacuum)

[peteletroll]

21 minutes ago, kspacc said:

ANY reduction in weight INCREASES Delta V.

Well, no, if you reduce weight by removing fuel :-)

[OHara]

10 hours ago, kspacc said:

but there is no explanation of why they made a unit 5 litres,

Squad's fuel 'units' are not any measure of volume.  We never get a way to directly measure fuel volume in the game, just its mass.  There is one tank called the 'Oscar' that seems to compress a unit of fuel into 2.5 litres, if you look at it size!  (The Oscar tank made small-craft challenges a bit less fun, because it was an almost-cheaty weird trick.)

KSP propellants are measured in terms of mass, with one 'unit' being 5kg whether it is liquid fuel or oxidizer.  Monopropellant is 4kg per unit, though.  Players have suggested to measure all fuel in kg or tonnes, if not in KSP1.x then in KSP2.   I suspect Squad wishes they had used tonnes in the beginning. 

You might be interested in reading the thinking that went into the mod SMURFF that adjusts the masses and ISPs so that KSP parts work like their look-alike real parts when the planets are modded to be the real solar system.   They reversed the game-play tweaks that Squad did for KSP.

[Single stage to ocean]

Wat u mean, u mean fuel tanks are to heavy?

[Dientus]

1 hour ago, kspacc said:

They have been syetmatically reducing the efficiency of all the engines in the game over the years, you used to be able to make  ion olanes that flew, now they are reduced so low you can't get any thrust, so it is not fixed or based on anything, and if it is arbitrary then there is no reason not to make it more realistic.

Theory: since it's approximately 1/10 scale anyway, they use abstract numbers and terms to slide difficulty up or down.

 

[Bej Kerman]

18 hours ago, kspacc said:

I have recently noticed that 300 units of liquid fuel weighs 1.5 tons.

So surely then it can't be in litres or gallons?

1 litre of aviation fuel weighs about 800g (0.8kg).

So 1 ton of fuel would be 1250 Litres.

Why is this?

And is there any chance of the weight being reduced in the game?

Does it matter? The game would be too easy if rockets were any lighter. I do recall that rockets have been made very inefficient and the gravitational constant very strong in KSP to counteract the miniaturized planets so that physics remains the same while burns don't take as long.

Edited March 29, 2021 by Bej Kerman

[Kerbart]

5 hours ago, kspacc said:

(...)

ANY reduction in weight INCREASES Delta V.

We are NOT talking about changes the AMOUNT of fuel, it is very simple.

(...)

A rocket engine works by expelling propellant at a certain velocity. Newton's second law of motion  dictates that this results in an opposite force which is what pushes your rocket - thrust.

Basically (and I'm taking some shortcuts here), F = exhaust velocity (m/s) × mass flow (kg/s).

The velocity is also referred to as specific impulse or I_sp  and is often divided by gravitational constant g to express it in seconds (otherwise you'll be stuck with either m/s or some archaic non-metric units like poundstones per teabreak or whatever they come up with. Seconds are the same everywhere). So, to determine thrust: I_sp × g x fuel flow.

Plug in the numbers for the Terrier and you get: 348 × 9.81 × (3.55 × 5)  (one fuel unit is 5kg) = 60 kN - right on spot.

 

Note that in this formula, fuel flow is measured in mass per second; it doesn't matter if that 17.75 kg/s is 1 gallon or 100 gallons per second. It's the mass that's counted.

So, if we redefine the fuel unit as 1kg per unit, then yes, our engijne only consumes  3.55 kg per second instead of 17.75. Our engine also produces only 12 kN instead of 60. And the rocket equation which tells us how much dV we get, is also slashed: DV = I_sp×g×ln(mass with fuel  / mass without fuel).

Remove weight anywhere else and your DV will increase. But remove fuel and your DV will decrease.

 

I think you're mistaking fuel volume for fuel amount. But volume can change with temperature and pressure; mass doesn't and it's what makes the engine work. Hence, it's mass that's the preferred measurement.

Edited March 29, 2021 by Kerbart

[kspacc]

42 minutes ago, Kerbart said:

A rocket engine works by expelling propellant at a certain velocity. Newton's second law of motion  dictates that this results in an opposite force which is what pushes your rocket - thrust.

Basically (and I'm taking some shortcuts here), F = exhaust velocity (m/s) × mass flow (kg/s).

The velocity is also referred to as specific impulse or I_sp  and is often divided by gravitational constant g to express it in seconds (otherwise you'll be stuck with either m/s or some archaic non-metric units like poundstones per teabreak or whatever they come up with. Seconds are the same everywhere). So, to determine thrust: I_sp × g x fuel flow.

Plug in the numbers for the Terrier and you get: 348 × 9.81 × (3.55 × 5)  (one fuel unit is 5kg) = 60 kN - right on spot.

 

Note that in this formula, fuel flow is measured in mass per second; it doesn't matter if that 17.75 kg/s is 1 gallon or 100 gallons per second. It's the mass that's counted.

So, if we redefine the fuel unit as 1kg per unit, then yes, our engijne only consumes  3.55 kg per second instead of 17.75. Our engine also produces only 12 kN instead of 60. And the rocket equation which tells us how much dV we get, is also slashed: DV = I_sp×g×ln(mass with fuel  / mass without fuel).

Remove weight anywhere else and your DV will increase. But remove fuel and your DV will decrease.

 

I think you're mistaking fuel volume for fuel amount. But volume can change with temperature and pressure; mass doesn't and it's what makes the engine work. Hence, it's mass that's the preferred measurement.

Mate, were not talking about amounts of fuel.

I am talking about how the fuel is set to be so heavy in the game compared to real life.

In real life a ton of fuel is a lot of fuel, in this game it is barely enough to get a jet engine to the nearest island and back.

Please, stop replying talking about amounts of fuel and Delta V, this is about how the game is set-up.

And it would increase Delta V anyway , because the same amount of fuel would weigh less if it were more realistic.

[kspacc]

3 hours ago, Bej Kerman said:

Does it matter? The game would be too easy if rockets were any lighter. I do recall that rockets have been made very inefficient and the gravitational constant very strong in KSP to counteract the miniaturized planets so that physics remains the same while burns don't take as long.

Yeah but they set Kerbin's gravity to be almost the same as Earth's, and it is really a huge amount of difference compared to real life how much fuel a ton is.

[kspacc]

6 hours ago, Single stage to ocean said:

Wat u mean, u mean fuel tanks are to heavy?

No, the fuel itself, compared to real life.

Aviation fuel for jet engines weighs 800kg per m3(1,000 litres),  in this game you get 200 units to 1,000kg (1 ton).

So if as OHara said 1 unit is 5 litres then the jet engines are buring 1L - 3.5L  per second!

No-matter which way you look at it, they have made either the fuel or the engines really unrealistically inefficient.

[kspacc]

6 hours ago, OHara said:

Squad's fuel 'units' are not any measure of volume.  We never get a way to directly measure fuel volume in the game, just its mass.  There is one tank called the 'Oscar' that seems to compress a unit of fuel into 2.5 litres, if you look at it size!  (The Oscar tank made small-craft challenges a bit less fun, because it was an almost-cheaty weird trick.)

KSP propellants are measured in terms of mass, with one 'unit' being 5kg whether it is liquid fuel or oxidizer.  Monopropellant is 4kg per unit, though.  Players have suggested to measure all fuel in kg or tonnes, if not in KSP1.x then in KSP2.   I suspect Squad wishes they had used tonnes in the beginning. 

You might be interested in reading the thinking that went into the mod SMURFF that adjusts the masses and ISPs so that KSP parts work like their look-alike real parts when the planets are modded to be the real solar system.   They reversed the game-play tweaks that Squad did for KSP.

Sure, but 5kg is a lot, that means you are burning 1kg-3.5kg per second minimum in the medium sized jet engines.

And a ton of fuel is hardly any fuel in this game,  in real life a small 1-2 man plane can't carry anything like a ton of fuel and can travel a few thousand miles easily.

I remember the Oscar-B tanks, they have been in the parts list since long before this game was released, but if a unit of fuel is 2.5L and weighs 5kg you can surely see the problem, that is only 500 litres per ton, when real-life aviation fuel is 0.8 tons per 1,0000 litres more than twice as much for the weight.

[paul_c]

You don't need to worry about the weight of fuel, as such. The two things - that you have to carry all that fuel, and that a big flamey bunch of hot gas comes out the nozzle at really high velocity - effectively cancel each other out. 

The important parameter is I_SP and its units are seconds, ie there is no kg in there because they cancelled out. I_SP is a function of fuel type and engine design - the higher the better.