Why is fuel so heavy?

[kspacc]

6 hours ago, peteletroll said:

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

That is reducing fuel, which is it's own variable in Delta V.

[kspacc]

1 minute ago, paul_c said:

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.

Which ever way you measure it, compared to real life either the fuel or the engines are extremely inefficient.

A ton of jet fuel goes a long way in real life, in the game it is barely enough to get one engine such a small distance you wouldn't even bother flying in real life.

[paul_c]

Its not real life though - its all scaled differently to make it easier as a game to play. In KSP, LKO orbital speed is ~2300m/s. In real life, to low orbit the Earth needs an orbital speed of ~7800m/s

[Bej Kerman]

2 hours ago, kspacc said:

5 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.

My point stands, fuel is still overpowered with how tiny the planets are here.

[OHara]

2 hours ago, kspacc said:

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

Yes, if KSP measured fuel in kg (or changes to measure it in kg in future) that does make comparisons much easier.

The 4.5-tonne Goliath engine, burning 2.9 kg/s to give 360 kN for takeoff, is comparable to the
Rolls-Royce on the A380, weighing 5t  and burning 5.8 kg/s to give 360 kN.

That burn rate is only for initial climb.  Both engines can throttle back significantly in cruise, but you need a bit more open throttle in KSP because the other the parts tend to be heavier, thus the required lift greater, thus the drag greater.  The game makes it a challenge to circumnavigate of Kerbin on a single load of fuel (only 2300 British miles).

[Kerbal Productions]

10 hours ago, Single stage to ocean said:

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

If it has some fuel inside it, yes. If not, it's dead weight.

[Kerbart]

7 hours ago, kspacc said:

Mate, were not talking about amounts of fuel.

How can you talk about reducing the weight of the fuel but “not changing the amount?” The two are literally intertwined as fuel amounts are measured by mass.

7 hours ago, kspacc said:

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

Please, please explain the rocket equation to me.  I can guarantee that fuel that “weighs less” will increase DV. For the same mass of fuel (required for the same DV) you will now need a bigger tank. Bigger tanks weigh more.

[HebaruSan]

A 747 burns 4 liters (3.2 kg) of fuel per second while cruising; a (metric) ton in about 5 minutes. The tanks can hold on the order of 200,000 liters (160 metric tons), which would last about 13 hours at that rate.

Kind of impressive that the discrepancy between KSP and real life is less than an order of magnitude, frankly.

[kspacc]

10 hours ago, HebaruSan said:

A 747 burns 4 liters (3.2 kg) of fuel per second while cruising; a (metric) ton in about 5 minutes. The tanks can hold on the order of 200,000 liters (160 metric tons), which would last about 13 hours at that rate.

Kind of impressive that the discrepancy between KSP and real life is less than an order of magnitude, frankly.

Exactly!

A 747 with 4 engines burns the same amount of fuel as ONE small to medium jet engine in the game.

The discrepency is about three fold, even if you round everything up, and allow for it not being Earth.

ONE normal sized jet engine used in a small 1-2 person aircraft that can't even carry half a ton can use that amount of fuel to fly thousands of miles in real life, and in the game half a ton gets you to the small island and not quite back, when it should easily get you to the desert airstrip.

They have made a fuel unit larger than a gallon, and made it less fuel energy than half a gallon.

17 hours ago, paul_c said:

Its not real life though - its all scaled differently to make it easier as a game to play. In KSP, LKO orbital speed is ~2300m/s. In real life, to low orbit the Earth needs an orbital speed of ~7800m/s

True, but the orbit science in this game is pure fiction anyway, so I just ignore it.

In reality NOTHING just hangs in orbit needing to be pushed back to earth, it takes constant energy to stay in orbit as it does in any flight, and the second that energy stops being expended the object is pulled back out of orbit towards the planet.

12 hours ago, Kerbart said:

How can you talk about reducing the weight of the fuel but “not changing the amount?” The two are literally intertwined as fuel amounts are measured by mass.

Please, please explain the rocket equation to me.  I can guarantee that fuel that “weighs less” will increase DV. For the same mass of fuel (required for the same DV) you will now need a bigger tank. Bigger tanks weigh more.

Come on mate, this is trolling!

We are talking about reducing the weight of a unit of fuel in the game.

DO NOT post another reply ignoring what the discussion is about or dismissing us being allowed to have that discussion please.

[Gargamel]

I'm still unclear as to what your suggestion or question is.   The game is setup for gameplay balance.   There is no corresponding real life value for a "unit".   A unit is just a unit.   Each unit of "Liquid Fuel" has X amount of mass.   Liquid fuel is just that, liquid fuel.  It's not JP2, JP4, or anything else; It's just "Liquid Fuel".  

If the amount of energy an engine could get out of a unit of liquid fuel changed, then planes would either be over or under powered.   Liquid fuel has the energy density that it does because that's what the game devs decided was best for gameplay. 

KSP is not a sim per se.   It's a game that takes Rocket Science and Orbital Mechanics, which are hideously complicated topics, and simplifies them down to the point where a younger player with a bit of smarts to them can pick it up and figure out the basics rather quickly.   KSP has a much steeper learning curve than most games, but far less than getting a doctorate in Aerospace Engineering would require.    It has found this nice little niche where the game can be played while still spurring an interest in further learning.   Part of that niche is the gameplay balance that requires the fuels in the game to be represented by generic substances; Liquid Fuel, Oxidizer, Ore, Monoprop, etc. 

If your request is for the values of Liquid fuel to change, for whatever reason, I doubt this will happen, as the game is balanced to achieve certain levels of gameplay, and changing some of the fundamentals of the game will affect that gameplay in ways the devs probably don't want. 

23 minutes ago, kspacc said:

In reality NOTHING just hangs in orbit needing to be pushed back to earth, it takes constant energy to stay in orbit as it does in any flight, and the second that energy stops being expended the object is pulled back out of orbit towards the planet.

This is not true at all.   That implies constant thrust is required to maintain orbit.   While real life space craft in Low Earth Orbit require occasional boosts due to the drag caused by the extreme upper atmosphere, if that drag wasn't present, the object would stay in orbit forever.   The Earth is in orbit around the sun, the Moon orbits the Earth, and neither of those have any source of thrust. 

[Deddly]

I don't have the figures in front of me, but I think I recall that it's the engines that are inefficient rather than the fuel being too heavy. As others have said, it is a matter of game balancing. Could it be balanced with a more realistic fuel efficiency for jet engines? That would be interesting to see! If you want to play around with the numbers to see what effect it has on the game, it should be a simple matter of editing a text file. If you do give it a go, I'd love to see your results. 

[OHara]

KSP's jet engines are quite efficient, it turns out, which is easier to see with the suggested conversion of fuel units to kg.

I'm flying right now at Mach 0.6 with a single Goliath (they don't like to go too fast) and that engine is burning 0.22 units = 1.1 kg of fuel per second, and producing 140 kN thrust (35% max. sea-level thrust).  The wiki gives fuel consumption at sea level where you get max thrust.  Fuel flow goes down with altitude, as it should; even in single engine prop planes you have to lean the mixture at altitude.

So the 'specific fuel consumption' (the inverse of Isp that is often quoted for jets) for the Goliath is 8 grams/s per kN --- lower than most jets, but believable for a high-bypass turbofan.

That 747 that @HebaruSan cited uses 4 engines with somewhat lower takeoff thrust than the Goliath.  Given the cited fuel flow in cruise, 3.2kg/s, and specified fuel consumption 10g/s/kN, the set of 4 must be producing about 320kN to balance drag (80kN each, or 35% max. sea-level thrust).  If the plane weights 400t, the lift must be 4000kN so that would be a lift-to-drag ratio of 12:1, which seems a bit high.  (The was a 767 called the 'Gimli Glider' that proved it could glide at 12:1, but at its most efficient speed, probably slower than cruise.  Maybe the Marketing Department had a hand in the cited fuel consumption.)

The folks making the mod SMURFF decided that KSP's jet engines were fine as-is.

Edited March 31, 2021 by OHara
filled in the missing math

[Kerbart]

20 hours ago, kspacc said:

Come on mate, this is trolling!

By who?

20 hours ago, kspacc said:

We are talking about reducing the weight of a unit of fuel in the game.

But that would accomplish... nothing? So now the same tank that holds, say, 720 units of fuel, would burn through it five times as fast as it only has 1/5th of its original content (measured in the way that matters: mass). You’d need five tanks to carry the same propellant mass. Five times the weight of those tanks. That will decrease your dv or range.

[kspacc]

11 hours ago, OHara said:

KSP's jet engines are quite efficient, it turns out, which is easier to see with the suggested conversion of fuel units to kg.

I'm flying right now at Mach 0.6 with a single Goliath (they don't like to go too fast) and that engine is burning 0.22 units = 1.1 kg of fuel per second, and producing 140 kN thrust.  The wiki gives fuel consumption at sea level where you get max thrust.  Fuel flow goes down with altitude, as it should; even in single engine prop planes you have to lean the mixture at altitude.

So the 'specific fuel consumption' (the inverse of Isp that is often quoted for jets) for the Goliath is 8 grams/s per kN --- lower than most jets, but believable for a high-bypass turbofan.

That 747 that @HebaruSan cited uses 4 engines with slightly lower takeoff thrust than the Goliath.  Given the cited fuel flow in cruise, the set of 4 must be producing about 320kN to balance drag.  If the plane weights 400t, the lift must be 4000kN so that would be a lift-to-drag ratio of 12:1, which seems a bit high.  (The was a 767 called the 'Gimli Glider' that proved it could glide at 12:1, but at its most efficient speed, probably slower than cruise.  Maybe the Marketing Department had a hand in the cited fuel consumption.)

The folks making the mod SMURFF decided that KSP's jet engines were fine as-is.

1 ton is 1,000 kg, which in reality is 1,250 litres of fuel, and in the game it is ONLY 200 units, there is no getting away from that fact.

You don't need a TWR of 1+ for a non VTOL, so a 747 doesn't need a KN of thrust per Kg of weight.

And you can see clearly when playing the game that planes can only fly a short distance on a large amount of fuel compared to in real life.

 

[kspacc]

2 hours ago, Kerbart said:

By who?

But that would accomplish... nothing? So now the same tank that holds, say, 720 units of fuel, would burn through it five times as fast as it only has 1/5th of its original content (measured in the way that matters: mass). You’d need five tanks to carry the same propellant mass. Five times the weight of those tanks. That will decrease your dv or range.

That is nonsense.

Reducing the weight of the fuel would in no way have any of the effects you have claimed, it would simply be more efficienty and increase Delta V.

[kspacc]

22 hours ago, Gargamel said:

I'm still unclear as to what your suggestion or question is.   The game is setup for gameplay balance.   There is no corresponding real life value for a "unit".   A unit is just a unit.   Each unit of "Liquid Fuel" has X amount of mass.   Liquid fuel is just that, liquid fuel.  It's not JP2, JP4, or anything else; It's just "Liquid Fuel".  

If the amount of energy an engine could get out of a unit of liquid fuel changed, then planes would either be over or under powered.   Liquid fuel has the energy density that it does because that's what the game devs decided was best for gameplay. 

KSP is not a sim per se.   It's a game that takes Rocket Science and Orbital Mechanics, which are hideously complicated topics, and simplifies them down to the point where a younger player with a bit of smarts to them can pick it up and figure out the basics rather quickly.   KSP has a much steeper learning curve than most games, but far less than getting a doctorate in Aerospace Engineering would require.    It has found this nice little niche where the game can be played while still spurring an interest in further learning.   Part of that niche is the gameplay balance that requires the fuels in the game to be represented by generic substances; Liquid Fuel, Oxidizer, Ore, Monoprop, etc. 

If your request is for the values of Liquid fuel to change, for whatever reason, I doubt this will happen, as the game is balanced to achieve certain levels of gameplay, and changing some of the fundamentals of the game will affect that gameplay in ways the devs probably don't want. 

This is not true at all.   That implies constant thrust is required to maintain orbit.   While real life space craft in Low Earth Orbit require occasional boosts due to the drag caused by the extreme upper atmosphere, if that drag wasn't present, the object would stay in orbit forever.   The Earth is in orbit around the sun, the Moon orbits the Earth, and neither of those have any source of thrust. 

No, because they ARE underpowered and I am suggesting making them sensibly powered.

And if you believe it is arbitarry then it would not matter.

The ONLY sensible argument was that the planet is smaller so they seriously reduced efficiency, but seeing as the game is not limited to the small planet that still does not make total sense.

The fact is that in reality they have given us 200 units of fuel per ton, which seems to be the equivolent of only 200 Litres,  when in reality it is 1,250 Litres of aviation fuel per ton.

[Kerbart]

47 minutes ago, kspacc said:

That is nonsense.

Reducing the weight of the fuel would in no way have any of the effects you have claimed, it would simply be more efficienty and increase Delta V.

The thrust of a rocket engine is calculated by the average exhaust speed (its I_sp) times the mass flow of the propellant. Reducing the mass of your fuel reduces thrust because you’re reducing the mass flow. Your rocket or airplane would likely not get off the ground with thrust cut by 80%, reducing DV/range to 0.

 

 

[Kerbart]

1 hour ago, kspacc said:

The fact is that in reality they have given us 200 units of fuel per ton, which seems to be the equivolent of only 200 Litres,  when in reality it is 1,250 Litres of aviation fuel per ton.

Why would a unit of fuel be 1 L? Everyone seems to agree it’s equivalent to 5 kg, which makes your 200 units perfectly equal to 1250 L of aviation fuel (assuming a density of .8 kg/L).

What you seem to want is that a unit would no longer be 5 kg, so that the same tank would show “1250 L” instead of “200 units,” everything else being the same.

Is that really it? Does it matter?

[Incarnation of Chaos]

11 minutes ago, Kerbart said:

The thrust of a rocket engine is calculated by the average exhaust speed (its I_sp) times the mass flow of the propellant. Reducing the mass of your fuel reduces thrust because you’re reducing the mass flow. Your rocket or airplane would likely not get off the ground with thrust cut by 80%, reducing DV/range to 0.

 

 

F=MA is a thing isn't it?

Rocket equation isn't that, but in the end all it's doing is taking into account the change in mass from the depleted fuel by using the natural log....

This is why Hydrolox has fantastic ISP, but poor thrust. It has such low mass per particle that even though it can be accelerated to decent speeds, the force produced can't approach heavier hydrocarbons.

KSP's fuel is fine, the tanks are too heavy. SMURFF would make OP happy in all likelyhood,

Sorry, iv'e been watching this for a while and just had to pop in.

[Kerbart]

Just now, Incarnation of Chaos said:

F=MA is a thing isn't it?

Rocket equation isn't that, but in the end all it's doing is taking into account the change in mass from the depleted fuel by using the natural log....

When you do the math, thrust = exhaust velocity x fuel flow is exactly that, and it only takes a bit of high school algebra to translate that into the rocket equation. You’re absolutely right.

The OP seems to be thinking that the volume of the fuel determines power, unaware that it’s really the mass that literally drives everything.

[jimmymcgoochie]

KSP is set in a solar system that is ~9% the size of the Sol system, yet Kerbin has the same gravity and surface atmospheric pressure as Earth which is over ten times larger. On Earth you need to accelerate to 7800m/s to stay in LEO compared to just 2200m/s around tiny Kerbin, so if KSP rockets had realistic mass fractions they would be hilariously overpowered. Just to put that into context, Pluto’s primary moon Charon is almost exactly the same size as Kerbin, but has 0.02g gravity or 50 times less.

Basically your point boils down to “the fuel units aren’t in litres”, which is entirely true- generic units of unspecified fuel (some people say kerosene, some say methane...) are all that’s needed; it doesn’t matter if that’s in litres, gallons or fluid ounces because what matters for rockets is fuel mass, not volume. Sure, fuel tanks have terrible mass ratios which limits their total delta-V, but that’s fine in a 1/10 scale solar system because you don’t need vast delta-V reserves to go places and the 9km/s you’d need to get into low Earth orbit can take you to Duna and back

[paul_c]

3 hours ago, kspacc said:

That is nonsense.

Reducing the weight of the fuel would in no way have any of the effects you have claimed, it would simply be more efficienty and increase Delta V.

PLEASE read the Wikipedia page on the rocket equation, or something like that, to learn why this isn't the case.

[Snark]

Some content has been removed. Let's please stick to the topic at hand and avoid personal remarks, please. Thank you.

[FleshJeb]

The average density of Kerbin is 58.48kg/liter, so I'd like to complain that the fuel is too light. /s

[king of nowhere]

On 3/29/2021 at 8:35 PM, kspacc said:

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.

 

oh, finally i can understand what you were asking.

and the answer is simple: planets in ksp are smaller. they decided that full realism would be too time-intensive, with a single orbital launch taking many tens of minutes. there are mods to restore that, but most people would prefer to do things faster. so, they decided that the whole system would be 1/10th of real size. Kerbin has 600 km radius while earth has a bit more than 6000. in real life, 600 km radius would be barely enough for a rocky object to have a round shape. Kerbin orbits the sun at 13.4 million km, the earth is at 160 million km. everything in ksp is smaller, so that travel times are shorter.

this, however, also reduces fuel consumption. it takes 11 km/s of deltaV to reach earth orbit. it takes 3.5 km/s to orbit kerbin. the requirements for interplanetary trips are also similarly smaller.

so, what they did was nerf engines and fuel. real life engines are lighter and give more thrust than ksp engines. ksp fuel tanks have more dry mass than real life fuel tanks. all this to ensure that the rockets required to do stuff in ksp would be somewhat realistic.

now, i'm not familiar with jet engines because that's a part of the game i don't use. but it makes sense that they would have lower efficiency. on earth, a range of 4000 km is a medium range international trip. if you're in the usa, it's maybe enough for a coast-to-coast. in ksp, it's enough for a full circumnavigation of the planet.

i'm not even sure you can circumnavigate the earth at the equator with a jet plane without refueling. on ksp you can. in ksp, you can make an ssto easily. in real life, you cannot - technically you could, but not without any sizeable payload.

so, fuel is actually more efficient in ksp than it is in real life; the numbers are a bit lower, but the requirements are much lower.

oh, ninja'ed by @jimmymcgoochie

Edited April 1, 2021 by king of nowhere