Liquid Fuel Mass

[Bluejayek]

I ran across something odd in my quest for the minimal orbital vehicle. Lets take a look at the stats of the FL-T250 and FL-T500. (Credit to UmbralRaptor for bringing this oddity to my attention)

FL-T250

Total Mass: 1.25

Dry mass: 0.2

Fuel Capacity: 250

Subtracting dry mass from total mass,

Fuel Mass: 1.05

Fuel Mass per unit of fuel: 0.042

FL-T500

Total Mass: 2.5

Dry mass: 0.3

Fuel Capacity: 500

Subtracting dry mass from total mass,

Fuel Mass: 2.2

Fuel Mass per unit of fuel: 0.044

Thats right... The mass per unit of fuel contained in the two tanks differs by a not insignificant 5%.

This does have implications for craft design. It means that it will ALWAYS be more efficient to put any half tanks on the bottom of stages, rather then the top, as the fuel burnt for the same amount of thrust from the larger tank will dump more mass off your craft.

I am curious as to what you guys think about this.

[sgtblazing]

You mislabled them. One would think the larger should be more economic as it has less container but more contents. I really wish they would add a system where stacking fuel tanks simply combined them, skipping the lid and base of the ones between. (would be the most economical of all )

[vexx32]

I believe the point is that fuel is heavier in the larger tank... which is interesting. And yes, I believe you have used the same label twice xD

Hmm. Odd.

[PakledHostage]

I noticed this so I bumped up the mass of the full 1/2 tank to 1.3 mass units in the tank\'s config file. It modifies the part, but not in a way that makes things easier.

This does raise an interesting point though: Should there be a standard fuel density for LFTs? The values (both for standard and mod parts) are currently all over the map.

[semininja]

There was a discussion about that a while back that led to an attempt at standardization, but it apparently didn\'t last very long...

[Bluejayek]

And yes, I believe you have used the same label twice xD

Woops. I copied the text because I was too lazy to rewrite the labels, and forgot to modify the titles. Fixed.

I agree with sgt that making one large tank rather then stacking little tanks would be very nice.

[Kosmo-not]

You get more delta-V out of full tanks than you do out of twice as many half tanks.

[UmbralRaptor]

You get more delta-V out of full tanks than you do out of twice as many half tanks.

Specifically, half-tanks have ~5% higher Isp, but the better mass ratio of full tanks outweighs it under most (all?) situations.

[Kosmo-not]

Specifically, half-tanks have ~5% higher Isp, but the better mass ratio of full tanks outweighs it under most (all?) situations.

Well, if you just need that half tank more, don\'t slap on a full tank. That is pretty much the only situation where you would want a half tank instead of a full tank. The other reason would be for balancing an asymmetrical rocket.

[NovaSilisko]

I am to blame here. I set the values of the tank before giving them to Harv (I just quickly made it along with the lander leg/small engine) and it looks like my rounding of numbers got a bit carried away...

[vexx32]

Perhaps a different solution is in order? The game should have a set value for the density of fuel, and each part then need only carry a full weight and an empty weight value. Then, the game can use this to work out how much fuel can be stored in the tank. This would stop any future rounding errors (although perhaps the fuel units value should be shown rounded to the nearest whole number, while the game does proper calculations for it...).

[UmbralRaptor]

I am to blame here. I set the values of the tank before giving them to Harv (I just quickly made it along with the lander leg/small engine) and it looks like my rounding of numbers got a bit carried away...

Yay, rounding error. ;P Drop the empty mass of the half-tank to 0.15, and they\'ll offer identical performance.

[Zephram Kerman]

Perhaps a different solution is in order? The game should have a set value for the density of fuel, and each part then need only carry a full weight and an empty weight value. Then, the game can use this to work out how much fuel can be stored in the tank. This would stop any future rounding errors (although perhaps the fuel units value should be shown rounded to the nearest whole number, while the game does proper calculations for it...).

I like this idea. It also allows for different fuels in the future. (Kerosene, hydrogen, oxygen, hydrazene, etc. all have different mass densities and energy densities. But I think you meant to assign only the empty weight; also assigning the full weight would defeat the purpose of your post.

[vexx32]

Oh, you mean just assign empty weight and fuel amount (in whatever units they use)? Yeah, that\'d probably make more sense than a full and empty weight, with an arbitrary fuel count xD

And if we end up with different fuel types, that can be set in the part\'s cfg as well. Perhaps in future, when cost and such is implemented, we can just put however much fuel in we want, rather than having all tanks totally full if we don\'t have the funds...