investigating Incorrect fuel density calculation in the Hornet

[Fresh]

The NATOPS manual says the Hornet has 1589 gallons of volume in the tanks. At 6.8 pounds per gallons specific density I get 10805 pounds of fuel on a standard ISA day. (NATOPS says 10810 pounds).

On a very hot day, say 50°C the volume of the fuel tanks stays constant, but the fuel denisty is less (fuel expands). Roughly it would be 6.6 pounds per gallon. So on this very hot day, the 1589 gallons should have a mass or about 10487 pounds.

In DCS at 15° C I can fit 10800 pounds of fuel in the Hornet, however at 50°C I can fit 11130 pounds of fuel.

DCS is calculating the fuel density and therefore the fuel mass wrongly in the Hornet. (I haven't tested if this is a Hornet only issue.)

See attached images.

image of specific density is unclassified and from 1968.

https://apps.dtic.mil/sti/tr/pdf/AD0291128.pdf

Thanks,

Fresh

[Tactical Defeat]

I just tried this with the OH-58D. Hot start with 90% gas at sea-level. And did a refuel to 100% see if there was any thing that changed. 

Note Mission editor weight indicated 661 lbs for all temps at 90% load. 736 lbs for 100%.

In the mission these were the indicated numbers via the panel

15c

Start:   661lbs indicated

Refuel: 730lbs indicated

-12c

Start:   661lbs indicated

Refuel: 730lbs indicated

50c

Start:    661 lbs indicated

Refuel: 730 lbs indicated

It may be module dependent. 

[Fresh]

I now tried a standard ISA day (15ºC at sea level) and spawned at 40,000 feet and at 4,000 feet. The resulting fuel mass (as indicated) changes with OAT/altitude. Unfortunately the higher(colder) you spawn, the less fuel you get, assuming the spawned volume is the same in the Hornet.

The NATOPS manual (2.2.10) says the fuel density can vary from 6.13 pounds/gallon at 100°F to 7.38 pounds/gallon at −40°F, the total internal fuel quantity with full tanks can vary from 9,740 pounds to 11,730 pounds for the F/A-18C.

I think DCS is correcting the volume for temperature, instead of the mass for temperature at mission start. If I fill the tanks full when I set 100% in the armament page or in the mission editor, I am expecting the volume to be full, i.e. constant 1589 gallons, and therefore the mass should vary. (It does vary, but the wrong way round). Once the mission starts the total mass of fuel should stay constant (minus burned/dumped/jettisoned/leaked fuel). The volume should change with temperature after mission start, not the mass.

Edited July 25, 2024 by Fresh

[Harker]

I also noticed this while doing missions in colder weather. The indicated fuel in lbs actually goes down when the temperature is lower, which contradicts what the NATOPS says.

 

Also, regardless of the NATOPS, JP-8 has a positive thermal expansion coefficient, so its density should go down with increasing temperature.