Fun fact ... SpaceX fueling speed

[Spinward Flow]

SpaceX is in the process of upgrading their cryogenic pumps and chillers to be able to deliver 4600 tons of liquified CH₄ fuel to a Starship stack in ~45 minutes, with a goal of pumping 100+ tons per minute.

For TL=8 cryogenic chemical propulsion systems this is an important upgrade, because the longer it takes to fuel the craft, the more of the liquified cryogenic CH₄ and O₂ will boil off, phase changing from liquid to gas (which must be vented from the pressure tanks) due to thermal transfer from the outside environment (see: frost ring on stainless steel outer skin). A faster chill and flow rate for these cryogenic liquids means less time for thermal transfer to happen and thus less boil off waste of propellants, meaning higher efficiency use of propellants and less waste.

Anyway, I figured that this data point could be of use to anyone who has ever wondered just how fast a fuel transfer at a starbase starport berth reasonably ought to be per fuel purification plant (200-1000 tons fuel capacity, per LBB5.80 RAW).

100 tons of L-H₂ per minute "sounds about right" for a fuel transfer rate from a starport's fueling infrastructure.
Also means that extremely large ships with enormous fuel tanks could take a much longer time to top up on fuel.

 

[whartung]

100 tons per minute is 200,000 lbs. That's 90,600kg. LH density is 71kg/m3. So, 90600/71 = 1276m3. A dTon is 14m3? So that 91dTons per minute?

Is that math right?

 

[Spinward Flow]

100 tons per minute is 200,000 lbs. That's 90,600kg.

200,000 lbs = 100 short tons (US)
220,400 lbs = 100 metric tons
224,000 lbs = 100 long tons (british imperial)

 

[Grav_Moped]

100 tons of L-H2 per minute "sounds about right" for a fuel transfer rate from a starport's fueling infrastructure.
Also means that extremely large ships with enormous fuel tanks could take a much longer time to top up on fuel.

Or... transfer could be done in parallel (multiple fuel lines to separate tank sections).

 

[nikodemus]

Sorry about the necro - new here and looking around

Ignoring the math, I think extremely fast fuel loading can indeed be justified by saying "look at Space-X today", but much slower ones can be justified by appealing to safety & maintenance concerns as well.

- A spaceport might have limited amount of berths with faster fuel lines, possibly reserved for military ships.
- A spaceport might be bribed to load fuel faster despite the regs, possibly damaging their equipment in process.
- Damaged equipment might make loading unusually slow.

Re. math, I would calculate based on the volume of methane. For 100 metric tons liquid methane per minute, I get 16dT per minute.

https://www.aqua-calc.com/page/density-table/substance/methane-coma-and-blank-liquid says liquid methane is 424kg/m3.

100 000 kg/min / 424kg/m3 ≈ 236m3/min

236m3/min / 14m3/dTon ≈ 16dT/min.

...but fore sure I could justify 100dT by citing TL12!

For my universe, I think 0.5-2t/min sounds about right for normal loading speed, and 5-20t/min for "high pressure milspec fuel lines". (Something ridiculous like 0.05t/min could be justified with damaged equipment...)

This means:

20t tank: 10-40 min on normal lines; 1-4 min on high pressure lines.
100t tank: 50-200 min on normal lines; 5-20 min high pressure lines.

So fueling is practically speaking fast enough that it doesn't need to be accounted for, but saying "you need to wait 20min for fueling to finish!" is also justifiable.

 

[whartung]

Ignoring the math, I think extremely fast fuel loading can indeed be justified by saying "look at Space-X today", but much slower ones can be justified by appealing to safety & maintenance concerns as well.

When fueling times becomes the bottle neck that's keeping ships from working, then they'll work to speed up the process.

A simple example is that airliners are fueled during the passenger switchover. I can't say the plane has never been ready and still waiting for fuel, but it's probably rare. They also seem to do it while passengers are still on the plane (either off- or on-boarding), and while loading luggage and cargo. So, I'm guessing it's a reasonably safe process.

The key point being that for the time being, fueling is "fast enough".

Why SX wants to fill a ship that quickly, I don't know, nor do I know any exigent hazards with fueling so fast. But I'm going to guess that the 45 minute fuel time isn't why folks are sitting around, tapping their feet waiting for launch. Perhaps it's a cooling/heating thing. Faster they can fuel, faster they can launch, don't need to design or worry about the rocket sitting on the pad for days with liquid hydrogen sitting in a thermos.

Perhaps there's savings in a design that can be fueled rapidly and launched in a few hours. If it takes longer, they simply ("simply") drain the tanks and do it again when ready to launch. Could be significant weight savings or something.

 

[kilemall]

OP covered the why succinctly.