Tanker Shuttle: Rate, Shortfall, and the Onboard Tank Trap
You are the second-arriving engine at a rural structure fire. The incident commander has set a required fire flow of 500 gpm to be sustained for at least 20 minutes. There is no hydrant; water comes from a fill site 1.5 miles away. Each of the three tankers now available carries 2,000 gallons usable. A full cycle takes 13 minutes: 4 minutes to fill, 3 minutes to travel loaded, 3 minutes to dump, 3 minutes to return. Your engine is on scene with 750 gallons in its own booster tank and is currently flowing 500 gpm. A fourth tanker is 10 minutes out.
1. How many tankers must be in the shuttle to hold 500 gpm, and what does each tanker actually deliver per minute?
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Per-tanker rate: each tanker is only productive one full cycle at a time, so its contribution is its usable load divided by the whole cycle, not just the dump time.
2,000 gal ÷ 13 min = about 154 gpm per tanker
Combined:
- 3 tankers: 3 × 154 = about 461 gpm - short.
- 4 tankers: 4 × 154 = about 615 gpm - sufficient.
Tankers required: 500 ÷ 154 = 3.25, and you cannot send a quarter of a tanker, so 4 tankers.
Cross-check against total demand: 20 min × 500 gpm = 10,000 gallons. Four tankers cycling for 20 minutes deliver roughly 615 × 20 = 12,300 gallons, which covers it; three deliver about 9,200 gallons, which does not.
Also worth stating to the IC: the cheapest way to improve any of these numbers is to shorten the cycle - a drive-through dump site or a second fill site usually beats adding apparatus.
2. For the first 10 minutes only three tankers are running. How much water must come out of your 750-gallon booster tank, and what is left when the fourth tanker arrives?
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The trap: it is tempting to say '500 gpm for 10 minutes is 5,000 gallons, so the tank is gone in 90 seconds.' That is wrong. The tankers are still delivering about 461 gpm; the booster tank only makes up the difference.
Shortfall: 500 − 461 = about 39 gpm
Drawn from the booster tank: 39 gpm × 10 min = about 390 gallons
Remaining: 750 − 390 = about 360 gallons
Interpretation for the IC: the buffer lasts roughly 360 ÷ 39 ≈ 9 more minutes at the same deficit rate. That is real margin, not a crisis - but it disappears instantly if a tanker breaks down, the fill site slows, or the fire flow is increased.
Judgment: report the numbers and recommend the fourth tanker be committed to the shuttle rather than held as a static resource, and that a second dump tank be set so a tanker never waits to dump.
3. Where do you set the dump site and portable tank, and what drives that choice?
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Drivers: the dump time is 3 of the 13 cycle minutes, so anything that shortens it raises every tanker's gpm. Placement also must not compromise fireground safety.
Place it:
- At your engine, so the hard suction or a short large-diameter line reaches the portable tank and the engine's intake is fed directly.
- In a drive-through configuration where the tanker enters, dumps while rolling or stops briefly, and exits forward without backing. A tanker that has to back is a 3-minute dump turning into 5 or 6.
- On firm, level ground clear of the collapse zone, the ladder truck's outrigger footprint, and the hosebed access of arriving companies.
- Downhill or level of the engine intake and close to it, minimizing suction lift.
Report to the IC: the chosen location, the entry/exit route, and the fact that the dump site - not the fire - now sets the sustained gpm.