AAE5301 Chap.4 Fleet Sizing, Assignment and Repositioning
Fleet Sizing, Assignment and Repositioning
Capacity, geography and time horizon
Fleet sizing converts demand, operating time, cycle time and a service target into an integer number of vehicles. Assignment matches vehicles or types to missions, while repositioning restores supply-demand balance across depots. The chapter emphasises rounding direction, conservation and cost horizons.
A system can own enough vehicles in total and still fail because they are in the wrong place or unavailable at the required time.
Size productive capacity, not nominal ownership
Begin with the usable operating window and a complete cycle that includes flight, loading, service, charging and required turnaround. Full cycles per vehicle round down; required served demand and the number of vehicles round up.
That calculation is a deterministic lower bound. Weather, maintenance, heterogeneous vehicles, charger limits and spatial imbalance should enter as scenarios, reserve rules or explicit constraints rather than an unexplained percentage added at the end.
Conserve vehicles through place and time
Assignment decides which vehicle or type performs each mission.
Repositioning changes where capacity is available but never creates fleet. Verify the complete post-move vector, shared movement resources and arrival times before comparing cost. When capital, daily operation and service penalties share an objective, convert them to the same horizon. A recommendation should show why one fewer vehicle fails and whether one more vehicle still creates useful service.
What this chapter covers
- 01
Productive cycles per vehicle
- 02
Service targets and integer rounding
- 03
Strategic and tactical fleet costs
- 04
Vehicle type assignment
- 05
Static and dynamic repositioning
- 06
Depot balance and conservation
Worked example · free
Compute a minimum daily fleet
- 2Compute complete cycles per drone.
- 2Round required served orders upward.
- 4Round fleet upward and test one fewer vehicle.
Key terms
- Productive Cycle
- One complete service movement including required flight, handling and turnaround time.
- Service Target
- The share or amount of demand the design commits to complete.
- Fleet Assignment
- The allocation of vehicle types or individual vehicles to flights, routes or locations.
- Static Repositioning
- A one-period redistribution based on expected demand and known supply.
- Post-move Balance
- Initial supply plus inbound movements minus outbound movements at a location.
Fleet Sizing, Assignment and Repositioning FAQ
Why do productive cycles round down?
A partial cycle cannot complete an order inside the operating period, so productive capacity uses the lower integer. Required served orders and fleet count round upward because the service promise and vehicle count cannot be fractional.
Can enough total fleet still fail demand?
Yes. Vehicles may be concentrated at surplus depots while shortages occur elsewhere, or they may be charging or assigned when needed. Repositioning and time-indexed balance restore the geography and timing omitted by an aggregate count.
What makes a repositioning plan feasible?
Outbound movements cannot exceed available supply, and initial supply plus inbound moves minus outbound moves must meet required demand at every depot. Total vehicles must remain conserved unless purchases, retirements or external transfers are explicitly modelled.
How should reserve fleet be justified?
Tie reserve to a named uncertainty or service rule, such as one maintenance outage, a high-demand scenario or a response-time target. Evaluate the minimum fleet across those cases and report how often reserve is used. An arbitrary percentage hides both the protected failure and the cost of protection.
Why must cost terms use the same horizon?
A purchase price, annual ownership charge, daily operating cost and per-mission penalty cannot be added directly. Convert each to the decision period with a stated life and operating-day convention. Then test whether the ranking changes under a reasonable alternative convention, especially when capital and operating costs are close.
When does fleet sizing need a time-indexed model?
Use time periods when departures, charging, maintenance or repositioning delay determine whether a vehicle is available. An aggregate daily count is insufficient if morning demand peaks before returned vehicles can recharge, or if a transfer arrives after its target window. Carry each depot's balance forward from one period to the next and link missions to occupation time.
Compare the result with the aggregate lower bound to quantify the cost of timing.
Exam move
For every fleet question, draw a four-line ledger: cycle time, cycles per vehicle, required served demand and fleet count. For repositioning, write the post-move vector before comparing cost. Verify one fewer vehicle or one omitted move to show why the recommendation is necessary. Add a column for each omitted source of downtime and decide whether it needs a constraint, scenario or caveat.
For cost questions, write the time basis beside every term before adding them. Finish with a marginal check: compare the chosen fleet against one fewer and one more vehicle. Name the resource that becomes binding next.
