Formula & Calculator
Specific Range
Distance traveled per unit weight of fuel consumed, at a given airspeed, fuel flow, and thrust setting.
Interpretation
Specific range: SR = V / (c·T), where V is speed, c is TSFC, T is thrust. It is the distance travelled per unit fuel mass. Example: V=250 m/s, c=0.00005 s⁻¹, T=5000 N → SR = 250/(0.00005×5000) = 250/0.25 = 1000 m/kg.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| SR | Specific range | m/N |
| V | True airspeed | m/s |
| c | Specific fuel consumption | 1/s |
| T | Thrust required | N |
What it means
Specific range is a measure of fuel efficiency for a jet aircraft: the distance it can cover per unit mass of fuel consumed. It is used in performance calculations, particularly in the Breguet range equation. For given fuel mass, higher SR gives longer range. SR depends on aircraft speed, drag, and engine efficiency. Optimising SR is a key objective in aircraft design and cruise planning. Understanding specific range is essential for mission analysis and fuel management.
Worked example
Specific Range – Two Examples
Real‑World| Parameter | Value |
|---|---|
| V | 230 |
| c | 2×10⁻⁵ |
| T | 20,000 |
| Parameter | Value |
|---|---|
| V | 240 |
| c | 1.8×10⁻⁵ |
| T | 22,000 |
Common mistakes
- Specific range: SR = V / (c · T).
- V: Airspeed (m/s).
- c: TSFC (1/s).
- T: Thrust (N).
- Units: SR is in m/kg (distance per unit fuel mass). Derivation: fuel flow = c·T (kg/s), distance per time = V, so distance per fuel = V/(c·T). Units: (m/s)/(kg/s) = m/kg.
- Used for range analysis.
Applications
Specific range, SR = V/(c·T), is the range per unit of fuel, a key efficiency metric. It is used in mission analysis to optimise cruise speed and altitude. Engineers use SR to compare aircraft designs and to select the best operating conditions for maximum range. By maximising SR, aerospace engineers can reduce fuel consumption and operating costs. The specific range is influenced by aerodynamics, engine efficiency, and weight, and is a central figure in aircraft performance.
- Aircraft range optimisation (cruise speed and altitude)
- Mission analysis and fuel planning
- Performance comparison of different aircraft
- Engine and airframe trade‑off studies
- Economic and environmental impact assessment
Frequently Asked Questions
It is the distance traveled per unit weight of fuel consumed, at a given airspeed, fuel flow, and thrust setting. It is a measure of fuel economy.
V = true airspeed (m/s)
c = thrust‑specific fuel consumption (TSFC) (kg/s·N)
T = thrust (N)
It directly affects the range of the aircraft. Maximising specific range is the goal for long‑range cruise.
- Confusing specific range (distance/fuel weight) with simple ground speed, ignoring fuel‑flow dependence entirely.
- Using the wrong units for c (e.g., per hour instead of per second).
- Assuming constant TSFC over the flight.
V = 250 m/s, c = 0.00002 kg/(s·N), T = 40,000 N. SR = 250 / (0.00002 × 40000) = 250 / 0.8 = 312.5 m per kg of fuel.
At higher altitude, the engine TSFC may improve, and the true airspeed for a given Mach increases, potentially increasing SR.
SR = V/(c·T) = (L/D)·V/(c·W) for level flight, so SR is proportional to L/D.
Fly at the speed that maximises V/(c·L/D), which is near the maximum L/D speed for jet aircraft.
Specific range is distance per fuel weight; specific fuel consumption is fuel weight per unit thrust per time. They are inverses (with speed).
As weight decreases (fuel burn), the required thrust decreases, improving SR. This is why aircraft have better range at the end of cruise.