Understand the relationship
The reasoning behind the result
Flow is an instantaneous operating quantity
F=ṁveffective=ṁIspg0
The thrust-derived mode divides net thrust by effective exhaust velocity. It assumes the supplied thrust and Isp represent the same constant operating condition and that the flow includes all expelled propellants represented by that thrust.
Known-flow mode uses the recorded or specified total flow directly. Neither mode infers startup losses, mixture changes, ullage, unpumpable residuals or thrust transients. Reserve is an explicit entered exclusion, not an automatically calculated tank or mission requirement.
A mass budget determines constant-flow powered time
tburn=(mloaded−mreserve)/ṁ
At constant positive flow, each second uses the same mass. Dividing usable mass by kg/s therefore gives seconds. The reserve remains unburned by definition. Zero usable propellant gives zero modeled burn time.
This duration is not a vehicle trajectory. Acceleration can change as total vehicle mass changes, and gravity and drag affect motion. The flow ledger makes no claim about attained speed, orbital maneuver completion or launch performance.
A schedule has a clock and a propellant ledger
mused,j=min(mremaining,j, ṁj·Δtj)
Each phase has a planned duration and flow. A powered phase is truncated if its required mass exceeds the remaining usable mass. A coast consumes no propellant but advances the clock while the sequence is still being executed. Once usable propellant is exhausted, later phases are marked unexecuted in this accounting model.
Powered time sums executed positive-flow durations; elapsed time also includes executed coasts. If the entered sequence ends with usable mass remaining, the remaining mass is shown without inventing an extension of the last phase. Requested mass for the full plan is reported separately so any shortfall remains inspectable.
Follow the numbers
A schedule that exhausts its usable propellant
- Load 1,000 kg and exclude a 100 kg reserve, leaving 900 kg usable.
- The first phase runs for 20 s at 10 kg/s and uses 200 kg, leaving 700 kg.
- A planned 100 s second phase at 10 kg/s requires 1,000 kg, but only 700 kg remains. It executes for 70 s. The sequence stops after 90 s, with the 100 kg reserve untouched; any later phase is unexecuted.
The calculator reports a partial phase and the missing propellant instead of presenting the full plan as completed.
Quick guide
How to use this calculator
- Enter loaded propellant and the portion deliberately excluded from use.
- For constant operation, supply either measured total mass flow or matched thrust and Isp.
- For a schedule, list phases in order, including zero-flow coasts where appropriate.
- Inspect phase completion, propellant remaining and powered versus elapsed time. A plan ending with fuel remaining does not determine a later exhaustion time.
Calculation method
Calculation and interpretation
Keep instantaneous propellant flow, powered time, elapsed schedule time and unburned reserve separate.
ṁ=F/(Isp·g0); usable mass=loaded−reserve; continuous burn time=usable mass/ṁ; each phase consumes flow×executed duration.
Worked example
A schedule that exhausts its usable propellant
The calculator reports a partial phase and the missing propellant instead of presenting the full plan as completed.
ṁ=F/(Isp·g0); usable mass=loaded−reserve; continuous burn time=usable mass/ṁ; each phase consumes flow×executed duration.
Supported inputs
Precision and limits
An entered engine model
These calculations do not establish combustion stability, nozzle operability, thermal margins, structural capacity, vehicle guidance or launch capability. Use consistent measured or specified performance at the same operating condition.
Effective exhaust and standard gravity
Effective exhaust velocity includes the thrust contribution represented by the entered performance. Converting Isp in seconds uses the fixed standard gravity 9.80665 m/s², even away from Earth; local gravity is a different quantity.
Piecewise accounting only
Each entered phase has constant flow. Phase transitions, restart capability, pressure transients, thermal behavior, tank feed behavior and guidance are outside this model. Stopping at reserve is an accounting assumption, not an engine cutoff prediction.
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