Station-keeping budgets

What you will accomplish

A delta-v and propellant budget for a whole fleet, with the one input that dominates it identified and swept rather than assumed.

Prerequisites

A constellation.
High-fidelity runs for where the ballistic coefficient comes from.

Steps

Run the budget


orbitforge stationkeep --constellation ph1.json --years 5


Station-keeping budget for `ph1`: 60 satellites over 5.0 years.
Per satellite: 5.21 m/s/yr (0.625 kg/yr propellant), capacity 172.7 m/s.
  drag-makeup: 5.21 m/s/yr, 1.096 m/s per cycle every 76.8 days
Fleet: 290.2 m/s/yr total delta-v, 34.78 kg/yr propellant.
Fuel margin covers the full horizon.

Check the capacity figure yourself

Verify the capacity before trusting the verdict

The whole “covers the full horizon” verdict rests on the 172.7 m/s capacity, and that number comes from the rocket equation with the defaults --isp-s 220, --dry-mass-kg 240, and --fuel-kg 20:

\Delta v = I_{sp} \, g_0 \ln\frac{m_{dry} + m_{fuel}}{m_{dry}}

220 \times 9.80665 \times \ln\frac{260}{240} = 172.7 \text{ m/s}

The tool reports 172.7 m/s and the closed form gives 172.7 m/s.

This takes thirty seconds and confirms the defaults are being applied the way you think they are. It is the same habit as checking nodal regression in Doppler and orbital elements.

Those defaults describe a roughly 260 kg spacecraft with a 220 s monopropellant system. If yours is different, every number on the page changes, and none of them will warn you.

The budget is linear in a coefficient you supplied

--cdam is the ballistic coefficient in m^2/kg, formed from the drag coefficient, the cross-sectional area, and the spacecraft mass. Sweeping it over 5 years:

`--cdam`	Per satellite	Fleet propellant
0.005	2.61 m/s/yr	17.44 kg/yr
0.01	5.21 m/s/yr	34.78 kg/yr
0.02	10.43 m/s/yr	69.17 kg/yr
0.04	20.85 m/s/yr	136.80 kg/yr

Exactly linear: each doubling of the coefficient doubles the delta-v, which is what drag makeup should do.

The entire budget is proportional to a number you typed in. The default of 0.01 is a plausible value, not a measurement of your spacecraft.

Two separate uncertainties multiply here. The ballistic coefficient depends on mass, area, and attitude, and atmospheric density is itself uncertain by tens of percent and varies strongly across the solar cycle.

A station-keeping budget is therefore a range, and the honest deliverable is the sweep above rather than any single row of it.

Run the case you expect and a pessimistic case, and size the tank for the pessimistic one.

When the tanks run dry, the annual figure changes meaning

Over a 10-year horizon:

`--cdam`	Fleet propellant	Verdict
0.02	68.40 kg/yr	Fuel margin covers the full horizon
0.03	101.47 kg/yr	Fuel margin covers the full horizon
0.04	120.00 kg/yr	Fuel-limited: tanks run dry after 8.3 years
0.05	120.00 kg/yr	Fuel-limited: tanks run dry after 6.6 years

The last two rows report the same 120.00 kg/yr for demands that differ by 25 percent.

That figure is 60 satellites times 20 kg of fuel, divided by the 10-year horizon. Once the fleet is fuel-limited, the propellant number stops being demand and becomes supply divided by the horizon.

Scaling from the 0.03 row, actual demand at cdam 0.04 is about 135 kg/yr, not 120. Comparing the two rows understates the increase, and does so in the optimistic direction.

Read the verdict line, not the propellant figure. “Tanks run dry after 8.3 years” is the result; 120.00 kg/yr is an artifact of the horizon you chose.

Delta-v per year, in the left column of the earlier table, stays honest throughout. It is the propellant figure that saturates.

What the cycle line tells you


drag-makeup: 5.21 m/s/yr, 1.096 m/s per cycle every 76.8 days

This is an operations input, not just a physics one. A 1.096 m/s burn every 76.8 days across 60 satellites is roughly one maneuver every 31 hours somewhere in the fleet, each one perturbing that satellite’s ephemeris and invalidating any conjunction screening that spans it.

Tightening --deadband-km makes burns smaller and more frequent for approximately the same annual delta-v, which trades propellant efficiency against operational load and ephemeris stability.

What is not modeled

The budget covers the drag makeup it names. Check whether your mission also needs, and whether this figure includes, any of:

Initial orbit acquisition and plane phasing after launch
Collision avoidance maneuvers, which are unscheduled by nature
End-of-life disposal, which for many regimes is the largest single item
Attitude control propellant, if it shares the tank
Residual and unusable propellant

Disposal in particular is worth costing separately and early, because it is a requirement rather than a choice and it competes for the same tank.

Checklist

Are --isp-s, --dry-mass-kg, and --fuel-kg your spacecraft?
Did you verify capacity against the rocket equation?
Did you sweep --cdam and report a range?
Are you reading the verdict line rather than a saturated propellant figure?
Is disposal delta-v budgeted somewhere?
Can operations absorb the maneuver cadence?

Next steps

stationkeep reference for the full flag set, including the geostationary slot options.
Screening for conjunctions, which the maneuver cadence above interacts with directly.

Question? Give us feedbackDocuments Varaha Constellation Designer main (pre-release)