Cannonball Drag And Solar Radiation Pressure

Octavian provides first-order drag and solar-radiation-pressure (SRP) models for preliminary mission design. The same force implementation is used by inertial coasts, finite burns, exact coupled relative motion, and numerical relative-element propagation.

Configure The Spacecraft And Environment

Spacecraft geometry belongs on Spacecraft; force selection belongs on Perturbations:

from octavian import Cannonball, Perturbations, Spacecraft

vehicle = Spacecraft(
    name="Inspector",
    dry_mass_kg=150.0,
    cannonball=Cannonball(
        drag_area_m2=4.0,
        drag_coefficient=2.2,
        srp_area_m2=5.0,
        reflectivity_coefficient=1.4,
    ),
)
forces = Perturbations(j2=True, drag=True, srp=True)

Both areas are constant projected areas. A positive mass and the corresponding positive area are required when a phase enables drag or SRP. Coast phases use the spacecraft's initial mass. Mass-carrying coasts and finite burns use the instantaneous propagated mass, so drag and SRP naturally increase as propellant is consumed.

Inertial Coast Or Burn

Attach the force model to ordinary Earth-centered dynamics:

from octavian import Dynamics, Phase

dynamics = Dynamics.for_body("earth", perturbations=forces)
coast = Phase(
    name="perturbed_coast",
    mode="coast",
    spacecraft=vehicle,
    dynamics=dynamics,
    tof_bounds_s=(1800.0, 3600.0),
)

Use the same dynamics and spacecraft in a finite_thrust phase. Thrust is added to the environmental acceleration, while spacecraft mass is depleted by the selected thruster.

SRP reads the Earth-centered Sun state from Octavian's bundled SPICE BSP. Set Mission.initial_epoch whenever SRP is active. Enabling srp=True samples the Sun but does not silently enable solar third-body gravity; add sun=True when both effects are desired.

Differential Relative Motion

Exact relative dynamics can use distinct chief and deputy properties:

from octavian import Dynamics

relative_dynamics = Dynamics.relative(
    chief_initial_state_eci=chief_eci,
    chief_spacecraft=chief_vehicle,
    propagation_mode="coupled_eci",
    perturbations=forces,
)

relative_phase = Phase(
    name="inspection_coast",
    mode="coast",
    spacecraft=vehicle,  # the deputy
    dynamics=relative_dynamics,
)

The solver propagates both absolute states, applies each spacecraft's ballistic properties, and converts the result to RIC. Omitting chief_spacecraft intentionally leaves the chief gravity-only while the deputy receives drag or SRP. CWH and native two-body RIC/ROE solver modes remain reduced models; select coupled_eci for perturbations.

For analysis without an optimization solve, the same properties work with relative orbital elements:

from octavian import EARTH
from octavian.relative import propagate_relative_orbital_elements

history = propagate_relative_orbital_elements(
    initial_roe,
    times_s,
    chief_initial_state_eci=chief_eci,
    mu_m3ps2=EARTH.mu_m3ps2,
    perturbations=forces,
    initial_epoch="2026-01-01T00:00:00Z",
    chief_spacecraft=chief_vehicle,
    deputy_spacecraft=vehicle,
)

See examples/composable/relative/27_cannonball_drag_srp.py for osculating D'Amico elements, RIC reconstruction, and plots.

Atmosphere Model

The default Earth model is a co-rotating, single-scale-height exponential atmosphere:

rho(h) = rho_ref exp(-(h - h_ref) / H)
a_drag = -0.5 rho Cd A / m |v_rel| v_rel

Supply an explicit model to change the assumptions:

from octavian import ExponentialAtmosphere, Perturbations

atmosphere = ExponentialAtmosphere(
    reference_density_kgpm3=1.0e-12,
    reference_altitude_m=400_000.0,
    scale_height_m=50_000.0,
    rotation_rate_radps=7.2921159e-5,
)
forces = Perturbations(drag=True, atmosphere=atmosphere)

Octavian will not apply the default Earth atmosphere to another central body. Provide atmosphere= explicitly for non-Earth drag studies.

The JSON/YAML schema uses the same separation:

spacecraft:
  inspector:
    dry_mass_kg: 150.0
    cannonball:
      drag_area_m2: 4.0
      drag_coefficient: 2.2
      srp_area_m2: 5.0
      reflectivity_coefficient: 1.4

dynamics:
  earth_perturbed:
    central_body: earth
    perturbations:
      j2: true
      drag: true
      srp: true

Set mission.initial_epoch in the same config whenever SRP is enabled.

Fidelity Boundary

These models intentionally omit:

• density tables, space weather, winds, and oblateness in the atmosphere;
• attitude-dependent projected area;
• self-shadowing, Earth eclipse, and penumbra transitions for SRP;
• detailed optical surface properties.

SRP points directly away from the Sun and follows inverse-square distance from the configured pressure at one astronomical unit. Use an external high-fidelity propagator when those omitted effects drive the design.