def _targetting(departure_body, target_body, t_launch, t_arrival):
"""This function returns the increment in departure and arrival velocities.
"""
rr_dpt_body, vv_dpt_body = _get_state(departure_body, t_launch)
rr_arr_body, vv_arr_body = _get_state(target_body, t_arrival)
# Compute time of flight
tof = t_arrival - t_launch
if tof <= 0:
return None, None, None, None, None
try:
(v_dpt, v_arr), = lambert(Sun.k, rr_dpt_body.xyz, rr_arr_body.xyz, tof)
# Compute all the output variables
dv_dpt = norm(v_dpt - vv_dpt_body.xyz)
dv_arr = norm(v_arr - vv_arr_body.xyz)
c3_launch = dv_dpt ** 2
c3_arrival = dv_arr ** 2
return (
dv_dpt.to(u.km / u.s).value,
dv_arr.to(u.km / u.s).value,
c3_launch.to(u.km ** 2 / u.s ** 2).value,
c3_arrival.to(u.km ** 2 / u.s ** 2).value,
tof.jd,
)
except AssertionError:
return None, None, None, None, None
def test_propagating_to_certain_nu_is_correct():
# take an elliptic orbit
a = 1.0 * u.AU
ecc = 1.0 / 3.0 * u.one
_a = 0.0 * u.rad
nu = 10 * u.deg
elliptic = Orbit.from_classical(Sun, a, ecc, _a, _a, _a, nu)
elliptic_at_perihelion = elliptic.propagate_to_anomaly(0.0 * u.rad)
r_per, _ = elliptic_at_perihelion.rv()
elliptic_at_aphelion = elliptic.propagate_to_anomaly(np.pi * u.rad)
r_ap, _ = elliptic_at_aphelion.rv()
assert_quantity_allclose(norm(r_per), a * (1.0 - ecc))
assert_quantity_allclose(norm(r_ap), a * (1.0 + ecc))
# TODO: Test specific values
assert elliptic_at_perihelion.epoch > elliptic.epoch
assert elliptic_at_aphelion.epoch > elliptic.epoch
# test 10 random true anomaly values
for _ in range(10):
nu = np.random.uniform(low=0.0, high=2 * np.pi)
elliptic = elliptic.propagate_to_anomaly(nu * u.rad)
r, _ = elliptic.rv()
assert_quantity_allclose(norm(r), a * (1.0 - ecc ** 2) / (1 + ecc * np.cos(nu)))
def test_cowell_propagation_circle_to_circle():
# From [Edelbaum, 1961]
accel = 1e-7
def constant_accel(t0, u, k):
v = u[3:]
norm_v = (v[0]**2 + v[1]**2 + v[2]**2)**.5
return accel * v / norm_v
ss = Orbit.circular(Earth, 500 * u.km)
tof = 20 * ss.period
r, v = cowell(ss,
tof.to(u.s).value,
ad=constant_accel)
ss_final = Orbit.from_vectors(
Earth, r * u.km, v * u.km / u.s)
da_a0 = (ss_final.a - ss.a) / ss.a
dv_v0 = abs(norm(ss_final.v) - norm(ss.v)) / norm(ss.v)
assert_quantity_allclose(da_a0, 2 * dv_v0, rtol=1e-2)
dv = abs(norm(ss_final.v) - norm(ss.v))
accel_dt = accel * u.km / u.s**2 * tof
assert_quantity_allclose(dv, accel_dt, rtol=1e-2)
def test_cowell_propagation_circle_to_circle():
# From [Edelbaum, 1961]
accel = 1e-7
def constant_accel(t0, u, k):
v = u[3:]
norm_v = (v[0]**2 + v[1]**2 + v[2]**2)**.5
return accel * v / norm_v
ss = Orbit.circular(Earth, 500 * u.km)
tof = 20 * ss.period
r0, v0 = ss.rv()
k = ss.attractor.k
r, v = cowell(k.to(u.km**3 / u.s**2).value,
r0.to(u.km).value,
v0.to(u.km / u.s).value,
tof.to(u.s).value,
ad=constant_accel)
ss_final = Orbit.from_vectors(Earth,
r * u.km,
v * u.km / u.s)
da_a0 = (ss_final.a - ss.a) / ss.a
dv_v0 = abs(norm(ss_final.v) - norm(ss.v)) / norm(ss.v)
assert_almost_equal(da_a0.value, 2 * dv_v0.value, decimal=4)
dv = abs(norm(ss_final.v) - norm(ss.v))
accel_dt = accel * u.km / u.s**2 * tof
assert_almost_equal(dv.value, accel_dt.value, decimal=4)
def test_propagation_hyperbolic():
# Data from Curtis, example 3.5
r0 = [Earth.R.to(u.km).value + 300, 0, 0] * u.km
v0 = [0, 15, 0] * u.km / u.s
ss0 = Orbit.from_vectors(Earth, r0, v0)
tof = 14941 * u.s
ss1 = ss0.propagate(tof)
r, v = ss1.rv()
assert_almost_equal(norm(r).to(u.km).value, 163180, decimal=-1)
assert_almost_equal(norm(v).to(u.km/u.s).value, 10.51, decimal=2)
def pqw(self):
"""Perifocal frame (PQW) vectors. """
if self.ecc < 1e-8:
if abs(self.inc.to(u.rad).value) > 1e-8:
node = np.cross([0, 0, 1], self.h_vec) / norm(self.h_vec)
p_vec = node / norm(node) # Circular inclined
else:
p_vec = [1, 0, 0] * u.one # Circular equatorial
else:
p_vec = self.e_vec / self.ecc
w_vec = self.h_vec / norm(self.h_vec)
q_vec = np.cross(w_vec, p_vec) * u.one
return p_vec, q_vec, w_vec
def test_propagation_hyperbolic():
# Data from Curtis, example 3.5
r0 = [Earth.R.to(u.km).value + 300, 0, 0] * u.km
v0 = [0, 15, 0] * u.km / u.s
expected_r_norm = 163180 * u.km
expected_v_norm = 10.51 * u.km / u.s
ss0 = Orbit.from_vectors(Earth, r0, v0)
tof = 14941 * u.s
ss1 = ss0.propagate(tof)
r, v = ss1.rv()
assert_quantity_allclose(norm(r), expected_r_norm, rtol=1e-4)
assert_quantity_allclose(norm(v), expected_v_norm, rtol=1e-3)
def guidance_law(ss_0, ecc_f, inc_f, f):
"""Guidance law from the model.
Thrust is aligned with an inertially fixed direction perpendicular to the
semimajor axis of the orbit.
Parameters
----------
ss_0 : Orbit
Initial orbit, containing all the information.
ecc_f : float
Final eccentricity.
inc_f : float
Final inclination.
f : float
Magnitude of constant acceleration.
"""
# We fix the inertial direction at the beginning
ecc_0 = ss_0.ecc.value
if ecc_0 > 0.001: # Arbitrary tolerance
ref_vec = ss_0.e_vec / ecc_0
else:
ref_vec = ss_0.r / norm(ss_0.r)
h_unit = ss_0.h_vec / norm(ss_0.h_vec)
thrust_unit = np.cross(h_unit, ref_vec) * np.sign(ecc_f - ecc_0)
inc_0 = ss_0.inc.to(u.rad).value
argp = ss_0.argp.to(u.rad).value
beta_0_ = beta(ecc_0, ecc_f, inc_0, inc_f, argp)
@state_from_vector
def a_d(t0, ss):
r = ss.r.value
v = ss.v.value
nu = ss.nu.value
beta_ = beta_0_ * np.sign(np.cos(nu)) # The sign of ß reverses at minor axis crossings
w_ = np.cross(r, v) / norm(np.cross(r, v))
accel_v = f * (
np.cos(beta_) * thrust_unit +
np.sin(beta_) * w_
)
return accel_v
return a_d
def a_d(t0, ss):
r = ss.r.value
v = ss.v.value
# Change sign of beta with the out-of-plane velocity
beta_ = beta(t0, V_0=V_0, f=f, beta_0=beta_0_) * np.sign(r[0] * (inc_f - inc_0))
t_ = v / norm(v)
w_ = np.cross(r, v) / norm(np.cross(r, v))
# n_ = np.cross(t_, w_)
accel_v = f * (
np.cos(beta_) * t_ +
np.sin(beta_) * w_
)
return accel_v
def a_d(t0, ss):
r = ss.r.value
v = ss.v.value
nu = ss.nu.value
alpha_ = nu - np.pi / 2
r_ = r / norm(r)
w_ = np.cross(r, v) / norm(np.cross(r, v))
s_ = np.cross(w_, r_)
accel_v = f * (
np.cos(alpha_) * s_ +
np.sin(alpha_) * r_
)
return accel_v
def _lambert(k, r1, r2, tof, M, numiter, rtol):
# Check preconditions
assert tof > 0
assert k > 0
# Chord
c = r2 - r1
c_norm, r1_norm, r2_norm = norm(c), norm(r1), norm(r2)
# Semiperimeter
s = (r1_norm + r2_norm + c_norm) * .5
# Versors
i_r1, i_r2 = r1 / r1_norm, r2 / r2_norm
i_h = np.cross(i_r1, i_r2)
i_h = i_h / norm(i_h) # Fixed from paper
# Geometry of the problem
ll = np.sqrt(1 - c_norm / s)
if i_h[2] < 0:
ll = -ll
i_h = -i_h
i_t1, i_t2 = np.cross(i_h, i_r1), np.cross(i_h, i_r2) # Fixed from paper
# Non dimensional time of flight
T = np.sqrt(2 * k / s ** 3) * tof
# Find solutions
xy = _find_xy(ll, T, M, numiter, rtol)
# Reconstruct
gamma = np.sqrt(k * s / 2)
rho = (r1_norm - r2_norm) / c_norm
sigma = np.sqrt(1 - rho ** 2)
for x, y in xy:
V_r1, V_r2, V_t1, V_t2 = _reconstruct(x, y, r1_norm, r2_norm, ll,
gamma, rho, sigma)
v1 = V_r1 * i_r1 + V_t1 * i_t1
v2 = V_r2 * i_r2 + V_t2 * i_t2
yield v1, v2
def a_d(t0, ss):
r = ss.r.value
v = ss.v.value
nu = ss.nu.value
beta_ = beta_0_ * np.sign(np.cos(nu)) # The sign of ß reverses at minor axis crossings
w_ = np.cross(r, v) / norm(np.cross(r, v))
accel_v = f * (
np.cos(beta_) * thrust_unit +
np.sin(beta_) * w_
)
return accel_v
def rv2coe(k, r, v, tol=1e-8):
"""Converts from vectors to classical orbital elements.
Parameters
----------
k : float
Standard gravitational parameter (km^3 / s^2).
r : array
Position vector (km).
v : array
Velocity vector (km / s).
tol : float, optional
Tolerance for eccentricity and inclination checks, default to 1e-8.
"""
h = np.cross(r, v)
n = np.cross([0, 0, 1], h) / norm(h)
e = ((v.dot(v) - k / (norm(r))) * r - r.dot(v) * v) / k
ecc = norm(e)
p = h.dot(h) / k
inc = np.arccos(h[2] / norm(h))
circular = ecc < tol
equatorial = abs(inc) < tol
if equatorial and not circular:
raan = 0
argp = np.arctan2(e[1], e[0]) % (2 * np.pi) # Longitude of periapsis
nu = (np.arctan2(h.dot(np.cross(e, r)) / norm(h), r.dot(e)) %
(2 * np.pi))
elif not equatorial and circular:
raan = np.arctan2(n[1], n[0]) % (2 * np.pi)
argp = 0
# Argument of latitude
nu = (np.arctan2(r.dot(np.cross(h, n)) / norm(h), r.dot(n)) %
(2 * np.pi))
elif equatorial and circular:
raan = 0
argp = 0
nu = np.arctan2(r[1], r[0]) % (2 * np.pi) # True longitude
else:
raan = np.arctan2(n[1], n[0]) % (2 * np.pi)
argp = (np.arctan2(e.dot(np.cross(h, n)) / norm(h), e.dot(n)) %
(2 * np.pi))
nu = (np.arctan2(r.dot(np.cross(h, e)) / norm(h), r.dot(e))
% (2 * np.pi))
return p, ecc, inc, raan, argp, nu
def test_propagation_mean_motion_parabolic():
# example from Howard Curtis (3rd edition), section 3.5, problem 3.15
p = 2.0 * 6600 * u.km
_a = 0.0 * u.deg
orbit = Orbit.parabolic(Earth, p, _a, _a, _a, _a)
orbit = orbit.propagate(0.8897 / 2.0 * u.h, method=mean_motion)
_, _, _, _, _, nu0 = rv2coe(Earth.k.to(u.km**3 / u.s**2).value,
orbit.r.to(u.km).value,
orbit.v.to(u.km / u.s).value)
assert_quantity_allclose(nu0, np.deg2rad(90.0), rtol=1e-4)
orbit = Orbit.parabolic(Earth, p, _a, _a, _a, _a)
orbit = orbit.propagate(36.0 * u.h, method=mean_motion)
assert_quantity_allclose(norm(orbit.r), 304700.0 * u.km, rtol=1e-4)
def set_frame(self, p_vec, q_vec, w_vec):
"""Sets perifocal frame.
Raises
------
ValueError
If the vectors are not a set of mutually orthogonal unit vectors.
"""
if not np.allclose([norm(v) for v in (p_vec, q_vec, w_vec)], 1):
raise ValueError("Vectors must be unit.")
elif not np.allclose([p_vec.dot(q_vec), q_vec.dot(w_vec), w_vec.dot(p_vec)], 0):
raise ValueError("Vectors must be mutually orthogonal.")
else:
self._frame = p_vec, q_vec, w_vec
if self._trajectories:
self._redraw()
def set_frame(self, p_vec, q_vec, w_vec):
"""Sets perifocal frame.
Raises
------
ValueError
If the vectors are not a set of mutually orthogonal unit vectors.
"""
if self._frame and self.trajectories:
raise NotImplementedError(
"OrbitPlotter2D does not support reprojecting yet"
)
if not np.allclose([norm(v) for v in (p_vec, q_vec, w_vec)], 1):
raise ValueError("Vectors must be unit.")
elif not np.allclose([p_vec.dot(q_vec), q_vec.dot(w_vec), w_vec.dot(p_vec)], 0):
raise ValueError("Vectors must be mutually orthogonal.")
else:
self._frame = p_vec, q_vec, w_vec
def set_frame(self, p_vec, q_vec, w_vec):
"""Sets perifocal frame if not existing.
Raises
------
ValueError
If the vectors are not a set of mutually orthogonal unit vectors.
NotImplementedError
If the frame was already set up.
"""
if not self._frame:
if not np.allclose([norm(v) for v in (p_vec, q_vec, w_vec)], 1):
raise ValueError("Vectors must be unit.")
if not np.allclose([p_vec.dot(q_vec),
q_vec.dot(w_vec),
w_vec.dot(p_vec)], 0):
raise ValueError("Vectors must be mutually orthogonal.")
else:
self._frame = p_vec, q_vec, w_vec
else:
raise NotImplementedError
def plot(self, orbit, *, label=None, color=None):
"""Plots state and osculating orbit in their plane.
Parameters
----------
orbit : ~poliastro.twobody.orbit.Orbit
Orbit to plot.
label : string, optional
Label of the orbit.
color : string, optional
Color of the line and the position.
"""
if color is None:
color = next(self._color_cycle)
self._set_attractor(orbit.attractor)
label = generate_label(orbit, label)
trajectory = orbit.sample()
trace = self._plot_trajectory(trajectory, label, color, True)
self._trajectories.append(
Trajectory(trajectory, orbit.r, label, trace.line.color)
)
# Redraw the attractor now to compute the attractor radius
self._redraw_attractor()
# Plot required 2D/3D shape in the position of the body
radius = min(
self._attractor_radius * 0.5, (norm(orbit.r) - orbit.attractor.R) * 0.5
) # Arbitrary thresholds
self._plot_point(radius, color, label, center=orbit.r)
if not self._figure._in_batch_mode:
return self.show()
def plot(self, orbit, *, label=None, color=None):
"""Plots state and osculating orbit in their plane.
Parameters
----------
orbit : ~poliastro.twobody.orbit.Orbit
label : string, optional
color : string, optional
"""
if color is None:
color = next(self._color_cycle)
self.set_attractor(orbit.attractor)
self._redraw_attractor(orbit.r_p * 0.15) # Arbitrary threshold
label = _generate_label(orbit, label)
trajectory = orbit.sample()
self._plot_trajectory(trajectory, label, color, True)
# Plot required 2D/3D shape in the position of the body
radius = min(self._attractor_radius * 0.5, (norm(orbit.r) - orbit.attractor.R) * 0.3) # Arbitrary thresholds
shape = self._plot_sphere(radius, color, label, center=orbit.r)
self._data.append(shape)
def e_vec(self):
"""Eccentricity vector. """
r, v = self.rv()
k = self.attractor.k
e_vec = ((v.dot(v) - k / (norm(r))) * r - r.dot(v) * v) / k
return e_vec.decompose()
def e_vec(self):
"""Eccentricity vector. """
r, v = self.rv()
k = self.attractor.k
e_vec = ((v.dot(v) - k / (norm(r))) * r - r.dot(v) * v) / k
return e_vec
def hohmann(cls, orbit_i, r_f):
r"""Compute a Hohmann transfer between two circular orbits.
By defining the relationship between orbit radius:
.. math::
a_{trans} = \frac{r_{i} + r_{f}}{2}
The Hohmann maneuver velocities can be expressed as:
.. math::
\begin{align}
\Delta v_{a} &= \sqrt{\frac{2\mu}{r_{i}} - \frac{\mu}{a_{trans}}} - v_{i}\\
\Delta v_{b} &= \sqrt{\frac{\mu}{r_{f}}} - \sqrt{\frac{2\mu}{r_{f}} - \frac{\mu}{a_{trans}}}
\end{align}
The time that takes to complete the maneuver can be computed as:
.. math::
\tau_{trans} = \pi \sqrt{\frac{(a_{trans})^{3}}{\mu}}
Parameters
----------
orbit_i: poliastro.twobody.orbit.Orbit
Initial orbit
r_f: astropy.unit.Quantity
Final altitude of the orbit
"""
if orbit_i.nu is not 0 * u.deg:
orbit_i = orbit_i.propagate_to_anomaly(0 * u.deg)
# Initial orbit data
k = orbit_i.attractor.k
r_i = orbit_i.r
v_i = orbit_i.v
h_i = norm(cross(r_i.to(u.m).value, v_i.to(u.m / u.s).value) * u.m ** 2 / u.s)
p_i = h_i ** 2 / k.to(u.m ** 3 / u.s ** 2)
# Hohmann is defined always from the PQW frame, since it is the
# natural plane of the orbit
r_i, v_i = rv_pqw(
k.to(u.m ** 3 / u.s ** 2).value,
p_i.to(u.m).value,
orbit_i.ecc.value,
orbit_i.nu.to(u.rad).value,
)
# Now, we apply Hohmman maneuver
r_i = norm(r_i * u.m)
v_i = norm(v_i * u.m / u.s)
a_trans = (r_i + r_f) / 2
# This is the modulus of the velocities
dv_a = np.sqrt(2 * k / r_i - k / a_trans) - v_i
dv_b = np.sqrt(k / r_f) - np.sqrt(2 * k / r_f - k / a_trans)
# Write them in PQW frame
dv_a = np.array([0, dv_a.to(u.m / u.s).value, 0])
dv_b = np.array([0, -dv_b.to(u.m / u.s).value, 0])
# Transform to IJK frame
rot_matrix = coe_rotation_matrix(
orbit_i.inc.to(u.rad).value,
orbit_i.raan.to(u.rad).value,
orbit_i.argp.to(u.rad).value,
)
dv_a = (rot_matrix @ dv_a) * u.m / u.s
dv_b = (rot_matrix @ dv_b) * u.m / u.s
t_trans = np.pi * np.sqrt(a_trans ** 3 / k)
return cls((0 * u.s, dv_a), (t_trans, dv_b))
def get_total_cost(self):
"""Returns total cost of the maneuver.
"""
dvs = [norm(dv) for dv in self._dvs]
return sum(dvs, 0 * u.km / u.s)
def bielliptic(cls, orbit_i, r_b, r_f):
r"""Compute a bielliptic transfer between two circular orbits.
The bielliptic maneuver employs two Hohmann transfers, therefore two
intermediate orbits are established. We define the different radius
relationships as follows:
.. math::
\begin{align}
a_{trans1} &= \frac{r_{i} + r_{b}}{2}\\
a_{trans2} &= \frac{r_{b} + r_{f}}{2}\\
\end{align}
The increments in the velocity are:
.. math::
\begin{align}
\Delta v_{a} &= \sqrt{\frac{2\mu}{r_{i}} - \frac{\mu}{a_{trans1}}} - v_{i}\\
\Delta v_{b} &= \sqrt{\frac{2\mu}{r_{b}} - \frac{\mu}{a_{trans2}}} - \sqrt{\frac{2\mu}{r_{b}} - \frac{\mu}{a_trans{1}}}\\
\Delta v_{c} &= \sqrt{\frac{\mu}{r_{f}}} - \sqrt{\frac{2\mu}{r_{f}} - \frac{\mu}{a_{trans2}}}\\
\end{align}
The time of flight for this maneuver is the addition of the time needed for both transition orbits, following the same formula as
Hohmann:
.. math::
\begin{align}
\tau_{trans1} &= \pi \sqrt{\frac{a_{trans1}^{3}}{\mu}}\\
\tau_{trans2} &= \pi \sqrt{\frac{a_{trans2}^{3}}{\mu}}\\
\end{align}
Parameters
----------
orbit_i: poliastro.twobody.orbit.Orbit
Initial orbit
r_b: astropy.unit.Quantity
Altitude of the intermediate orbit
r_f: astropy.unit.Quantity
Final altitude of the orbit
"""
if orbit_i.nu is not 0 * u.deg:
orbit_i = orbit_i.propagate_to_anomaly(0 * u.deg)
# Initial orbit data
k = orbit_i.attractor.k
r_i = orbit_i.r
v_i = orbit_i.v
h_i = norm(cross(r_i.to(u.m).value, v_i.to(u.m / u.s).value) * u.m ** 2 / u.s)
p_i = h_i ** 2 / k.to(u.m ** 3 / u.s ** 2)
# Bielliptic is defined always from the PQW frame, since it is the
# natural plane of the orbit
r_i, v_i = rv_pqw(
k.to(u.m ** 3 / u.s ** 2).value,
p_i.to(u.m).value,
orbit_i.ecc.value,
orbit_i.nu.to(u.rad).value,
)
# Define the transfer radius
r_i = norm(r_i * u.m)
v_i = norm(v_i * u.m / u.s)
a_trans1 = (r_i + r_b) / 2
a_trans2 = (r_b + r_f) / 2
# Compute impulses
dv_a = np.sqrt(2 * k / r_i - k / a_trans1) - v_i
dv_b = np.sqrt(2 * k / r_b - k / a_trans2) - np.sqrt(2 * k / r_b - k / a_trans1)
dv_c = np.sqrt(k / r_f) - np.sqrt(2 * k / r_f - k / a_trans2)
# Write impulses in PQW frame
dv_a = np.array([0, dv_a.to(u.m / u.s).value, 0])
dv_b = np.array([0, -dv_b.to(u.m / u.s).value, 0])
dv_c = np.array([0, dv_c.to(u.m / u.s).value, 0])
rot_matrix = coe_rotation_matrix(
orbit_i.inc.to(u.rad).value,
orbit_i.raan.to(u.rad).value,
orbit_i.argp.to(u.rad).value,
)
# Transform to IJK frame
dv_a = (rot_matrix @ dv_a) * u.m / u.s
dv_b = (rot_matrix @ dv_b) * u.m / u.s
dv_c = (rot_matrix @ dv_c) * u.m / u.s
# Compute time for maneuver
t_trans1 = np.pi * np.sqrt(a_trans1 ** 3 / k)
t_trans2 = np.pi * np.sqrt(a_trans2 ** 3 / k)
return cls((0 * u.s, dv_a), (t_trans1, dv_b), (t_trans2, dv_c))
def lagrange_points_vec(m1, r1, m2, r2, n):
"""Computes the five Lagrange points in the CR3BP.
Returns the positions in the same frame of reference as `r1` and `r2`
for the five Lagrangian points.
Parameters
----------
m1 : ~astropy.units.Quantity
Mass of the main body. This body is the one with the biggest mass.
r1 : ~astropy.units.Quantity
Position of the main body.
m2 : ~astropy.units.Quantity
Mass of the secondary body.
r2 : ~astropy.units.Quantity
Position of the secondary body.
n : ~astropy.units.Quantity
Normal vector to the orbital plane.
Returns
-------
list:
Position of the Lagrange points: [L1, L2, L3, L4, L5]
The positions are of type ~astropy.units.Quantity
"""
# Check Body 1 is the main body
assert (
m1 > m2
), "Body 1 is not the main body: it has less mass than the 'secondary' body"
# Define local frame of reference:
# Center: main body, NOT the barycenter
# x-axis: points to the secondary body
ux = r2 - r1
r12 = norm(ux)
ux = ux / r12
# y-axis: contained in the orbital plane, perpendicular to x-axis
def cross(x, y):
return np.cross(x, y) * x.unit * y.unit
uy = cross(n, ux)
uy = uy / norm(uy)
# position in x-axis
x1, x2, x3, x4, x5 = lagrange_points(r12, m1, m2)
# position in y-axis
# L1, L2, L3 are located in the x-axis, so y123 = 0
# L4 and L5 are points in the plane of rotation which form an equilateral
# triangle with the two masses (Battin)
# sqrt(3)/2 = sin(60 deg)
y4 = np.sqrt(3) / 2 * r12
y5 = -y4
# Convert L points coordinates (x,y) to original vectorial base [r1 r2]
L1 = r1 + ux * x1
L2 = r1 + ux * x2
L3 = r1 + ux * x3
L4 = r1 + ux * x4 + uy * y4
L5 = r1 + ux * x5 + uy * y5
return [L1, L2, L3, L4, L5]
def _plot_position(self, position, label, colors):
radius = min(
self._attractor_radius * 0.5, (norm(position) - self._attractor.R) * 0.5
) # Arbitrary thresholds
self._draw_point(radius, colors[0], label, center=position)
def compute_flyby(v_spacecraft, v_body, k, r_p, theta=0 * u.deg):
"""Computes outbound velocity after a flyby.
Parameters
----------
v_spacecraft : ~astropy.units.Quantity
Velocity of the spacecraft, relative to the attractor of the body.
v_body : ~astropy.units.Quantity
Velocity of the body, relative to its attractor.
k : ~astropy.units.Quantity
Standard gravitational parameter of the body.
r_p : ~astropy.units.Quantity
Radius of periapsis, measured from the center of the body.
theta : ~astropy.units.Quantity, optional
Aim angle of the B vector, default to 0.
Returns
-------
v_spacecraft_out : ~astropy.units.Quantity
Outbound velocity of the spacecraft.
delta : ~astropy.units.Quantity
Turn angle.
"""
v_inf_1 = v_spacecraft - v_body # Hyperbolic excess velocity
v_inf = norm(v_inf_1)
ecc = 1 + r_p * v_inf ** 2 / k # Eccentricity of the entry hyperbola
delta = 2 * np.arcsin(1 / ecc) # Turn angle
b = k / v_inf ** 2 * np.sqrt(ecc ** 2 - 1) # Magnitude of the B vector
# Now we compute the unit vectors in which to return the outbound hyperbolic excess velocity:
# * S goes along the hyperbolic excess velocity and is perpendicular to the B-Plane,
# * T goes along the B-Plane and is parallel to _some_ fundamental plane - in this case, the plane in which
# the velocities are computed
# * R completes the orthonormal set
S_vec = v_inf_1 / v_inf
c_vec = np.array([0, 0, 1]) * u.one
T_vec = np.cross(S_vec, c_vec) * u.one
T_vec = T_vec / norm(T_vec)
R_vec = np.cross(S_vec, T_vec) * u.one
# This vector defines the B-Plane
B_vec = b * (np.cos(theta) * T_vec + np.sin(theta) * R_vec)
# We have to rotate the inbound hyperbolic excess velocity
# an angle delta (turn angle) around a vector that is orthogonal to
# the B-Plane and trajectory plane
rot_v = np.cross(B_vec / b, S_vec) * u.one
# And now we rotate the outbound hyperbolic excess velocity
# u_vec = v_inf_1 / norm(v_inf) = S_vec
v_vec = np.cross(rot_v, v_inf_1) * u.one
v_vec = v_vec / norm(v_vec)
v_inf_2 = v_inf * (np.cos(delta) * S_vec + np.sin(delta) * v_vec)
v_spacecraft_out = v_inf_2 + v_body
return v_spacecraft_out, delta
