keplerian trajectories
TRANSCRIPT
1
KEPLERIAN TRAJECTORIES
SOLO HERMELIN
2
SOLO
Table of Contents
2. Newton’s Laws
3. Newton’s Law of Universal Gravitation4. Two Body Problem
4.1 Polar Representation4.2 Conservation of Angular Momentum
KEPLERIAN TRAJECTORIES
1. Kepler’s Laws
4.3 Conservation of Mechanical Energy
4.4 Integration of Equations of Motion
4.5 Velocity on the Trajectory
4.6 Specific Mechanic Energy
4.7 Eccentricity Vector
4.8 Orbit Determination from Initial Conditions
4.9 Flat Earth Approximation4.10 Orbital Trajectories
4.11 Time of Flight on an Elliptic Orbit
5. References
3
SOLO
Kepler’s Laws
KEPLERIAN TRAJECTORIES
Johannes Kepler1571 - 1630
Tycho Brache1546 - 1601
From 1601 to 1606 Kepler tried to fit various geometrical curves to Tycho Brache’sdata on Mars orbit.
4
SOLO
Kepler’s Laws
KEPLERIAN TRAJECTORIES
1. The orbit of each planet is an ellipse, with the sun at a focus.
2. The line joining the planet to the sun sweeps out equal area in equal times.
3. The square of the period of a planet orbit is equal to the cube of its mean distance to the sun.
b
a
a dAh
dt2
b
a 2/322a
GMGM
abTP
b
5
SOLO
Newton’s Laws of Motion
KEPLERIAN TRAJECTORIES
1. Every body continues in its state of rest or of uniform motion instraight line unless it is compelled to change that state by forcesimpressed upon it.
2. The rate of change of momentum of a given body is proportional to the force impressed on the body and is in the same direction as the force.
3. To every action there is always an equal reaction
“THE MATHEMATICAL PRINCIPLES OF NATURAL PHILOSOPHY”1687
Isaac Newton1643-1727
6
SOLO
Newton’s Law of Universal Gravitation
KEPLERIAN TRAJECTORIES
“THE MATHEMATICAL PRINCIPLES OF NATURAL PHILOSOPHY”1687
Isaac Newton1643-1727
Any two body attract one on other with a force proportional to theproduct of the masses and inversely proportional to the square of the distance between them.
EQPOISSON
G
Gr
GM
r
GMg
gmr
GMmr
r
mMGF
4&&
1
2
2
228 /1067.6 gmcmdyneG The Universal Gravitational Constant
Instantaneous propagation of the force along the direction between the masses (“Action at a Distance”)
GF
GF
M m
7
SOLO
Two Body Problem
KEPLERIAN TRAJECTORIES
Assumptions
1. The bodies are spherical symmetric.
2. The gravitational forces are the only forces acting on the bodies.
Mr
mr
r
M
m
Cr
Let
Mr
-the position vector of the mass M relative to an inertial point.
mr
-the position vector of the mass m relative to an inertial point.
Mm rrr
-the relative position vector of the mass m relative to M.
r
rr
:1 -unit vector in direction. r
Let apply the Newton’s Law of Universal Gravitation to describe the forces each mass applies on the other.
rr
mGr
r
r
r
MmGrM
rr
MGr
r
r
r
MmGrm
MM
mm
1
1
22
22
8
SOLO
Two Body Problem (continue – 1)
KEPLERIAN TRAJECTORIES
rr
mGr
r
r
r
MmGrM
rr
MGr
r
r
r
MmGrm
MM
mm
1
1
22
22
Mm
rMrmr Mm
C
The Center of Mass of those masses is
MmGrr
rr
MmGrrr Mm
:1122
From those equations we obtain:
and
0
Mm
rMrmr Mm
C The Center of Mass is not accelerating
9
SOLO
Two Body Problem (continue – 2)
KEPLERIAN TRAJECTORIES
Polar Representation
Define
r
- Angular Velocity Vector
nrR 11
Decompose in and perpendicular to direction. r
r
r
M
m
r1
n1
R
t1
Since we can define a right handCartesian system using vector product:
011
rn
rnt 11:1
tnnrnnntd
d
nrtnrttttd
d
trnrrrrtd
d
RR
RR
R
1111111
11111111
1111111
Differentiating we obtain:
10
SOLO
Two Body Problem (continue – 3)
KEPLERIAN TRAJECTORIES
Polar Representation (continue – 1)
or
n
t
r
n
t
r
td
d
R
R
1
1
1
00
0
00
1
1
1
rrr 1
Therefore
trrrrrrrr 1111
nrtrrrrr
nrrtrrtrrrr
R
R
1121
11111
2
rrr 1
vtrrrr
11
rr
nrtrrrrrr R 111212
2
r
M
m
r1
n1
R
t1
11
SOLO
Two Body Problem (continue – 4)
KEPLERIAN TRAJECTORIES
Conservation of Angular Momentum
Vector Solution
Cross-multiply the equation of motion by
rr
r 12
r
03
rrr
rr
Define the specific angular momentum as: h
vrrrh
:
Differentiate h
0
rrrr
td
hd
Therefore is a constant vector in space that is perpendicular to thetrajectory vectors and , therefore those vectors (and the trajectory) remainin the plane defined by the initial conditions
vrh
r
v
000 tvtrth
12
SOLO
Two Body Problem (continue – 5)
KEPLERIAN TRAJECTORIES
Conservation of Angular Momentum (continue – 1)
Polar Coordinate Solution
nrtrrrrrvrh 1111 2
The equation of motion in polar coordinates is
rr
nrtrrrrrr R 111212
2
from which
02 rr andr
rdd2
Integrated both sides
ConstrhConstrConstr 222 lnlnlnlnln
1
000 rr RR 2
constntn
td
dR 1011
also
constnrvrh 12 therefore
13
SOLO
Two Body Problem (continue – 6)
KEPLERIAN TRAJECTORIES
Conservation of Mechanical Energy
Vector Solution
Dot-multiply the equation of motion by
rr
r 12
r
rrr
rr
3
Use vrvr &
03
rrr
vv
Use also the identity aaaaaaaaaaaa
011
111
022
022
3
r
v
td
d
rtd
dv
td
drr
rvv
const
r
vE
energypotentialspecific
energykineticspecific
2:
2
We can see that
E is the specific mechanical energy.
14
SOLO
Two Body Problem (continue – 7)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion
Vector Solution
Cross-multiply the equation of motion with the specific angular momentum
rr
r 12
h
hrr
hr
3
rrtd
drhrh
td
drhrhr
td
d
The left side is
hrrr
rrhrrrrrrrhr
0
3
0
The right side can be written
r
r
td
dr
r
rv
rrrrrv
rvrrrrv
rvrr
rhr
r
2
2
3333
Equaling both sides gives
r
r
td
dhr
td
d
Integrating both sides Br
rhr
where is the constant of integration B
15
SOLO
Two Body Problem (continue – 8)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion (continue – 1)
Vector Solution (continue – 1)
Dot-multiply this equation by :
Br
rhr
r Br
r
rrhrr
Use 2hhhhrrhrr
rr
rr
pBrBr cos
to obtain pBrrh cos2 or
pp e
p
B
hr
cos1cos/1
/2
where/: 2hp
/: Be
p
parameter (semi-latus rectum)eccentricityperiapsis angle
16
SOLO
Two Body Problem (continue – 9)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion (continue – 2)
Polar Coordinate Solution
The equations of motion in polar coordinates are:
22
rrr
Consthr 2
Let define a new variable: r
u1
:
from which urrrr
u 22
1
If , we can write 00 h
hr 2 and 0
h
d
udhu
hr
Differentiating again we obtain
02
222
22
2
2
2
hd
uduh
r
h
d
udh
d
udhr
We also have 32
4
22 uh
r
hrr
The first equation of motion becomes 02322
222
huuh
d
uduh
17
SOLO
Two Body Problem (continue – 10)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion (continue – 3)
Polar Coordinate Solution (continue – 1)
Therefore
32
4
22 uh
r
hrr
The solution of this equation is
02322
222
huuh
d
uduh
02
32
2
hh
ud
ud
221 cos
1
hCC
ru
where C1 and C2 are constant of integration
pCp
eC
hp 212
,:,:Define
to obtain pe
pr
cos1 This is the equation of a conic section in polar coordinates.
18
ConeApex C
ConeAxis
Generators
BaseCircle
SOLO
A right circular cone is a cone obtained by generators (straight lines) passing througha circle, and the apex C that is situated on the normal to the circle plane and passingtrough the center of the circle. β is the angle between the cone axis and the generators.
CONIC SECTIONS
CuttingPlane
generating a"hyperbola"
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
CuttingPlane
generating a"parabola"
CuttingPlane
generating a"ellipse"
CuttingPlane
generating a"circle"
CuttingPlane
generatingtwo
"lines"
2
2
2
0
lines
line
po
22
12
int2
P
F
F*
CuttingPlane
(Hyperbola)
RightCircular
Cone
Hyperbola2
Branches
C
Ellipse
Parabola
CuttingPlane
(Ellipse)
CuttingPlane
(Circle)
CuttingPlane
(Parabola)
By cutting the right circular conic by a plane we obtain different conic sections, as afunction of the inclination angle α of the plane relative to the base of the conic sectionand the angle β between the generators and the base.
The discovery of theConical Sections isattributed to the greekMenachmus who livedaround 350 B.C..
19
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
CuttingPlane
generating a"circle"
0
RightCircular
Cone
C
CuttingPlane
(Circle)
2
SOLO
The conical sections are:
CONIC SECTIONS
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
CuttingPlane
generating a"ellipse"
2
C
Ellipse
CuttingPlane
(Ellipse)
1. Circle if the cutting plane is normal to the cone axis (α=0) and is above or bellow the apex.
2. Ellipse if the cutting plane is inclined to the basis at an angle that falls short of the angle between generators to the base (α<π/2-β) (in greek word elleipsis means falls, short or leaves out.
3. Hyperbola if the cutting plane is inclined to the basis at an angle that exceeds of the angle between generators to the base (α>π/2-β)(in greek word hyperbole means excess.
4. Parabola if the cutting plane is parallel to a generator of the right circular cone (α=π/2-β) (in greek word parabole is the origin of the words parabola and parallel.
5. A point- apex (α<π/2-β), one straight line (α=π/2-β), two straight lines (α>π/2-β), if the cutting plane passes through the apex and intersects the cone basis.
CuttingPlane
generating a"hyperbola"
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
2
P
F
F*
CuttingPlane
(Hyperbola)
RightCircular
Cone
Hyperbola2
Branches
C
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
CuttingPlane
generating a"parabola"
2
C
Parabola
CuttingPlane
(Parabola)
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
CuttingPlane
generatingtwo
"lines"
lines
line
po
22
12
int2
C
CuttingPlane
generatingtwo
"lines"
CuttingPlane
generating a"hyperbola"
RightCircular
Cone
ConeApex
ConicalSection
C
ConeAxis
CuttingPlane
generating a"parabola"
CuttingPlane
generating a"ellipse"
CuttingPlane
generating a"circle"
CuttingPlane
generatingtwo
"lines"
2
2
2
0
lines
line
po
22
12
int2
P
F
F*
CuttingPlane
(Hyperbola)
RightCircular
Cone
Hyperbola2
Branches
C
Ellipse
Parabola
CuttingPlane
(Ellipse)
CuttingPlane
(Circle)
CuttingPlane
(Parabola)
20
SOLO
Two Body Problem (continue – 13)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion (continue – 6) pe
pr
cos1
Conic Section (continue – 2)
pp
r d
directrixfocus conicsection
x
y
P1
Q1
r1
t1
periapsis
Let definee
pd :
and rewrite pe
edr
cos1 as follows prd
re
cos
From which we obtain the following definition of conic sections:
Conic Section A conic section is a planar curve, such that the ratio of distances, for any point on the curve, to a fixed point F* (focus) and to aline directrix is constant and equal to e.
21
SOLO
Two Body Problem (continue – 14)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion (continue – 7)
Conic Section (continue – 3)
Let write the conic section equation in cartesian coordinates using
QP 1,1
QPt
QPr
pp
pp
1cos1sin1
1sin1cos1
and p
p
ry
rx
sin
cos
pe
pr
cos1Substitute those in the equation rewritten as
prer p cos
xepyx 22to obtain
Squaring both sides gives 22222 2 xexpepyx
or 2222 21 pyxpexe
22
SOLO
Two Body Problem (continue – 15)
KEPLERIAN TRAJECTORIES
Integration of Equations of Motion (continue – 8)
Conic Section (continue – 4)
For e ≠ 1 can we can write
2222 21 pyxpexe
2
2
2
2222
2
22
1111
e
p
e
eppy
e
epxe
We see that for p ≠ 0 ( h ≠ 0 )
1 0e222 pyx circle
2 10 e 1
11
1
2
2
2
2
2
e
py
e
pe
epx
ellipse
3 1epe
ypx
2
22 parabola
4 1e hyperbola1
11
1
2
2
2
2
2
e
py
e
pe
epx
F*
directrix2
directrix1
F
P
x
yx=cx=-c
e
ax
e
ax
Hyperbola
a
Circle
y
x
FF*
P M
x
yr
0
a
b
directrix2directrix
1
e
ax
e
ax
x=-c x=c
Ellipse
r
Parabola
px
y
directrix
M
F
23
SOLO
Two Body Problem (continue – 16)
KEPLERIAN TRAJECTORIES
Velocity on the Trajectory
Let find the velocity on the trajectory in and coordinates
tr 1,1
QP 1,1
QyPxtrrrrv 1111
QrrPrr pppp 1cossin1sincos
From the equation of motion
pe
pr
cos1 hr 2 /: 2hp
we obtain
p
p
p
pr
ep
p
eh
r
h
e
ep
td
d
d
drrv
sin
sin
cos1
sin22
pt epr
p
r
hrrv cos1
2
24
SOLO
Two Body Problem (continue – 17)
KEPLERIAN TRAJECTORIES
Velocity on the Trajectory (continue – 1)
Let substitute those results in the velocity equation to obtain the componentsin coordinates.
QP 1,1
Qep
Pp
QyPxv pp 1cos1sin11
2/12222/1222/122 cos1sin pptr eep
yxvvv
2/12 cos21 peep
v
pp
r d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
v
rv tv
periapsis
The velocities at the periapsisand apoapsis are
p
p
ep
va 1 e
pv p 1
25
SOLO
Two Body Problem (continue – 18)
KEPLERIAN TRAJECTORIES
Specific Mechanic Energy
The specific mechanic energy is
ap
e
p
e
p
e
r
v
r
vE
a
a
22
11
2
1
22:
2222
or
ar
vE
22:
2
where we used 21 eap
From the last equation we get a
pe 1
Substitute and to obtain/: 2hp Ea 2/
2
221
hE
e
26
SOLO
Two Body Problem (continue – 19)
KEPLERIAN TRAJECTORIES
Eccentricity Vector
We defined the eccentricity as a scalar /: Be
Let define the eccentricity vector as /: Be
where is defined as B
Br
rhr
Therefore
Pevvr
rvr
r
rvvrvr
r
rvrv
r
rhve 1
22
The last equation is obtained from the definition of , and , and canbe checked by substituting the range and velocity vectors in coordinates,in previous equation.
B
e p
QP 1,1
27
SOLO
Two Body Problem (continue – 20)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions
Given the initial position and velocity vectors , find the orbital parameters 00 ,vr
pr
d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
vrv
tv p
1 From the specific angular momentumof the orbit we can find 00 vrh
01 00
hh
vrR
2hp
2 From the specific mechanical energy on an elliptic orbit
ar
vvE
22 0
00
we obtain
000
2 vvr
a
28
SOLO
Two Body Problem (continue – 21)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (continue – 1)
pr
d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
vrv
tv p
3 The eccentrity vector of a Keplerian trajectoryis given by
from which
Pevvrr
rvve 1
10000
000
ee
01
ee
eP
PRQ 111
pe
pr
cos1
The position and velocity vectors are given by
QPrr pp 1sin1cos
QePp
QyPxv pp 1cos1sin11
29
SOLO
Two Body Problem (continue – 22)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (continue – 2)
pr
d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
vrv
tv p
4 Initial trajectory angle , on the conicsection, from the periapsis is given by
p 01
1
111
0
01
0
01
cos
sintan
11sin
1cos
Rr
rP
r
rP
One other way to find is by using
RQP 1,1,1
QePvp
QPr
1cos1sin
1sin1cos1
110
110
or
01011
01011
cos1sin1cos1
sin1cos1cos1
vp
rQe
vp
rePe
1
1
sin
cos
e
1
1
cos
sin
30
SOLO
Two Body Problem (continue – 23)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (continue – 3)
pr
d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
vrv
tv p
Using 1
0 cos1
e
pr
we finally obtain
010
01
010
01
cossin
1
sincos
1
vp
rr
pQ
vp
rr
p
eP
010
01
010
01
0110
011
sincos11
sincos1cos1
sincoscossinsinsincoscos
1sin1cos
vp
rrr
p
r
vp
rrre
p
r
vp
rrre
p
r
QPrr
Therefore
31
SOLO
Two Body Problem (continue – 24)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (continue – 4)
pr
d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
vrv
tv p
where we used
0110
011
0110
011
coscossinsinsin
coscossinsinsincoscossin
1cos1sin
vep
rr
p
ee
p
vep
rr
p
ee
p
QePp
v
010
01
0
1
0
00
010
01111
0110
0111111
cos11sin1
cos1
cos11cos1sincos1sin
cos1cos1
sinsincoscossinsin
vp
rr
pre
pr
vr
vp
rr
p
eee
p
vep
r
rp
ee
p
10
1111000
sin
1cos1sin1sin1cos
ep
r
QePp
QPrvr
32
SOLO
Two Body Problem (continue – 25)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (continue – 5)
pr
d
directrixfocus conic
section
x
y
P1
Q1
r1
t1
vrv
tv p
Summarize
010
01 sincos11 vp
rrr
p
rr
010
010
10
00 cos11sin1
cos1 vp
rr
pre
pr
vrv
33
SOLO
Two Body Problem (continue – 26)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (Second Method)
Start from the solution for Keplerian orbit 221 cos
1
hCC
r
where C1 and C2 must be determined from initial conditions
0r - initial range from the center of mass M
0v - initial velocity magnitude
000002 cosvrvrrvrh
0 - initial trajectory angle (between velocity vector and the horizon plane)
Force Center
FiringPoint
0r
0v
pr
0
p
r
0
Trajectory
Periapsis
34
SOLO
Two Body Problem (continue – 27)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (Second Method - 1)
We have 0
20
21
022
02
0
21 cos
1cos
coscos
1
rCC
vrCC
r
where0
20
/:
r
v
Let differentiate 21212
212sinsinsin
1CChCCrrCC
r
r
rdt
d
or 21000 sincos CCvrr
Let use now the initial conditions to find C1, C2.
0
20
2010 cos
1cos
1
rCC
r 002010000 sinsincos vCCvrr
or
02
00201 cos
11cos
rrCC
00
201 tan1
sin r
CC
35
SOLO
Two Body Problem (continue – 28)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (Second Method - 2)
We have
0
200
201 cos
11cos
rrCC
00
201 tan1
sin r
CC
0
20
000
00
200
02
002010201
02
02001
cos
1sintan
1cos
cos
11
cos
1sinsincoscos
cos
1cos
1
rrrr
rCCCC
rCC
r
0
00
02
0
0
0000
02
00
cos
cos
cos
cos1
cos
sinsincoscos
cos
cos1
r
r
which, when developed further, gives
36
SOLO
Two Body Problem (continue – 29)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (Second Method - 2)
0
00
0
2
00
cos
cos
cos
cos1
r
r
Finally
0000
02
0
coscoscos1
cos
rr
37
SOLO
Two Body Problem (continue – 30)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (Second Method - 3)
0000
02
0
coscoscos1
cos
rr
20
20
20
22
002
0002
02
02
022
002
000
0000
1coscossin
cos1cossincossin1coscossin1
cos1cossincossin1
coscoscos1
The denominator can be expressed as
02
0 cos: rp
1cos21cos2cos1coscossin: 02
02
0222
02
02
022 e
We define
1cos
cossintan:
02
0010
p
pp ee
cos1cos1
coscoscos1
00
0000 to obtain
38
SOLO
Two Body Problem (continue – 31)
KEPLERIAN TRAJECTORIES
Orbit Determination from Initial Conditions (Second Method - 4)
0000
02
0
coscoscos1
cos
rr
The denominator can be expressed as
pp ee cos1cos1coscoscos1 000000
We obtain again the fact that the trajectory is a conic section given by
pe
pr
cos1
We can also write
21cos21
cos
1: 0
020
20
2
rr
e
pa
and
02
0 cos: rp 1cos2: 0
2 e
39
SOLO
Two Body Problem (continue – 32)
KEPLERIAN TRAJECTORIES
Flat Earth Approximation
We assume that , so we can use10
20000 2
11cos&sin
20
0200
0
0002
0022
0
02
0
0000
02
0
cos
11
2
1tan1
cossin2
11cos
2
1cos
coscoscos1
cos
r
r
rr
and
where gr
v
rr
v
r
v
0
20
20
0
20
0
20
The order of magnitude of λ is 64
000,1
10000,400,6122
0
0
v
gr
40
SOLO
Two Body Problem (continue – 33)
KEPLERIAN TRAJECTORIES
Flat Earth approximation (continue – 1)
Therefore if γ is not close to 90° , we can assume that , and then1cos
1
02
0
220
20
200000
022
0
020000
022
0
02000
0
cos2
1tan
cos2
1tan1
cos2
1tan1
v
grrr
v
grr
v
grr
r
Use
to obtain
0000 && rxzRrzRr ee
022
0
200
cos2
1tan
v
gxxzz
41
SOLO
Two Body Problem (continue – 34)
KEPLERIAN TRAJECTORIES
Flat Earth approximation (continue – 2)
We obtained the Flat Earth approximation of the trajectory, that is a parabola.
022
0
200
cos2
1tan
v
gxxzz
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5 Vacuum Ballistic Trajectories
g
V
2
20
g
V
2
20
022
0
200 cos2
V
gxtgxzz
g
V 20
0V
00z
42
SOLO
Two Body Problem (continue – 35)
KEPLERIAN TRAJECTORIES
Orbital Trajectories
Let discuss in more detail the Keplerian trajectories.
Ellipse (e < 1) ( Kepler’s First Law ) and Circle ( e = 0 )
x
y
21 eap
eac
a a
2/121 eab
eara 1 earp 1
rFOCUS
EMPTYFOCUS
Apoapsis RadiusPeriapsis Radius
ApoapsisPeriapsisGeometric Center
DirectrixDirectrix
c
epd /Semimajor
axis
Semiminor axis
v
nv
rv
P1
Q1
1
11
1
2
2
2
2
2
e
py
e
pe
epx
We found that the ellipse equation in cartesian coordinates is given by:
43
SOLO
Two Body Problem (continue – 36)
KEPLERIAN TRAJECTORIES
Orbital Trajectories (continue – 1) Ellipse (e < 1) ( Kepler’s First Law ) and Circle ( e = 0 )
x
y
21 eap
eac
a a
2/121 eab
eara 1 earp 1
rFOCUS
EMPTYFOCUS
Apoapsis RadiusPeriapsis Radius
ApoapsisPeriapsisGeometric Center
DirectrixDirectrix
c
epd /Semimajor
axis
Semiminor axis
v
nv
rv
P1
Q1Define:
21:
e
pa
semimajor axis
21:
e
pb
- semiminor axis
eae
epc
21: - distance of geometrical
center from focus
e
pd : - distance of directrix
from focus
eara 1: - apoapsis radius
earp 1: - periapsis radius
- true anomaly
- flight-path angle
2/pa rra
papa rrrre /
ererabeap ap 11/1 22
From those relations we get:
122
b
y
a
cx
and the ellipse equation will be:
44
SOLO
Two Body Problem (continue – 37)
KEPLERIAN TRAJECTORIES
Time of Flight on an Elliptic Orbit
From the equation 2rh we can write h
Ad
h
drdt 2
2
where is the area defined by the radius vector as it moves through an angle
2
2
drAd
d
pp
r
focus conicsection
xy
P1
Q1
r1
t1
v
rv tv
d
drAd 2
2
1
periapsis
This proves the 2nd Kepler’s Law that equal area are swept out equal in equal timesby the radius vector.
45
SOLO
Two Body Problem (continue – 38)
KEPLERIAN TRAJECTORIES
Time of Flight on an Elliptic Orbit (continue – 1)
The period of the orbit depends only on the major axis of the ellipse a.
p
pa
h
eaa
h
ea
h
baT
eap
ph
2/3122/322
21
21
222
or 2/32 aT
The period of an elliptical orbit T is obtained by integrating from Θ= 0 to Θ=2π , and the radius vector sweeps the area of the ellipse A = π a b.
This proves the Kepler’s third law: “the square of the period of a planet orbit is equalTo the cube of its mean distance to the sun”.
46
SOLO
Two Body Problem (continue – 39)
KEPLERIAN TRAJECTORIES
Time of Flight on an Elliptic Orbit (continue – 2)
Let draw an auxiliary circle of radius a, and the same center O as the geometric centerof the ellipse.
x
y
eac
a a
2/121 eab
r
FOCUS
EMPTYFOCUS
c
P1
Q1
a
F
Q
O VS
E
P
Let take any point P on the ellipse withpolar coordinates r,Θ and define the point Q on the circle at the same coordinate x as P.
Eeary
ra
xea
a
xby
Eaa
xay
ellipse
ellipse
circle
sin1sin
sin111
sin12
2
22
2
2
2
2
The angle E of OQ with x axis is called theeccentric anomaly.
aeEarxellipse coscos
47
SOLO
Two Body Problem (continue – 40)
KEPLERIAN TRAJECTORIES
Time of Flight on an Elliptic Orbit (continue – 3)
Let compute
x
y
eac
a a
2/121 eab
r
FOCUS
EMPTYFOCUS
c
P1
Q1
a
F
Q
O VS
E
P
0cos11
sin1sincos1cos
1
2
22
2
EEEeea
EEeaEaEEeaaeEa
xyyxvreah ellipseellipseellipseellpse
We obtain n
aEEe :cos1
3
pttntEetE sin
Integrating this equation gives
Kepler’s Equation
where tp is the time of periapsis ( E (tp) = 0 )
48
SOLO
Two Body Problem (continue – 41)
KEPLERIAN TRAJECTORIES
Time of Flight on an Elliptic Orbit (continue – 4)
From
x
y
eac
a a
2/121 eab
r
FOCUS
EMPTYFOCUS
c
P1
Q1
a
F
Q
O VS
E
P
Eeary
aeEarx
ellipse
ellipse
sin1sin
coscos
2
we have Eea
EeEeaEeaaeEar
cos1
coscos21sin1cos2/1222/12222
Therefore
cos1
sin1sin
cos1
sin1sin
cos1
coscos
cos1
coscos
22
e
eE
Ee
Ee
e
eE
Ee
eE
Ee
Ee
Ee
eEEe
sin1
cos11
sin1
coscos1
sin
cos1
2tan
22
From
2tan
1
1
2tan
E
e
e
or
and are always in the same quadrant.2
2
E
49
SOLO
References
2. Battin, R.H., “An Introduction to the Mathematics and Methods of Astrodynamics”, AIAA Education Series, AIAA, Washington. DV., 1987
KEPLERIAN TRAJECTORIES
3. Battin, R.H., “Astronautical Guidance”, McGraw Hill, 1964
1. Bate, R.R., Mueller, D.D., White, J.E., “Fundamentals of Astrodynamics”, Dover Publications, New York, 1971
4. Chobotov, V.A., Editor, “Orbital Mechanics”, AIAA Education Series, , AIAA,2nd Edition, 1996
5. Reagan, F.J., “Re-Entry Vehicle Dynamics”, AIAA Education Series, , AIAA,1984
6. Reagan, F.J. and Anandakrishnan, S.M., “Dynamics of Atmospheric Re-Entry”, AIAA Education Series, , AIAA, 1993
7. Montenbruck, O. and Eberhard, G., “Satellite Orbits”, Springer-Verlag, 2000
8. Kaplan, M.H., “Modern Spacecraft Dynamics and Control”, John Wiley & Sons,1976
April 18, 2023 50
SOLO
TechnionIsraeli Institute of Technology
1964 – 1968 BSc EE1968 – 1971 MSc EE
Israeli Air Force1970 – 1974
RAFAELIsraeli Armament Development Authority
1974 – 2013
Stanford University1983 – 1986 PhD AA