#### Abstract

Numerical and first-order analytical results are presented for optimal low-thrust limited-power trajectories in a gravity field that includes the second zonal harmonic in the gravitational potential. Only transfers between orbits with small eccentricities are considered. The optimization problem is formulated as a Mayer problem of optimal control with Cartesian elementsβposition and velocity vectorsβas state variables. After applying the Pontryagin Maximum Principle, successive canonical transformations are performed and a suitable set of orbital elements is introduced. Hori methodβa perturbation technique based on Lie seriesβis applied in solving the canonical system of differential equations that governs the optimal trajectories. First-order analytical solutions are presented for transfers between close orbits, and a numerical solution is obtained for transfers between arbitrary orbits by solving the two-point boundary value problem described by averaged maximum Hamiltonian, expressed in nonsingular elements, through a shooting method. A comparison between analytical and numerical results is presented for some maneuvers.

#### 1. Introduction

This paper presents numerical and analytical results for optimal low-thrust limited-power transfers between orbits with small eccentricities in a noncentral gravity field that includes the second zonal harmonic in the gravitational potential. This study has been motivated by the renewed interest in the use of low-thrust propulsion systems in space missions verified in the last two decades due to the development and the successes of two space mission powered by ionic propulsion: Deep Space One and SMART 1. Several researchers have obtained numerical and sometimes analytical solutions for a number of specific initial orbits and specific thrust profiles in central or noncentral gravity field [1β26].

It is assumed that the thrust direction is free and the thrust magnitude is unbounded [5, 27], that is, there exist no constraints on control variables. It is supposed that the orbital changes caused by the thrust and the perturbations due to the oblateness of the central body have the same order of magnitude. The optimization problem is formulated as a Mayer problem of optimal control with position and velocity vectorsβCartesian elementsβas state variables. After applying the Pontryagin Maximum Principle [28], successive Mathieu transformations are performed and a suitable set of orbital elements is introduced. Hori methodβa perturbation technique based on Lie series [29]βis applied in solving the canonical system of differential equations that governs the optimal trajectories. For transfers between close quasicircular orbits, new set of nonsingular orbital elements is introduced and the new Hamiltonian and the generating function, determined through the algorithm of Hori method, are transformed to these new elements through Mathieu transformations. First-order analytical solutions are obtained for transfers between close quasicircular orbits, considering maneuvers between near-equatorial orbits or nonequatorial orbits. These solutions are given in terms of systems of algebraic equations involving the imposed variations of the orbital elements, the duration of the maneuvers, the effects of the oblateness of the central body, and the initial values of the adjoint variables. The study of these transfers is particularly interesting because the orbits found in practice often have a small eccentricity, and the problem of slight modifications (corrections) of these orbits is frequently met [5]. For transfers between arbitrary orbits, a slightly different set of nonsingular elements is introduced, and the two-point boundary value problem described by averaged maximum Hamiltonian, expressed in nonsingular elements, is solved through a shooting method. A comparison between analytical and numerical results is presented for some maneuvers.

#### 2. Optimal Space Trajectories

A low-thrust limited-power propulsion system, or LP system, is characterized by low-thrust acceleration level and high specific impulse [5]. The ratio between the maximum thrust acceleration and the gravity acceleration on the ground, , is between and . For such system, the fuel consumption is described by the variable defined as
where *Ξ³* is the magnitude of the thrust acceleration vector , used as control variable. The consumption variable is a monotonic decreasing function of the mass of space vehicle:
where is the maximum power, and is the initial mass. The minimization of the final value of the fuel consumption is equivalent to the maximization of .

The general optimization problem concerned with low-thrust limited-power transfers (no rendezvous) will be formulated as a Mayer problem of optimal control by using Cartesian elements as state variables. Consider the motion of a space vehicle powered by a limited-power engine in a general gravity field. At time , the state of the vehicle is defined by the position vector , the velocity vector , and the consumption variable . The geometry of the transfer problem is illustrated in Figure 1. The control is unconstrained, that is, the thrust direction is free, and the thrust magnitude is unbounded.

The optimization problem is formulated as follows: it is proposed to transfer the space vehicle from the initial state at the initial time to the final state at the specified final time , such that the final consumption variable is a minimum. The state equations are where is the gravity acceleration. For transfers between orbits with small eccentricities, the initial and final conditions will be specified in terms of singular orbital elements introduced in next sections. It is also assumed that and the position of the vehicle in the initial orbit are specified.

According to the Pontryagin Maximum Principle [28], the optimal thrust acceleration must be selected from the admissible controls such that the Hamiltonian function reaches its maximum. The Hamiltonian function is formed using (2.3), where , and are the adjoint variables, and dot denotes the dot product. Since the optimization problem is unconstrained, is given by The optimal thrust acceleration is modulated [5], and the optimal trajectories are governed by the maximum Hamiltonian function , obtained from (2.4) and (2.5):

The consumption variable is ignorable, and is a first integral. From the transversality conditions, ; thus, Equation (2.6) reduces to The consumption variable is determined by simple quadrature.

For noncentral field, the gravity acceleration is given by [30] with the disturbing function defined by where is the gravitational parameter, is the mean equatorial radius of the central body, is the coefficient for the second zonal harmonic of the potential, is the latitude, and is the Legendre polynomial of order 2. is a row vector of partial derivatives, and superscript denotes its transpose.

Using (2.8) and (2.9), the maximum Hamiltonian function can be put in the following form: where is the undisturbed Hamiltonian function, and are disturbing functions concerning the oblateness of the central body and the optimal thrust acceleration, respectively. and are assumed to be of the same order of magnitude.

#### 3. Transformation from Cartesian Elements to a Set of Orbital Elements

Consider the canonical system of differential equations governed by the undisturbed Hamiltonian : where is the unit vector pointing radially outward of the moving frame of reference (Figure 2). The general solution of the state equations is well known in Astrodynamics [31], and the general solution of the adjoint equations is obtained through properties of generalized canonical systems [32], as described in the appendix. Thus, where and are unit vectors along circumferential and normal directions of the moving frame of reference, respectively; is the semimajor axis, is the eccentricity, is the inclination of orbital plane, is the longitude of the ascending node, is the argument of pericenter, is the true anomaly, is the eccentric anomaly, is the mean anomaly, is the mean motion, and , and so forth are functions of the elliptic motion which can be expressed explicitly in terms of the eccentricity and the mean anomaly through Lagrange series [31]. The true, eccentric and mean anomalies are related through the equations: The unit vectors , and of the moving frame of reference are written in the fixed frame of reference as

Equations (3.2)β(3.5) define a Mathieu transformation between the Cartesian elements and the orbital ones . Since the Hamiltonian function is invariant with respect to this canonical transformation, it follows, from (2.11) through (3.5), that where with the disturbing function given by

The new Hamiltonian function , defined by (3.8)β(3.10), describes the optimal low-thrust limited-power trajectories in a noncentral gravity field which includes the perturbative effects of the second zonal harmonic of the gravitational potential.

#### 4. Averaged Maximum Hamiltonian for Optimal Transfers

In order to eliminate the short periodic terms from the maximum Hamiltonian function , Hori method [29] is applied. It is assumed that is of zero-order and is of the first-order in a small parameter associated to the magnitude of the thrust acceleration, and has the same order of .

Consider an infinitesimal canonical transformation: The new variables are designated by the prime. According to the algorithm of Hori method [29], at order 0, denotes the new undisturbed Hamiltonian. Now, consider the canonical system described by : general solution of which is given by The subscript 0 denotes the constants of integration.

Introducing the general solution defined above into the equation of order 1 of the algorithm of Hori method, it reduces to According to [29], the mean value of must be calculated, and, is obtained through integration of the remaining part. and are given by the following equations: Terms factored by have been omitted in equations above, since only transfers (no rendez-vous) are considered [2].

It should be noted that the averaged maximum Hamiltonian and the generating function become singular for circular and/or equatorial orbits. In order to avoid these singularities, suitable sets of nonsingular elements will be introduced in the next sections.

#### 5. Optimal Transfers between Close Quasicircular Orbits

In this section, approximate first-order analytical solutions will be obtained for transfers between close quasicircular orbits.

##### 5.1. Transfers between Close Quasicircular Nonequatorial Orbits

Consider the Mathieu transformation [33] defined by The new set of canonical variables, designated by the double prime, is nonsingular for circular nonequatorial orbits.

Introducing (5.2) into (4.6) and using the expansions of the elliptic motion in terms of eccentricity and mean anomaly [31], one finds that the new averaged maximum Hamiltonian and the generating function are written up to the zeroth order in eccentricity as Double prime is omitted to simplify the notation.

For transfers between close orbits, and can be linearized around a suitable reference orbit, and an approximate first-order analytical solution can be determined. This solution is given by where is the vector of initial value of the adjoint variables, is a symmetric matrix concerning the optimal thrust acceleration and is a vector containing the perturbative effects of the oblateness of the central body. The variables and are introduced to make the adjoint and the state vectors dimensionless, and the variables and are introduced to simplify the matrix . The adjoint vector is constant. The matrix and the vector are given by where with is the initial time, is the final time, and . The overbar denotes the orbital elements of the reference orbit.

The analytical solution defined by (5.4)β(5.7) is in agreement with the ones obtained through different approaches in [6, 10]. In [6] the optimization problem is formulated with Gauss equations in nonsingular orbital elements as state equations, and in [10] Hori method is applied after the transformation of variables. Equations (5.4)β(5.7) represent a complete first-order analytical solution for optimal low-thrust limited-power transfers between close quasicircular nonequatorial orbits in a gravity field that includes the second zonal harmonic in the gravitational potential. These equations contain arbitrary constants of integration that must be determined to satisfy the two-point boundary value problem of going from the initial orbit at the time to the final orbit at the specified final time . Since they are linear in these constants, the boundary value problem can be solved by simple techniques.

An approximate expression for the optimal thrust acceleration can be obtained from (3.5) and (5.2), by using the expansions of the elliptic motion, and it is given, up to the zeroth order in eccentricity, by

According to Section 2, the optimal fuel consumption variable is determined by simple quadrature of the last differential equation in (2.3), with given by (5.8). An approximate expression for is given by

For transfers involving a large number of revolutions, (5.4)β(5.7) can be greatly simplified by neglecting the short periodic terms in comparison to the secular ones, and, the system of algebraic can be solved analytically. Thus, Introducing these equations into (5.8) and (5.9), and are obtained explicitly in terms of the imposed variations of the orbital elements, the perturbative effects due to the oblateness of the central body and the duration of the maneuver:

##### 5.2. Transfers between Close Quasicircular Near-Equatorial Orbits

Consider the Mathieu transformation [33] defined by The new set of canonical variables, designated by the double prime, is nonsingular for circular equatorial orbits.

Introducing (5.14) into (4.6) and using the expansions of the elliptic motion in terms of eccentricity and mean anomaly [31], one finds that the new averaged maximum Hamiltonian and the generating function are written up to the zeroth order in eccentricity and first-order in inclination as Double prime is omitted to simplify the notation.

For transfers between close orbits, and can be linearized around a suitable reference orbit, and an approximate first-order analytical solution can be determined. This solution is given by where is the vector of initial value of the adjoint variables, is a matrix concerning the optimal thrust acceleration, and is a vector containing the perturbative effects of the oblateness of the central body. The matrix and the vector are given by where The adjoint variables are where . The overbar denotes the orbital elements of the reference orbit. Equations (5.20) do not contain higher order terms in ββas appear implicitly in the solution presented in [7].

The analytical solution defined by (5.17)β(5.21) is in agreement with the ones obtained through different approaches in [7, 10]. In [7] the optimization problem is formulated with Gauss equations in nonsingular orbital elements as state equations, and in [10] Hori method is applied after the transformation of variables. Equations (5.17)β(5.21) represent a complete first-order analytical solution for optimal low-thrust limited-power transfers between close quasicircular near-equatorial orbits in a gravity field that includes the second zonal harmonic in the gravitational potential. These equations contain arbitrary constants of integration that must be determined to satisfy the two-point boundary value problem of going from the initial orbit at the time to the final orbit at the specified final time . Since they are linear in these constants, the boundary value problem can be solved by simple techniques.

As described in Section 5.1, approximate expressions can be obtained for the optimal thrust acceleration and the fuel consumption . These expressions are given by

For transfers involving a large number of revolutions, (5.17)β(5.21) can be greatly simplified by neglecting the short periodic terms in comparison to the secular ones, and, the system of algebraic can be solved analytically. Thus, The subscript denoting the constants is omitted. Introducing these equations into (5.22), and are obtained explicitly in terms of the imposed variations of the orbital elements, the perturbative effects due to the oblateness of the central body and the duration of the maneuver: with and .

##### 5.3. General Remarks on Optimal Transfers between Close Quasicircular Orbits

Considering the analytical solutions described above, general remarks about the effects of the oblateness of the central body on optimal transfers between close quasicircular orbits can be stated.

Matrices and , defined by (5.5) and (5.18), respectively, can be decomposed into two square matrices and . This fact shows that there exists an uncoupling between the in-plane modifications and the rotation of the orbital plane. Similar results were obtained by Edelbaum [1] and Marec [3] for transfers in a Newtonian central field. There is a normal component of the optimal thrust acceleration that counteracts the perturbative effects due to the oblateness of the central body. This normal component is proportional to . In general, the maneuvers in a noncentral gravity field are more expensive than the maneuvers in Newtonian central field, taking into account that the perturbative effects due to the oblateness of the central body must be counteracted. Fuel can be saved for long-time maneuvers which involves modification of the longitude of the ascending node if the terminal orbits are direct () and , and if the terminal orbits are retrograde () and . The extra consumption needed to counteract the perturbative effects due to the oblateness of the central body reaches its maximum for maneuvers between orbits with or of inclination and its minimum (null extra consumption) for maneuvers between equatorial or polar orbits (the gravity field is symmetric in these cases). As the transfer time increases, the extra fuel consumption increases. The extra fuel consumption is greater for transfers between low orbits.#### 6. Optimal Long-Time Transfers between Arbitrary Orbits

In this section the two-point boundary-value problem for long-time transfers between arbitrary orbits is formulated. In Section 7, this boundary-problem problem will be solved numerically through a shooting method, and the solution will be compared to the analytical ones in the case of orbits with small eccentricities.

Since the averaged maximum Hamiltonian, given by (4.6), becomes singular for circular and/or equatorial orbits, a set of nonsingular elements is introduced.

Consider the Mathieu transformation [33] defined by This transformation of variables is the same one given by (5.12), excepting the fast-phase, which is unnecessary (short-periodic terms have been eliminated). Double prime designates the new variables.

Introducing (6.2) into (4.6), is written as Double prime is omitted to simplify the notation. Note that (5.15) is a simplification of (6.3) for quasicircular near-equatorial orbits.

The two-point boundary-value problem for long-time transfers between arbitrary orbits is formulated by the following system of canonical differential equations: where and , and the boundary-conditions with initial time and final time . The explicit form of state and adjoint equations is derived by using MAPLE Software 9, and they are not presented in text because they are extensive.

#### 7. Results

In this section, numerical and analytical results are compared for maneuvers described in Tables 1 and 2. Earth is the central body, such that . In Table 1, orbital elements of initial and final orbits are defined for a maneuver of modification of all orbital elements considering different values of transfer durations and ratios . In Table 2, orbital elements of initial orbit, values of transfer duration and ratio , where denotes the orbital element that is changed in the maneuver, are defined for small (orbit corrections) and moderate amplitude maneuvers of modification of semimajor axis and inclination of the orbital plane. Results are presented in Figures 3 through 8 using canonical units. In these units, and . For example, in metric units, taking corresponds to , and (time units) corresponds to 24.20 hours (1.008 day), and corresponds to , and (time units) corresponds to 964.45 hours (40.185 days). Thus, maneuvers described in the tables below have duration of 1 to 74 days, approximately. On the other hand, considering that the value of is approximately given by (short periodic terms are neglected), has values between and (canonical units) for all results presented in Figures 3β5. These values of magnitude of the thrust acceleration are typical for LP systems [5, 27].

**(a)**

**(b)**

**(c)**

**(d)**

**(a)**

**(b)**

**(c)**

**(d)**

Figure 3 shows the consumption as function of ratio and values of for the maneuver of modification of all orbital elements described in Table 1. Figures 4 and 5 show the consumption as function of ratio and for the small and moderate amplitude maneuvers described in Table 2, considering that the maneuvers are performed in central or noncentral gravity field. In these figures, solid line represents analytical solution, and dashed line represents numerical solution. Analytical solutions, including short periodic terms, are obtained by solving the linear system of algebraic equations given by (5.4)β(5.7) (transfer between nonequatorial orbits) with the semimajor axis and the inclination of the reference orbit defined by and , respectively. In order to include the short periodic terms, it is assumed that the initial position of the vehicle is the pericenter of the initial orbit. Variables and are transformed into nonsingular variables and to compare the analytical results with the numerical results. Numerical solutions are obtained by solving the two-point boundary value problem described by (6.4) and (6.5) through a shooting method [34].

Considering the maneuvers described in Tables 1 and 2, Figures 3β5 show that analytical and numerical solutions yield quite similar values of the consumption in the following cases: (i) maneuvers of modification of all orbital elements with ratio ; (ii) maneuvers of modification of semimajor axis with ratios and ; (iii) maneuvers of modification of inclination with ratios and , for all values of . For maneuvers with very high ratio , analytical and numerical solutions only yield similar results for the same maneuvers described above for