# Vibroimpact Dynamics of a Tethered Satellite System

Authors: B. S. Yu; H. Wen
Journal: Shock and Vibration
 (2017)

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## Abstract

This paper presents the vibroimpact dynamics of an in-plane tethered subsatellite caused by sudden braking during deployment or retrieval. The full dynamics of the subsatellite are composed of its free-flight and the instantaneous impacts. At the moment of impact, the reflective angle of the subsatellite is envisioned to be equal to its incident angle such that the impact law is obtained. Then, the stability of the periodic vibroimpacts is analyzed using the composite Poincaré map. Further, the vibroimpact responses that do not exceed a specified region are numerically determined via the cell mapping method.


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## Body

## 1. Introduction

The vibroimpact phenomena of tethered satellites have been observed in experiments since the first on-orbit launch. In the H-9M-69 and S-520-2 missions, for instance, the deployment process was braked suddenly when the tether was deployed to only 38 m and 65 m, and then a series of impacts occurred [1]. In the TSS-1 mission, impact-like phenomena were observed when the space tether was jammed unexpectedly by a screw installed at an inappropriate position [2]. Moreover, a similar impact behavior was witnessed in a multiapplication survivable tether (MAST) experiment [3]. Furthermore, numerical simulations have indicated that vibroimpacts occurred at the end of the free deployment of a flexible tethered satellite system, as shown in Figure 1 [4]. Matunaga et al. [5] pointed out that the impact phenomena in a tethered satellite system are caused by a sudden tensioning force in the tether. To avoid the impact due to the slack of the tether, Wen et al. [6, 7] designed a nonlinear control law with a positive tension constraint for the deployment. Markeyev [8] analyzed the stability of the vibroimpact oscillation of an ideal tethered particle under a uniform gravitational field in terms of a strict analytical solution of periodic motion.Figure 1
Vibroimpacts of a subsatellite [4].The aim of this paper is to reveal the impact dynamics of a tethered subsatellite system under tether constraint. The paper is organized as follows. First, the velocities of the subsatellite before and after impact are obtained. Then, a set of composite Poincaré maps is established. Next, the periodic vibroimpacts and their stability via the composite Poincaré map are analyzed. The cell mapping method is used to determine the parameter regions where the impact occurs. Finally, case studies are numerically examined.


## 2. Impact Problem of a Tethered Subsatellite

An in-plane tethered satellite system orbiting Earth under consideration consists of a mother satelliteM, a subsatellite S, and a flexible tether, as shown in Figure 2. The satellites connected by tether are regarded as particles. The tether can be deployed/retrieved by a deployment/retrieval device mounted on two satellites. The mass of the mother satellite is sufficiently larger than that of the subsatellite, so the motion of the mother satellite is immune to the subsatellite motion. Furthermore, it is assumed that the mother satellite operates in a circular orbit with an angular velocity Ω around the Earth. To describe the motion of the system, an orbital reference frame o-xy with coordinate origin o in the mother satellite is established, wherein the x-axis points are in the opposite direction of the motion of the mother satellite, and the y-axis is on the orbital plane and perpendicular to the x-axis.Figure 2
Impact process of a tethered subsatellite system.Provided that the deployment or retrieval process is stopped suddenly at timetk with the tether length l(tk)=L, there follows a subsatellite impact under the tether constraint C(x,y)=x2+y2-L=0, as illustrated in Figure 2.The impact is divided into the perfect elastic and inelastic impacts, that is, impact without/with damping due to the elasticity of the tether. As shown in Figure2, the incident and reflective angles before and after impact are equal to each other; that is, φin=φre. By letting x1=x, x2=x˙, x3=y, and x4=y˙, the impact law of the subsatellite can be obtained as follows:(1)x1+tk=x1-tk,x2+tk=e·cos2θk-sin2θkx2-tk+2sin⁡θkcos⁡θkx4-tk,x3+tk=x3-tk,x4+tk=e·2sin⁡θkcos⁡θkx2-tk+sin2θk-cos2θkx4-tk,where the superscripts “−” and “+” represent the moment before and after impact, respectively, and e is the factor of restitution in impact. The pitch angle of the tether at the impact moment tk is(2)θk=π-arccos⁡x3-tkL·signx1-tk.Equation (1) can be rewritten in the following form:(3)x+tk=x1+tkx2+tkx3+tkx4+tk=10000e·cos2θk-sin2θk0e·2sin⁡θkcos⁡θk00100e·2sin⁡θkcos⁡θk0e·sin2θk-cos2θkx1-tkx2-tkx3-tkx4-tk≜Gx-tk,where cos⁡θk=-x3-(tk)/L and sin⁡θk=x1-(tk)/L.After one impact, the subsatellite could freely move without constraint until the beginning of the next impact. For such a free-flight phase, by the straightforward application of Hill’s equations, the motion equations of the subsatellite are as follows:(4)x˙1=x2,x˙2=2Ωx4,x˙3=x4,x˙4=3Ω2x3-2Ωx2,where the dot represents the derivative with respect to time t. Equation (4) can be expressed in the following state equation:(5)x˙=Fx,t=Ax,where x=x1x2x3x4T and(6)A=10000002Ω00100-2Ω3Ω20.By taking x+(tk) as the initial states, the analytical solutions of motion of the subsatellite can be obtained as(7)x≜fx+tk,t=1-3t+4sin⁡ΩtΩ6Ωt-sin⁡Ωt21-cos⁡ΩtΩ0-3+4cos⁡Ωt6Ω1-cos⁡Ωt2sin⁡Ωt02cos⁡Ωt-1Ω-3cos⁡Ωt+4sin⁡ΩtΩ0-2sin⁡Ωt3Ωsin⁡Ωtcos⁡Ωtx1+tkx2+tkx3+tkx4+tk.Equations (3) and (7) give the vibroimpact dynamics of the subsatellite.


## 3. Stability of Periodic Vibroimpacts

The Poincaré map will be used to analyze the stability of the periodic vibroimpacts. The Poincaré section is defined as(8)Σ=x1,x2,x3,x4Cx1,x3=0.The full dynamics of the subsatellite are determined by the following two maps:(9)Pf:Σ+⟶Σ-,Pi:Σ-⟶Σ+,where Pf and Pi represent the maps for the free-flight and impact phases of the subsatellite, respectively. The composite Poincaré map is(10)P=Pni∘Pnf∘⋯∘P1i∘P1f.Clearly, the fixed pointx- of the composite map corresponds to a vibroimpact oscillation of period T-. Taking the Taylor expansion of P in the vicinity of the fixed point gives the following:(11)Px,T--Px-,T-=Jx-,T-x-x-,where T-=∑k=1nT-k, and T-k is the free-flight time of the subsatellite after the kth impact. Based on the perfect elastic impact, J(x-,T-) is the Jacobian of (10) calculated at x- using the chain rule; namely,(12)Jx-,T-=Jix-tnJfT-n⋯Jix-t1JfT-1with(13)JfT-k=Dfx+tk-1,T-k=1-3T-k+4sin⁡ΩT-kΩ6ΩT-k-sin⁡ΩT-k21-cos⁡ΩT-kΩ0-3+4cos⁡ΩT-k6Ω1-cos⁡ΩT-k2sin⁡ΩT-k02cos⁡ΩT-k-1Ω-3cos⁡ΩT-k+4sin⁡ΩT-kΩ0-2sin⁡ΩT-k3Ωsin⁡ΩT-kcos⁡ΩT-k,Jix-tk=DGx-tk=10000x3-tk2L2-x1-tk2L20-2x1-tkx3-tkL200100-2x1-tkx3-tkL20x1-tk2L2-x3-tk2L2x1-tkx2-tkx3-tkx4-tk=1000-2x1-tkx2-tk+x3-tkx4-tkL2x3-tk2-x1-tk2L22x2-tkx3-tk-x1-tkx4-tkL2-2x1-tkx3-tkL200102x1-tkx4-tk-x2-tkx3-tkL2-2x1-tkx3-tkL2-2x1-tkx2-tk+x3-tkx4-tkL2x1-tk2-x3-tk2L2.Thus, the stability of the periodic vibroimpacts can be determined by the roots of the JacobianJ(x-,T-), λj(j=1,2,3,4); that is,(14)λjmax<1asymptotically  stable,λjmax>1unstable,λjmax=1undetermined.


## 4. Algorithm of Parameter Domain

To obtain an insight into the vibrodynamics of a tethered system, the parameter domain state not exceeding a specified regionΠ is calculated using the cell mapping method [9]. The specified region Π is also a system state in an expected pitch angle or radial impact velocity range of the subsatellite. According to the polar coordinate system shown in Figure 2,(15)x=lsin⁡θ,y=-lcos⁡θ.The study on the parameter domain is divided into two parts. First, the three-dimensional state space is divided intoNc=nθinθ˙inl˙i cells as cj(j=1,2,…,Nc) with nθi, nθ˙i, and nl˙i elements in θ, θ˙, and l˙. The sink cell is defined as cNc+1=Nc+1. The minute flow chart of the cell mapping algorithm is given in Figure 3, where the group numbers Gr(cq) are assigned for cell sequences {cq}, and the state information Da(cq) is recorded, so that the maximum value for each cell mapping can be obtained based on Gr(cq) and Da(cq). After completion of the calculation, the states of all cells can be analyzed. The corresponding cell will be not in the domain of attraction (i.e., the parameter domain) as long as the maximum of any mapping cell of the cell exceeds the specified region Π. Hence, the domain of attraction is dependent upon the attractor, all cells of which are maintained in the specified region Π.Figure 3
Flow chart of the cell mapping algorithm.To extend the application to all system states(θ,θ˙,l,l˙), a similar flow process of the cell mapping method is conducted. The ranges [θmin,θmax], [θ˙min,θ˙max], [lmin,lmax], and [l˙min,l˙max] are uniformly divided into nθ, nθ˙, nl, and nl˙ elements. Then, the state space of the system is divided into Nz=nθnθ˙nlnl˙ cells as zj(j=1,2,…,Nz). Similarly, the sink cell is defined as zNz+1=Nz+1. For each cell, zj(j=1,2,…,Nz), the corresponding cell cq=I(zj) can be computed by (7), as shown in the dashed box of Figure 3. Finally, the parameter domain for the cells of all system states is acquired via Gr(cq) and Da(cq).


## 5. Case Studies

### 5.1. Nonlinear Dynamics

Suppose that the tethered system moves in a circular orbit at an altitude of 300 km with an orbital angular velocityΩ = 1.1591×10-3 rad/s. The total tether length L=10km, and e=1.The initial velocity is set as[x20+,x40+]T=[-10m/s,10m/s]T. The bifurcation responses of the pitch angle in Figure 4(a) show that the pitch angle for the impact moment increases with an increase of x10+. The pitch angle will particularly exist throughout [-π,π] once the abscissa x10+>7113m. Figure 4(b) shows the bifurcation for x40+=2m/s, in which the critical value of the impact point’s coordinate is (8627 m, −5057 m). To elaborate the relationship between the coordinate position and the sudden change, the mechanical energy E of the subsatellite in the orbital frame is introduced [10](16)E=V+T=-μEmSRE+μEmSL22RE31-3x30+L2+12mSx20+2+x40+2,where μE=3.986×1014m3/s2 is the gravitational parameter of the Earth, RE=6371km is the mean radius of the Earth, and mS=50kg is the mass of the subsatellite. The critical values E0c of the initial energy are -2.987554706×109J±10J. The phenomenon that the pitch angle is throughout [-π,π] will appear when the initial energy of the system is larger than E0c.Figure 4
Bifurcations of pitch angles with increase ofx10+.
(a)
x
40
+
=
10
m
/
s
(b)
x
40
+
=
2
m
/
sFigure5 gives the bifurcation responses of the pitch angle when the initial positions (x10+,x30+) are (0,-L). Figure 5 shows that the amplitudes of pitch angle θk decrease suddenly if the initial velocities x20+>-17.44m/s or x20+>-20.00m/s. Both the absolute values of the critical velocities can be calculated as approximately (x20+)2+(x40+)2=20.10m/s, and the critical energy is in accord with only E0c. Consequently, it can be concluded that the initial energy E0 plays an important role in the impact dynamics of the tethered satellite system.Figure 5
Bifurcations of pitch angle with increase ofx20+.
(a)
x
40
+
=
10
m
/
s
(b)
x
40
+
=
2
m
/
sFor the above-determined system parameters, the periodic, quasi-periodic, and irregular motions will appear as the initial energy changes. Forx10+=0, x20+=-2.12496m/s, x30+=-L, and x40+=10m/s, for example, a periodic motion is exhibited in Figure 6. Figure 6(a) plots the motion trajectory of the subsatellite in the orbital frame o-xy. As shown in Figure 6(a), the periodic motion is constituted by one free-flight and one impact. The subsatellite flies with only the Coriolis force acting along the solid curve and comes back to the initial impact position (0,-L) for 561s. Figure 6(b) gives the coordinate positions of 1000 impacts. For a clearer observation, the abscissas of the impacts are presented in Figure 6(c). The abscissas converge within the range of (-0.03m,0.03m). The phase points on C(x) are drawn in Figure 6(d). The converging phase points indicate that this is a periodic motion with one impact.Figure 6
Periodic motion with one impact.
(a)
Trajectory of the subsatellite in the orbital frame
(b)
Position coordinates of impacts
(c)
Change ofx1 coordinate of impacts
(d)
Phase points onC(x)The stability of the periodic motion is studied via the Poincaré map in (12). The eigenvalues of the Jacobian can be calculated as λ1=0.5360, λ2,3=0.5202±0.9136i, and λ4=-1.6880, with λ1=0.5360, λ2,3=1.0513, and λ4=1.6880. This means that the periodic motion with one impact is unstable. With the initial state of [0m,-2.2m/s,-L,9.98376m/s]T, the impact positions are located on a segment of C(x), and the phase trajectory is a closed curve, as shown in Figure 7.Figure 7
Unstable impact motion.
(a)
Impact position coordinates
(b)
Phase trajectory onC(x)A periodic motion with four impacts is depicted in Figure8. Figure 8(a) presents the flight trajectory of the subsatellite. As seen in Figure 8(a), starting with the initial point 0, the motion of the subsatellite without the tether constraint will impact at points 1, 2, and 3 and return to the origin. Figure 8(b) shows the coordinate position of the impacts. The change of state x2 with each impact is displayed in Figure 8(c). The phase trajectory on C(x) is shown in Figure 8(d), where the four converging points correspond to the states of four impacts. Similarly, the roots of the Jacobian of the periodic motion can be obtained as λ1=0.0284, λ2=-0.0981, λ3=10.8714, and λ4=-32.9549. Clearly, λ3,4>1 means that the periodic motion is unstable.Figure 8
Periodic motion with four impacts.
(a)
Trajectory of the subsatellite
(b)
Position coordinates of impacts
(c)
Change ofx2 of impacts
(d)
Phase trajectory onC(x)A quasi-periodic motion is shown in Figure9. With the initial state [0m,-5m/s,-L,10m/s]T, Figure 9(a) depicts the coordinate positions of the impacts. The impacts are all situated on a segment of C(x), and Figure 9(b) shows the phase points on C(x). As observed from Figure 9(b), the phase points constitute a closed curve, which concludes that this is a quasi-periodic motion.Figure 9
Quasi-periodic motion.
(a)
Position coordinates of impacts
(b)
Phase trajectory onC(x)Irregular motion will occur if the initial mechanical energy exceeds the critical valueE0c. With the assumption of the initial state [0m,-20m/s,-L,10m/s]T, an irregular motion is presented in Figure 10. The coordinate positions of the impacts are distributed densely on the entire C(x), as shown in Figure 10(a). Figure 10(b) sketches the distribution of the phase points on C(x). Evidently, an oscillation with such a wide range caused by the irregular motion might cause serious disaster to other spacecraft in the neighborhood of the tethered system.Figure 10
Irregular motions.
(a)
Position coordinates of impacts
(b)
Phase trajectory onC(x)In terms of the above irregular motion, a numerical case for the impacts with damping is given in Figure11. The recovery factor in the impact was set as e=0.999; that is, there exists energy dissipation of 0.1% for each impact. The change of x1 coordinate in each impact is presented in Figure 11(a). The corresponding phase trajectory on C(x) is drawn in Figure 11(b). It can be observed from Figure 11(b) that the impact points eventually converge to origin (0,0).Figure 11
Irregular motions with damping factor.
(a)
Change ofx1 coordinate of impacts
(b)
Phase trajectory onC(x)


### 5.2. Parameter Domain

The parameter domain is analyzed through the cell mapping method. First, the state space of impacts onC in the polar coordinate system is discussed. The study areas of the states are θ∈[-π/2,π/2], θ˙∈[-10-3rad/s,10-3rad/s], and l˙∈[0,18m/s], and the state variables are uniformly divided into 120, 120, and 90 elements, respectively. Thus, the state space of the system on C has 1296000 three-dimensional cells. Then, the corresponding parameter domain (i.e., domain of attraction) can be calculated provided that the specified region (i.e., attractor) is given.For the specified regionΠ={(θ,θ˙,l,l˙)∣θ∈[-π/6,π/6],l=L}, the effect of the radial impact velocity is as shown in Figure 12. As seen from Figure 12, the section area θ-θ˙ of the parameter domain decreases as l˙ increases. Therefore, the expansion of the range of pitch angle with the initial kinetic energy increase is in accordance with the above statement on the initial system energy. For l˙=6m/s, a comparison of the parameter domain with different specified regions is presented in Figure 13, where the specified regions are Π1={(θ,θ˙,l,l˙)∣θ∈[-π/6,π/6],l=L}, Π2={(θ,θ˙,l,l˙)∣θ∈[-π/8,π/8],l=L}, and Π3={(θ,θ˙,l,l˙)∣θ∈[-π/12,π/12],l=L}. The section area of the parameter domain decreases with the decrease of the specified region.Figure 12
Effect of radial impact velocity on the parameter domain.Figure 13
Comparison of parameter domains based on different specified regions.The influence of the tangential velocityθ˙ on the parameter domain is shown in Figure 14. Figure 14 shows that the maximal section area of the parameter domain appears when θ˙=-5×10-4rad/s because the flight of the subsatellite after impact is towards the positive direction of θ˙ under the Coriolis force. The effect of pitch angle on the parameter domain is provided in Figure 15.Figure 14
Effect of tangential velocity on the parameter domain.Figure 15
Effect of pitch angle on the parameter domain.Considering the deployment/retrieval process of the tether before its first impact, the tether length variablel is introduced. The parameter domain on all the system states (i.e., θ, θ˙, l, and l˙) is proposed using the above results for the domain on C. The study areas of the pitch angle and pitch angular velocity are θ∈[-π/2,π/2] and θ˙∈[-10-3rad/s,10-3rad/s], respectively, and both are divided into 120 elements. Moreover, the ranges of the radial position and radial velocity are l∈[L/20,L] and l˙∈[-18m/s,18m/s], respectively, both of which are divided into 30 elements. Accordingly, the state space of the subsatellite is divided into 12960000 four-dimensional cells.The parameter domain determined by the specified regionΠ={(θ,θ˙,l,l˙)∣θ∈[-π/6,π/6],l=L} is depicted in Figure 16. For all of the state cells with an initial radial position l0>8km and an initial radial velocity l˙0=0.6m/s, the parameter domain corresponds to the blue zone in Figures 16(a) and 16(b). These figures show that the contrasted red zone decreases as l0 decreases or l˙0 increases.Figure 16
Dependence of parameter domain in the rangeθ∈[-π/6,π/6].
(a)
Effect of initial radial position of subsatellite on parameter domain
(b)
Effect of initial radial velocity of subsatellite on parameter domainIn addition, it is important to study the specified region based on the radial impact velocity. The break of the space tether will occur once the tether cannot undergo the impact force dependent upon the impact velocity, which might cause more serious damage. The parameter domain governed by the specified regionΠ={(θ,θ˙,l,l˙)∣l=L,l˙<15m/s} is drawn in Figure 16. Let initial radial velocity l˙0=0.6m/s, so the effect of the initial radial position on the parameter domain is presented in Figure 17(a). With the assumption of the initial radial position l0>8km, the effect of the initial radial velocity on the parameter domain is shown in Figure 17(b). The consequence of the numerical simulations indicates that the parameter domain decreases with the decrease of l0 or with the increase of l˙0.Figure 17
Dependence of parameter domain on radial impact velocitiesl˙<15m/s.
(a)
Effect of initial radial position of subsatellite on the parameter domain
(b)
Effect of initial radial velocity of subsatellite on the parameter domainOnce the system state cell is inside a parameter domain, the state of the system will be retained in a specified region even if the tether loses control abruptly. Hence, the parameter domain can be treated as a constraint condition to design the tether deployment/retrieval control law, which is capable of maintaining the subsatellite in an expected region.


## 5.1. Nonlinear Dynamics

Suppose that the tethered system moves in a circular orbit at an altitude of 300 km with an orbital angular velocityΩ = 1.1591×10-3 rad/s. The total tether length L=10km, and e=1.The initial velocity is set as[x20+,x40+]T=[-10m/s,10m/s]T. The bifurcation responses of the pitch angle in Figure 4(a) show that the pitch angle for the impact moment increases with an increase of x10+. The pitch angle will particularly exist throughout [-π,π] once the abscissa x10+>7113m. Figure 4(b) shows the bifurcation for x40+=2m/s, in which the critical value of the impact point’s coordinate is (8627 m, −5057 m). To elaborate the relationship between the coordinate position and the sudden change, the mechanical energy E of the subsatellite in the orbital frame is introduced [10](16)E=V+T=-μEmSRE+μEmSL22RE31-3x30+L2+12mSx20+2+x40+2,where μE=3.986×1014m3/s2 is the gravitational parameter of the Earth, RE=6371km is the mean radius of the Earth, and mS=50kg is the mass of the subsatellite. The critical values E0c of the initial energy are -2.987554706×109J±10J. The phenomenon that the pitch angle is throughout [-π,π] will appear when the initial energy of the system is larger than E0c.Figure 4
Bifurcations of pitch angles with increase ofx10+.
(a)
x
40
+
=
10
m
/
s
(b)
x
40
+
=
2
m
/
sFigure5 gives the bifurcation responses of the pitch angle when the initial positions (x10+,x30+) are (0,-L). Figure 5 shows that the amplitudes of pitch angle θk decrease suddenly if the initial velocities x20+>-17.44m/s or x20+>-20.00m/s. Both the absolute values of the critical velocities can be calculated as approximately (x20+)2+(x40+)2=20.10m/s, and the critical energy is in accord with only E0c. Consequently, it can be concluded that the initial energy E0 plays an important role in the impact dynamics of the tethered satellite system.Figure 5
Bifurcations of pitch angle with increase ofx20+.
(a)
x
40
+
=
10
m
/
s
(b)
x
40
+
=
2
m
/
sFor the above-determined system parameters, the periodic, quasi-periodic, and irregular motions will appear as the initial energy changes. Forx10+=0, x20+=-2.12496m/s, x30+=-L, and x40+=10m/s, for example, a periodic motion is exhibited in Figure 6. Figure 6(a) plots the motion trajectory of the subsatellite in the orbital frame o-xy. As shown in Figure 6(a), the periodic motion is constituted by one free-flight and one impact. The subsatellite flies with only the Coriolis force acting along the solid curve and comes back to the initial impact position (0,-L) for 561s. Figure 6(b) gives the coordinate positions of 1000 impacts. For a clearer observation, the abscissas of the impacts are presented in Figure 6(c). The abscissas converge within the range of (-0.03m,0.03m). The phase points on C(x) are drawn in Figure 6(d). The converging phase points indicate that this is a periodic motion with one impact.Figure 6
Periodic motion with one impact.
(a)
Trajectory of the subsatellite in the orbital frame
(b)
Position coordinates of impacts
(c)
Change ofx1 coordinate of impacts
(d)
Phase points onC(x)The stability of the periodic motion is studied via the Poincaré map in (12). The eigenvalues of the Jacobian can be calculated as λ1=0.5360, λ2,3=0.5202±0.9136i, and λ4=-1.6880, with λ1=0.5360, λ2,3=1.0513, and λ4=1.6880. This means that the periodic motion with one impact is unstable. With the initial state of [0m,-2.2m/s,-L,9.98376m/s]T, the impact positions are located on a segment of C(x), and the phase trajectory is a closed curve, as shown in Figure 7.Figure 7
Unstable impact motion.
(a)
Impact position coordinates
(b)
Phase trajectory onC(x)A periodic motion with four impacts is depicted in Figure8. Figure 8(a) presents the flight trajectory of the subsatellite. As seen in Figure 8(a), starting with the initial point 0, the motion of the subsatellite without the tether constraint will impact at points 1, 2, and 3 and return to the origin. Figure 8(b) shows the coordinate position of the impacts. The change of state x2 with each impact is displayed in Figure 8(c). The phase trajectory on C(x) is shown in Figure 8(d), where the four converging points correspond to the states of four impacts. Similarly, the roots of the Jacobian of the periodic motion can be obtained as λ1=0.0284, λ2=-0.0981, λ3=10.8714, and λ4=-32.9549. Clearly, λ3,4>1 means that the periodic motion is unstable.Figure 8
Periodic motion with four impacts.
(a)
Trajectory of the subsatellite
(b)
Position coordinates of impacts
(c)
Change ofx2 of impacts
(d)
Phase trajectory onC(x)A quasi-periodic motion is shown in Figure9. With the initial state [0m,-5m/s,-L,10m/s]T, Figure 9(a) depicts the coordinate positions of the impacts. The impacts are all situated on a segment of C(x), and Figure 9(b) shows the phase points on C(x). As observed from Figure 9(b), the phase points constitute a closed curve, which concludes that this is a quasi-periodic motion.Figure 9
Quasi-periodic motion.
(a)
Position coordinates of impacts
(b)
Phase trajectory onC(x)Irregular motion will occur if the initial mechanical energy exceeds the critical valueE0c. With the assumption of the initial state [0m,-20m/s,-L,10m/s]T, an irregular motion is presented in Figure 10. The coordinate positions of the impacts are distributed densely on the entire C(x), as shown in Figure 10(a). Figure 10(b) sketches the distribution of the phase points on C(x). Evidently, an oscillation with such a wide range caused by the irregular motion might cause serious disaster to other spacecraft in the neighborhood of the tethered system.Figure 10
Irregular motions.
(a)
Position coordinates of impacts
(b)
Phase trajectory onC(x)In terms of the above irregular motion, a numerical case for the impacts with damping is given in Figure11. The recovery factor in the impact was set as e=0.999; that is, there exists energy dissipation of 0.1% for each impact. The change of x1 coordinate in each impact is presented in Figure 11(a). The corresponding phase trajectory on C(x) is drawn in Figure 11(b). It can be observed from Figure 11(b) that the impact points eventually converge to origin (0,0).Figure 11
Irregular motions with damping factor.
(a)
Change ofx1 coordinate of impacts
(b)
Phase trajectory onC(x)


## 5.2. Parameter Domain

The parameter domain is analyzed through the cell mapping method. First, the state space of impacts onC in the polar coordinate system is discussed. The study areas of the states are θ∈[-π/2,π/2], θ˙∈[-10-3rad/s,10-3rad/s], and l˙∈[0,18m/s], and the state variables are uniformly divided into 120, 120, and 90 elements, respectively. Thus, the state space of the system on C has 1296000 three-dimensional cells. Then, the corresponding parameter domain (i.e., domain of attraction) can be calculated provided that the specified region (i.e., attractor) is given.For the specified regionΠ={(θ,θ˙,l,l˙)∣θ∈[-π/6,π/6],l=L}, the effect of the radial impact velocity is as shown in Figure 12. As seen from Figure 12, the section area θ-θ˙ of the parameter domain decreases as l˙ increases. Therefore, the expansion of the range of pitch angle with the initial kinetic energy increase is in accordance with the above statement on the initial system energy. For l˙=6m/s, a comparison of the parameter domain with different specified regions is presented in Figure 13, where the specified regions are Π1={(θ,θ˙,l,l˙)∣θ∈[-π/6,π/6],l=L}, Π2={(θ,θ˙,l,l˙)∣θ∈[-π/8,π/8],l=L}, and Π3={(θ,θ˙,l,l˙)∣θ∈[-π/12,π/12],l=L}. The section area of the parameter domain decreases with the decrease of the specified region.Figure 12
Effect of radial impact velocity on the parameter domain.Figure 13
Comparison of parameter domains based on different specified regions.The influence of the tangential velocityθ˙ on the parameter domain is shown in Figure 14. Figure 14 shows that the maximal section area of the parameter domain appears when θ˙=-5×10-4rad/s because the flight of the subsatellite after impact is towards the positive direction of θ˙ under the Coriolis force. The effect of pitch angle on the parameter domain is provided in Figure 15.Figure 14
Effect of tangential velocity on the parameter domain.Figure 15
Effect of pitch angle on the parameter domain.Considering the deployment/retrieval process of the tether before its first impact, the tether length variablel is introduced. The parameter domain on all the system states (i.e., θ, θ˙, l, and l˙) is proposed using the above results for the domain on C. The study areas of the pitch angle and pitch angular velocity are θ∈[-π/2,π/2] and θ˙∈[-10-3rad/s,10-3rad/s], respectively, and both are divided into 120 elements. Moreover, the ranges of the radial position and radial velocity are l∈[L/20,L] and l˙∈[-18m/s,18m/s], respectively, both of which are divided into 30 elements. Accordingly, the state space of the subsatellite is divided into 12960000 four-dimensional cells.The parameter domain determined by the specified regionΠ={(θ,θ˙,l,l˙)∣θ∈[-π/6,π/6],l=L} is depicted in Figure 16. For all of the state cells with an initial radial position l0>8km and an initial radial velocity l˙0=0.6m/s, the parameter domain corresponds to the blue zone in Figures 16(a) and 16(b). These figures show that the contrasted red zone decreases as l0 decreases or l˙0 increases.Figure 16
Dependence of parameter domain in the rangeθ∈[-π/6,π/6].
(a)
Effect of initial radial position of subsatellite on parameter domain
(b)
Effect of initial radial velocity of subsatellite on parameter domainIn addition, it is important to study the specified region based on the radial impact velocity. The break of the space tether will occur once the tether cannot undergo the impact force dependent upon the impact velocity, which might cause more serious damage. The parameter domain governed by the specified regionΠ={(θ,θ˙,l,l˙)∣l=L,l˙<15m/s} is drawn in Figure 16. Let initial radial velocity l˙0=0.6m/s, so the effect of the initial radial position on the parameter domain is presented in Figure 17(a). With the assumption of the initial radial position l0>8km, the effect of the initial radial velocity on the parameter domain is shown in Figure 17(b). The consequence of the numerical simulations indicates that the parameter domain decreases with the decrease of l0 or with the increase of l˙0.Figure 17
Dependence of parameter domain on radial impact velocitiesl˙<15m/s.
(a)
Effect of initial radial position of subsatellite on the parameter domain
(b)
Effect of initial radial velocity of subsatellite on the parameter domainOnce the system state cell is inside a parameter domain, the state of the system will be retained in a specified region even if the tether loses control abruptly. Hence, the parameter domain can be treated as a constraint condition to design the tether deployment/retrieval control law, which is capable of maintaining the subsatellite in an expected region.


## 6. Conclusions

An impact law of an in-plane tethered subsatellite system is given. The periodic vibroimpact motions are investigated using the composite Poincaré map. The periodic, quasiperiodic, and irregular vibroimpacts such as one impact or four impacts are found. A parameter domain is proposed to acquire the undesired pitch angle and radial impact velocity. In addition, the method of investigating the impact dynamics can be extended to the out-of-plane motions of the tethered system. More complicated nonlinear behaviors appear if the energy dissipation of the space tether caused by impacts is considered.



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*Source: 8748094-2017-11-21.xml*