Chapter 7
Q. 7.1
Spacecraft A is in an elliptical earth orbit having the following parameters:
h=52 059 km²/s , e=0.025724 , i=60°, Ω=40°, ω =30°, θ=40° (a)
Spacecraft B is likewise in an orbit with these parameters:
h = 52 362 km²/s, e = 0.0072696, i = 50°, Ω= 40°, ω = 120°, θ = 40° (b)
Calculate the velocity v_{rel})_{xyz} and acceleration a_{rel})_{xyz} of spacecraft B relative to spacecraft A, measured along the xyz axes of the co-moving coordinate system of spacecraft A, as defined in Figure 7.1.

Step-by-Step
Verified Solution
From the orbital elements in (a) and (b) we can use Algorithm 4.2 to find the position and velocity of the spacecraft relative to the geocentric equatorial reference frame. Omitting those calculations, we find, for spacecraft A,
r_{A}=-266.74\hat{I} + 3865.4 \hat{J}+ 5425.7\hat{K}(Km) (a)
v_{A}=-6.4842\hat{I}-3.6201\hat{J}+ 2.4159( km/s) (b)
and for spacecraft B,
r_{B}=−5890.0\hat{I}− 2979.4\hat{J}+ 1792.0\hat{K} (km) (c)
v_{B}=0.93594\hat{I}− 5.2409\hat{J}- 5.5016\hat{K}(Km/s) (d)
According to Equation 2.15, the accelerations of the two spacecraft are
\ddot{r}= – \frac{\mu }{r^{3} }r (2.15)
a_{B}=-\mu \frac{r_{B}}{\left\|r_{B}\right\|^{3} }= 0.0073377\hat{I}+0.0037117\hat{J}− 0.0022325\hat{K}(km/s^{2}) (f)
The unit vector \hat{i} along the x axis of spacecraft A’s rigid, co-moving frame of reference is
\hat{i}=\frac{r_{A}} {r_{A}}=-0.040008\hat{I}+0.57977\hat{J}+0.81380\hat{K} (g)
Since the z axis is in the direction of h_{A}, and
h_{A}=r_{A}\times v_{A}=\left|\begin{matrix} \hat{I} & \hat{J} & \hat{K} \\ -266.74 & 3865.4 & 5425.7 \\ -6.4842 &-3.6201 &2.4159 \end{matrix} \right| =28980\hat{I}-34537{I}+26030 \hat{K}(Km/s^{2})we obtain
\hat{K}=\frac{h_{A}}{h_{A}}=0.55667\hat{I} -0.66341 \hat{J}+0.5000\hat{K} (h)
Finally,\hat{j} = \hat{K}\times \hat{i} ,so that
\hat{j}=−0.82977\hat{I} – 0.47302\hat{J}+0.29620\hat{K} (i)
The angular velocity Ω of the xyz frame attached to spacecraft A is given by Equation 7.1,
\Omega =\frac{r_{0}\times v_{0}}{r^{2}_{0} } (7.1)
\Omega =\frac{ 28 980\hat{I}-34537\hat{J}+26030\hat{K}}{6667.1^{2}}= 0.00065196\hat{I}− 0.00077698\hat{J}+ 0.00058559\hat{K} (rad/s) (j)
We find the angular acceleration \dot{\Omega } using Equation 7.5,
\dot{\Omega } =-\frac{2(r_{0}\cdot v_{0})}{r^{2}_{0} } \Omega (7.5)
\dot{\Omega }=-\frac{2(844.41)}{6667.1^{2}}(0.00065196\hat{I} − 0.00077698\hat{J}+ 0.00058559\hat{K})= −2.4763(10^{-8})\hat{I}+ 2.9512(10^{-8})\hat{J} − 2.2242(10^{-8})\hat{K} (rad/s^{2}) (k)
According to Equation 1.38, the relative velocity relation is
v=v_{O}+\Omega \times r_{rel}+v_{rel} (1.38)
v_{B}=v_{A}+\Omega \times r_{rel}+v_{rel} (I)
where r_{rel} and v_{rel} are the position and velocity of B as measured relative to the moving xyz frame attached to A. From (a) and (b), we have
r_{rel} = r_{B} − r_{A} = −5623.3\hat{I}− 6844.8\hat{J}− 3633.7 \hat{K} (km) (m)
Substituting this, together with (b), (d) and (j) into (l), we get
0.93594 \hat{ I }-5.2409 \hat{ J }-5.5016 K =(-6.4842 \hat{ I }-3.6201 \hat{ J }+2.4159 \hat{ K }) +\left|\begin{matrix} \hat{I} & \hat{J} & \hat{K} \\ 0.00065196 & −0.00077698 & 0.00058559 \\ -5623.3 & −6844.8 & −3633.7 \end{matrix} \right| +v_{rel}Solving for v_{rel} yields
v_{rel}= 0.58865\hat{I} − 0.69692\hat{J} + 0.91414 \hat{K} (km/s) (n)
The relative acceleration formula, Equation 1.42, is
a=a_{O}+\dot{\Omega }\times r_{rel}+\Omega \times (\Omega \times r_{rel})+2\Omega \times v_{rel}+a_{rel} (1.42)
a_{B}=a_{A}+\dot{\Omega }\times r_{rel}+\Omega \times(\Omega \times r_{rel})+2\Omega \times v_{rel}+a_{rel} (o)
Substituting (e), (f), (j), (k), (m), and (n) into (o), we get
0.0073377\hat{I} + 0.0037117\hat{J} − 0.0022325\hat{K}= 0.00035876\hat{I} + 0.0051989 \hat{J} − 0.0072975\hat{K}
+(0.00065196 \hat{I} -0.00077698 \hat{J} +0.00058559 \hat{K})
\times \left|\begin{matrix} \hat{I} & \hat{J} & \hat{K} \\0.00065196 & -0.00077698 & 0.00058559 \\ -5623.3 & -6844.8 & -3633.7 \end{matrix} \right|
+2 \left|\begin{matrix} \hat{I} & \hat{J} & \hat{K} \\0.00065196 & -0.00077698 & 0.00058559 \\ 0.58865 & -0.69692 & 0.91414 \end{matrix} \right| +a_{rel}
Carrying out the cross products, combining terms and solving for a_{rel} yields
a_{rel}=0.00043984 \hat{I} − 0.00038019\hat{j} + 0.000017988\hat{K} (km/s^{2}) (p)
From (g), (h), and (i), we see that the orthogonal transformation matrix [Q]_{Xx} from the inertial XYZ frame into the co-moving xyz frame is
\left[Q\right]_{Xx}= \left[\begin{matrix}−0.040008 &0.57977 &0.81380 \\ −0.82977& −0.47302& 0.29620\\ 0.55667& −0.66341& 0.5000 \end{matrix} \right]To get the components of r_{rel} , v_{rel} , and a_{rel}, along the axes of the co-moving xyz frame of spacecraft A, we multiply each of their expressions as components in the XYZ frame [(m), (n) and (p), respectively] by [Q]_{Xx }as follows:
