Question 6.14: Orbit 1 has angular momentum h and eccentricity e. The direc......

Orbit 1 has angular momentum h and eccentricity e. The direction of motion is shown. Calculate the Δv required to rotate the orbit 90° about its latus rectum BC without changing h and e. The required direction of motion in orbit 2 is shown.

Step-by-Step
The 'Blue Check Mark' means that this solution was answered by an expert.
Learn more on how do we answer questions.

By symmetry, the required maneuver may occur at either B or C, and it involves a rigid body rotation of the ellipse, so that vr and vv_r \text { and } v_{\perp} remain unaltered. Because of the directions of motion shown, the true anomalies of B on the two orbits are

θB)1=90θB)2=+90\left.\left.\theta_B\right)_1=-90^{\circ} \quad \theta_B\right)_2=+90^{\circ}

The radial coordinate of B is

rB=h2μ11+ecos(±90)=h2μr_B=\cfrac{h^2}{\mu} \cfrac{1}{1+e \cos ( \pm 90)}=\cfrac{h^2}{\mu}

For the velocity components at B, we have

vB)1=vB)2=hrB=μhvrB)1=μhesinθB)1=μehvrB)2=μhesinθB)2=μeh\begin{aligned}& \left.\left.v_{\perp_B}\right)_1=v_{\perp_B}\right)_2=\cfrac{h}{r_B}=\cfrac{\mu}{h} \\\\& \left.\left.\left.\left.v_{ r _B}\right)_1=\cfrac{\mu}{h} e \sin \theta_B\right)_1=-\cfrac{\mu e}{h} \quad v_{ r _B}\right)_2=\cfrac{\mu}{h} e \sin \theta_B\right)_2=\cfrac{\mu e}{h}\end{aligned}

Substituting these into Eqn (6.19), yields

Δv=(vr2vr1)2+v12+v222v1v2cosδ\boxed{\Delta v=\sqrt{\left(v_{ r _2}-v_{ r _1}\right)^2+v_{\perp_1}^2+v_{\perp_2}^2-2 v_{\perp_1} v_{\perp_2} \cos \delta}}                                    (6.19)

ΔvB=[vrB)2vrB)1]2+vB)12+vB)222vB)1vB)2cos90=[μeh(μeh)]2+(μh)2+(μh)22(μh)(μh)0=4μ2h2e2+2μ2h2\begin{aligned}\Delta v_B & =\sqrt{\left.\left.\left.\left.\left.\left.\left[v_{ r _B}\right)_2-v_{ r _B}\right)_1\right]^2+v_{\perp_B}\right)_1^2+v_{\perp_B}\right)_2^2-2 v_{\perp_B}\right)_1 v_{\perp_B}\right)_2 \cos 90^{\circ}} \\\\& =\sqrt{\left[\cfrac{\mu e}{h}-\left(-\cfrac{\mu e}{h}\right)\right]^2+\left(\cfrac{\mu}{h}\right)^2+\left(\cfrac{\mu}{h}\right)^2-2\left(\cfrac{\mu}{h}\right)\left(\cfrac{\mu}{h}\right) \cdot 0} \\\\& =\sqrt{4 \cfrac{\mu^2}{h^2} e^2+2 \cfrac{\mu^2}{h^2}}\end{aligned}

so that

ΔvB=2μh1+2e2\boxed{\Delta v_B=\cfrac{\sqrt{2} \mu}{h} \sqrt{1+2 e^2}}                           (a)

If the motion on ellipse 2 were opposite to that shown in Figure 6.34, then the radial velocity components at B (and C) would in the same rather than in the opposite direction on both ellipses, so that instead of Eqn (a) we would find a smaller velocity increment,

ΔvB=2μh\Delta v_B=\cfrac{\sqrt{2} \mu}{h}
6.34

Related Answered Questions