Gravity Assist Maneuver

7/29/2019 Gravity Assist Maneuver

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Interplanetary Mission using

Planetary Gravity AssistManeuver


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Introduction

One problem with interplanetary travel is that it

takes so long.

The alternative to spending long periods in

heliocentric transfer orbits to reach other planets isto increase speed.

Speed can be increased by expending more fuel,but it can also be increased by use of planetary

gravity assist maneuver.


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Gravity Assist Maneuver

Also known as planetary flyby trajectory,

slingshot maneuver, swingby trajectory.

Useful in interplanetary missions to obtain a

velocity change without expending propellant.

This free velocity change is provided by the

gravitational field of the flyby planet and can beused to lower the Dv cost of a mission.


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This gravity assist technique uses a planets

gravitational field and orbital velocity to slingshota spacecraft, changing its velocity (in magnitude

and direction) with respect to the Sun.

Gravity-assisted velocity changes are not totally

free. Actually, the spacecraft steals velocity from

the planet, causing the planet to speed up or slow

down ever so slightly in its orbit around the Sun.

Gravity assist can also bend the spacecrafts

trajectory.


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How does Gravity Assist Work?

As a spacecraft enters a planets sphere of

influence, it coasts on a hyperbolic trajectoryaround the planet.

Then, the planet pulls it in the direction of theplanets motion, thus increasing (or decreasing) its

velocity relative to the Sun.

As it leaves the SOI at the far end of the

hyperbolic trajectory, the spacecraft has a new

velocity and direction to take it to another planet.


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Spacecraft passing Spacecraft passing

behind a planet in front of a planet


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Slingshot referenced to the planet Mars

1 2P PV V

1 12sine


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Slingshot referenced to the Sun

out inV V


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Slingshots do not change the velocity relative to

the planet.

As the probes velocity has increased its kinetic

energy must have changed relative to the Sun.Energy must be conserved, so this energy must

have come from somewhere. The kinetic energy has

come from the planet, deflecting its orbit by a tinyamount.


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Transfer to Jupiter

with a Mars Flyby


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Orbit pumping (A gravity assist that changes the

magnitude of a spacecrafts velocity).

Orbit cranking (Using a planets gravity to change

the direction of travel).


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Reference(s)

Douglas J Hopper, Interplanetary Flight Using the

Planetary Gravity Assist Maneuver, A Project Report.Website:http://www.dept.aoe.vt.edu/~cdhall/courses/mec

h533/Reports92b.pdf

http://www.philsrockets.org.uk/interplanetary.pdf

http://www.faa.gov/other_visit/aviation_industry/designe

es_delegations/designee_types/ame/media/Section%20III.4

.1.6%20Interplanetary%20Travel.pdf
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