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Chapter 9
astronautical engineering GR2
39
Astronomy
Graduate
03/14/2013

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Term
launch window
Definition
period of time when we can launch a spacecraft directly into a speficied orbit from a given launch site
Term
apparent solar day
Definition
marking the time between the Sun's successive passages above a certain point by observing shadows on a sundial
Term
Mean Solar Day
Definition
since the Earth rotates on an ellipse, each apparent solar day's length varies; to compensate for this variation, we take the average of the lengths for one year to get a "mean solar day;" little more than 360 degrees; exactly 24 solar hours
Term
Greenwich Mean Time (GMT)
Definition
the local mean time solar time at the Prime Merdian, as the 0 degree longitude line as the international reference point to start the 24 time zones around the world
Term
Sidereal Time
Definition
the angle between a longitude line and the vernal equinox; "related to the stars"
Term
Sidereal Day
Definition
the successive passages of the vernal equinox over a specific longitude; exactly 360° rotation; little less than 24 solar hours
Term
Local Sidereal Time (LST)
Definition
the time since the vernal equinox last passed over the local logitude
Term
Earth's Rotation Rate
Definition
15 degrees per hour
Term
How to solve for Time in Degrees
Definition
Time (hours) x 15 degrees/hour
Term
How to solve for Time in Hours
Definition
Time (degrees) / [15 degrees/hour]
Term
If Lo = i, how many launch opportunities per day?
Definition
1
Term
If Lo < i on a prograde orbit, how many launch opportunities per day?
Definition
2
Term
If Lo < 180° - i on a retrograde orbit, how many launch opportunities per day?
Definition
2
Term
Launch Window Sidereal Time (LWST)
Definition
the degrees between the vernal equinox direction (I vector) to the point where the launch site passes through the orbital plane; function of Ω and δ (launch window location)
Term
Ascending-node opportunity
Definition
the launch site opportunity closer to the ascending node
Term
Descending-node opportunity
Definition
the launch site opportunity closer to the descending node
Term
spherical triangle
Definition
triangle on the surface of a sphere; different from planar triangles
Term

α

Definition
Inclination auxiliary angle; angle between the equator and ground trace of the orbit at the ascending node
Term
γ
Definition
launch-direction auxiliary angle; angle between the lauch site longitude line and ground trace of the orbit
Term
δ
Definition
launch-window location angle; along the equator, between the node closest to the launch opportunity being considered and the longitude where the orbit crosses the lauch site latitude
Term
Lo
Definition
launch-site latitude; the length of the local longitude line between the launch site and the equator
Term
β
Definition
launch azimuth; the direction to launch; angle from true north at the launch site, clockwise to the launch direction
Term
Computing β and LWST depends on what 3 things?
Definition

1. Direct or indirect orbit

2. Ascending node or descending node

3. Launch-site in northern or southern hemisphere

Term
If Lo > i how many launch opportunities per day?
Definition
0
Term
perturbations
Definition
changes in COEs caused to unintentional outside forces
Term
Which 2 COEs change to atmospheric drag and how do they change?
Definition
a (smaller); e (smaller, AKA tried to become a circle)
Term
oblateness
Definition
Because Earth is spinning on an axis, and is mostly comprised of water, it is not actually spherical; instead it is "squashed" or "oblate"
Term
J2 effect
Definition
J2 is a constant describing the size of the bulge in the mathematical formulas used to model the oblate Earth
Term
What 2 COEs change due to Earth's oblateness and how?
Definition
Ω, at its nodal regression rate which causes it to decrease in prograde orbits and increase in retro grade orbits (also increases if the orbit is low in altitude and low in inclination, no affect on polar orbits); ω, at its perigee rotation rate (usually given)
Term
sun-synchronous orbit
Definition
takes advantage of eastward nodal progression in order to make the orbital plane maintain the same orientation to the Sun
Term
Molniya orbit
Definition
Russian for "quick as lightening;" inclination is 63.4° so that the ω doesn't change due to Earth's oblateness, and was useed to get great coverage of the Northern hemisphere where the apogee is
Term
solar radiation pressure
Definition
can cause long-term orbital perturbations and unwanted spacecraft rotation
Term
third-body gravitational effects
Definition
(Moon, Sun, planets, ect) can perturb orbits at high altitudes and on interplanetary trajectories
Term
unexpected thrusting
Definition
cause by either outgassing or malfunctioning thrusters; can perturb orbits or cause spacecraft rotation
Term
Hohmann Transfer
Definition
the most fuel efficient transfer; uses an ellitptical transfer orbit tangent to the initial and final orbits
Term
4 assumptions to Hohmann transfers
Definition

2 instantaneous burns

tangential orbits

in the orbital plane

co-apsidal orbits (major axis aligned)

Term
impulsive burn
Definition
instantaneous burn
Term
transfer orbit
Definition
to get from 1 orbit to the transfer orbit, we change the orbit's energy (by an amount ΔV1) and then when the spacecraft gets to orbit 2, we change its energy again (by changing its velocity by an amount ΔV2)
Term
ΔV
Definition
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