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A rendezvous usually takes place in orbit, e.g. when spacecrafts are travelling to a space station. If two spacecraft are close enough to each other (< 300m) and travel in similar orbits they are said to rendezvous. In such a situation, both spacecraft can stay close to the space station with minimal corrections (linear [[RCS]]).
 
A rendezvous usually takes place in orbit, e.g. when spacecrafts are travelling to a space station. If two spacecraft are close enough to each other (< 300m) and travel in similar orbits they are said to rendezvous. In such a situation, both spacecraft can stay close to the space station with minimal corrections (linear [[RCS]]).
  
There are two ways to bring about rendezvous:
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There are two possible ways to get to rendezvous:
 
* direct insertion into the space station's orbit, or
 
* direct insertion into the space station's orbit, or
 
* insertion into a catch-up orbit.
 
* insertion into a catch-up orbit.
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===Catch-up orbit===
 
===Catch-up orbit===
[[Image:Catch-up_orbit.gif|right|thumb|250px|Example of a catch-up orbit]]
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[[Image:Transfer_orbit.gif|right|thumb|250px|Example of a transfer orbit]]
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The best launch window for a catch-up orbit is one that places the ship into the same orbital plane as the target but with a significantly lower orbit. The important measure is the difference between the [[large semiaxis|large semiaxes]], since the large semiaxis defines the time needed for one revolution around [[Earth]].
The best launch window for a catch-up orbit is one that places the ship into the same orbital plane as the target but with a significantly lower orbit. The important measure is the difference between the [[semi-major axis]], since the semi-major axis defines the time needed for one revolution around [[Earth]].
 
  
 
The point at which the spacecraft should leave the (almost circular) catch-up orbit can be calculated using this formula:  
 
The point at which the spacecraft should leave the (almost circular) catch-up orbit can be calculated using this formula:  
  
<math>\Delta \varphi = 2 \pi \left [ 1 - ( 2 m - 1 ) \left ( \frac {a}{r_z} \right )^{\frac{3}{2}} \right ] </math>
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<math>\Delta \varphi = \pi \left [ 1 - ( 2 m - 1 ) \left ( \frac {a}{r_z} \right )^{\frac{3}{2}} \right ] </math>
  
 
where
 
where
 
* <math>\Delta \varphi</math> - the [[distance angle]] between the chasing spacecraft and the target spacecraft.
 
* <math>\Delta \varphi</math> - the [[distance angle]] between the chasing spacecraft and the target spacecraft.
 
* <math>m</math> - the number of [[apoapsis|apogee]] passes on the [[transfer ellipse]].
 
* <math>m</math> - the number of [[apoapsis|apogee]] passes on the [[transfer ellipse]].
* <math>a</math> - the semi-major axis of the transfer ellipse.
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* <math>a</math> - the large semiaxis of the transfer ellipse.
 
* <math>r_z</math> - the radius of the target orbit (assumed circular).
 
* <math>r_z</math> - the radius of the target orbit (assumed circular).
  
When this angle is reached, the chasing spacecraft performs a prograde burn to enter the planned transfer ellipse. This is called the ''intercept burn''.
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When this angle is reached, the chasing spacecraft performs a prograde burn to enter the planned transfer ellipse. This is called the ''intercept burn''.  
  
 
===Rendezvous burn===
 
===Rendezvous burn===
[[Image:Target first rendezvous.gif|right|thumb|250px|Example of correcting a late arrival of the chaser]]
 
[[Image:Chaser first rendezvous.gif|right|thumb|250px|Example of correcting a early arrival of the chaser]]
 
 
 
When reaching the final apogee pass of the transfer ellipse, all possible errors will usually bring the spacecraft away from its desired course (including errors because the initial orbits have not been perfectly circular). Because of that influences, the next orbit is usually not the final target orbit, but again a less eccentric chasing orbit. The parameters of this orbit can be calculated using the following formula:
 
When reaching the final apogee pass of the transfer ellipse, all possible errors will usually bring the spacecraft away from its desired course (including errors because the initial orbits have not been perfectly circular). Because of that influences, the next orbit is usually not the final target orbit, but again a less eccentric chasing orbit. The parameters of this orbit can be calculated using the following formula:
  
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* The target arrives at the target point before the chasing spacecraft:
 
* The target arrives at the target point before the chasing spacecraft:
 
<math>a = r_z \cdot {}^3 \sqrt {\left ( 1 - \frac{1}{n} \cdot \frac{ \Delta \varphi}{2 \pi}\right )^2}</math>
 
<math>a = r_z \cdot {}^3 \sqrt {\left ( 1 - \frac{1}{n} \cdot \frac{ \Delta \varphi}{2 \pi}\right )^2}</math>
 
The variable <math>n</math> again describes, how many orbits the chasing craft should wait in the immediate orbit until it meets the target. 
 
 
If the time to rendezvous is shorter than one [[orbital period]], its possible to ignore most orbital mechanics and instead assume both craft are in a gravity-free space. That concept gets used very often in various [[radar]] rendezvous guidance systems ([[Gemini]], [[Soyuz]]). Instead of making orbital maneuvers, which would need fast computers to calculate, the chasing spacecraft uses a [[Doppler effect|doppler radar]] to measure the angle, distance and rate of change of the distance. By keeping the angle between [[line of sight]] to the target (measured by the radar) and its own velocity vector constant ([[proportional guidance]]), the chasing spacecraft approaches the target very effectively.
 
 
When better computers are available, its also possible to solve [[Hills equations]], which describe the movement of an object relative to another in an orbit. This is more effective in terms of fuel and more accurate, but requires better onboard computers, as it requires solving differential equations.
 
  
 
===Rendezvous===
 
===Rendezvous===
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When the other spacecraft is far away (such as on the other side of the planet), a more fuel-efficient way to get there may be via a ballistic trajectory. This can be thought of as "throwing" the spacecraft towards the target, and letting it fall close to it. On planets with no atmosphere the [[Map MFD]] can be used to estimate and adjust the ballistic trajectory and the landing point.
 
When the other spacecraft is far away (such as on the other side of the planet), a more fuel-efficient way to get there may be via a ballistic trajectory. This can be thought of as "throwing" the spacecraft towards the target, and letting it fall close to it. On planets with no atmosphere the [[Map MFD]] can be used to estimate and adjust the ballistic trajectory and the landing point.
 
The first and so far only historic example of a surface rendezvous was the landing of [[Apollo 12]], where its LM landed only about 200 m away from its target, the lunar probe [[Surveyor]] 3.
 
 
Another kind of "surface rendezvous" happened in August 1990 in the [[Kennedy Space Center]]. When the [[space shuttle]] Atlantis (STS-43) rolled back from [[pad 39]] to the [[VAB]] for repairs, after a fuel leak and hail damage, it passed close to the space shuttle Discovery (STS-41), which headed for the same pad for its scheduled launch.
 
 
==See also==
 
* [[Rendezvous MFD]], an [[Orbiter]] implementation of [[Hills equations]] in a MFD.
 
* [[STS guidance MFD]] and [[Soyuz guidance MFD]], two similar MFDs which implement automatic rendezvous and station keeping.
 
 
{{HasPrecis}}
 
 
[[Category: Articles]]
 
[[Category:Celestial mechanics]]
 

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