Editing Space rendezvous (section)

==== R-bar approach====
The R-bar approach consists of the chaser moving below or above the target spacecraft, along its radial vector. The motion is [[orthogonal]] to the orbital velocity of the passive spacecraft.<ref name=wertz2003/><ref name=pearson1989/><!-- ref 'pearson1989' provides examples of V-bar and R-bar approaches, without definitions -->
When below the target the chaser fires radial thrusters to close in on the target. By this it increases its altitude. However, the orbital velocity of the chaser remains unchanged (thruster firings in the radial direction have no effect on the orbital velocity). Now in a slightly higher position, but with an orbital velocity that does not correspond to the local circular velocity, the chaser slightly falls behind the target. Small rocket pulses in the orbital velocity direction are necessary to keep the chaser along the radial vector of the target. If these rocket pulses are not executed (for example due to a thruster failure), the chaser will move away from the target. This is a ''natural braking effect''. For the R-bar approach, this effect is stronger than for the V-bar approach, making the R-bar approach the safer one of the two.{{citation needed|date=June 2014}}
Generally, the R-bar approach from below is preferable, as the chaser is in a lower (faster) orbit than the target, and thus "catches up" with it. For the R-bar approach from above, the chaser is in a higher (slower) orbit than the target, and thus has to wait for the target to approach it.{{citation needed|date=June 2014}}

[[Astrotech Corporation|Astrotech]] proposed meeting ISS cargo needs with a vehicle which would approach the station, "using a traditional nadir R-bar approach."<ref>
{{cite conference
 |last        = Johnson
 |first       = Michael D.
 |author2     = Fitts, Richard
 |author3     = Howe, Brock
 |author4     = Hall, Baron
 |author5     = Kutter, Bernard
 |author6     = Zegler, Frank
 |author7     = Foster
 |author8     = Mark
 |title       = Astrotech Research & Conventional Technology Utilization Spacecraft (ARCTUS)
 |book-title  = AIAA SPACE 2007 Conference & Exposition
 |page        = 7
 |place       = Long Beach, California
 |date        = September 18, 2007
 |url         = http://pdf.aiaa.org/preview/CDReadyMSPACE07_1808/PV2007_6130.pdf
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20080227050750/http://pdf.aiaa.org/preview/CDReadyMSPACE07_1808/PV2007_6130.pdf
 |archive-date = February 27, 2008
 |df          = mdy-all
}}</ref> The nadir R-bar approach is also used for flights to the ISS of [[H-II Transfer Vehicle]]s, and of [[SpaceX Dragon]] vehicles.<ref>''Rendezvous Strategy of the Japanese Logistics Support Vehicle to the International Space Station,'' [http://adsabs.harvard.edu/full/1997ESASP.381..103Y] {{Webarchive|url=https://web.archive.org/web/20210505175034/http://adsabs.harvard.edu/full/1997ESASP.381..103Y|date=May 5, 2021}}</ref><ref>''Success! Space station snags SpaceX Dragon capsule'' [http://news.cnet.com/8301-11386_3-57441570-76/success-space-station-snags-spacex-dragon-capsule/] {{Webarchive|url=https://web.archive.org/web/20120525171424/http://news.cnet.com/8301-11386_3-57441570-76/success-space-station-snags-spacex-dragon-capsule/|date=May 25, 2012}}</ref>