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When performing space rendezvous operations close to the target vehicle (0-5 m relative distance), pursuer thruster escape nozzle gases can hit the target if firings are done in the direction of docking axis. Eventually, this firings could damage the target surfaces (by heating or erosion), or provoke some attitude and orbit changes of the target (slightly).

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My question is: how current and past space vehicles with rendezvous capabilities (Soyuz, ATV, Dragon, Space Shuttle,etc) have deal with this issue since following an R-bar or V-bar rendezvous requires to perform braking firings when the pursuer is very close to the target?. Are target surfaces (ISS,etc) designed to cope with this plumes?.

Figure extracted from: "Fast, Safe, and Propellant-Efficient Spacecraft Planning under Clohessy-Wiltshire-Hill Dynamics"

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  • $\begingroup$ Heat should be no problem if cold gas thrusters are used. $\endgroup$ – Uwe Nov 22 '17 at 10:54
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    $\begingroup$ Shuttle used a "low-z" mode where the rcs jet selection logic minimized the plumes along the z axis (towards the target). Shuttle's jets were set at rather odd angles making this possible. $\endgroup$ – Organic Marble Nov 22 '17 at 13:01
  • $\begingroup$ @OrganicMarble, could you provide link of a document, web, etc, explaining the selection logic you are talking about?. $\endgroup$ – Julio Nov 22 '17 at 16:35
  • $\begingroup$ I have a vague memory of a Gemini program documentary where astronauts had to alert mission control to the firing of the RCS during EVA but unfortunately I have no references for it. In the meantime I did a search and found this ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930013116.pdf relating to the STS / Space Station Freedom approach. $\endgroup$ – Puffin Nov 22 '17 at 21:53
  • $\begingroup$ I'm not in a position to do amy research for the next few days (camping trip) If you can't google up anything on low z I'll post something when I get back. $\endgroup$ – Organic Marble Nov 22 '17 at 22:29
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NOTE: My answer applies specifically to Space Shuttle (STS) operations.

In general, it is quite safe to say that it is never desireable that the chaser plume the target to any significant degree during rendezvous/proximity operations.

A cursory overview of the Space Shuttle Orbiter's Reaction Control System (RCS) is shown below (page taken from a 2002 version of the Shuttle Crew Operations Manual).

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The next diagram is a slide presented during a 1996 briefing given to an Astronaut Candidate class. It gives the reader an idea about the orientation of the various jets.

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The "normal" way to thrust toward the Orbiter's +Z axis was to utilize the upward pointing RCS jets, naturally (the next two images below are extracted from the Rendezvous/Proximity Operations Workbook (dated 1985):

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When suitably close to the target, the crew would select "LOW Z" mode, which operated as explained below:

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    $\begingroup$ It looks like the thrusters essentially vector diagonally to the x-z directions in such a way that the thrusters avoid impinging the target and so that the net force results in the z direction only. Is my understanding correct? $\endgroup$ – Paul Nov 25 '17 at 2:37
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    $\begingroup$ @Paul, you are correct - and you can see that this is a rather inefficient way to induce an acceleration in the +z direction. Inefficient but necessary... $\endgroup$ – Digger Nov 25 '17 at 6:10
  • $\begingroup$ This answers my question. I imagine that other space vehicles with rendezvous capabilities have similar procedures to the Space Shuttle one. $\endgroup$ – Julio Nov 27 '17 at 11:24
  • $\begingroup$ I'm curious - why not slow to a safe speed while still outside the danger bubble, and let the orbiter drift over to and grapple the target? $\endgroup$ – Vikki Feb 29 '20 at 19:09
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    $\begingroup$ @Sean While some docking operations (e.g. Space Shuttle Orbiter/ISS) require a closing velocity along a very specific direction to enable latching, "free-flyer" grapple operations (specifically, in said Orbiter case, utilizing the RMS) required that relative velocity be practically nulled. Due to the constraints of orbital mechanics, "drifting over" to the target in a such way that all relative velocities and accelerations are nulled when grapple distance is achieved, is quite difficult, if not impossible to pull off. $\endgroup$ – Digger Mar 1 '20 at 20:19

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