How dangerous are RCS thrusters, in a vacuum, to A. other spacecraft, especially when docking, and B. astronauts in EVA suits? How far away do you have to be to be safe / do RCS thrusters need to be rigorously shut down and safed before an EVA is carried out near them? Does the gas from these very small hypergol engines expand quickly enough to be minimally hazardous if it impinges on something?

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    $\begingroup$ Combustion in a rocket engine is never perfect, which means that an RCS thruster that uses hydrazine or a hydrazine derivative as a monopropellant might deposit some rather nasty chemicals on the target vehicle or EVA crew member, some of which are carcinogenic. The deposition problem can be even worse in the case of biprop RCS thrusters that use (for example) a hydrazine derivative and oxides of nitrogen. Depositing red fuming nitric acid on a vehicle or an EVA crew member is an extremely bad idea. $\endgroup$ Feb 28, 2020 at 18:21
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    $\begingroup$ @David Hammen would this deposited stuff not just evaporate into space in short order? $\endgroup$
    – ikrase
    Feb 28, 2020 at 20:14
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    $\begingroup$ @ikrase it's a serious concern. See pdf page 148 of the EVA checklist here nasa.gov/centers/johnson/pdf/492872main_EVA_G_H_20.pdf $\endgroup$ Feb 28, 2020 at 20:29
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    $\begingroup$ @OrganicMarble Looks like it indeed is, though it maybe can be mitigated by "washing" the space suit with a heat lamp? $\endgroup$
    – ikrase
    Mar 1, 2020 at 2:04
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    $\begingroup$ @ikrase there have been a couple incidents with NH3 on the ISS. I think they have just baked out in the sun. csmonitor.com/Science/2015/1106/… $\endgroup$ Mar 1, 2020 at 2:51

3 Answers 3


For Shuttle EVA, the Space Shuttle Flight Rules show that the safe distance for a suited crewmember was 27 feet for the main jets, 3 feet for the vernier jets, and 3 feet for the APU exhaust. It's rule A-15-22 in the document. This rule is probably a simplified version of what would actually be managed to; I remember seeing "Keep Out Zones" (KOZ) defined for all kinds of things on the Orbiter.

The buzzword for this is "plume impingement" if you want to google for other specific cases.

Extracted from an answer to In space, how far from the nozzle is the exhaust from a space-craft capable of causing damage? to focus on just the RCS. If the crew needed to EVA inside the KOZ, the jets in question would be disabled to keep them from firing. From the flight rule rationale:

These distances are conservative estimates based on the thermal effects of the jets on the EMU, particularly the visor, which is most sensitive to thermal extremes. These numbers were generated from analytical tests which modeled the heat flux from the respective jets and the thermal characteristics of the visor. Crew injury/EMU damage may result from hydrazine brought into the cabin after being picked up from an operating APU. Flight Rule {A6-151A}, RCS JET DRIVER MANAGEMENT [CIL], requires primary RJD’s to be powered off while the crew is outside the payload bay envelope. The EVA checklist “CONTINGENCY DAP/JET CONFIG” directs the crew as to which jets to inhibit or power off based on EVA crew location.

This answer describes the shuttle RCS, thrust levels, etc.

An example of the Primary RCS KOZ generated in DOUG.

enter image description here

For shuttle plume impingement on other vehicles see this answer.

  • $\begingroup$ Would shuttle payloads with their own RCS also produce their own keep-out zones while packed in/docked to/being grappled by the orbiter (to guard for the possibility of an uncommanded payload RCS firing)? $\endgroup$
    – Vikki
    Feb 29, 2020 at 19:07
  • $\begingroup$ @Sean True answer, I don't know. It was a long time ago that Shuttle grappled or deployed anything with jets on it (Hubble had none). We did use SPAS and SPARTAN as payloads in generic robotics training and from looking at those procedures, the jets on the payloads were always inhibited until ungrapple and backaway. The first indication that they had messed up on STS-87 was that the SPARTAN's jets didn't fire after release. $\endgroup$ Feb 29, 2020 at 20:37
  • $\begingroup$ One more question: Pragmatically, how does the actual-danger-zone for the jets flare outward? $\endgroup$
    – ikrase
    Mar 5, 2020 at 6:05
  • $\begingroup$ I'm not sure; from the flight rule I expected a 27 foot radius spherical zone, but the ones in the DOUG graphics are cylinders with cones at the base; that may reflect a more realistic interpretation of the "conservative estimate" mentioned in the rule. In practice I would bet they disabled the jet if the crew was expected to get anywhere near it. $\endgroup$ Mar 5, 2020 at 13:00

A space ship may need some protection against the heat of its own RCS thrusters.

The Apollo Lunar Module was covered with multiple layers of thermal insulation foil. Close to the RCS thrusters the top layers were made from from nickel and the high temperature alloy inconel.

enter image description here

enter image description here

The thermal blanket consists of multiple-layered (at least 25 layers) of aluminized sheet (mylar or H-film). Each layer is only 0.00015 inch thick and is coated on one side with a microinch thickness of aluminum. To make an even more effective insulation, the polymide sheets are hand crinkled before blanket fabrication. This crinkling provides a path for venting, and minimizes contact conductance between the layers. Structures with a high thermal conductivity, such as antenna supports and landing gear members, that pass through the thermal blanket also have thermal protection. Individual blanket layers are overlapped and sealed with a continuous strip of H-film tape.

Mylar sheets are used predominantly in those areas where temperatures do not exceed 300° F. In areas where higher temperatures are sustained, additional layers of H-film are added to the mylar sheets. H-film can withstand temperatures up to 1000° F, but, because it is a heavier material, it is used only where absolutely necessary. Certain areas of the ascent stage are subjected to temperatures as high as 1800° F due to CSM and LM RCS plume impingement. These areas are thermally controlled by a sandwich material of thin nickel foil (0.0005 inch) interleaved with lnconel wire mesh and lnconel sheet.

300 °F is about 150 °C, 1000 °F is 540 °C and 1800 °F is 980 °C.


See this related question.

  • $\begingroup$ This is interesting -- I have never seen a picture of the LM descent stage with dark-colored foil insulation on one side. I knew about the rigid shields of course, but not the heat-resistant foil. Figured that blasting away foil would be more of a problem than melting it. $\endgroup$
    – ikrase
    Mar 5, 2020 at 6:06
  • $\begingroup$ @ikrase Some more images with the dark foil: 1, 2, 3, 4. $\endgroup$
    – Uwe
    Mar 5, 2020 at 11:08

An interesting case occurred during re-entry of the apollo-soyuz CM, where they failed to disable the RCS before opening the snorkel valves to repressurize the cabin. As the outside air rushed in, some RCS exhaust gas also entered the cabin, causing one astronaut to pass out and created breathing difficulties for them for a couple of days afterwards.


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