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A recent tweet by @NASASpaceflight about STS-70 deployment of TDRS-G (TDRS-7) says:

STS-70 - Shuttle Discovery - Launched 13 July 1995 from KSC 39B.

Terence T. Henricks commanded her 21st launch and the last of seven shuttle missions to carry a Tracking and Data Relay Satellite (TDRS) - which were vital for Shuttle/Earth comms, and still are today for the ISS.

The last of four file photos is the one below. What is happening here? What kind of exhaust is this, and in what direction is the thrust? It looks like it's going straight "up" but without depth information it could be "back" as well. Also, what camera might have taken this photo, and where would it be?

enter image description here

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    $\begingroup$ I'm sure @OrganicMarble will have the definitive answer, but I think this is an upward-firing RCS thruster, back-lit by the sun while the orbiter itself is in the shadow of its own wing, photo taken by a crew member from an aft window of the crew compartment through the open payload bay. $\endgroup$ – Russell Borogove Sep 5 '18 at 4:40
  • $\begingroup$ I can add nothing to this, write it up. $\endgroup$ – Organic Marble Sep 5 '18 at 14:02
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    $\begingroup$ Highly related: space.stackexchange.com/questions/8358/… $\endgroup$ – Organic Marble Sep 5 '18 at 14:04
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    $\begingroup$ @OrganicMarble hmm... nota bene all over again, (or is that déjà vu)? Since both the source of the exhaust and the location of the camera is described there, would it make sense to dupe this to that? $\endgroup$ – uhoh Sep 5 '18 at 15:19
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    $\begingroup$ I think it's not quite a dupe because the other question is more general -- it could cover larger upper-stage engines, different propellants, etc. I'm not 100% confident in my analysis of the lighting (and what's the orange glow on the vertical stabilizer?) but I'll go ahead and write it up. $\endgroup$ – Russell Borogove Sep 5 '18 at 15:21
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What is happening here? What kind of exhaust is this, and in what direction is the thrust?

The photo shows the exhaust plume(s) from the upward-facing RCS thrusters on the right-hand OMS pod. There are three R-40A thrusters in that position.

I think the plume is backlit by the sun in this shot, while the body of the orbiter is in the shadow of its own wing; I'm not sure what the orange glow around the vertical stabilizer is. It looks firey, but I've never seen any indication that the stabilizer gets scorched during flight, and such RCS firings are frequent. Possibly some kind of shockwave interaction with the exhaust reflected off the stabilizer?

Nearly half the exhaust product of MMH/NTO combustion is water; the visible plume here is largely made of condensed water droplets and/or ice crystals. Some amount of unburned MMH and NTO would also be present in the plume.

It looks like it's going straight "up" but without depth information it could be "back" as well.

There are rearward-firing RCS thrusters on the pod, but from this angle the plume from those would appear nearly circular, rather than conical, and largely masked by the OMS pod.

Also, what camera might have taken this photo, and where would it be?

This would be a handheld camera operated by a crew member at one of the aft viewing windows on the flight deck. Jenkins says Hasselblad 500EL/S 70mm and Nikon F4S 35mm SLR cameras were normally carried. I don't know if there was a camera mounted in the payload bay itself, but if so it would have been a relatively low-resolution CCTV camera.

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  • $\begingroup$ The position of the aft RCS stingers (they protrude aft from the OMS pods) means that jet impingement on the vertical tail from upfiring jets was inevitable. qph.fs.quoracdn.net/… $\endgroup$ – Organic Marble Sep 5 '18 at 17:21
  • $\begingroup$ I'm just curious about the mechanism by which impingement of a clear/whitish plume on the tail produces orange glow. Ice crystals of unburned props revaporizing/shattering/remixing and combusting? $\endgroup$ – Russell Borogove Sep 5 '18 at 17:43
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    $\begingroup$ It looks a lot like the atomic oxygen glow from the TPS. s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/… My guess is that it's related to that. nature.com/articles/354048a0 The "nature" abstract says that shuttle thrusters "probably" produce a component of the glow. $\endgroup$ – Organic Marble Sep 5 '18 at 18:02

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