4
$\begingroup$

This answer shows the first image below of a vintage view of the ISS. Only one of each triplet of thermal radiators is extended from each TRRJ.

The ISS' solar panels articulate to modulate the incidence of sunlight in order to present maximum cross-section when power is needed most and possibly less cross-section when power generation isn't needed and power is dumped into ballast resistors, as well as to sometimes decrease aerodynamic drag and need to reboost when the station is in eclipse.

But my thinking is that

  1. radiators are designed to be fairly solar-blind so presumably one wouldn't gain a lot by tilting them away from the Sun,
  2. The position of the Earth (also a source of radiative loading) is always the same relative to the ISS' orientation, or at least it better be!
  3. "the cold of space" in which to radiate is always the upper half hemisphere (see 2.)

So why do the ISS' TRRJs need to tilt?

From Wikipedia's External Active Thermal Control System :

Heat Rejection - Ammonia passes from the ATA through a two way path of the Flex Hose Rotary Coupler (FHRC) where heat captured while passing through the Heat Exchangers is directed to be expelled through the Heat Rejection System Radiators (HRSRs). The radiators are rotated by the Thermal Rotary Radiator Joint (TRRJ) which continually rotates the radiator wing for optimum cooling.

enter image description here

enter image description here

Source open for full size view!

enter image description here

Source

$\endgroup$
  • 2
    $\begingroup$ You might want to re-examine assumption #1. $\endgroup$ – Organic Marble Feb 11 at 4:53
  • 1
    $\begingroup$ @OrganicMarble that's exactly why I've put it out there as part of the question. $\endgroup$ – uhoh Feb 11 at 5:14
  • 2
    $\begingroup$ It's late here, more tomorrow. $\endgroup$ – Organic Marble Feb 11 at 5:15
6
$\begingroup$

The "tilt angle" of the beam that the radiators are mounted on is called "gamma" in ISS parlance. The device is called the Thermal Radiator Rotary Joint (TRRJ) and the part of the device that passes the fluid connections across to the moving part is called the Flex Hose Rotary Coupler (FHRC).

("Alpha" is the rotation angle of the outer truss segments that the solar panels are based on, and "beta" is the rotation angle(s) of the solar arrays on their long axes. These are the three main ISS articulation angles)

The overall goal for gamma is to keep the radiator facing edge-wise to the Sun during the Sun side of the orbit and face-wise to the Earth during the shadow side of the orbit. Software to accomplish this (called Radiator Goal Angle Calculation (RGAC)) ensures the radiators stay cold enough so the heat can be rejected but warm enough so that the ammonia does not freeze.

However, just as in the solar panel positioning (see How are the orientations of the ISS' eight independent solar arrays optimized?) there are complications.

A major complication is that the FHRC, unlike the solar array joints, cannot rotate continuously. The solar array joints utilize slip rings, but the FHRC uses hoses that wrap and unwrap, and they cannot do so forever. So at some point, the software must command a "radiator flip" to unwind the FHRC.

So the RGAC uses GNC inputs to calculate the Theoretical Gamma to meet the overall goal, plus complications: For example, it tries to avoid flipping the radiators too soon before entering eclipse or near orbital noon. It also tries to avoid excessively fast rotations. After all the complications are taken into consideration, a commanded gamma angle is issued to the hardware.

This picture shows the underside of the truss and a crewmember working on a FHRC.

enter image description here

This graphic shows the hose wrap limits inside the FHRC.

enter image description here

Source: this paper page 16 and personal notes.

$\endgroup$
  • 2
    $\begingroup$ oh what a tangled hose we weave... I'm glad I didn't have to write that software! I noticed from your source that the panels are coated with Z-93 which I am sure has been discussed in older posts; this or some similar material (high visible reflectance plus high thermal-infrared emittance) See also ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990021250.pdf $\endgroup$ – uhoh Feb 11 at 14:59
  • 2
    $\begingroup$ While this goes into great detail regarding how the TRRJs are intended to operate, in practice, they just park them at specific angles for weeks or months at a time. I think they used to do autotrack, but they don't anymore. $\endgroup$ – Tristan Feb 11 at 21:42
  • 1
    $\begingroup$ @Tristan interesting, another example of how my information is becoming obsolete :) Consider posting an answer about the current operations. $\endgroup$ – Organic Marble Feb 11 at 22:13
  • 1
    $\begingroup$ @Tristan please do! keep an eye on "assumption #1" in the question ;-) If the answer to "Why do the ISS' Thermal Rotary Radiators need to tilt?" ends up being "It turns out they don't need to." then that's the answer! $\endgroup$ – uhoh Feb 11 at 23:27
  • 1
    $\begingroup$ Or perhaps the cooling needs are reduced with only three crew on board, allowing the thermal system to be placed in an orientation that is less than optimal as far as cooling capabilities are concerned but also reduces wear & tear. $\endgroup$ – David Hammen Feb 12 at 17:24

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.