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According to this answer, for the question How often does ISS require re-boosting to higher orbit?,

During Space Shuttle years, small re-boosts were also performed by the Shuttle Orbiters (according to Wikipedia, they had 232 kg of fuel available for that)...

Which engines did it use for this purpose? I think, using the forward or rear reaction control thrusters would cause lateral stresses on the docking port, due to the torque provided, since the thrust vector doesn't pass through the centre of mass of the whole system (ISS+Space Shuttle Orbiter). Or in other words, if these engines are fired they will cause the ISS to spin about its centre of mass, in addition to the re-boost (which will be less effective as at some time the engines will fire retrograde due to the spin).

The following image shows the Space Shuttle Endeavour docked to the International Space Station (ISS), flying at an altitude of approximately 354 km (220 miles), and was taken by Expedition 27 crew member Paolo Nespoli from the Soyuz TMA-20 following its undocking on May 23, 2011.

enter image description here

It can be seen, that neither forward or rear reaction control thrusters can't be used alone. They must be used in addition to other thrusters to nullify the torque, and I think that will be an energy-intensive process. Instead, they could have transferred the fuel from the orbiter to the propulsion module of the station, where the thrust vector is along the centre of mass and doesn't cause any torques. I think, there were no thrusters powerful enough facing the heat shield side to provide orbital re-boost. So, How was the Space Shuttle Orbiter used for the same?

Vishnu
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1 Answers1

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No propellant was ever transferred from the Orbiter to the ISS.

Shuttle reboosted ISS using the Reaction Control System (RCS) jets.

The small 24 lbf vernier RCS jets were used.

The steps for executing the reboost were called out in the Flight Plan. Here's an example from STS-130. Note that it was done at the very end of the docked phase (straddling the Farewell ceremony).

enter image description here

The first procedure executed H/O ATT CONTROL CMG TA to ORB gives control of the entire mated stack to the Orbiter's control system.

This procedure is number 3.110 in the International Space Station ISS/Shuttle Joint Operations Book and takes the following actions:

  1. Verified the Orbiter control system was in free drift
  2. Configured the ISS control system to free drift
  3. Gave mated stack control to the Orbiter using VRCS jets

The next procedure MNVR REBOOST maneuvers the stack under Orbiter VRCS control to the reboost attitude specified on the 2nd line and sets the Digital Autopilot (DAP) to maintain that attitude. The Orbit Ops Checklist shows a diagram of the attitude. Note Configuration 3 is called out in the Flight Plan (for the next step).

enter image description here

Next the procedure AUTO REBOOST from the Orbit Ops checklist is executed. The Digital Autopilot settings are also found in the Orbit Ops checklist as follows:

enter image description here

Two down-firing jets were used simultaneously on this flight.

Other VRCS jets fired as needed to control undesired rotations of the stack and maintain the commanded attitude.

After the reboost was complete, control of the stack was handed back to the ISS control system.

Organic Marble
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  • According to my understanding of your answer, the Space Shuttle pulls the station and not pushes it, since the thrusters used point in the direction of the station and not away from it. If this is the case, will in not cause additional stress on the docking port? – Vishnu Oct 24 '19 at 14:14
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    The jets used were tiny. Total force < 100 lbf. – Organic Marble Oct 24 '19 at 14:14
  • Thanks, but I think pushing the station is better than pulling it. I don't want to harm the locks in the docking ports in addition to the air pressure inside. Was the shuttle equipped with thrusters which can push the station in docked mode (I don't find any thrusters on the heat shield)? If yes, why didn't they use these instead of pulling? – Vishnu Oct 24 '19 at 14:20
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    I'm stating what actually was done. – Organic Marble Oct 24 '19 at 14:47
  • Ok, thanks. May I ask what were those removed images showing different orientations relative to earth? Is that how the station rotates on firing the thrusters? – Vishnu Oct 24 '19 at 14:53
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    @Intellex The 75 pounds of thrust applied through the docking port is nothing compared to the force of the air pressure (14.7 psi $\times \frac{pi}{4}$ * (31 in)$^2$ $\approx$ 11,000 lb) trying to blow the port apart. – Tristan Oct 24 '19 at 17:40
  • Thanks for the update. Could you please tell what is meant by "free drift"? Does it mean that the hatches on both sides were closed or in other words - ready to undock - configuration? – Vishnu Oct 25 '19 at 04:56
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    When a vehicle is in "free drift," it means that it's set not to control itself with its thrusters. You put both vehicles into drift and then set the shuttle orbiter to take control. – Erin Anne Oct 25 '19 at 10:08
  • I don't understand how the four configurations are used. Does the process cycle through them one after another? I don't understand the 4° at all, and no amount of coffee has helped. – uhoh Oct 26 '19 at 02:41
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    @uhoh they are 4 different sets of parameters for different uses. Only CONFIG 3 was used for this reboost task. It is called out explicitly in the flight plan. – Organic Marble Oct 26 '19 at 02:43
  • @OrganicMarble, Could you please tell whether the hatches on both sides were closed during this procedure? – Vishnu Oct 26 '19 at 16:27
  • @Intellex referring to page 2-34 in the linked Flight Plan, you can see hatch closure after the reboost. – Organic Marble Oct 26 '19 at 16:29
  • Thanks for your reply. I actually meant, closing the hatch for additional safety during reboost - In case the shuttle undocks accidentally due to force loads (the previous comments have stated this force is small compared to the atmospheric pressure, but I think the mass of station need to be considered), closed hatches prevent depressurization on both sides. Am I overthinking on this fact? – Vishnu Oct 26 '19 at 16:33
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    Maybe overthinking it a little. The ISS is big and floppy, they don't push on it too hard. Unless they mess up, of course: http://www.nbcnews.com/id/28998876/ns/technology_and_science-space/t/shaking-space-station-rattles-nasa/ – Organic Marble Oct 26 '19 at 16:40
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    @OrganicMarble I have heard secondhand stories of ISS structural engineers accusing GNC engineers of deliberately finding resonant modes with their attitude maintenance. As a GNC person I found the idea delightful :D – Erin Anne Oct 27 '19 at 01:36