# How was the Space Shuttle Orbiter used for ISS Reboost?

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.

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?

• Great question!!! – uhoh Oct 24 '19 at 8:44

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).

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).

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:

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.

• The jets used were tiny. Total force < 100 lbf. – Organic Marble Oct 24 '19 at 14:14
• I'm stating what actually was done. – Organic Marble Oct 24 '19 at 14:47
• @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
• 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
• @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 2:43