Issue 111

After the loss of Space Shuttle Columbia (1 Feb 2003), a pre-docking with ISS maneuver was designed and implemented to give the ISS crew a close up look at the condition of the Shuttle before it docked. It was called the R-bar pitch maneuver (RPM), some folks called it the rendezvous pitch maneuver or backflip. The RPM was a maneuver performed by the Space Shuttle as it rendezvoused with the ISS prior to docking. After loss of Columbia, 21 Shuttle missions went to ISS and did the RPM before docking. The Shuttle performed a backflip to show the ISS crewmembers all surfaces of the Shuttle after it achieved orbit. They wanted to ensure there was no damage while ascending to orbit. The key thing they were looking for was the condition of the underside and leading edge where extreme heat is experienced during re-entry. Based on the insight and information gained during the rendezvous pitch maneuver, NASA could decide if the Shuttle was not safe for entry at the end of its mission with ISS. They may have then decided either to wait on the ISS for a rescue mission or do a spacewalk to attempt to repair the damage. Here’s a short video of the Shuttle (STS-120) doing the RPM:

The RPM gives me a chance to use the Ten One Rule of Thumb that gives us some simple rule of thumb rendezvous and proximity operations insight. Basically, the Shuttle arrives just below the ISS, about 200 meters below (on the R-bar, the NASA coordinate frame used has Radial down, and in a circular orbit). Before it drifts forward due to being below ISS, the Shuttle then does the RPM or backflip as the ISS crew photos and inspects the condition of the Shuttle. ISS is in about a 400 KM circular orbit and the Shuttle is coplanar with it, but arrives at 200 meters below. The RPM talks a few minutes and then the Shuttle then maneuvers up to a position in front of the ISS, on the V-bar…the ISS Velocity Vector.

Well, here’s a scenario that they didn’t do, but it enables me to do a refresher on the Ten One Rule of Thumb. So, what if the Shuttle didn’t move to the “in front on the V-bar position?” It would move ahead of ISS per the Ten One rule of thumb, which says per orbit (ISS orbital period in this case is 92.4 minutes) the distance moved in-track will be TEN TIMES the radial offset. So, that means after 92.4 minutes the Shuttle will have advanced ahead 2000 meters (or 2 Km) … which is 10 x the 200 meter radial offset. Hooray for the TenOne rule of thumb.

Let’s see if you got this Ten One rule of thumb down. Here’s another “they didn’t do it that way” scenario, but, it’s a “do you know your TenOne rule of thumb?” What if the Shuttle was 1 Km below the ISS orbit and 30 Km behind right now. It’s in a circular orbit that was matched in orbit plane. When would it move to a position below ISS? Using the Ten One rule of thumb we see the Shuttle will advance 10 Km per orbit (why? Because it’s 1 Km below, so 10 X 1 Km, gives us the 10 Km per orbit (92 minutes or so). How long until the Shuttle is below ISS and can execute the RPM? It takes 3 orbits and using the ISS orbit period of 92.4 minutes we see that’s 277 minutes (4:37). This enables the Shuttle to close the 30 Km behind the “just below the ISS” position. Now, like I said, this is not how they did it, but you see how we apply the TenOne rule of thumb. Actually, the Shuttle would be too far away for a effective RPM inspection. I bet you are all saying “well, let’s do a Hohmann Transfer and raise our altitude to the 200 m below altitude. Yup, we could do that and hooray for you remembering Walter Hohmann’s transfer.

Your Ten One Rule of Thumb Refresher is complete and you also learned one of the key flight safety things done by the Shuttle late in its life supporting the ISS operations.