# What does “supersonic large amplitude ID maneuver PTI” mean, and what does a tumble motor do?

In this question I linked to a YouTube video of the Ares X-1 launch. You can start listening at 02:00 for the point where these are mentioned:

"Supersonic large amplitude ID maneuver PTI" and "a tumble motor ignition"

What do these mean?

cued at 02:00:

"tumble motors" were solid rocket motors similar to the Booster Separation Motors used on regular shuttle SRBs, but mounted in such a way to cause the booster to tumble in an attempt to prevent it from stabilizing aerodynamically on the downward leg of the trajectory. (from the "first stage" part of this article) Normal shuttle SRBs weren't fitted with tumble motors, but early shuttle External Tanks (ETs) had a tumble valve which was supposed to serve the same purpose for the ET by venting residual gas in a radial direction. The valves were problematic and were deleted after a few flights1, apparently without much effect. Tumbling is desired during reentry to help ensure that the entry trajectory is predictable "since satellites during a tumbling re-entry generally do not produce significant average lift forces acting against gravity [and therefore] a simplified ballistic re-entry approach can be used to model the trajectory and flight dynamics." (from Safety Design for Space Operations, Chapter 9, Re-entry Operations Safety)

A PTI is a Programmed Test Input, used throughout the Shuttle program to introduce small preplanned deviations into the control system or trajectory and take data on the resulting response of the system. The specifics of the PTIs on this flight are described in this paper. There were 3 called out on the sound track, all are in the paper. The one you mention is described as

... a large amplitude (nominal peak 0.35 degree) sweep from 75-85 seconds and is intended to identify aerodynamic response as well as control stability margins.

Much, much more detail is available in the paper if you are interested. I've excerpted one graph from the paper showing some actual trajectory data versus what it would have been if the PTI hadn't been incorporated.

1 Space Shuttle Missions Summary page 2-38

• The paper is incredible! There is an amazing amount of explanation there, and the figures are both beautiful and informative. For the tumble motors - I don't understand the benetif(s) of avoiding aerodynamic stabilization - is it possible to add something? – uhoh Apr 26 '16 at 2:19
• I believe it's simply that a tumbling body averages out to no lift and therefore is more predictable whereas a non tumbler could theoretically stabilize in some kind of lifting attitude and fly off unexpectedly. I will look for a reference. – Organic Marble Apr 26 '16 at 2:24
• @uhoh reference located, sentence added to answer. – Organic Marble Apr 26 '16 at 3:04
• Wow! Now I regret not breaking that out as a separate question, something like: "Why is it important that large objects tumble upon re-entry?" so that it would stand out more - I don't see any questions about re-entry tumble, and yet it's obviously a pretty important concept to people who have to deal with re-entry (and unwilling participants as well). I'm not sure if I still have my "Skylab is Falling!" tee-shirt, but it was definitely a news item in many countries back then. – uhoh Apr 26 '16 at 5:02
• @uhoh, indeed, Skylab was commanded to tumble for entry. spaceref.com/iss/skylab.deorbit.html – Organic Marble Apr 26 '16 at 20:15