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Technical Article A2

Columbia's Final Attitude Changes

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Factors affecting Columbia's final attitude:

The official report does not contain any information on Columbia's final attitude before breakup, but the sudden application of Mach 18 aerodynamic forces laterally to the shuttles airframe is the only stress mode that would result in the sudden and catastrophic breakup that occurred.  Based on Fig. A9 which shows the shuttle as having a constant negative Yaw Moment the initial conclusion would be that Columbia's final Yaw Angle would have been -90˚ (the shuttle traveling at a right angle to the flight path with the right hand side facing windward).

Some observed factors relating to the Columbia's final attitude:

  1. Extensive right hand Yaw Jet firing after 13:59:00.
    (RCS Yaw Jets R2R and R3R fired for a full 8 seconds in an attempt to counter the increasing negative Yaw Rate)

  2. 1Amateur video with close up taken from Colony Texas, (near Dallas).
    (Video appears to shows the Columbia traveling at a right angle to the
    flight path with the left hand side facing windward or Yaw = +90˚ in complete contradiction to what     Fig. A9 would lead us to believe)

  3. Debris field data indicates that the shuttle's left wing broke up before the right.
    (At Yaw = +90˚ the right wing would be protected while the left is exposed to
    the Mach 18 airflow, see Fig. B3)

Based on these observations the effects of the Yaw Jet firing need to be determined.

  1. See, Technical Article TA-A3; Analysis of Reentry Video Taken From Colony, Texas.

 

The effects of yaw jets on flight control:

Fig. TA-A2-1

From document NASA_TM-1987-88281.pdf

Fig. TA-A2-2

From document NASA_TM-1987-88281.pdf, Fig. 29
(Data is from a bank maneuver at Mach 24)

The Plots of either Roll Moment Vs. Time or Yaw Moment Vs. Time as the shuttle performs flight maneuvers should typically look similar to the 5 graphs of Fig. TA-A2-1.  The data should show a repeating pattern that changes from positive to negative as the shuttle rolls left then right and back to the left again.

These diagrams may be used to confirm that no flight maneuvers were performed during STS-107.  When comparing the diagrams to Fig. A9 it is obvious that Fig. A9 does not contain any areas that resemble the curves to the left, even during the period where a roll reversal is said to have occurred.

A set of graphs were derived from empirical data before STS-1 to predict how the RCS Yaw Jets would control the shuttle's yaw rate during reentry.  The derived data matched the actual flight data very closely.  Fig. TA-A2-2 contains two sets of data from a Mach 24 bank maneuver.  The graphs are analyzed here to determine if the relationship between Yaw Jet firing and Yaw Rate can be used to derive a function that can predict the shuttle's yaw rate for yaw jet firings with different parameters.

The increase in Yaw Rate based on the firing of 2 right rear Yaw Jets for a total of 5.5 seconds is 3˚ per second.  Because the relationship between Yaw Rate and time is linear it can be used as a Rate Function to determine how other firing durations would increase or decrease the Yaw Rate.

0.2727˚ per second/duration of jet firing (seconds)/No. of jets

When the shuttle is in an environment free of gravity and atmosphere this Yaw Rate would continue until the opposing Yaw Jets are fired to counter it.  During atmospheric flight other factors such as the use of control surfaces and drag forces would either increase or decrease the Yaw Rate once the jets have stopped firing.  Therefore, during atmospheric flight it is nearly impossible to determine how the Yaw Rate changes after the Yaw Jets stop firing unless the value is recorded.

The derived Rate Function can then be used to determine how Yaw Jet firing events affected Columbia's attitude during reentry.  The most significant jet firing would be events 38 and 39 on Table A3 where Yaw Jets R2R and R3R fired for nearly 8 seconds continuously.  All of the other jet firing events were too short of duration to have an effect on the shuttle's trajectory.

These values are then inserted into the derived Rate Function to find the final Yaw Rate after jet firing,

0.2727 * 8 * 2 = 4.36˚ per second positive yaw

By multiplying the final Yaw Rate produced by the jet firing with the period of time that Yaw Rate was maintained and adding to this the initial Yaw angle whether it was positive or negative will give the final Yaw angle of the shuttle produced by that Yaw Jet firing.

See explanation for RCS thruster location and direction of vehicle travel at bottom of section.

 

Columbia's final attitude:

Columbia acquired a negative Yaw Rate of unknown value early in reentry due to an incomplete flight maneuver.  The left hand elevon was dropped at 13:49:32 to give the negative Yaw and positive Roll required to begin a Roll to the right.  After this no further commands were given for the completion of the maneuver or to correct the adverse Yaw and Roll rates.  The official investigation declares the existence of these Yaw and Roll trends by including various styles of dimensionless charts which are stated to be graphical representations of the established Aero Moments, see Fig. A9 and Fig. TA-A2-4.  The absolute value of the Aero Moment is never provided with the graphical data so assumptions about the Yaw and Roll rates must be made to establish the effect these Aero Moments had on the shuttles flight attitude.

Assumptions:

By 13:59:00 the negative Yaw Rate would have placed Columbia's nose to the left of the flight path by an undetermined angle, possibly as much as 45˚ or Yaw = - 45˚, (this yaw angle is only an assumption based on a worst case and may be less).

13:59:30 is the point where RCS Yaw Jets R2R and R3R began their 8 second continuous firing to correct the negative Yaw but it is also the exact moment where the Colony video begins.  This does not provide enough time for the shuttle's attitude to change and stabilize to what appears during the zoom portion of the video.  The RCS jet firing telemetry data, events 38 and 39 at 13:59:30 on Table A3, are then moved back by 32 seconds, (see Update 05/15/2004).  The RCS R2R and R3R jet firings then start at 13:58:58 and end at 13:59:06.

The maximum amount of time required to redirect the shuttle's Yaw angle from - 45˚ to the final value of +90˚ is assumed to be 30 seconds, (this corresponds to an end time for the Yaw rotation of 13:59:28 which is 2 seconds prior to the recording start time of the Colony video).

The assumed initial values can then be inserted into the derived Yaw Rate Function to get the final Yaw angle,

(4.36˚ x 30 seconds) - 45˚ = +90˚

This places the Columbia at a right angle to the flight path with the left hand side facing fully windward.  Although the timeline seems a little tight with only 2 seconds to spare before the start of the Colony video, these times are based on a worst case scenario.  The elapsed time of 30 seconds from the beginning of Yaw jet firing at 13:58:58 to Columbia's final position at 13:59:28 may have been only 15 to 20 seconds.  The Columbia may have been sitting at its final position of Yaw = +90˚ by 13:59:15.

Based on the calculations done above it is entirely possible that some attempt to correct an uncontrolled negative Yaw rate is what placed the Columbia in the final position of +90° Yaw.  A subsequent vehicle breakup with the left wing dispersing its material first would be the next logical event.  Although the conclusion reached using the simple derived Rate Function is entirely possible it requires some assumptions that have an unknown accuracy.  Fortunately an independent source of evidence exists that relates the final calculated Yaw angle to an eye witness account.  An amateur video taken of Columbia that begins after that final RCS jet firing and just a few seconds before vehicle breakup includes a high level zoom of the orbiter that clearly shows its attitude to be +90° Yaw or flying sideways with the still intact left wing facing windward.  The details of the Colony Texas video are on Technical Articled TA-A3.

The animation of Fig. TA-A2-3 describes a scenario such as the one described above where some degree of negative Yaw developed during the first several minutes of reentry.  This negative Yaw is suddenly counteracted by firing the right rear Yaw Jets for 8 seconds continuous.  Unfortunately the uncontrolled over correction resulted in the Columbia being left in the worst possible position, turned sideways to the Mach 18 airflow.  Essentially all of the aerodynamic normal accelerations are now being taken laterally on the shuttles airframe.  This is something that the orbiter was never designed for and the fact that the shuttle would breakup would have been a forgone conclusion to any flight engineer watching.

Fig. TA-A2-3

 

 

Attitude Graphs From the Official Investigation

The following four charts are from the official final report and are intended by that investigation to add legitimacy to its findings.  The definition of how the data is presented is contained in Vol. 5 Appendix G13 of the official final report.  The explanation for how the values for the coefficients are arrived at is quite complicated and would not add to the clarity of the information on this website.  If this information is required it can be found in the section of the final report mentioned above.  This is the same data presented in the graph of Fig. A9 with a clearer representation and added information.  It has been observed that the coefficient values shown on these charts from STS-107 are not outside the range of values collected from other shuttle missions.  This observation is verified by Time Line and Ground Track documents from the official investigation that at no point refer to the Yaw and Roll trends as being significantly greater than that recorded during any other shuttle mission, (until the last couple of minutes of reentry).  The fact then that the Columbia's flight control system was unable to control the attitude errors is further evidence that this portion of the shuttle's avionics was severely damaged early during reentry.  The main value of these charts then is the same as that of Fig. A9 which is to show the Yaw and Roll trends over the course of reentry.

Fig. TA-A2-4

Yaw and Roll Yaw Over Time Roll Over Time Pitch Over Time

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