How Aeromoments Would Have Affected Columbia's Flight

Technical Article A1

Aero Moment Definition and its Effect on Flight Control

An Aero-Moment, sometimes also called an Aero-Torque, is simply a way of describing the leverage that is required to steer an aircraft such as the Space Shuttle. For instance moving control surfaces such as the Space Shuttle's left elevons down into the airflow will result in an uneven force couple that will have a tendency to turn or rotate the aircraft to the left about its center of gravity, (negative Yaw), and also result in a positive Roll. An Aero-Moment may also be the result of damage to one of the aircrafts surfaces that disrupts its aerodynamics. This damage causes a drag force that is located some distance from the aircrafts center of gravity having an effect similar to the movement of a control surface altering the course and attitude. It is then up to the pilot or autopilot to calculate the value of the damage induced Aero-Moment and determine the required corrective action. Other factors such as wind may alter the attitude of an aircraft and or force it to drift off course.

For measuring course and attitude changes the Space Shuttle has three different types of gyroscopic sensors which collectively detect motion in any of the six degrees of freedom. Data from the sensors is sent to the GPC's where the Flight Control software calculates the magnitude of the required restorative moment and decides how to respond. Whether the corrective action takes the form of firing the corresponding RCS Jets or movement of the appropriate control surface depends on the phase of reentry flight.

The following sensors detect course and attitude changes,

Inertial Measurement Units (IMU's)
Senses the vehicle's attitude in inertial space relative to the flight path.

Accelerometer Assemblies (AA's)
Detects changes in the Yaw, Pitch or Roll angles.

Rate Gyro Assemblies (RGA's)
Detects linear motion along any of the three primary X, Y or Z axis.

The graph of Fig. A9 represents Aero-Moment coefficients calculated by the shuttle's flight control software using off nominal attitude and course values detected by the gyroscopic sensors as described above. Attitude and course errors may be the result of many different external events, therefore the sensors onboard the Space Shuttle do not directly record Aero-Moment data. The avionics system uses only the knowledge of how the shuttle's attitude and course have changed to calculate an Aero-Moment sufficient to counter the changes and restore the orbiter to its proper course and attitude. The flight control system then sends a signal to one or more of the control surfaces ordering it through some degree of rotation. Exactly how much the control surfaces must rotate to create the required Aero-Moment depends on the atmospheric density which is a function of aircraft altitude. AltTA-A1_AeroMoments.htmhough the Digital Auto Pilot was unable to send the command to correct the errors to either the RCS system or the control surface actuators, the data was still transmitted to Mission Control.

Fig. TA-A1-1

The main motions controlled by the shuttle's RCS jets and various control surfaces are Yaw (rotation about the Z axis), Pitch (rotation about the Y axis) and Roll (rotation about the X axis).

Whatever change in attitude is being made, or if multiple changes are being made simultaneously, any movements of the shuttle will be made about it's center of gravity.

The Yaw diagram to the left shows how closely the Yaw angle is related to the Sideslip angle Beta. Pitch angle is also directly related to the Angle of Attack.

Fig. TA-A1-2

Fig. TA-A1-3

The diagram below is a key to indicate how the various groups of RCS thrust jets affect the shuttle's rotation in the three primary rotational axis Yaw, Pitch and Roll. Thrust from forward and aft jets can also be combined to increase the rate of rotation if required. RCS jets may be used for translational motion as well when Fore and Aft jets on the same side are combined. This will cause the shuttle to translate along any of the three primary axis while holding its Yaw, Pitch and Roll angles constant.

Location Designations For RCS Thrusters

In engineering terminology a "moment" is a term that represents the leverage created by a force and a lever arm of some particular length. Documents produced by the C.A.I.B. often use both terms, moment and torque, to describe the same action. Although the two terms have similar meanings, torque is most often used to describe the power output of a rotating shaft or flywheel.
In the case of an Aero-Moment the force would be the extra drag created by the change in position of a control surface, the thrust from a maneuvering jet or damage to the surface of an aircraft which affects its aerodynamics. The location of the force would be at the centroid of the control surface, the jet thrust cone or the point of damage. The lever arm would be the distance from the force to the aircrafts center of gravity.

The conclusions reached by the official investigation revolve around the existence of a heavily damaged left wing leading edge RCC panel 7, 8 or 9. The official final report gives the distinct impression that the graphed data shown in Fig. A9 represents off nominal Aero-Moment coefficient data resulting directly from the damaged RCC panel. This data often appears in different formats throughout the final report.

As explained previously in this section, the shuttle's avionics system calculates a restorative moment based on off nominal attitude and flight path data recorded by the gyroscopic sensors. It was also stated that the changes which occur in the attitude and course parameters may be due to several different factors affecting the shuttle's flight path. Therefore the shuttle could not have determined the off nominal Aero-Moment coefficients due to a damaged RCC panel because the avionics system had no way of knowing that the panel was damaged. The system can only determine the total off nominal Aero-Moments due to all factors including any possible damage without knowing the extent or contribution of any one factor in particular.

The diagram below Fig. TA-A1-4 explains the mechanics of Aero-Moments and in particular the Yaw moment that would be created by damage to the left wing leading edge at RCC panel 7, 8 or 9.

The Plan View (upper left) describes how the Yaw Moment drives the negative Yaw rate.

[ MYaw = (FDx)(DY) ]

View B - B shows how the Roll moment drives the positive Roll rate.

[ MRoll = (FDz)(Dz) ]

For these cases it is impossible to predict the Aero-Moment values. Although the distance from the force line of action to the shuttle's C.G. is known, the magnitude of the drag force originating in the RCC panel is impossible to determine. This is due to the initial nature of the damage as well as the fact that the official scenario states that the size of the damaged area steadily increases throughout the course of reentry.

Fig. TA-A1-4

Because the negative Yaw trend may be one of the most important factors resulting in the breakup of Columbia the diagram of Fig. TA-A1-5 complements Fig. TA-A1-4 by describing the Yaw moments resulting from movement of the left elevons and or firing of the right rear RCS Yaw Jets. In the case of the elevons the moment is created by the differential between the rotation angle of left and right. If all 4 elevons are already rotated down by 2° from 0° then the left side would need to be rotated a total of 5° from 0° to achieve the elevon rotation of 3°.

Fig. TA-A1-5

The Yaw Moment Coefficient CN typically relates to an angle of rotation of a control surface such as the elevons. Depending on some other variables such as atmospheric density etc. this coefficient can be used with a graph or an equation to determine the total Yaw Moment. An important observation within the diagram is that the drag force located at the damaged area, FDx has a line of action that coincides with the force that would be induced by the deflection of the left elevon. Therefore it is equally possible that the Yaw and Roll trends of Fig. A9 that lead to the breakup of Columbia were caused by movement of the control surfaces and not damage resulting from a foam debris strike during launch and ascent.

Another dimension of interest is from the center of the RCS Yaw Jets to the Center of Gravity (DRCS or also DX) is 595.220". Each RCS Jet provides 870 vacuum pounds of thrust. Our countering moment based on the amount of thrust from the Yaw Jets and distance to the C.G. can then be calculated.

The moment created by operating all four RCS Yaw Jets is then

MRCS = (870)(595.220)(4) = 172,613 Ft.-Lbs.
For two jets MRCS = 86,307 Ft.-Lbs.

The conclusion of the reentry data and event analysis conducted in the sections above is that a significant disconnect existed between the attitude changes that were being recorded by the Columbia's avionics system and the shuttle's response to that information. The various RCS Jet firings that occur prior to the 8 second continuous RCS burn during the reentry do not appear to be a response to the negative Yaw trend. The firing durations are too short and occasionally the wrong jets are being fired. It also does not appear that any commands were sent to the shuttle's control surface actuators that were attempts to correct the errors. Therefore the most important issue resulting from the analysis of the Yaw and Roll data along with any control surface movements was the inability of the flight control system to correct the compounding Yaw and Roll errors. Although the source of the Yaw and Roll moments may be ambiguous, resulting from either wing damage or elevon movement, the loss of flight control after 13:51:00 (EI+411) appears to be a major event. The cause of the avionics failure onboard Columbia must be determined.