Appendix B Flare And Shallow Glide Slope
In the early design of the FSGS geometry, it was decided to use a two-flare approach instead of one flare. The one flare design was more sensitive to trajectory and vehicle disturbances near the ground, and it provided no stable flight time just prior to T/D. Using an initial pullup circle for a reference profile minimizes the maximum load factor and eases the monitoring task because the pullup g's are nearly constant. Also, using an IGS before an FF provided a stable flight time, which was strongly desired by the shuttle pilot in both the CSS and AUTO mode of flight. This time is used to make a final energy assessment and to establish a controlled approach to the runway T/D zone.
With the decrease of the OGS from -24° in the pre-ALT days to the current - 20°, the pullup circle radius has increased so that the nominal pullup g's are now approximately 0.33 compared to 0.5 pre-ALT. The 0.5g value is the nominal design limit, but with today's geometry it no longer affects the trajectory design. The initial preflare altitude has increased since that same time, due to the pilot's concern that a few seconds delay in the start of the flare while descending at -185 fps can make the pullup maneuver very demanding and can result in an uncomfortably low pullout altitude.
The autoland guidance FSGS phase starts at 2000 ft altitude. In the reference geometry the pullup circle tangent to the OGS is at 1700 ft altitude. An early STS-l design had the flare altitude at 1750 ft because, among other reasons, that was halfway between the cockpit instrumented altitude readouts.
During the early part of the FSGS phase, the velocity is increasing to ~ 310KEAS due to the SB's retraction at 3000 ft altitude. As the flightpath angle nears the minimum steady-state value (approximately - 12 deg at approximately 1300 ft altitude), the velocity begins to decrease. Generally the transition to the IGS begins at 200 ft altitude.
The FSGS exponential and the FF trigger geometries are designed to give the pilot at least 5 sec of stable flight time on the IGS before the FF maneuver on the nominal trajectory. Stable flight time is defined as the time when the pitch rate is less than 0.5 deg/sec (Figure B.3-1). The lightweight nominal trajectory yields 8 sec stable time on the IGS. Once the vehicle is on the IGS and the landing gear is down and locked, the deceleration is fairly constant at just over 4 KEAS/sec. Each additional second the vehicle remains airborne results in a 4 kts slower T/D velocity. The stable flight time must be curtailed due to the velocity lost during that time. But a minimum amount of stable time is required for the crew to assess the trajectory energy state and to make any final adjustments necessary to achieve a smooth, safe landing. For an autoland to T/D flight mode, this stable time is also required for the crew to assess the auto guidance final approach to the runway.
Another time measurement that has a similar connotation is stable time to T/D. Stable time to T/D is defined as when the pitch rate is less than 0.5 deg/sec until T/D. The nominal trajectory yields 17 sec of stable time to T/D. There are several conditions which compress these stable times: winds, atmosphere, energy, SB retract angle, and the close-in aimpoint geometry. The main reason for using the close-in aimpoint is to increase T/D energy by 1000 ft. This is partially achieved by cutting in half the time spent on the IGS. Any attempt by the crew to try to maintain the normal 8 sec of stable time on the IGS, when flying the close-in aimpoint, will negate some of the expected T/D energy gain.
The pilot can visually monitor the tracking of the IGS with the ball/bar landing aid. The height and separation of these lights are computed so that the pilot's eye position is such that he sees the ball superimposed on the bar if the vehicle is ideally tracking the reference IGS geometry. Most runways also have painted markers at the nominal 2500-ft T/D location to give the pilot's a downrange point of reference.
The IGS intercept is currently anchored at 1000 ft downrange. The intercept used for the first two space shuttle flights was 1500 ft. The stronger than predicted aerodynamic ground effects are the reason for moving the IGS back 500 ft. The STS-2 preflight predicted auto T/D range was 4000 ft, which was beyond the nominal designed range of 3000 ft predicted for STS-1. Using the current aerodynamic data modifications results in a nominal T/D range of 2500 ft.
The IGS has buried in it a concept explained by the late A. Moyles/ Rockwell International that involves time to go and a comfort zone. Time to go is the altitude divided by the altitude rate. Coming off the pullup circle to the exponential, the pilot is comfortable with a trajectory that has 4 to 7 sec time to go. Trajectories that have less than 4 sec time to go give the pilot a feeling that there is not enough time to make a correction before the vehicle hits the ground. Conversely when the time to go is greater than 7 sec, the pilot feels that the trajectory is being stretched and too much energy is being dissipated before T/D. The nominal trajectory is designed to stay between 4 and 7 sec time to go on the IGS and generally triggers FF as it decreases from 5 to 4 sec. (Figure B.3-2).
- Figure B.3-1 Stable Time on the IGS (5 sec minimum)
Figure B.3-2 IGS Altitude - Altitude Rate Design Philosophy
Figure B.3-2 IGS Altitude - Altitude Rate Design Philosophy
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