January 25, 1963

MEMORANDUM for Chief, Research Division
Subject: Preliminary evaluation of X-15 Flight No. 3-14-24

1. Flight 3-14-24 was flown on January 17, 1963, by Joseph A. Walker for the purposes of:

2. All pre-launch operations were normal except for the MH-96 FCS airborne analyzer. Several functions of the AFCS were not checked by the analyzer during the airborne check, possibly because a relay did not function properly. This situation was not detected at the time of the flight and became known only after flight data records were reviewed. The situation could not be duplicated during subsequent ground checks, however, a very thorough circuit and component check of the analyzer will be made to determine and correct the situation.

3. The launch was made in the vicinity of Delamar Lake at an altitude of 45,000 feet. Rotation to the climb attitude was much faster than planned and contributed in part to the overshoot in maximum altitude. Also, the engine was unavoidably operated for about 5 seconds longer than planned, so that the maximum altitude was about 20,000 feet higher than expected.

Approximately four minutes after launch, during the early part of the entry, the #1 APU performance had deteriorated to the extent that the #1 alternator dropped off the line and could not be reset. Loss of the alternator produced no noticeable electrical transients and did not appear to affect the operation of the aircraft electrical equipment. The #1 hydraulic pump failed approximately 30 seconds after the #1 alternator, however, sufficient hydraulic pressure was retained in the #1 system to operate the ball nose and yaw damper servo for about 4 additional minutes. The ball nose and yaw damper servo were inoperative for only the last one minute of the flight. The most noticeable affect that the loss of #1 APU had on the flight was in the operation of the flaps. Normally, the flaps extend in about 10 seconds. On this flight, the flaps required almost 24 seconds to fully extend. This may not be entirely attributed to the loss of one hydraulic system, but may be caused by a faulty flap actuator, since only 15 seconds were required to fully extend the flaps in a similar situation on flight 3-10-19. It is suggested that the flap actuator be thoroughly inspected prior to the next flight of X-15-3.

The characteristic high dynamic pressure limit cycle oscillation in the AFCS was again reported by the pilot during both the climb out and during the entry. Maximum dynamic pressure was attained during the entry and exceeded 1800 psf. At this most severe condition, the limit cycle characteristics were almost identical to those reported on flight 3-13-23. The AFCS limit cycle will probably still be acceptable at a dynamic pressure of 2000 psf, since there was still some gain margin above the minimum attainable for the system during the flight.

The entry for this flight was made at an initial angle of attack of about 27° with about 18° of speed brakes. As the dynamic pressure increased, the angle of attack was decreased to about 22° until the entry was essentially completed. The maximum normal acceleration was about 3.7g during the entry. Lateral-directional stability during the entry was excellent. The previously experienced sideslip oscillation during entry did not manifest itself during this entry. The pilot felt that lateral control and stability, although very good, were the most sensitive during the entry. The data indicates that pilot lateral control early in the entry together with the normal reaction control system lag, produced a low amplitude roll oscillation at 0.6 cps in which the roll attitude never exceeded ±4°. As the aerodynamic control effectiveness increased, the tendency to over control ceased. Sideslip excursions during the entry were small, generally less than 2°, and were not periodic. Some correlation between sideslip and lateral control was noted but not to the extent previously observed. The data from this flight clearly indicates that the ventral-off configuration yields the superior entry.

Early in the entry a 12-13 cps oscillation, lasting about 15 seconds, occurred in the ball nose. At this time, the angle of attack was about 25° and the dynamic pressure was rapidly increasing from about 20 psf to greater than 100 psf. This oscillation has previously been observed on all entries at comparable conditions and precludes the use of the AFCS a Hold mode.

4. Reaction controls were used during this flight and functioned satisfactorily. BCS effectiveness was about 25fi below the expected values for all three axes. An estimated 40 pounds of peroxide were used by the reaction controls during the flight. Most of the fuel was used for roll control.

The large deviations in sideslip or yaw angle (±13°) during the ballistic portion of the flight could be expected in view of the limited use of yaw reaction controls by the pilot. The yaw rate exceeded the dead band for the reaction control yaw damper only twice during the ballistic portion of the flight. The dead band is ±1.5°/sec. Use of the heading hold mode of the AFCS would minimize yaw excursions and maintain heading, and consequently sideslip, within 3°.

5. The automatic AFCS disengage switch was again used during the landing for this flight. The maximum stabilizer deflection during the landing was only -5°. Landing gear loads should be comparable to those of flight 3-13-23.

Elmor J. Adkins

Aerospace Engineer