Re-entry and landing

A nominal re-entry could be initiated whenever Buran’s ground track carried it over or near one of three runways available in the Soviet Union. The primary landing site was at Baykonur, with back-up sites available in the Soviet Union’s Far East and in the Crimea (see Chapter 4). The ship had a maximum cross-range capability of 1,700 km, but that was only required for an emergency return back to Baykonur after a single revolution when the vehicle was launched into a polar orbit. For more common inclinations below 65° a cross-range capability of 1,050 km was sufficient.

Deorbit preparations began with the crew realigning the GSPs, retracting an­tennas, closing the payload bay doors, and preparing hydraulic systems for re-entry. When descending from an altitude of 250 km, about one hour would elapse between the deorbit burn and touchdown, with the vehicle covering a total distance of about 20,000 km and reducing its speed from Mach 25 to zero. After the deorbit burn, performed with the help of the DOM engines, Buran needed some 25 minutes to reach the official boundary between space and the atmosphere at 100 km, at which point it was still at a range of 8,500 km from the runway. It was only then that the three Auxiliary Power Units were activated.

The return through the atmosphere was divided into three phases: “Descent”, “Pre-Landing Maneuvering”, and “Approach and Landing”. These correspond roughly to the three major phases of a Shuttle Orbiter return (“Entry”, “Terminal Area Energy Management”, and “Approach and Landing”).

“Descent” was the hypersonic phase of the re-entry (Mach 28-Mach 10 at 100km-20km altitude) where the vehicle was exposed to the highest temperatures and achieved maximum cross-range. The flight control system guided the orbiter through a tight corridor limited, on the one hand, by altitude and velocity require­ments (in order to make the runway) and by thermal constraints, on the other hand. Buran’s angle of attack was kept at a high value (39°) during most of this phase to keep the temperatures within acceptable limits, while roll reversals were used to bleed off air speed and thus reduce kinetic energy. When the vehicle reached Mach 12, the angle of attack was gradually lowered from 39° to 10° to increase the lift-to-drag ratio. As the atmosphere thickened, the ship gradually transitioned from 20 aft attitude control thrusters to conventional aerodynamic control surfaces. The thrusters were used up to an altitude of 10 to 20 km. Between altitudes of about 80 and 50 km Buran was enveloped in a sheath of ionized air that blocked all communications with the ground. After coming out of the blackout, the ship’s RDS system began beaming pulses to transponders on the ground to furnish the on-board computers with range data. Azimuth and range data from the more traditional RSBN beacon navigational aid system were only used as a back-up to the RDS data.

During the “Pre-Landing Maneuvering” phase (Mach 10-Mach 2 at 20 km-4km altitude) Buran gradually transitioned from hypersonic to supersonic speeds and lined itself up with the runway for the final approach and landing. At this stage it intercepted one of two so-called “Heading Alignment Cylinders’’ (TsVK), imaginary cylinders to align the vehicle with the runway. Which of the two was chosen mainly depended on the wind direction. By the end of this phase Buran reached an “entry point’’ 14.5 km from the runway to begin the final descent. Primary navigational input throughout this phase still came from the RDS rangefinder system, backed up by the RSBN for azimuth and range data and by the RVB high-altitude altimeter and

SVSP air data system for altitude data. The SVSP probes were deployed at an altitude of 20 km.

The Approach and Landing phase saw the orbiter moving from hypersonic to subsonic speeds and finally coming to a stop on the runway. It began with a steep glideslope of —17° to —23° degrees (depending on landing mass), allowing the ship to correct any small trajectory errors it still had at the entry point. At an altitude of 400-500 m a pre-flare maneuver was started to position the vehicle for a shallow glideslope of —2° in preparation for landing. A final flare at an altitude of 20 m led to touchdown some 1,000 m past the runway threshold at a speed between 300 and 330 km/h. Wind speed limits were 5m/s for tail winds, 20m/s for head winds, and 15m/s for crosswinds. After touchdown, speed was brought down to zero by the brake chutes and the main gear brakes, with the speed brakes only used in manual landings. Steering during roll-out was provided by the nose gear steering system and by differential braking. The maximum roll-out distance was 1,800 m. The navigation aids during Approach and Landing were the RMS microwave system for altitude and azimuth, the RDS rangefinder system for range and the RVM low-altitude altimeter for altitude.

The landing could be performed in automatic, flight director, or manual mode. Automatic mode was the preferred mode even for manned missions (see Chapter 7). Flight director systems, also used in aviation, provide visual indications on the pilots’ displays of what the autopilot would want to do if it were flying the vehicle under the current settings. In other words, the pilots fly the vehicle manually but are guided by the autopilot. Simulations showed that the use of this mode throughout descent would be monotonous and tiring and should be restricted to the final approach and landing, especially if visibility was poor. Moreover, this mode did not give the crew the necessary psychological comfort because it could not always anticipate unexpected events. In manual control the pilots themselves determined the flight path using information on the expected touchdown point and remaining energy and also by relying on navigational aids, outside visual clues, and data uplinked from the ground. If all that information was available to them, they could switch to manual mode at an altitude of about 20 to 30 km. In emergency situations they could land the vehicle using only navigational aids or information provided by Mission Control [30].