THE BOR-4 TEST VEHICLE

Heat shield testing went much further than the experiments with the OK-TVA and OK-TVI models at TsAGI and NIIkhimmash. Smaller pieces of thermal insulation were tested in plasma generators at TsNIIMash in Kaliningrad, the Institute of Mechanical Problems in Moscow (IPMekh), and other test stands at NPO Molniya in Tushino and the Siberian Scientific Research Institute of Aviation (SibNIA) in Novosibirsk. Besides that, Ilyushin-18D and MiG-25 aircraft were used to test tiles and felt reusable surface insulation at subsonic and supersonic speeds. The thermal insulation was installed on areas of the aircraft that were subjected to the highest dynamic pressure and acoustic loads from the engines. NASA similarly tested Shuttle tiles on F-15 Eagle and F-104 Starfighter jets.

Since the US Space Shuttle’s Thermal Protection System was very similar to that of Buran, the Russians probably watched the first Shuttle missions with more than casual interest. In a 1984 National Intelligence Estimate on the potential for the transfer of US space technology to the Soviet Union, the CIA concluded that the Soviets had benefited considerably from surface heating data from the STS-2 and STS-3 missions publicly released by NASA in June 1982. The report quoted NASA officials as estimating that the data could save the Soviets the equivalent of $750 million in R&D cost and considerably reduce development time [16].

Whether that was true or not, it didn’t stop the Russians from pursuing their own test program. Unlike NASA, the Russians had the unique opportunity to test Buran’s heat-resistant materials during actual re-entries from orbit using scale models of the canceled Spiral spaceplane. In the late 1960s and early 1970s they had already flown
scale models known as BOR (Bespilotnyy orbitalnyy raketoplan or “Unmanned Orbital Rocket Plane”) on suborbital trajectories. These were BOR-1 (a wooden mock-up), BOR-2 (a 1:3 scale model), and BOR-3 (a 1: 2 scale model) with ablative heat shields. In 1975 plans were completed at the Flight Research Institute (LII) for an orbital test bed known as BOR-4, which was a 1: 2 scale model of Spiral. Even though the future of the Spiral program was very much in limbo by this time, BOR-4 test flights would also have been applicable to the giant lifting body proposed by NPO Molniya in early 1976. When that was dropped in favor of the delta-wing Buran, it looked as if BOR-4 would remain on the drawing boards forever.

However, the Russians soon realized that in order to test the heat shield they did not necessarily need a vehicle that exactly copied Buran’s outlines. The most impor­tant thing was to ensure that the heat-resistant materials would be exposed to the same type of temperatures for about the same period of time. Moreover, the nose section of the BOR-4 test vehicle did more or less match the contours of Buran’s nose section. Therefore, it was decided in 1977 to develop two types of scale models in support of the Buran program:

– BOR-4, a 1: 2 scale model of the Spiral spaceplane, to test Buran’s heat shield materials.

– BOR-5, a 1: 8 scale model of Buran, to test its aerodynamic characteristics.

Design

Just like the Spiral spaceplane, BOR-4 was a flat-bottomed lifting body with a vertical fin and foldable wings. It was 3.859 m long with a launch mass of about 1,450 kg

and a landing mass of 795 kg. The BOR-4 vehicles made it possible to test the three main types of thermal insulation used on Buran—namely, tiles, felt reusable surface insulation, and reinforced carbon-carbon. Black tiles (using both the TZMK-10 and TZMK-25 substrate) covered the belly, white tiles (with TZMK-10 substrate) were installed on the sides, and ATM-19 felt insulation protected the upper part of the vehicle. Carbon-carbon GRAVIMOL material was used only on the nosecap, since the wing leading edges were too thin for installation of such material.

The tiles were not applied directly to the BOR’s airframe, but to a thin layer of aluminum of the same composition as that used in Buran’s airframe. In between this aluminum layer and the actual airframe was an ablative heat shield material (PKT-FL) that had been planned for the original BOR-4 vehicle to be flown in the framework of the Spiral program. This provided the necessary redundancy in case any of the Buran heat shield material burnt through during re-entry. The area between the nosecap and the airframe was filled with insulating material made of heat-resistant fibers. Since the wings were much thinner than the rest of the airframe, they were filled with a porous felt material impregnated with a water-based substance. Evaporation of that substance provided enough cooling for the wing during re-entry in case the Buran thermal protection material proved ineffective.

BOR-4 was equipped with 150 thermocouples, installed mainly on the airframe and just under the coating of some of the tiles. In addition to that it had acceler­ometers, angular velocity sensors, pressure sensors, and sensors that indicated the position of the wings. Information obtained from the sensors was recorded on board and sent back to earth in “packages” to tracking ships and also to a ground station during re-entry.