In 1973 the VPK initiated the BOR (Bespilotniye Orbitalniye Raketoplan) program to
investigate the hypersonic aerodynamic characteristics of lifting-body and heat shield
materials for the crewed Spiral lifting body spacecraft.
1:3 and 1:2 subscale models of the Spiral were built, which had, unlike the full-scale
model, fixed wings and were designated BOR (unpiloted orbital rocketplane). BOR-1, BOR-2, and BOR-3 were increasingly sophisticated models of the configuration, flown on suborbital
trajectories.
After the cancellation of Spiral in favour of the Buran, BOR-4 subscale spaceplanes
were used to test heat shield materials developed for Buran. Certain essential tests of
these heat shield materials could not be done in the lab. These included interaction with
the plasma sheath during re-entry, chemical disassociation effects, etc. The BOR-4 was
clad in 118 tiles of the type developed for Buran as well as carbon-carbon nose cap and
leading edge.
As the BOR-4 were intended for orbital flight, they featured braking engines for
deorbiting. After only one orbit of the earth, the spacecraft would deorbit, perform a
gliding re-entry, followed by parachute deployment, splashdown in the ocean, and recovery
by Soviet naval forces. A total of seven BOR-4 were built. Of these one flew a suborbital mission and four performed successful orbital test flights. These flights examinated the vehicle during reentry at speeds of from Mach 3 to 25 and altitudes of 30 to 100 km.
The BOR-4 gathered useful material and environmental data for the Buran orbiter program.
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