Osoaviakhim-1 was a record-setting, hydrogen-filled Soviet high-altitude balloon designed to seat a crew of three and perform scientific studies of the Earth's stratosphere. On January 30, 1934, on its maiden flight which lasted over 7 hours, the balloon reached an altitude of 22,000 metres (72,000 ft). During the descent the balloon lost its buoyancy and plunged into an uncontrolled fall, disintegrating in the lower atmosphere. The three crew members- I.D. Usyskin, A.B. Vasenko, and P.F. Fedoseinko- were probably incapacitated by high g-forces in a rapidly rotating gondola, failed to bail out, and were killed by a high-speed ground impact.
100,000 sets were issued, and Scott '12 prices the unused set at $150.00. The vast majority were probably used as postage and discarded. The set has obvious appeal to Aviation topicalists.
Note that a Perf. 14 set, which is probably a hundred times scarcer than the normal set (Perf. 11), was also issued. The price of the Perf. 14 set (Scott #C50a-52a) is truly stratospheric, however, as it cats. at $30,110.00 for unused.
The market for better Russian stamps from the Czarist through Stalin periods is very hot right now. With 142 million people, Russia is the 8th or 9th largest economy in the world, with vast reserves of natural resources and a highly educated population. Since the collapse of the Soviet Union, Russia has experienced several major economic crises in its transition to capitalism, including a major economic contraction of about 8% in 2009 due to the global financial crisis. Even accounting for 2009, annual GDP growth over the last 5 years has averaged about 3.4%. The country is still plagued by corruption and organized crime, making it somewhat reminiscent of America during its "Wild West" and Robber-Baron periods. Nevertheless, the middle class has grown from just 8 million people in 2000 to 55 million in 2006.
I favor all scarce sets of Russia, as I believe that it is likely both its economy and stamp collecting population will grow substantially over the next decades.