I'm counting 23...
Valentine Vonderenko
March 23, 1961- Russia- Soyuz
Oxygen rich flash fire inside capsule prior to launch
Virgil "Gus" Grissom - Edward White - Rodger Chaffe
January 27, 1967- United States- Apollo 1
Oxygen rich flash fire inside capsule prior to launch
Vladimir Komarov
April 23, 1967- Russia- Soyuz-1
Parachutes failed to deploy correctly on reentry resulting in catastrophic vehicle crash
Col. Yuri Alexeyevich Gagarin
March 27, 1968- Russia- MiG-15 crash during training
Georgi Dobrovolsky - Viktor Patsayev - Vladislav Volkov
June 30, 1971- Russia- a valve opened prematurely upon reentry
fatally depressurizing the Vostok capsule
McNair, Onizuka, Resnik, Scobee, Smith, McAuliffe, Jarvis
January 28, 1986- United States, Space Shuttle Challenger- STS51L- O-ring failure on booster rocket
initiated catastrophic destruction of entire shuttle vehicle seconds after launch
Rick Husband, Kalpana Chawla, William McCool, back row, David Brown, Laurel Clark, Michael Anderson and Ilan Ramon
February 1st, 2003- United States, Space Shuttle Columbia- STS107- Heat tile failure on left wing
initiated catastrophic destruction of entire shuttle vehicle upon reentry (to be confirmed)
to all of them
Daniel