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Vanguard TV5, also called Vanguard Test Vehicle Five, hoped to be the second successful flight of the American Vanguard rocket following success of Vanguard 1 on Vanguard TV-4. Vanguard TV-5 launched on April 28, 1958. It was launched from Launch Complex 18A at the Cape Canaveral Air Force Station. The rocket was unsuccessful in its attempt to place an unnamed satellite, Vanguard 2A, into orbit. There was a lost of control of the vehicle, the third stage did not separate and failed to ignite, thus failed to achieve orbit. Vanguard TV5 only reached an altitude of 550 km (340 mi), the goal was 3,840 km (2,390 mi) to orbit. This was set back in the space race between the USA and Soviet Union.
The goal Vanguard TV5 satellite, Vanguard 2A, was to measure the X-ray emission from the sun and its effects on the Earth's atmosphere. This was the same test as Vanguard Lyman-alpha satellites, but at different wavelengths. The detectors were two identical ionization chambers sensitive to X-ray wavelengths produced in solar flares (2 to 8 Å, or 200 to 800 pm). The ionization chambers were located 120 deg apart in the equatorial plane of the satellite and received a maximum signal when an ion chamber tube "looked" toward the sun. The instrumentation measured the 1 to 8 Å (100 to 800 pm) X-ray flux, and recorded the peak solar flare intensity by means of a peak-reading memory device, during the daylight portion of each orbit. With the failure of Vanguard TV5, this same test was sussfully put in to orbit on Vanguard 3 flown on Vanguard rocket SLV-7 on September 18, 1959.