Bumber-WAC was the first large two-stage liquid-propellant rocket. The first stage was a captured german V-2 rocket. The warhead was removed and the nose rebuilt to a launching compartment with blast ports.
The second stage was a modified WAC Corporal sounding rocket. The WAC was equipped with four fins with 50% more total area, than the original WAC.
After some test launches the fifth Bumber-WAC 5 was launched On February 24, 1949 from White Sands proving ground. It reached a record altitude of 393 km/244 miles. This record held for 8 years until 1957.
Bumper-Wac number 7 and 8 were flown for speed not for altitude. There was a risk that parts of the rocket would land outside the White Sands area. Instead the Bumper-Wac tests were moved to a sparsely populated costal area, called Cape Canaveral. Bumper-WAC 7 was the first rocket to be launched for the cape.
The program was officially concluded in July 1950.
Books and articles:
Germany's Secret Weapons by Roger Ford
V-2 Ballistic Missile 1942-1952 by Steven J. Zagola
Dawn of the Rocket Age by Joachim Engelmann
The Rocket - The History and Development of Rocket and Missile Technology by David Baker.
Rockets of the world by Peter Alway,
NASA Sounding Rockets, 1958-1968: A Historical Summary
Vanguard - a history
Directory of U.S. Military Rockets and Missiles