Enzian

According to The Last Year of the German Army by James Lucas (Arms & Armor Press, 1994; ISBN: 1-85409-334-7):

The Enzian was a ground-to-air missile carrying a 500kg warhead. The initial launch was made by four solid-fuel rockets, and as they burned out liquid-fuelled rockets took over and carried the missile to its objective. Accuracy was to be achieved by one of the special types of newly developed fuses, infra-red, thermal, or acoustic. The Enzian showed promise but the programme was shut down during January 1945.

Thanks to Michael Behm for this info.

And this, provided by Joseph Andrew Bono:

On the name Enzian in Gravity's Rainbow, in Guided Weapons (Burgess, Eric, Macmillan Company, New York, 1957) Burgess says:

Enzian was a ground-to-air pilotless aircraft which had a similar outline to the Me 163 target-defence interceptor fighter, for it was essentially a small aircraft with sharply swept-back wings. As in the Me 163, two ailerons were incorporated for control purposes, but at launching Enzian was mounted on a large inclined platform from which take-off at a high angle was assisted by means of solid-propellant boosters mounted in pairs at the wing roots. Several versions of this weapon were being developed . . . the missile had not found operational use by the end of World War II. (page 133)

An interesting addendum is the Natter "a somewhat similar but larger weapon. It differed from Enzian in that it was designed to carry a human pilot who could take control after an almost vertical and automatically-controlled ascent." (134)

The Natter rocket plane (a one shot plane that required the pilot to parachute back to Earth) was the last attempt the WWII German government made to create a rocket powered manned plane. The only test flight of the Natter was a disaster - the cockpit came off in mid ascent, killing the pilot. The Natter is mentioned just one page before the Enzian in Von Braun's "History of Rocketry and Space Travel," one of Pynchon's sources for Gravity's Rainbow.