This is the X-34 Technology Testbed Demonstrator being mated with the L-1011 mothership.
EC99-44944-40
This is the X-34 Technology Testbed Demonstrator being mated with the L-1011 mothership. The X-34 will demonstrate key vehicle and operational technologies applicable to future low-cost reusable launch vehicles.March 11, 1999NASA Photo / Tom Tschida› X-34 Project Description