The primary objective of the EDOMP was to ensure that the crew could safely land the Orbiter and perform an emergency egress after a 16-day mission. Some of the considerations that drove the decision about what type of vehicle to purchase and develop for immediate access were the following:
Several variations of available vehicles were considered. These included the airport passenger transporter (APT) with 568 square feet, and the Aircraft Service Vehicle (ASV) with 360 square feet of available space. The ASV was determined to be insufficient, primarily because of its size. The APT was selected and modified to meet the unique requirements. Both CTVs (KSC and EAFB) were fully self-contained, single-operator, self-propelled units with internal environmental control systems. Each CTV could be raised by as much as 11 feet via a self-contained lift system to dock with the Orbiter hatch or the BDCF. A telescoping gangway was provided for docking ingress and egress, and a stairway in the rear of the vehicle provided an alternate exit. The passenger seats were removed and provisions for large recliner-type chairs, a refrigerator, restroom, emergency medical equipment and other improvements were added.
Although the two CTVs were originally different models of a Plane Mate APT, after modification they provided similar capabilities. The CTVs have been used since June 1991 (STS-40) and have proven to be very effective. Use of the CTVs contributed to enhanced emergency medical capability, improved crew comfort, enhanced medical data collection capability, and reduced time from wheels stop to data collection.