The US Navy investigated the use of the DC-9-20 for the COD mission in the early 1970's. According to Great Airliners Series: Volume 4:-
"The most noticeable exterior change was the re-positioning of the nose wheel 12 feet 6 inches farther aft. The gear strut could also be extended by two feet to give the aircraft a 6-degree nose-up attitude for a catapult launch. local structural strengthening was required for both the retractable tail hook and the catapult hold-back attachments and heavier landing gear struts were substituted to absorb the deck-landing loads.
Provisions were made for an in-flight refueling probe located below and ahead of the pilot's windscreen and the fitting of pylon attachment points some 12 feet inboard from each wing tip. These pylons were designed to carry a pod containing a standard USN
'buddy' refueling package consisting of a pump and drum-mounted flexible hose with an attached drogue basket. An additional 780-gallon tank in the forward hold plus a second 1,000-gallon unit in the rear hold would have permitted the tanker to off-load more than twice the fuel volume of either the Skywarrior or the tanker version of the A-6 Intruder.
One other exterior change was the elimination of the heavy thrust reversers, replaced by simple straight exhaust cones. The thrust reverser were of no use beacuse instant full power for a 'bolter' - due to a missed approach or the failure of the hook to engage the wires - was required. The aircraft would be stopped during deck landings by the arrestor hook only. The MTOW for shore based operations was 110,000 pounds, but catapult limitations reduced on-board MTOW to 104,000 pounds. The carrier landing weight was restricted to 75,000 pounds, versus 93,400 pounds ashore. Overall COD modifications raised the empty weight 3,000 pounds; nearly half the increase came from the stronger landing gear. The built-in airstairs were also removed to save weight, and replaced by a standard crew ladder. The engines were JT8D-9s.
As the aircraft would normally spend minimal time aboard the ships, a folding wing system was deemed to be unnecessary. A typical turnaround time for the C-9 (COD), from landing until being catapulted off was intended to be 30 minutes. Because of the cargo door location, the C-9 could be parked with the tail overhanging the deck edge while safely off-loading and loading personnel and cargo.
The folding wing aspect was explored - the wing fold point being between the elevators and flaps, and the wings folding 115 degrees. For deck parking, the folded span was 58.5 feet. A more complex change moved the hinge line inboard to give a folded span of just 47 fet and, with the nose gear strut extended to lower the tail, the aircraft could be taken down to the hangar deck on larger carriers."
Apparently, the winding down of the Vietnam campaign removed the need for this development.