WASHINGTON (AP) _ NASA said Tuesday the first post-Challenger space shuttle will have an escape hatch but that no decision has been made on the rocket assist necessary to eject the crew safely in an emergency.

The shuttle Discovery, which is scheduled to make the first flight in June 1988, will be changed significantly from the orbiters that flew before the Challenger explosion in January 1986 grounded the system, said a report to President Reagan by NASA administrator James C. Fletcher.

Twenty major changes have been made in the shuttles' main engines to increase their operating life, safety and reliability; the ship will have a new re-entry heat protection system in the area where the wing joins the fuselage; the external fuel tank lines will be strengthened and the solid fuel rocket boosters will be radically redesigned.

The 191-page document sent to the White House is NASA's report on how it implemented the changes recommended by the Rogers Commission, which investigated the explosion of the Challenger and deaths of its crew of seven.

One of the commission's nine recommendations was that NASA provide a crew escape system for level controlled flight. Astronauts always feared that if they had to ditch in the ocean during an emergency return, the ship would break up on impact.

''A final decision to implement a space shuttle crew escape system has not been made,'' the report said. But, it added, ''the jettisonable hatch modification has been approved and will be installed prior to the first flight.''

Such a system would not have saved the Challenger crew. NASA said it has a study under way to evaluate the feasibility of a method to use while the solid rockets are thrusting in the first two minutes of flight.

A major problem in designing any escape system is that it must ensure that the crew member does not smash against the shuttle's wing as soon as he or she leaves the cabin.

NASA said it assessed several manual approaches for reducing that potential contact, including a ''deployable tunnel'' through which crew members would pass and an extendable rod or rope to guide the escaping astronaut.

There has been concentration, however, on a system to use ejection seats, rocket-powered extraction of seated astronauts, a bail-out from the bottom and rocket-powered extraction through the side hatch.

A new ejection seat design, to hurtle five crew members through a blow-away top of the shuttle hatch, would not be ready until the mid-1990s, NASA said. The same time period would be required to develop a rocket-powered system to remove up to six seated crew members.

In the so-called ''bottom bail-out'' a panel would be opened on the bottom of the orbiter to deploy a guide chute. NASA said those modifications could be done by 1989, but it would make too many changes in the vehicle.

The most likely concept - and the one NASA is working on - would be an escape through the side hatch using extractor rockets. Up to eight astronauts could get out that way in two minutes if the shuttle were in controlled gliding flight at 20,000 feet and a speed of 200 mph.

The procedure, NASA said, would be to equalize the cabin pressure with that outside and to blow the hatch.

''Once the hatch is jettisoned, the crew moves to the hatch area and climbs onto a guide ramp,'' the report said. ''Each crew member attaches a tractor rocket pendant to their parachute-survival pack.'' The crew member fires the rocket, then parachutes to the ocean.

NASA says it plans to test the concept in aircraft tests using dummies.