The airman who twice went down to take readings in the vapor-filled, Titan II missile complex near Damascus, Ark., last September and almost died from injuries when it exploded during his second trip has been reprimanded by the Air Force, according to informed sources.

Sgt. Jeffrey K. Kennedy, who had been considered one of the heroes of the accident, was given an official letter of reprimand last month, sources said, for the first entry, which he did on his own, disregarding an order to stay out of the complex.

In that effort, just three hours after the fuel leak began, kennedy, wearing only a gas mask, went down an escape hatch into the underground Titan launch control center, which had been evacuated by the crew. Kennedy emerged unharmed within a few minutes and brought back valuable readings of guages that measured pressures in the silo.

Kennedy's second entry, five hours later, took place when he and his partner, Senior Airman David L. Livingston, were ordered into the complex through the main entryway by Lt. Gen. Lloyd R. Leavitt Jr., vice commander of the Strategic Air Command, who wanted additional vapor measurements.

The explosion occurred just as the pair had come back to the surface after finding unsafe vapor levels inside the complex.

Livingston died the next day from vapors inhaled after the blast, and 20 other airmen were injured, several permanently.

Kennedy's leg was broken and he received numerous wounds from flying chunks of cement and inhaled a heavy dose of the poisonous vapors when the blast tore off his safety suit helmet. Sources said he may have suffered permanent head and other injuries from the accident. Kennedy now is awaiting an Air Force examination that could lead to a medical discharge.

It was also learned yesterday that the airman who dropped the nine-pound wrench socket that punctured the missile's fuel tank and started the accident also faces possible disciplinary action.

Airman David P. Powell, Air Force sources said, has been charged with disobeying technical orders governing the types of equipment used inside the silo. Rather than accepting the Air Force charge, Powell, sources said, has demanded a court-martial and his military superiors reportedly are studying his request.

An Air Force spokesman at Little Rock Air Force Base said yesterday in a telephone interview that whatever actions were taken were "internal Air Force" business and would not be discussed "because of the privacy act."

Powell had carried a ratchet wrench into the silo, rather than a torque wrench, as required by the orders.

Both wrenches look somewhat alike, both have heavy sockets, which have been knowned to drop off, and both had been used for the job Powell was assigned to do -- at least up until two months before the accident, when orders were rewritten to say that only the torque wrench was to be used.

Powell told Air Force investigators three weeks after the accident that he had been on duty 12 straight hours when he went into the Damascus silo. He realized he had the wrong wrench, but was already in his safety suit below ground ad so went ahead, he told investigators.

Neither Kennedy nor Powell was available yesterday for comment.

According to the Air Force investigation, when Kennedy emerged from his first, unauthorized trip into the launch control center, his superior told him he had comitted "a safety violation." SAC regulations require two men to enter the complex together.

Leavitt told investigators that although Kennedy's information "certainly added to our information base," the sergeant "should not have done that. . . ." a

After Kennedy's first entry, Leavitt order that no one go into the complex without a direct command from SAC headquarters.

When another entry into the complex was planned, Kennedy argued unsuccessfully that it should go through the escape hatch as he had done.

Other Air Force sources in Arkansas contacted by telephone the past few days described morale low as a result of the accident and the personnel actions taken by SAC, which commands the Titan II missile wing at Little Rock.

A month ago, Col. John T. Moser, commander of the wing at the time of the accident, was abruptly reassigned.