The Soviets are having a lot of trouble with their newest and closest equivalent of the U.S. MX missile.

U.S. officials disclosed yesterday that the Soviet SSX24, which the Pentagon says is about the size of the MX and burns the same kind of rubbery, solid rocket fuel, failed when flight tested last week.

This was the third failure in four flight tests, officials said, indicating that the Soviets are still having trouble switching from their liquid-fuel rockets to solid-fuel ones.

Solid fuel is preferred for military use because it can be bounced around or stored for long periods without exploding or leaking. All U.S. submarine and land-based intercontinental ballistic missiles are solid fueled, except the old Titans, which are being phased out.

"It's hard to say what went wrong," said one missile specialist of the Soviets' latest SSX24 test, "because they encrypt so much of their telemetry."

Engineers rig the missiles to send back radio signals during test flights to indicate what is working and what is not. President Reagan complained on May 17 that the United States was having trouble obtaining information about recent Soviet missile tests.

The second strategic arms limitation treaty, SALT II, which both the United States and Soviet Union have agreed to observe even though the Senate did not approve it, limits encryption during flight tests. While authorizing some encryption, SALT II states that it is not allowed "whenever such denial" of flight test information "impedes verification of compliance with the provisions of the treaty."

The Soviets, officials said, have been encrypting the telemetry not only on the big SSX24 but a smaller solid-fuel missile under development, called the PL5 by the U.S. intelligence community.