Of a type of hypersonic most likely.
On August 8, Russia's Ministry of Defense reported that the explosion at the test site in the Arkhangelsk region occurred during the tests of a liquid-propellant jet engine....the explosion occurred during the test of a rocket with a radioisotope element in the engine. "The tests were held on the offshore platform. After the tests were completed, the rocket fuel ignited, and an explosion followed," the department said.
Thanks...I remember that, but didnt look it up.