The Russian people are still waiting for answers

When they examined the wreck of the submarine, investigators discovered that inside the torpedo a stainless steel pipe containing HTP had burst.

An inquiry concluded that this was what had caused the accident, although the precise mechanism remained a mystery.

Nearly half a century later, the Kursk disaster prompted Maurice Stradling, a torpedo designer and former lecturer at the Royal Naval Engineering College in Plymouth, to look again at how HTP could have caused an explosion on the Sidon.

Simple error

Mr Stradling concluded that HTP splashing over reactive metals inside the torpedo created a gas that blew open the torpedo casing.

He believes there are similarities between the Kursk disaster and the sinking of the Sidon.

The engine would have over-revved and the HTP pipe would have burst... allowing HTP to spray into the inside of the torpedo hull...

Maurice Stradling

On the morning of the Russian tragedy, the Kursk was preparing to test-fire a torpedo. Mr Stradling proposes that the disaster began when someone made the simple error of starting the torpedo engine too soon - while it was still inside the submarine.

"If the torpedo was accidentally started then because the propellers were not in the water there would be nothing to control the speed of the engine," Mr Stradling told the BBC. "The engine would have over-revved and the HTP pipe would have burst... allowing HTP to spray into the inside of the torpedo hull...and the whole ghastly chain of events would be in place."

Nobody will know for sure unless remains of the submarine's front compartment are raised. Although the Russians are trying to lift the Kursk, they plan to leave the front of the submarine on the seabed.

And, although the Royal Navy stopped using HTP in its torpedoes after the Sidon sinking in 1955, the Russians still use it, unlike almost all other countries.

The Horizon Special: What Sank The Kursk? can be seen on BBC Two at 2100 BST on Wednesday, 8 August