The two experimental HTP submarines, HMS Explorer and HMS Excalibur, also suffered from considerable problems once they were commissioned into the Royal Navy in 1956 and 1958. Both submarines were designated primarily as anti-submarine targets, but they were rarely used due to the high cost of HTP. HMS Explorer only managed 22 hours of exercises during its first commission, while HMS Excalibur only achieved 100 hours. They were also unsurprisingly regarded as unsafe. In Explorer, the hydrogen peroxide was fed into a catalyst chamber where oxygen became disassociated from water with a great release of heat. The resulting steam and oxygen were then passed into a combustion chamber where sulphur-free fuel was injected which burnt and considerably raised the temperature. Water was then injected to cool the gas, producing yet more steam, which was then used to drive a turbine. The steam was subsequently condensed in a condenser where carbonic acid was removed and then injected back into the combustion chamber again while the carbonic acid was pumped into the sea. The whole process of starting and running the HTP machinery in Explorer was known as 'fizzing' and to the unwary bystander 'fizzing' in harbor was 'like a preview of doomsday.' The sight of exhaust gases, emerging at speed, towered above the submarine in great plumes of grey smoke, and was accompanied by a roar which shook windows a hundred yards away. When Explorer first 'fizzed' after joining the 3rd Submarine Squadron at Faslane, HMS Adamant's officer of the watch was so convinced that the submarine was about to explode that he called out the fire and emergency party and summoned the local fire brigade.
The volatile HTP could only be stored in containers and passed through pipes made of 'compatible' materials such as glass, porcelain, PVC, some forms of rubber, certain types of stainless steel, and, for a limited exposure time, aluminum. It reacted vigorously with incompatible materials, such as mild steel, brass, wood, clothing, or human tissue, instantly producing both heat and oxygen- two of the three essentials to establish combustion. On both Explorer and Excalibur the HTP was carried in 54 special bags, outside the submarine's pressure hull. Filling these bags with HTP was a dangerous operation in itself as the bags had a worrying tendency to explode. During sea trials in February 1957, one of Explorer's HTP bags burst, exploded and flooded much of the HTP system with sea water. 'Any small leak in any of the plastic fuel bags needed a docking to change the whole lot,' recalled Michael Wilson, one of Explorer's COs. 'It was VERY frustrating.' Explorer was eventually confined to a small timber jetty a few hundred yards from Adamant and awarded the nickname 'Exploder.'
At sea, however, HMS Explorer's performance was both impressive and complicated. The HTP propulsion machinery gave short periods of very high underwater speeds. The same weight of hydrogen peroxide provided 35 times the energy that could be stored in an electric battery. But the HTP system suffered from repeated breakdowns and was notoriously unreliable. Those in charge of operating and maintaining it resorted to unusual practices to carefully nurture the equipment. 'If I, as Engineer Officer, failed to do my usual rounds and do my daily obeisances, the turbines would not perform' remembered John Pratt (hereafter referred to under his pen name, John Winton), one of the specially trained and highly attuned engineers who served on board HMS Explorer. 'They would not, in any case, perform on Sundays or holy days; break-downs on those days happened too often to be coincidence. Once, after we had slogged for 36 hours into a raging Atlantic gale, neither turbine would start. Later, I checked and found it was Yom Kippur.' It sometimes took weeks for Explorer to accept a new operator and superstition was widespread. Some members of the ship's company were forbidden to move aft of the Control Room bulkhead while Explorer was 'fizzing' because of the so-called 'evil eye' effect. Despite the dangers inherent in operating both Excalibur and Explorer, their crews grew very fond of the two submarines. 'We did not look upon her as being dangerous. The crew took the bangs and fires as a matter of course,' recalled another of Explorer's COs, Commander Christopher Russell.
One of the most dangerous incidents on board HMS Explorer occurred on 5 October 1961, off the Mull of Kintyre. Explorer was fizzing on the surface, acting as a target for another, dived submarine. 'It was the first "fizz" of the day, indeed the first for many days, after lengthy and exhausting repairs,' wrote Winton. 'There was much jubilation on the turbine platform when both turbines got under way and settled down to the required r.p.m. with only the minimum of bangs and alarms. It seemed that for once we were going to have a good day.' After 15 minutes' fizzing, the watch keepers in the Control Room became concerned about the volume of smoke pouring down the conning tower. The First Lieutenant, a new arrival, was standing at the foot of the tower ladder with painful eyes, struggling to catch his breath. He mistakenly assumed that the smoke was a normal occurrence in Explorer, having heard that anything was possible as far as HTP was concerned.
As carbon dioxide poured into the submarine the equipment designed to measure the gas content showed such unprecedented results that all 3 indicators were reported as defective. 'Looking back now, it does seem that I was extraordinarily slow to take the point which was being hammered in on me from all sides,' reflected Winton. But he, along with the rest of the crew inside the submarine, was suffering from the effects of carbon dioxide poisoning: headaches, dizziness, and nausea. Their judgment and reasoning were also impaired. As he recalled:
I myself felt perfectly fit, although one or two men around me were screwing up their eyes in concentration and complaining of slight headaches. But there seemed no reason to stop the turbines. It cost our department so many back-breaking man hours to maintain them, and we had to overcome so much 'bad joss' to start them, that subconsciously we all must have resisted the idea of stopping the turbines unnecessarily or prematurely.'
Explorer's CO was with the Navigating Officer on the bridge, where there was no sign or smell of gas. When he descended into the submarine he found a Control Room that was full of smoke and a number of crew members asking to be relieved. The CO immediately ordered a full stop and evacuated the submarine, and as the crew clambered onto the casing some were very sick. Others lay face down on the casing, their foreheads pressed into their fists. A few just sat, looking bewildered. HMS Explorer was decommissioned in June 1963, followed by HMS Excalibur in May 1964.
