Three points to make here.
1. This was a DASO test for HMS Vanguard which tests the onboard procedures for lunching a missile, it passed. The test concluded the moment cleared the missile tube. Vanguard is now qualified and will rejoin the patrol schedule as planned.
2. This was an American telemetry missile, no different from any of those used in any other test firing of which there have been dozens, this is not a generic fault and should not generate alarm about the missile in general, certainly not the operation missiles on patrol.
3. What is known of the investigation* into the previous failure indicates it was a telemetry issue which automatically terminated the test by preventing the booster ignition. Where the telemetry problem occurred will undoubtedly not be released, but a second occurrence, if related, will probably mean changes to test equipment and/or procedures - but not related to the operations of the deterrent itself.
*
https://committees.parliament.uk/ora...nce/6604/html/
Q20 Douglas Chapman: Can I ask a very quick question about Vengeance? Sir Michael Fallon made it clear yesterday that it was back in operation and had been certified and so on. Where a problem exists, such as misfiring or the issue we had with this particular launch, how can that be certified as being okay in terms of its ability to do the job it is required to do if there had been a failure in the test? Who would tick the box and sign that off? Does it go as far as the Prime Minister, or is it at a more naval end?
Lord West: Well, it would never be the team on their own. Basically, the submarine was put in the right position. It was in the right mode. Everything was done correctly, with all the right firing checks. Everything was done internally and the missile was fired. It fired properly and went up in the air, so that was a correct firing. You know that you will fire a missile properly. From everything that has been said, it sounds as though there was an issue with telemetry within that missile. If you are not 100% certain, you do not even take a risk. It sounds as though they were not 100% certain of some of the telemetry and therefore took it down. That is the missile itself. That is an American issue.