Sea Skua Failure
There is a good description including a video of a Sea Skua being launched from a Lynx helicopter.
http://www.mbda.net/site/FO/scripts/siteFO_contenu.php?lang=EN&noeu_id=113
The missile has been in service for nearly 25 years and has been very reliable.
At launch the missile drops well clear of the helicopter before the rocket booster fires. From the shots I have seen at least 10m. I think that this is to guard against the missile pitching-up and striking the rotor blades.
From the report it suggests that the main motor did not fire; however this may not be a fault with the missile.
Sea Skua is a semi-active missile that uses the Ferranti (Now SELEX) Seaspray Radar to illuminate the target and on board receivers (front & rear) to home onto the target. (The same way as the Sparrow AIM 7E Air to Air Missile). The radar is used to illuminate the target; the missile is initially powered by the Helicopter and locks onto the target. To fire the missile the pilot has to select arm on the Master Arm switch and pull the trigger to start the launch sequence. (There are timing checks to ensure that the pilot has actually pressed the trigger, about 50ms). The first thing to be done to get the missile power supply running. I am not sure what type of power supply is currently used on this missile, it could be a gas grain generator or thermal battery; away they work in a similar fashion, on receipt of the power supply start signal they fire and after a short time (300-600ms) begin producing power and send a Powered up signal back to the Helicopter; if this signal is lost during the remainder of the firing sequence the launch is aborted.
The next step is to release the weapon and get it clear of the Helicopter. There are two things to do, firstly release the attachment points between the missile and the launcher, usually this in done by firing cartridges which generate gas and activate the release hooks; the second task is to clear the weapon from the launcher, some systems use an active ejector to push the missile away from the platform. This is often used on fighter aircraft where one fin of the missile is recessed inside the fuselage of the aircraft. The firing wire was mounted on the foot of the ejector so that the motor fire signal could only be given after the ejector had reached full extension.
I forget the exact details of the Sea Skua Lynx launcher, but there will be a separate fire wire, which cannot be activated until the missile is clear of the platform.
Once the missile is clear but while the Fire Wire is still attached, the Motor Fire Start/Enable signal will be sent by the Helicopter, this could be a high current signal to actually start the rocket motor or a low level enable signal. I suspect the latter, because this would let the missile activate a timer allowing the missile to drop away from the Helicopter before firing the rocket motor. This would account for the usual long delay before the main motor fires.
Solid fuel rockets are reliable. I suspect that something has gone wrong with the firing sequence, either the Helicopter hasn’t sent the Motor Fire signal or the missile electronics have failed to relay the signal to the rocket motor.
It was a good call not to fire the second missile.
The next step will be for the engineers to check out the missile firing system on the Helicopter, unfortunately the other half the problem is at the bottom of the sea and is probably irretrievable.
The good news is that I would have been a Telemetry round, which would monitor what was going on in the missile and should enable the engineers to establish exactly, what, went wrong.
My guess is something simple like a broken motor fire wire in the missile or possibly a failure of the missile power supply at the moment of launch, I doubt that is was the rocket motor that failed.