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A Martlet 3A g-hardened rocket being loaded into its sabot. The Martlet 3A would coast ballistically, after being fired from the cannon, for about 14 seconds, at which time the rocket would ignite. The maximum acceleration load achieved, while maintaining the integrity of unsupported rocket grain, was 5500 gs. With a supported grain structure, the rocket grain can withstand higher in bore accelerations in the range of 12 to 14,000 gs.

(see the Martlet 3A during launch in the figure below)

Launching of the Martlet 3A -- note the sabot petals and pusher plate coming away from the rocket that has just been launch-assisted by a cannon. Another way to think of the cannon in rocket launch assists is as the first stage of a multi-stage rocket -- it's just that this first stage (the cannon) remains in contact with the ground.