Miss Young, 44, a member of the Dangerous Sports Club which pioneered bungee jumping, was hurled 70ft into the sky at 50mph by the contraption.
But the fun event near Bridgewater in Somerset, turned to disaster when she landed on a safety net, as planned, and then bounced 30ft to the ground.
Environmental health officers with West Somerset District Council have launched an investigation into the stunt in case it breaks health and safety at work regulations.
Miss Young, a citizen's advice bureau manager, was only the third person to volunteer for the catapult, invented by her scrap-dealer boyfriend Richard Wicks and mechanic David Aitkenhead.
"For some peculiar reason, which we have yet to establish, she recoiled a couple of feet farther than anyone has before and flipped over the edge of the net to the ground," said Mr Aitkenhead, who has also been fired from the machine.
She landed on soft, muddy earth after narrowly overshooting a buffer zone of tyres designed to break the 15ft fall from the net.
The £3,000 wooden catapult had been tested over many months using tyres and dummies weighing the same as Miss Young.
Mr Aitkenhead made the first successful flight last month.
On Sunday, in a paddock at Mr Wicks' Stogursey home, Miss Young prepared for her turn.
She had donned body armour, a rugby helmet - and a Coldstream Guard's coat, as Dangerous Sports Club members traditionally wear smart jackets for their events.
But onlookers said she looked petrified before sliding into the catapult's sling.
Mr Wicks, 29, who wants to market the device for dangerous sports enthusiasts, said Miss Young had made it clear she did not want to try it again.
"I blame myself," he said. "I wish it had happened to me but it won't stop the project."
"I think we need a bigger net."
Miss Young was taken to Musgrove Hospital in Taunton with a fractured pelvis but her friends say she could be out of hospital in a few days and back at work for the Citizens Advice Bureau in two weeks.