Hardened Shells

My brother is working on a new fuel for his rocket car. He's brilliant, he really is. So brilliant that... I think that new fuel he's designing is strong enough to eat right through the car's chassis! I want to reinforce its fuel tanks before that happens. The tortoises out on the flats have very hard shells. I want to use them to create new tanks for our rocket car. Tanks that can hold my brother's new fuel. You look responsible enough. Will you get me those shells?