What do engineers need to ensure there is sufficient of when designing a roller coaster?

Mechanical Energy (required for each hill, loop, helix, etc.)

Remember the law of conservation of energy. As you travel on a roller coaster, your kinetic energy and gravitational potential energy changes but have the same total. Depending on the track and the friction on it, you need to make sure that the gravitational potential energy does not "overpower" its kinetic energy and that it ensures a consistent, smooth ride.