In the first V1 mission on London (II Act, A New Kind of Weapon), shhot down all the Waves of V1 rockets, alternate between full (to reach then) and minimum throttle (after they break formation and when close to them) - if one get past you, tail them and shoot it down before they hit the Parlament (you will known if you fail because the health Bar will decrease, and you will get a optional objective failure message).