If you are having trouble finishing the really difficult levels (5, 9, 11), the new version of Double Star should help. Changes:
(1) your ship starts out with 10% more energy and stronger shields;
(2) your ship starts out with 12 photon torpedoes;
(3) the time to complete a mission is longer;
(4) a problem with the enemy tractor beams was fixed.
Get version 1.1.1 from the Google Play Store. See https://play.google.com/store/apps/details?id=com.wglxy.starship