When you refer things as they were at a time in the past. I have never seen a spacesuit. On July 4th, I went to Nasa. Up to that time, I had never even seen a spacesuit. Now, of course, I've seen hundreds. . Until we came along, you had not received an answer. Now, you have received an answer.

The present perfect (I have done) is used for completed actions during a period of time ending at the present moment. "Have you worked hard all week?" (= Have you worked hard during the period from Monday until now?) The past perfect (I had done) is used for completed actions that occured prior to another time in the past. "Last Friday, my boss gave me the afternoon off because I had worked hard all week." (= Last Friday, my boss gave me the afternoon off; Before that, I worked hard from Monday until the time he gave me the afternoon off.) Sometimes, the past perfect is used by itself with an implicit second time in the past. "He had worked hard all his life." (until he retired, until he died) are understood as the second point in time. The past perfect is also used in newspapers and novels to report the “past in the past.” “Joe had worked hard all his life. When he went to work on Monday, he thought about his upcoming retirement on Friday.” Your example has an implicit second time in the past. "I had never seen a spacecraft" (before my trip to the space museum)

Hi there! If you go with 'I had never seen a spacecraft 'option, you describe a prior state of ignorance which ended at your past reference time, that's why you need to add it (something like " I hadn't seen a spacecraft BEFORE") You should use a present perfect if you decide to do without any time reference phrase.

'I have' is the more common usage. For 'had', you can use it as 'I had never seen this before'.