Before adding the suffix "less", flaw means a fault or imperfection. When you add the suffix, you get flawless, which means "without any imperfections", or "perfect".
Kennedy's speech is very persuasive as it appeals to the emotions and national pride of the audience. First of all, we have to take into consideration the context in which this speech was delivered. Kennedy pronounced this words on September 12th of 1962, amidst the Cold War and only a year after the Berlin wall had been built. President Kennedy was then in a difficult position in which he had to both keep the spirits of the population up and transmit a sense of safety and control in front of the growing threat of Communism. Also, he had to demonstrate both authority and humility in order to contrast with the authoritarian methods of the communist regimes.
In order to achieve this, he used both rhetoric and evidence. Kennedy lists a set of milestones for humanity and scientific development and places the landing of an unmanned spacecraft into Venus as the latest, most recent achievements. In this way, by deciding to scale down all of the achievements of humanity to a unit of time that is easy to grasp and understand, such as 50 years, Kennedy places the United States as a small but yet important cog in the advancement of humanity and traces a line of continuity between the early pioneers who "conquered" the country and these new pioneers (both in scientific and military power) that are meant to conquer space. These choices are in line with the objective of the speech, that is to present America as a powerful yet humanistic nation in front of the enemy, who was portrayed as dictatorial and merciless.
By establishing this parallelism between those who "moved forward" in the past to conquer the country (especially in Texas, the land of Stephen Fuller Austin and the Old Three Hundred) and those who were "moving forward" at that time to conquer space, he traces a continuity between the original spirit of the nation and the current spirit that had to be maintain in order to face the "new ills (...), new ignorance, new problems, new dangers" that were a result (either direct or indirect) of the Cold War.
To sum up, Kennedy's speech succeeds at engaging an audience that is both in awe and afraid of the state of affairs of the world at that time. The space race worked as an agglutinant force to bring the nation together to admire the marvelous advancement of technology and, at the same time, served as a demonstration of power that attempted to sooth the fears of the population, who were afraid of the potential threat of the USSR.
