Generally science projects, especially at the college level, involve a theory or hypothesis and then involves work (which can be mathematical, research, or hands on physical) to prove or disprove the hypothesis.
When you received the assignment, you probably got a list of questions or goals of the assignment.
What none of us replying to Mohinish knows is the nature of this "Expo", the tone or theme of this "National Science Day", and the reason for his participation. Was it an assignment? Were there goals spelled out? Is it required to follow or demonstrate the scientific method?
It could be that the matters of hypothesis and experiment are not required, that a demonstration of engineering methodology is acceptable. One of the mantras in an engineering analysis or investigation is "Don't make a science project out of it."
Boatgeek's questions are good ones:
What did you learn/what new tools did you use?
What went wrong and how did you fix it?
Why did you do something different than everyone else?
Why is developing new skills in rocketry/motor design/3-D printing important?
Also good would be:
∙ What's different about this rocket from others?
∙ Why are those differences valuable?
∙ What sorts of analysis and testing* did you do during the design, and why?
∙ If there were test failures, what did you learn from them?
∙ What sorts of analysis and testing did you
not do, and why were they not necessary?
∙ How did your rocket's performance compare to your predictions?
∙ What can you learn from the differences between prediction and performance?
If you're supposed to be following scientific method, and didn't start with a hypothesis to be tested, then you're in trouble trying to add one later. In fact, you're totally screwed trying to add one later, because that's not how the scientific method works**. (A scientist doing research who did the experiment first and formulated the hypothesis later would be utterly disgraced.)
On the other hand, if a demonstration and description of engineering methods is OK, then it's not a problem to add the description afterward; you did the work, so now you just have to say what you did and why.
* Testing looks a lot like conducting an experiment with "it will do what I intended" as the hypothesis, but it's really not the same thing.
** On the other hand, "Try it and see what happens" can also be real science. Before the hypothesis come some observations, and setting up conditions to make useful observations (which also looks a lot like experiments and tests, but is also something different) is legitimate. The little kid who turns over rocks to see what's under them
is doing science. Unfortunately, this project doesn't seem to fit that model, as everyone knows what happens when you stick fins and a nose cone on a motor and light it.