1) You say that you are very interested in this topic, but make sure you can handle the maths involved. It seems to be quite a difficult topic for you if it was done only by HL students. Also, make sure that you understand every single step of maths involved and don't just reproduce calculations from a website. Regurgitation of calculations does not make a well-scoring IA. You have to show understanding of the maths involved. Also, try and see if you can bring in other areas of maths into this.
2) This seems like a good topic. Its simple, and you say that you are interested in space travel, so you could possibly bring in that excitement you have. Don't worry if you can't relate it to the IB MATH SL syllabus. It can be from elsewhere. You just need to make sure you demonstrate an understanding of the math involved. The only problem I see is that you mention it is a "question of calculus". Just ensure that your IA doesn't turn into problem-solving. We do that in our exams.
3) Again don't worry if the math is beyond the syllabus, just ensure that you understand it.
4) I am not a physics person, but this seems advanced and requires a lot of effort. Don't want to put you off given how much interest you have in this topic. But you really have to brainstorm the calculations required and how you would go about it. If you can pull this off, it sounds amazing. But again, talk to your teacher about this one and find more information if you can. With how much you shared, there isn't much anyone call tell you.