This story was my very first contact with SF. It was at least 61 years ago. I remember reading it in my parents' "old" apartment and we moved out of it in December 1962.
It was a novel, in a hardcover. There was a nice picture, probably on a dust-jacket rather than on the hardcover itself. It represented a rocket on a dark space background with a planet (Earth ?). I suppose there were stars, too, on the background for aesthetical reasons, but really, I don't remember such fine details.
I read it in French, of course, I was just about 9 years old at that time. And I am pretty sure it was the French original and even that the book begins in France.
The main protagonist is enlisted into a special school for future astronauts. He learns a lots of topics, including physics. I remember a course about gravitation and the teacher mentioning "potentiel vectoriel". I liked the sound of it, so I remember. It took me many, many years after that to find out that, yes, there is such a notion as "vector potential", for magnetism and electromagnetism. The gravitational potential, however, is not a vector but a scalar in Newtonian physics, and in General Relativity gravitation it is described by a tensor. BTW in French also the correct term is "potentiel vecteur", not "potentiel vectoriel". This is an example of science in that book : interesting but usually not correct.
I definitely remember an attempt at sabotage. In space, the saboteur would put himself into danger so I suppose this was while still at school. The main protagonist has a hunch at who the saboteur was but no certainty. The reason of the sabotage was never clear to me.
Then the main protagonist is ready and starts to travel all across the solar system. To the Moon and Mars, that was already quite common by that time. I think they travel to Venus too. Already when I was 9, I was an astronomy buff and I knew the temperature on Venus was incompatible with tourism. But I took it as "poetic license" and it did not disturb me too much. I don't remember any detail of these travels.
Towards the end of the book he travels very far, beyond Jupiter and maybe beyond Saturn. I remember someone saying that the answer of Earth to a question they asked would take hours to do the roundtrip at the light of speed. At some point in time and space (not necessarily near Jupiter or Saturn) there is a very serious problem with a rocket. IIRC the problem is not with the rocket where the main protagonist is in. On the contrary, his rocket goes to help the rocket with the problem. But it might be the other way round, I am not really sure.
Since it was a YA story, it ends well, but again I forgot the details.
