I note that how long is the aerodynamic heating during reentry, do solid propellant engine needs optimum temperatureand what is the safe temperature range of spacecraft propellant storage tanks are all closed. They seem neatly on topic to me, but my experience is mostly on math.stackexchange. Yes, there are many different technologies involved, and the answers have a range depending on the technology, but I think a useful answer can be given. For the first, OP had heard a figure of 15 seconds and asked if that was close. It seems reasonable to me to look at a few mission profiles and extract a heating period to show that they are longer than 15 seconds. For the second, I would guess the combustion process probably doesn't care much about propellant temperatures and it is more about other parts of the mission (o-rings, for example). For the third, OP recognizes that there are a number of propellants, but a few sentences on cryo vs storable liquids, the temperature range of storables, etc. would seem useful. Is the community view that these should be closed?
OK, let's go top down.
The first one you mention, How long is the aerodynamic heating during reentry? is closed as too broad because there is no way to give even rough estimates for an unknown, yet specific launch vehicle that OP later mentions in the comments, and then rather asks for a simulation software (a whole different question), where he would have to put in all the figures we were asking for in the comments, instead of either mentioning them, or editing the question to be answerable in general sense (which was also suggested). It could as well be closed as unclear, but as it is, it can't be expected to give any meaningful answer without prolonged discussion, if OP serves new requests and information at will, without needed cooperation towards providing an answer for a difficult, and rather unclear question. Stack Exchange isn't a discussion forum, so questions need to be specific and to the point, you know that. And until the question is clear, it's better left closed and authors of them make their mind up, before one or more answerers go into trouble of approaching it from the angle that's not relevant to what it's author had in mind. Or worse, later changes the question completely, after the first one was already answered, and question and its answers become detached from each other - i.e. a broken thread, useless to future visitors, and anyone else that took their time for it.
Do solid propellant engine needs optimum temperature? is both too broad due to all the possible propellants used, tank capacity, maximum pressure, and thousands of other details, as well as impossible to asnwer in broader sense due to the same reason. For example, when you say liquid hydrogen, we know that it's the most efficient at certain temperature, so it will likely be preheated, but it will be stored at cryogenic temperatures as a liquid, or even as slush hydrogen near its triple point (which depends on the pressure, as I'm sure you'll be avare). Different tanks will have different capacity (fuel wet mass), internal pressure they can sustain (also depends on external tank insulation, as well as between other cryo tanks used, e.g. liquid oxygen), all of it resulting in pretty much different optimal temperatures per each launcher stage ever used. Too broad, and clearly not suitable to answer in general sense either, since I barely scratched the surface there, and for a single propellant. OP is one of our regulars, also in our chat, he knows where to complain about it, if he deemed it necessary. He didn't.
What is the safe temperature range of spacecraft propellant storage tanks? is actually similar in its broadness (and nature) to the second one. It is not nearly specific enough to be meaningfully answered without requiring either one brave answerer writing a short novel, or many many answerers writing incomplete answers, then competing between each other for user approval. Unacceptable, especially if the same question could have easily been edited to be more specific, but it never will be because its author didn't even register with our website, and each new time this user starts a new user session, the system won't have ways to know it's indeed the same person. Shame, but that's what sometimes happens. And even if it got answers, and a complete and useful one would have to be something really fantastic, none could later be accepted by the question author.
Anyway, these are just some thoughts on why these questions were closed. Since I closed only one of the three, exact reasoning for the other two might be different. There are a few things to be said about all of them tho, namely that;
- That all three example questions you list were closed by all three moderators, one by each, and for similar concerns means that the moderation team is coherent. That's actually fantastic, given we barely get a chance to exchange a few words with each other.
- The fact that you're interested in detailed reasons for their closure and if there is something we feel could change is admirable (that's why I upvoted your question), and this concern could perhaps be used to instead ask new, more specific questions that can be answered in reasonable length contributions of our members, and provide answers which could still be helpful to the authors of the questions that were closed as too broad.
- Questions can and will be too broad, and if their authors take notice of suggestions in the comments, we can all work together towards narrowing down its scope and provide good answers too. I can attest to that through my experience with our website, and some of this is also proven by the fact that you exposed here three such questions, not fifteen of them. Out of over a thousand question that we have, that's again fantastic and means many questions must have been eventually reopened, after they were edited by OP or the community to fit our requirements. There were many like that, but I don't have any statistics on me at the moment to confirm that with hard numbers.
- If community feels otherwise than what the moderator's decision was, they can reopen the question themselves through voting. None of these questions have a single reopen vote, never had, and their closure as too broad wasn't contested by their authors either. This to me means we're all in agreement. Again, that's fantastic, and we're achieving some quorum on moderation issues with the whole Space Exploration community too.
So, thanks for your concern, noted and appreciated, but as a moderator I don't see much cause for concern. Perhaps your question here exposed quite the contrary even. But if someone else thinks otherwise, I'll be of course glad to hear suggestions to how they think the site should be moderated better, questions closed more appropriately, and so on. The latter goes also for all our community here, since even if the question is closed fast by a moderator (to prevent someone going into trouble of writing hit and miss answers and communicate the need for specificity to its author), it can later be reopened if the community feels otherwise. And moderator actions can always be questioned in our main chat room too, where there's almost all the time someone to chat with. About site issues, and pretty much anything else (mostly the latter tho). Nobody has ever been treated any differently for pointing out site issues in a fair and constructive manner before.
To the first, saying merely that the figure of 15 seconds isn't exactly right does not really answer the OP's whole question and is not particularly useful to the site.
For the second, your point actually highlights the lack of specificity in the question.
As to the third, a summary might be possible but would not be very useful in the context of safety that the OP raises in the question.