I want to build the gravitron tech. so I can build RIP's but it requires 300000 energy
what is the quickest way to get that much energy?

Solar sats on a hot planet. And remember, it's total energy; so you can have mines on the planet using energy.

also it's a good idea to find out the exact amount of solar sats needed (normally around 14.000) then stockpile the res, keeping it safe in fleetsaves then ordering all the solar sats at once. instead of continually building solar sat over a few weeks and end up with them getting crashed.

would a planet in the fourth space be hot enough?

It depends. Temperature is listed right above developed fields on the overview. If possible you want the higher number to be in the 100's.

once you have gravition you might also want a friend to destroy your solar sats for you... before someone else gets them,

What does it mean gravitron tech has a high risk of being detected?

It just means it can be risky to build all the sats because somebody could see them, destroy them, and send you back to square one.

pls how can i fleetsave . wen i start buildin solar ships, sombdy distoyd dem

You can't FS solar sats. They're stationary.

You want to spam them all in the quickest time span. I would build to Nanite 4 or 5 and then use a ruby to boost ship production (might want to use another one to boost energy output, too, so you won't need to spam quite so many sats) - then spam all of them, at once.