I can't find any grammar rules about transformation of つ in っ in compound words. For example 橘花 (WW2 japanese jet). 橘 is きつ and 花 is か. Pronunciation is not きつか but きっか (some people say きつか is ok too, but きっか is actually right). Or words with kanji 絶 (ぜつ): 絶望 (ぜつぼう), 絶滅 (ぜつめつ), but 絶息 (ぜっそく), 絶交 (ぜっこう). Can anyone explain please?
These are instances of sokuon (consonant gemination).
Unfortunately, there aren't any simple go-to predictive grammar rules that explain the phenomenon of consonant gemination in Japanese (check here). As explained on Wiki here, つ and く are sometimes converted to a geminate consonant (marked by the small tsu) when they occur mid-word. This happens in some words (絶交 zekkou, 絶息 zessoku) and not in others (絶滅 zetsumetsu, 絶望 zetsubou) depending on the phonemic environment within the word. It's definitely an interesting aspect of Japanese linguistics but it would require some dedicated study to understand the complexity of the rules which determine these changes. If you are just looking to learn which words it happens in and which it doesn't, it's probably more practical to just learn them case by case.
This is discussed in some detail in these answers, which you might find useful: