In English, what are the vowel rules for long words?

In English, what are the vowel rules for long words?
 

I know most of the vcv, cvcc rules, etc. but what about for words with more than one syllable?

For instance, in the word "desperation", the "a" is long. But why not the "e" in between "p" and "r"? "E-r-a" is vcv just like "a-t-I" is. Does it have to do with stress?

This is because in English "er" "ir" and "ur" and pronounced as the same short sound. When I was in Kindergarten learning how to spell I vaguely remember learning this as called the "her, first, nurse" rule. The IPA symbol is ɜː
I think this is what you were asking. If not, I apologize and tell me what I misunderstood so I can get you the answer you need.

Not what I was asking, but good info.

I'll give a few more examples

"Periodical". The "e" is a long vowel. But the "I" sandwiched in between "d" and "c" is a short vowel. Why?

"Saturday". Why is the first "a" short?

"Elephant". Why is the first e short?

You are not at all going to like the answer, but we literally memorize how each word is spelled because the way we pronounce it isn't necessarily going to adhere to a set pronunciation standard. The vowels is unstressed syllables are often reduced to another vowel, all the way down to what we call the schwa sound.

In the stressed syllables, the short vowels frequently appear in syllables that end in a consonant.
Sat-ur-day (the first syllable is pronounced the same way as the word 'sat').

el-e-phant (the first syllable has a short 'e' sound - the second 'e' is the schwa sound - that is the same vowel sound heard in the last syllable)

In the word per-i-od-i-cal. The 'ee' sound is said twice, in the first and second syllables, but the short 'i' sound is said in the fourth syllable. Why? Because that is the way we pronounce the word in American English.

des-per-a-tion (short 'e', followed by the schwa sound in the second syllable)
We generally say a long 'a' sound when the following syllable is 'tion'. Except not in 'ration' which, oddly, doesn't rhyme with 'nation'.

Okay, I understand now. I am going to let someone much more qualified than me answer this, but I will say that English is notorious for having exceptions to its rules. This is due to it having borrowed many words from different sources. I looked up periodical's origin and it was Greek, which probably is why the "e" is long and the "i" is short and the reason that the "per" is pronounced as if it is "peer" (or "pir" if it was read in Spanish). In Spanish, Greek words ending in "a" are masculine, hence "el drama" so it's not only English that suffers from Greeks breaking the rules :p

I'm a math guy who's recently been interested in linguistics...the rules of math are much more orderly than the seemingly random rules of language