There's defiantly a mention of it in 'Terra Firma' when D.K. and Laura poking around the his module and asks what those modifications are.
"I'm not sure what I believe anymore. All this crap you've tacked on!"
"Hetch drive.- FTL."
"It can't work. It's theoretically impossible."
"Theory is wrong."
There's a handful of other mentions
which are easy to miss and they never get into any explanation. I gather the writers didn't care very much and good on them too.
Only Leviathans are capable of starburst; everyone else is stuck with Hetch which is much slower.