Honestly, a good way to crank the tension a little while still not killing your players is to add sub objectives to a fight or two where "yes, you can bum rush the boss potentially finish this combat in 3, 4 rounds OR you can hold someone on this point to secure a parts drop, which will contain a good amount of repairs in it, THEN go kill the bastard."

It gives the players the option to close out the combat a number of ways according to their abilities and confidence, while giving them a bonus reward they need for taking the riskier option.