How do you make completely invulnerable sub?

[Col7777]

I used to have an indestructible sub in SH2, I used it for testing certain things out, the reason was if I got destroyed too early it meant I had to keep reloading the scenario to try out what I was testing again.
I'm not 100% sure but may be altering the armour so your sub takes more hits till it is destroyed, there may be something a lot more simple I'm not sure.