B5 style drives

We've been toying with the idea of going to B5 style drives, and
what we're looking at is as follows.

A drive costs 4% of the mass of your ship +3% per thrust.

A destroyer, which will go boom soon anyway, would probably
get 1 drive.  So your thrust 6 destroyer uses (4% + 18%) or
22% of its mass for drives, saving 8%, a box or two.  If
it fails a drive threshold it has no drive.  Not a biggy as
it would probably be a mission kill anyway.

A Cruiser needs a bit of redundancy, so we put two seperate
thrust 2 drives.  This gives is (2x4% + 2x(2x3%) ) or
20% for drives.  Dead even with the current setup.  Note it
could still lose both drives by failing thresholds twice,
but this is less likely.  This also balances a bit the 
fact that it is cheaper for this ship to have thrust 6
drives.

That dreadnought that you really want powered could be thrust
4 with 1 thrust two drive and two thrust 1s.  This would
be (10% + 2x7%) or 24%.  A bit more mass for a bit more
redundancy.

Any thoughts?

Roger