When failure is not an option, stacking skill bonuses

There are some jobs in which failure is simply not an option. Players have to be able to succeed at all costs. To encounter challenges in a game in which failure means more than TPK (see image bellow), means players must have a high, almost certain, success rate. On the other hand if success is assured where is the challenge in the adventure? If success is a sure thing because failure is so catastrophic, doesn't the game just become a story in which the GM just narrates the outcome of one success after another?

Or is there a way to stack bonuses so skill checks are quite certain to succeed while still having that little margin for error that makes your heart skip a beat when you roll the dice?

I'm working on some skill mechanics to allow for characters to add their skill bonuses to a task, work together and make success almost certain. That almost might seem like too certain a success rate, until you see what's at stake if you fail (once again see image at left).



So here are some ideas to make this work:

• Die roll with a lot of steps, like d100 vs d20 which has only one fifth as many.
• Diminishing returns which means a bell shape distribution.
• Characters can stack their skill bonuses and attribute bonuses to the roll. There will still be a chance of failure given the diminishing returns.

Things I want this to work for:

Cut the blue wire

Weapon disarming in Weapons Free may be a very common task. Having help from an external source may add bonuses to your explosive skill check. Failure to cut the right wire or follow the proper disarming procedure can be quite catastrophic.

Identify agent

Someone is ill, what biological agent is it? Running some tests in a team of medics should add bonuses to solving the issue quicker. Failure to isolate the pathogen and find a way to stop its propagation can be quite catastrophic. Even if just confined to the unit in the adventure it would still be a game stopper.

Close quarter combat
The odds of putting a bullet in a target should be astronomically high at close quarter range, but drop off drastically as range increases. Skill should play a strong role here. At longer ranges bonuses from equipment like scopes should add up to make long range shots easy for those trained in their usage. Failure to be the first to hit its target when entering a room can be quite painful for the character that fails to take the target out.

Thoughts? How many games have you played that allow many characters to chip in on a die roll to increase their odds while still keeping a margin for error that makes the game exciting?

Image sources

http://gizmodo.com/5867017/taking-apart-nukes-is-probably-a-stressful-job

http://www.rpgpost.com/topic/15232-inspiration-strikes-21/