1. Each engine have limited number of available ignitions.
2. Engines requires resources (hypergolic fluid) and ignitors (and also electricity) to be activated. The required amounts of consumables can be configured in CFG. (Example MM patch included, additional explanation see below).
3. If lack of ignitors or hypergolic fluid - ignition will fail.
4. Its functionality could be restored using reloading by kerbalEVA from additional containers (included in pack).
5. Simplified ullage simulation available. If it is on, fuel have two condition s: "stable" (while under gravitation or acceleration) and "unStable" (if weightlessness). For "unStable" state - you can specify a chance of succesfull ignition (0.2 or 20% in example). Better way - to have small special engines for fuel rebound (sit down) down and reignition in weightlessness.
6. Of course, here are unlimited consumables if you'll use Launch clamp. This way onboard resources will not be consumed on launch. (useless to my mind, but let it be) can embed YouTube videos or screenshots. Be creative.

You can check out the SpaceDock markdown documentation for tips.

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