Struggling with Forward Motion

I've wanted a system for movement and combat in zero-g for my sci fi games. I do not want a cumbersome subsystem with lots of little caveats to remember. 

It has to be dead simple; the inputs (interacting with the system at the table) have to be grounded enough in common understanding that the system feels natural to use, and the outputs (the effects on characters) have to interact with the game system in a way that feels impactful, and reminds us of the inhospitable, unnatural environment we are trying to navigate.

This is written for use with Stars Without Number, but should be pretty easily adaptable for any game that uses Saves, Attacks Rolls, and Ability Tests. 

//Movement and Combat in Zero-G

Instability. In zero-g, the careless and the untrained can quickly lose control of the situation and themselves. PCs in zero-g gain an Instability Die each time they…

➤ Make a Melee Attack       ➤ Fire a Bow or Thrown Weapon     ➤ Take Damage from an Explosion^

➤ Fire a Firearm        ➤ Take damage from a Melee or Thrown Weapon*

^: The character also moves in a straight line away from the blast until they reach an obstacle or stop themselves.

*: Blunt, “bludgeoning” weapons only. Blades and bullets may risk a tear in your vac suit, but they don’t destabilize.

Instability Die. Rolled alongside all Attack Rolls, Skill Checks, and Evasion Saves, and subtracted from the result. The die begins as a d4. Whenever that PC would gain Instability, the die is stepped up by one size - first to a d6, then finally to a d8.  At the GM’s discretion, Burst Fire with a firearm may count as two instances of Instability.

Reducing and Preventing Instability. If something stable is within arm’s reach, characters can use a Move Action to reduce the Instability Die by one step - d8 to d6, etc - or a Main Action to remove the Instability Die completely. 

With a means of propulsion, like maneuver jets, or by venting 2 hours’ worth of oxygen from a tank, characters can use a Move Action to completely remove the Instability Die.

Holding on to something stable prevents Instability gain, but precludes the use of two-handed weapons or tools. 

Movement. Characters may use a Move Action to push off from a surface, and continue moving in that direction until they reach a fixed object. Turns in combat still comprise 10 meters of movement. While in motion, characters may use a Move Action to grab a stable object in arm’s reach and come to a stop.

Characters may crawl along basically any surface, provided some kind of handholds are present. They do so at half their normal movement rate and must use both hands.