A rocket, or in this case a rocket thruster uses newton 3rd law to provide motion. Unfortunately, powder toy doesn't allow motion unless using frme, pstn, or powders. But you can simulate how it would thrust using pressure or other measurements. The best way to make a thruster is out of walls or dmnd as some propellants can melt even ttan. A good fuel is loxy and nitr as under certain conditions they may explode (especially using portals.) You can use many other fuels to experiment with. A good rocket design is very narrow in a triangle shape (you can make it taller by holding the control key and scrolling the mouse wheel.) Uses nitr and loxy supplied by portals. I'd recommend putting the loxy and nitr in separate containers with clone and make the entire loxy container -273.15 C and a portal in the thruster of the same temp. From here you should have a working thruster.