So I’ve seen engines like these which use gaseous oxygen as an oxidizer and then a solid tube as a fuel. But do these types of hybrid engines have any possibility of creating a useful amount of thrust? I’m asking this because the pressure of the oxygen being fed into the engine is low (I estimate between 70-150 psi) so if the engine somehow did produce a useful amount of thrust (enough to launch a model rocket) wouldn’t the pressure from the combustion travel back into the oxygen line?
Yes, the chamber pressure in a pressure-fed engine must be lower than the propellant supply pressure. For example, the space shuttle Orbital Maneuvering System engine operated at a chamber pressure of ~130 psi and was fed from tanks pressurized to ~250 psi.
Another issue that is going to limit the thrust from that demonstration engine is that it doesn't have a nozzle, just an opening.
Source for OMS data, section 2.18
These types of hybrid engines can create a useful amount of thrust because the combustion chamber is not a pressure amplifier, it is a volume amplifier. The total area of the input ports is much less than the area of the exhaust port, consequently, the velocity of the gases flowing through the exhaust is much higher than the fluid/gas entering the combustion chamber.
Yes, the pressure in the combustion chamber is less than the pressure in the oxidizer supply line and the pressure at the exhaust port is lower yet.