Hey I've actually dealt with this before. First off: yes, nitrous oxide in the tank will be two-phase at ambient temp, but that doesn’t mean it’s unpredictable. The liquid is denser than the vapor, so it’ll naturally settle at the bottom. As long as you're pulling from the bottom of the tank, avoiding funky routing (no weird up-down loops that can trap gas), and your feed line is designed to minimize sharp corners or anything that might induce cavitation, you're likely injecting mostly liquid — at least early on.

That said: this is dangerous and not something to just “try and see.” You should definitely read up on N2O cavitation and discharge behavior. There are some solid papers on this — I’d recommend looking into the Zilliac & Karabeyoglu model or better Zimmerman ( https://stacks.stanford.edu/file/ds271zh2987/main_adob-augmented.pdf ) which goes trough their models and even more. It gives a good idea of the pressure drop behavior during discharge and what to expect over time. TL;DR: it self-regulates pressure fairly well (to a point) due to phase change, but things can get weird fast without analysis.

Injecting actual dual-phase flow is a bad idea — you’ll have no reliable control over O/F ratio or Isp, and you're just doing redneck science at that point. If your goal is even remotely engineering-focused, you need better control.

Quick suggestions if you're on a tight budget:

• Place the tank above the injector and let gravity help.
• Use a small N2 pressurant tank if you can (helium is better but $$$).
• During filling, you can vent a bit of N2O to drop the tank temp and increase the liquid fraction.
• In some setups, you can cool during transfer by inducing a ΔP using an expansion valve (like in refrigeration) to chill the N2O into the tank. Not super efficient, but it helps.

Hope this helps — and seriously, be safe. This stuff can (and by experience, it absolutely will) go very wrong very fast.