Liquid engines are very complex... they might SEEM outwardly simple, but they are not. The experts like Goddard, Von Braun, Oberth, Korolev, Glushko, Isayev, etc. experimented with liquid rocket engines for YEARS before getting ones that would operate safely without melting down or exploding outright. Establishing safe, smooth, steady combustion at startup is absolutely critical and the difference between a motor that explodes and one that actually thrusts. Of course things can go wrong after startup that can quickly melt an engine down, or cause it to explode, but startup is usually the first MAJOR hurdle...
Liquid propellants are incredibly volatile. "A tank of oxygen" won't cut it... that's GASEOUS oxygen (GOX or GO2) and for a rocket engine you need LIQUID oxygen, which is cryogenic and INCREDIBLY volatile in it's liquid form (basically anything liquid oxygen comes into contact with that's flammable will cause it to burst into flames... go to YouTube and look up "liquid oxygen fires" and you'll see what I mean... even a hot diamond dropped into liquid oxygen will burn away to nothing... even the oil from your fingerprints, if left uncleaned in an LOX line, can combust or explode under the right conditions...) Room temperature storable propellants (such as rocket grade hydrogen peroxide, red fuming nitric acid, nitrogen tetroxide, hydrazine, etc.) are either extremely volatile or highly toxic, or both. Some storable propellants aren't so bad, like Nitrous Oxide, which is what makes the liquid part of a hybrid actually possible.
You have to force the propellants into the combustion chamber against the pressure inside the chamber, with sufficient pressure to atomize or mix the propellants in the combustion chamber, and feed it at predictable rates, IN THE PROPER PROPORTIONS. An engine running oxidizer-rich will very likely melt down or explode due to the excess heating and corrosive effects of the extremely hot excess oxidizer. The metering of the propellants and the valving is no small undertaking in itself. Metering propellants through an orifice can give markedly different delivery rates based on the pressure differential across the orifice opening (delivery doubles as pressure squares). To pressurize the propellants, you either need something that's self-pressurizing (like nitrous oxide) by boiling off as a low cryogen as head pressure in the tank reduces from propellant expulsion from the tank, or you need a pressurant gas (for say kerosene or other non-cryogenic propellants that do not self-pressurize by boiloff at standard temperatures.) This would require the design and inclusion of a pressurization system to inject the propellant(s), with their associated lines, valves, regulators, etc. All this would be necessary for even a "simple" pressure-fed rocket engine-- a pump fed engine at this scale would be an extremely advanced design and not in the realm of a hobbyist's capabilities.
Besides, it violates all the safety codes, and IF you could build such a motor that would operate safely and reliably, it would cost literally hundreds or thousands of dollars and STILL be incredibly complex for other hobbyists to use, even compared to the already complex operation of "relatively simple" hybrid motors, which are WAY less complicated than a liquid rocket motor would EVER be...
Later! OL JR
