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I have been wondering about why oxyhydrogen (HHO), which is a ~2:1 H2/O2 stoichiometric mixture, hasn't been used as a fuel? I know it has been used for bottle rockets and other types of small scale rockets, and I know hydrogen and oxygen have been used as fuel (like in the delta IV) but could it be used for a large scale rocket? Would there be any problems with just one fuel tank holding both the oxidizer and fuel (maybe it would be dangerous to keep them together?) Would there be any changes needed to engines to use HHO as a fuel?

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    $\begingroup$ As a gas, it's prohibitively voluminous for rocket use; as a liquid, it's incombustible water. $\endgroup$ Feb 20, 2018 at 21:22
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    $\begingroup$ what if you condensed it? $\endgroup$
    – jasper
    Feb 20, 2018 at 21:26
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    $\begingroup$ You would need more energy to power the electrolysis than you'd get back from combustion; if you have that much electrical power at hand you'd be better off using an arcjet. (see space.stackexchange.com/questions/12372/…) $\endgroup$ Feb 20, 2018 at 21:29
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    $\begingroup$ @jasper I did, all the reputable links I found talk about debunked water fueled cars, or the use of HHO as a fuel additive not as an actual fuel. $\endgroup$
    – Lex
    Feb 20, 2018 at 22:05
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    $\begingroup$ HHO gas isn't practical to store/handle at the scale needed for anything more than a toy rocket; it would also be kinda dangerously explosive. If you chilled it down to liquefy it for more practical storage, the oxygen would condense, then freeze solid before the hydrogen condensed. So, you'd want/need to keep the liquid hydrogen and oxygen in separate tanks, which is what rockets do. $\endgroup$
    – Anthony X
    Feb 21, 2018 at 3:43

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Oxyhydrogen is an extremely bad idea for a fuel. It is highly explosive, and too dangerous to be used on rockets.

But rockets today do use liquid hydrogen as fuel which is oxidized by liquid oxygen(Lox/LH2) as it is cheap and provides extremely high thrust and can be controlled to burn steadily. It is one of the most powerful liquid propellants.It also yields the highest specific impulse of any currently used rocket propellant.

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Oxyhydrogen is not fuel in the strict sense of that term. Instead, it is an explosive mixture, made from stoichiometric proportions of fuel and oxidizer. As such, it could theoretically be used in reciprocating internal combustion (IC) engines, like those used in cars. These engines normally use explosive mixtures made of atomized fuel droplets finely dispersed in atmospheric air, but note how this mixture is prepared in situ and utilized immediately afterwards. It is partly due to technical reasons, as car fuel usually tends to be liquid in room temperature and solid in temperature of oxygen's boiling point, and it would make no sense to attempt storing an aerosol of fuel droplets nor reinforce ordinary car engines enough to withstand pressures needed to contain liquid oxygen in room temperature.

Nonetheless, even if it was technically possible to do in cars, it would not be done because such a car would be a horribly dangerous piece of equipment presenting perpetual explosion risk, potentially lethal to anyone within the radius of several dozens of meters. Absolutely unacceptable from the public safety's viewpoint, and the driver surely would also not appreciate having to operate a vehicle that was actually a ticking explosive device in disguise, a structure straight from the forbidden chapter of civil engineer's handbook. I'm half kidding, of course, as some currently-in-use solutions are honestly not that much better. Not only that, but HHO mixture does not deflagrate: it detonates instead, and that is highly undesirable in car combustion engine. Not only it wastes energy, but damages the engine as well. That phenomenon is named engine knocking, and to put things in perspective about how undesirable it is, certain individuals in the past, about whom I cannot find enough rotten things to say about, had come into conclusion that contaminating our entire environment with a neurotoxic heavy metal is a reasonable price to pay for preventing that phenomenon from happening in car engines. Yikes.

However, rocket engines operate differently than those of cars. While most of them are also classified as IC engines, the are not reciprocating IC engines but continuous combustion IC engines. Instead of going through intermittently periodic cycle of igniting an explosive mixture of fuel and oxidizer inside a chamber, and then harnessing the power of said explosion to force a piston downward in engine's cylinder, rocket engines harness the thermal energy released by combustion; the resulting pressure in the combustion chamber is converted to thrust by ejecting the combustion products through the nozzle, according to the recoil principle.

In case of HHO, it is reasonable to assume what you imagine was a monopropellant engine. Continuous combustion makes it somewhat more troubling for HHO to be used in this context, because as mentioned, it is an explosive: once ignited, this mixture does not really combust, but outright detonates. That's wonderful if you want to perform a demolition or break rocks for an excavation, but not if you want to cleanly and relatively safely generate thrust. That is difficult to control, and you surely need to be able to control that in order to finely tune the thrust generated by engine and throttle the engine if needed.

Let's compare HHO to other monopropellant, like hydrogen peroxide. Water solutions of hydrogen peroxide in concentrations higher than 68% are also explosive, potentially being able to be exothermically converted entirely into hot water vapor and oxygen. However, it is possible to localize where the decomposition happens by the use of a catalyst bed. It facilitates controlled decomposition that could be regulated, and thus allows you to throttle the engine if needed.

On the other hand, in case of HHO engine I cannot possibly imagine how one could safely localize the the reaction. Any potential flashback would go back inside the fuel storage chamber -- and have in mind that the velocity of detonation front of HHO gas is around 1.8 to 3.8 km/s. Once a flashback happens and does not get arrested before reaching the fuel tank, your whole remaining amount of fuel would release its energy all at once, ripping the whole rocket apart.

Here is a video of NightHawkInLight building and using a HHO torch. WARNING: this is extremely dangerous, absolutely do not attempt recreating that yourself! In his setup, he uses a thin metal nozzle made of a syringe with a needle of miniscule diameter (0.6 mm) to increase the velocity of gas flow in order to prevent flashbacks. The metal nozzle also serves as a radiator, facilitating the dissipation of flame's heat and thus slowing down the potential backward movement of the flame front; he has also stuffed some aluminium wool into the syringe to further facilitate heat dissipation in order to reduce the chances of flashbacks. Nonetheless, he still uses two separate flashback arrestors, wears goggles, stores the HHO producing unit in a metal cage, and uses a steel sheet to shield himself from it. He also happens to have committed a big whoopsie, as he appears to be using stainless steel electrodes for water electrolysis. Stainless steel generally should not be used for aqueous electrolysis because it contains a significant fraction of chromium which, depending on the electrolytic cell's pH and voltage, could potentially get oxidized on the anode to the highly toxic, carcinogenic, and corrosive Cr(VI) form (hexavalent chromium), namely chromate and dichromate anions: $\text{CrO}^{2-}_{4}$ and $\text{Cr}_{2}\text{O}^{2-}_{7}$. Any solutions containing Cr(VI) are extremely harmful to the environment with long lasting effects and cannot be disposed of by pouring them down the drain; one needs to give them to a chemical waste company and pay a fee to have such solutions properly and safely disposed of. Do not recreate; not only you could potentially produce volumes of hazardous waste in the form of forbidden cancer juice, but if anything goes wrong, you may also go blind, deaf, or even end up being stuck sniffing flowers from below the ground.

That is why HHO is not used as fuel. It is inherently dangerous to store any non-trivial amount of oxyhydrogen gas for extended periods.

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HHO mixture. Energy per gram: 112KJ. TNT. Energy per gram: 4KJ.

A HHO mixture contains everything it needs to combust, all tightly packed together. We generally call such materials explosives.

Furthermore, you've got a big storage problem. The melting point of oxygen is far above the critical point of hydrogen, no liquid can exist.

We already have an example that approximates HHO as a gas: A potato cannon. It's far more powerful with HHO than the normal hydrocarbon in air, but it still doesn't have enough energy to be a useful rocket.

That leaves solid. Below 14K you can have solid HHO but you have nothing to moderate the burn, you would get a mushroom cloud, not a space flight.

In practice hydrolox engines don't mix them until they enter the combustion chamber. It means two pumps and two tanks but it will actually get you up there.

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