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According to my understanding (which may be incorrect), the cryogenic form and liquid form of propellant both use a fluid as an oxidizer and fuel. Research tells me crygenic propellant is more efficient and provides more thrust in comprison to normal liquids. What is the reason behind that and what is main difference between these two?

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  • $\begingroup$ Kerosene is a liquid... at room temperature. Not very cryogenic. $\endgroup$
    – RonJohn
    Commented yesterday

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Cryogenic propellants are a subset of liquid propellants1.

Cryogenic just refers to materials kept at very low temperatures. Cryogenic propellants are those such as hydrogen, oxygen, or methane, that are gases at room temperature, but liquify when very cold. You can put more mass of liquid in a tank than gas, and rockets need to carry a lot of fuel to get anywhere.

The most common non-cryogenic liquid propellants (also known as "storable propellants", because they can be stored for long periods without active refrigeration) are fuels in the hydrazine family (MMH, UDMH, or a mix), used with oxidizers like nitrogen tetroxide. These are unpleasantly toxic, but they are convenient to use because the propellants are hypergolic, meaning they ignite as soon as they mix, and the rocket can remain fueled for long periods of time, which is convenient for military missiles.

One of the most common fuel-oxidizer combinations is kerosene fuel (aka RP-1 or RG-1) with liquid oxygen. In this case the oxidizer is cryogenic and the fuel is not.

Different propellant types perform differently because of their particular chemistries, rather than their temperatures -- if you built a rocket that used room-temperature, gaseous hydrogen and oxygen, the engine performance would be similar to a cryogenic one, or even slightly better (but the rest of the rocket would be much larger and heavier, since rocket stages are 90%+ fuel tank.)

The metric for engine efficiency is called specific impulse. Impulse is the product of thrust and time; specific impulse is how much impulse you get per unit of propellant mass. Higher specific impulse means you can get more thrust, or the same thrust for longer. Specific impulse is often measured in seconds, meaning "how many seconds can the rocket produce one pound of force using one pound[-mass] of fuel".

Specific impulse can also be measured in velocity units such as meters per second, and for a pure rocket, the specific impulse is the same as the average velocity of the exhaust.

Peformance-wise, the highest-performing practical propellant combination for a chemical rocket is the cryogenic liquid hydrogen/liquid oxygen mix, with a specific impulses of around 420-450 seconds depending on the engine design. This combination is what's most often meant when people say "cryogenic propellants", which explains where you got the notion that cryogenic propellants are more efficient.

Cryogenic methane-oxygen is slightly less efficient than hydrogen/oxygen, with specific impulses in the 330-380 second range.

Below this are the half-cryogenic kerosene-oxygen mix and non-cryogenic UDMH-NTO, with typical specific impulses in the 260-320 second range.

Solid rockets are the least efficient, with specific impulse typically around 250 seconds, but they have a good thrust-to-cost ratio, so they're often seen used as boosters for part of the first-stage burn. Most modern military missiles use solid rockets instead of storable liquids for safety reasons.

  1. Nearly; apparently there have been experiments with cryogenic solid propellants, but no production rocket uses them.
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    $\begingroup$ I will note that your first line is not entirely accurate: there have been studies into cryogenic solid propellants. (Using frozen Methane-Oxygen, IIRC) $\endgroup$ Commented 2 days ago
  • $\begingroup$ So noted, thanks. $\endgroup$ Commented 2 days ago
  • $\begingroup$ Let's add - specific impulse is directly proportional to exhaust velocity. Pumping the same amount of thermal energy into lighter particles results in higher exhaust velocity. Light propellants tend to produce lighter exhaust particles, but also tend to be gaseous in ambient temperature, so to pack a significant amount of them you need to liquefy them. Liquefying by compression would require extremely heavy heavy-duty pressure tanks, so liquefying by cryocooling is the way. $\endgroup$
    – SF.
    Commented yesterday
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    $\begingroup$ There's a survivorship bias aspect to the "cryogenic propellants are more efficient" idea - nobody talks about cryogenic propellants that don't provide higher performance than storables, because there would be no point to using them. $\endgroup$
    – Cadence
    Commented yesterday
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A cryogenic liquid is a very cold liquid. The term is now generally used for gases that cannot be liquefied by pressure alone at room temperature, or to put it another way gases that have critical temperatures below room temperature.

Examples are oxygen, hydrogen and methane. If these gases are compressed at room temperature without additional cooling they will never liquify. The gases just become denser and denser until they can be described as super critical fluids but they never go through a phase change as in gas > liquid. These supercritical fluids tend to require huge pressures making storge problematic. It is much easier to cool the gases down and let them condense into traditional (cryogenic) liquids.

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  • $\begingroup$ I don't think the critical question is critical point. hydrogen for example can be liquid at normal temp by pumping up the pressure. it's just that lowering the temp is more efficient $\endgroup$ Commented yesterday
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    $\begingroup$ @Oscar Smith no it can't. That's the whole point, if you compress hydrogen at room temperature you will not see any phase change to a liquid the gas will just get denser and denser until it morphs slowly into supercritical hydrogen. researchgate.net/profile/Rami-Eid-4/publication/362724024/… $\endgroup$
    – Slarty
    Commented yesterday

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