Liquid Propellants

Rockets that need higher energy propellants and controllability (throttling or restart capability) often choose liquid propellants. Generally speaking, liquid propellant combinations has higher energy levels (specific impulse) than solid propellant mixtures. A high energy propellant combination often used in launch vehicles is the mixture between oxygen and hydrogen. Depending on the mixture ratio, the specific impulse level can reach above 4300 m/s (438 s). This is 1.7 times more efficient than a modern solid propellant.

When we are talking about liquid propellants, we are often dividing them into the following parts:

a) Room storable propellants

b) Cryogenic propellants

c) Monopropellants

Liquids that holds a liquid state at room temperature is said to be room storable. These types of propellants are often easier to handle and store. Generally, they have a lower energy level than cryogenic type of propellants.

Example on some room storable propellants are listed in the table below:

ReactantFormulaTypeDensity @ +20 ºC [kg/m3]Characteristics
87.5 % Hydrogen Peroxide0.875H2O2·0.125H2OOxidizer1376Transparent, decomposes, monopropellant, non-toxic.
Nitrogen TetroxideN2O4Oxidizer1440High density, narrow temperature range, very toxic
Nitric AcidHNO3Oxidizer1549Corrosive, wide temperature range.
HydrazineN2H4Fuel1005Very toxic, monopropellant, low density, low temperature range.
RP-1CH1.97Fuel580Broad temperature range, low cost, very low density.
Unsymmetrical Dimethylhydrazine (UDMH)(CH3)2NNH2Fuel856Broad temperature range, very toxic.
MethaneCH4Fuel424Broad temperature range, poor density, low cost.

Liquid propellants in cryogenic state means that the temperature required for liquid state is well below the room temperature. Typically, a gas like hydrogen or oxygen becomes a liquid when chilled down to a low enough temperature.

ReactantFormulaTypeDensity [kg/m3]Characteristics
OxygenO2Oxidizer1140Boils at temperatures above −183 ºC when at atmospheric pressure. Low cost.
Liquid FluorineF2Oxidizer1636Boils at temperatures above −185 ºC when at atmospheric pressure. Very reactive, very toxic and high cost. Favorable density.
HydrogenH2Fuel71Boils at temperatures above −253 ºC when at atmospheric pressure. Very reactive. Low cost. Poor density.

The above listed propellants may all be used in particular combinations: a particular oxidizer against a particular fuel. The specific impulse level generated based on the propellant combination chosen but also on the mixture ratio. In rocketry we define the mixture ratio (O/F) as the relationship between the propellant mass flow of oxidizer over mass flow of fuel. The table below lists some optimum mixture ratios for different propellant combinations when combustion occurs at 3.45 MPa pressure. Specific impulse will increase somewhat at higher chamber pressures and opposite.

PropellantO/Foptimum [–]Flame temperature [ºC]Vacuum specific impulse [s]
Oxygen + Hydrogen3.82530435
Nitrogen Tetroxide + RP-13.53000297
90 % H2O2 + RP-17.02500298

In liquid rocket engines the reactants are stored in their respective tanks. Then they are either pushed by pressure or pumped into the combustion chamber where they are mixed and burned. By having the reactants separated makes the liquid rocket engine very safe compare to the solid propellant rocket motor where the propellant is premixed and stored inside a single compartment. The downside is increased complexity since several tanks/pressure vessels are needed alongside tubing and valves. On top of that we also need electronic sensors and control devices.

That said, an important benefit with liquid rocket engines is that they are much more controllable with respect to thrust modulation and the ability to turn the engine off — and even back on again — features which are extremely difficult to achieve in solid propellant motors. Liquid rocket engines often are used in launch vehicles, spacecrafts and landers.

A third type of liquid propellants are named monopropellants. This means propellants that alone can perform a exothermal decomposition process releasing energy that can be utilized for creating thrust. The following table lists some frequently used monopropellants:

ReactantCatalystDecomposition temperature [ºC]Specific impulse [s]Characteristics
87.5 % Hydrogen PeroxideSilver670150Non-toxic, medium performance, low temperature and low cost.
HydrazineShell4051100230Very toxic, high performance and high cost.

Monopropellants just needs a single tank. However, they do need a feed system, often in the form of a gaseous feed system using ether helium or nitrogen gas. In order to convert the liquid into a thermally hot gas suitable for expansion, the liquid needs to come into contact with a material that can lower the molecules’ activation energy to enter into a self-sustained decomposition process. Silver is an excellent catalytic material when using hydrogen peroxide at concentrations less than 91 % (rest is water). At higher peroxide concentrations the decomposition temperature will approach or exceed the melting point of silver. For such cases another type of material will have to be used.

Hydrogen peroxide decomposes into oxygen and steam (water vapor). There is no combustion going on — and therefore no burn — but the amount of generated hot gas is more than sufficient to produce thrust from expansion through an ordinary nozzle.

Hydrazine decomposes in a more complex way involving an endothermic reaction which has to be minimized. It is a two-stage process. The decomposition process is outside the scope here, but in a well-designed thruster (injector and catalyst bed system) the specific impulse can reach 230 s. Most modern spacecrafts and satellites use hydrazine monopropellant thrusters for orientation control. Typically, the thrust generated are from 1 N to 400 N.

Hydrogen peroxide as monopropellant are seeing renewed interest due to its lower cost and non-toxicity compare to the state-of-the-art Hydrazine, in particular so for launchers attitude control systems. Typical thrust levels for such applications are 10 to 250 N.

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This article is part of a pre-course program used by Andøya Space Education in Fly a Rocket! and similar programs.