Rocket Science — Basics — Part 8— Liquid Propulsion Engines 🚀🧑🚀
In this blog post, we will discuss about liquid propulsion engines.
We know that thrust is nothing but mass flow rate multiplied by exhaust velocity. If we need more thrust, we can either increase the mass flow rate or exhaust velocity.
Mass Flow Rate — It is the rate at which we use the mass (fuel + oxidizer). We can increase the mass flow rate by adding twice the fuel and oxidizer or have same engine twice but it has disadvantages mainly, rockets now can only run for 50% as long and it causes the double acceleration in half time is same as the rocket with one engine. We can say that it does not increase the delta-v (increase in acceleration).
Exhaust Velocity — It is the speed at which the mass (fuel + oxidizer) leaves the rocket. It is very difficult to increase it because, the amount of energy that we get during the combustion process is very precise and very difficult to increase it. For example, combining hydrogen (fuel) and oxygen (oxidizer) atoms we get water as the output with exactly 241.8 kJ (kilo Joules). Now, we are left with only an option of using the thermal energy produced to increase the exhaust velocity through efficient Nozzle design. Usually, we need a huge mass flow rate for launching rocket from the ground and not that much once it is in space.
Above is the schematic diagram of liquid propulsion engine. In this type of engine, the fuel as well as the oxidizer are liquids. They are mixed in the combustion chamber and results in exothermic chemical reaction creating hot gas that exhausted out from the nozzle.
We can also see from the diagram, both fuel and oxidizer are passed to the combustion chamber through pumps which can be controlled. Unlike solid rocket motors, here we can control the flow of fuel and oxidizer even completely stopping it to enter the chamber and restarting it.
Unlike solid propellants, the exhaust velocity will be in range of 3000 to 4800 m/s which is better than solid rocket motors.
Most of the recent rockets are using liquid propellants even falcon heavy from SpaceX. Even liquid propellant engines are used in space as thrusters in order to make tiny maneuvers in the orbit. Falcon series of rockets from SpaceX reuse their first stage of rocket after their launch which are safely controlled to land on ground station.
There are some disadvantages also in liquid propellants — The liquid oxidizer (usually oxygen) should be kept in -183⁰ C. Small failure in pumps can be devastating to the mission. Need to minimize Sloshing (shown below in figure) in the tanks during the mission.
Also liquid fuels are harmful and can be fatal if there is any leakage.
The nozzle needs to be very light as possible and it should not melt in the heat. We need to use pumps (usually turbo pumps) to get the fuel and the oxidizer into the combustion chamber. The liquids must be of very high pressure when they enter the combustion chamber. Why it needs to be at a very high pressure 🤔?
It is because at very low pressure, the pressure from the combustion chamber will make the liquids go backwards thereby blocking the fuel and oxidizer pipes. The way it works is using the high pressure gas that is a output combustion between the fuel and oxidizer to drive the turbine. This turbine then powers the pump, which moves the liquid fuel and oxidizer into the combustion chamber. The design of these pumps must accommodate extreme conditions, including high temperature differentials and the need to prevent cavitation — an issue that can lead to mechanical failure and mission failure. By using turbopumps instead of pressurized tanks, rockets can reduce overall weight, which is crucial for achieving the necessary thrust for launch. Engine cycles in rocket propulsion refer to the specific methods used to drive the turbopumps that supply propellants to the combustion chamber. The most commonly used engine cycle for rockets is staged combustion cycle.
In this type of cycle, small amount of propellant (usually fuel rich) is combusted in a pre-burner before entering the main combustion chamber. This allows for higher pressure and better efficiency as all propellants are utilized for thrust. This type of cycle increases the specific impulse (measure of fuel efficiency).
The disadvantage of this staged cycle is that the hot and pressurized gases produced in the pre-burner create harsh conditions for turbine components and plumbing systems.
We also need fuel injectors to mix the fuel and oxidizer. It can be electronically adjusted. This is important say when we want to mix the fuel and oxidizer ratio different for climatic conditions. The fuel is converted into fine droplets to be mixed efficiently with oxidizer which improves the efficiency of the system. This process is called atomization.
We can make use of our propellants (usually fuel) by plumbing it around the nozzle to cool it from melting because of heavy heat as you can see from the above diagram. This type of cooling is called as regenerative cooling and it is very efficient. As the propellant flows through the channels, it absorbs heat from the nozzle walls, thus cooling them down. The heated propellant is then injected into the combustion chamber, where it contributes to combustion efficiency.
Overall, liquid propellant engines works more efficient than solid propulsion motors.
In the next section we will discuss about hybrid engines and lasers & big guns.
Thanks for reading!
