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Jet Propulsion

Let’s recap a bit:

The burning of fuel creates gases at high pressure, which exit from the exhaust nozzle and push the rocket forward. As gases exit the rocket, a reaction force (thrust) pushes on the rocket making it go forward. The faster the gases are expelled from the rocket, the greater the thrust. This is the case for ROCKET PROPULSION.

Again, ROCKET PROPULSION.

Yes, jet propulsion systems are for aircrafts, but there is a tiny detail we miss when we study about these 2, COMPLETELEY different systems.

There is no atmosphere in space. It’s a vacuum. So it solely works on the Newton’s Third law of motion- For every action, there is an equal and opposite reaction. For accelerating the spacecraft, one needs to supply higher thrust than that of drag forces and gravitational force acting on it.

Fa = -(Fr)

The acting force is the force exerted by one body on another, while the reacting force is exerted by the second body on the first. Although these forces have equal magnitude and occur in opposite directions, they never cancel each other because these forces always act on two different objects.

So, we’ll rename it as Rocket propulsion:

Rocket propulsion

In fact, jet engines and rockets operate on the same general physics principle. Both eject fuel out the back. The momentum imparted to this exhaust is equal to the momentum gained by the vehicle, thus making the vehicle go forward.

One difference between rockets and jets is found in the type of fuel they burn. Jet engines are air breathers. They take in air (which contains oxygen needed for combustion), mix it with fuel, burn it to increase the pressure, and exhaust the spent gases out the back at a high rate of speed. This high-speed ejection of mass propels the plane forward.

Rockets do almost the same thing with two exceptions:

• They carry their own oxygen along with them

• A rocket does not have wings that add lift

On the space shuttle, you notice an orange tank, which actually contains separate tanks of hydrogen and oxygen. These two ingredients are mixed in the liquid-fuel rocket engine, burned, and expelled out the nozzle. The white, solid-fuel rocket on each side contains a chemical mixture in which the oxidizing agent is part of the fuel. Rocket fuel can burn without external oxygen being present.

(As a side note, once a solid fuel rocket is ignited, it cannot be turned off. Jet engines must have outside oxygen from the air.)

So we conclude that:

• It carries its own oxidizer and fuel.

• It can operate in atmosphere and space.

• Its performance does not depend on altitude and flight speed.

• It can operate at any flight velocity (Mach1 number: M = 0–25).

• Rate of climb increases with altitude.

Most of the mass of a rocket before launch is propellant. Much of the rest—supporting structure, tanks, pumps, engines, and more—is useless once the fuel has burned. To avoid having to carry all that excess weight into space, rockets often have several stages, or sections, each of which drops away after use. Such as boosters and first-stage engines, along with thermal insulation and fairings.

This helps reduce the weight of the rocket, which will help in using less fuel, a.k.a. improving the efficiency of the rocket.

Rocket equation:

It describes the relationship between a rocket's change in velocity (Δv) and its initial and final mass. It is known as Tsiolkovsky rocket equation.

In order to understand the principle of rocket propulsion, Konstantin Tsiolkovsky proposed the famous experiment of "the boat". A person is in a boat away from the shore without oars. They want to reach this shore. They notice that the boat is loaded with a certain quantity of stones and have the idea of quickly and repeatedly throwing the stones in succession in the opposite direction. Effectively, the quantity of movement of the stones thrown in one direction corresponds to an equal quantity of movement for the boat in the other direction (ignoring friction / drag).

To use the conservation of momentum, we must compare the initial and final total momentum; however, there is a minor complication in this. Mass is not ejected one-by-one in a rocket, instead it is fired continuously

where,

Δv: is the change in velocity of the rocket.

ve: is the exhaust velocity of the rocket (the speed at which the exhaust gases are expelled).

mi: is the initial mass of the rocket (before burning fuel).

mf: is the final mass of the rocket (after burning fuel).

The change in velocity (Δv) is proportional to the exhaust velocity and the natural logarithm of the ratio of initial to final mass. This means that a higher exhaust velocity and a greater mass ratio (mi / mf) will result in a larger change in velocity.

Rocket science isn’t necessarily JUST about rockets, but everything related to rockets and what they do in space. We’ll be discussing about satellites in the next newsletter. So, stay tuned!

Hasta la vista!

Who am I?

I am Anvita. Born and brought up India. I’m currently in high school (HSC) , and have a strong interest in Physics and Astronomy. I am an artist, love playing volleyball and I read, occasionally. I am also a foodie, so I like to cook sometimes!

If you want to catch up with me, you should check out my:

LinkedIn: http://www.linkedin.com/in/anvita-srivastava-311b86325

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Understanding movements:

If you’re familiar with graphs and number line, you might already know axis. There are 2 main axis when we talk about 2 dimensions; the x-axis and the y-axis. When we put things into real life considerations, we add another axis, the z-axis, which determines a 3 dimensional figure/shape. So, imagine the movement of a bird in 3D space. It can possibly move forward, up, down, left and right.

Similarly, a rocket/spacecraft also moves in these directions. A rocket can mainly maneuver in the following ways: FORWARD, LEFT, RIGHT, UP, DOWN and:

• Roll: Rotation (along the axis)

• Yaw: Rotation (left or right)

• Pitch: Rotation (up and down)

These 3 rotations happen along the axes x, y and z. Just like the movement of a bird. And so, these are called the “Degrees of Freedom”.

Reaction Control System (RCS)

In space, one of the most effective ways to reorient a spacecraft is by firing small thrusters that are strategically positioned on its body. The Space Shuttle RCS (reaction control system) is required to provide propulsion thrust for precise attitude control and three-axis translation. This capability must be provided for:

External tank separation, On- Orbit operation, Deorbit and Reentry

In order to meet these requirements a bipropellant, pressure-fed system utilizing monomethyl hydrazine as fuel, and nitrogen tetroxide as oxidizer has been selected.

PURPOSE:

1. Control orientation (pitch, yaw and roll)

2. Perform small maneuvers for docking, orbital adjustments, and payload deployment.

3. Stabilize the spacecraft during various phases of flight.

RCS has various components;

• Thrusters- RCS thrusters are small engines used for precise attitude control and small translational maneuvers, allowing for docking, course corrections, and emergency maneuvers. Types of RCS thrusters:

  1. Cold Gas thrusters- Uses pressurized gas, like nitrogen.

  2. Bipropellant thrusters- Use a combination of fuel and oxidizer, like monomethyl hydrazine and nitrogen tetroxide.

• Reaction Wheels- These are electrically driven devices that use spinning wheels (like flywheels) to store and change angular momentum, allowing for precise control of a spacecraft's orientation (pitch, roll, and yaw).

• Sensors- Sensors are used to calculate the attitude and rate of change.

• Control system- A computer system that processes sensor data and commands the thrusters or reaction wheels.

RCS works by 2 ways:

1. Attitude control- Attitude control refers to the process of controlling the orientation of an object, particularly spacecraft or satellites, with respect to a reference system (like the celestial sphere or a target). By firing thrusters in specific directions, the RCS can create torques that rotate the spacecraft to the desired orientation.

2. Translation- Making precise adjustments to a spacecraft's position, such as during docking or orbital maneuvers. By firing thrusters in a specific direction, the RCS can create thrust that moves the spacecraft in that direction.

Orbital Maneuvering System (OMS)

The OMS is the system of hypergolic1 liquid-propellant rocket engines used on space shuttles. It provides propulsion for the space shuttle vehicle during the orbit phase of flight. It is used for orbital insertion maneuvers after the main propulsion system has shut down. It is also the primary propulsion system for orbital transfer maneuvers and the deorbit maneuver.

The OMS is located in two independent pods in the aft end of the orbiter on either side of the vertical tail. Each OMS pod contains one OMS engine, a fuel tank, an oxidizer tank, and a helium tank, along with propellant feed lines and other supporting equipment.

The OMS engines use monomethyl hydrazine as the fuel and nitrogen tetroxide as the oxidizer. The propellants are hypergolic, which means that they ignite when they come in contact with each other, therefore no ignition device is needed. This adds reliability to the system, but the propellants are very corrosive.

Each OMS engine has a gaseous nitrogen tank that provides pressurized nitrogen to operate the engine valves and purge the fuel line after burn completion. The OMS engine does not have propellant pumps; propellant flow to the engines is maintained by pressurizing the propellant tanks with helium.

An automatic computer sequence controls all OMS burns. It is in the OMS engine that the fuel is burned with the oxidizer to produce thrust

.

Thrust Vector Control (TVC)

TVC maintains the vehicle's correct attitude for the thrust duration by rotating (gimballing) the thrust chamber or by redirecting the exhaust-gas flow so the thrust generates a vehicle torque.

Common methods for directing the exhaust gas are moveable vanes or injection of another fluid into the flow. TVC is typically used to control pitch and yaw attitude on boosters and upper stages.

A pair of linear actuators2 is positioned to allow rotation of the engine or the nozzle on its bearings along orthogonal planes to move the resulting thrust vector.

I am really hesitant in writing about jet propulsion, as understanding physics about it is very vast and extensive. Jet propulsion honestly deserves a post of its own, so that’s what the next newsletter will be about.

hasta la vista!

STRUCTURE OF ROCKETS

Whenever you think of the word “rocket”, you must imagine a cylindrical shape structure with a curved cone on the top, and wide cones on the bottom. Something like this:

Quite an accurate imagination, though you can’t deny the fact that your imagined rocket looks like a baby shark. Which reminds me, when we talk about sharks, fishes or even birds, they all have one thing in common: A STREAMLINE BODY.

And that is what rockets have too! This streamline shape helps in cutting through the air more easily and move forward. It helps in experiencing less friction with the air around. Makes them more aerodynamic. Your favorite sports car has also been built in a streamline shape.

A rocket is mainly divided into 6 parts:

1. Nose cone- It is the top most part of a rocket, designed to adjust oncoming airflow behavior and minimize aerodynamic drag. What is drag, if you may ask? Drag is an aerodynamic force that opposes the upward movement of a rocket.

   The nose cone is made of carbon composite because it has the lowest overall weight of all available materials. It has to be the strongest part since it has to go through:

   • Rapid temperature changes

   • High speed/velocity

   • Surface cracking due to high thermal conductivity

   • Thermal shock

   • Large heat flux

     

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2. Payload system- It is the part of rocket which holds the primary objective of the mission. They can have:

   • Scientific instrument- They are for the understanding of the universe. They mostly carry telescopes, spectrometers, radio beacons, solar radiation monitor, terrain mapping camera etc.

   • Communication equipment- Since we are sending a mission to space, we need to communicate with our rocket, which is quite obvious. We receive picture and data through communication equipment such as radars, radio antenna, modems, transponders etc.

   • Navigation instruments- It is crucial to know to position of our rocket to navigate it. We put star trackers, gyroscopes etc.

   • Observation payloads- Anything that needs to be seen or observed and sent to us through visual devices, such as cameras, radios, ladars, radars etc.

   • Human payloads- They carry the needs oh human beings in space. This can be any life support, waste deposition, thermal control etc.

     

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3. Guidance system- The guidance system of a rocket includes very sophisticated sensors, on-board computers, radars, and communication equipment. The guidance system has two main roles during the launch of a rocket; to provide stability for the rocket, and to control the rocket during maneuvers. It has 2 main roles to perform:

   • To provide stability

   • To control the rocket during maneuvers

     

     i) Movable fins used to adjust to the amount of aerodynamic force applied.

     ii) In a gimbaled thrust system, the exhaust nozzle is moved, thus, the direction of the thrust is changed relative to the center of gravity of the rocket.

     iii) The right vernier rocket engine has been fired to cause the nose of the larger rocket to move to the right. This helps in changing the direction of the rocket.

     iv) Small thrust vanes were placed in the exhaust stream of the main rocket to deflect the thrust and produce a control torque.

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4. Body of the rocket- It is the part that holds the propulsion system of the rocket. It is usually made up of titanium and aluminum because it has to be strong enough to provide structure and both the materials are strong and quite lightweight.

   

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5. Propulsion system- It is the system that powers a rocket. It is made up of various engines, fuel, tanks, pumps etc. The most of the rocket is the propulsion system. It is so, because to launch something so heavy, that’s facing great amounts of friction, atmospheric pressure, extreme temperature rise, going against the earth’s gravity and needs minimum 11.2 km/s speed to even escape the earth, would need a large amount of fuel.

   

   We need to clear something first. Both rocket and jet engines use jet propulsion to develop thrust. Rocket propulsion differs from its air-breathing cousin mainly in that rockets have self-contained propellants1. This feature allows operation at high altitudes and in space.

   Now you must be thinking that a rocket needs a FUEL and an OXIDIZER in order to provide energy, right? So what is the “propellent” or “propulsion” part?

   Well, fuel and oxidizers are the propellants. Fuel is the substance that burns to create the expanding gasses that power a rocket engine. In order for anything to burn, you need oxygen or something chemically similar: an oxidizer. Oxidizers become necessary in the vacuum of space, where there is no oxygen to help ignite the fuel.

   Propane (C₃H₈; Propellant A-108), butane (C₄H₁₀; Propellant A-17), and isobutane (C₄H₁₀; Propellant A-31) are the most commonly used hydrocarbons.

   A separate ignition system, called igniter, is assembled in the rocket motor to achieve the task of starting propulsion.

   The thrust of a rocket engine is the force that propels the rocket forward. It is measured in pounds or Newtons. The thrust of a rocket engine depends on the mass of the exhaust gases, the speed of the exhaust gases, and the area of the nozzle. We will discuss about these factors in the next part.

   

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6. The Fins- Just like sharks and fishes, rockets too need fins for changing direction and moving on the path desired. They control the direction and stability of a rocket. You might have seen that a rocket, in most cases, have 3 fins. Why?

   This is just some simple grade 10th physics. Vectors. In the direction, one fin presents minimum surface, the other two fins combine vectored forces to make up for it. Its now that 4 finned rockets are more in use.

   In a 4 finned rocket, 2 pairs of fins are placed perpendicular to each other, so in the direction, one pair presents minimum stabilizing surface to the airstream, the other presents its maximum surface.

   

   Why are fins important?

   • As air flows over the fins it creates pressure on the surface of the fins that hold the rocket straight. Without them, the slightest bit of wind can destabilize the rocket.

   • IF a rocket tries to move up and straight, without fins, it will move off the upward course, spiral, and may turn back downward due to air pressure, inertia, propulsion, and gravity all working against it.

   • The fins also serve a structural role by providing support and rigidity to the lower portion of the rocket. They help distribute and manage the loads experienced during launch, ascent, and re-entry.

     

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     That’s all for today. You now have the basic knowledge about a rocket’s structure. Quite interesting, right? We will talk more about rockets in the third and the last half of part 1.

     hasta la vista!

In this monthly newsletter, you’ll get to know about the basics of various innovations done in the history of space technology, that has forever revolutionized the way that we study, and learn about our solar system.

Now shall we begin?

What is a Rocket?

We have to reach space somehow. So a rocket is a type of jet-propulsion device, which is used to carry people/ satellites/ rovers into the space, by using large amounts of either solid or liquid propellants1 that provides it fuel for combustion.

“a large cylinder-shaped object that moves very fast by forcing out burning gases, used for space travel or as a weapon” is how the Cambridge dictionary defines it.

Did you know that people got the idea of rockets through a firework?

History of Rockets:

The principles of rocketry were first tested more than 2,000 years ago, but it's only been within the past 70 years or so that we have built rockets to explore space.

Archytas, a Greek philosopher and mathematician, showed off a pseudo-rocket: a wooden pigeon suspended on wires. The pigeon was propelled by escaping steam. Around 300 years after the pigeon experiment, the Greek mathematician Hero of Alexandria is said to have invented the aeolipile (also called Hero's engine)- which is a sphere shaped device that sat on a pool of boiling water. Further developments in early rocket technology were recorded in the 9th century when Chinese monks developed what is now called "gunpowder,". They used it to make fireworks. The first military use of rocket technology was also recorded in China later in 1232 CE. Smithsonian researchers led by Frank Winter, former curator of rockets at the Smithsonian’s National Air and Space Museum, dug deeper into these early "rockets".

Escape Velocity:

Rockets require a a very huge kick start in order to leave the surface of the earth. It is because of the gravitational field on the surface of the earth.

Escape velocity is known as the minimum velocity needed by anything to be projected to overcome the gravitational pull of the planet. In simple words, this is the minimum velocity needed to get out of earth and not fall back.

This is the equation for escape velocity:

Where, V= escape velocity

G= gravitational constant = 6.674 × 10^-11 m³ kg^-1 s^-2

M= mass of earth = 5.97219 × 10²⁴ kilograms

R= radius of earth = 6,378 km

Since all the values are already known, we are able to find out the value of escape velocity.

= 11.2 km/s

Now, we call rockets as a “jet-propulsion device”, but, what does it mean?

Jet-Propulsion-

A propulsion system is a machine that produces thrust2 to move an object forward. It works on the Newton’s Third Law3 of action and reaction. Jet propulsion include the jet engine used for aircraft propulsion.

Propellants

Rockets have a very different way of using fuel. It is not like a vehicle that you see on the road, such as a car or a bus.

A propellant is any gas, solid or liquid which expands in order to move another object. There are mainly 3 types of rocket propellants:

1. Liquid-fuel rocket engines: A liquid-propellant rocket engine burns a liquid oxidizer, usually liquid oxygen, with a fuel consisting of liquid hydrogen, kerosene, or methane. The fuel is delivered to the combustion chamber using gravity, acceleration, pressure, or turbopumps4. The combustion produces great amounts of exhaust gas at high temperature and pressure.

2. Solid-fuel rocket engines: Solid-propellant rocket engines use a mixture of solid oxidizer and solid fuel, called the fuel grain. The grain is cast inside a cylindrical casing with a cylindrical hole running down its length, called the combustion chamber. The first solid-fuel rockets used gunpowder. Now, propellant grain uses a wide variety of complex chemicals.

3. Hybrid rocket engines: Hybrid-propellant rocket engines use a solid fuel and a liquid or gaseous oxidizer. The oxidizer, usually liquid oxygen or hydrogen peroxide, is injected into the cylindrical combustion chamber that runs down the length of the cylinder.

How does propellant work?

This is exactly how propellants are stored in a rocket. Fuel, and a source of oxygen, called an oxidizer, are mixed and exploded in a combustion chamber. The combustion produces hot exhaust which is passed through to accelerate the flow and produced thrust. For a rocket, the accelerated gas, or working fluid, is the hot exhaust produced during combustion. This way, the propellant is able to provide large amounts of power for the rocket to move towards space.

We will talk about “how a rocket works” and more, in the second half of part-1. Hope this article was helpful for you to understand the basics about a Rocket. See you in the next one.

hasta la vista!