Rockets, What You Should Know Information from http://www.qrg.northwestern.edu/projects/vss/docs/Propulsion/1-how-are-rockets-designed.html Blast Off! To understand moreabout rockets, one needs to understand physics. Sir Isaac Newton Sir Isaac Newton (4 January 1643 – 31 March 1727) was an English physicist, mathematician, astronomer, natural philosopher, and alchemist, regarded by many as the greatest figure in the history of science. His treatise Philosophiae Naturalis Principia Mathematica, published in 1687, described universal gravitation and the three laws of motion, laying the groundwork for classical mechanics. Sir Isaac Newton Inertia Inertia (the property of matter by which it retains its state of rest or its velocity along a straight line so long as it is not acted upon by an external force) is a word we use when we talk about matter and movement. Velocity is in mechanics is the time rate of change of position of a body in a specified direction. Basically, our idea of inertia goes back to Sir Isaac Newton's first two laws of physics: 1. An object at rest tends to stay at rest. 2. An object in motion tends to stay in motion. Inertia Inertia is a object's reluctance to change its state of motion: From a state of rest to motion or vice versa. Matter is anything you can touch. Inertia If you want to overcome inertia, you have to apply a force. A force will make something that is still start to move, like flicking a wad of paper with a pencil will make it move. Also force, due to resistance, will slow or stop something that is already moving. The wad of paper will be slowed by resistance made by rubbing up against the air it is passing through. What is mass? We use the word mass to talk about how much matter there is in something. (Matter is anything you can touch physically.) On Earth, we weigh things to figure out how much mass there is. The more matter there is, the more something will weigh. Often, the amount of mass something has is related to its size, but not always. A balloon blown up bigger than your head will still have less matter inside it than your head (for most people, anyhow) and therefore less mass. What is mass? The difference between mass and weight is that weight is determined by how much something is pulled by gravity. If we are comparing two different things to each other on Earth, they are pulled the same by gravity and so the one with more mass weighs more. But in space, where the pull of gravity is very small, something can have almost no weight. It still has matter in it, though, so it still has mass. Why is mass important? Mass is important because of two major factors affecting how things move in space: inertia and gravity. The more mass something has, the more of both it experiences. That is why heavy things (things with a lot of mass) are hard to move. When an object is sitting still, it resists moving, and the more mass it has the more it resists. The amount of thrust needed to move something and how fast it ends up moving are both directly tied to its mass. Why is mass important? On the other hand, once something massive starts moving, it is very hard to stop. This is also due to the relationship between mass and inertia. Gravity is also proportional to how much mass each thing has. The bigger an object is, the larger the gravitational pull it exerts. Why is mass important? Because of gravity and inertia, the more massive something is, the harder it is to get into space, the harder it is to keep it there, and the harder it is to move it where you want it to go when it is there. For that reason, a lightweight spacecraft is better than heavy spacecraft. Every Action has an Equal and Opposite Reaction? This isthe third of Sir Isaac Newton's laws of physics, and one that is very important to space flight. Here's how it works. If you push on anything, it pushes back on you. That's why if you lean against the wall, you don't just fall through it. Every Action has an Equal and Opposite Reaction? The wall pushes back on you as hard as you push on it, and you and the wall stay in place. If you throw something, you put more force behind it than just leaning on it, so it pushes back with more force. This is hard to observe, because usually, if you throw something away from you, the friction between you and the floor makes resistance to keep you in place. Every Action has an Equal and Opposite Reaction? Every Action has an Equal and Opposite Reaction? If you take away the friction and try again, you will move away from the thing you threw as much as it moves away from you. The bigger the push, the bigger the push back. That's why cannons and guns recoil. As the cannon ball flies on one direction, the cannon moves in the opposite direction. If we turn the cannon up on end, it gets a little closer to how a rocket works. The force that pushes the cannon ball down also pushes the cannon up. But since the cannon is bigger than the cannon ball it has more inertia acting to keep it in one place. What is gravity? Gravity is a force pulling together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly. What is gravity? Mass is how we measure the amount of matter in something. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces. What is gravity? In addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth. Is there gravity in space? There isgravity everywhere. It gives shape to the orbits of the planets, the solar system, and even galaxies. Gravity from the Sun reaches throughout the solar system and beyond, keeping the planets in their orbits. Gravity from Earth keeps the Moon and human-made satellites in orbit. Is there gravity in space? It is true that gravity decreases with distance, so it is possible to be far away from a planet or star and feel less gravity. But that doesn't account for the weightless feeling that astronauts experience in space. The reason that astronauts feel weightless actually has to do with their position compared to their spaceship. We feel weight on Earth because gravity is pulling us down, while the floor or ground stop us from falling. Is there gravity in space? We are pressed against it. Any ship in orbit around the Earth is falling slowly to Earth. Since the ship and the astronauts are falling at the same speed, the astronauts don't press against anything, so they feel weightless. Is there gravity in space? Is there gravity in space? You can feel something very like what the astronauts feel for a moment in a fast-moving elevator going down or in a roller coaster, when you start going down a big hill. You are going down rapidly, but so is the roller coaster or the elevator so for a second you feel weightless. How do objects travel in space? Objects in space follow the laws or rules of physics, just like objects on Earth do. Things in space have inertia. That is, they travel in a straight line unless there is a force that makes them stop or change. The movement of things in space is influenced by gravity. Gravity is an important force that can change the course of bodies in space or pull them off of one course, or even cause them to crash together. How do objects travel in space? While some objects in space travel in irregular paths, most (especially our near neighbors in space) tend to travel in orbits around the Sun or around planets. The orbits are usually close to circular, but are actually slightly flattened ellipses. What is an orbit? An orbit is a regular, repeating path that an object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the moon, or human (or extraterrestrial?) -made. What is an orbit? In oursolar system, the Earth orbits the Sun, as do the other seven planets. They all travel on or near the orbital plane, an imaginary disk-shaped surface in space. All of the orbits are circular or elliptical in their shape. In addition to the planets' orbits, many planets have moons which are in orbit around them. How are rockets designed? Rocket designers want the rocket to do the best job possible for its mission. The performance of rocket engines can be measured in several ways, and the designer must decide which kinds of performance he or she would like the rocket to emphasize. Some important questions for rocket engine designers are the following: How are rockets designed? How powerful is the rocket; how much thrust can the motor produce? This is important because the rocket must be powerful enough to counteract Earth's gravity, and get its payload (the stuff that the spacecraft is carrying) into orbit, or even out of orbit! How are rockets designed? What is the power-to-weight ratio? This is important because the heavier the engine is, the harder it will be to get the spacecraft into space. However bigger (heavier) engines can be much stronger than small light ones. If you make a light enough spacecraft, it may not have enough thrust. So if a rocket is heavy, it must be strong, and if it is weak, it should be light. How are rockets designed? What is the speed of the exhaust gases? The faster the exhaust gasses stream out, the more thrust, and thus the faster the ship goes forward. How long can it run? The rocket has to get its payload to its destination against gravity. If the rocket runs out of oomph too quickly, the rocket may fall back to Earth or put its payload into a completely wrong orbit. How are rockets designed? No rocket design or kind of propellant will give the best answer to all of these questions. There are always tradeoffs; depending on what the satellite needs different kinds of rockets are chosen. The designer must choose which qualities are most important to his or her design and this changes depending on the rocket's intended purpose. Sometimes a single mission will have more than one propulsion system for different kinds of propulsion. Types of Propellants The solid motor is used mainly as a booster for launch vehicles. Solid motors are almost never used in space because they are not controllable. The boosters are lit and then they fire until all the propellant has burned. Their main benefits are simplicity, a shelf life which can extend to years as in the case of missiles, and high reliability. Types of Propellants Liquid motors come in many shapes and sizes: Most of them are controllable (can be throttled up and down), restart-able, are often used as control and maneuvering thrusters. Liquid thrusters can be broken into three main types: monopropellant, bipropellant, and cryogenic thrusters. Monopropellants only use one propellant such as hydrazine. Bipropellants use a fuel and an oxidizer such as RP-1 and H2O2. Types of Propellants Liquid Motors Continued: Cryogenic systems use liquefied gases such as LiH and LOX (liquid hydrogen and liquid oxygen). Cryogenic means super-cooled. You would have to super-cool hydrogen and oxygen to make them liquids. With each step from monopropellant to bipropellant to cryogenic the thruster complexity goes up but the performance also goes up. Types of Propellants Cold-gas motors have controllability similar to liquids but are the simpler and lighter. They are basically a high pressure tank with switches which flip between the open and shut state. They function a little like spray paint, with the contents under pressure inside, and when the valve is opened, they stream out. Types of Propellants Ion engines are vastly different from chemical (solid, liquid) engines in that they are low thrust engines which can run for extended periods of time. The length of use of chemical engines is usually from seconds to days while the length of use of ion engines can be anywhere from days to months. How does propulsion work? Propulsion moves things like spacecraft or jet planes forward by pushing something out of the back. Think of a balloon that you blow up and then release. The air rushing out of the back pushes the balloon forward. This happens because of a phenomenon described by Sir Issac Newton: "every action has an equal and opposite reaction." Every Action has an Equal and Opposite Reaction? We would need a larger force to push the cannon a great distance. If we could make a long continuous hot explosion in the cannon, instead of one quick one, we could push the cannon a far distance. The air that is heated would push out the back, pushing the cannon in the opposite direction. This is how jets work as well as how rockets get into space. Remember, because every action has an equal and opposite reaction something will go forward if it is pushing matter behind itself. How can something as small as an atom move a space craft? Anything witha propulsion system works when something (usually a gas-- sometimes a liquid) pushes out of it. This makes thrust. Any gas or liquid is made of atoms, so jet engines, the space shuttle, and Fourth of July fireworks are all pushed forward by atoms shooting out of them. How can something as small as an atom move a space craft? Everything from fireworks to space shuttles are moved by atoms. There are two important factors: how many atoms are being used and how fast they are going. In space shuttle launches, the fuel flow rate at launch is about 10 tons a second. This means that for each second of the launch a space shuttle burns 10 tons of fuel. That's a huge amount of atoms!