What is Rocket motion: Definition and 25 Discussions

A rocket (from Italian: rocchetto, lit. 'bobbin/spool') is a projectile that spacecraft, aircraft or other vehicles use to obtain thrust from a rocket engine. Rocket engine exhaust is formed entirely from propellant carried within the rocket. Rocket engines work by action and reaction and push rockets forward simply by expelling their exhaust in the opposite direction at high speed, and can therefore work in the vacuum of space.
In fact, rockets work more efficiently in space than in an atmosphere. Multistage rockets are capable of attaining escape velocity from Earth and therefore can achieve unlimited maximum altitude. Compared with airbreathing engines, rockets are lightweight and powerful and capable of generating large accelerations. To control their flight, rockets rely on momentum, airfoils, auxiliary reaction engines, gimballed thrust, momentum wheels, deflection of the exhaust stream, propellant flow, spin, or gravity.
Rockets for military and recreational uses date back to at least 13th-century China. Significant scientific, interplanetary and industrial use did not occur until the 20th century, when rocketry was the enabling technology for the Space Age, including setting foot on the Earth's moon. Rockets are now used for fireworks, weaponry, ejection seats, launch vehicles for artificial satellites, human spaceflight, and space exploration.
Chemical rockets are the most common type of high power rocket, typically creating a high speed exhaust by the combustion of fuel with an oxidizer. The stored propellant can be a simple pressurized gas or a single liquid fuel that disassociates in the presence of a catalyst (monopropellant), two liquids that spontaneously react on contact (hypergolic propellants), two liquids that must be ignited to react (like kerosene (RP1) and liquid oxygen, used in most liquid-propellant rockets), a solid combination of fuel with oxidizer (solid fuel), or solid fuel with liquid or gaseous oxidizer (hybrid propellant system). Chemical rockets store a large amount of energy in an easily released form, and can be very dangerous. However, careful design, testing, construction and use minimizes risks.

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  1. gggnano

    What's wrong with this math: A lot of thrust and almost no fuel?

    So according to this if you need a force that can lift a human and even an extremely ultra-light craft (1500 N) and have a velocity of 200 m/s you can eject extremely small amount of fuel through the nozzle: 0.0007 or less kg per second for as long as you're using 15 MPa pressure which is doable...
  2. gggnano

    Can I make a rocket by mixing aluminum + iodine (+ water catalyst)?

    Basically it's a well known reaction, many kids on youtube make it for the views: I am thinking, since AlI3 sublimes can it be put in say tungsten bottle (is this even a thing??) and then used as a rocket the same way Na+H2O or HCL+Al rockets are used - the gas escapes? This phase...
  3. Father_Ing

    How Does Rocket Ejection Mass Affect Its Velocity in Space?

    Consider a rocket with mass ##m## in space is going to move forward. In order to do so, it needs to eject mass backwards. Let the mass that is ejected has velocity ##u## relative to the rocket. What is the equation for the final velocity? It is said that after ##dt## second, the rocket will...
  4. O

    I Is the classical ideal rocket thrust equation correct?

    Hello! I have recently found this fascinating article: https://zenodo.org/record/3596173#.YJ1ttV0o99B The author claims that classical equation for rocket thrust in incorrect because F is not equal to ma for a changing mass. Neither my professors nor me can see any errors. Do you think this...
  5. Physyx

    How to model a rocket equation from the derivative of momentum?

    I am using the derivative of momentum (dp/dt) with Newton’s 3rd Law with the gravitational force of Earth. F - [Force of gravity on rocket] = dp/dt F - (G * m_e * m_r / r2 ) = v * dm/dt + ma F = Force created by fuel (at time t) G = Gravitational Constant m_e = Mass of Earth m_r = Mass of...
  6. Abhishek11235

    Can Lagrangian Method be Applied to Solve Rocket Motion Equations?

    Homework Statement While solving equation of rocket motion with Newton's law in 1-d,I pondered to apply Lagrangian method on this. However, I didn't get correct result. Because I can eliminate last 2nd equation using last equation and get some other equation which is certainly not rockets...
  7. bob14

    Error When Deriving the Rocket Equation

    Homework Statement I want to try to derive the rocket equation and then add additional effects like gravity, air resistance, etc. Here's the equation that I found online: Homework Equations p (momentum) = mass * velocity F (force) = Δp / Δt The Attempt at a Solution [/B] pi = mv Here is...
  8. Dopplershift

    What is the Initial Mass of Fuel Needed for a Rocket to Reach 0.1c?

    The problem states: Typical chemical fuels yield exhaust speeds of the order of 103 m/s. Let us imagine we had a fuel that gives v0 = 3 × 105 m/s. What initial mass of fuel would the rocket need in order to attain a final velocity of 0.1c for a final mass of 1 ton? I derived the equation in the...
  9. J

    How Is Rocket Speed and Height Calculated Under Varying Thrust and Mass?

    Homework Statement A rocket with initial mass of m0. The engine that can burn gas at a rate defined by m(t)=m0-αt, and expel gas at speed (relative to the rocket) of u(t)=u0-βt. Here, m0, α, u0, and β are all constants. Assume the lift-off from ground is immediate a) The rocket speed v(t)=...
  10. R

    Simulating a water rocket motion using a computer programme

    Homework Statement I am trying to model a water rockets motion using a computer programme in order to compare it to my experimental data. I am studying physics at degree level but I have no knowledge of programming and am finding this particularly difficult. I have thought about using...
  11. NATURE.M

    Rocket Motion in interstellar space

    So suppose an astronaut in interstellar space has gas ejecting from her propulsion system. So the gas would cause her to move forward by some distance, d. Then, the F_{thrust} acting on her must be constant (assuming the amount of gas ejected per unit time is constant, and the speed it is...
  12. E

    Rocket Velocity Determination with Constant Mass Loss and Resistive Force

    Homework Statement Consider a rocket is subject to linear resistive force, f = -bv . \dot m is constant. Use the equation: m \dot{v} = -\dot{m }v + f to determine the velocity of the rocket : since the rate of mass lost is constant let \dot{m} =k vex = nuzzle velocity v = \frac{k}{b}...
  13. B

    Rocket motion - conservation of momentum

    Homework Statement Find the speed v of the rocket when the mass of the rocket = m. The rocket starts from rest at with mass M. Fuel is ejected at speed u relative to the rocket. Homework Equations m1v1 = m2v2 The Attempt at a Solution In the textbook, it starts off with a moving...
  14. I

    Rocket Motion - calculating distance

    I'm trying to solve a rocket problem and have found no clear answer on how to do this. If I have a rocket in space (no significant gravitational forces, no drag, etc.) with a known constant thrust (T), mass flow rate (mdot), exhaust velocity (Ve), total initial mass including propellant (Mi)...
  15. A

    Understanding Rocket Motion: Conservation of Momentum and Gravity's Impact

    In the consideration of rocket motion we always use the principle of the Conservation of Linear Momentum.This principle holds true only for isolated systems.But the initial stage of rocket motion is strongly affected by gravity-----which is definitely an external force [if we consider the rocket...
  16. T

    How Is the Rocket's Acceleration Calculated After the Parachute Opens?

    Homework Statement What is the acceleration of the rocket at a time of 5seconds after the parachute opens. Homework Equations F=ma The Attempt at a Solution I simply thought the acceleration would be 9.8m/s/s
  17. D

    How Do You Calculate Rocket Motion Parameters Over Time?

    Hi there I'm having a bit trouble understanding the rocket motion and how the find the velocity, acceleration and travelheight as a function of time. First, i assume that the velocity of the ejected mass relative to the rocket is constant. Secondly, i assume that the rocket mass changes with...
  18. T

    Calculating Final Speed & Angle of a Rocket Motion

    Homework Statement A rocket is traveling with a speed 20 km/s in a non-gravity space. To fix the direction of motion, it turns on an engine, which pushes the gasses with constant speed 3 km/s w.r.t. the rocket perpendicularly to the direction of its motion. The engine is on till the mass of...
  19. P

    Rocket Motion Problem: Finding Maximum Speed and Height at Burnout

    Homework Statement A rocket of initial mas M, of which M-m is fuel, burns its fuel at a constant rate in time tau and ejects the exhausts gases with constant speed u. The rocket starts from rest and moves vertically under uniform gravity . Show that the maximum speed achieved by the rocket...
  20. H

    Solving for Launch Speed and Height of a Toy Rocket

    A toy rocket moving vertically upward passes by a 2.2 m high window whose sill is 9.0 m above the ground. The rocket takes 0.14 m/s} to travel the 2.2 m height of the window. What was the launch speed of the rocket? Assume the propellant is burned very quickly at blastoff. How high will...
  21. S

    Solving Rocket Motion: Initial and Final Mass Calculations

    Homework Statement A rocket on its launch pad has a mass of 20,000kg. The engine fires at t=0s and produces a constant force of 500,000 N straight up. The engines fire for 1 minute during which the entire 10,000kg of fuel on poard is consumed and expelled from the rocket at a constant rate...
  22. M

    How Does Gravity Affect a Model Rocket's Upward Acceleration?

    1. A shuttle bus slows down with an average acceleration of -1.8m/s^2. How long does it take the bus to slow from 9.0m/s to a complete stop? For this question, we can immediately say that the final velocity is 0m/s because it stops. Now let's take a look at the second question: A robot probe...
  23. D

    Solving a Rocket Motion Problem

    Ok here's a question i had for homework, but i am having some trouble with it: a rocket is fired from rest with an acceleration at=22.8m/s at an elevation angle of theta=62.4. the rocket travels in a straightline for 8.95 seconds, when the fuel runs out. at this time it becomes a projectile...
  24. E

    How Does Rocket Propulsion Work in the Vacuum of Space?

    For a rocket in free space, it seems that P = mv + (m_0 - m)(v-u) where m is the mass of the rocket, v is its speed, m_0 is its initial mass, u is the speed of the propellent relative to the ship. But in solving dP/dt=0, I get that the velocity is linear in m. There seems to be a mistake...
  25. S

    Rocket Motion: Solving the Rocket Equation for Lift-Off

    This question was posed in class the other day for extra credit: A rocket with initial mass 70000kg burns fuel at a rate of 250kg/s; it has an exhaust velocity of 2500m/s. If the rocket is at rest, how long after the engines fire will the rocket lift off? I've been trying to solve it...