Forces on rocket

-

Forces on rocket is another interesting topic.

Aerodynamic forces are generated and act on a rocket as it flies through the air. Forces are vector quantities which means that they both have a magnitude and a direction. Action of force is described both by the magnitude and the direction. In flight, rocket is under influence of four forces: weight, thrust, lift and drag, see Applications of aerodynamics.

Four forces on a rocket; source: NASA

The magnitude of the weight depends on the mass of the whole rocket. The weight force is always in direction towards the center of the Earth and acts through the center of gravity, depicted by the yellow dot in the Figure above. 

The magnitude of the thrust depends on the mass glow rate though the engine and the velocity and pressure at the rear of the nozzle. The thrust force acts along the longitudinal axis of the rocket. 

The lift of a rocket is a side force used to stabilize and control the direction of flight. Lift occurs when a flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton’s third law of action and reaction.

Forces acting on a rocket at take-off; source: What are Isaac Newton's Laws of Motion? (science-sparks.com)


While most aircraft have a high lift to drag ratio, the drag of a rocket is usually much greater than the lift.

You can imagine that drag is like an aerodynamic friction. One of the main source is the skin friction between the air molecules and the solid surface of the moving rocket. It is interaction between a solid and a gas, so the magnitude of the skin friction depends on both properties of the solid and the gas. The solid would produce less skin friction if it the surface is smooth, waxed, than rough surface. In case of the gas, the magnitude depends on the viscosity of the aur and the relative magnitude of the viscous forces to the motion of the flow, which is express as the Reynolds number. 

Along the surface, a boundary layer of low energy flow is generated, and the magnitude of the skin friction depends on the state of this flow. We can also think of drag as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the rocket and is called form drag. As air flows around the body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. We can determine the magnitude of the force by integrating, or adding up the local pressure times the surface area around the entire body. The base area of a model rocket produces form drag.

Determining the size of the drag force is very difficult in reality. The size of the drag changes depending on the thrust setting and the flow of gases at the base of the rocket. When the engine is operating, the drag is usually low. But when the engine is off during the coast phase of a model rocket, the entire base of the rocket produces a large drag. Drag is most often determined by wind tunnel testing a model of the rocket.

source: NASA

The launch is one of the biggest phases. It is the moment where the rocket has to carry all the fuel and oxidant which it will need to reach space, so the the weight is at a maximum and a huge amount of thrust is needed just to get it moving. As the rockets liftoff it consumes fuel on the way through the exhaust, so its weight is reduced. The same amount of thrust will stand for accelerating of the rocket. 




Comments

Post a Comment

Popular posts from this blog

Apollo 8 delta-v calculation

-
Image
The Apollo 8 Apollo 8 mission launched on December 21, 1968, and it was the second crewed spaceflight mission flown in the Apollo space program. The first mission, Apollo 7, stayed in Earth orbit. Apollo 8 mission was the first ever mission to leave the Earth's gravitational sphere, and to reach the Moon. The crew orbited the Moon ten times and returned home. It went without the landing. However, the mission was the first mission to see and take a photo of the far side of the Moon.  The mission consisted of three astronauts: Frank Borman, James Lovell, and William Anders.  Apollo 8 was the third flight and the first crewed launch of the Saturn V rocket.  Earthrise: Taken from Apollo 8; source: Wiki When building a rocket, it is a good thing to do staging. Staging basically means throwing away big, expensive parts of your rocket while the rest continues on the way up. The main advantage is not to carry dead weight.  When one has multiple powered stages, the question a...
Read more

Exercise on single stage vs. two-stage rocket

-
Image
Exercise: 1. Single stage rocket:  Before a rocket begins to burn fuel, the rocket has a mass of  m r,i =2.81×10 7 kg, of which the mass of the fuel is  m f,i =2.46×10 7 kg. The fuel is burned at a constant rate with total burn time is 510 s and ejected at a speed v e = 3000 m/s relative to the rocket. If the rocket starts from rest in empty space, what is the final speed of the rocket after all the fuel has been burned? What we know: initial rocket mass m r,i =2.81×10 7 kg initial mass of the fuel m f,i =2.46×10 7 kg t = 510 s v e  = 3000 m/s So, we can calculate the dry mass of the rocket m r,d =m r,i -m f,i =0.35×10 7 kg. The mass ratio is then R = m r,i  /m r,d =8.03. So, the final speed of the rocket is then: V r,f =Δv r =v e lnR=6250m/s. 2. Two-stage rocket:  Now, we have similar example, the same rocket as in the previous example, but the fuel will be burnt in two stages ejecting the fuel in each of the stages at the same relative speed (v e  =...
Read more

Specific impulse

-
Image
Specific impulse  I was many times using the term specific impulse. So, let's speak about it in more detail.    Specific impulse (usually abbreviated I sp ) is a measure of how efficiently a reaction mass engine, such as a rocket using propellant or a jet engine using fuel, generates thrust. A propulsion system which has a higher specific impulse uses the mass of the propellant more efficiently than a propulsion system with lower specific impulse. Now, in the case of a rocket, this means less propellant is needed for a given delta-v, so that the vehicle attached to the engine can more efficiently gain altitude and velocity. For all vehicles specific impulse (impulse per unit weight-on-Earth of propellant) in seconds can be defined by the following equation: Thrust is force, the thrust obtained from the engine (newtons or pounds force), g 0 is the standard gravity, which is nominally the gravity at Earth's surface (m/s2), I sp is the specific impulse measured (secon...
Read more