All About Rockets and Other Space Craft

Focuses 3 and 4

Law of Conservation ofMomentum and the

changing acceleration of a rocket

Before the engine fires:

• A rocket is generally made up of an engine and fuel. The engine’s purpose is to burn the fuel and release the products out the end of the rocket.

• The fuel within the rocket is comprised of both liquid hydrogen and liquid oxygen.

• Consider a rocket at rest in space that has no outside forces acting upon it. Therefore as the object is stationary the momenta of both the engine and fuel are zero, thus the total momentum of the rocket is zero and the law of conservation of momentum applies.

After the engine fires:

• As the engine is fired fuel is released out the end of the rocket, thus giving the fuel a momentum toward the left (as shown in diagram)

• As the total momentum of the rocket is zero this means that the engine has an equal momentum in the direction opposite to the fuel so that:

procket = - pfuel

Analyse the changing acceleration of a rocket during launch in terms of the forces experienced

by astronauts.

• Two forces act upon an astronaut: 1. The upward thrust (T)2. The downward weight (W or mg)

• Newton’s second law is used to derive a simple expression for the acceleration of a rocket that is launched directly up.

a = ∑F/m = (T-mg)/m• If the mass of the rocket

decreases during flight and the thrust remains constant, the acceleration of the rocket (and astronauts) increases.

• Thus the force experienced increases

Sourced from: http://hsc.csu.edu.au/physics/core/space/9_2_2/922net.html

• There are three stages:

– On launch pad• The downwards force of gravity is equal to the upwards force

of the seat. The astronaut is not really aware of any force. She would say “it’s just normal”

– During launch• The upwards force of the rocket makes the upwards force of

the seat considerably greater than the downwards pull of gravity. The astronaut feels like she is being pushed into the seat

– After launch, when blasters are not firing• There is no force due to the seat and no force due to gravity so

the force on the astronaut is zero – she feels like she is “floating”

True weight is a constant, evenly

spread influence that occurs when gravity

is applied on an object. Due to its constant, evenly spread nature it

cannot be felt. When an external force acts

upon an object causing it to change its motion, apparent

weight is experienced.

G Force

The g force is your apparent weight as a

proportion of your true weight

G force = (mg + ma)/mgOr G force =

g is acceleration due to gravity, m = mass of astronaut.

g

ag weightnormalweightapparent

__

• Based on Newton’s first law, apparent weight occurs because an object will resist change in motion aka inertia!

• When an astronaut is being launched into orbit, her weight is a downwards force on the seat, the seat also has an upward force on her. Since the rocket is accelerating upward, the seat has more upwards force on her body than gravity has a downwards pull on her body.

15. Discuss the effect of the earths orbital motion and its rotational motion on the

launch of a rocket

The Motion of the Earth

• Earth revolves around the Sun at approximately 107000 km/hr relative to the Sun.

• In addition the Earth rotates once on its axis per day so that the point on the equator has a rotational velocity of approximately 1700km/hr relative to the sun.

• Hence the Earth is itself a moving platform with two different motions which can be exploited in a rocket launch to gain a boost in velocity.

The Motion’s Effects on Rocket Launch

• In order to achieve the velocity needed for a stable orbit, Engineers exploit the Earth’s rotation by launching in the direction of rotation.

• This will allow the rotational velocity of the launch site to add up with the orbital velocity of the rocket relative to the sun.

• In similar way, engineers planning a rocket mission heading further into space can exploit the Earth’s revolution around the Sun by planning the launch for a time of year when the direction of the Earth’s orbital velocity corresponds to the desired direction. When the rocket is launched up into the orbit it is allowed to proceed around its orbit until the direction of its orbital velocity corresponds with the Earth’s. Its engines are then fired to push it out of orbit and further into space.

• In this way the Earth’s orbital velocity relative to the sun adds to the rocket’s orbital velocity relative to the Earth to produce a higher overall velocity.

• The favourable periods of this planning are referred to as launch windows.

Discuss issues associated with safe re-entry into the Earth’s atmosphere and landing on the Earths surface

• When entering the earth’s atmosphere the space craft must lose velocity in order to hit the atmosphere at the right angle,

• if this angle is too small the space craft will fail to re-enter the atmosphere

• If the angle is too large the amount of heat/g-forces will be too great and the space craft and its passengers may not survive.

• Re-entering the earths atmosphere can be very dangerous, due to the large amount of heat created . This is because of the high velocity the rocket travels at and because it collides with the particles in the earths atmosphere.

• To combat this issue and to stop the rocket from burning up on re-entry

• heat shields are used to spread the heat evenly over the rocket.• by going slower to reduce the velocity that the rocket is

travelling at.

G-forces encountered at re-entry, because of the rapid deceleration of the craft, can cause the occupants in the craft to lose consciousness or die.

When re-entering the earth’s surface the space craft lowers itself, as the space craft comes closer to the earth’s surface it slows down at the last stage of the crafts descent parachutes are released to slow the craft further, at last it will splash down into the ocean where it waits to be recovered by a naval vessel

Slingshot Effect.

Identify that a slingshot effect can be provided by planets for

space probes.

The slingshot effect involves a probe approaching a planet into a hyperbolic orbit to gain kinetic energy from the planet’s gravitational field. It is also known as a planetary swing or a gravity-assist manoeuvre.

The slingshot effect allows satellites to change speed and direction. The change in direction is due to the gravitational field which applies a force to the

space probe which changes its direction. The change in speed is because, relative to the sun, the passing space probe has gained velocity from the

moving planet.

Change in Direction:

Image from: http://www.boredofstudies.org/wiki/index.php?title=The_Solar_System_is_held_together_by_gravity

Image from: http://www.boredofstudies.org/wiki/index.php?title=The_Solar_System_is_held_together_by_gravity

Change in Speed: