02 Spaceflight - Part 1.pdf

Lecture 2 Spaceflight  Fly spacecraft into or through outer space Spaceflight is an application of science and technology to fly spacecraft into or through outer space.  Outline Phases – Rocket launching – Outer space – Orbit – Orbital maneuver ...

Lecture 2 Spaceflight  Fly spacecraft into or through outer space Spaceflight is an application of science and technology to fly spacecraft into or through outer space.  Outline Phases – Rocket launching – Outer space – Orbit – Orbital maneuver  Launching Spaceflight can be achieved conventionally by rocket launching, which provide the initial thrust to overcome the force of gravity (i.e. weight) and propel a spacecraft from the surface of Earth.  Rocket A rocket engine produces thrust by reaction to exhaust expelled at high speed (according to Newton’s third law). https://www.grc.nasa.gov/www/k-12/rocket/TRCRocket/rocket_principles.html  Propellant carried within User:Surachit, CC SA 1.0, via Wikimedia Commons Rocket engines work entirely from propellant carried within the vehicle. – A rocket design can be as simple as a cardboard tube filled with gunpowder. Fuel – Chemical rockets are the most Oxidizer common type of high power rocket, typically creating a high speed Combustion chamber exhaust by the combustion of fuel Exhaust with an oxidizer.  Can fly in a vacuum A rocket accelerates without using any surrounding air. A rocket can fly in the vacuum of space. Rockets work more efficiently in a vacuum and incur a loss of thrust due to atmospheric drag.  Outer space Outer space (or simply space) is the expanse that exists beyond Earth’s atmosphere and between celestial bodies.  Lacks a well-defined physical boundary The transition between Earth’s atmosphere and outer space lacks a well-defined physical boundary, with the air density steadily decreasing with altitude.  Kármán line The Kármán line, an altitude of 100 km above sea level, is a conventional definition of the edge of space. It is named after Theodore von Kármán, who calculated a theoretical limit of altitude for airplane flight above Earth. – An aircraft is a vehicle that is able to fly by gaining support from the air.  Legal & regulatory purposes The Kármán line is has no distinct physical significance in that there is a rather gradual difference between the characteristics of the atmosphere at the line. It is mainly used for legal and regulatory purposes of differentiating between aircraft and spacecraft, which are then subject to different jurisdictions and legislations.  Summary Edge of space (domain multi- disciplinarity) – Outer space is the expanse that exists beyond Earth’s atmosphere (science) – Kármán line: altitude limit of airplane flight (technology) – Legal and regulatory purposes  Orbit An orbit is the curved trajectory of an object, such as the trajectory of Earth around the Sun, or of a spacecraft around Earth. For most situations, orbital motion is adequately approximated by Newtonian mechanics.  Newtonian mechanics There are a few common ways of understanding orbits: – A force, such as gravity, pulls an object into a curved path as it attempts to fly off in a straight line. – As the object is pulled toward the massive body, it falls toward that body. However, if it has enough tangential velocity it will instead continue to follow the curved trajectory caused by that body indefinitely.  Newton’s cannonball model As an illustration of an orbit around Earth, the Newton’s cannonball model may prove useful. This is a ‘thought experiment’, in which a cannon on top of a tall mountain is able to fire a cannonball horizontally at any chosen muzzle speed. user:Brian Brondel, CC BY-SA 3.0, via Wikimedia Commons  Low initial speed If the cannon fires its ball with a low initial speed, the trajectory of the ball curves downward and hits the ground (A). https://www.youtube.com/watch?v=ALRdYPMpqQs  Sufficient speed As the firing speed is increased, the cannonball hits the ground farther (B) away from the cannon. If the cannonball is fired with sufficient speed, the ground curves away from the ball at least as much as the ball falls—so the ball never strikes the ground. It is now in what could be called a non-interrupted or circumnavigating, orbit.  Sub-orbital spaceflight A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the surface of Earth. Hence, it will not complete one orbital revolution. This is usually because of insufficient orbital speed.  Orbital spaceflight An orbital spaceflight is a spaceflight in which a spacecraft is placed on a trajectory where it could remain in space for at least one orbit. The spacecraft reaches the minimal orbital speed required for a closed orbit.  Kepler’s laws of planetary motion When it is assumed that the spacecraft is subject only to the gravitational force of Earth (i.e. an engine thrust is not present), the orbit is described by Kepler’s laws of planetary motion.  Thrust When an engine thrust is present, Newton’s laws still apply, but Kepler’s laws are invalidated. When the thrust stops, the resulting orbit will be different but will once again be described by Kepler’s laws.  Applied at only one point in the orbit If thrust is applied at only one point in the spacecraft’s orbit, it will return to that same point on each subsequent orbit, though the rest of its path will change.  In a direction opposite to the motion Thrust From a circular (red) orbit, thrust applied in a direction opposite to the spacecraft’s motion changes the orbit to an elliptical (yellow) one. The resultant orbit will be smaller than the original circular orbit.  In the direction of the motion From a circular (blue) orbit, thrust applied in the direction of the spacecraft’s motion also creates an elliptical (yellow) orbit. The resultant orbit will be larger than the original circular orbit. Thrust  Orbital maneuver An orbital maneuver (otherwise known as a burn) is the use of propulsion systems to change the orbit of a spacecraft.  Transfer orbit A transfer orbit is an intermediate elliptical orbit that is used to move a spacecraft in an orbital maneuver from one circular, or largely circular, orbit to another. – One cannot move from one circular orbit to another with only one brief application of thrust. There are several types of transfer orbits, which vary in their energy efficiency and speed of transfer.  Hohmann transfer orbit The Hohmann maneuver is accomplished by placing the spacecraft into an elliptical transfer orbit. The maneuver uses two impulsive engine burns: the first establishes the transfer orbit, and the second adjusts the orbit to match the target. MenteMagica, CC BY-SA 3.0, via Wikimedia Commons  Bi-elliptic transfer The bi-elliptic transfer consists of two half-elliptic orbits. From the initial orbit, a first burn boost the spacecraft into the first transfer orbit. A second burn sends the spacecraft into the second elliptical orbit. A third burn injects the spacecraft into the desired orbit. AndrewBuck, CC BY-SA 4.0, via Wikimedia Commons  Energy efficiency & speed of transfer While they require one more engine burn than a Hohmann transfer and generally require a greater travel time, some bi- elliptic transfers require a lower amount of total energy than a Hohmann transfer.  Summary Spaceflight – Phases (celestial mechanics) Rocket launching Outer space – Kármán line (domain multi- disciplinarity) Orbit Orbital maneuver  Lecture 2 – Part 2 Spaceflight (cont.) – Interplanetary spaceflight – Applications Space tourism – Space pollution & atmospheric reentry  Interplanetary spaceflight Interplanetary spaceflight is travel between the planets of our Solar System. CactiStaccingCrane, CC BY-SA 4.0, via Wikimedia Commons  Sun’s gravitational pull Due to the Sun’s gravitational pull, a spacecraft moving farther from the Sun will slow down, while a spacecraft moving closer will speed up.  Extremely large amount of fuel A spacecraft desiring to transfer to a planet closer to the Sun must decrease its speed with respect to the Sun by a large amount in order to intercept it, while a spacecraft traveling to a planet farther out from the Sun must increase its speed substantially. Simply doing this by brute force – decelerating or accelerating in the shortest route to the destination – would require an extremely large amount of fuel.  More fuel is needed to put the fuel into orbit And the fuel required for producing these velocity changes has to be launched along with the spacecraft, and therefore even more fuel is needed to put both the spacecraft and the fuel required for its interplanetary journey into orbit. Fuel is Put the fuel needed into orbit  Techniques to reduce the fuel requirements Thus, several techniques have been devised to reduce the fuel requirements of interplanetary travel.  Hohmann transfer orbit 2nd burn For many years economical interplanetary travel meant using the Hohmann transfer orbit. It is an elliptical orbit which forms a tangent to the starting and destination orbits. 1st burn  Planets are moving at different speeds Earth is moving around the Sun at a different speed than the planet to which the spacecraft is travelling (in accordance with Kepler’s third law). https://youtu.be/3PCdNaUaYt4  Alignment of the two planets The alignment of the two planets in their orbits is crucial – the destination planet and the spacecraft must arrive at the same point in their respective orbits around the Sun at the same time. Phoenix7777, CC BY-SA 4.0, via Wikimedia Commons  Launch windows Launch windows for a Martian expedition This requirement for alignment gives rise to the concept of launch windows.  Synodic period Launch periods are periodic according to the synodic period. The synodic period is the elapsed time where planets return to the same kind of location. – For example, in the case of Mars, the period is 780 days (2.1 years). – 3D Solar System Viewer | TheSkyLive.com Takes ½ of the orbital period of the outer orbit The Hohmann transfer takes an amount of time similar to ½ of the orbital period of the outer orbit. – A spacecraft traveling from Earth to Mars via this method will arrive near Mars orbit in approximately 8.5 months. In the case of the outer planets this is many years – too long to wait.  Gravity assist A gravity assist is a type of spaceflight flyby which makes use of the orbit around the Sun and gravity of a planet to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.  Spacecraft gains velocity The “assist” is provided by the motion of the gravitating body as it pulls on the spacecraft. This Photo by Unknown Author is licensed under CC BY-SA  Planet loses velocity The momentum gained by the spaceship is equal in magnitude to that lost by the planet, so the spacecraft gains velocity and the planet loses velocity (according to the conservation of momentum). However, the planet’s enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small.  Planets are seldom in the right places The main practical limit to the use of a gravity assist maneuver is that planets are seldom in the right places to enable a voyage to a particular destination. There are years when the planets are scattered in unsuitable parts of their orbits.  Summary Interplanetary spaceflight (domain multi-disciplinarity) – Sun’s gravitational pull (science) – Techniques (economics) Hohman transfer orbit – Travel time (science) Gravity assist  Applications for spaceflight Current and proposed applications for spaceflight include: – Space tourism – Satellites (Lecture 3) – Space exploration (Lectures 4 – 9) – Protecting Earth from potentially hazardous objects (Lecture 10) – Space colonization (Lecture 11)  Space tourism Space tourism is human space travel for recreational purposes. There are several different types of space tourism, including orbital, suborbital and lunar space tourism.  Sub-orbital tourist flights Sub-orbital tourist flights will initially focus on attaining the altitude required to qualify as reaching space. The flight path will be either vertical or very steep, with the spacecraft landing back at its take-off site. https://www.youtube.com/watch?v=jbMoKbpo5tY  High-altitude part of the flight is in free fall To minimize the required fuel, the high-altitude part of the flight is made with the rockets off. A spacecraft with propulsion off is in free fall, a motion where gravity is the only force acting upon the spacecraft. – An object in the technical sense of the term “free fall” may not necessarily be falling down in the usual sense of the term. – Technically, if gravity is the only influence acting, an object is in free fall even when moving upwards.  Passengers will experience weightlessness If gravity is the only influence Same deceleration acting, then all objects (the spacecraft and passengers) “fall” (decelerate) at the same rate. The passengers will experience weightlessness. – The passengers inside the spacecraft appear to be floating. https://www.youtube.com/watch?v=TBBTJxjO-zE  Orbital tourist flights Space Adventures, a space tourism company, offers orbital spaceflight missions to the International Space Station. SpaceX, an spacecraft manufacturer, launch service provider and satellite communications company, offers paid crewed spaceflights for private individuals. Tourists visiting the International Space Station The International Space Station (ISS) is a space station maintained in Earth orbit. 13 space tourists have visited the ISS. https://www.youtube.com/watch?v=xg9R4yykvqU  Private orbital spaceflight This Photo by Unknown Author is licensed under CC BY-NC Inspiration4 was a 2021 human spaceflight operated by SpaceX. The trip was the first orbital spaceflight with only private citizens aboard. The mission spent almost three days in orbit https://www.youtube.com/watch?v=_T-exE-DRRo  Lunar tourism Lunar tourism may be possible in the future if trips to the Moon are made available to a private audience. Krückstock, CC BY-SA 3.0, via Wikimedia Commons  Natural attractions Stigmatella aurantiaca, CC BY-SA 3.0, via Wikimedia Commons Two natural attractions would be available by circumlunar flight or lunar orbit, without landing: – View of the far side of the Moon Due to gravitational locking, as the Moon orbits Earth, it always keeps the same face turned towards the planet. The far side of the Moon is never visible from Earth. – View of the Earth rising and setting against the lunar horizon https://www.youtube.com/watch?v=H1KWtG66lEQ  Environmental pollution While spaceflight altogether pollutes at a fraction of other human activities, it still does pollute heavily if calculated per passenger. – Rockets mostly exhaust greenhouse gases and sometimes toxic components. – Most rockets are made of metals that can have an environmental impact during their construction.  Space pollution Brocken Inaglory, CC BY-SA 3.0, via Wikimedia Commons Some spacecraft remain in space practically indefinitely, which has created the problem of space pollution in the form of satellite flare (light pollution) and space debris, which is a hazard to spaceflight.  Atmospheric reentry Otherwise spacecraft are terminated by atmospheric reentry.  Complete disintegration Objects entering an atmosphere experience atmospheric drag, which puts mechanical stress on the object, and aerodynamic heating—caused mostly by compression of the air in front of the object, but also by drag. These forces can cause loss of mass (ablation) or even complete disintegration of smaller objects.  Spacecraft cemetery If spacecraft do not disintegrate, their reentry is mostly controlled to safely reach a surface by landing or impacting, often being dumped into the oceanic spacecraft cemetery. – The spacecraft cemetery is a region in the southern Pacific Ocean. – The area is roughly centered on the oceanic pole of inaccessibility, the location farthest from any land.  Summary Spaceflight – Interplanetary spaceflight Domain multi-disciplinarity – Applications Space tourism – Attractions – Space pollution & atmospheric reentry