Introducing the Hohmann Transfer: A Spacecraft's Energy Saving Route to its Destination

Space exploration has always been a fascinating subject for many people. With the advancements in technology, humans have been able to send spacecraft and rovers to explore and study planets beyond Earth. One of the most important aspects of space exploration is the journey to the destination. In this article, I will introduce the Hohmann transfer, a technique that enables spacecraft to travel efficiently between two celestial bodies.

What is the Hohmann Transfer?

The Hohmann transfer is an orbital maneuver that is used to move a spacecraft from one circular orbit to another circular orbit of a different radius. This maneuver was first described by Walter Hohmann, a German scientist, in 1925. The Hohmann transfer consists of two main burns - the first burn is used to transfer the spacecraft from its initial circular orbit to an elliptical transfer orbit which is known as the Hohmann ellipse, and the second burn is used to transfer the spacecraft from the elliptical transfer orbit to the final circular XXXXXXXXe suggestion was really surprising because at that time the artificial satellite hadn't be invented.

Basic elements of an orbit

There are six basic elements of an orbit. semi major axis a, eccentricity e, inclination i, argument of periapsis ω, longitude of ascending node Ω, true anomaly φ.The last three one affect the state of the orbit in three XXXXXXXXXXXXXnce the Hohmann Transfer is based on the same flat,I will just talk about the first three elements.

semi major axis a:it is a half of the lager radii which pass the focuses of the XXXXXXXXXX determines the size of the orbit.

eccentricity e:the ratio of the distance between two focuses and the major axis . it determines the shape of the XXXXXXXXe smaller of the eccentricity of an ellipse orbit,the closer it can be seen as a XXXXXXXXX circle is a special kind of ellipse,its eccentricity is 0.

inclination i: it indicates how inclined the orbital plane was relative to the reference plane,which usually means the angle perpendicular to the orbital plane vector.During the process of the Hohmann Transfer,the inclined orbit needs to be the same.

How Does It Work

 The Hohmann transfer works by taking advantage of the different speeds of circular orbits at different radii. When a spacecraft is in a circular orbit, it travels at a constant speed. However, the speed changes as the spacecraft moves to an elliptical transfer orbit. This change in velocity is achieved by firing the spacecraft's engines in the direction of motion, which increases the velocity of the spacecraft. When the spacecraft reaches the furthest point in the elliptical orbit, known as the apogee, the engines are fired again which causes it to enter into the final circular orbit.Otherwise,the spacecraft will remain in the transfer orbit.

The transfer orbit is initiated by firing the spacecraft's engine to add energy and raise the apogee. When the spacecraft reaches apogee, a second engine firing adds energy to raise the perigee, putting the spacecraft in the larger circular orbit.

In addition,the direction of the acceleration need to be in the tangent of the direction of the original orbit,due to the purposes of saving fuel and safety.Just let me give an example,look at the graph XXXXXXXXere is a car wants to leave the autodrome,if there is a ramp at the tangent of the autodrome,the driver can get out easily.However, if the ramp is to the other directions of the orbit,the whole progress would be more difficult,the driver needs to change both the direction and the magnitude of the XXXXXXXXXXX’s very dangerous when the vehicle is traveling at a high speed. 

Although Hohmann transfers use the minimum amount of energy and fuel to reach a new orbit, they also require plenty of time. For missions with time constraints, a short transfer time can be achieved at the cost of more fuel.

Instead of using an elliptical transfer orbit that just reaches the outer orbit, using a transfer ellipse which extends past the outer orbit will result in faster transfer times. This gives us the general coplanar transfer.

Conclusion

The Hohmann transfer is a very important technique for space exploration.

Some alternatives to the Hohmann transfer are the bi-elliptic transfer and the  the gravity assist maneuver.

The bi-elliptic transfer requires a smaller amount of delta-v (change in velocity) than the Hohmann transfer but needs more time and is more complex.

The gravity assist maneuver is also called the gravitational XXXXXXXXXXXX is used by Voyager1 and Voyager2. In a gravity assist maneuver, a spacecraft uses the gravitational pull of a planet or other celestial body to change its speed and trajectory. It is possible to change the spacecraft's direction or speed without using any propellant.

Each of these techniques has its own advantages and disadvantages, and the choice of which one to use depends on a variety of factors, including the mission requirements, the amount of propellant available, and the specific orbital characteristics of the spacecraft and the destination. Lastly！Don’t forget the fuel required to set up the engine need to be invoved in the total energy cost.Otherwise,the astronauts will be in trouble in the space.