Spacecraft Maneuvers

Last time we learned about orbits, this time we'll look at what I believe to be some interesting maneuvers spacecraft have used, both in real space missions and in science fiction, to go from planet to planet or to change orbits. Read on!



In order to change orbits, a spacecraft has to change its speed. To change speed, one has to use fuel (or some other energy source) to either slow down or speed up. Now, fuel is expensive, as we saw in this post, so we want to use the least amount of fuel possible to change orbits.

Hohmann Transfer

The most fuel efficient method of doing this is to perform a maneuver called a Hohmann Transfer. As you can see in the diagram below, the Hohmann transfer involves two changes of speed. First, the spacecraft adds a bit of speed, changing it from orbit 1 (green) to orbit 2 (yellow). To change the spacecraft's path back into a more circular one, it has to slow down so that it doesn't fly past orbit 3 (red).

You might be wondering why we wouldn't simply just use one straight burst of speed to move from orbit 1 to orbit 3. Well, as I mentioned before, the Hohmann transfer is the most fuel efficient. According to the article above, a low-energy straight burn can take longer and cost ~141% more fuel in some cases. Of course this all varies but by utilizing the spacecraft's momentum, by simply nudging the spacecraft a couple of times, we use less fuel than one straight burn.

Because the last post was a little light on the math, I have included an example below of the math and fuel required for a transfer between two orbits above Earth.

In the above example, if you, a 165 pound person, was orbiting some 2000 km above the Earth and you wanted to change to be orbiting around 9000 km, going from an MEO, or Middle-Earth Orbit (No, not Lord of the Rings!) to a higher Earth orbit, as we learned about last time, you would need about 90 lbs of some of the best rocket fuel money could buy. Obviously, this situation is simplified, but the fact remains that even being the most fuel efficient, it still takes a lot of fuel.

This transfer can be used to change from any type of orbit; it's simply like changing lanes while driving. Additionally, one can use this maneuver to go to other planets. Since all objects in the solar system orbit the Sun, we can use the Hohmann transfer to change between orbiting with Earth, to orbiting with any other planet.

Now, I make it sound easy, but really it is not. Let's say we wanted to go to Mars. Well, it would make sense that we would want Mars to be as close to Earth as possible, or at least on the same side of the Sun as us. This is known as conjunction, and I refer you to a diagram on the matter, here. Earth and Mars move at different speeds around the Sun and thus conjunction is not that common. So, we have to wait for a launch window, a time when it would be most efficient to go to Mars. Of course we can go whenever we want but again, fuel is expensive.

Then, Mars comes around the Sun and we are in conjunction with it. Well, we still have to line things up. Performing some calculations one of my professors calls "orbital billiards" or "orbital gymnastics", we have to accurately predict where Mars will be by the time our spacecraft has had a chance to make it there, and we have to plan our path so we can use the least amount of fuel to leave our own orbit but still catch up with Mars. The same math applies from before, we just simply change the variables to include the orbits of Earth and Mars.

A space mission is like playing with dominoes; it's a delicate balance of so many elements and if one thing goes wrong, the entire operation can fail. Worse than that, often in space mission design there is no best choice. An engineer has to simply choose the better of two designs, and balance the risks of their choices against the advantages of other components. That is why even with almost 60 years in the space industry, failures are still very common. Getting to Mars is easy on paper, but take a look at this website which lists all the missions, and failed missions, to Mars. The most recent of which was the Russian Phobos-Grunt mission which failed to perform the first part of the Hohmann transfer and remained stuck in low-Earth orbit.

All this time, we've been talking about a spacecraft using its fuel, its energy, to change orbit or to reach other planets. But what about the planets themselves? Can't those big rocks/gas giants do anything to help us? Well, yes, they can.

The Slingshot Effect

In a move popularized most prevalently in Star Trek: The Voyage Home, a spacecraft can use the gravity of a planet, star, or other massive body to add energy and speed to its flight in a move known as a Gravity Assist or the Slingshot Effect. I prefer the latter because it sounds cooler.

Like many a concept, the Slingshot Effect is simple, but brilliant in its application. By flying toward a planet (or other massive body), the gravitational attraction pulls the spacecraft, thereby increasing its speed. This same gravity bends the path of the spacecraft around the planet and, as long as it is moving fast enough, it shoots around the other side moving faster than it had before. It's kind of like drafting, as seen in racing. Of course, in Star Trek: The Voyage Home, they used the Slingshot Effect to go back in time, but putting that aside, the maneuver is a great way to steal speed from a planet or star.

The Gravity Assist/Slingshot Effect has been used to great success in many space missions. In order to gain the speed needed to reach the outer planets and eventually leave the solar system, the Voyager probes, after blasting off from Earth, looped around Jupiter and Saturn. The Cassini probe has been shooting around the moons of Saturn since 2004 using the Slingshot Effect.

For more visual examples, check out this little video, or refer to this video from Booby Trap, a Star Trek: The Next Generation episode. The Enterprise becomes trapped in an asteroid field containing devices which sap their thrusters' energy. In order to escape, well, watch the scene, it's one of my favourites!

You could also remember the "lunar boost" scene from Armageddon. (Of course, Armageddon has been called the most inaccurate space movie ever made, but if you excuse the dramatic license of the scene, the technique is still shown.)

Sidenote: Something I've cleared up with many people over the years is the fact that the Space Shuttle is incapable of going to the Moon. It is barely capable of flying above low-Earth orbit. Why? Because it's too massive. The fuel and thrust needed to give that much mass enough speed for a higher orbit or to go to the Moon is so outside its design specifications that it is, in fact, not possible at all. I just wanted to make that clear because it's one of those myths which became stuck in society and was just accepted as fact.

The Slingshot Effect is great for changing the path of a spacecraft or to give it a great boost of speed. But, sometimes, a concern for a space mission is the threat of moving too fast. The transfer from one orbit to another, or from planet to planet, is meant to be smooth, again like changing lanes on the highway. Sure, we could simply blast off from Earth, in a straight, or curved, line toward our destination, but beyond the impracticality of such an approach, it can sometimes backfire. If we're moving too fast as we approach a planet, we might shoot right past it unless we consume a lot of energy slowing down so as to either land or fall into orbit around it.

For years, engineers struggled with this as they tried to put a spacecraft around Mercury. Finally, after several fly-bys, the Messenger probe slowed itself down using 31% of its original fuel supply, putting it in orbit around Mercury.

Aerobraking

Too bad for those engineers that Mercury doesn't have an atmosphere. One technique theorized to slow a spacecraft down is to drag it through the gases of a planet's atmosphere. This is known as Aerobraking and has been used on 4 different space missions around Earth, Venus, and Mars. It has also been portrayed in several pieces of science fiction including Robert Heinlein's novel Space Cadet, Arthur C. Clarke's novel 2010: Odyssey Two, and the 4th episode of Stargate Universe.

Skip Re-Entry

The antithesis of this maneuver is something known as skip re-entry in which a spacecraft uses aerodynamic lift, or simply the sheer boundary of an atmosphere to skip off. This has been performed countless times in actual space missions to reduce the speed of an incoming vessel, skipping it around Earth in its orbit, allowing it to land where and when it is supposed to.

It was also in the episode of Star Trek: The Next Generation entitled Coming of Age in which a cadet steals a shuttlecraft and then finds himself being pulled too fast toward a planet. Captain Picard directs him to point the spacecraft more sharply toward the planet, which allows it to skip like a rock on a pond instead of crashing onto the planet below, thus saving the cadet's life. I tried to find a video, but couldn't. The episode is on Netflix if you're really interested, although I doubt your enthusiasm is that high.

And that concludes this look into some interesting spacecraft maneuvers which have been or will soon be used by real and fake space missions. I hope you have found it as entertaining and enlightening as I have. I've been reading a book on artificial gravity and so, if I don't become too busy, I might write my next engineering post about that. Should you have any comments or questions, post them here or email me and, as always...

Thanks for reading!