Altaïr
Space Stig, Master of gravity
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Today I'd like to show you a powerful space maneuver that will save you a LOT of fuel during your future missions: the gravitational slingshot.
This presentation is the result of a collaborative work:
- The original text and diagrams are from me
- @TtTOtW and @8bitCosmonaut translated my text from "frenglish"
- @Ana Imfinity and @8bitCosmonaut verified the content and added some precisions
- @Aiden @Danny Batten @Gecko Gekkota @Earlmilly1 @GuHP20 and @SupremeDorian read the presentation, tried to practice and gave feedback
Special thanks to all of them
First of all, I recommend that you familiarize with the Oberth effect first: https://jmnet.one/sfs/forum/index.php?threads/advanced-techniques-the-oberth-maneuver.1215/
The gravitational slingshot is a very powerful technique, but also a tricky one, whereas the Oberth maneuver is a more basic one that still allows some substantial benefits.
Done? Ok, let's go!
In this text I'll use the following abbreviations:
LEO: Low Earth Orbit
SOI: Sphere Of Influence
Regarding the lowest and highest points of an orbit, for more precision I'll use the following terms:
- periapse and apoapse when the ship is orbiting a planet or a moon
- perihelion and aphelion when the ship is orbiting the Sun
What's the gravitational slingshot?
Basically, the gravitational slingshot, also known as gravity assist, consists of making a fly-by of a planet (or a moon) and letting the gravity curve your trajectory so that it provides you a free speed gain.
Here is an easy example to reproduce. With a small rocket, first aim for the Moon as if you were going for a Moon mission:
Then, once you entered the Moon's SOI, just let your ship continue on its trajectory until it leaves:
Hey, your orbit is much higher than before now! What's interesting is that you didn't spend a drop of fuel to do that! That maneuver is called a gravitational slingshot.
Something important to notice is that your fly-by must be done over the rear side of the Moon. By "rear", I mean the direction the Moon comes from. If you make a fly-by over the front side, the effect will be reversed.
Now, generally speaking, maybe we could exploit this effect to our advantage... But to anticipate our future trajectory we will have to understand how it works first! What happens exactly? Isn't it weird that the ship magically gains speed by just flying over the Moon? And why does the result depends on the side you fly over?
To explain this I'll use an analogy. This will look completely disconnected from our problem at first, but this is an easier experience to understand, as the gravitational slingshot obeys a similar logic.
Let's make an experience!
Now, let's forget spaceflight for a while, and let's play ball for a moment. You see that truck parked in front of you? What will happen if you shoot your ball against it?
You don't expect anything special to happen right? Here is the experience:
Indeed, the ball just bounces against the truck, and that's all...
But hey, what would happen if we threw it against the truck while it's driving? Well, let's try:
Wow, now the ball bounces at a much faster speed! What happened is that because of the truck's speed, there was a higher relative speed between the ball and the truck. Thus the ball received a greater impact from the truck and bounced stronger. Seems logical right?
Now, let's try the same experience, but with the truck driving away. What do you think it would happen?
Correct, now the ball bounces at a reduced speed. In this case, the relative speed was reduced, and then the impact was lower.
Is that OK? So let's get back to our Spaceflight Simulator!
Oh, before that, just a warning: in case somebody would be tempted to reproduce that experience in real life, be warned this is a bad idea! You would expose yourself and other people to danger (you probably have no idea how much speed a ball thrown at a fast moving vehicle could acquire), so please, DON'T DO IT! This is really a bad idea.
OK, let's go on now.
So, How the gravitational slingshot works?
Let's think about it... What happens during that fly-by of the Moon we were talking about? First, the ship enters the SOI at low speed, and as it gets lower it accelerates right? OK, but once it passed the periapse, the altitude increases again, and the speed diminishes until you reach the SOI limit. Now, your speed is the same as when you entered the SOI, so we should not be able to gain speed like that...
Indeed, but... Hey wait, the Moon is moving right? Of course it does, when you look at it from outside it orbits around the Earth, so yes it's constantly moving!
Remember how the speed of the truck influenced the speed at which the ball bounced? This is exactly the same phenomenon here. Of course, the Moon doesn't impact your ship (Well, generally it doesn't... ) like the truck with the ball, but the Moon still attracts your ship and manages to deflect its trajectory, so the same reasoning applies.
Now we understood how it worked, let's see exactly how you should proceed to use this to your advantage. Let's suppose our ship is cruising in space, and is about to encounter the Earth. Let's use that opportunity to perform a slingshot!
See how the Earth deflects the ship's trajectory and increases its speed?
I've also included a ghost ship in the animation. Unlike the real ship, it's unaffected by the Earth's gravity, and keeps going in a straight line. If you carefully observe the 2 ships, you will notice that the real ship is much faster in the end.
What is important here is flying over the side the Earth comes from, so that it deflects your ship in the direction the Earth is moving (towards the right here).
Now, what if on the contrary we want to reduce our speed? In this case you just have to fly over the other side:
See? Now the ship's trajectory is deflected in the opposite direction. As a result, the ship is considerably slowed down.
Is it OK? Then you get it! I'll just summarize what you need to remember, and we'll see in practice.
Now, let's practice!
To finish that tutorial, let's see an example of an easy and popular use of that maneuver. Let's suppose you want to go to Mercury. As Venus is quite massive and is approximately halfway on your path, this is a very good opportunity to use the gravity assist maneuver.
To do this, first get an encounter with Venus, as if you actually wanted to go there:
You may notice that I made my injection burn slightly after the transfer window, which is why my trajectory goes below Venus orbit before the encounter. The reason for this is because the slingshot from Venus will be more efficient after that. Otherwise I wouldn't reach Mercury directly. The counterpart is that I had to burn a longer time to get onto that transfer trajectory, but because of the Oberth effect the difference is actually pretty small.
Then, which side should you fly over? Mercury is closer to the Sun than Earth, so we want to decrease our speed. In this case you have to make a retrograde fly-by, so that your trajectory is deflected backwards:
Adjust your periapse so that it is just above Venus' atmosphere, and let the magic happen:
See? This makes a Mercury mission considerably easier!
To get it right, you can first practice with the Moon, as this is a close and easy target, and then, make some tries with more ambitious missions!
This presentation is the result of a collaborative work:
- The original text and diagrams are from me
- @TtTOtW and @8bitCosmonaut translated my text from "frenglish"
- @Ana Imfinity and @8bitCosmonaut verified the content and added some precisions
- @Aiden @Danny Batten @Gecko Gekkota @Earlmilly1 @GuHP20 and @SupremeDorian read the presentation, tried to practice and gave feedback
Special thanks to all of them
First of all, I recommend that you familiarize with the Oberth effect first: https://jmnet.one/sfs/forum/index.php?threads/advanced-techniques-the-oberth-maneuver.1215/
The gravitational slingshot is a very powerful technique, but also a tricky one, whereas the Oberth maneuver is a more basic one that still allows some substantial benefits.
Done? Ok, let's go!
In this text I'll use the following abbreviations:
LEO: Low Earth Orbit
SOI: Sphere Of Influence
Regarding the lowest and highest points of an orbit, for more precision I'll use the following terms:
- periapse and apoapse when the ship is orbiting a planet or a moon
- perihelion and aphelion when the ship is orbiting the Sun
What's the gravitational slingshot?
Basically, the gravitational slingshot, also known as gravity assist, consists of making a fly-by of a planet (or a moon) and letting the gravity curve your trajectory so that it provides you a free speed gain.
Here is an easy example to reproduce. With a small rocket, first aim for the Moon as if you were going for a Moon mission:
Then, once you entered the Moon's SOI, just let your ship continue on its trajectory until it leaves:
Hey, your orbit is much higher than before now! What's interesting is that you didn't spend a drop of fuel to do that! That maneuver is called a gravitational slingshot.
Something important to notice is that your fly-by must be done over the rear side of the Moon. By "rear", I mean the direction the Moon comes from. If you make a fly-by over the front side, the effect will be reversed.
Now, generally speaking, maybe we could exploit this effect to our advantage... But to anticipate our future trajectory we will have to understand how it works first! What happens exactly? Isn't it weird that the ship magically gains speed by just flying over the Moon? And why does the result depends on the side you fly over?
To explain this I'll use an analogy. This will look completely disconnected from our problem at first, but this is an easier experience to understand, as the gravitational slingshot obeys a similar logic.
Let's make an experience!
Now, let's forget spaceflight for a while, and let's play ball for a moment. You see that truck parked in front of you? What will happen if you shoot your ball against it?
You don't expect anything special to happen right? Here is the experience:
Indeed, the ball just bounces against the truck, and that's all...
But hey, what would happen if we threw it against the truck while it's driving? Well, let's try:
Wow, now the ball bounces at a much faster speed! What happened is that because of the truck's speed, there was a higher relative speed between the ball and the truck. Thus the ball received a greater impact from the truck and bounced stronger. Seems logical right?
Now, let's try the same experience, but with the truck driving away. What do you think it would happen?
Correct, now the ball bounces at a reduced speed. In this case, the relative speed was reduced, and then the impact was lower.
Is that OK? So let's get back to our Spaceflight Simulator!
Oh, before that, just a warning: in case somebody would be tempted to reproduce that experience in real life, be warned this is a bad idea! You would expose yourself and other people to danger (you probably have no idea how much speed a ball thrown at a fast moving vehicle could acquire), so please, DON'T DO IT! This is really a bad idea.
OK, let's go on now.
So, How the gravitational slingshot works?
Let's think about it... What happens during that fly-by of the Moon we were talking about? First, the ship enters the SOI at low speed, and as it gets lower it accelerates right? OK, but once it passed the periapse, the altitude increases again, and the speed diminishes until you reach the SOI limit. Now, your speed is the same as when you entered the SOI, so we should not be able to gain speed like that...
Indeed, but... Hey wait, the Moon is moving right? Of course it does, when you look at it from outside it orbits around the Earth, so yes it's constantly moving!
Remember how the speed of the truck influenced the speed at which the ball bounced? This is exactly the same phenomenon here. Of course, the Moon doesn't impact your ship (Well, generally it doesn't... ) like the truck with the ball, but the Moon still attracts your ship and manages to deflect its trajectory, so the same reasoning applies.
Now we understood how it worked, let's see exactly how you should proceed to use this to your advantage. Let's suppose our ship is cruising in space, and is about to encounter the Earth. Let's use that opportunity to perform a slingshot!
See how the Earth deflects the ship's trajectory and increases its speed?
I've also included a ghost ship in the animation. Unlike the real ship, it's unaffected by the Earth's gravity, and keeps going in a straight line. If you carefully observe the 2 ships, you will notice that the real ship is much faster in the end.
What is important here is flying over the side the Earth comes from, so that it deflects your ship in the direction the Earth is moving (towards the right here).
Now, what if on the contrary we want to reduce our speed? In this case you just have to fly over the other side:
See? Now the ship's trajectory is deflected in the opposite direction. As a result, the ship is considerably slowed down.
Is it OK? Then you get it! I'll just summarize what you need to remember, and we'll see in practice.
- As we saw it, what's important is the direction in which the ship's trajectory is deflected: in the direction towards which the planet moves? It will speed up! In the opposite direction? It will slow down!
- the more you lower the periapse, the more your trajectory will be deflected. Thus you will generally want to fly over the planet as close as possible.
- Your trajectory will be more deflected by a heavy planet than by a light one. Imagine the result with Jupiter!
- something important to notice is that the speed gained doesn't depend on the mass of your ship. This means that the result will be the same with a small capsule or with your biggest space station. This makes that maneuver particularly valuable for heavy ships!
Now, let's practice!
To finish that tutorial, let's see an example of an easy and popular use of that maneuver. Let's suppose you want to go to Mercury. As Venus is quite massive and is approximately halfway on your path, this is a very good opportunity to use the gravity assist maneuver.
To do this, first get an encounter with Venus, as if you actually wanted to go there:
You may notice that I made my injection burn slightly after the transfer window, which is why my trajectory goes below Venus orbit before the encounter. The reason for this is because the slingshot from Venus will be more efficient after that. Otherwise I wouldn't reach Mercury directly. The counterpart is that I had to burn a longer time to get onto that transfer trajectory, but because of the Oberth effect the difference is actually pretty small.
Then, which side should you fly over? Mercury is closer to the Sun than Earth, so we want to decrease our speed. In this case you have to make a retrograde fly-by, so that your trajectory is deflected backwards:
Adjust your periapse so that it is just above Venus' atmosphere, and let the magic happen:
See? This makes a Mercury mission considerably easier!
To get it right, you can first practice with the Moon, as this is a close and easy target, and then, make some tries with more ambitious missions!
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