Altaïr
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Hi,
Today I'd like to show you an advanced space driving technique, that allows to save fuel if used properly. It's based on Oberth effect, that sounds counterintuitive at first, but you probably already used without being aware of it.
For those who are not familiar with this, I will use the acronym SOI that means "Sphere Of Influence".
What is the Oberth effect
The Oberth effect is simply the fact that burning when you are next to a planet (or any body) will have a greater impact on your trajectory than if you burnt far from that planet.
The Oberth maneuver consists simply in exploiting that effect to your advantage.
For example, when your ship is in LEO and you aim for the Moon, you burn in the transfer window until your trajectory enters the Moon's SOI right? Congratulations, you've just used the Oberth maneuver!
Indeed, another strategy could be to raise slightly your apoapsis, and to circularize your orbit before going higher. Because you would lose the Oberth effect doing so, this would actually consume more fuel in the end.
The consequence is that when you are on a high orbit and you want to modify your trajectory, it can be interesting to first lower your periapsis, and then perform your maneuver when you have reached it. What sounds counterintuitive is that it's true even if you mean to go higher!
I'll show you with an example.
Example
Let's say you have that ship, which is orbiting the Earth at the same height as the Moon:
We want to send that ship as close as possible to the Sun. So we want it in Sun orbit, and to lower the periapsis as much as possible. The apoapsis will stay at the Earth level.
For this we are going to test several strategies. To compare them, we will watch the periapsis in the end of course, but what is more representative is the speed at the apoapsis.
Indeed, the Earth orbits the Sun at 4650 m/s, and the ship has a delta-V of 2738 m/s. So at first, we can legitimately expect that in the end, our speed at the Earth level won't be lower than 1912 m/s (the difference).
Let's see this.
FIRST STRATEGY: the deep space maneuver
This strategy consists in burning just enough to quit the Earth's SOI, and to burn retrograde once we are in solar orbit. Here is the result of this maneuver:
Our periapsis is 248.47 Mm, and our speed is 1896 m/s at the apoapsis. Strangely, this is slightly lower than the 1912 m/s predicted...
Now, let's try something else.
SECOND STRATEGY: the direct burn
Now, let's keep things simple, and let's burn directly from our position until we have no fuel left:
In the end, our periapsis is 171.00 Mm, and the speed at the apoapsis is 1600 m/s. This is significantly better than before, as this is nearly 300 m/s lower.
But can we do better? Now, let's make a last tentative!
THIRD STRATEGY: the Oberth maneuver
Now, we are going to test a strategy that sounds crazy at first: instead of burning prograde, we'll first burn retrograde to adjust the periapsis at the Earth level:
What have we done??! We've just wasted fuel!
Wait, let's try that Oberth maneuver now. Once we reach the periapsis, we burn everything we have, and here is the result:
And our periapsis is finally at 2731 km, our speed at the apoapsis is 521 m/s! We nearly reached the Sun surface! This is clearly better.
Conclusion
From the 3 strategies, the Oberth maneuver was by far the best one. Remember, we expected a speed of 1912 m/s at the apoapsis. The first strategy gave practically this result, the second one allowed to gain 296 m/s, and the Oberth maneuver 1375 m/s.
The reason why we finally achieved a better speed than expected in the first 2 cases is because even at the Moon level we benefit from a reduced Oberth effect.
Now you are aware of that possibility, try to use it at your advantage during your missions!
Today I'd like to show you an advanced space driving technique, that allows to save fuel if used properly. It's based on Oberth effect, that sounds counterintuitive at first, but you probably already used without being aware of it.
For those who are not familiar with this, I will use the acronym SOI that means "Sphere Of Influence".
What is the Oberth effect
The Oberth effect is simply the fact that burning when you are next to a planet (or any body) will have a greater impact on your trajectory than if you burnt far from that planet.
The Oberth maneuver consists simply in exploiting that effect to your advantage.
For example, when your ship is in LEO and you aim for the Moon, you burn in the transfer window until your trajectory enters the Moon's SOI right? Congratulations, you've just used the Oberth maneuver!
Indeed, another strategy could be to raise slightly your apoapsis, and to circularize your orbit before going higher. Because you would lose the Oberth effect doing so, this would actually consume more fuel in the end.
The consequence is that when you are on a high orbit and you want to modify your trajectory, it can be interesting to first lower your periapsis, and then perform your maneuver when you have reached it. What sounds counterintuitive is that it's true even if you mean to go higher!
I'll show you with an example.
Example
Let's say you have that ship, which is orbiting the Earth at the same height as the Moon:
We want to send that ship as close as possible to the Sun. So we want it in Sun orbit, and to lower the periapsis as much as possible. The apoapsis will stay at the Earth level.
For this we are going to test several strategies. To compare them, we will watch the periapsis in the end of course, but what is more representative is the speed at the apoapsis.
Indeed, the Earth orbits the Sun at 4650 m/s, and the ship has a delta-V of 2738 m/s. So at first, we can legitimately expect that in the end, our speed at the Earth level won't be lower than 1912 m/s (the difference).
Let's see this.
FIRST STRATEGY: the deep space maneuver
This strategy consists in burning just enough to quit the Earth's SOI, and to burn retrograde once we are in solar orbit. Here is the result of this maneuver:
Our periapsis is 248.47 Mm, and our speed is 1896 m/s at the apoapsis. Strangely, this is slightly lower than the 1912 m/s predicted...
Now, let's try something else.
SECOND STRATEGY: the direct burn
Now, let's keep things simple, and let's burn directly from our position until we have no fuel left:
In the end, our periapsis is 171.00 Mm, and the speed at the apoapsis is 1600 m/s. This is significantly better than before, as this is nearly 300 m/s lower.
But can we do better? Now, let's make a last tentative!
THIRD STRATEGY: the Oberth maneuver
Now, we are going to test a strategy that sounds crazy at first: instead of burning prograde, we'll first burn retrograde to adjust the periapsis at the Earth level:
What have we done??! We've just wasted fuel!
Wait, let's try that Oberth maneuver now. Once we reach the periapsis, we burn everything we have, and here is the result:
And our periapsis is finally at 2731 km, our speed at the apoapsis is 521 m/s! We nearly reached the Sun surface! This is clearly better.
Conclusion
From the 3 strategies, the Oberth maneuver was by far the best one. Remember, we expected a speed of 1912 m/s at the apoapsis. The first strategy gave practically this result, the second one allowed to gain 296 m/s, and the Oberth maneuver 1375 m/s.
The reason why we finally achieved a better speed than expected in the first 2 cases is because even at the Moon level we benefit from a reduced Oberth effect.
Now you are aware of that possibility, try to use it at your advantage during your missions!
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