### Author Topic: Orbital Mechanics and energy  (Read 801 times)

#### SkipMorrow

##### Orbital Mechanics and energy
« on: 04/04/2018 04:06 PM »
Let's say I am in an elliptical orbit at apoapsis and I want to circularize my orbit. I burn my engines prograde for a while which raises the periapsis. If I timed it right, then my orbit will be circular. But I wonder, what's going on with the energy in this scenario? If I burn my engines, I am adding energy. So is it KE or PE? My altitude didn't change, so it can't be PE. The altitude will certainly be different in 180 degrees, but right now, I'm still at the same altitude, so my PE must still be the same. I think this is right, but who knows. Please correct me if I am wrong.

Now this is where I get confused. As for KE, when I burn my engines, I don't think my speed can increase because my altitude hasn't changed. Altitude is connected to speed. You can't change one without changing the other.

So, if everything I have written is correct, then where is the new energy right after I do the burn?

#### e of pi

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##### Re: Orbital Mechanics and energy
« Reply #1 on: 04/04/2018 04:15 PM »
Now this is where I get confused. As for KE, when I burn my engines, I don't think my speed can increase because my altitude hasn't changed. Altitude is connected to speed. You can't change one without changing the other.
I think what you're misunderstanding is that while indeed mean altitude is connected to speed, velocity is changing constantly in a non-circular orbit as KE trades off with PE. The total energy is a constant, and at apogee it's more in PE while at perigee it's more in KE. Thus, when you do a burn at apogee to circularize, you're adding velocity and KE, which when you reach what would have been your periapsis is now the added PE to keep you at a higher altitude.

#### freddo411

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##### Re: Orbital Mechanics and energy
« Reply #2 on: 04/04/2018 04:50 PM »

Now this is where I get confused. As for KE, when I burn my engines, I don't think my speed can increase because my altitude hasn't changed.

No, your speed will increase.   The difference in your elliptical orbit, and your circular orbit, is the speed at that particular place and time.

You are increasing your kinetic energy.

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