Flight Question Orbital Speeds

[Viatrix]

I'm sorry if this is a basic question on some FAQ somewhere, but although I flatter myself I have some reasonable (if a little seat-of-the-pants) understanding of how objects move in space, one thing does puzzle me a little; just because I can't make my head reconcile two apparently contradictory conditions.

When I'm in orbit around the Earth, I'm going at a certain speed to keep a circular path. When I want to raise my orbit, I turn prograde and boost, increasing my speed. Yet, when I use a formula to work out the required speed for a circular orbit at a given height, I find the required speed is slower.

Can someone tell me how to fit Tea and No Tea into my inventory at once please? I appreciate it may simply be that the speed being measured is speed against the ground - but for an orbit at 35,000km, the formula doesn't give me zero, which I'd expect in that case. So what are the reference frames for the speeds involved in these calculations?

Again, sorry if it's a basic, many-times-answered question, and sorry if I'm writing in a bit of a hurry.

:tiphat:

 

[jedidia]

When I'm in orbit around the Earth, I'm going at a certain speed to keep a circular path. When I want to raise my orbit, I turn prograde and boost, increasing my speed. Yet, when I use a formula to work out the required speed for a circular orbit at a given height, I find the required speed is slower.

the completely counterintuitive law of orbital mechanics: to speed up, slow down.

it isn't only groundspeed that decreases. It's got to do with orbital energy. If you boost your engines at periapsis, you put more energy into your orbit. That means that your apoapsis increases. Now, if you leave it that way, your speed at periapsis will indeed be higher than it was before.

However, when you are in an excentric orbit, your vessel slows down towards the apoapsis, since it's pulled back by the earths gravity. It would get all that speed again on the descent, but if you circularize your orbit, that won't happen anymore, since in a circular orbit the velocity always stays the same. In a nutshell, it is because all the velocity you spent on reaching the higher orbit has been converted into orbital energy.

 

[Shinrar]

Apoligizes in advance for lack of citing of sources, I'm on my cellphone at work...

Anyways: Groundspeed isn't relevent, because the same principles will apply to a body regardless of how fast you're rotating it. The key things to concern yourself with are as follows:

Forget how fast you're moving at any point - the AREA of your orbit determines your average speed. Obviously, if you're in the lower parts, you're moving faster then average (more kenetic energy, less potential). That extra speed means you're moving faster then what would be required to maintain an Ecc:0 Orbit. Higher up, you're moving slower then requried to maintain that orbit, so you start to fall lower again.

I think your formula is only good for circular orbits - most forumlas I've worked with involved the area of the orbit to determine various factors; which it sounds like is why you've having a Tea/No Tea issue.

 

[Viatrix]

However, when you are in an excentric orbit, your vessel slows down towards the apoapsis, since it's pulled back by the earths gravity. It would get all that speed again on the descent, but if you circularize your orbit, that won't happen anymore, since in a circular orbit the velocity always stays the same. In a nutshell, it is because all the velocity you spent on reaching the higher orbit has been converted into orbital energy.

I believe the word I'm looking for here is "doh".

Now you've told me, it's quite obvious. Ain't it always the way? :rofl:

I'm putting that energy into lifting the orbit, and then lifting the other side of the orbit when at apoapsis, rather than making me go faster.

Forget how fast you're moving at any point - the AREA of your orbit determines your average speed.

My apologies to Mr Kepler - I think it's his laws I completely forgot about there, wasn't it?

Heh. Thank you both for your replies. I'll just slope off and orbit Neptune for a day or two and say no more about it. :lol:

 

[Tommy]

Keep in mind that an orbit's specific energy is a function of both velocity (kinetic energy) and radial distance (potential energy). A higher orbit has a higher energy, but will have a less of that energy in kinetic form and more of it in potential form. For instance, an orbit at 2000k alt will have a lower velocity than one at 200k - but has much more potential energy, resulting in a higher overall energy.

Remember that gravity is subject to the inverse square law - the farther from the body, the less "pull" it has on you. This means less centrifugal force is needed to counteract the gravity, and less velocity is needed to generate the reduced centrifugal force.