Thread 16829188

>Energy is conserved, and potential energy transforms into kinetic energy when you lower your height and kinetic energy is transformed into pontential energy when you gain height
then why do I need to spend delta v to come back to a previous orbit?
I understand why I need to burn fuel to reach a moon orbit if I'm in a low orbit around Earth, I need to climb the gravitational well, but then returning to Earth from the moon should be free, I already have the potential energy, so returning should make me orbit the Earth at super fast speeds.
But instead I need to add kinetic energy to climb down the gravity well, isn't that weird?

Anon I like your line of thinking.
Maybe spend more time formulating it with proper mathematics, and you will either see the error, or will realize that space is fake and gay.
Godspeed anon!

>>16829188 (OP)
You need to escape the moon's gravity well

>>16829188 (OP)
spend a weekend playing Kerbal space program and it'll be really intuitive after that

>>16829188 (OP)
Higher orbit means more momentum. You need to apply a counter force to shed that momentum so you can start falling down.

>>16829188 (OP)
to drop down to a lower orbit, you first slow down at apoapsis. this puts you into an elliptical orbit.
then you wait until periapsis. at that point, you slow down again, such that you circularize your orbit.
viola, you are in a lower orbit now.
if you are re-entering an atmosphere, you can aerobrake to save fuel.

>>16829188 (OP)
Decent observation for a high school physics student, but you are over simplifying how this works. If you want to change from one stable orbit to another, you need to find the proper La Grange point. Moving through space isn't a simple free fall you can calculate with algebraic kinematics and mgh=1/2 mv^2

>>16829188 (OP)
Also, you always have frictional/heat losses because there's no such thing as a perfect vacuum

>>16829351
anon, one works out all the hohmann transfer stuff using conservation of energy and momentum.
patched conics got us to the moon, and that model doesn't have lagrange points.

>>16829355
I didn't mean to imply that these conservation laws and equations weren't used, just that the math and additional components were more complicated than high school algebra.
Also, maybe I wrongly assumed that OP was speaking about moving satellites between stable orbits.

>>16829188 (OP)
Energy wise it's free transfer, but you still need to physically change your orbit for anything to happen and to change your orbit you need to use energy. You are confused because on earth things that you send up just naturally come down again but that's no longer true in space with orbits.

>>16829355
You are responding to a bot.

>>16829188 (OP)
So you only expend energy climbing up a ladder, not down, you don't need energy to stabilize your descent?

>>16829188 (OP)
theres no way this a real thread

>>16829188 (OP)
>but then returning to Earth from the moon should be free
It is free. Your orbit will return you right where you started from if you don't alter it at another point. But you will return with orbital velocity.
The problem isn't getting back to the same point, the problem is slowing down enough to capture/land. Escape velocity is a LOT higher than landed surface velocity.
If the ground wasn't solid, we'd be going too slow to stay at this orbital height. It's counter-intuitive, but that's how it is

>>16829193
Doesn't that defer to the larger body? The moon is just in as much orbit as you are.

>>16829325
the problem is that you have too much kinetic energy.
then you burn fuel creating more energy.
then the kinetic energy disappears

how is this possible? energy is not a vector like linear momentum, so where does it go when you slow down?

>>16829676
Into the propellant.
When you fire forward to increase your orbital energy, you throw propellant behind you. This steals kinetic energy from the propellant and gives it to the rocket
When you fire backward to reduce your orbital energy, you shoot propellant forward. From the propellant's perspective, you "throw" the rocket backward to increase the energy of the propellant.

If you look at the rocket + propellant system, you're always converting chemical energy to increase your total kinetic energy. From the rocket's perspective alone, you are sometimes spending chemical energy to reduce your kinetic energy, which intuitively is strange.

>>16829423
You could just jump off the ladder or just let go, its not like you are falling into the ground in space

>>16829355
LOL
Nope.
Orbital mechanics is a fraud.
If you create a model based on forces, orbits are not stable unless they are attached by a string.

>>16829578
>Escape velocity is a LOT higher than landed surface velocity.
What people rarely mention is that the escape velocity for an object in orbit is typically very small. Aka orbits are highly unstable, and only exist as mathematical perfection, rather than physical reality.

ORBITAL MECHANICS IS A FRAUD, BECAUSE ORBITS ARE NOT STABLE:
Orbits exist in three cases:
(1) E < Escape Velocity: This is like two magnetic balls trying very hard to pull eachother together. It is very hard to make the magnetic balls smash into eachother, because orbits are unstable.
(2) E = Escape Velocity (including effects of the potential energy well). This is the case of an object having the perfect amount of energy.
This case is also unstable, because as soon as the object loses the tiniest amount of energy, to something like heat, then the object has less energy than escape velocity, so it is doomed to smash into the other object. And if it has the tiniest amount more energy it will fly off into space.
(3) E > Escape Velocity: The orbit is unstable because energy is greater than escape velocity. This is like tossing two magnetic balls at eachother, but there isn't enough magnetic force to smash them together.

Hopefully this helps you realize that orbital mechanics is fake and gay, because orbits are unstable, and can only exist by assuming they exist due to conservation of energy, rather than creating a model based on forces, where instability becomes extremely obvious.

If orbital mechanics is a lie.
Then the solar system is fake.
Then space is fake.

>>16829845
Yes you are, you will be falling to the largest nearby mass and you could just crash into that too instead of having a steady approach.