First US lunar lander in more than 50 years rockets toward moon with commercial deliveries

submitted by neolib from apnews.com

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[–]Alienhunter 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (7 children)

Assuming the Jetski has an infinite amount of water, it would work in space as there's minimal friction to overcome, merely on the third law principal.

A Jetski also would be propelled forward in air as well however this thrust is simply insufficient to counter both gravity and friction.

We tend to visualize this as a push against something in terrestrial situations but it isn't how it actually works. Think about how if you use a water hose at high pressure you can feel the push back before the stream is in contact with anything. Or for a more extreme situation, a gun.

You could easily use a gun to propel yourself in space same as throwing a rock or just pissing would also produce thrust. It's just not going to be very effective when compared to a rocket.

The rockets used in a vacuum do need to be designed differently than ones used in atmosphere for practical purposes. (You can't light a firework in space for example because there's no oxygen to combust) this is solved in space rocket designs by having the reaction take place in a chamber that has the necessary agents for combustion already prepared.

Essentially you bring your own air to "push off against".

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (6 children)

A Jetski also would be propelled forward in air as well however this thrust is simply insufficient to counter both gravity and friction

The actual work done to move the jetski in water is not produced by the equal and opposite push from the water jet.

Be honest about this. The propulsion of a water jet on air vs water jet in water is incomparable. The amount of work done (force x distance) in the air is literally negligible.

This same principle applies in space, where the is literally nothing to push against. Zero work can be done on the vacuum of space. It literally vacuums the exhaust gases out of the rocket as quickly as they are produced.

Literally vacuumed out.

Also, Gravity doesn't disappear in space. The Earth's radius is 7000 miles, and the atmosphere is 100 miles max.
The rocket is still feeling 95% of gravity's effects continuously.

Meanwhile, the rocket gases are vacuumed out of the rocket engine.

Bro. Every bit of interplanetary space travel is 110% impossible with rocket propulsion.

It's horrid reality. But it's the actual factual truth of physics.

[–]Alienhunter 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (5 children)

The actual work done to move the jetski in water is not produced by the equal and opposite push from the water jet.

That's how it would work in a vacuum though. And that's how rockets work.

If you've got unlimited rocks you could just build a space vehicle that consists of a catapult, hurling rocks into the black to push you forward. Slowly and surely. Doesn't matter what the shit is, you take some mass and expell it in the opposite direction of where you want to go.

The problem in real world examples is we don't have unlimited fuel, fuel is heavy and it's very difficult to carry enough of it to go to wherever you want since you need to accelerate to about 40,000 kph to escape velocity.

Low Earth orbit is like 30,000 kph for example so you're already 3/4ths of the way there with that. Gravitational pull at orbital velocities really is less about overcoming the pull and more about shaping your orbital vector.

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (4 children)

That's how it would work in a vacuum though. And that's how rockets work.

Then why wouldn't it work with water in air on a jet ski? It's the identical principle.

And liftoff consumes the 1st stage rocket in mid atmosphere less than 25 miles.

There's more than 200,000 more miles to the moon with the second stage rocket, and maybe the main unit rocket.

You think they can travel another 100,000 miles directly against 95% gravity? The Earth's radius is 7000 miles.

The rocket leaves the atmosphere at 7100 miles from Earth's center of G. The inverse square law of gravity doesn't shrink significantly for a huge distance.

It's easily 8-9 newton's of constant gravity, with a seriously diminished propulsion; assuming your model theory. Hopefully, we're at least in agreement about this.

In reality, the vacuum of space would draw out the rocket exhaust as quickly as it's produced.

The foundational propulsion mechanism between a rocket and a jet engine is the same.

The method of generating thrust differs, one is strictly combustion.
The other is a positive feedback loop of compressing ambient air (oxygen source) in a turbine, and mixing in fuel for enhanced combustion, which generates increased airflow and compression, [in positive feedback loop] until the sufficient thrust is generated.

Both depend upon high velocity gaseous exhaust applying force against the atmospheric environment.

I realize you attribute the equal and opposite concept in space, but that can't happen.

Pressure requires a surface to compress against. The atmosphere is compressed against the Earth's surface and gravity.

There's no opposing compression surface in a rocket nozzle cone. The vacuum draws it out without resistance.
Zero compression. Zero thrust.

[–]Alienhunter 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (3 children)

Again you are simply misunderstanding the third law of motion. No atmosphere is necessary to push against. A jet engine would work fine in a vacuum as a source of thrust assuming you brought your own air to use. (In reality since a Jet engine needs to suck in air from the surrounding atmosphere to work it won't work in space) rockets simply bring everything with them they need to function in space.

You also aren't fighting directly against Earth gravity. Once you achieve orbital velocity which in terms of energy needed is absolutely the biggest hurdle, then you need relatively little energy and thrust to make course corrections as you're in a vacuum condition with no air resistance and therefore no drag. In an orbital velocity that 95% of normal ground level acceleration works to keep you in an orbit and also can be used to gravitationally slingshot you out further if you are smart about how you go about it.

Keep in mind that you are at about 75% of escape velocity in low earth orbit. The amount of energy you need to escape from low earth orbit is far lower than the amount of energy you need to reach low earth orbit in the first place. The amount you need to reach the moon is even less than that as the moon is still in earth orbit.

It's one of the counter intuitive peculiarities of orbital physics we don't consider in terrestrial situations. Same as how the energy required to leave the solar system entirely is much lower than the energy required to go to the sun despite the fact the sun is much closer. If you wished to send a probe to impact the sun you'd have to work against the Earth's velocity to place the probe in an orbit that had the probe lose most of its relative velocity against the sun which is much harder to do than get the probe a bit more velocity to do a slingshot maneuver around the sun and achieve a parabolic orbit that will see it fly out uncontrollably into the void.

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (2 children)

Again you are simply misunderstanding the third law of motion. No atmosphere is necessary to push against.

Your premise assumes that the equal and opposite reaction in the third law will apply force in a vacuum.

For a gas to apply force, it has to apply pressure. But had molecules can only apply pressure to one another if they're contained.

The atmosphere functions at a container with back pressure at 14 lbs/(in2).

The inertia of atmosphere (resistance to gaseous expansion), applies back pressure to the exhaust gases, which applies back pressure against the rocket surfaces, and the equal and opposite sequence does work on the rocket and it's is moved.

Combustion in a vaccine doesn't have an enclosure of atmosphere.

In a vacuum, as soon as the molecules ignite and expand they are forced into empty space without resistance.

No inertia of gases in the atmosphere to create back pressure. The gases expand without resistance into the vacuum of space.

No back pressure, and no compression of gases, and no build up of pressure against the surface of the rocket.

No work applied to the rocket. No movement.

There's no way around this reality. The rocket exhaust won't build up back pressure (thrust), which means there's no efficiency.

Bro. This is physical reality. I don't like it either, but there's nothing for it.

[–]Alienhunter 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

This is very easy to prove with a vacuum chamber and some pressurized spray can of any sort. Place the pressurized spray can onto a cart or suspend it from the top of the contained, then depressurize it and see if it moves.

There's no need for back pressure of any sort. The initial push is from expelling the matter in the first place.

[–]Tom_Bombadil 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

There's no need for back pressure of any sort.

This is the error in your assessment.

Here's an exceedingly clever experiment, which stimulates the removal of the atmosphere with a rocket.

Rockets can't work in the vacuum of space science experiment.

Notice how the rocket jumps backwards when the parachute comes out. The equal and opposite reaction that doesn't occur with the rocket.