Physics question: why do larger rockets travel farther?

Fart

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There is something I can't understand, why can larger rockets travel farther than smaller ones? Larger rockets have more fuel but they have more weight also, If you have two rockets that are the same proportion but one is twice has large why would travel farther? My point is it takes a certain amount of calories to move a certain amount of mass why would it matter if there's a lot of calories and mass?
I know larger rockets travel farther into the sky, what am I missing?
 

Jimcat

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First, have you considered changing your name?

As for your question: if you're talking about rockets launched from Earth, remember that the thrust of the rocket is working against Earth's gravity. If the rocket doesn't push up, gravity will pull it back down.

So, all the way up, you've got (rocket providing acceleration up) minus (gravity providing acceleration down). The rocket has to burn fuel constantly to keep the rocket going up. Once you stop burning fuel, the acceleration up will stop. Gravity, however, will keep pulling it down. At this point one of three things will happen.

1. The rocket will keep losing upward velocity until it reverses direction and falls back to Earth.

2. The rocket will have enough speed that it will keep going forward even though Earth's gravity is pulling it down. Essentially, it is constantly falling toward the Earth but never hitting it. This is what we call an orbit.

3. The rocket will have enough speed that it will never fall back to Earth. It has passed the escape velocity of Earth's gravitational field.

Now, to keep thing simple, let's assume you're trying to get an equal mass up there for all three cases. Think of the Apollo missions for a real-world example. The early missions just did docking practice in Earth orbit, so they didn't need to reach escape velocity. They used the smaller Saturn IB rocket (space buffs can feel free to correct me on rocket designations). The moon missions, though, needed to get that same payload (the Command/Service Modules and Lunar Module) to the moon, which meant that they needed extra thrust to reach escape velocity. Extra thrust means you need to burn more fuel. So they needed a bigger rocket, the Saturn V, in order to burn more fuel to get them to that higher speed.

And don't forget that fuel itself is more mass to lift, so you need even more thrust at the beginning. That's why you use multi-stage rockets. No sense in lifting empty fuel tanks and heavy motors once you've burned off so much fuel. The greatest thrust is needed at the very start of the launch, when you're lifting a huge amount of mass from zero velocity against all of Earth's gravity. As the rocket continues to burn fuel, the whole device gets lighter, the speed increases, and less thrust is needed to keep it going upward. So you dump your big first-stage tanks and motors and keep going with a lighter load.

There are many, many fascinating books about rockets and space launches that could explain this in greater detail if you want. Try your local library.

The bottom line is: the higher up you need to go, the longer you need thrust, and thrust requires fuel. So payloads going higher up require bigger fuel tanks to get them there.
 

Fart

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You have two rockets they are both the same design and have the same proportions of fuel (forget about fuel tanks) but one is twice has large. The larger rocket has twice has much fuel and can produce twice has much thrust but it has twice has much mass so gravity pulls twice has much. shouldn't it go the same speed has the smaller one since it has twice has much thrust but has twice has much work to do? and shouldn't it run out of fuel the same time since it has twice has much fuel but burns it twice has fast to produce twice the thrust to push twice the mass?

What am I missing here?
(I know larger rockets can travel farther)
 

GenghisK

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At least with the same speed, in balistic flight the heavier has more inertial energy, hence will go farther. Btw, talking about rockets, it seems that prehistoric people already invented that...
 

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pavelsu

khoi
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:lol: I am sure that the system of the picyure works, maybe today it is not used too much because it is very expensive (needs too many male elephants ) :lol:
 

Håkon

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Fart,
you're missing the fact that gravity gives everything the same acceleration downwards (9.8 m/s/s), which means that a larger rocket does not need to fight against larger gravity. It still has to lift twice as much, but the gravity is not larger.
 

kundor

Chairman of the H.I.V.E.
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You are all missing the obvious answer: A rocket that is twice as large carries 8 times as much fuel.
You have two rockets they are both the same design and have the same proportions of fuel (forget about fuel tanks) but one is twice has large. The larger rocket has twice has much fuel and can produce twice has much thrust but it has twice has much mass so gravity pulls twice has much.
That is your mistakes. Something twice as large (2 times wider, 2 times taller, 2 times deeper) carries *8* times as much fuel.
It is the same as with screen resolution: 1280x960 is twice the resolution of 640x480, but has 4 times as much area. (it's only squared, instead of cubed, because it's in 2 dimensions.) this applies for all proportional changes.
 

GenghisK

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You all worth being Tsiolkovsky successors :)
Oh Håkon, I just forgot something. Theoretically, yes, it shouldn't matter about the size, but actually their always a bit of air resisting, from atmosphere. Therefore, to get another example, a feather should fall with the same speed than a steel ball, when dropped from the same height. But actually they don't...
 

Blitz79

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The fundamental consideration which everybody (I think) has overlooked is that gravitational field strength decreases in accordance with the inverse square of the distance. This is more important than it at first appears.
 
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