Rockets - when do they get too big?

[ReallyReallyGood]

Hi,

Having watched the awesome Falcon Heavy launch, I was wondering... at what point does the power generated by a rocket at launch, pushing against the Earth and its atmosphere, have a perceivable impact on the Earth's orbit, or even the tectonic plate it is sitting on?

[The Dangerous Elk]

or buy better weed

[Einion Yrth]

Never.

HTH

[RobDickinson]

When they mass as much as a mountain range?

Now put the crack pipe away

[RobDickinson]

Earth mass:
5.972 × 10^24 kg

Fh thrust
2.5 10^6 kg

[Moonhawk]

Whilst every rocket launch will have an effect - I don't think it'll ever be perceivable.

Saturn 5 produced about 3579 tonnes of thrust

A Maersk Triple E-class container ship displaces about 55,000 tonnes unladden.

Mount Everest which essentially floats along with the rest of the Himylayas on a techtonic plate weighs an estimated 300 billion tonnes just on it's own. That's equivalent to the thrust of about 100 million Saturn 5's.

Edited by Moonhawk on Monday 18th June 12:10

[Toltec]

It is like a fruit fly trying to lift a cubic metre of rice pudding by gripping the skin on top.

[The Dangerous Elk]

Toltec said:

It is like a fruit fly trying to lift a cubic metre of rice pudding by gripping the skin on top.

I had one that could do that as well but not funny when they let go !

[annodomini2]

Rockets get too big when they cannot generate enough thrust to get themselves off the ground.

[Simpo Two]

Moonhawk said:

Whilst every rocket launch will have an effect - I don't think it'll ever be perceivable.

Saturn 5 produced about 3579 tonnes of thrust

A Maersk Triple E-class container ship displaces about 55,000 tonnes unladden.

Mount Everest which essentially floats along with the rest of the Himylayas on a techtonic plate weighs an estimated 300 billion tonnes just on it's own. That's equivalent to the thrust of about 100 million Saturn 5's.

I think the mass of something sitting on the Earth is not the same as th emass of something blasting away from it.

Presumably the Earth has some way of resettling its orbit, because in theory every weight you fire away from it would affect its orbit.

[RobDickinson]

That really only matters if it escapes earth's orbit, which very little does, immeasurable relatively .

[Funky Squirrel]

What if everybody in the world jumped at once?

[The Dangerous Elk]

Funky Squirrel said:

What if everybody in the world jumped at once?

whilst all on holiday on the Isle of White

[Halmyre]

Simpo Two said:

Moonhawk said:

Whilst every rocket launch will have an effect - I don't think it'll ever be perceivable.

Saturn 5 produced about 3579 tonnes of thrust

A Maersk Triple E-class container ship displaces about 55,000 tonnes unladden.

Mount Everest which essentially floats along with the rest of the Himylayas on a techtonic plate weighs an estimated 300 billion tonnes just on it's own. That's equivalent to the thrust of about 100 million Saturn 5's.

I think the mass of something sitting on the Earth is not the same as th emass of something blasting away from it.

Presumably the Earth has some way of resettling its orbit, because in theory every weight you fire away from it would affect its orbit.

But that's countered by the 40,000 tons of mass that's added to the earth every day by meteorite impact. Which in turn is countered by the loss of 50,000 tons of gas...

https://gizmodo.com/5882517/did-you-know-that-eart...

I agree with the comments about wasting helium...

[anonymous-user]

annodomini2 said:

Rockets get too big when they cannot generate enough thrust to get themselves off the ground.

Doesn't the geometric Surface Area to Volume ratio relationship mean that bigger rockets are better? (more fuel / payload per unit surface area)

[Simpo Two]

RobDickinson said:

That really only matters if it escapes earth's orbit, which very little does, immeasurable relatively .

That makes good sense. And whilst meteorites presumably land evenly from all directions, extra-orbital rockets only take off from certain very few places... suggesting a net force albeit tiny in the scheme of things.

Max_Torque said:

annodomini2 said:

Rockets get too big when they cannot generate enough thrust to get themselves off the ground.

Doesn't the geometric Surface Area to Volume ratio relationship mean that bigger rockets are better? (more fuel / payload per unit surface area)

That's my hunch. But perhaps only up to a point - consider an earlier technology and the Spruce Goose.

[Sparkov]

The Dangerous Elk said:

Funky Squirrel said:

What if everybody in the world jumped at once?

whilst all on holiday on the Isle of White

Or Rhode Island?

[Toltec]

Simpo Two said:

I think the mass of something sitting on the Earth is not the same as th emass of something blasting away from it.

Presumably the Earth has some way of resettling its orbit, because in theory every weight you fire away from it would affect its orbit.

No and Yes. If there are no relativistic effects then the mass is the mass whether it is on the surface, in or blasting into orbit. In reality the mass is reducing rapidly, because it will be burning a lot of fuel and hence throwing mass away.

[Toltec]

Max_Torque said:

annodomini2 said:

Rockets get too big when they cannot generate enough thrust to get themselves off the ground.

Doesn't the geometric Surface Area to Volume ratio relationship mean that bigger rockets are better? (more fuel / payload per unit surface area)

That's my hunch. But perhaps only up to a point - consider an earlier technology and the Spruce Goose.

Like most things there is a balance, you need to be able to produce enough thrust to accelerate for long enough to reach the velocity you need.

If the thrust per unit fuel burnt remains constant then if the rocket is heavier you either need to burn longer or produce more thrust to reach the velocity you need, both will burn more fuel. There is a thing called the Rocket Equation that allows you to work out if your rocket will manage this or not.

At one end the rocket may lift off, but will accelerate so slowly that it runs out of fuel before reaching escape velocity, at the other it will accelerate very quickly, but not for long enough. The latter is a bit more complex, to accelerate more rapidly the structure of the rocket must be stronger to withstand the forces which means the mass of the structure becomes a higher proportion of the take off mass. There is also air resistance to consider, if you accelerate so rapidly that you are still in the atmosphere when you run out of fuel the rocket will begin to slow down again, also the faster you go at lower altitude the stronger and therefore heavier the rocket need to be to withstand the force due to aerodynamic drag.

In some ways this is quite similar to making a car go faster, you need more power so you use a bigger engine which burns more fuel, needs more cooling, a stronger transmission and body, which will make the car bigger and heavier so there is more drag and it is harder to accelerate, etc, etc.

[Simpo Two]

Toltec said:

No and Yes. If there are no relativistic effects then the mass is the mass whether it is on the surface, in or blasting into orbit. In reality the mass is reducing rapidly, because it will be burning a lot of fuel and hence throwing mass away.

Though if you can liken firing a rocket into space to firing a bullet from a gun there is recoil - Newton et at.