Rockets - when do they get too big?

[Toltec]

Simpo Two said:

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

Unfortunately you cannot do that

https://en.wikipedia.org/wiki/Gyrojet

one of the key ideas was to reduce recoil.

[annodomini2]

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)

Typically larger rockets are more cost efficient, $/lb to LEO.

But there is a point when the material factors take over and you need to add more structure due to the increasing mass, to the point where as mentioned, it either cannot make orbit or just won't get off the ground.

The aerodynamic bits are a factor, but is less significant to mass as the bulk of acceleration is performed in the upper atmosphere.

[eharding]

The Sea Dragon would have been an interesting beast, had they ever made it:

https://en.wikipedia.org/wiki/Sea_Dragon_(rocket)

On the same scale as a Saturn V, you begin to see that we might be approaching "too big" territory:

[Toltec]

annodomini2 said:

Typically larger rockets are more cost efficient, $/lb to LEO.

But there is a point when the material factors take over and you need to add more structure due to the increasing mass, to the point where as mentioned, it either cannot make orbit or just won't get off the ground.

The aerodynamic bits are a factor, but is less significant to mass as the bulk of acceleration is performed in the upper atmosphere.

It is not so much an issue overcoming drag at lower altitudes as much as the structural strength needed to handle the drag and thrust forces being applied between the nose and engines. You are balancing the extra mass of structure against the extra mass of fuel, particularly the mass in the second stage, which as you say, really gets to work in the upper atmosphere.

I remember reading somewhere that in scale the tanks are thinner than a coke can.

[Beati Dogu]

The Atlas Agena rocket had to be kept pressurised even when empty of fuel, in order to maintain structural strength.

Otherwise....

https://youtube.com/watch?v=imkdz63agHY

[RobDickinson]

Beati Dogu said:

The Atlas Agena rocket had to be kept pressurised even when empty of fuel, in order to maintain structural strength.

Otherwise....

https://youtube.com/watch?v=imkdz63agHY

I think the f9 does too it's pretty common

[hidetheelephants]

That was why Blue Steel/Saturn V etc had corrugated sides, to try and keep a semblance of rigidity while not upright, pressurised and full of fuel.

That Sea Dragon is completely bonkers; I like it.

Edited by hidetheelephants on Friday 29th June 14:57

[Eric Mc]

RobDickinson said:

I think the f9 does too it's pretty common

I didn't think so.

Most rockets can stand upright even if empty. The Atlas was exceptionally thin skinned (in an attempt to keep structural weight down).

[Eric Mc]

hidetheelephants said:

That was why Blue Steel/Saturn V etc had corrugated sides, to try and keep a semblance of rigidity while not upright, pressurised and full of fuel.

Not the whole structure. Corrugations were placed where structural strength was needed. The Shuttle External Tank had corrugations on the "Intertank" section between the liquid oxygen and liquid hydrogen tanks. But the whole tank didn't need to be pressurised when they were moving it about.

[HRL]

Simpo Two said:

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

But would you notice that recoil if the gun was more than 6,000,000,000,000,000,000,000,000 times heavier than the bullet?

[Kccv23highliftcam]

hidetheelephants said:

That was why Blue Steel/Saturn V etc had corrugated sides, to try and keep a semblance of rigidity while not upright, pressurised and full of fuel.

That Sea Dragon is completely bonkers; I like it.

Edited by hidetheelephants on Friday 29th June 14:57

Not if your'e aquatic....

[Flibble]

HRL said:

Simpo Two said:

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

But would you notice that recoil if the gun was more than 6,000,000,000,000,000,000,000,000 times heavier than the bullet?

Seems unlikely given the recoil velocity would be well under an atom's width per second.

[The Dangerous Elk]

Toltec said:

Unfortunately you cannot do that

https://en.wikipedia.org/wiki/Gyrojet

one of the key ideas was to reduce recoil.

More Wiki inaccuracy (not unique really), Walther and WASAG were developing 9mm rocket rounds in Germany during 1942-5
http://gigconceptsinc.com/Rocket-German-1a1a.html



[RULE of Thumb No 1...The Yanks do not invent anything]

Edited by The Dangerous Elk on Monday 2nd July 16:07

[paulrockliffe]

Toltec said:

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.

I think that essentially the issue comes down to the fact that the mass of fuel you haven't burnt yet needs to be accelerated by the fuel that has been burned until it's ready to be burned. It's dead weight until it can be put into the engine and some of it won't go in for quite some time.

If you could dump all the fuel in at once and harness all that energy instantaneously it wouldn't matter. It's not hugely dissimilar to the F1 when the commentators are going on about the 100kgs of fuel off the start line - it all needs to be accelerated around the track until it's needed, so the cars are slower.