Rocket Body Shape Curve

[Sam-urai]

Hey, guys. This is my first post here but I have a question which I am hoping someone could answer for me.

There is a certain common feature lots of the Russian rockets have like the Long March versions and the Falcon Heavy from SpaceX where the head of the rocket has a bigger diameter than the middle and then the end of the rocket has a diameter similar to the head. So the rocket has a thinner part in the middle. Is there any aerodynamic reason for this or is it simply to use up less material? I have no idea about rocket shapes and have every little experience with stuff like this but it would be great if someone could shed some light on this for me.

[magnemoe]

Practical reasons, Falcon 9 is 3.6 meter in diameter to be easy to transport on road. Russia has the same constrain but larger as they can not transport over sea. 
However you make fairing larger as you want larger payloads. 

Falcon 9 has the same diameter all the way past fairing, some rockets like Atlas has small SRB at bottom making that thicker Other like Ariadne and SLS has large SRB who in practice act as first stage. Soyuz has four large liquid boosters and an narrow core. Falcon heavy and Delta heavy has two liquid fuel boosters who are very simlar to the core. 

In short if you need any sort of boosters they are put at the bottom making the bottom wider. This is not aerodynamic efficient but far more practical. 

Saturn 5 and N1 both tapered towards the top as they was mostly rocket with an fairly small payload who had to go all the way to the moon. New Glen and starship are so wide they don't bother with an larger fairing. 

[kerbiloid]

Just an existing rocket was too narrow for the cargo.

[magnemoe]

9 minutes ago, kerbiloid said:

Just an existing rocket was too narrow for the cargo.

Sometimes however many rockets like Falcon 9 has always had that fairing. Some like Atlas has two fairing options. Boosters tend to become larger or more numerous then you get new versions of rockets however. 

Generaly streamlining is not critical for rockets other than to keep max-q down as they don't stay long in the dense atmosphere. You don't bother with sharp tips on fairings even if this help then going supersonic and almost all supersonic planes has pointed noses as from you get supersonic to drag is not an issue any longer is short. 

 

[kerbiloid]

2 minutes ago, magnemoe said:

Sometimes however many rockets like Falcon 9 has always had that fairing.

Because it's hard to imagine a 10+ t payload narrower than 3.5 m. Or 30+ t for a trifalcon.

[Sam-urai]

Thanks for your quick responses, so if the middle slim part isn't nessesary for the aerodynamics then why don't they just have the same diameter the whole way down the body and have the rocket not as tall as that would reduce skin friction? Is it to keep the centre of mass up higher?

[magnemoe]

34 minutes ago, Sam-urai said:

Thanks for your quick responses, so if the middle slim part isn't nessesary for the aerodynamics then why don't they just have the same diameter the whole way down the body and have the rocket not as tall as that would reduce skin friction? Is it to keep the centre of mass up higher?

Thick rockets or rocket parts are harder to transport, if you can you down want to go past 3.6 meters. An long rocket is also more natural stable as you say. 

[kerbiloid]

Also the lesser is the set of used diameters, the less various equipment they should have for manufacturing and servicing the rockets. The length is less important.

[magnemoe]

44 minutes ago, kerbiloid said:

Also the lesser is the set of used diameters, the less various equipment they should have for manufacturing and servicing the rockets. The length is less important.

Also true, Soyuz is the odd duck here together with N1 who is also Soviet. Other rockets with tapering tends to have it at inter-stages. 

[peadar1987]

On 8/6/2019 at 4:51 PM, magnemoe said:

Also true, Soyuz is the odd duck here together with N1 who is also Soviet. Other rockets with tapering tends to have it at inter-stages. 

As far as I'm aware the N1's odd shape was because the tanks were spherical, as opposed to cylindrical, as the USSR didn't have the required materials knowledge to produce sufficiently strong cylinders. The big taper in each stage was a consequence of having two spheres of different sizes to hold fuel and oxidiser.

[mikegarrison]

Since it hadn't been mentioned, rockets often contain pressurized tanks. Spherical is the best for this, but a cylinder with spherical ends is pretty good too. A cone-shaped tank, however, is more complex because the stresses are not as simple as for a sphere or a cylinder.

 

-Oh wait, I hadn't read the very last post. Dang.

Edited August 8, 2019 by mikegarrison

[magnemoe]

7 hours ago, peadar1987 said:

As far as I'm aware the N1's odd shape was because the tanks were spherical, as opposed to cylindrical, as the USSR didn't have the required materials knowledge to produce sufficiently strong cylinders. The big taper in each stage was a consequence of having two spheres of different sizes to hold fuel and oxidiser.

Think I have seen it on blueprints but not thought about it.
One obvious downside is that you loose out on the balloon tank effect where the pressure inside the tank add to the load bearing of stage. 
No its not as extreme as the old atlas steel tanks but wonder if an Falcon 9 first stage could handle the weight of the upper stage and payload  at max g force without pressurization. 

2 minutes ago, mikegarrison said:

Since it hadn't been mentioned, rockets often contain pressurized tanks. Spherical is the best for this, but a cylinder with spherical ends is pretty good too. A cone-shaped tank, however, is more complex because the stresses are not as simple as for a sphere or a cylinder.

How does the Soyuz work with its pointed and slanted boosters? yes they look cool but probably is not an very efficient design. 
Also an older design, yes it might well have evolved over time to. 

[mikegarrison]

18 minutes ago, magnemoe said:

How does the Soyuz work with its pointed and slanted boosters?

Dunno. It's not impossible to make odd-shaped pressurized tanks. Just harder (and therefore heavier).

Edited August 8, 2019 by mikegarrison