Rocket Nose Cones

[Spaced Out]

 So I couldn't help but noticing that the Delta IV, and Delta IV Heavy have somewhat rounded nose cones, which I have heard are better at less than supersonic velocities, and pointed ones are good at supersonic velocities. The Falcon 9 fairings havs one in the middle. Can someone explain why all of this is? And what is the best design for a nose cone for a typical rocket?

Edited August 19, 2017 by Spaced Out

[cubinator]

My guess is it would depend on the acceleration and ascent profile of your rocket.

[MatterBeam]

6 hours ago, Spaced Out said:

 So I couldn't help but noticing that the Delta IV, and Delta IV Heavy have somewhat rounded nose cones, which I have heard are better at less than supersonic velocities, and pointed ones are good at supersonic velocities. The Falcon 9 fairings havs one in the middle. Can someone explain why all of this is? And what is the best design for a nose cone for a typical rocket?

Pointy nose cones are useful for supersonic flight in relatively low altitudes (10-20km). Space is at 100km. A typical rocket only spends a few seconds in the lower atmosphere after it has passed Max-Q, so you want to optimize the cones for the long time you spend at subsonic speeds and low atmosphere. Their higher drag at supersonic speeds is insignificant above 20km as the air becomes so thin, aerodynamics matter much less. 

[YNM]

Rocket aerodynamics are lame. All you need to do is avoid failure, not having the least drag. Pointy things are more prone to failure, probably - hence round.

[Nibb31]

Nose cones and fairings are expensive. A rounded nose cone might be marginally better, but will use more material and could be heavier, more expensive and harder to produce than a simple cone. In engineering, there is no single best design. Engineering is about meeting requirements, and each R&D team has different requirements. Therefore, an actual design is always a matter of compromise and defining priorities: performance, cost, schedule, pick two. This is why different engineering teams figure out different solutions depending on their different requirements, and some rockets are cones and others are ogives.

Edited August 20, 2017 by Nibb31

[mikegarrison]

Pointed is somewhat better in supersonic flow, but only if the angle of the cone is less than the Mach angle. But weight is also important, and so is strength and volume and cost.

[kerbiloid]

Probably just because a shroud with rounded nose can be filled with cargo more completely, with less empty space

Spoiler

vs

 

Though, depends on the cargo design.

Spoiler

Unlikely aerodynamics plays much role here.
It plays role when choosing exact cone angle or exact rounded shape.

[sevenperforce]

A cone with a sharp transition will have more parasitic drag than a cone with a smooth/rounded transition, due to boundary-layer separation at the interface.

[Kryten]

Most modern fairings have a rounded 'ogival' fairing, it's just a lot of rockets use old designs. The atlas 5 pointed fairing is a direct descendent of the original atlas-centaur fairing from the 60s, and the DIVH fairing was designed for Titan IV in the eighties.

[Snark]

I don't actually know the answer to the question.  But I wonder if it might have to do with saving mass?

Curved things like domes tend to be stronger than straight things, for a given thickness of structural members.  It's why bridges have arches.  It's a big part of why domed buildings are a thing.  It's why eggshells are surprisingly strong, given how thin they are.

So, one idea (which I don't know to be the case, but it seems plausible to me) is that perhaps by making them curvy like that, they increase the strength, which allows them to be made out of thinner materials, which in turn allows them to be lighter, and my impression is that mass is the kingpin bogeyman of rocket design.  Seems like an awful lot of "why are rockets designed thus-and-so?" questions come down to "because it saves mass".

The vast majority of the rocket's dV on the way to orbit happens up where the air is pretty darn thin, verging on nonexistent, so I would imagine that "it's lighter" would tend to trump "it's aerodynamic", as long as the aerodynamics aren't egregiously bad.

I wonder if this might be it?

[GDJ]

Straight sided cones are easier to manufacture, therefore used on large rockets, and aerodynamically they are better than a blunt end.

Best shape for aerodynamics: Von Karman or LV Haak for supersonic.
Best shape for subsonic: Elliptical 4:1

Worst shape (beyond a flat surface) straight sided cone
2nd worse: 3:1 Olgive.

[kerbiloid]

Any nose shroud is a sequence of shapes: long cylinder + cone (+ cone + cone + ...) + hemisphere.

They just vary the number of intermediate cones according to professional superstitions and patience.

As we can see, results are more or less the same.

Edited August 23, 2017 by kerbiloid

[NSEP]

I dont know if this is the reason why they do not use pointy noses, but its all i can think of right now.