Just how pointy does a rocket's nosecone need to be?

Question

There are some beautiful images of the Qu8k rocket launch on this web page and I show a few below. There's a video (below) and the PDF Qu8k Final By Derek Deville, November 27, 2011

The stainless steel point of this rocket's nosecone appears to be quite sharp and replaceable, which made me wonder just how pointy does a rocket need to be?

Would a 1cm radius rounded nosecone tip perform significantly different than this sharply pointed tip for a rocket like this? What about for a larger, heavy orbital rocket; do they need to be sharp as well?

Click below for full size

Answer 1

As rockets get larger, the importance of drag relative to mass decreases (drag runs generally proportional to cross section and surface area while mass runs generally proportional to volume). Hence big rockets tend to have less pointy payload fairings, which provide more useful volume by mass, as Hobbes' answer shows.

Qu8k was a small rocket developed with the sole goal of reaching 100,000 feet (30km) altitude under the Carmack Micro Prize rules; it was designed over a weekend and took some departures from normal amateur rocketry conventions in the name of reducing drag:

Most pads require use of a launch lug or rail guide. These protruding parts on a rocket create lots of drag. By removing these parts, simulations showed I was able to increase peak altitude by over 10,000 feet. The desire to keep drag low and have full control over when the pad would be setup and available for launch drove me to make my own launch tower.

The Wikipedia nose cone design article that Hobbes links mentions that conic tips are often chosen for ease of manufacture over more complex shapes, and this was probably the driving factor for Qu8k. This chart from that page shows that cones are actually poor performers in the transonic regime. Qu8k reached its top speed of around mach 2.8 at only 17,000 feet (5km), meaning that it spends a lot of time in dense air at speeds where a conic nose is not ideal:

Comparison of drag characteristics of various nose cone shapes in the transonic to low-mach regions. Rankings are: superior (1), good (2), fair (3), inferior (4).

So presumably the tradeoff here was that Deville knew he could fabricate a conic nose cone, and his simulations said it would go to 100,000 feet, so that's the route he took; I imagine that with more time and effort he could have made a very slightly smaller rocket with a Haack/von Kármán nose and reached the target altitude, but in rocket development it's very rare that you're ever optimizing for only one factor.

The chart doesn't show the performance of spherical-tip conics separately from pure conics, but I suspect that a 1cm radius tip would have been an insignificant difference on this rocket: a little more time in the shop grinding the tip down, and a little less likely to accidentally puncture something while being handled, and still capable of breaking 100,000 feet.

Answer 2

Blunted noses, especially in the case of the Shuttle Orbiter, are a thermal consideration as well. To put it simply, when a vehicle travels at supersonic speeds, there is a shockwave produced by the nose “because the air wouldn’t otherwise get out of the way fast enough”. This wave produces a large component of total drag, especially at hypersonic (Mach > 5) speeds. Having a more slender, smooth airframe mitigates this drag.

Much of the energy of air particles impacting this shockwave is turned to heat, which ionizes the gas and produces the “flames” seen around reentering vehicles. With a slender, pointed nose, the shockwave isn’t as immediately intense so it sticks closer to the airframe, especially at the tip. The heat at the center of the shockwave is very intense, enough to melt or weaken most materials known to man.

To avoid the tip melting, the nose can be made blunt. A blunt tip produces an immediately stronger shockwave, which is pushed out further from the nose. While this increases total drag, there is a weight and heat savings from not having to insulate/dissipate the heat at the nose tip.

There is a trade-off here that must be evaluated design by design to decide what tip structure and material is best.

Answer 3

Orbital rockets generally don't have such sharp noses. Here's an Ariane 5 as an example:

You can see there's a large radius on the tip. Eyeballing from a scale model I have handy, I'd say that radius is about 0.5 m.

The Falcon 9 fairing has a large radius too:

The Apollo LAS had a sphere embedded in the tip.

The nose design with the lowest drag seems to be the Haack/von Karman design, which uses a rounded tip.

Answer 4

As a counterexample to some statements in the other answers....the actual extreme tip of the shuttle stack was surprisingly sharp....

(personal photo)