why are the noses of rocket, shuttles, planes, missile(…) half spheres instead of spikes?


why are the noses of rocket, shuttles, planes, missile(…) half spheres instead of spikes?

In: 5557

A long thin spike means you have more surface area and more weight and most of it isn’t going to be useful space to put anything in. For subsonic flight a blunter nose is actually better because it has a lot less surface area to create drag than a pointy nose, and it makes the aircraft overall shorter and lighter.

For supersonic flight pointier noses work better.

Blunt noses are fine if not preferable at low speeds because the airflow can start to move out of the way before it hits the aircraft. The air in front of the craft pushes on the air further ahead to allow for a smooth transition.

Supersonic travel does not have this. The air is ‘notified’ of the oncoming plane right as the plane comes up on it because pressure waves cannot travel out ahead of the plane. This makes pointier designs better in this region.

At very *very* high speed, a blunted nose forces the shockwave to form farther in front of the vehicle, protecting it from the heating of the air that forms at the shock.

A long, pointy nose is great for supersonic travel because it pierces through the air and helps dissipate the shockwaves experienced past the sound barrier (think *Concorde*). However, it’s worse for subsonic speeds because there’s more surface area than on a blunt nose, and therefore more drag. They’re only used on craft expected to spend most of their time travelling faster than the speed of sound.

To your examples: neither rockets nor the space shuttle travelled supersonically for enough time for it to make much of a difference; by the time they’re going fast enough to really get the benefit of a pointed nose, they’re pretty much out of the atmosphere so air resistance is nil anyway. On top of that, weight savings are everything in spacecraft, a few kilogrammes saved on takeoff might equal a few extra tonnes of payload you can get into orbit.

As for missiles, they’re small enough and travel for such a short amount of time that they wouldn’t see much benefit from a pointed nose. Again, not worth it – a missile is fired and hits its target in a matter of seconds.

All of these types of vehicles experience different environments, so it’s important to study them individually.

Regular rockets have to work under their biggest limitation, mass. They don’t spend much time in the thicker parts of the atmosphere, as they launch straight up and only start pitching over to horizontal gradually after a few dozen seconds have gone by. As such they’re better off with the reduced mass of a rounder nose than with marginal gains from a slightly more aerodynamic sharper nose that weighs significantly more.

The shuttle is a special case because, unlike regular rockets, the orbiter itself has to survive re-entry. At the speeds at which it hits the atmosphere, the compression of air forming shockwaves in front of the orbiter causes it to heat up to extremey high temperatures. With a sharp nose, those shocksaves stay very close to the spacecraft, which means the heat transfers faster into it. A rounded nose moves those shockwaves further away, creating a buffer layer of air that slows down the heat transfer. This is also why all space capsules have a rounded, almost blunt bottom.

Subsonic aircraft like passenger jets are built to be as efficient as possible above anything else, because that saves fuel and fuel costs money. Conversely to common intuition, below Mach 1 rounded noses are actually more aerodynamically efficient.

Lastly for missiles and other supersonic aircraft, you’ll find their noses are indeed sharp as you would expect. This is because sharp noses are the most aerodynamically efficient at supersonic speeds, but these aircraft don’t go fast enough for heat to be a major concern.

In addition to aerodynamics, a blunt nose allows more storage space for the same mass of payload. Unless you can get your payloads to be all pointy, that’s a lot of wasted space and weight at the tip. So we do a tradeoff, not hemispherical, but not very pointy either.

Its aerodynamic shape design depending on the application. To get attach or detach shock waves during supersonic, transonic and hypersonic speeds determine the design. https://en.m.wikipedia.org/wiki/Bow_shock_(aerodynamics)





On a rocket a shape of the nose is irrelevant, because it’s gonna leave thick layers of the atmosphere shortly after start. It’s the shape that allows more payload and some aerodynamics. A blunt cone is enough.

For the shuttle the main reason for blunt shape is the thermal barrier. It’s the one coming after super-sonic barrier and is experienced at hyper-speeds. The shuttle have to slow down from orbital speed of 8 kilometers per second to a speed of a plane in order to land.

Sharp pointy shape have heat energy acting on the surface exceeding the limits modern materials can take. If you look at the heat diagram for a sharp cone, the side surfaces close to the tip and the tip itself take enormous amount of heating. Blunt shape for comparison can reduce the stress something like 6 times. That’s the only reason as far as I know. Sorry, I don’t have the exact numbers.

EDIT: expanded a bit.

The “angle of attack” is not always directly ahead, so a needle-bow won’t always be pointed directly into the flow.

If you’re in a ship, trying to cruise due north at 30mph, and you have a 10mph current flowing from west to east, the bow of your ship isn’t going to be pointing north. It’s going to be pointed about 30 degrees west if north. If you trail a kite behind the ship, the kite is going to be directly south of you; you’re going to be headed directly north. The water is going to be hitting the right side of your bow, not straight on the point of the “needle”. The needle won’t be cutting through the water efficiently; it will be pushed through the water sideways.

Aircraft have the same issue. You can see it most clearly during a [crosswind landing](https://www.youtube.com/watch?v=w4EQuM_t8Fo). The aircraft is “crabbing” into the wind, cutting across it sideways rather than head-on. It happens at cruising altitude, too, when the winds aren’t parallel with the direction of travel.

A rounded, bulbous bow is *almost* as efficient as a needle bow when traveling directly forward, but it doesn’t lose that efficiency as the wind is coming in sideways.

Some aircraft do indeed use a needle-like nose. Aircraft designed for sustained supersonic flight have a needle-like nose. ELI5, these planes fly so much faster than the wind can blow that the apparent wind is always nearly directly ahead. (They also manage the supersonic shockwave, but that’s well beyond the scope of your original question.)

Great question. Long story short it’s thermal control. Blunt nose designs are only appropriate for really really high-speed, or hypersonic stuff. Pointy noses are great until heating becomes an issue. A blunt nose is pushing back the boundary layer flow and to keep the shockwave stood off from the surface or ‘stagnation point’.

Heat concentrates at the tip on a spike, because of basically friction. Blunt noses make the shock wave form in front of the body, and also more surface area to disperse heat.

Source: Former rocket scientist turned AI enthusiast.


For an e-like-I-work-for-nasa explanation nasa has a [free book](https://www.nasa.gov/connect/ebooks/coming_home_detail.html) published that details how they landed at the rounded shape, plus lots of material engineering challenges for figuring out how to make a spacecraft capable of surviving many reentries

“it is too round at the top it needs to be pointy!”

“Round is not scary, pointy is scary!”