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Why are sphere shaped missiles impractical? I am assuming the main points are that there wouldn't be enough space for fuel, and that the sphere is not the most aerodynamic of shapes. So in order for this to be viable fuel would have to be considerably dense, warhead considerably powerful, and guidance systems considerably generous to ensure the missile doesn't have the accuracy of a deflating balloon. Are my assumptions correct? Are there any major points I am missing? I'm sure it's a stupid idea, but I'd like to know why.

Why do I want a spherical missile? In order to have more of them, of course!

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    $\begingroup$ They're not impractical just dated. We used to call such things cannonballs. Not all missiles are self propelled. $\endgroup$
    – candied_orange
    Aug 23, 2016 at 15:29
  • $\begingroup$ Stability for self propelled missiles. $\endgroup$
    – Jax
    Aug 23, 2016 at 16:20
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    $\begingroup$ Very good question, but I do have to ask why having spherical missiles would let you have more of them. Spheres are not the most efficient for packing. $\endgroup$
    – Cort Ammon
    Aug 23, 2016 at 17:23
  • $\begingroup$ In a space environment, a spherical shape has the most volume for the least surface area, saving materials and costs. As noted, it isn;t an efficient way to pack the weapons bay or fill volume inside the ship. $\endgroup$
    – Thucydides
    Aug 24, 2016 at 2:43
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    $\begingroup$ @celtschk Not necessarily: missile "an object that is forcibly propelled at a target, either by hand or from a mechanical weapon." A lot of people associate the word missile with guided missile because they hear the word mentioned in the name of weapon systems that are guided. The word has a long and rich history outside of this context. $\endgroup$
    – candied_orange
    Aug 24, 2016 at 11:58

7 Answers 7

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Nice question.

I think Aerodynamics is the main factor here. Bearing in mind that, as the object flies through the air, wind resistance slows the object down; having a spherical object with a large surface area will mean a much shallower angle in which to be able to fire and still keep distance.

enter image description here

With the other points here, it's not too bad. Pirates were pretty accurate with cannon-balls, they had to be to make each shot count at sea.

As for power, well the cannonball was a huge lump of iron or bronze traveling very fast (fast enough to punch through wooden ships and stone fortresses from a distance); Admittedly this fuel source was the kinetic energy released from the gunpowder they used. The more powder, the more powerful the shot.

That said, there's a famous thought experiment that boiled down to "If you can build a cannon powerful enough and fire a cannonball from high enough, it'll have enough momentum to make orbit" (Newton's Cannonball)

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  • $\begingroup$ You beat me to the punch with the answer! $\endgroup$
    – Chris J
    Aug 23, 2016 at 15:23
  • $\begingroup$ @ChrisJ Heh. Sorry :P $\endgroup$
    – Raisus
    Aug 23, 2016 at 15:24
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There are many factors to consider, but one that catches my attention is stability. To be stable in flight, the center of pressure needs to be behind the center of mass. The further it is behind the center of mass, the more stability you have. This is why rockets and missiles have fins: at most angles of attack, the fins move the center of pressure far enough back to be stable. This is also visible in pop-bottle-rockets. The long stick that they are attached to acts as a rudder of sorts, trailing behind the center of mass (which is near the front of the stick because that's where the propellant is).

If you have a long slender missile, you can have smaller fins and still retain stability. Your sphere may need rather large fins to avoid tumbling in the turbulent air.

The closest example I can think of this in real life is the Fat Man, the bomb dropped on Nagasaki. Note the very large square of fins they put on it to keep it stable:

enter image description here

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    $\begingroup$ the Fat Man isn't a missile tho $\endgroup$
    – enkryptor
    Aug 23, 2016 at 17:36
  • $\begingroup$ @enkryptor It's still the closest I could find. The aerodynamics are similar, there's just no propulsion on the bomb. It shows how large the stabilizing part of the missile would have to be if you made one. $\endgroup$
    – Cort Ammon
    Aug 23, 2016 at 17:38
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    $\begingroup$ Which is funny because in early drop tests (before the finalized fins) the Fat Man would miss its target by half a kilometer, though it had an air blast diameter of 3.4 km. The fins increased bombing accuracy down to .25 km. To the people of Nagasaki this was not a significant improvement. $\endgroup$
    – Xplodotron
    Aug 23, 2016 at 17:51
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Aerodynamics

Simply a sphere causes more drag, as you suppose. This is just as much an issue of how it impacts the air (nose) as how deals with the trailing portion (tail). It'd take more energy to move it with no gain.

Guidance

It's far easier to guide a missile when you treat it something like an arrow. Shoot, even WWI/WWII era bombs had a fin and weighted nose which made them act something like arrows when simply dropped. This consistency allows for it to be easier to guide it.

Size

By making it a sphere (even hand waving the previous two requirements) you can't reduce the actual mass of the object. A sphere will be wider, and facilities (aircraft and ground facilities) will have to be built larger to accommodate for this, or your payload would have to be smaller. It simply isn't cost effective.

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Your rationale, "more of them", ought to be reconsidered. Other shapes (like cylinders) pack more tightly than spheres do. Given that the spherical missiles & the cylindrical missiles have the same volume of warhead & propellant, you can store more cylinders in your magazine than spheres.

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Earlier question to ask, perhaps: are we flying through a planetary atmosphere? In space, the drag and aerodynamics don't become important until one gets to serious fractions of light-speed.

Also, the classical missile has an orientation to its direction of flight. This reduces the number of sensors and protections needed (like a heat shield) and defines where they can be located on the missile. A comparable sphere would need to orient itself or have more sensors looking in all directions. Cannonballs didn't need sensors and were effective no matter which surface made impact.

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Basically, I believe it boils down to aerodynamic efficiency. A sphere has too much drag and that causes too much fuel expenditure and limits speed.

Here is a thought: it might be useful to have a spherical missile if you wanted to use the shape to stabilize the missile for re-entry. Say you have a number of missiles in space and you want to launch them at a target on the surface. They get into a degrading orbit and aerobrake thanks to their spherical shape. At a lower altitude, a rocket motor cuts on and the still-descending missile can dodge anti-missile defenses and power it's way to the target (slower than a conventional missile). A cluster of these might even be pretty hard to deal with.

There you go: use case for a spherical missile. (Obviously, it would have to be covered in a very heat resistant shell, but NASA has been experimenting with this for a while, so it's not out of the realm of possibility).

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  • $\begingroup$ Why would a sphere be more helpful for this though? And even if it was you'd most likely jettison the spherical shell (or hemispherical would be enough) once ready to start maneuvering in atmosphere. $\endgroup$
    – Tim B
    Aug 23, 2016 at 16:31
  • $\begingroup$ Could jettison it, but maybe the design complexity (cost) involved in using a discarding sabot type design don't give you so much performance that it's worthwhile (you want to crank them out cheaply in large numbers). Spheres are actually pretty stable reentry vehicles. Lots of space ship designs out there center on using a sphere because it maximizes internal volume, minimizes external area, and is aerodynamically stable. Like an Apollo capsule, it would just float down on a given trajectory. $\endgroup$
    – JBiggs
    Aug 23, 2016 at 17:18
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Aerodynamics, Guidance/accuracy, Fuel are all good considerations. First of all a sphere is not a bad idea - pirates not withstanding - they seem to do OK with spheres in baseball. Guidance, including spin is not a challenge for a good pitcher. Pitchers practice targeting all the time and it's the construction and weight of the baseball that aids in the pitcher's ability to deliver it accurately - within inches. So you just need the right construction, with the right targeting mechanism. Now if you want to hurl that baseball across continental, planetary or interplanetary distances, you need great propulsion - what will you use? Could you start out with a larger sphere and have it be self-consuming, that is have it be it's own fuel and payload at the same time?

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