12

For example, Tomahawk Cruise Missile, or Harpoon anti-ship missile. They fly straight and level for the most part of their flight regime. I think if they had a fuselage optimised for level flight then they could have a greater range. Something of a lifting-fuselage.

I know that even their cylindrical body generates lift at some angle of attack but it comes at the expense of drag, so a more optimised shape could result in a greater L/D ratio and hence a longer range.

I think that space and volume constraints may be an issue, like a circular cross section probably has the largest volume to store whatever.

Tony Stark
  • 429
  • 4
  • 11
  • 5
    These missiles are nowhere near comparable. The AMRAAM flies at Mach 4 while the Tomahawk is subsonic. The former is unlikely to benefit from lots of extra drag in exchange for some additional lift in horizontal flight, while it might make sense for the latter. Consider narrowing your question down. – TooTea Oct 31 '22 at 11:51
  • 4
    I suppose that some missiles spin around their axis to augment their stability, just like a bullet. A cylindrical shape makes in that case sense. – sophit Oct 31 '22 at 13:08
  • 2
    ease of construction, and thus cost considerations no doubt come into play. The ALCM is triangular mostly because it was designed for packing on a rotary launcher, and a triangular shape allows for more missiles to be packed on that so the shape made sense there, not for aerodynamic reasons. – jwenting Oct 31 '22 at 14:13
  • 4
    Once you reach the point of having "enough lift", there's no benefit to "more lift". Instead, the design focus becomes "less drag". – Mark Oct 31 '22 at 21:37
  • @Mark I think a cylindrical fuselage creates lift by being at certain AoA all the time, if the fuselage created more lift then it could be at lower AoA and create less drag. What do you think? – Tony Stark Oct 31 '22 at 21:46
  • @TonyStark, I think that a fuselage at zero AoA will generate less drag than one at a non-zero AoA. – Mark Oct 31 '22 at 21:49
  • 1
    I don't know how much of a contributing factor this is, but some (maybe most or all) big munitions need to be periodically rotated while stored to prevent the composition of their propellants and explosives settling into layers. Easier to build racks that allow this if they're cylindrical. – Grimm The Opiner Nov 02 '22 at 07:59
  • Not only round, but they also need to be pointy at the top or they will just bounce back to from where it was launched. – winny Nov 02 '22 at 15:24
  • 1
    @winny damn! Admiral General Aladeen was onto something – Tony Stark Nov 02 '22 at 19:31

5 Answers5

39

Rockets with solid fuel must be round, or they would weigh more. Remember that the rear part is filled with fuel which gradually burns away, so the whole fuel container has to withstand the pressure which in the end propels the missile. Only a cylindrical body will do this efficiently. Depending on the routing of cables and the position of actuators, the aeroshell (the outer skin that defines the body of the missile) can be different from the casing of the rocket motor, however, structural efficiency favors to have a cylindrical body.

The throat of the rocket nozzle allows to position the actuators of the rear surfaces within the cylindrical contour and sometimes you will see a cable conduit running along the outside of an otherwise cylindrical fuselage.

For the turbojet-propelled missiles: their body shape is determined by their storage. Submarine-launched cruise missiles have the same diameter as a torpedo, for very practical reasons: They are meant to be launched from torpedo tubes.

Air-launched cruise missiles which are stored on rotary launchers have a rounded trapezoidal cross section, so more of them can be stored in a constrained space.

Regarding the myth of the efficiency-enhancing lift-producing fuselage: This has been laid to rest for more than half a century by now. What counts is the lift-to-drag ratio and producing lift is trivial when the minimal speed during your mission is somewhere around Mach 0.5.

At high dynamic pressure everything will produce ample lift. Now the goal has to be to do so with minimal drag. Using proper wings makes lift production very efficient, and the cross section of the fuselage will not have much influence here. As a slender body the fuselage produces lift mostly where its width increases, regardless of cross section shape.

Peter Kämpf
  • 231,832
  • 17
  • 588
  • 929
  • I am not thinking of a blended wing design, more like a flat bottom and a curved upper surface, maybe some splines on the side. I aim to fit it in the same profile as a conventional cylinder. Also, can you please clarify on why producing lift is trivial? The way I see it, a pure cylinder can be Aerodynamically optimised such that it produce more lift, less drag, and the internal volume is not less than 90% of a cylindrical profile. – Tony Stark Oct 31 '22 at 19:57
  • 5
    @Peter Kämpf the aeroshell and the cylinder containing the rocket fuel are normally not the same. If it were, you would see an racetrack containing conduits for ignition and actuators, like you see on big rockets (like the Falcon 9). I would therefore put this down as an "argument" rather then an reason... With your permission I would extend my answer with this "argument" – U_flow Oct 31 '22 at 20:15
  • Aren't cruise missiles generally launched vertically (unlike torpedos)? Or are you saying they repurpose torpedo tubes as vertical launch tubes? – johnDanger Oct 31 '22 at 21:02
  • 5
    @johnDanger: There are submarines with dedicated vertical missile launch systems, but there are also submarines where the torpedo tubes are the only way to launch something out of the boat. – Jörg W Mittag Oct 31 '22 at 22:05
  • 3
    @U_flow: Good point! No need to ask for permission, though. – Peter Kämpf Nov 01 '22 at 07:10
  • 3
    @TonyStark At high dynamic pressure everything will produce ample lift. Now the goal has to be to do so with minimal drag. Using proper wings makes lift production very efficient, and the cross section of the fuselage will not have much influence here. As a slender body the fuselage produces lift mostly where its width increases, regardless of cross section shape. – Peter Kämpf Nov 01 '22 at 07:49
  • 2
    @PeterKämpf I did not want to look like a copy cat :D. – U_flow Nov 01 '22 at 10:12
  • I'm not happy with the very first part of your answer: can you please update it with the good point of @U_flow? – sophit Nov 02 '22 at 13:15
  • Actually, if I recall correctly, the big reasons for a cylindrical cross-section are that it has the highest volume to surface area ratio, which is good for reducing drag with the additional bonus that a cylindrical structure is very rigid and resilient to bending moments and torsion. – Romeo_4808N Nov 06 '22 at 13:20
  • Yes! But X-15 had a cylinder, no area ruled fuselage, while SR-71 had the lower side shaped as a high speed boat, it seems generating some lift, besides the Reynolds number issues. Does a half cylinder shape, round side up, deserves testing vs a vulgar round fuselage? Me-262 fuselage was somehow a Rouleaux triangle, as Wankel engine Rotor. Did it generate lift? Blessings + – Urquiola Mar 10 '23 at 09:04
  • 1
    @Urquiola The SR-71 bottom was shaped to avoid horizontal radar reflections. Lift at supersonic speed is determined by angle of attack and at high Mach (>2) area ruling is no longer necessary. The fuselage shape has little influence; one configuration where shaping the bottom (of the wing and intakes, however) helped was the XB-70. The Me-262 fuselage was shaped to accommodate two turbojets F-80 style, but during engine development they grew in diameter so they had to be hung under the wings. Fuselage shape was kept to avoid lengthy redesign. – Peter Kämpf Mar 11 '23 at 12:35
16

There are several reasons why most missiles have a round shape. I try to list (some of them) in roughly ascending order of importance:

  • Round shapes with wings are easy to calculate: Well known rough calculation methods exist which tell you which performance you can expect from a tubular shape at which you attach some wings. For example Missile Datcom is extensively used to calculate some performance numbers.

  • For the rocket engine, normally solid fuel is used. As the canister containing this fuel has to withstand the pressure of the rocket fuel while burning, a circular cross-section is optimal. However the aeroshell and the cylinder containing the rocket fuel are normally not the same. If it were, you would see an racetrack containing conduits for ignition and actuators, like you see on big rockets (like the Falcon 9).

  • Wings are efficient: Instead of going through the hazzle of designing, testing and manufacturing a complicated lifting body for which you probably still need wings, a tubular body is well tested and worked in the past. Wings are very well understood, and a lot of expertise exists to design these.

  • Wings can be deployed: You can unfold a wing, but you cannot unfold a lifting body. This is also important for my last argument.

  • A round shape is easy to manufacture: A tube with a pointy nose and some fins is easy and cheap to manufacture. As missiles are produced in fairly high numbers, ease of manufacture is therefore an important consideration.

  • A circular body is easy to be containerized into some launch/deployment canister. Missiles and similar weapons are often delivered in some canister out of which it is then deployed. The missile has to fit in this container. A circular cross-section is the most efficient way to put such a missile into this canister, as it has the least surface for the most amount of space (which also optimized aerodynamics). This containerization is also important for folding out wings and fins. Taken from https://en.wikipedia.org/wiki/Tomahawk_(missile)#/media/File:USN_Tactical_Tomahawk_launch.jpg

However all of these are only arguments for a circular crosssection. With the proliferation of modern CFD methods for example, it becomes much easier to design more complex shapes. More modern missile designs may feature quadratic or triangular shapes, e.g. the AGM-86 ALCM (Air Launched Cruise Missile) as pointed out by WPNSGuy.

U_flow
  • 3,640
  • 2
  • 9
  • 24
8

And some missiles are NOT cylindrical. Here, 6x AGM-86 ALCM (Air Launched Cruise Missile) on a B-52 left wing pylon.

enter image description here

photo courtesy of NARA - https://nara.getarchive.net/media/technicians-check-the-left-wing-pylon-of-a-92nd-bomb-wing-b-52g-stratofortress-6d1f18

WPNSGuy
  • 8,624
  • 2
  • 18
  • 42
  • The Taurus KEPD 350 (another air-launched cruise missile) has a similar body shape. – Mookuh Nov 01 '22 at 06:30
  • 1
    What are we looking at here? Three missiles attached to a support? Two? Or is the whole thing one missile? – WaterMolecule Nov 01 '22 at 22:01
  • 5
    @WaterMolucule - That's actually 6x missiles. 3 front and 3 behind. Mounted on a B-52 wing pylon. – WPNSGuy Nov 01 '22 at 22:15
  • 2
    Note that these missiles were designed for fitting 8 on a rotary launcher in the bomb bay. That's the main reason for the body shape, another shape wouldn't fit that number in the bomb bay so this one maximises the number of missiles that can be carried internally. This was especially important for the B-1 as it was initially designed for internal weapons only. – jwenting Nov 02 '22 at 04:22
0

Another factor: Missiles need a nice, smooth skin for aerodynamic reasons. You use less skin to contain a round missile than you use to contain a non-round missile. Thus the round missile flies a bit further than the otherwise-identical non-round missile.

Loren Pechtel
  • 509
  • 2
  • 6
-4

Missiles are essentially rockets, and rockets don't have lifting surfaces for the simple reason that they don't need to generate lift, because the thrust that their engine(s) produce is more than able to overcome both gravity and aerodynamic drag.

Ian Kemp
  • 294
  • 1
  • 10
  • 4
    Both types of missiles mentioned by the OP (Tomahawk and Harpoon) have lift-generating wings. It's simply not true that rockets categorically don't have lifting surfaces, or that they cannot benefit from lift. – Nuclear Hoagie Nov 02 '22 at 15:41
  • Some rockets/missiles are expected to follow a ballistic trajectory, so don't need wings. The remainder are intended to 'fly', but that is not directly connected to the propulsion type. The Space Shuttle was a rocket, and is a classic example of a rocket-with-wings. – MikeB Nov 03 '22 at 13:50