So that's where the basic familiar rocket shape comes from. Keeping the bottom end of the Sears-Haack shape cylindrical, or even flaring it out slightly, rather than boat-tailing, improves stability dramatically. For stability, you want to keep the center of pressure behind the center of gravity - this is why darts, arrows, and many rockets have fins at the back the fins add drag at the base, moving the center of pressure back and keeping the projectile stable. In the same way that the weight of all the rocket components acts through the center of gravity cg, the aerodynamic forces act through a single point called the center of pressure cp. The tapered "boat-tail" at the back of the Sears-Haack shape, though, moves the center of aerodynamic pressure forward. As a model rocket flies through the air, aerodynamic forces act on all parts of the rocket. (Note that the proportions of this particular example aren't part the definition of the Sears-Haack shape for minimal drag you'd have to have an impractical body of infinite length and infinitesimal cross section.) Sears-Haack is pretty similar to the German V-2 rocket body. So what am I missing? Where in my broad estimations / calculations have I made naïve assumptions?įor supersonic flow, the Sears-Haack body offers less drag than the shorter teardrop that's optimal in the subsonic regime. Now, rocket designers are reasonably smart folks, and have probably thought of this So, for a single stage rocket, I would assume a bullet shaped rocket with something like a 3:1 length to diameter ratio would be ideal for an Earth launched rocket.Īnd for a multistage rocket, the top stage would have something like a 3:1, and the lower stages something like 1:1 - 2:1 length to diameter ratios. Fabricating a sphere is much harder than fabricating a cylinder.An airfoil shape has less drag than a sphere.
Launching a rocket requires passing through the atmosphere.I.e., Surface area (dry mass) to volume (fuel mass) ratios would be worse in a tall skinny rocket. Which, if it operated in a vacuum, would call for a spherical tank. What I mean is that an ideal rocket would have as little mass used for the tank structure, and hold the maximum amount of fuel. However, to me, many rockets seem very tall and skinny. Obviously there are many factors that go into the design of a rocket.
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