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When a rocket is traveling through an atmosphere, the component of the aerodynamic force in the direction of motion is called drag, and the component perpendicular to that is called lift. Usually a rocket is pointed in nearly the same direction as motion, but not necessarily exactly. See the NASA illustration below.

For simulation, and for flight guidance and control, consideration of lift is essential. Lift is therefore important.

My question though: is lift ever useful when rockets are traveling in an atmosphere?

And here, I mean "rocket-shaped" rockets; not car-shaped, or plane-shaped rockets, but the long, approximately cylindrically symmetric ones that tend to fly from low altitudes up into space. (yes, and more frequently these days, back down again).

Image from https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html

NASA Rocket Forces

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    $\begingroup$ I assume you are ground ruling out Pegasus as a "plane-shaped rocket"? $\endgroup$ – Organic Marble Jul 9 '16 at 17:40
  • $\begingroup$ @OrganicMarble for here, for now, yep. A search did lead me to this interesting video: https://www.youtube.com/watch?v=oY3GclS5VUQ, and then to this question. $\endgroup$ – uhoh Jul 10 '16 at 2:01
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    $\begingroup$ If there are fins, and they deflect the airflow to cause a force perpendicular to the direction of flight (say, for steering), that would be lift. It happens to also cause drag. $\endgroup$ – Steve Jul 10 '16 at 10:16
  • $\begingroup$ This is related to the CG and CP mentioned in the down-voted answer (which I up-voted). This could also happen, in the passive sense, to a cylindrical rocket without fins. $\endgroup$ – Steve Jul 10 '16 at 10:21
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    $\begingroup$ It is lift, even if it's a momentary steering force. It only acts in one direction. $\endgroup$ – Steve Jul 10 '16 at 10:23
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Angling to get lift is going to increase the atmospheric cross-section of the rocket and so increase drag. For any reasonable angle of attack, the drag force is going to be much larger than the lift force, so I believe that for powered ascent it makes the most sense to minimize drag, which means zero AoA and zero lift. This also, as Mark Adler notes, minimizes lateral stresses on the vehicle, allowing structural weight to be minimized.

Note that the total drag losses on a large rocket in ascent to LEO are small -- for Saturn V, about 0.5% of the total ∆v expenditure. Since low-AoA lift is a small fraction of drag, any gain from body lift would be extremely small.

Of course, there have to be minor excursions from zero AoA during a powered ascent (though hopefully only small ones); these should be positive AoA in order to take advantage of what little lift there is.

That said, lift is useful for steering during unpowered flight; the Falcon 9 first stage uses body lift to control its downrange flight to an ASDS. It's also very important for control of reentry vehicles, but those aren't "rocket-shaped rockets" so they're outside of the scope of your question.

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  • $\begingroup$ An argument for the use of lift might go as follows. Drag vs angle is almost flat (quadratic or at least symmetric) near zero angle, while lift is roughly linear. Therefore sufficiently small angles will give some "almost free" lift relative to the change in drag. The counter argument given by @MarkAdler as you mention is that carrying the increased mass of the airframe necessary to accommodate the additional side loading will end up costing more than any lift could offset. $\endgroup$ – uhoh Jul 10 '16 at 1:07
  • $\begingroup$ The smallness ∆v impact of drag on the Saturn V is good to know! I'm interested in the other end of scale, small(est) launch vehicles where drag will become much more significant. I found this question and answer on the smallest examples. I remember seeing a question about smallest possible launch to orbit vehicle, but I can't find it now. $\endgroup$ – uhoh Jul 10 '16 at 1:13
  • $\begingroup$ Also - kudos for noticing that the answer to "is lift ever useful when rockets are traveling in an atmosphere?" is actually yes, an example being assistance in re-entry control of rocket bodies - the most notable being Falcon 9's 1st stage return to earth and controlled landing on a bulls-eye for re-use. :-) $\endgroup$ – uhoh Jul 10 '16 at 1:20
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    $\begingroup$ Once I master orbital insertion guidance in my sim, maybe I'll tackle simulation of lift (currently it pretends AoA is always zero) and see if there's advantage to maintaining a small positive angle. $\endgroup$ – Russell Borogove Jul 11 '16 at 15:38
  • $\begingroup$ Just fyi I happened to hear Declan Murphy of flightclub.io mention the use of lift in F9 return trajectory in the TMRO episode The beautiful data of rocket launches after 32:10. youtube direct link: youtu.be/WpcbFdSB_Ik?t=1930 $\endgroup$ – uhoh Feb 6 '17 at 7:14
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As far as I know, no. In order to make a cylindrical rocket as light as possible, they are flown to minimize the side loads to the structure -- as close to a zero angle of attack as possible. If they wanted to use lift, it would increase the mass of the structure to be able to take substantial drag forces from the side. There would not be sufficient benefit from the lift of a cylindrical structure to offset that mass increase.

Wings or other lifting surfaces on the other hand (which you are ruling out), could potentially provide some benefit in the early parts of ascent. That was the case for Pegasus.

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  • $\begingroup$ I really like this answer - two sentences to run through the physics argument and practical engineering conclusion. As easy to remember as a slam dunk. https://en.wikipedia.org/wiki/Slam_dunk#Notable_Dunks $\endgroup$ – uhoh Jul 10 '16 at 0:46
  • $\begingroup$ I had to give the 'accept' to @RussellBorogove 's answer. While - as you correctly surmised - my primary interest is in powered launch, the question doesn't actually have that restriction. $\endgroup$ – uhoh Jul 10 '16 at 1:27
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You said it yourself, Lift is important for guidance and control. As a matter of fact a rocket is designed in such a way, that the center of pressure is aft of the center of gravity. The distance between CG and CP is also called the caliber stability margin measured in rocket caliber.

You can use the fins of a rocket to control the direction and magnitude of the lift force and therefore change the positon of the center of pressure and therefore the stability of your rocket in the atmosphere.

Hence, yes, lift is useful.

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  • $\begingroup$ @Steve convinced me that you have a point. Many if not most rocket designs used for Space Exploration have given up on passive stability like that, and instead use lots of sensors, a computer, and thrust vectoring. But you are right, lift can be used to stabilize fight also. I can't undo my down-vote until the question is edited (stackexchange UI enforces that) - if you can make even a trivial change, then I can do so. $\endgroup$ – uhoh Jul 10 '16 at 10:51
  • $\begingroup$ OK I did a nominal edit so I could change my vote. $\endgroup$ – uhoh Jul 11 '16 at 8:24

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