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Consider a reentry vehicle that looks like this

enter image description here

If this re-entered the atmosphere from orbit, presupposing a sufficiently forward CG, could it stabilizer into a nose-first lifting attitude?

I'm thinking that as the AoA increases, the s shape will begin to behave like a canard planform, where the nosecone is the canard as it has a higher AoA (the bottom surface does, with the top surface being inactive), while the main body is the main wing.

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    $\begingroup$ Previous discussions have somewhat agreed that non-aviation aerodynamics questions are offtopic here. I can see how this one might be borderline so I'll not mod-hammer it just yet. Lets see what the community decide but please all review the linked meta (and consider voting on the answers you agree/disagree with if it's the first time you've seen it). $\endgroup$
    – Jamiec ♦
    Commented May 23, 2023 at 11:00
  • $\begingroup$ The question of on/off topic hinges on the distinction of whether this is an "aircraft". Technically that is something which gains lift from the air - this qualifies. Others distinguish that it must be piloted either directly or remotely, a bullet does not qualify. Is a reentry vehicle "piloted"? Hence my assertion that this is borderline. $\endgroup$
    – Jamiec ♦
    Commented May 23, 2023 at 11:04
  • $\begingroup$ This is basically what the Falcon 9 booster does, except it has both reaction control thrusters as well as active aerodynamic control surfaces to control attitude, so it doesn't need to be aerodynamically stable. $\endgroup$
    – Jörg W Mittag
    Commented May 23, 2023 at 12:49
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    $\begingroup$ Instinctively I would answer no. That's why rifle barrels have rifling, and why footballs are thrown with a spiral. Because there is no aerodynamic means to provide stability, so they must be stabilized gyroscopically. And instinctively, if the CG was forward it would stabilize like a dart. Aerodynamically seeking the path of least resistance. Because what property of a forward CG might produce a positive AOA? Conceptually it doesn't seem worthy of serious consideration. $\endgroup$
    – Michael Hall
    Commented May 24, 2023 at 17:41
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    $\begingroup$ @MichaelHall sometimes I try to steer the OP in the right direction, but your advice is well taken, and I will work on being more concise. $\endgroup$
    – Robert DiGiovanni
    Commented May 25, 2023 at 18:59

1 Answer 1

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Definitely No!

Your rocket qualifies as a slender body. Slender body theory says that the center of pressure is near the tip, regardless of Mach number. This means that a small side force caused by a nonzero angle of attack or sideslip will amplify this angle. In other words, the rocket will be unstable in pitch and yaw. Catastrophically so if you are in a re-entry.

Most lift will occur on the nosecone and the rear body will contribute almost none. A finless rocket cannot be stabilized at any angle of attack but will tumble uncontrollably.

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  • $\begingroup$ Won't the CP move back as the AoA rises, until it reaches the CG? $\endgroup$
    – Abdullah is not an Amalekite
    Commented May 25, 2023 at 2:08
  • $\begingroup$ So, now we add stabilizer fins to help control these tendencies, or keep some instability a for better manuvering. $\endgroup$
    – Robert DiGiovanni
    Commented May 25, 2023 at 6:25
  • $\begingroup$ @Abdullah No, it will stay where it is until you are almost orthogonal to the flow. Remember what happens in a deep spin: Only the airplane nose will experience significant aerodynamic forces (apart from simple drag), the rest will have fully separated flow. The final posture will be orthogonal to the flow and spinning, after lots of tumbling around. Unless it breaks apart during the tumbling phase, which is very likely. $\endgroup$
    – Peter Kämpf
    Commented May 25, 2023 at 10:14
  • $\begingroup$ Why would we care about flow separation at those speeds? $\endgroup$
    – Abdullah is not an Amalekite
    Commented May 25, 2023 at 10:17
  • $\begingroup$ @Abdullah We care about what happens at the opposite end of flow separation: Massive heat and aerodynamic loads over the full surface of the rocket. And the aero loads are purely drag. I understand your question that you care about lift. $\endgroup$
    – Peter Kämpf
    Commented May 25, 2023 at 17:43

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