Some observations about the aerodynamics of Blue Origin's New Shepard.
I think that aerodynamic stabilization is much more important to New Shepard than Falcon 9. Falcon 9 did eventually add aerodynamic control surfaces in the form of its famous "grid" or "X-wing" control vanes. These are deployed and moved hydraulically, and help to stabilize and steer the descending booster. But they were a late addition to the design, and I think F9 was originally intended to mostly use thrusters for attitude and roll control during descent.
New Shepard, on the other hand, seems to have been designed with a number of important aerodynamic surfaces from the beginning.
If you don't have a bunch of childhood model rocket experience to give you an intuitive grasp of what makes a rocket aerodynamically stable, here's a primer.
The first important characteristic of a rocket is its center of gravity. A rocket will tend to rotate about its CG. A simple tube with its CG in the middle will have no real stability at all, and will simply tend to spin freely about its center. That's because the other important axis of a rocket, its center of pressure, is also smack in the middle of a simple tube.
The center of pressure, or CP is the balance point of all the aerodynamic forces acting on a rocket. Adding things to that tube, like fins, nose cones, payload fairings, etc, will move the CG. On a rocket (or other moving object that you want to be aerodynamically stable, like a bomb dropped from a plane, or a common pub-dart from a dart game), to make something aerodynamically stable we generally want the center of gravity well in front of the center of pressure.
We can move the center of gravity by moving around the parts of the rocket, though in a space booster, which is mainly a flying fuel tank, there's not a lot of room to play. And of course, as the fuel burns, the center of gravity changes, moving backwards as the tanks empty from the top down.
The rocket's payload though, is generally at the front, and it's generally heavy, and it becomes a greater portion of overall spacecraft weight as the spacecraft climbs, so that's good for stability.
But generally, moving the CG forward isn't enough. You need to use aerodynamic surfaces to move the CP rearward. The New Shepard has small fins at the rear that seem to be for this purpose. I don't see any obvious control surfaces, or mechanisms to steer them, so I assume they're simply passive guidance fins, like you'd find on a Estes model rocket kit. They don't steer the rocket, they just keep it moving more or less straight up.
But at peak altitude, everything about New Shepard changes. The fuel is mostly gone, so the tanks are very light, and much of the weight is in the pumps, piping, and rocket motors at the back of the rocket. This is even more true when the heavy capsule separates. At this point, my ballpark guess is that the CG is well rearward on the rocket. I'd say no more than a third of the way from the rear of the rocket, and maybe even further back.
But, at this point, they want New Shepard to fall tail first. Those fins that helped on the way up are now on the wrong end to make that happen. But if I'm right, those fins are also much nearer the center of gravity now, and that reduces their effectiveness. The body of the rocket acts like a teeter-totter hinged at the center of gravity. Forces that act far from that rotation point have more effect that those that are very close to it. The rear fins (which you'll note, are not located all the way back on the rocket body, probably for this very reason) are largely canceled out by the rearward movement of the center of gravity.
Now, look at the ring around the front of the rocket. THIS IS ANOTHER SET OF FINS. Yeah, it's a "ring fin," similar to the fin cans and rings seen at the rear of some air-dropped bomb designs. Why doesn't this ring fin make New Shepard fly backwards, or at least become unstable, on the way up? Well, it's because the top opening of the ring fin is plugged by the base of the passenger capsule on the way up. Air flows around the outside of the fin rather than through it, effectively canceling it out during descent. Once the capsule separates, air is free to flow through the lower opening of the ring, and suddenly New Shepard has a big new fin located as far as possible above the CG, where it will do the most good.
This gives New Shepard aerodynamic stability causing it to tend to fall tail first.
Note in the landing photo, there are also some additional aerodynamic surfaces: Eight rectangular flaps that deploy from the outside of the ring fin. I don't know if these act as drag brakes, add to descending stability, act as aerodynamic controls like F9s grid fins, or some combination of the previous. I don't see them moving in the video, so I'm guessing they're again just passive controls that deploy and don't move thereafter, but I can't be sure.
Anyway, there's a lot of very clever aerodynamic trickery here, and if I'm reading things right, it's all done very simply with a minimum of control or deployment mechanics. #space #blueorigin #newshepard #spacex #falcon9