Rocket vs Re-entry Capsule Aerodynamics

[Wheffle]

I don't study aeronautics, but I like to understand at least the basics of different subjects, just for peace of mind. Playing around with FAR, I've realized that while launching rockets, if the payload at the top of the rocket is dragging too much (not using fairings for blunt payloads), the rocket will be unstable and want to flip and go up butt-first, placing the higher drag at the back. However, I've also realized that single capsules will want to fall blunt-end first during re-entry, which seems to be the opposite.

In my mind, I can sort of visualize the air flow, and it makes sense, but it still bugs me that in one case, the draggiest end wants to go last, and in the other case, the draggiest end wants to go first. Could someone more knowledgeable than me explain the difference out?

[goldenpeach]

I'm not a specialist, but the cause of this might be the other force on your rocket(weight, center of pressure etc...)

[Idobox]

I think it has to do with the position of the center of mass.

With your rocket, your high drag thingie is in front of the centre of mass, and tries to get behind it.

With the capsule, I think the centre of mass is very low, so the blunt end doesn't have a lot of fulcrum, and the rest still has significant drag and more fulcrum, acting like a tail fin.

[RocketTurtle]

I think it has to do with the position of the center of mass.

With your rocket, your high drag thingie is in front of the centre of mass, and tries to get behind it.

With the capsule, I think the centre of mass is very low, so the blunt end doesn't have a lot of fulcrum, and the rest still has significant drag and more fulcrum, acting like a tail fin.

Yep. This really defeats the 10 character limit.

[Starwhip]

That's exactly how that works. Which is why you have small payloads with aerodynamic designs.

[K^2]

I think it has to do with the position of the center of mass.

That's a factor, but it's not quite that simple. If you naively put center of drag at the center of the heat shield, then no matter how low center of mass is, center of drag is always in front of it, so the system will be unstable.

What really happens is that center of drag shifts around as the capsule tilts, providing stability. Reasons why it works are way beyond the scope of this topic. But a simple analogy is the way a boat floats. If you try to tilt the boat to the right, center of buoyancy also shifts to the right, providing stability. Up to a point, of course. If you rock the boat too much, it will capsize. Same thing can happen to re-entering capsule.

[PakledHostage]

What really happens is that center of drag shifts around as the capsule tilts, providing stability.

IIRC, there are two regions of stability for a re-entering American style capsule. The larger region surrounds the blunt end first orientation, but the capsule would also be aerodynamically stable re-entering pointy end first. This is the less well known reason for why the capsule's centre of mass is offset from the axis of symmetry. The offset centre of mass eliminates the dangerous second region of stability.

Edited December 5, 2013 by PakledHostage

[Wheffle]

I didn't think about the center of mass, I guess in KSP with a single part it just never occurred to me that it could be off-center. That makes a lot of sense. A little bit of a slow moment for me. Awesome replies, thanks all.

One of the reasons I love FAR (and KSP in general) is because of this... I love the extra challenge of figuring out the more realistic physics.

[K^2]

IIRC, there are two regions of stability for a re-entering American style capsule. The larger region surrounds the blunt end first orientation, but the capsule would also be aerodynamically stable re-entering pointy end first. This is the less well known reason for why the capsule's centre of mass is offset from the axis of symmetry. The offset centre of mass eliminates the dangerous second region of stability.

I figured there might be a case of bi-stability, but I wasn't sure, so I didn't want to say anything. The problem, though, is that even if you eliminate that point-first stability, you can still have a dynamic instability. Id est, if you start with a bit of a tumble, it's likely that it would grow only worse. Offsetting CM wouldn't really change that. But I guess you can try to eliminate or reduce this effect by careful choice of the heat shield curvature and the aspect ratio of the capsule.

Is the other reason lift, by the way? Seems like a capsule with an offset CM is going to glide a little better, which should allow for a slightly gentler deceleration profile.

[PakledHostage]

Is the other reason lift, by the way? Seems like a capsule with an offset CM is going to glide a little better, which should allow for a slightly gentler deceleration profile.

Yes, that is the better known reason why the centre of mass was offset. In the case of the Apollo capsules, it allowed for adjustment to the re-entry trajectory depending on which way the vessel was rotated about its axis of symmetry.

[rdfox]

I doubt that the offset center of mass *completely* eliminates the secondary region of stability, but I don't doubt that it greatly reduces it. Of course, if you're coming in without a working RCS to help make sure you enter Blunt End First, you've got a whole world of OTHER problems to deal with...

Trivia note: The concept of blunt-body re-entry vehicles was discovered during research into how to deliver nuclear weapons by missile. Early experiments with reentry vehicles that were "intuitively" aerodynamic--i.e., pointy end first--were frustrating because they kept disintegrating due to heat buildup. However, one test had an accident that saw the RV come in blunt end first, and survive essentially unscathed. The surprised engineers did some analysis, and realized that coming in pointy-end first had lower drag... and thus let speed build up a lot more in the upper- and mid-atmosphere, so heat built up much quicker and higher and caused thermal failures of the casing that resulted in the vehicle breaking up. Coming in blunt-end first, with its high drag coefficient, allowed the vehicle to start to slow down much earlier, thereby greatly reducing peak thermal loading and making it more likely to survive. This is why so many early ballistic missiles had blunt noses--that was the blunt end of their warhead, positioned up front to simplify the job of having it come in that way, rather than requiring a turnaround maneuver in space.

Modern ballistic missile warheads, for the record, come in pointy-end first, but that's an adaptation made in the late 60s as anti-ballistic missile technology was being explored; they chose to switch to that configuration for two reasons. First, the much higher descent speed of the pointy RV made for a much more difficult intercept problem for an ABM, and second, coming in pointy-end first would allow the use of fins or vanes on the RV (apparently never deployed operationally--but then, we don't know that much about RV design for security reasons!) to allow terminal maneuvering, initially intended to try and dodge ABMs, but later considered to improve accuracy. They can do this because someone figured out that the depleted uranium used as a neutron reflector in the warhead, if shaped properly, would also make a dandy ablative/heatsink-type thermal protection system for the rest of the warhead, even coming in pointy-end first...

[K^2]

The surprised engineers did some analysis, and realized that coming in pointy-end first had lower drag... and thus let speed build up a lot more in the upper- and mid-atmosphere, so heat built up much quicker and higher and caused thermal failures of the casing that resulted in the vehicle breaking up.

Speed doesn't build up. Gravity-induced speed boost for a ballistic warhead is minimal. On the other hand, the thing is already traveling at 5km/s+. So it's all about deceleration profile. Atmosphere gets thinner exponentially with altitude, so going into thick atmosphere with a bit more speed lets you reach much thicker atmosphere still carrying a lot of speed. Starting to decelerate early lets you avoid that.

and second, coming in pointy-end first would allow the use of fins or vanes on the RV (apparently never deployed operationally--but then, we don't know that much about RV design for security reasons!) to allow terminal maneuvering, initially intended to try and dodge ABMs, but later considered to improve accuracy.

Maneuvering warheads were deployed on ICBMs in USSR.

[Neil1993]

I don't know too much about re-entery, but I might be able to help with rocket ascent. A flying body has two centers that you need to consider; the cente of mass and the center of pressure. You already know what the center of mass is. The center pf pressure can be considered as the point where the total sum of a pressure field acts on a body.

In a rocket exhibiting positive stability (this is what you want) the center of gravity will be closer to the nose than the center of pressure. Why is this so? when a rocket is perturbed and at a non-zero angle of attack (the airstream isn't parallel to the longitudinal axis of the body) the body generates lift, as anything at a non-zero angle of attack would. This lift, which acts at the center of pressure, restores the rocket to a zero angle of attack. Here's a picture to make this more clear:

Side note: you can have unstable rockets that still fly. They just require a ridiculous amount of computer control to keep going straight.

Edited December 17, 2013 by Neil1993

[K^2]

Neil, as I pointed out in an earlier post, center of pressure for a descending capsule is bellow center of mass. So capsule stability is a far more interesting problem. If CoP was fixed relative to the capsule, it'd flip. Fortunately, CoP shifts as capsule tilts. And it shifts in such a way as to provide a region of dynamic stability.

[Neil1993]

Neil, as I pointed out in an earlier post, center of pressure for a descending capsule is bellow center of mass. So capsule stability is a far more interesting problem. If CoP was fixed relative to the capsule, it'd flip. Fortunately, CoP shifts as capsule tilts. And it shifts in such a way as to provide a region of dynamic stability.

I was addressing Wheffle's comment regarding launching rockets and I admit that I know little about reentry aerodynamics. Most of what I have studied are the dynamics of ascending rockets, specifically rockets using passive stabilization.

As well, while I imply the general assumption that the CoP remains stationary, this is never the case. An ascending rocket's CoP will change for different angles of attack and airspeeds. However, assuming that a rocket has a stationary center of pressure is probably good enough for KSP.

I will definitely look into reentry aerodynamics some more. It is not something I know enough about.

Edited December 17, 2013 by Neil1993

[K^2]

As well, while I imply the general assumption that the CoP remains stationary, this is never the case. An ascending rocket's CoP will change for different angles of attack and airspeeds. However, assuming that a rocket has a stationary center of pressure is probably good enough for KSP.

Yeah, shifting CoP for a rocket can be pretty much disregarded until you start talking about large displacement angles or precision control.

[Neil1993]

Yeah, shifting CoP for a rocket can be pretty much disregarded until you start talking about large displacement angles or precision control.

One case where it could matter, is with neutral stabiliy. In this case, the CoP and CG have the same location. This means that the rocket will shift between positive and negative stability depending on the flight conditions leading a very erratic and unpredictable flight path.

Note: As a general rule, a rocket of this kind will tend to fly towards the nearest crowd of bystanders. Should a crowd of bystanders be unavailable, it will fly towards the nearest easily damaged property.

[K^2]

Oh, I know. I've built my share of model rockets that vote a resolute "undecided!" on the question of stability. Anyone who thinks they fly in a completely random direction has never launched one.

[SpaceScout]

While going up you want to increase the speed as much as possible (keeping it in stable position). But during a reentry you want to hit the earth with with the least possible velocity possible and so its designed that way.

And so the draggiest end is made to go at the front during reenty.

[p1t1o]

34 minutes ago, SpaceScout said:

While going up you want to increase the speed as much as possible (keeping it in stable position). But during a reentry you want to hit the earth with with the least possible velocity possible and so its designed that way.

And so the draggiest end is made to go at the front during reenty.

Welcome to the forums SpaceScout!

This question was settled a couple of years ago, earning your post the title of "Necro"

Just FYI, take a read of the thread as the reason entry vehicles are often "blunt" is more complicated than one might think. Technically, the centre of drag is still behind the centre of mass for a stable blunt body.