Passive Thermal Control and Coning

[Peter B]

I've just been reading (again) Murray and Cox's "Apollo - The Race to the Moon". In one of the chapters dealing with Apollo 13 they talk about the problem of restarting PTC after the "PC+2" burn. In particular, they describe how the first attempt ended in failure, with the spacecraft coning.

I have two ideas in my head about what coning involved, and I'm not sure which one is right. A: the spacecraft rotated around an axis, but that axis wasn't the spacecraft's longitudinal axis; B: the spacecraft's axis of rotation was the same as its longitudinal axis, but the axis of rotation was itself precessing. Or was it C: something else?

Can anyone help me, or (even better) point to some graphics to illustrate what was happening, please?

Thank you.

[smartcooky]

I've just been reading (again) Murray and Cox's "Apollo - The Race to the Moon". In one of the chapters dealing with Apollo 13 they talk about the problem of restarting PTC after the "PC+2" burn. In particular, they describe how the first attempt ended in failure, with the spacecraft coning.

I have two ideas in my head about what coning involved, and I'm not sure which one is right. A: the spacecraft rotated around an axis, but that axis wasn't the spacecraft's longitudinal axis; B: the spacecraft's axis of rotation was the same as its longitudinal axis, but the axis of rotation was itself precessing. Or was it C: something else?

Can anyone help me, or (even better) point to some graphics to illustrate what was happening, please?

Thank you.

AIUI coning is the precession of the axis of the spacecraft around its direction of motion while it spins on it own axis. I have always visualised it as a slow version of what happens when a spinning top slows down.



This is my opinion, but there are others on here who have far more expertise than me, and may offer a better explanation/description.

[ka9q]

That's basically it.

If you start an object spinning about some axis, it will naturally tend to convert that spin to the axis with the greatest moment of inertia as that's the lowest energy state it can reach while still conserving its original angular momentum.

The longitudinal axis of the LM/CSM stack had a much lower moment of inertia than the other two axes so the longitudinal spin desired for PTC wasn't stable. It always wanted to go into an end-over-end tumble. If it were very carefully spun up along the longitudinal axis it would stay there for a while, but eventually various energy dissipation mechanisms (mainly propellant sloshing) would cause it to precess. If left uncorrected, it would eventually convert to a flat spin.

Astronaut Don Petit did some wonderful videos from the ISS to demonstrate the physics of rotating bodies. Some, like a camera lens, were rigid so they tended to keep spinning in whatever orientation they were started. But others, like a partially full water bottle (a good model for the Apollo service module), when started in a longitudinal roll, very quickly converted into a flat end-over-end spin.

 

[Chew]

Pettit did a whole bunch of fascinating experiments. I recommend watching all of his videos.

Germane to this discussion is his spinning raw versus hard boiled simulated eggs:
http://www.youtube.com/watch?feature=player_detailpage&v=2w_X0E0pxu8#t=414s

[Nowhere Man]

If you start an object spinning about some axis, it will naturally tend to convert that spin to the axis with the greatest moment of inertia as that's the lowest energy state it can reach while still conserving its original angular momentum.

The mission experts ran into this with Explorer 1.  It was supposed to spin around its long axis, but started precessing.  Surprised the heck out of them and prompted some more research into rigid body dynamics.

Fred

[Noldi400]

Is this a variation of the phenomenon called "inertial coupling" that kept trying to kill the early super- and hyper-sonic test pilots?  (And all too often succeeding.)

Or as Scott Crossfield reportedly described it, "The damn thing came uncorked on me.".

[ka9q]

Surprised the heck out of them and prompted some more research into rigid body dynamics.

If the body were absolutely rigid, it would have continued to spin just fine. You need some sort of energy dissipation mechanism to convert spin to the axis with the greatest moment of inertia, and on Explorer 1 the antennas probably provided it.

That wasn't the last time that NASA "rocket scientists" forgot some basic physics. It also happened during the early and nearly disastrous Gemini EVAs.

[Glom]

Also, Gene Kranz recounts Gemini 4 when the spacecraft attempted to turn around and rendezvous with its booster. Thrust towards it right? Get a bit of relative velocity towards it? Well no, because thrusting toward the booster was causing the orbital speed to decrease, lowering the orbit, causing an increase of orbital speed thus increasing velocity away from the booster.

[cjameshuff]

Also, Gene Kranz recounts Gemini 4 when the spacecraft attempted to turn around and rendezvous with its booster. Thrust towards it right? Get a bit of relative velocity towards it? Well no, because thrusting toward the booster was causing the orbital speed to decrease, lowering the orbit, causing an increase of orbital speed thus increasing velocity away from the booster.

That was the first thing that came to mind when I read ka9q's remark. I don't know if the pilots didn't want to be told how to fly or if somehow nobody thought about the differences from moving around down on Earth, but it seems like they really had a lot of trouble that should have been easily avoidable.

[ka9q]

Yes, this was one of the stories that made me smile a bit knowing the history of the conflict between the intuitive, seat-of-the-pants pilots on the one side and the methodical, theory-based engineers on the other.

Being written for a popular audience, The Right Stuff naturally made fun of the engineers, so being an engineer myself it was good to see that the pilots also got egg on their faces from time to time.

[ka9q]

Here's why non-rigid rotating objects in space tend to precess until they're spinning around their preferred axes.

Angular momentum is I*ω, where I is the moment of inertia and ω is the rotation rate. It is always conserved, so if I increases, ω must decrease and vice versa.

The kinetic energy in a rotating object is 1/2 * I * ω², which does not increase linearly with angular velocity.

When a rotating object flexes inelastically, it turns some of its kinetic energy into heat. That heat radiates away to space, never to return, and that's why every system tends to seek its lowest energy state.

To decrease energy while also conserving angular momentum, the spinning object must decrease 1/2 * I * ω² while keeping I*ω constant, and the only way it can do that is to increase I while decreasing ω by the same ratio. So if it's not already spinning around the axis with the largest I, it will seek to do so.

Once it is spinning around the axis with the largest moment of inertia, it has reached its lowest energy state and it will continue to spin stably about that axis.

[smartcooky]

Is this a variation of the phenomenon called "inertial coupling" that kept trying to kill the early super- and hyper-sonic test pilots?  (And all too often succeeding.)

Or as Scott Crossfield reportedly described it, "The damn thing came uncorked on me.".

Not the same IMO

Inertia coupling takes place when aerodynamic stability is lost through being overcome by the inertia of the fuselage. The aircraft ends up uncontrollably rotating in roll, pitch and yaw, and with no lift being generated as a result, it plummets to the ground.

It happened a lot during development of aircraft like the F-104 Starfighter; the American aircraft design industry's proof to the world that, given a big enough engine, even a brick will fly. This aircraft wasn't nicknamed "the widowmaker" for nothing. The addition of the "stabilator" the horizontal surface on the tail, helped to reduce the susceptibility to inertial coupling.

Chuck Yeager had something similar happen in an NF-104 (an F-104 with a rocket over the tailpipe), and the incident was dramatized well in this scene from the movie "The Right Stuff"

I think it also happened to him in an X-1.

[Noldi400]

Is this a variation of the phenomenon called "inertial coupling" that kept trying to kill the early super- and hyper-sonic test pilots?  (And all too often succeeding.)

Or as Scott Crossfield reportedly described it, "The damn thing came uncorked on me.".

Not the same IMO

Inertia coupling takes place when aerodynamic stability is lost through being overcome by the inertia of the fuselage. The aircraft ends up uncontrollably rotating in roll, pitch and yaw, and with no lift being generated as a result, it plummets to the ground.

It happened a lot during development of aircraft like the F-104 Starfighter; the American aircraft design industry's proof to the world that, given a big enough engine, even a brick will fly. This aircraft wasn't nicknamed "the widowmaker" for nothing. The addition of the "stabilator" the horizontal surface on the tail, helped to reduce the susceptibility to inertial coupling.

OK, that makes sense. After I thought about it, the coupling phenomenon is an aerodynamic problem, which (duh!) naturally enough requires atmosphere.

Chuck Yeager had something similar happen in an NF-104 (an F-104 with a rocket over the tailpipe), and the incident was dramatized well in this scene from the movie "The Right Stuff"

I think that was more of a problem with not being high enough to maneuver with his RCS and reentering flat instead of nose first, which meant he was unable to get his engine restarted which in turn left him with no hydraulic pressure. Control surfaces stuck in a full climb position make it a tad difficult to dead stick.

I think it also happened to him in an X-1.

IIRC, it was the X-1A and he was the first pilot to experience it, after setting a new speed record of about Mach 2.5 (?I think).  One of the other top test pilots - Crossfield maybe - supposedly later said "He shoulda been dead. Hell, anybody but Yeager would have been dead!".