@Greg: Okay, I've got it now. Yes, I think you are correct about not being to impart a spin about the longitudinal axis of the rocket. Maybe there are small hydrazine type jets that can impart enough spin to stabilize it about that z axis, especially since the rocket's body is symmetrical about the z axis.
Yaw would be motion about the z axis or "normal axis" i.e. the axis of rotation would be through your head to your feet. I see no way to impart a spin with a single gimbaled motor about this axis, unless it uses a vane to react with the thrust.
I'm not sure, but I think you might mean roll. In any case, it's just one more degree of freedom. Try to imagine that you are balancing the broom stick which is constrained to move in just one plane; e.g., it might be contained by two strips of metal such that it can only "yaw" as you are facing it. The broom head is the "nose" and the whole broom stick swings left and right like a pendulum.
Now, remove the metal strips so that you are trying to balance the stick which can now "pitch" forward and backwards as well as "yaw" left and right. You have just added one more degree of freedom.
Now, imagine that the broom head is a pair of wings, but now in addition to trying to control the pitch and yaw motions, you are also trying to keep the broom head from "rolling". In this case, it is not gravity that is trying to destabilize the broom, but it could be wind turbulence that is trying to "roll" the broom about the long axis of the stick. That is just yet another degree of freedom.
This is exactly what a flight control system tries to do. The rocket may or may not have to control its rolling motion. Some satellites use the roll motion as a stabilizing system, in the manner of a gyroscope.
@Greg: Yes. It's exactly as you say. It is much like your trying to balance a broom stick on the tip of your finger. You try to move your finger in X-Y plane to keep it balanced. There is nothing preventing you from also moving your finger up (z direction) at the same time as you balance the stick and keeping it pointed in the desired direction.
Is that sort of clear?
How exactly do these platfroms "fly" ? Can a single rocket engine gimbal to provide thrust about the lateral and longitudinal axis, yaw and still remain stable while asceding/decending?
Adam,
You are correct. I left out a big chunk in the middle, which you filled in. A finned rocket (or missile) gets its
static stability only if it is flying through air (or some other fluid). Fins would be useless in space.
The center of pressure does have to be, typically, a body diameter of more (rule of thumb), behind the center of gravity, in order to keep the rocket stable, just as you said. Therefore, in order for it to return to the ground safely, a control system would have to be employed. The gimballed rocket motor would be in the control loop,
just as you stated. So, that, in a roundabout way, is an answer to Jack Crossfire's need to explain why a
powered landing is required. Thanks for your elucidating input.
It is indeed remarkable the way the gimballed motor is used. I've seen, on Masten Space Systems; website,
a movie simulation of one of their rockets going into space and then returning to the ground by flying
backwards to a controlled landing, ala the lunar lander. Great stuff, these low-budget control systems.
My understanding is that orbital rockets are inherently unstable (provides them the ability to steer, without the negative effects of things like wings and big fins). From here on out, I am making this up, but might be right- seems like there would be a minimum of 3 vectors acting on a rocket.
+ Gravity Vector acting on the center of mass
+ Thrust vector acting at the rear/bottom of the craft
+ Aerodynamic vector acting at the center of pressure
As the rocket is taking off and landing, there is no aerodynamic affect, so it is purely the Gravity and Thrust vector acting on the device which has a similar stability problem to a segway but without any DoF constrained. Which is a nice way of saying that it can only achieve stability through a sophisticated control system
As it gets flying, and is flying straight it is possible that the center of thrust is not far behind the center of pressure (and the effect of gravity gets small) making the rocket sorta stable and the fins can compensate if the rocket decides to tumble. The trick with this rocket is that there are no fins, and the cross-section is very fat making it unstable at any speed.
One thing to note is that many rockets use gimbal rocket motors, just not to such cool effect
Jack,
So, Armadillo Aerospace is involved in the Northrop Grumman Lunar Lander Challenge. They got 2nd place in
2009 to Masten Space System's attempt for Level 2 of the Challenge.
It also occurs to me that a rocket is normally statically stable if it is moving forward. So, in returning to earth it
would have to be pointed in a nose-down attitude in order for it to be stable on its return to earth. A soft
landing with this attitude would be impossible without an active element. The gimballed rocket motor is the
only active element that can be used for a stable descent back to the ground.in an upright attitude.
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Now, remove the metal strips so that you are trying to balance the stick which can now "pitch" forward and backwards as well as "yaw" left and right. You have just added one more degree of freedom.
Now, imagine that the broom head is a pair of wings, but now in addition to trying to control the pitch and yaw motions, you are also trying to keep the broom head from "rolling". In this case, it is not gravity that is trying to destabilize the broom, but it could be wind turbulence that is trying to "roll" the broom about the long axis of the stick. That is just yet another degree of freedom.
This is exactly what a flight control system tries to do. The rocket may or may not have to control its rolling motion. Some satellites use the roll motion as a stabilizing system, in the manner of a gyroscope.
Is that sort of clear?
You are correct. I left out a big chunk in the middle, which you filled in. A finned rocket (or missile) gets its
static stability only if it is flying through air (or some other fluid). Fins would be useless in space.
The center of pressure does have to be, typically, a body diameter of more (rule of thumb), behind the center of gravity, in order to keep the rocket stable, just as you said. Therefore, in order for it to return to the ground safely, a control system would have to be employed. The gimballed rocket motor would be in the control loop,
just as you stated. So, that, in a roundabout way, is an answer to Jack Crossfire's need to explain why a
powered landing is required. Thanks for your elucidating input.
It is indeed remarkable the way the gimballed motor is used. I've seen, on Masten Space Systems; website,
a movie simulation of one of their rockets going into space and then returning to the ground by flying
backwards to a controlled landing, ala the lunar lander. Great stuff, these low-budget control systems.
+ Gravity Vector acting on the center of mass
+ Thrust vector acting at the rear/bottom of the craft
+ Aerodynamic vector acting at the center of pressure
As the rocket is taking off and landing, there is no aerodynamic affect, so it is purely the Gravity and Thrust vector acting on the device which has a similar stability problem to a segway but without any DoF constrained. Which is a nice way of saying that it can only achieve stability through a sophisticated control system
As it gets flying, and is flying straight it is possible that the center of thrust is not far behind the center of pressure (and the effect of gravity gets small) making the rocket sorta stable and the fins can compensate if the rocket decides to tumble. The trick with this rocket is that there are no fins, and the cross-section is very fat making it unstable at any speed.
One thing to note is that many rockets use gimbal rocket motors, just not to such cool effect
So, Armadillo Aerospace is involved in the Northrop Grumman Lunar Lander Challenge. They got 2nd place in
2009 to Masten Space System's attempt for Level 2 of the Challenge.
It also occurs to me that a rocket is normally statically stable if it is moving forward. So, in returning to earth it
would have to be pointed in a nose-down attitude in order for it to be stable on its return to earth. A soft
landing with this attitude would be impossible without an active element. The gimballed rocket motor is the
only active element that can be used for a stable descent back to the ground.in an upright attitude.
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