New poll regarding ARCA stabilization method

Why exactly does any unguided rocket fly straight? What makes an arca rocket worse than a regular unguided rocket? An estes rocket has the same problem as a "pendulum" rocket - when the angle of the rocket changes, the rocket wants to keep going in the new direction. Fins should only exacerbate this. So what keeps a regular rocket straight?

I have just typed "arca fallacy" and Google found somebody back in 2006 who, talking about Stabilo 1, says:

meiza: 10/17/2006 09:48 PM »

That's a fallacy. A rigid tractor is no more stable than a pusher. When the rocket turns, the rigidly attached engine turns too and always thrusts through the center of the rocket. It is neutrally stable or however it's said.

Balancing a standing stick on a finger is different, since the force is always upwards regards to gravity: when the stick tilts, the force points upwards, not along the stick, tilting the stick even more. It is unstable. If you hold the stick from the top, it's the opposite of course and you get a pendulum.
http://forum.nasaspaceflight.com/index.php?topic=4457.0

So, these guys from ARCA have been told for quite some time to take care of fallacies, unfortunately to no avail.

Not exactely!

When you launch an ordinary rocket using a launch rod, you oblige its "Center of Gravity (CG)" to follow the direction in which the stick points to.
Random atmospheric perturbations can make the symmetry axis of the rocket shift from its initial direction (same as velocity vector) with a certain angle but these perturbations do not change the direction of the speed vector. The CG of the rocket continues to follow the initial direction of the launch platform.
A couple of forces appears which make the symmetry axis of the rocket align again to the speed vector. See the explanations from this link:
http://exploration.grc.nasa.gov/educati ... tstab.html

It may seem incredible but if a well built rocket (CG ahead "Center of Pressure (CP)") traveling vertically at great speed is forced by a thruster in the nose to rotate 30 degrees. Strong aerodynamic forces in conjunction with gravity will align again the rocket with respect to the vertical axis.

A rocket without fins will have most likely its CP ahead of CG, a case that make it unstable. Also a rocket flaying in vacuum will not have a CP at all which make it absolutely unstable to any stray forces.

I'm not sure if your questions were meant to be rhetorical, but I'll answer them anyway. An Estes rocket is designed to fly in the atmosphere, and not in space. It is aerodynamically stable. The fins keep the rocket pointed toward the direction of travel. If the rocket thrust is slightly off-center it will cause a slight tilt in the rocket, but the fins will keep the rocket pointed in the correct general direction.

The upper stages of Helen are intended to operate above the atmosphere. At these altitudes there are no correcting forces due to air. An Estes rocket would be unstable. A pendulum rocket would be unstable also. Any off-center thrust would cause an Estes rocket or a pendulum rocket to spin around. In the case of the pendulum rocket it will spin in one direction, and the Estes rocket would spin in the opposite direction.

One way to remove the torque component is to put the motor at the center of mass of the rocket. I am guessing that the Helen rocket is designed this way. By adjusting the lengths of the tethers, the center of mass can be positioned at the end of the rocket nozzle. If the rocket remains in a straight line the torque would be cancelled out. However, with the increased aerodynamic drag and the exhaust hitting the lower stages, this is not a very efficient method. There is also a lot of uncertainty about the effects of the exhaust hitting the tethered stages. At the very least, I believe there will be a slow rotation of the rocket (i.e., it is unstable).

No, those were not rhetorical questions. I don't know that much about the physics of rocketry. The rocket is being launched from high up - that is why arca is ok with using a non aerodynamic shape. And the rocket exhaust isn't pointed at the rocket, but is angled away.

The first stage is pointed out so that the rocket doesn't hit the balloon. However, the animation shows the rocket turning straight up after clearing the balloon, and the exhaust is aimed directly at the lower stages.

[Edit} The rocket is launched at an altitude of 14,000 meters, where the atmospheric pressure is about 1/6 of the pressure at sea level. This will still be a significant aerodynamic factor at this altitude.

Can we talk about Center of Mass, Gravity of an articulated body?
From what I know CG apply only to things that are or can be approximated by "solid rigids".
It looks like a body like this: "two metal bars joined together at one end with a spherical articulation" has a variable center of mass or does not have such a center.

This is just a question. The intuition can mislead me.

Yes, even objects that aren't physically attached have a center of mass as a group. As an example, the moon orbits around the center of mass of the earth/moon system, and not around the center of the earth. The center of mass of the earth/moon system is located between the center fo the earth and the surface in line with the moon.

Centre of gravity is a simplification though, one that works perfectly for rigid objects, but becomes less useful the less rigid the object is. Imagine a tennis ball and a piece of rope, both hanging in zero G. You push away the tennis ball. A simulation based on centre of gravity will work fine to predict what will happen. Now push at some point on the rope. You could compute the centre of gravity for the rope in its original configuration, but that won't do you much good if you want to predict how it will move when pushed. You need a more complex model for that.

Dear chandrayaan, do you have a twin brother, named Spalsh? Have a look: http://www.googlelunarxprize.org/forum- ... php?t=1816 (second post)
How would it be if you and your twin brother would start building something instead of analyzing what others already built?

Could someone from ARCA confirm that the Cg must be located at the end of the motor nozzle? This would be an interesting experiment to try. I am thinking about modifying my original Quad Pod pendulum rocket by moving some of the mass above the motor. If that works I might try a tethered rocket also, but I could have trouble getting it past the launch safety officer.

What about CG?
Any system composed of material points can have an instantaneous CM but this does not mean it is a solid rigid.

A system made of two metal bars, as the one I described, does not seem to posses a CG (a point around which the system always rotates no matter where you apply the perturbation force).
In the case of solid rigid bodies such a point exists and is unique.

1. Dave, theoretically it is quite easy to put the Center of Gravity of the (straight) assembly at the aft of the first stage, but remember that stage is consuming fuel and its CG moves significantly, since most of the weight shall be fuel.
2. That aft position of CG provide for very short moment arm that would "stabilize" the upper stage.
In fact, the rotation of the stage will never be around its CG, but around of a momentary centre of gyration which is situated away from CG or from the point of connection with the tether. This is a complex motion of the solid with the "fixed" point, but that point has also a complex motion. Reducing to one plane, the stage would have a combination of two plane-parallel motions.
3. I designed, made and flown many rockets, including "tractor" types but in all cases it was a stabilizing moment that kept a predictable path. At no moment I would design a configuration such as Helen's (hopefully no one needed...). For low acceleration rockets, only the inverted pendulum + gimbaled motor(s) method worked.
4. Lack of information is driving people think away from the real situation that may occur at the launch this October, next May or whenever the ideal conditions will be met. Maybe ARCA team loves this discussion...
5. Stabilo, in original configuration may be stable, if traction provided by those 4 nozzles is modified by throttling or canting. Of course doing this will drastically impede performance (anyway very low due to canting) and implies an active stabilization device (gyros or optical/magnetometer) but again we cannot say for sure what's inside of that structure.

Matricon,

I'm just trying to understand how ARCA's stabilization method works. You make a good point that the mass of the active stage drops as the propellant is consumed. One way to maintain the Cg at a constant point would be to shorten the tether during the motor burn. I wonder if ARCA has considered that.

Dave

See point 4. above
Giving the mass ratio they indicated and the need of a tether much longer than the rockets, the situation you suggested is impossible in practice. CG of assembly is somewhere well under the aft of the upper stage, at least at the beginning. Making the tether shorter or longer means very complicate mechanisms and doesn't help. That 4-point suspension together with the supposition that the lower stages stay approximately vertically is everything they have in hand. I bet they did not made any test flight on scaled model.