Pendulum Munlander (Stock)

[Duban]

Your standard rocket is an inverse pendulum with the thrust coming from the bottom of the rocket and gravity pulling it down. This makes the design unstable at high thrust-weight ratios and prone to tipping over uncontrollably. Here is my experiment to make a design with more of the thrust at the top for stability.

I had to work around two main challenges.

1) The fuel of the craft drains from the top down. This means stages have to be jettisoned from the top down.

2) Dropped stages from above tend to crash into the stages below them.

This makes the vessel somewhat complicated to pilot. Stages need to be dropped 1 fuel tank after they stop working. Also, the third stage in needs to be dropped at about 60% thrust or the entire thing will explode. You also might need to activate RCSs about 3 stages in as the late stages are unstable, the balancing thrusters have been jettisoned.

The ship itself isn\'t remarkable, but it\'s an interesting design concept.

[Xivios]

It\'s also a flawed concept, in fact, Wikipedia has a very nice article on exactly this, the Pendulum Rocket Fallacy (and before anyone jumps down my throat for using wikipedia, note that the article is referenced).

That said, making it work definately qualifies as Kerbal to me 8).

[corinoco]

My \'This Way Up\' design explored the same challenges; and yes, I already knew about the Rocket Pendulum Fallacy - but clearly Jeb hasn\'t heard of it.

Very impressive to get all the way to the Mun with your design though; I was never able to keep the upper stages stable for long enough to achieve an accurate trans-Munar orbit.

[Duban]

It\'s also a flawed concept, in fact, Wikipedia has a very nice article on exactly this, the Pendulum Rocket Fallacy (and before anyone jumps down my throat for using wikipedia, note that the article is referenced).

That said, making it work definately qualifies as Kerbal to me 8).

Good point. When i think about it the torque a pendulum gives changes changes based on the angle of the string while a rocket engine\'s torque does not change. Still, it\'s an interesting design concept that has some advantages.

[corinoco]

Still, it\'s an interesting design concept that has some advantages.

I certainly agree that being able to look up into your own engines is an advantage in a rocket.

[dvangel]

It\'s also a flawed concept, in fact, Wikipedia has a very nice article on exactly this, the Pendulum Rocket Fallacy (and before anyone jumps down my throat for using wikipedia, note that the article is referenced).

That said, making it work definately qualifies as Kerbal to me 8).

The Problem with the Pendulum Rocket Fallacy is that it assumes the rocket is a rigid body, but this is not the case for Kerbal rocketry.

When the rocket is no longer a rigid body, the two cases become very different.

For engines on the top, gravity acts on the center of mass of the rocket, and the thrust acts in an opposite direction in the opposite direction of this force. (That is, gravity points down from the center, and thrust points up upwards of the center.) These forces are thus in a stable equilibrium.

When this is reversed, with gravity pointing downward above the thrust pointing upward, this causes an unstable equilibrium.

The rigidity of a rocket renders moot any unstable equilibrium that might exist in this case. But because the rocket is not rigid, any slight wobbles will necessarily cause a net torque on the rocket.

I performed a test for this. I made two 30m tall rockets, one with engines at the bottom, the other with engines at the top. for the engines at the top case the rocket tended to go straight up, but for the engines at the bottom case the rocket curled up and crashed into the ground.