I was reading this comment about the Pendulum rocket fallacy under this answer and looked at the picture (below) as I've seen it (or similar) in science books all my life, and I realized that I don't understand how that thing works. I do understand that the engine is near the top, and at least one of the propellant tanks is at the bottom, but does that large "cage" that Goddard is holding on to stay on the ground? The equivalent of the "rocket body" is only the two vertical rods - held at their midpoints? When launched, did it just slide freely through those those U-shaped holders?

And what is the stuff at the bottom - ground level? Is it just holding the rocket from tipping?

And finally, did Goddard really not have the "Use of basic Newtonian mechanics"? Or is it really that the point of this configuration was to first demonstrate and study successful launch and engine function under acceleration in flight, and once that was off the ground, the study of in-flight stability* could be done with next generation designs?

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above: Cropped details of a photo of Goddard and Rocket originally from NASA and described as "Dr. Robert H. Goddard and a liquid oxygen-gasoline rocket at Auburn, Massachusetts. 8 March 1926 (according to National Air and Space Museum)"


2 Answers 2


The A-frame is the "launch pad" and stays on the ground. The bit at the top is the engine; the two vertical rods (pipes) deliver gasoline and LOX from the fuel tanks at the bottom. The tank assembly has a conical shield on the top to deflect exhaust.

The Wikipedia article on Goddard describes the rocket rather well. It also strongly implies that the tractor layout was expected to produce stability:

This layout is no longer used, since the experiment showed that this was no more stable than placing the combustion chamber and nozzle at the base. By May, after a series of modifications to simplify the plumbing, the combustion chamber and nozzle were placed in the now classic position, at the lower end of the rocket.

It is generally more difficult to build a fuel-at-the-bottom tractor rocket (because fuel needs to move up) and less efficient (because exhaust needs to be diverted sideways, reducing thrust as the cosine of the deflection angle), so you wouldn't do it if you didn't expect an advantage.

The trajectory taken by this first rocket probably owed more to a burn-through of the engine nozzle than to the pendulum fallacy, however.


This image shows what's at the bottom:

Goddard rocket diagram

That's the line for filling the LOX tank.

  • 2
    $\begingroup$ This drawing is amazing! Now I can really begin to imagine what it must have been like - to think up something like this, to address all the technical problems of making it work, and then to actually make it work! $\endgroup$
    – uhoh
    Aug 26, 2016 at 16:25

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