Mastodawn

"To achieve a stable flight without the need for fins, the rocket's heavy motor was located at the top, fed by lines from liquid oxygen and gasoline fuel tanks at the bottom."

Umm, actually, no. That's not how that works. That's what he intended to have happen, and it sounds like common sense, but this is actually the classic #PendulumFallacy of #rocketry:

https://en.wikipedia.org/wiki/Rocket#Pendulum_rocket_fallacy

Date: 2026 March 28
URL: https://apod.nasa.gov/apod/ap260328.html
Title: Robert Goddard and Nell

(No disrespect intended to Mr. Goddard. He was an epic pioneer, and the very best of us learn by making mistakes and analyzing the outcome to learn from them.)

#NASA #Astronomy #PictureOfTheDay #KSP #KerbalSpaceProgram

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Mark W. Gabby-Li 🐌3d ago

@rl_dane What if you actually *made it a pendulum*? Had the engine on gimbals so that the heavier propellants would be pulled by gravity toward the earth? Not really necessary but an amusing idea.

Even if you gimbal it, it won't act like a pendulum, because the engine isn't actually a fulcrum.

Simple fins are far more effective than any attempt at mechanically-controlled gimballing.

P.S., the reason that the engine doesn't work like a pendulum (fulcrum relative to the Earth) is that there is nothing that constrains it to an earth-bound frame of reference. Eh, I'm explaining this poorly, because I'm not a physics guy.
Let's say you put wheels on the rocket and it takes off horizontally like a plane. What would force it to fly upright? Even if you attach the gimbal to a counterweight, the counterweight would "feel" that the base of the rocket was "down" due to thrust, moreso than earth's gravity.

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Mark W. Gabby-Li 🐌3d ago

@rl_dane I bet it would be more unstable. My point is that it is a fun thought experiment, not that it would actually work better. I guess one could try it in Kerbel?

Go for it! KSP is pretty accurate, physics-wise.

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Nick 'The Viking'3d ago

@rl_dane

I have objections to those free body diagrams.

Will circle back later.

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Wasted Radiance 3d ago

@rl_dane

It says the description is written by an astronomer, so it's not that surprising that it miscommunicates an astronautical concept. Astronomy and astronautics are interdependent fields, but very different disciplines.

I still expect better from NASA, though.

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unit327 2d ago

@rl_dane Yes but also kinda no... not related to the thrust but drag does play into it.

Throw a dart forwards and the heavy, low drag bit at the front will keep it pointed into the wind. Throw it backwards instead and the lightweight, high drag bit will flip it around.

Rockets in ksp tend to be like a dart thrown backwards, because the engines are usually the heaviest part, especially once most of the fuel is gone. The pictured design with the engine at the top would actually help with that.

Fins usually offset that, but yes, I've done the "Rocket flip" many times in KSP 🙃

SSTOs are even harder, because you can't shed engines or fuel tanks to manage the CoG/CoM.

So, yes, having the CoM ahead of the engine does give it a bit more stability, but probably not as much as Mr. Goddard was initially hoping. 😁