The Most Common Mistake In Rockets

THE GRAVITY JETPACK:
Wow, this has blown up in the past day for no reason. I made this 2 years ago when Gravity only had about 6 videos and I hadn't heard of them yet. They're definitely the closest thing to flying like Iron man and I'm a big fan. I don't want to cut the Hacksmith part of this video because it's been well discussed in these comments, but I would like to update my points.

1. I don't like how harsh I was, I should comment on the Hacksmith's design without being rude. Unfortunately, it's too late to make changes to this video.
2. The gravity suit is a great example of how people are rigid enough to fly with thrust on their limbs. (2/3 of the thrust is on the pilot's arms in this suit)
3. The gravity jet pack is much more practical than my idea of boots.
- (I was concerned a jetpack would have to be bulky and have arms that extend away from you, but this is a very sleek design. Very Iron man)

Today I explain one of the oldest mistakes in rocketry: The Pendulum Rocket Fallacy.

James Bruton and Ivan Miranda's Rocket Collaboration:

The Hacksmith Videos:

Flyboard Air:
(This one was out before the hacksmith video, but didn't appear in my video)

Flyboard Water:

Skiing Video (Because why not?)

(0:00) - Introduction
(0:53) - What is the Pendulum Rocket Fallacy?
(2:07) - Why doesn't it work?
(4:13) - Real-world examples
(4:59) - The Hacksmith Videos
(6:59) - I didn't know about the Gravity suit yet
(7:55) - Outro

[ConHathy]

ON THE GRAVITY JETPACK:
Wow, this has blown up in the past day for no reason. I made this 2 years ago when Gravity only had about 6 videos and I hadn't heard of them yet. They're definitely the closest thing to flying like Iron man and I'm a big fan. I don't want to cut the Hacksmith part of this video because it's been well discussed in these comments, but I would like to update my points.

1. I don't like how harsh I was, I should comment on the Hacksmith's design without being rude. Unfortunately, it's too late to make changes to this video.
2. The gravity suit is a great example of how people are rigid enough to fly with thrust on their limbs. (2/3 of the thrust is on the pilot's arms in this suit)
3. The gravity jet pack is much more practical than my idea of boots.
- (I was concerned a jetpack would have to be bulky and have arms that extend away from you, but this is a very sleek design. Very Iron man)

[JoshuaZitoJZ]

This is the best explanation I've seen and it deserves more views

[hedgehog]

Wow it is frustrating to see the hacksmith just consistently ignore the advice of the people who are clearly more educated than him on this topic. Also this is a really good explanation and I think using the example of some people who got it wrong helps a lot in explaining it.

[HarshColby]

Came from Tom Stanton's Super Capacitor Rocket video, in Camp 1. Left in Camp 2. Thanks for the great explanation.

[akshaynayal]

I was watching this video "The Dumbest Mistakes In Space Exploration", and at number 3 they mention pendulum rocket fallacy . I Didn't understood this that time, so I searched "pendulum rocket fallacy" and watched 3 videos but they all are too much technical, means they literally talking in language of rocket science and in another video he is explaining with quadcopter and blackbox software and weird quadcopter's values graphs .

but then I found this video and it was just uploaded 2 days ago .

Great explanation, great example, great video and nice astronaut character

Thank you

subscribed

[steelbrg]

Regarding the iron man guy I'm not sure that its such a bad idea. As you said: if you can point the engines at the top to always be down the pendulum effect works. That's basically what he is doing with his hands while in flight. He wont have the engines locked in place in relation to his body. He is trying to hover and we have better fine motor control in our arms/hands than we have in our legs. I don't really see the problem.

[kelsiehogan]

This is so helpful for my class omg... finally... I’ve been looking for an explanation like this

[musaran]

I think the problem with rocket boots (vs board) is we have poor/weak feet "swivel" control, especially laterally.
In normal time they are more like passively leveling with the terrain.

[m.streicher]

Weird to see such a great video produced 2 years ago and yet you only have 1.93k subs... + me of course.

[FufuFang]

This is really good quality explanation

[demacherius]

Thank you for the video. I needet 3 Versions to get the point. Your drawing with the forcearrows made it klick.

[jasonycw]

I like the first half explaining why it won't work, but feels like a miss on the second part on explaining why how it works

[TH-wrdv]

actually I think that there is still point to put edf to top of "rocket" (human body). Yes pendulum rocket fallacy apply when motor is mounted stationary to solid piece. How ever when motor is top its thrust vector can always point up to get balanced vehicle. But when it is bottom you lost some of upwards trust to compensate reverse pendulum angle error because you need generate sideways acceleration to fix that error. Anyway at computer stabilized system it is so marginal problem it does not matter (if your pids are properly configured). But when you do it by hand angle errors can come more significant and then it starts matter more. Also it is more intuitive for human brain to keep thrust vector pointing up than do reverse pendulum balancing.

[ModernClassic]

The toy helicopter remains upright when upside down because of gyroscopic rigidity in space. That doesn't invalidate the other points, but the dynamics of propellers vs. turbines vs. rockets for propulsion are very different. (Ask a pilot about it.)

[dead_warrior_wae]

The only problem with rocket boots is that even though the pendulum rocket fallacy applies, because we have ankles and knees, a feedback loop of angle shift - knee/ankle bend - angle shift - knee/ankle bend would make it very hard for someone that doesn't have solid ankles and knees to be stable.


edit: also hips, strong hips

[JasperBlomme]

Hi, and thank you for the interesting video!! I'm a science teacher and at our school we're experienced in stratosphere balloon projects. This time we are planning to launch a model rocket at an alt. of 40 km. We were planning to use a pendulum rocket, but last night I could not sleep, thinking there was something wrong with the physics. So this morning I decided to do some research. Your movie explains it very well! But now our problem remains: How to stabilize our rocket in near vac (Cp irrelevant)? Gimballing the engine is out of our capacities I'm afraid.. Better to keep Cm close rather then far to location of thrust? Or doesn't it matter and should those just be aligned perfectly? Maybe you have other thoughts? Kind regards! Jasper from Belgium. :-)

[TheMarrt]

So basically, when you want to stabilize the rotation of an object, you need to have means of applying different rotational forces. If you can't, you can't stabilize it.
Parachutes and pendulums *do* apply differing rotational forces by their topology.

[petersmythe]

"it makes the design much more complicated"
Yes. Especially since with modern rockets, the turbopump would still need to be at the bottom or this would be an ullage machine. *VAPOR IN FEED LINES, ENGINE SHOTDOWN* every single time.

[arcanealchemist]

I have a question. I know this video is over a year old, but I'm having a hard time with this. I understand that having your thrust on top does little to nothing to help stabilize your rocket, the force diagrams support this.


However, doesn't putting all the weight at the bottom still help significantly? if I were to drop two sideways cylinders from my roof, one which had all its weight at the bottom, and one which had all its weight at the top, the one that's heavy at the bottom would "self level" while the other would fall "upside down" I guess this isnt really part of the pendulum fallacy?

[TnEighty]

There are some reasons I think the hover shoes thing isn't the best idea. The main one is that most people dont have the strength to squezze their legs together against their body weight. From your even half way splits position its extreamly difficult to slide your feet back together, also your ankles are not nearly as strong side to side as they are forwards and backwardsthough I suppose they could just be rocket ski boots basically so you dont roll your ankle also unless your feet are together or equally far apart you start to roll over