Tuesday, December 30, 2014

Why Do They Go Up?

I'm sure some won't agree with this explanation. For me it explained the basics of thrust.

It's 1969, I'm in my eighth grade Science class.
I thought I knew everything about model rocketry. I'd been building them for a year and probably had a few dozen flights under my belt.

I was talking to my science teacher about my new hobby.
He asked: "What makes your rockets move?"
I tried to explain: "The thrust gasses push against the air outside the rocket. That friction makes the rocket go up and fly."

He said: "No, no exactly." He added something like: "The model moves because the pressure inside the engine isn't even."
What? I still didn't get it. (That catalog drawing with all the arrows inside the cutaway engine didn't make sense.)

He grabbed a piece of paper and pencil and drew some stick figures in a box.
The box has no bottom, their feet are on the floor under the box sides.
(I'll try to explain what he showed me.) 

"Here's Jim and Fred in a big cardboard box. They have a bet who will break through the box first. Jim is on the left, Fred is on the right. Both are pushing hard on opposite walls of the box.
First to break through their side wall wins."

"Fred (on the right) doesn't know that Jim (on the left) has installed a trick door on his side."

"With both pressing hard on their sides,
Jim hits the switch and the trick door on Fred's side pops open!
Which way does the box move?
With the door open and the back pressure released out that door, Jim's push to the left is what moves the box!"

Flip the box vertically, imagine it's a rocket engine.
"Think of the open door as the rocket nozzle. Fred's pushing effort is the escaping gas out the nozzle."
It's the upward push that gives the rocket it's movement.

This was one of those "AH HA!" moments.
My science teacher explained it in a way I could visualize and understand.


  1. Not quite right explanation. It is simple reaction -- as the rocket motor expels the gasses out the nozzle, the motor itself is pushed along by that same force it uses to push the gasses out. You maximize the thrust by maximizing the speed of the gasses being thrown out the back -- if it was mere internal pressure alone, then having a gaping hole would suffice (i.e. no need for nozzle that would otherwise "restrict" the flow). The nozzle is designed to increase the kinetic energy (i.e. the speed at which the gasses escape). Much as putting your thumb over the end of the garden hose makes the water stream out faster, the convergent part of the nozzle acts to increase the speed of the jet of gasses coming out. The divergent part acts to convert as much of the energy that would cause the gasses to simply expand radially into moving the gasses mostly straight out the back.

    1. Hi Naoto,
      It may not be the best explanation but it was the first time I understood "Opposite and equal reaction". I was just sharing a rocketry story when things "clicked" for me.

    2. The unfortunate part is your science teacher got it wrong -- of course it might not all be his fault, I'd seen a number of books that use this same (incorrect) explanation of how rockets work.

  2. One experiment that could be performed by throwing an object while wearing roller skates. The force you're applying to throw the object is also acting upon you, which causes you to be pushed along in the opposite direction. The harder you throw the object, the faster you'll be pushed along.

  3. A little less precarious experiment would be to use some sort of slingshot mechanism on a free-rolling cart that would eject a smallish object along the direction the cart would roll. As the smallish object is thrown out, the cart will be propelled in the opposite direction. The "thrust" experienced could be increasing the mass of the smallish object being ejected, or by increasing the speed at which the object is ejected.