How do rockets work?

4 03 2013

As science presenters we are always being asked to take complicated concepts and simplify them for the audiences we are working with. The process of simplifying concepts is not an easy one. There is a constant balance to be struck between ‘dumbing down’ and ‘over-complicating’ whilst trying remaining ‘accessible’ but ‘correct’. Sometimes it can be impossible.

As an example of the perils of simplification I’ll ask a question. How do rockets work? Most people would give you the following answer:

Rocket engines work by action and reaction. Rocket engines push rockets forward simply by throwing their exhaust backwards extremely fast. [Wikipedia: Rocket]

So is that the answer? Is it as simple as referencing Newton’s Third Law and being done with it? Well to make it a bit more accessible most people would then trot out the classic balloon/basketball ‘real-world’ comparison:

When you let the air out of a balloon, the balloon doesn’t just sit there – it flies around the room. The action is the air rushing out of the balloon, and the reaction is the balloon being forced in the opposite direction. Second, imagine you are standing on a skateboard and you throw a football as hard as you can to your friend. You won’t just sit there – you will roll a bit in the opposite direction of your throw. The action is your throwing the football, and the reaction is your movement in the other direction. [Curious About Astronomy]

So far, so good. Up until a few months ago, I, like most people, would have left it at that however on a training job in the Middle East this subject came up*. Someone who I respect and defer to in their scientific knowledge and experience threw in a totally different explanation- the rocket is pushed along by an imbalance of pressure:

In a closed [combustion] chamber, the pressures are equal in each direction and no acceleration occurs. If an opening is provided in the bottom of the chamber then the pressure is no longer acting on the missing section. This opening permits the exhaust to escape. The remaining pressures give a resultant thrust on the side opposite the opening, and these pressures are what push the rocket along. [Wikipedia: Rocket]

Image

We have two competing explanations. Instead of the rocket working by throwing their exhaust gases backwards this explanation suggests they are being pushed by an imbalance of pressure. At face value these are totally at odds with each other. I was certainly very confused to hear this alternate explanation and it made me (reluctantly*) re-examine my idea of what was going on.

They are so different is one ‘right’ and one ‘wrong’? Well, no. One seems more correct than the other but neither actually do a very good job. The issue here lies with that complication-accessibility/dumb down-correct dilemma we face as science presenters.

The bald something goes one way so something goes the other explanation, whilst seeming accessible and simple is not actually very helpful. It explains what you see- a huge plume of flame going one way and the rocket going the other- but not what is actually happening. What is it that is actually making the rocket move? This explanation has been dumbed down too far.

The particles hitting against the inside of the combustion chamber explanation does take us further towards an idea of what is actually pushing the rocket along. For something to move there must be a force acting on it (Newton’s First Law) and it suggests where that force comes from (the particles of expanding gases hitting the inside of the chamber). It is more ‘correct’ but it is over simplified. It fails to take into consideration what we obviously see (the nozzle and the plume of exhaust) and half of the actual generated thrust**.

After trawling many popular science websites and reading as much as I could to the limits of my physics the best description I have found of what we see when a rocket takes off and what happens inside the rocket to make it move is this:

The rocket pushes on its exhaust. The exhaust pushes the rocket, too. The rocket pushes the exhaust backward. The exhaust makes the rocket move forward. [NASA- Rocketry]

For a general lay-audience of intelligent ten year olds and up this is accessible and correct, it is simple but not dumbed down. It covers Newton’s first law and deals with the fact that something must be pushing the rocket for it to move (The rocket pushes on its exhaust. The exhaust pushes the rocket, too.). And it covers Newton’s third law, the equal and opposite part (The rocket pushes the exhaust backward. The exhaust makes the rocket move forward.)

It doesn’t fall into the trap of our second explanation saying that the push only happens at a specific point opposite the opening of the combustion chamber because that isn’t correct; and it doesn’t fail to explain what is actually happening by saying ‘because’ the exhaust goes backward the rocket goes forward. It correctly identifies the equal and opposite forces involved.

The lesson here is we must always be careful that in our attempt to consider the needs of our audience we don’t lose sight of correct science. The ‘dumbing down’ and ‘over-complicating’, ‘accessible’ but ‘correct’ dilemma is not an easy one to overcome. We must not dumb down to the point that the science is no longer meaningful (eg: one thing goes one way so another thing goes the opposite) but then we must not over-simplify to the point that we are not actually including the obvious effects of what is happening (eg: it is only the particles opposite the opening that is pushing it along).

In a previous post I have spoken of Bruner who suggested anyone is capable of learning any material so long as the instruction is organized appropriately. Perhaps it is not possible for everything but that doesn’t mean we don’t have a responsibility, as science presenters, to try.

*The exercise of arriving at this explanation was far more interesting to me than the explanation itself. I had a devil of a time putting aside my firmly held belief in the first explanation. The process of beginning to see that there was a problem with my state of knowledge, to then be motivated to look further in an unbiased way, to then trying to include both explanations might seem easy and obvious from reading the above post but actually it was time-consuming and painful.

** ‘About half of the rocket engine’s thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle.’ [Wikipedia: Rocket engine]

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