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Rocket science isn’t rocket science, if you get my thrust

How hard can it be getting an object from A to B at great speed? KARI/HANDOUT

You know, it’s odd being a rocket scientist. The people you meet assume you’re not just smart, but some super-colossal paragon of intelligence with the wits of an atomic lovechild of Albert Einstein and Isaac Newton.

But, after a bit of study into the matter, it doesn’t take a rocket scientist to spot that the science just ain’t that tough.

A rocket works by throwing stuff in one direction (this “stuff” is called the exhaust), and using the recoil generated by expelling the exhaust to move in the opposite direction.

The physics is identical to a gun firing, and the shooter being pushed backwards by the recoil, as is required by Newton’s Third Law of Motion and the Law of Conservation of Momentum.  The amount of recoil generated, or in the case of a rocket, thrust, depends on how much fuel you burn in a given time, and how fast the exhaust leaves the rocket motor. The equation that describes this is called, funnily enough, the thrust equation.

Key to the equation is the fact that the force is opposite in direction to the direction of the exhaust flow, since every action must have an equal and opposite reaction.

The equation tells us a couple of important things about how rockets work.

1) The amount of thrust you generate depends on how quickly you’re burning fuel and throwing the exhaust out the back of the rocket.

You generate ten times as much thrust when you throw 10kg of exhaust out the back of the rocket motor every second than when you expel 1kg per second.

This is what we mean by “mass flow rate”, and it explains why chemically-fuelled rockets are so popular: they have a high mass flow rate, and so generate large amounts of thrust.

But, as they have low exhaust velocities, chemical rockets aren’t particularly efficient, which brings us to:

2) The faster the rocket exhaust leaves the rocket, the more efficient your motor is. For a given mass flow rate (say 1kg per second) you get 10 times as much benefit from an exhaust stream moving at 10km a second than you would from an exhaust stream moving at 1km a second.

Since the amount of fuel you can carry on a rocket is limited, this means a rocket motor that creates a fast-moving exhaust stream is very desirable, since faster exhausts mean more thrust for a given mass of fuel burnt.

More thrust means more acceleration, and more acceleration results in a greater change of velocity, otherwise known as Δv (“delta-vee”).

Engineers talk in terms of the change in velocity, or Δv, needed to reach a destination as a measure of the difficulty in getting the object to where they want it.

Since it takes about 10km/sec of Δv to get into orbit, you can see why a lot of fuel must be burnt just to get into space.

If you want to go anywhere afterwards, you have to carry along the fuel for that, too, which means extra fuel just to lift that fuel, and so on ad nauseam.

So, describing how to loft a football-sized object into space isn’t exactly rocket science, just a bit of classical mechanics. The engineering that goes into making one of those babies fly … now that’s hard!

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