Liftoff: How to launch a rocket

23 July 2019

Interview with 

Hugh Hunt, University of Cambridge


A rocket, with space shuttle attached, blasting off from Earth


The distance from Earth is not always the same – it varies because the lunar orbit is not circular, but elliptical. At its closest, it’s about 356 000 km away, and at its farthest: 407 000 km. But, either way, travelling to this rocky destination really is rocket science, as Izzie Clarke learned from Hugh Hunt from the University of Cambridge.

Hugh - We're here in the Engineering Department of Cambridge University which is where I work. And it's the fiftieth anniversary of the launch of the Saturn V rocket that took the Apollo 11 spacecraft up to the moon. Now that launch is going to happen - 50 years ago - in about three hours’ time.

Izzie - So it is… we are recording this on the 16th of July, so exactly 50 years ago. So tell us about the rocket. What type of rocket did the crew go up in?

Hugh - Well the rocket itself was the Saturn V, a huge great big tank of fuel. The fuel was basically kerosene; more or less the same, a bit like petrol, bit like diesel fuel, nothing particularly special but with liquid oxygen. And to this day the rockets SpaceX are using are exactly that, they have kerosene and liquid oxygen. It turns out to be a really efficient fuel. A million gallons, 4 million litres of this fuel; boy, it was like, that’s an explosion waiting to happen. That takes the rocket up to the upper edges of the atmosphere, at which point that stage falls off and falls to the ground. There's a couple more stages left which are hydrogen- and oxygen-fuelled stages. It's important to have different stages because you don't want to carry the weight of your fuel tanks all the way to the moon. And they did this 50 years ago! These engineers were just fantastic.

Izzie - And why is oxygen so important in this launch?

Hugh - If you burn a fuel in air, air is 80% nitrogen, and nitrogen is like a waste of... it's just added weight. If you really want to get fast, energetic combustion you need to have pure oxygen. Pure oxygen is what's needed for the fastest, rocket-type combustion. Now aircraft use air, they don't carry pure oxygen with them, and that's why aircraft just don't quite go so fast. So rocket-propelled machines - it's all because of the oxygen.

Izzie - And so what's going on in our rocket to make sure we get to whatever our destination might be?

Hugh - So the rocket is employing Newton's Third Law, which is that every action has an equal and opposite reaction.

Izzie - It's a bit like when a rifle is fired. The bullet rockets out of one end and the gun kicks back into your shoulder, just using the expanding gases caused by an explosion. Just not quite on the scale of a rocket.

Hugh - Now a rocket is throwing out the exhaust from burning the kerosene and the oxygen at a really high speed. The faster you propel this stuff out the more thrust you'll get. Lots of fuel going at high speed propels you up.

Izzie - Because I think a common misconception is that when a rocket goes up it's literally pushing against the ground.

Hugh - A rocket when it goes up doesn't push against the ground. Even in outer space you get thrust. When they were orbiting around the moon they were able to eject thrust with little rockets to accelerate the lander, and all sorts of things to manoeuvre the spacecraft.

Izzie - Enough theory. Time to launch some rockets. But we needed a crew for this and thankfully… we had some help from St Paul's Primary School in Cambridge. So what's the mission plan? We set up our own little launch pad in the courtyard. We placed our long thin pointed tube rocket, made from cardboard, on the pad; sitting it on top of a thin cylinder of some compressed air, just waiting to be shot skyward. A trusty bike pump would compress the air down and hold it in a chamber. All that was left to do was push an exciting-looking red button to unleash the air and launch the rocket.

Schoolchildren - Five! Four! Three! Two! One! Whoa!

Izzie - Alright! It's coming back down, it’s coming back down.

Hugh - Wow! That was high.

Izzie - That was really high!

Hugh - That was really high! That was twice the height of the building!

Izzie - Safe to say it went well. But what makes a rocket a good rocket, like our one?

Hugh - A good rocket has to have a propellant which is ejected at the highest possible speed, and it's got to be quite heavy as well. But it's a tradeoff. You don't want to carry really, really heavy stuff with you. So it's the mass flow rate times the speed that gives you the biggest thrust.


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