Heres an idea, lets use a highly combustable liquid propellant to cool down an extremely hot rocket engine. “Wait, what? You want to fight fire with soon-to-be fire? Preposterous! You have lost your mind David, I am leaving!”
That seems like a reasonable reaction to the first sentence, but just bear with me for a moment. This is the technique which Nasa used to launch 135 space shuttle missions [1]. Now you may be wondering why they used technique, whats the point? Well the “…temperatures in the main engine combustion chamber can reach as high as 3,315.6 degrees Celsius” [2], which is far higher than its melting point, and melting engines could cause disastrous consequences whilst trying to fly to space. Cleary there is a need to reduce this temperate somehow, however adding more weight to the shuttle in the form of cooling systems would be impractical, and somewhat counter productive.
The space shuttle needs to achieve an escape velocity (the speed in which it is travelling in order to escape the pull of earths gravity) of at least 25,000 mph [4], which requires a tremendous amount of fuel to achieve; the external tank of a space shuttle has a gross weight of 719,116kg, which is 1,991,603 litres (which is the equivalent of 336,454,162 teaspoons, not that you would use tea spoons to fill an external fuel tank…). All of this fuel is burnt in 8.5 minutes [3], in order to lift the 2000 tonnes [5] of mass which comprises the entire shuttle and its payload to a speed of around 17,5000 mph [6]. This doesn’t include the solid rocket busters which also play a vital role in achieving escape velocity, but more on that another time.
As you can see, the space shuttle is already a heavy machine which requires a lot of fuel in order to achieve lift off, adding more weight to the space shuttle is not going to work. More weight requires more fuel, which in itself is weight, you can see where this is going. But what if you had available 1,450,063 litres [3] of the worlds second coldest liquid [2], liquid hydrogen, at a temperature of -258.2 degrees Celsius [2]? Liquid hydrogen is one of the core fuels used to power the shuttle engines, and is already in the external tank ready to serve its purpose of providing thrust. It would be an awful shame to let it only fulfil one job, why not use it to cool down the engines before it is burnt? Multi purpose rocket fuel, that doesn’t add to the overall mass of the rocket, to cool down the engines? Jackpot.
But How?
During flight, some of the liquid hydrogen is pumped to the exhaust of the engine, as opposed to the combustion chamber, which is usually where you would expect it to go. Figure 1 shows a diagram of this process, whilst figure 2 shows an example of an engine with the pipes clearly visible. After the liquid hydrogen has reached the edge of the exhaust, it turns around and travels back down the length of the engine (as shown in figure 2), on the inside; finally heading towards the combustion chamber, where it can fulfil its primary purpose of proving thrust, and thus speed.
What this process achieves is to allow for heat transfer to take place between the very hot metal, and the very cold liquid hydrogen, which results in a colder engine. The liquid hydrogen has to travel down the length of of the engine exhaust in order for it to reach the combustion chamber; whilst it is on its journey it is going to pick up some heat from the engine, as the two temperates vary significantly, due to the law of thermal dynamics. This causes the engine to lose temperature, and the liquid hydrogen to raise in temperature. Shortly thereafter, however, the fuel is burnt and thus the rising temperate of the liquid hydrogen has little negative impact. The liquid hydrogen is consumed by the engines so fast, in fact, that they would “drain an average family swimming pool in under 25 seconds” [2]. I’m not sure what size constitutes an “average size family swimming pool”, but it sounds rather impressive non the less. In fact, this entire process of cooling an engine is rather impressive; Colour me impressed.
[1] Nasa (2015) The Shuttle [Online]
[2] Nasa (2009) Space Shuttle [Online]
[3] Nasa (2006) Space Shuttle – The External Tank [Online]
[4] Explain That Stuff (2015) – Space Rockets [Online]
[5] ESA (2015) – Tribute to the Space Shuttle [Online]
[6] Nasa(2012) – Exploring Space Through Math [PDF]
[Fig1] Nasa (2010) – The 21st Century Space Shuttle [Online]