The rocket systems that we have discussed are fundamentally simple in concept, but as we have already said at the beginning of this chapter, it is the engineering implementation of these simple concepts that represent the major challenges for launch vehicle designers and manufacturers. The SSMEs, with a specific impulse of around 450 seconds, are considered to be the best you can do currently for a chemical propulsion system. They are designed to be reusable, which is extraordinary when you consider the mechanical and thermal stresses involved in their 8 minutes of operation from ground to orbit.
The shuttle has three liquid-fueled SSMEs, and the description of the attributes of just one of these fills me with admiration for the engineers who have transformed the concept into reality! The combustion chamber operates routinely at a temperature of around 3300°C, which is approximately twice the melting point of steel. To get the high exhaust velocity, the combustion chamber pressure is equivalent to about 200 times atmospheric pressure. With this magnitude of pressure, the propellant feed system has to be substantial to be able to push the fuel and oxidizer into the chamber against that pressure. The turbopumps that perform this function rotate at about 37,000 rpm to provide chamber inlet pressures of 305 atmospheres for the liquid oxygen and 420 atmospheres for the liquid hydrogen, with a total fuel flow rate of 470 kg/sec—nearly half a metric tonne a second! The resulting thrust level is around 2 MN, with a nozzle exhaust velocity of roughly 4500 m/sec (14,800 feet/sec).
At the moment of takeoff, the space shuttles engines are throwing an awful lot of hot gas down into the flame trench of the launch pad, about one and a half metric tonnes per second of exhaust gases, traveling at about 4500
m/sec, from the three SSMEs, and about 8 metric tonnes per second at a speed of 2500 m/sec (8200 feet/sec) from the two solid propellant boosters— a little bit more of a kick than our 50-g high-velocity bullet traveling at 1500 m/sec! There is clearly a lot of destructive power here that needs to be managed to prevent damaging the launch pad. To deal with this, what might be called the "space shuttle swimming pool" comes to the rescue. In a recent IMAX big screen 3D film of the International Space Station, one spectacular sequence showed the launch of a space shuttle at close quarters. The controlled power of the vehicle was overwhelming to watch! But one striking thing that came over clearly was the cascade of water that is released into the flame trench from the "swimming pool" just prior to ignition of the SSMEs. The energy in the high-speed jets of hot gas from the engines is consumed in the process of converting all that water into steam rather than causing significant damage to the launch pad.
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