The Efficiency Mismatch: Miles Per Gallon vs. Gallons Per Second in NASA Space Shuttles

There is a common misunderstanding when it comes to understanding the fuel efficiency of NASA space shuttles. While the concept of miles per gallon (mpg) is familiar and easy to grasp, applying it to the shuttle's operations is fundamentally flawed. Instead, we must explore the unique context and physics involved in rocket fuel consumption and efficiency.

Theoretical Fuel Consumption

During the launch phase, the space shuttle consumed an enormous amount of fuel. For every 68 miles it traveled upwards, it used approximately 7700 gallons of liquid oxygen (LOX) and 383000 gallons of liquid hydrogen (LH2). It's important to note that this calculation includes the fuel burned by the solid rocket boosters, which consumed 11000 pounds per second for 123 seconds. However, when we consider the entire launch sequence, the question of fuel efficiency becomes even more complex.

Real-World Fuel Efficiency

When we talk about fuel efficiency in the context of the space shuttle, traditional measures like miles per gallon are largely irrelevant. Rockets operate in fundamentally different conditions compared to car engines. The space shuttle's engines are designed for rapid acceleration and not for maintaining a constant speed. Therefore, the concept of miles per gallon becomes meaningless in this context.

Instead of trying to calculate miles per gallon, it is more relevant to consider the fuel consumption in terms of gallons per second. The total fuel usage during the launch phase was staggering, with the space shuttle consuming an average of thousands of gallons of fuel per second. This highlights the immense power and energy requirements during the initial stages of the launch, where the shuttle must rapidly climb through the Earth's atmosphere and achieve orbital velocity.

Orbital Efficiency and Specific Impulse

Once the shuttle reaches orbital velocity, it no longer needs to burn fuel to maintain its trajectory. In fact, it can coast indefinitely with minimal fuel consumption. This unique characteristic of rockets means that their efficiency is best described by specific impulse, a measure of engine efficiency that compares the change in momentum to the mass of fuel used.

Specific impulse is defined as the number of seconds that a unit of fuel can accelerate its weight under standard Earth gravity. For the solid rocket boosters of the space shuttle, the specific impulse was 250 seconds. This value is relatively low compared to the liquid hydrogen and liquid oxygen engines, which, once above the atmosphere, achieved a specific impulse of 453 seconds. This corresponds to each ton of fuel being able to provide a significant amount of thrust to accelerate its own weight by 9.8 meters per second every second.

Atmospheric Efficiency

The specific impulse of rocket engines is generally lower in the atmosphere due to the additional work required to push through the air. This means that the efficiency of rocket engines is best measured in the vacuum of space, where the absence of air resistance allows for higher specific impulse values.

In conclusion, the traditional concept of miles per gallon does not accurately represent the fuel efficiency of NASA space shuttles. Instead, specific impulse provides a better measure of efficiency for rocket engines. The high fuel consumption during the launch phase and the unique trajectory requirements of reaching orbit mean that the shuttles operated in a way that made miles per gallon an inappropriate metric. Understanding the unique physics involved in rocket propulsion is crucial for evaluating and optimizing their performance.