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0 A space shuttle should take about 10 hours to get to the moon. Why does it take 3 days?

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Special Blog Post BY:  C Stuart Hardwick, Award-Winning Scifi Author  Sept-28-2019

    Q: A space shuttle should take about 10 hours to get to the moon.

Incorrect. A space shuttle could never reach the moon, not in a zillion years, not unless it’s carried inside some much larger spacecraft that can reach the moon. Besides, the nice people in the museum won’t let you have their space shuttle.

But I digress.

    Q: Why does it take 3 days?

You are confusing maximum speed with average speed.

Like the space shuttle, the Apollo spacecraft went into low earth orbit at around 17,000 mph. Unlike the shuttle (which was then out of fuel except for what it needed to maneuver and reenter) an Apollo mission still had a nearly full S-IVB upper stage. When that fired, it accelerated itself, the SLA housing the lander, the lander (the LEM), and the Command and Service Modules (the CSM) to about 24,000 mph (11 km/s), fast enough to reach the moon in ten hours. But as soon as the engine cut off, the spacecraft started slowing down—still under the influence of Earth’s gravity. It slowed and slowed until it was barely moving (in space terms). At a mere 2,400 mph, it started accelerating again, this time toward the moon under lunar gravity.

And that’s why it took three days (and will always take three days for a free return trajectory, regardless of spacecraft). The maximum speed at orbital insertion or trans-lunar injection is not the speed for the duration of the flight.

I mention free return trajectory because, as a genuine rocket scientist recently explained to me at a conference, we are unlikely to want to send humans to either the moon or Mars on any other kind of trajectory, any time in the foreseeable future. A free return trajectory is one that will always take you back home if you just do nothing—useful in case anything goes wrong.

Original Source: The original article on Quora will open in a new window CLICK HERE 

Used for educational purposes.