Atlas V makes you feel like you’re trippin

I have just stumbled upon what is (inadvertently) my second jaw-dropping video shared on my blog in a very short time, what is only about the most beautiful thing that I have ever seen in the sky. Below is a video of the February 11 launch of the Solar Dynamics Observatory upon an Atlas V rocket. The launch is exciting as always but at about the 1:50 mark in the video the most amazing thing happens, here have a look, I’ll explain later;

Isn’t that something? Ok, so what exactly happened there? Well, when the Atlas V rocket had accelerated fast enough for long enough it reached a velocity known as the speed of sound. What happens at this point is rather technical, but let me break it down for you. As a rocket (or an aircraft or any other craft for that matter) accelerates and it’s velocity (speed) increases it’s nose begins to push on the air in front of it, this causes an increase of air pressure in front of the craft. The faster you go, the more the pressure climbs until **WHAM** you reach a velocity of 340.29 m/s (or about 1200 km/h) and you unleash a sonic boom!

An aeroplane approaches the speed of sound. Courtesy of Wikimedia

An FA-18 lets you see the speed of sound. Courtesy Wikimedia

Would you like to work out an approximate value for how fast the Atlas V rocket in this video was going? It’s easy, I promise!

First, I’m going to make a few assumptions about the motion of the rocket which will greatly simplify our calculations and won’t affect the estimated acceleration all that much. I am going to assume that the rocket’s movement is in one dimension only. This means that I am going to neglect the horizontal motion and only consider the vertical motion. I will also assume that the rocket accelerates uniformly and that air resistance does not play any role. I know that this will not give me the exact value of the rocket’s acceleration, but I’m only interested in an estimate anyway!

I will use one of the equations of kinematics for one-dimensional motion, namely;

v = vi+at

That equation says “The final velocity is equal to the initial velocity plus acceleration multiplied by time”. I will rearrange the equation to isolate the acceleration component as the subject of the equation;

a = (v – vi)/time

That is, “the acceleration is equal to the change in velocity divided by the change in time”. Now let’s put in some values, we will use the initial velocity of the rocket vi = 0 m/s, time elapsed from the launch to the sound barrier t = 72 s;

a = (340.29 m/s) / (72 s)

a = 4.73 m/s^2

From this we know that the rocket was accelerating at approximately 5 meters per second every second. One meter per second is equal to 3.6 km/h, so the acceleration of the rocket is very very fast!

When a fast moving rocket hits a wispy cloud bank and disturbs the water crystals within them you see the sky ripple like we did on the 11th of February and you feel as though you’ve just dropped a hit of Acid.

For more awesome pics, check out the Flickr site of George Privon.

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3 responses to “Atlas V makes you feel like you’re trippin

  1. Amazing video! And I actually understood that explanation…. thanks Angie. (I didn’t even allow the formulae to scare me off).

    The picture of the FA-18 jet is astonishing – is that water-vapour expanding? Does the shock wave cause the water to suddenly condense out of the air? How does it do that?

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