dynamics and space

applications of satellites

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By now you should have watched the video about satellites.  This screenshot showing a satellite passing over the Highlands was taken from about 17 minutes into the programme – did you notice at the time?

 

 

 

 

 

 

 

It was quite eye-opening to see just how much modern society relies on satellite technology.  I’ve got some more examples of the uses for satellites below.

A satellite moves horizontally at constant speed but also accelerates vertically towards the planet’s surface due to gravity.  Thankfully, the curvature of the Earth means the satellite doesn’t crash but keeps on orbiting the planet.

Satellites can be used for environmental purposes, such as

monitoring volcanoes

weather forecasting
Screen Shot 2015-10-07 at 23.22.35

 

 

 

 

 

 

 

 

climate change

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re-entering the atmosphere

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In space there is no air resistance to oppose motion, so the Space Shuttle orbiter could travel at very high speeds, up to 17,000 mph!  At these speeds, the orbiter experienced enormous air resistance as it descended into the Earth’s atmosphere at the end of its mission.

Air resistance is just like any other form of friction – it converts kinetic energy into heat energy.  The effect of this heat energy is demonstrated in this video clip taken by a Canadian police car camera.  It shows a meteor burning up in the atmosphere above Edmonton.

Thankfully most meteors do burn up in the atmosphere, although the dinosaurs were not so lucky.

The high temperatures created during re-entry ionised the gas around the orbiter and this is often seen as a bright light in NASA cockpit videos, such as the one shown below.

To protect the vehicle and its crew from these high temperatures, the underside of the orbiter was covered by a layer of heat resistant tiles called the thermal protection system.  This NASA clip explains how the tiles are constructed and arranged on the underside of the orbiter.

When Columbia was launched in 2003, something fell against the insulation on the left wing and knocked off some of the tiles.  This hole in the thermal protection system caused Columbia to explode over the US as it re-entered the atmosphere.  There is a wikipedia article about the Columbia disaster.

Video footage of NASA’s Houston control room from the morning of the disaster was included in the BBC Horizon documentary Final Descent – Last Flight of Space Shuttle Columbia.

WARNING: This last film is an excerpt from the Horizon programme and includes genuine cockpit video that was found in the wreckage, with some clips of the crew’s final minutes before they were killed.

There is a good description of the Space Shuttle at How Stuff Works.

Before the space shuttle, each spacecraft was designed to be used only once and it was only the capsule containing the crew that returned to Earth. This was a small conical vehicle that had a thick heat shield on its base to withstand the heat of re-entry.

drawing of Apollo capsule re-entering the atmosphereartist’s impression of Apollo capsule re-entering the atmosphere

An ablative heat shield was used for these capsules. The material covering the base was designed to heat up until it sublimed (changed from solid to gas). The latent heat of sublimation is much greater than that required for fusion or vaporisation, so much more heat energy could be absorbed by the shield material as it changed state. Obviously there is a catch…the longer the shield protects the astronauts, the thinner it becomes! Here is an image of a Gemini IV capsule on display at the Smithsonian National Air and Space Museum showing what was left of the heat shield after successful return to Earth.

Gemini IV crew capsule photo: Richard Kruse

 

Newton’s third law of motion

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Today we examined the importance of Newton’s 3rd law of motion. In our discussions, different explanations for the motion of jets and rockets were proposed and considered. The front runners were;

  1. at launch, the ground pushes back against a rocket
  2. during flight, air pushes back against a plane

Unfortunately, the lack of ground and air (or any other gas) meant that neither of these models were able to explain the propulsion of an object in space. It was at this point we remembered Newton’s 3rd law of motion (or here with non-rocket examples).

You’ve got to be careful with Newton’s 3rd law of motion, it’s easy to get confused. Bonus question: What’s wrong with this explanation?

I found a photograph that provides a stunning visualisation of Newton’s 3rd law in action during the launch of a DeltaIV rocket. You can read the details of setting up for this photo here.

Delta_4-Heavy_DSP-23

image courtesy of Ben Cooper, Launchphotography.com

The photo was taken at very short range (about 30m) from the launch site and clearly shows hot gases being forced out of the exhaust at high speed. When a rocket forces out gas, the expelled gas pushes back on the exhaust with an equal force. Since the exhaust is part of the rocket’s structure, the entire rocket is propelled in the opposite direction to the gas.

It is this pushing back on the exhaust that provides thrust for a rocket. It doesn’t matter if the rocket is on the launch pad, in mid air or outer space. As long as it can push gas out of the exhaust, it will be able to propel itself forwards using Newton’s 3rd law of motion.

We don’t normally get a clear view of the hot gases being forced out of a rocket in launch photographs. A lot of the smoke seen in images like the one shown below is actually steam.

NASA_launch_Mars_rovers

NASA/courtesy of nasaimages.org

There are two main sources of steam during launch. The most obvious is the burning of fuels but NASA also soaks launch platforms with water just before and after launch so that the massive sound waves don’t damage the vehicle being launched. There is a wikipedia article on the use of water during space shuttle launches.