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DEFCON

Space Physics Question

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Does anyone want to try to answer this question; I can't?

I was watching a video regarding the space shuttle. When the vehicle ascends to a particular altitude the main engines shut down and the external tank is jettisoned.

At this point the vehicle has zero thrust, it's in an essentially a zero g state and a near vacuum.

Why or how is it then that gravity is somehow capable of grabbing the external tank and causing it to 'fall back' into Earth's atmosphere while the Shuttle itself somehow manages to elude the gravitational force and avoid following the tank in its return trajectory?

   

 

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At main engine cut-off the shuttle is on a suborbital trajectory, along with the tank.  About 30 minutes later the tank is released and the shuttle orbital maneuvering engines are fired to raise the trajectory of the shuttle.  The tank remains on the suborbital trajectory which puts it back into the atmosphere.  As the tank heats up the contents cause it to burst which leads to an explosion which reduces the size of the bits.

Good question, and thanks to Wiki for the answer!

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Wait, there's more! I read a few years ago about main engine thrust at booster separation. Full thrust was not sufficient to orbit the shuttle with external tank attached after the boosters separated. The shuttle actually descended into a sub orbital trajectory with the main engines at full thrust until it ejected enough of its own mass (external tank contents) to achieve the correct acceleration to continue to orbit. This took a minute or two. There's a differential equation in there somewhere.....I just know it!

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Interesting Planett, but a question arises; if the main engines have enough thrust to get the vehicle and tank to that point in the ascent phase, how could it run out of poop at a time when weight, mass and aerodynamic drag are both decreasing exponentially?

The thrust of an orbital maneuvering engine is 6K in a vacuum. https://www.google.ca/#q=thrust+of+space+shuttle+orbital+maneuvering+engines

The thrust of a main engine is approximately 500K in a vacuum. The throttle is reduced to 65% power, or about 325K ilb. of thrust about 7 seconds prior to main engine cut-off (meco). http://spaceline.org/rocketsum/main-engines.html

The tank itself weighs almost 1.7M lbs. at liftoff and burn off eliminates nearly 1.6M lbs. of propellant before meco. http://www.nasa.gov/returntoflight/system/system_ET.html

This would mean that dumping the tank which couldn't weigh more than a couple hundred lbs., or so, at this point, allows one, sometimes two 6000 lb. thrust engines to lift a mass to orbit that 1.5 million lbs. of thrust couldn't, which doesn't make sense?

When I asked the question this morning I wasn't aware of the role of the 'orbital maneuvering engines' in the ascent phase. We'd have to know by what amount the shuttle's mass has been reduced at meco to conduct a numbers crunch, but even in spite of the reduction in the vehicle's weight, it's still hard to imagine 6000 to 12000 lb. of thrust actually being enough juice to make the final lift.

 

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Isn't it that the main engines 'lift' the rocket into suborbital, then the maneuvering engines 'accelerate' the shuttle into final orbit?  No 'lifting' to be done?

 

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When the shuttle is launched it has to accelerate from zero to somewhere in excess of 17000 mph to achieve breakaway velocity. IOW's, the rocket motors have to keep the machine in a constant state od acceleration throughout almost all of its ascent. For instance, the Starship will fall back to Earth reliably every time because it never achieves the speed necessary to defeat gravity and the spin of the planet.

A vehicle's final position in orbit represents a vertical vector, which can be depicted by drawing a line between the shuttle's position in orbit and a given spot on the surface of the Earth, say the launch pad. Put simply, the vehicle has to be lifted by force to that point in space.

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It only "loses poop" when it drops the boosters.... for a short while afterward it's thrust/weight ratio is insufficient to continue the climb.... until the Shuttle's main engines alone burn enough of the fuel to get that ratio back.

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Mitch's explanation was what I was getting at. Solid Rocket Boosters separate, and the main engines are insufficient for just a small period? I'll try to track down the old reference textbook. Ugh!

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Mitch's explanation was what I was getting at. Solid Rocket Boosters separate, and the main engines are insufficient for just a small period? I'll try to track down the old reference textbook. Ugh!

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Defcon was correct, the orbiter accelerates and climbs from SRB separation at 2min and 28 miles to 6 min and 80 miles altitude. The assembly will then descend to 72 miles while reaching peak acceleration of 3g. By 8 min the main engines are shut down and the tank is jettisoned allowing orbital engines to do their thing. The 8 mile descent stayed in my memory, the rest didn't. 

Info from Space Shuttle, Christopher Chant, 1984.

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