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 Subject: Space Shuttle re-entry speeds Category: Science > Technology Asked by: publish1000-ga List Price: \$26.00 Posted: 01 Nov 2005 08:07 PST Expires: 01 Dec 2005 08:07 PST Question ID: 587531
 ```Why do they have to come back to earth so fast? Can't they just slow down in orbit or even stop and point the nose down and gently glide back to earth? I remember a test pilot that went up in a balloon to the edge of space and free fell back to earth and he never burned up> Then the shuttle would not need all those heat tiles.```
 Subject: Re: Space Shuttle re-entry speeds Answered By: tutuzdad-ga on 01 Nov 2005 09:59 PST Rated:
 ```Dear publish1000-ga; You posed a great question. The answer is really much simpler than it first appears. The US Space Shuttle uses a rocket propulsion system to get into its final orbit in its approach to Earth. Other than that the Shuttle is essentially falling and during re-entry the aircraft is actually a glider. The reentry speed is largely dictated by the Earth gravitational pull and given the physics of such a reentry the speed is typically approximately 25,405 feet per second (17,322 statute miles per hour). The Shuttle has relatively small rockets that can be fired to get the aircraft in a position to maximize the drag that aids in slowing the vehicle down enough so the pilot can select an appropriate trajectory and target (called the deorbit maneuver). Even with this at their disposal it only decreases the Shuttle?s velocity approximately 300 feet per second (205 mph) for reentry. This in turn allows the crew to select the best angle of approach and dissipate the reentry speed to a safe level to ideally ensure that the vehicle does not ?burn up?. This maneuver is called a hypersonic split-S maneuver, or a kind of zigzag flight pattern that generates additional drag in the same way that a skier going down a hill executes the maneuver to decrease his speed. At 17,500 miles per hour there simply isn?t enough time, given the distance of the craft from the Earth and the Earth?s gravitational pull to execute enough split-S maneuvers to slow the vehicle down more than they already do. To further simply the explanation, a downhill skier is essentially experiencing a controlled fall. If, for example, a skier is careening down a one mile long slope at 500 miles per hour, assuming he doesn?t kill himself in the process, is able to pull off the maneuver, he might have enough time at that speed to execute one split-S before he reaches the end of his glide path. This maneuver will slow his descent a certain percentage but it is impossible to slow down more because there simply isn?t enough time from beginning to end to perform the maneuvers enough times to bring him to a halt or slow him down significantly to make his decent a leisurely glide. What?s more, objects in space are not stationary. They may ?appear? to be stationary but like a boat under the influence of a stream, objects in space are orbiting or falling (due to gravitational pull) or they are propelling under their own power. The shuttle drifts in space in the same way that a boat drifts in the stream. In space there is no physical drag factor so brakes and the like, in the conventional sense, cannot function and would achieve nothing. The only drag in space is the drag created by oppositional forces. In other words, for the boat to give the appearance that it has stopped, it would need to exert an equal amount of propulsion against the force of the stream. In essence, the boat would have to speed upstream under enough power to cause it to remain in one place, but not more lest it travel upstream. While it appears motionless it is effectively moving forward just enough to maintain its seemingly stationary position. For the Shuttle to do this against the force of the earth?s gravitational pull, it would have to have en enormous store of fuel that exceeds, under current technology, the weight restrictions that would allow us to feasibly get the vehicle in space to begin with. Even if, theoretically of course, we were to create some type of refueling station in space that the Shuttle could dock with just prior to it?s return and take on additional fuel to create enough drag through opposing propulsion, the concept would still be unfeasible. The amount of fuel it would take to pull this off successfully would be prohibitive because it would like trying to put enough fuel in a small boat to propel it hard enough to stabilize it and cause it to maintain a seemingly stationary position midway up the face of Niagara Falls. It simply could not carry enough fuel or exert enough thrust to do that. In the end, the best we can do under current technological constraints is to slow the Shuttle enough to control the fall until it reaches the Earth?s lower atmosphere where it can fly like a normal airplane and the low atmospheric laws of physics take over. By then of course the fall is already technically over and the rest of the journey is flight. I hope you find that my answer exceeds your expectations. If you have any questions about my research please post a clarification request prior to rating the answer. Otherwise I welcome your rating and your final comments and I look forward to working with you again in the near future. Thank you for bringing your question to us. Best regards; Tutuzdad-ga ? Google Answers Researcher INFORMATION SOURCES SPEED REGIMES http://www.grc.nasa.gov/WWW/K-12/airplane/hihyper.html CBC NEWS ?Coming home: steps of a shuttle's re-entry? http://www.cbc.ca/news/features/shuttle_columbia/columbia_reentry.html NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (NASA) http://spaceflight.nasa.gov/shuttle/reference/shutref/sts/requirements.html SEARCH STRATEGY SEARCH ENGINE USED: Google ://www.google.com SEARCH TERMS USED: Space shuttle Reentry Speed Descent split-S maneuver```
 publish1000-ga rated this answer: `Thank you for the quick response.`

 Subject: Re: Space Shuttle re-entry speeds From: kottekoe-ga on 01 Nov 2005 19:41 PST
 ```A shorter way to say this: Orbital velocity is ~17,000 miles per hour. To get out of orbit you can burn a little bit of fuel to lower your orbit enough to encounter the drag of the atmosphere and then use that drag to slow you from 17,000 mph to zero. Alternatively, in principle, you could burn a lot of fuel to slow your velocity to zero before hitting the atmosphere and then glide back at low speed. To do the latter would require approximately the same amount of fuel that it takes to get into orbit in the first place. Imagine the enormous size of rocket that would be required to put the entire space shuttle with fully loaded external tank and solid rocket boosters into orbit. Not a sensible way to go!```
 Subject: Re: Space Shuttle re-entry speeds From: publish1000-ga on 03 Nov 2005 17:53 PST
 ```Ok, I see what you are getting at. BUT, what if you insert your craft into orbit in the direction the earth below you is turning, and achieve the same speed as the earth turns, seems to me you could hover over florida and point your nose down and push off at about 25 miles an hour. Your craft would pick up speed gently and just glide in circles till you reach the ground. Somewhere I have heard of shuttles doing 24000 miles an hour, up there in space. My whole work place thanks you for your input. I don't know about this. You know a lot more that I do. What do you think.... David```
 Subject: Re: Space Shuttle re-entry speeds From: kottekoe-ga on 03 Nov 2005 21:51 PST
 ```publish1000: The only way to be in orbit and be stationary with respect to a spot on the earth's surface is to be in geosynchronous orbit, at an altitude of 22,000 miles. This is far beyond the capability of the shuttle, which can only reach an altitude of about 200 miles. Even if you could get that high, you couldn't hover over Florida, only over a spot on the equator. If you were in a cricular orbit far above the atmosphere, pointed the craft down, and turned on the rocket to give yourself a small downward velocity, you would only succeed in slightly perturbing the orbit into a ellipse with your altitude decreasing for a while, then increasing, reaching a higher maximum altitude, then decreasing again until it crossed the original orbit exactly one orbit later. This is easier to explain with a diagram and a bit confusing if you haven't thought about orbital mechanics before.```
 Subject: Re: Space Shuttle re-entry speeds From: iang-ga on 04 Nov 2005 10:34 PST
 ```There's a java applet at http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=24 that shows how the orbit of a satellite depends on its velocity. There's another one at http://lectureonline.cl.msu.edu/~mmp/kap7/orbiter/orbit.htm which does the same thing, but in a slightly different way - more fun, but less informative Ian G.```
 Subject: Re: Space Shuttle re-entry speeds From: publish1000-ga on 04 Nov 2005 16:22 PST
 ```My friends and I want to thank each of you for helping out with my question. You have answered the question on orbital speeds that we have kicked around for years, thank you very much. Now we can think about women. David```
 Subject: Re: Space Shuttle re-entry speeds From: tutuzdad-ga on 04 Nov 2005 16:36 PST
 `We've answered those question here too. ;)`