Shuttle re-entry

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mirak

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Could someone who knows please feed my feeble mind. I understand the shuttle has to orbit pretty fast to catch up too and dock with the Int. space station but upon re-entry why can't the shuttle slow down considerably to reduce friction when decending through the atmosphere?<br /><br />Paul S.
 
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nacnud

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No, it not practical to carry enough propellant to slow the shuttle enough to reduce heating. Also friction is not the main cause of heating during re-entry. The heating is because the air in front of the craft is compressed as it doesn’t have time to get out of the way. If you compress something into a smaller space it heats up, hence the heat of re-entry. Link for more on compression heating.
 
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mirak

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Other questions. Do objects need to travel fast to stay in orbit longer or does it matter? Are they traveling so fast because of the speed at which they broke through the atmosphere on the way into space? I think we need to find a slower way to break through earths atmoshere on the way up, thus a slower re-entry.<br /><br />Paul S.
 
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drwayne

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Paul,<br /><br /><br />Think about it this way. To get to orbit, the shuttle used those two huge solids up, plus all the fuel and oxidizer from that huge tank.<br /><br />To slow down a lot, you would need a sizeable fraction of that same amount of resources. Plainly, it does not have it, nowhere near.<br /><br />What it can do is slow down enough to just start to catch the atmosphere. In this case, drag is our friend. The resistance of the air will scrub off practically all of the energy given the shuttle by those resources I mentioned. Essentially for free. (Other than dealing with the heating)<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>
 
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drwayne

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Paul,<br /><br />You must travel at a high speed in order to stay in orbit. Travel any slower, gravity pulls you back. <br /><br />In general, higher altitude orbits last longer, less drag.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>
 
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spacefire

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nacnud, the stagnation temperature is proportional with the <b>square</b> of the Mach number. That means that great reductions in heating can be obtained by reducing the Mach number slightly. Wouldn't a small amount of propellant, used at key points duriong re-entry (after all, speed of sound varies with air temperature) to decelerate the craft in bursts, might make a big difference in the amount of reentry heating suffered by the TPS? <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>
 
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scottb50

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If you had enough energy available you could descend and land vertically and not care anything about heating. The problem is that would take a tremendous amount of propellant. If you could slow well below orbital velocity will still above the atmosphere you would still generate significant heat entering the atmosphere, look at SS-1, at apogee it is not going all that fast, but gravity accelerates it on the way down.<br /><br />You would have to lift a lot of propellant to have a significant effect.<br /><br />The speed of sound is just a reference point, it varies to a certain point and then stays constant, outside the atmosphere it is irrelevant. The Shuttle scrubs off velocity by banking steeply as it descends, creating more drag. A forward firing engine to do the same thing would be prohibitive.<br /><br />At any altitude, above a body, it takes a specific speed to orbit at that altitude. You could orbit the Moon at a very low altitude, as long as you go fast enough and don't hit anything. In an atmosphere you have to counter drag and that can be affected by Solar conditions, the upper atmosphere expands. The ISS needs reboosting because it is fairly low, the Hubble not as much because it is higher.<br /><br />Skylab should have stayed in orbit longer, but increased solar activity caused it to re-enter before Shuttle could get to it. <div class="Discussion_UserSignature"> </div>
 
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thinice

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Slowing down every additional 10 m/s per second adds extra G for everyone onboard.
 
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nacnud

Guest
Hence the '<font color="yellow">10 m/s per second<font color="white">'<br /><br />but what does this have to do with the price of fish?</font></font>
 
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thinice

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<i>that's if you slow down by 10m/s... in one second</i><br /><br />And what did you mean by "decelerating the craft in bursts"? Slowing down by 1cm/s for a half of a day?
 
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captsolo

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Could anyone tell how fast does Shuttle gain speed when taking off?<br /><br />I was surprised to hear on TV that they are experiencing only 3 G during the liftoff. Since plane acrobatic pilots experience up to 9-10 G I expected that Shuttle is also rising at quite high acceleration.<br /><br />Or is so high acceleration not necessary to get into the orbit?<br /><br />Best,<br />CaptSolo
 
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drwayne

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Large accelerations are not neccessary and not desirable, because of the strain that they put on men and equipment.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>
 
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najab

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It's not 3G all the way, 1.5G is probably a good approximation of average acceleration.<br /><br />1.5G is ~15m/s/s.<br /><br />The Shuttle ascent lasts 8 and a bit minutes, call it 500 seconds.<br /><br />So the final velocity is approximately: 500 * 15 = 7 500m/s. = Orbital velocity.
 
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captsolo

Guest
Thanks. <br /><br />Is there a graph available on the web showing how the acceleration changes during take-off?<br /><br />Tried to find it on the web, but did not get anything meaningful. I recall seeing here a graph of how Shuttle main engine power is adjusted to compensate for hot or cold SRB performance, so people here probably know better than Google. <img src="/images/icons/smile.gif" /><br /><br />Best,<br />CaptSolo
 
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captsolo

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Thanks for the insight, SG! <img src="/images/icons/smile.gif" />
 
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viper101

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I've always thought about how amazing it would be to orbit the moon at very low altitudes. <br /><br />I'm willing to bet that 95% of the population has no idea what an orbit actually is or how it works /> I must give the "if you threw this ball even harder" speech about 6 times a year. <br /><br />A friend of mine once asked me - following the SS1 flight 'how did they keep that ship from just floating away once it was in Space'. I explained, as only a skydiver can, what a harsh mistress gravity can be. <img src="/images/icons/wink.gif" />
 
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nacnud

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What did I say? I think I would love sky diving, but my budget won't allow it. I'll stick to climbing for now, unless I can find a cheap paragliding school <img src="/images/icons/smile.gif" />
 
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