# rocket's in space

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#### tomorows_scientist

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<p>ok in space if u shoot a rocket off will the rocket keep going the same speed the whole time even when it is beeing propelled or does it just keep speeding up(example: ok so u got flames shooting out the back of ur jet plane and they just keep shooting, out on earth of course there is air to slow you down. but up in space if u just kept your flames shooting out wich i dont think is possible do to there not beeing any oxygen but lets just say would you keep speeding up because there is nothing there to slow you down or does the flame only have a certian amount of fource behind it and there for it can "top out" in speed and go no faster).</p><p>lol idk if you can really understand what im saying lol its all up in my head just kinda hard to try and explain it.</p> <div class="Discussion_UserSignature"> </div>

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#### Saiph

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<p>as long as you are applying a force, and creating a NET force, you will accelerate (speed up).</p><p>The problem with rockets and planes on earth is, as you pointed out, air.&nbsp; The pressure of air against the vehicle acts to counter any force our engines produce, often resulting in no NET force (engine force - air force = 0!)&nbsp; If we turn off the engines, all we have left is the force due to the air...which acts to slow the vehicle down.</p><p>In space there is no air.&nbsp; So if you keep your rocket engines on (which supply their own oxygen btw, so no problem there) you will continue to speed up.&nbsp; If you turn your engines off, theres nothing to slow you down...so you'll just coast along forever.</p><p>The complicating factor, of course, is gravity applies a force, and makes things a little less straightforward.&nbsp; But the general principles still apply.</p><p>&nbsp;&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>ok in space if u shoot a rocket off will the rocket keep going the same speed the whole time even when it is beeing propelled or does it just keep speeding up(example: ok so u got flames shooting out the back of ur jet plane and they just keep shooting, out on earth of course there is air to slow you down. but up in space if u just kept your flames shooting out wich i dont think is possible do to there not beeing any oxygen but lets just say would you keep speeding up because there is nothing there to slow you down or does the flame only have a certian amount of fource behind it and there for it can "top out" in speed and go no faster).lol idk if you can really understand what im saying lol its all up in my head just kinda hard to try and explain it. <br /> Posted by tomorows_scientist</DIV></p><p>&nbsp;</p><p>The reason jets have a "top out" speed is because they have air intake engines.&nbsp; If the the speed of the exhaust equals the speed of the aircraft (the speed of the intake), the will zero net force applied.&nbsp; All the ingredients for a rocket engine are carried on board and are used by applying a direct force against spacecraft inside the combustion chamber. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;The reason jets have a "top out" speed is because they have air intake engines.&nbsp; If the the speed of the exhaust equals the speed of the aircraft (the speed of the intake), the will zero net force applied.&nbsp; All the ingredients for a rocket engine are carried on board and are used by applying a direct force against spacecraft inside the combustion chamber. <br /> Posted by derekmcd</DIV></p><p>But also using the jet engine principle, as soon as your rocket reached the velocity that is equivalent to the rocket's exhaust gas velocity, there would no longer be net force and your acceleration stops.</p><p>The maximum velocity is the exhaust velocity (the velocity may still be increased by a gravity well, like Jupiter's huge gravity, if the gravity well is nearby the rocket). </p>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>But also using the jet engine principle, as soon as your rocket reached the velocity that is equivalent to the rocket's exhaust gas velocity, there would no longer be net force and your acceleration stops.The maximum velocity is the exhaust velocity (the velocity may still be increased by a gravity well, like Jupiter's huge gravity, if the gravity well is nearby the rocket). <br /> Posted by aphh</DIV></p><p>&nbsp;</p><p>The only limiting factors for a rocket in space, are the amount of fuel it can carry or the speed of light.&nbsp; A rocket in space, outside of any significant gravity well, could produce 1 pound of thrust and still accelerate.&nbsp; This is because everything needed for thrust is self contained.<br /> </p><p>A jet engine is limited by how fast it can accelerate air through the engine.&nbsp; If the acceleration out the back in form of exhaust is equal to the speed of the intake of air, you have zero net thrust.&nbsp; </p><p>Think of holding a fan in your hand.&nbsp; If you hold your hand still, you will feel it push against your hand (thrust).&nbsp; If you move your hand at the same speed in the opposite direction of the blowing air, you will negate the force of the thrust.&nbsp; However, if you enclose the fan and your hand inside a a self containing system that provides its own air (not relying on intake), the acceleration is limited only by how much air you can bring with you.</p><p>Maybe someone with a better understanding of fluid dynamics has a better way to explain it.&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;The only limiting factors for a rocket in space, are the amount of fuel it can carry or the speed of light.&nbsp; <br />Posted by derekmcd</DIV></p><p>Action - Reaction principle states that for every action you will have a reaction. With chemical rocket your reaction is limited by the exhaust velocity, not the amount of thrust. Thrust is a net force that determines how rapidly you can accelerate the mass.</p><p>The lighter the mass, the more rapid acceleration.</p><p>Acceleration, however, will no longer occur once the mass reaches the same velocity of the gasses expanding and shooting out of the nozzle. There will no longer be noticeable reaction in vacuum, as you have already reached the velocity produced by the action. In athmosphere your plane will maintain it's maximum velocity using continuous thrust. </p><p>This is why we need propulsion with faster exhaust velocity. Even smaller force over long period will accelerate a mass in the vacuum of space, but the determining factor for the maximum velocity will still be the action, or how quickly the particles shoot out the back, i.e. the exhaust velocity.&nbsp;</p><p>Reaction can not exceed action, meaning the rocket can't go faster than the gasses expand in the combustion chamber and shoot out the back.&nbsp;</p>

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#### MeteorWayne

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> With chemical rocket your reaction is limited by the exhaust velocity, not the amount of thrust. Thrust is a net force that determines how rapidly you can accelerate the mass.The lighter the mass, the more rapid acceleration.Posted by aphh</DIV><br /><br />Sort of. The lighter the mass you are accelerating, that true. As far as the accelarting force, however, &nbsp;F=m*a.</p><p>The lighter the exhaust particle's mass, the faster you can accelerate the exhaust, but that does not necessarily mean more thrust which is goverened by the above equation.</p><p>You are applying the mass to two different things, that of the body being accelrated, and the massXvelocity of the thrusting mass. You need to clearly state whether you are talking about the spacecraft, or the rocket exhaust.</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

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#### Saiph

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<p>The final velocity of the exhaust has nothing to do with the top speed of either the jet, or the rocket.&nbsp; Before you slap you hands over your ears, hear me out.</p><p>It is true that jets can go no faster than their exhaust velocity..but that is not <em>because</em> of the exhaust velocity.&nbsp; It has to do with the forces the engines exert on the propellants.</p><p>In a jet engine the air being brought in is grabbed, compressed, energized (the combustion chamber), <strong>accelerated</strong>, and expelled. If the air comming into the engine is at the same maximum speed the engine can expell it, then the air is not accelerated by the engine.&nbsp; No acceleration of the air means no force exerted upon the air by the engine. No force exerted by the engine on the air, means no force on the airplane by the engine either! &nbsp;</p><p>So the engine isn't exerting any net force on the airplane...which means there is no acceleration of the airplane, meaning a constant velocity.&nbsp; This occurs at the point where the intake velocity is equal to the exhaust velocity, but that isn't the true cause, just the conditions present.</p><p>&nbsp;</p><p>In a rocket, that carries it's propellant, the engine is <strong>always</strong> exerting a force upon the propellant.&nbsp; The rocket exhaust velocity is measured in the frame of the rocket, and in that same frame, the propellant has a <strong>zero</strong> velocity relative to the rocket.&nbsp; The engine takes the fuel, accelerates it in the combustion chamber, and expells it at the exhaust velocity...this does not change regardless of how fast the rocket is going.&nbsp; The fuel is always taken from a relative velocity of zero, and <strong>accelerated</strong> to the exhaust velocity.&nbsp; This exerts a force on the fuel by the engine, which transfers it to the rocket itself.&nbsp; This net force produces an acceleration, and increase in velocity.</p><p>&nbsp;</p><p>If you'd like I can also argue both these cases from a conservation of momentum point of view.&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### aphh

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<p>Can reaction exceed action? I would think this is against Newton's law. Vacuum or carrying the propellant does not change the properties of action and thereby reaction. <br /><br />Action is the combustion in the rocket's combustion chamber and reaction is the rocket's acceleration. Even if we had a unlimited amount of fuel to carry with the rocket, after a certain point it would make no use to burn it as the action - reaction maximum has been achieved.</p><p>Is this not true?&nbsp;</p><p>Edit: I understand what you're saying, though, that the exhaust velocity of a rocket always adds because the relative velocity of the fuel is 0 compared to the rocket body before combustion. So the amount of fuel that can be carried around and the mass of the fuel are the determining factors of the top velocity of a rocket?</p><p>* It is true that jets can go no faster than their exhaust velocity. </p><p>For a whort while they could, using G as the additional booster. Just like boosting the velocity of a spaceprobe using a gravity well of a planet.&nbsp;</p>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Can reaction exceed action?&nbsp; <br /> Posted by aphh</DIV></p><p>&nbsp;</p><p>No, it can't.&nbsp; I think you might be confusing acceleration and overall velocity.&nbsp; While a rocket in space may have a limit to how fast it can <strong><em>increase</em></strong> it acceleration, when this max is reached, it is still accelerating, thus increasing its velocity.&nbsp; The increase of acceleration may be limited by its thrust, but the amount of time it can spend accelerating is only limited by how much propellent it can carry.</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;No, it can't.&nbsp; I think you might be confusing acceleration and overall velocity.&nbsp; While a rocket in space may have a limit to how fast it can increase it acceleration, when this max is reached, it is still accelerating, thus increasing its velocity.&nbsp; The increase of acceleration may be limited by its thrust, but the amount of time it can spend accelerating is only limited by how much propellent it can carry.&nbsp; <br /> Posted by derekmcd</DIV></p><p>Thanks, I think I see it now. <br /><br />Given the limited amount of fuel that can be carried around, the overall velocity is limited anyway. The more fuel you carry, the more mass you need to accelerate. </p>

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#### Saiph

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<p>I think you've got it now </p><p>This last point is why engineers care about better propulsion systems with higher exhaust velocities.&nbsp; These new engines make better and more efficient use of the fuel provided.</p><p>Otherwise you have to deal with the more fuel, bigger ship, requiring more fuel....issue and trying to strike a balance between desire, cost, and feasability.&nbsp;</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### aphh

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<p>The efficiency of the fuel and propulsion is the number one issue in getting humans and infrastructure to Mars. If we could make the leap in just weeks instead of months, that would solve most of the big problems in human spaceflight.</p><p>While waiting for faster interplanetary travel, I propose a moonbase. Grander than what Dr. Evil had. Only 3 days one leg.&nbsp;</p>

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#### neilsox

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>ok in space if u shoot a rocket off will the rocket keep going the same speed the whole time even when it is beeing propelled or does it just keep speeding up(example: ok so u got flames shooting out the back of ur jet plane and they just keep shooting, out on earth of course there is air to slow you down. but up in space if u just kept your flames shooting out wich i dont think is possible do to there not beeing any oxygen but lets just say would you keep speeding up because there is nothing there to slow you down or does the flame only have a certian amount of fource behind it and there for it can "top out" in speed and go no faster).lol idk if you can really understand what im saying lol its all up in my head just kinda hard to try and explain it. <br />Posted by tomorows_scientist</DIV></p><p>There&nbsp;are no simple answers to your questions. Space typically has less gravity than Earth, but there is some gravity, but air resistance is negligible. The gravity and the thrust add vectorally. If the thust is weak as an ion engine or solar sail, the speed may decrease rather than increase. You could even be traveling backwards, and sideways&nbsp;while thusting forward. The top out occurs rarely: More often you are powering away from a planet so the gravity is decreasing, so you are acellerating and thus increasing speed with respect to the planet below even if the speed of the exhaust is slower than your speed with respect to the planet.</p><p>Space vehicles typically do not use the oxygen in the air, but there are some designs which are air breathing until they reach an altitude of 20 or 30 miles after which the air is too thin to provide sufficient oxident from the air. I don't know of any air breathing first stage in use at present.&nbsp;&nbsp; Neil</p>

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#### DrRocket

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<p>Neglecting outside forces such as gravity, in a vacuum the speed of a rocket is given by the rocket equation:</p><p>delta V = Isp x ln(final mass/initial mass)</p><p>Here Isp (specific impulse) is the velocity of the exhaust gasses relative to the rocket.&nbsp; One can calculate Isp from knowledge of the thermodynamic properties of the propellant and the expansion ratio of the nozzle, but generally it varies like the square root of the temperature divided by the molecular weight of the gas.&nbsp; So the velocity boost is determined by propellant chemistry, nozzle design, and the ratio of propellant to inert weight.&nbsp;</p><p>In deriving this equation it is NOT true that F=ma.&nbsp; This is because Newton's equation is actually F = dp/dt where p is momentum.&nbsp; This reduces to F = ma for the case of constant mass.&nbsp; But rockets do not have constant mass.</p><p>If you do not neglect gravity then the speed depends on the trajectory and the rate at which the propellant is consumed.&nbsp; This is basically because when one goes "up" carrying propellant some of&nbsp; the energy goes into lifting the propellant as well as the structure.&nbsp; In that case, neglecting air resistance the optimal strategy is to burn the propellant very quickly -- an impulsive burn.&nbsp; But if you throw in air resistance, the situation changes again since air resistance goes up like the density of the air time the square of velocity and the air is denser lower in the atmosphere.&nbsp; When you get this complicated, detailed trajectory programs are used to determine the optimal trajectory and burn rates.</p> <div class="Discussion_UserSignature"> </div>

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#### origin

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Neglecting outside forces such as gravity, in a vacuum the speed of a rocket is given by the rocket equation:delta V = Isp x ln(final mass/initial mass)Here Isp (specific impulse) is the velocity of the exhaust gasses relative to the rocket.&nbsp;&nbsp; <br />Posted by DrRocket</DIV></p><p>OK, I am perfectly willing to ask a stupid question.</p><p>When you say "the velocity of the exhaust gases relative to the rocket" are you talking about the rocket body or the inertial frame of the rocket.&nbsp; Because I am somewhat confused as to the ultimate answer.</p><p>Geez, this isn't rocket science, oh, wait&nbsp;, I guess it is....</p> <div class="Discussion_UserSignature"> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>OK, I am perfectly willing to ask a stupid question.When you say "the velocity of the exhaust gases relative to the rocket" are you talking about the rocket body or the inertial frame of the rocket.&nbsp; Because I am somewhat confused as to the ultimate answer.Geez, this isn't rocket science, oh, wait&nbsp;, I guess it is.... <br />Posted by origin</DIV></p><p>I am talking about the rocket body.&nbsp; A reference frame attached to the rocket is not interial -- the rocket is accelerating (at least we hope so).&nbsp; It is actually an interesting exercise, left to the reader, to write the equations of motion in an intertial reference frame, and then transform them to the frame of the rocket.</p><p>What you might find interesting (or maybe not) is to look at the momentum of the rocket, in an inertial reference frame as the acceleration occurs.&nbsp; Towards the end of burn the rocket velocity is increasing, but the momentum is actually decreasing.&nbsp; This occurs when the forward velocity of the rocket, in an intertial reference frame, equals the rearward velocity of the exhaust gasses, relative to the&nbsp;body of the rocket.&nbsp; The result is that at that point the exhaust gasses are moving in the same direction as the rocket, just not as fast.&nbsp; So the momentum of the&nbsp;gasses excaping at that point and aftereward is in the&nbsp;same direction as the momentum of the rocket body.&nbsp; Conservation of momentum requires that the momentum of the rocket&nbsp;plus the remaining fuel be&nbsp;decreasing.&nbsp; Alternatively, you can look at&nbsp;it as the point at which the decrease in mass overcomes the increase in velocity to result in a net decrease iin momemtum.&nbsp;<br /></p> <div class="Discussion_UserSignature"> </div>

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#### origin

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I am talking about the rocket body.&nbsp; A reference frame attached to the rocket is not interial -- the rocket is accelerating (at least we hope so).&nbsp; It is actually an interesting exercise, left to the reader, to write the equations of motion in an intertial reference frame, and then transform them to the frame of the rocket.What you might find interesting (or maybe not) is to look at the momentum of the rocket, in an inertial reference frame as the acceleration occurs.&nbsp; Towards the end of burn the rocket velocity is increasing, but the momentum is actually decreasing.&nbsp; This occurs when the forward velocity of the rocket, in an intertial reference frame, equals the rearward velocity of the exhaust gasses, relative to the&nbsp;body of the rocket.&nbsp; <strong>The result is that at that point the exhaust gasses are moving in the same direction as the rocket, just not as fast.</strong>&nbsp; So the momentum of the&nbsp;gasses excaping at that point and aftereward is in the&nbsp;same direction as the momentum of the rocket body.&nbsp; Conservation of momentum requires that the momentum of the rocket&nbsp;plus the remaining fuel be&nbsp;decreasing.&nbsp; Alternatively, you can look at&nbsp;it as the point at which the decrease in mass overcomes the increase in velocity to result in a net decrease iin momemtum.&nbsp; <br />Posted by DrRocket</DIV><br /><br />Ahhh, the light finally turns on.&nbsp; Thank you. <div class="Discussion_UserSignature"> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>When you get this complicated, detailed trajectory programs are used to determine the optimal trajectory and burn rates. <br /> Posted by DrRocket</DIV></p><p>&nbsp;</p><p>Way to complicate my perfectly simple understanding of how they work. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-tongue-out.gif" border="0" alt="Tongue out" title="Tongue out" /> </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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