APU Exhaust Thrust

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bobw

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http://uplink.space.com/showthreaded.php?Cat=&Board=sciastro&Number=439735&page=&view=&sb=&o=&vc=1<br /><br />Shuttle_guy wrote something very interesting, in the thread above, and I have a few questions... sort of spawned. I have been wondering about it for ten days now; it's burning a hole in my brain. Please, anyone, feel free to reply. This post isn't specifically aimed at shuttle_guy. <br /><blockquote><font class="small">In reply to:</font><hr /><p>One side note: The APU exhaust (all 3) are near the vertical stabilizer (rudder) pointing in the same direction thus they are propulsive. This propulsive force is used to pitch the Orbiter around after the de-orbit OMS burn rather than using the RCS thrusters. Some engineers are clever.<p><hr /></p></p></blockquote><br />I have always thought that as the shuttle orbiter travels doors-down it must be rotating, about it's long axis, at the rate of 1 rev/orbit. Does it take a lot of correction burns to keep it that way or is it the rotation pretty stable once the astronauts get it going?<br /><br />The de-orbit burn is conducted doors-down, engines forward but sort-of pointed up. I figgured that the one rev/orbit rotation would automatically carry the orbiter to a bottom-down, nose forward attitude for re-entry after half an orbit. I guess it is more complicated than that. How much of a rotation does the APU exhaust thrust actually cause? 1/4? 1/2? Do they actually have to correct for overshoot? Just a general figgure would be great.<br /><br />Clever, indeed! Those rocket scientists think of everything.<br /><br />Thanks, all.<br /> <div class="Discussion_UserSignature"> </div>
 
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tzero

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"I have always thought that as the shuttle orbiter travels doors-down it must be rotating, about it's long axis, at the rate of 1 rev/orbit. Does it take a lot of correction burns to keep it that way or is it the rotation pretty stable once the astronauts get it going? <br /><br />The de-orbit burn is conducted doors-down, engines forward but sort-of pointed up. I figgured that the one rev/orbit rotation would automatically carry the orbiter to a bottom-down, nose forward attitude for re-entry after half an orbit. I guess it is more complicated than that. How much of a rotation does the APU exhaust thrust actually cause? 1/4? 1/2? Do they actually have to correct for overshoot? Just a general figgure would be great. "<br /><br /><br />I am not sure what your asking with regard to the payload bay toward the earth attitude...what I can say is there are several orital attitude modes such as "free drift" which essentially does not hold any set attitude..."gravity gradient", where gravity is used to maintain a stabilized attitude without the use of thrusters..."thermal control" or barbecue where the orbiter is rolled to maintain equal heating..."LVLH" (local vertical local horizontal) where the same attitude relative to the ground is maintained thru-out the orbit...and various other modes.<br /><br />As for the deorbit burn this is usually a heads down, retro-grade (i.e. tail forward) burn. Once the burn is complete the crew will enter an "item 27 execute" which is a computer keyboard sequence that tells the flight control system to maneuver to entry attitude by firing the RCS (reaction control system) thrusters. More specifically since entry interface is still some 20 minutes away the orbiter will move to a wings level, nearly zero yaw attitude while the orbiter slowly drifts in pitch to the point where it is 40 degrees nose up at the time of entry interface. Once in entry attitude the RCS jets will fire to maintain attitude within certain limits called deadbands. While pr
 
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bobw

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Sorry. I meant to type the CANCELATION of the 1 rev/orbit rotation prior to the deorbit burn.<br /><br />If I understand correctly, the APU thrust is very small but in the right direction.<br /><br />Thanks so much.<br /><br />I don't know the technical name for the mode, but it seems like the orbiter spends a lot of time with the doors toward the earth, probably LVLH, e.g. when the orbiter is in range of TDRS and tramsmits pictures of earth from a payload bay camera. In order to take these pictures the orbiter must rotate 1 rev/orbit relative to its center of gravity. <br /><br />I was wondering, I guess, if the orbiter just rotates at that rate or if it takes a lot of RCS help to keep it in that position. <div class="Discussion_UserSignature"> </div>
 
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vogon13

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The APU thrust has to be very small.<br /><br />The hydrazine fuel is decomposed on a platinum (IIRC) catalyst and the resulting reactants spin a small turbine wheel at up to 70,000 RPM. The efficiency of the device is related to the delta pressure across the turbine. <br /><br />Ideally, (not too possible in the real world) all the energy in the fuel would be imparted to the turbine wheel and the exhaust would dissipate at zero velocity.<br /><br />Not too handy for pitching 100 tons of orbiter.<br /><br />In the real world, there is some back pressure on the turbine wheel, and the exhaust works as advertised.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>
 
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bobw

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It has to be a lot more than an ion engine. Three pounds/minute at 1000 degrees F. plus cooling water. A 135 horsepower hydraulic pump is a pretty substantial unit and the orbiter has three. I have watched the plumes after landing, on TV, many times. Like a car exhaust, I guess. You are right that it probably doesn't do much to rotate 100 tons.<br /><br />TZERO wrote that burn to entry interface is about 20 minutes. I never paid attention but my guess was about twice that long so no way the orbiter would just line up by itself. I was way wrong about that.<br /><br />Pointing the exhaust up at the tail is way better than pointing it two ways to cancel. It must save some RCS propellent. I remember when I would have typed "fuel". I love this place <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>
 
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bobw

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I bet the equivalent thrust would really mess up docking. Heck, I remember on the radar mapping mission that long boom had a thruster that malfunctioned. What was that? The weight of a piece of paper; like an ion engine? The commander had the crew on the exercise bike to save fuel; so every little bit does count.<br /><br />Does anybody care to hazard a guess to compare the thrust of the APU exhausts to the leaks on Apollo 13? <div class="Discussion_UserSignature"> </div>
 
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bobw

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Maybe I watch too many movies but it said they had a lot of thruster activity and were always close to gimbal lock while the tanks were venting; wicked shimmy. They were not out of control like the Gemini 8 on skylab but I thought they were getting pushed around. <div class="Discussion_UserSignature"> </div>
 
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davf

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Perhaps a 747. Point of trivia: I don't believe Concorde has an APU due to weight/space saving issues... the two inboard engines are started with start-carts while on the gate prior to pushback.
 
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davf

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I thought the main contributer there was the fact that they were trying to orient the vehicle using the LM thrusters which were way off the c of g.
 
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CalliArcale

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That was mostly after the SM was done venting on Apollo 13. While the SM was venting, they did indeed have a serious control problem. I was just reading Lovell's description of the event and it did not sound very fun. Remember, they were supposed to be in a "barbecue" mode, so the loss of control didn't just mean their trajectory adjustments would be off; it meant they would die if it couldn't be corrected soon. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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drwayne

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Interesting Side Note:<br /><br />I can't recall where I read it, but the problem of using the LM attitude jets was not unconsidered when Apollo 13 came about.<br /><br />Thre were programs in the works to use the LM with attachments where the command service module would dock (a big telescope leaps to my memory among other things). So some software had been written to deal with the issue of using the LM jets to control the LM with a mass similar to the command service module hanging out there.<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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davf

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I wonder if they had started development of an APU and then had to drop it later on in Concorde's development cycle?<br /><br />
 
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