throttle rocket thrust

[gtnick]

Does anyone know how throttling thrust during launch effects the rockets ISP (aka specific impulse)?

 

[henryhallam]

In principle throttling down shouldn't affect Isp; in practice an engine will perform most efficiently at a particular thrust level so there might be a slight drop in Isp when the engine is throttled down (probably not much more than 5%ish).<br /><br />When the thrust is reduced gravity losses also increase since the vehicle is not accelerating "as quickly as possible". This is independent of Isp. But as the vehicle is travelling more slowly, drag losses are reduced so in some cases it's conceivable that you would save energy by throttling down, especially near Qmax.

 

[mlorrey]

This all depends on how the throttling is done. Since Isp really suffers if the exhaust pressure drops below ambient at the nozzle edge, and first stage engines are usually designed for low level work, if your throttling is accomplished by reducing both fuel and oxidizer, then you should not throttle to a point where nozzle exit pressure is less than ambient pressure.<br />However, another means of throttling a rocket engine is by changing the fuel/oxidizer mixture ratio: the farther the ratio gets from a stoichiometrically perfect ratio, you get less burning but the same volume or mass of material through, so the exhaust is less hot. This also hurts Isp, but I can't comment as to which method is better. <br /><br />As long as you throttle down when you are above the engine's design point altitude, and do not let exit pressure drop below ambient, then you should not lose Isp.

 

[vogon13]

SSME only throttles oxider, the fuel (LH2) remains constant. The LH2 is light enough that the weight penalty carrying it is not large. Also, the fuel rich mixture protects engine components from chemical attack from the O2.<br /><br />The Challenger Accident Report noted engine damage consistent with loss of fuel first, and subsequent O2 damage from the engine combustion cycle going fuel lean during the final seconds of powered flight.<br /><br />I don't know if anyone has ever evaluated an engine design with a variable area throat, or if the mechanical complexity would negate the small loss of Isp during throttling conditions.<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>

 

[mlorrey]

Rather that the seams between parts would leak gasses given the high chamber pressures. Jet engines can do variable choke nozzles because they operate at relatively low pressure ratios, which is why the nozzle exit is generally equal to or less than engine afterburner diameter. Bell nozzles can be a problem with this, but annular and linear aerospikes can do it well.<br /><br />I think the trick would be to come up with a material with high temperature resistance that also has a very high coefficient of expansion: as you shift the f/o ratio toward and away from stoichiometric, combustion temperature would change, and thus the diameter of the throat as the material expanded and contracted.

 

[propforce]

<font color="yellow">SSME only throttles oxider, the fuel (LH2) remains constant.</font><br /><br />You are talking about the preburner. The main combustion chamber stay within the mixture ratio (MR) of 6 +/- 1% and deviates only to 1.7% at 65% power level. So for all practical purposes, the chamber MR remains relatively constant.<br /><br /><font color="yellow">The Challenger Accident Report noted engine damage consistent with loss of fuel first, and subsequent O2 damage from the engine combustion cycle going fuel lean during the final seconds of powered flight. </font><br /><br />That's because the SRB burn through right next to the fuel tank instead of LOX tank. One should not take this as an indication on how the engine operates.<br /><br /><font color="yellow">I don't know if anyone has ever evaluated an engine design with a variable area throat, or if the mechanical complexity would negate the small loss of Isp during throttling conditions.</font><br /><br />IMO, for all practical purpose, the engine throttle for a reason during a flgiht trajectory, such as to reduce the max Q level. As such, it's a transient problem and mostly done during the first stage burn. To design an elaborate variable geometry configuration to take case of a problem that really does not exist, is not economically feasible. <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

<font color="yellow"><br />IMO, for all practical purpose, the engine throttle for a reason during a flgiht trajectory, such as to reduce the max Q level. As such, it's a transient problem and mostly done during the first stage burn. To design an elaborate variable geometry configuration to take case of a problem that really does not exist, is not economically feasible.<br /></font><br /><br />On the other hand, being able to throttle down while retaining good efficiency is very important for a lunar lander.

 

[drwayne]

Deep throttling was a big consideration in the Apollo decent module engine design.<br /><br /><img src="/images/icons/smile.gif" /><br /><br />Wayne<br /> <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[mlorrey]

Engines designed to operate in a vacuum can deep throttle very easily, simply because there is zero ambient pressure: your nozzle exhaust pressure could be measured in millibars and you'd still be operating at optimum Isp. There is no need of variable geometry engines in space.<br /><br />Using aerospike engines, however, is very helpful because, typically, one usually uses a number of thrusters arranged either in a ring, or in two rows. One can throttle down simply by shutting some of the thrusters off, or, if each thruster can throttle to about 75%, computer controlled throttling should enable completely variable throttling down to a few thrusters.<br /><br />For instance, an annular aerospike with 32 thrusters (8 per quadrant) where, lets say each thruster is 10,000 lb thrust, and can be throttled down to 7500 lb thrust. Such an engine can be uniformly throttled from 30,000 lb up to 320,000 lb., a range of 9.33-100%.<br /><br />Such an engine, while having a lot of parts due to the multiple thruster chambers, wouldn't be terribly complex primarily because control would be done by software, and parts duplication adds to safety because one or more thruster failure does not mean failure of the whole engine, thus allowing much greater capability for intact aborts of launch missions.

 

[propforce]

<font color="yellow">On the other hand, being able to throttle down while retaining good efficiency is very important for a lunar lander. </font><br /><br />Oh I see, you were talking about a lander engine <img src="/images/icons/smile.gif" /><br /><br />Actually efficiency is not really a "prime" consideration during the throttle down process. It's the ability to control thrust with fast feedback that is the prime consideration, that's why a hypergolic engine has advantage there. <div class="Discussion_UserSignature"> </div>

 

[Testing]

Atlas is also throttled by the LOX valve. I do not know the actual in flight %. We run full travel during vibe to verify feedback pot integrity. <div class="Discussion_UserSignature"> </div>

 

[propforce]

<font color="yellow">Atlas is also throttled by the LOX valve. I do not know the actual in flight %. We run full travel during vibe to verify feedback pot integrity. </font><br /><br />Cool. Thanks.<br /><br />Is that on the RD-180? <div class="Discussion_UserSignature"> </div>

 

[danwoodard]

>>It's the ability to control thrust with fast feedback that is the prime consideration, that's why a hypergolic engine has advantage there.<br /><br />LOX/Methane is being proposed for the CEV lunar lander. Would this also be easy to throttle quickly?

 

[mlorrey]

My experience working with LOX in the USAF trained me that LOX is pretty hypergolic with just about any petrochemical compound. For example, safety rules mandated you service LOX with unpolished combat boots, because the polish will literally detonate your boot if you dripped LOX on it, or so the safety boffins claimed. As my experience was more practical than scientific, is there anyone here who can provide more information about this?

 

[propforce]

Oh yeah, if you drop little dropplet of LOX on the ground it will literally explode. If you step on a pool of LOX on the ground, it may blow your foot off. I was told that in an actual case of LOX spillage in Santa Susana, helicopter was used to air-lift technicians out of the area instead of having them risking walking out.<br /><br />I personally have witness GOX explosion,, though not LOX but still powerful enough to leave you a very vivid impression. A ~200# GOX regulator simply disappeared along with 50 yds of stainless steel piping. <br /><br />Contrary to what most believe, the LH2 is relatively safe even in air as long as you don't have a spark source or any electro-static discharges. One time during testing at Sycamore Canyon, the vented LH2 went up the exhaust and came down as little rain. I'd figure how often does one get this experience, so I walked into a fine mist of LH2 rain <img src="/images/icons/laugh.gif" />. The little LH2 dropplets sting a bit on your skin but was quickly vaporized by the body heat. I came through unharm, though some would question my mental stability after having witnessed such an event. But hey, that's why I am a prop engineer !! <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> </div>

 

[propforce]

LOX compatibility is very strict in a launch vehicle.<br /><br />Every surfaces that LOX may touch will be cleaned according to a strict standard. Basically no oil films, or rubber elastomeric materials to come in contacts with LOX. The LOX tank made of aluminum will be inspected via a "black light" to ensure no leftover oil or grease films. All personnel will wear protective gloves to ensure no skin oil or grease get left behind. <div class="Discussion_UserSignature"> </div>