rcpilz@ablnc.ATT.COM (Robert C. Pilz) (10/15/87)
One thing about the crash has been haunting me. When I saw the replays of the explosion, I heard the commands to the crew to be at 105% throttle. They acknowledged that. Then they were told to go to 80% (or so). They acknowledged that. Then they were told (very quickly) to go back to 105%. At this point, the explosion happened. This was never discussed in the national news. At least I never picked up on it. What were they (the flight people) trying to do? I have a strong feeling that they knew something was wrong at that point and were trying to do something? Was the extra boost supposed to put out the flame? Use up the fuel? Could that have made things worse? Nothing was said. I have not had a chance to read any of the detailed reports. Is any of this covered/explained in them? I'm not trying to find someone else to blame, I'm curious to what extent people in those last few seconds had any ideas of getting them out of danger. I'm not trying to open new wounds. But areas like this need to be addressed beyond fixing the boosters. There need to be better plans to abort the mission. ....ihnp4!ablnc!rcpilz
asgard@cpro.UUCP (J.R. Stoner) (10/16/87)
in article <340@ablnc.ATT.COM>, rcpilz@ablnc.ATT.COM (Robert C. Pilz) says: > One thing about the crash has been haunting me. When I saw > the replays of the explosion, I heard the commands to the > crew to be at 105% throttle. They acknowledged that. Then they > were told to go to 80% (or so). They acknowledged that. Then > they were told (very quickly) to go back to 105%. At this point, > the explosion happened. > This was never discussed in the national news. At least I never > picked up on it. What were they (the flight people) trying to > do? I have a strong feeling that they knew something was wrong > at that point and were trying to do something? > ....ihnp4!ablnc!rcpilz [Henry et al - correct me if I am very wrong] It does not have anything to do with the particular flight. You will note from programmed flight profiles that a mid-launch throttle-down followed by a throttle-up is normal procedure. The reason is that a layer of the upper atmosphere (stratosphere?) is a region of particularly violent aerodynamic pressures on the structures (where the jet stream resides? Enquiring minds want to know :-) So the engines are throttled down to minimize vibrations on the orbiter so that velocity is still high enough to allow continuing upward motion but not enough to allow orbital insertion. After this violent atmosphere is passed the engines are throttled up to orbital insertion level. All this occurs before SRB sep or MECO. ...ihnp4!hplabs!ptsfa!cpro!asgard. -- "To prevent having to tell fools to RTFM don't let on you WTFM to begin with." J.R. Stoner asgard@cpro.UUCP asgard@wotan.UUCP P.S. I help CompuPro make computers. They do not help me make my opinions.
jhb@athena.TEK.COM (John Burgess) (10/16/87)
In article <340@ablnc.ATT.COM> rcpilz@ablnc.ATT.COM (Robert C. Pilz) writes: > >One thing about the crash has been haunting me. When I saw >the replays of the explosion, I heard the commands to the >crew to be at 105% throttle. They acknowledged that. Then they >were told to go to 80% (or so). They acknowledged that. Then >they were told (very quickly) to go back to 105%. At this point, >the explosion happened. Depending on what you mean by "very quickly", it sounds quite routine. The way I've heard it is this: 1) It is possible, using full throttle, to exceed the aerodynamics of the system early on in the flight. They start at full throttle, then they reduce power when the speed gets "real high", finally, as the air thins out more, they can go back to full power. 1a) So how do they reduce throttle of the solid boosters? The boosters are built with concentric circular layers of propellant so, when they are launched, there is a hole down the middle of the propellant. This burns from the inside out at a known rate. During manufacturing some of the layers have inhibitors built in so that, at the right time, the burn rate decreases and power goes down. 2) The earlier shuttle engines were'nt as powerful as the newer ones used recently. But in order to use a constant frame of reference, they still use the max power of the older engines as the starting point. So, it is not uncommon to hear "105%" throttle. And if you believe that ... As stated, this is what I've heard, and it does sound plausible enough to me that I'll beleive it and repeat it. -------- John Burgess; tektronix!athena!jhb CAE Systems Division, Tektronix, Inc Phone: 629-1150; Del Sta 92-822
karn@faline.bellcore.com (Phil R. Karn) (10/17/87)
During a normal launch the crew does little more than watch the displays and twiddle their thumbs; all engine control (gimbaling, throttling, etc) is completely under onboard computer control. Control of a launch vehicle is something that humans do very badly but computers do extremely well, so there's no practical alternative. The crew has the ability to take manual control, but that is little more than a last-ditch backup. Even aborts are done under computer control, at least as long as engines are firing. The "callouts" you hear during a launch are simply confirmations of events that happen automatically; no one is pushing any buttons. The reduction of engine throttle during the first minute of flight is programmed in to reduce the stresses on the launcher as it hits "Max-Q", the region of maximum aerodynamic pressure. Once altitude has increased sufficiently the throttle may again be increased because the less dense atmosphere no longer generates enough drag to worry about. The SRBs are also "throttled back" during this time, but this is not under control of the onboard computers; their thrust-vs-time curves are established by the way propellant is loaded into them. Phil
mike@ames.arpa (Mike Smithwick) (10/18/87)
In article <340@ablnc.ATT.COM> rcpilz@ablnc.ATT.COM (Robert C. Pilz) writes: > >One thing about the crash has been haunting me. When I saw >the replays of the explosion, I heard the commands to the >crew to be at 105% throttle. They acknowledged that. Then they >were told to go to 80% (or so). They acknowledged that. Then >they were told (very quickly) to go back to 105%. At this point, >the explosion happened. >This was never discussed in the national news. At least I never >picked up on it. What were they (the flight people) trying to >do? I have a strong feeling that they knew something was wrong >at that point and were trying to do something? Was the extra >boost supposed to put out the flame? Use up the fuel? Could >that have made things worse? >Nothing was said. I have not had a chance to read any of the >detailed reports. What you noticed is merely standard procedures for a shuttle launch. First, when you hear the engines running at "105%", that means that they are running at 105% of their originally intended performance, and not that the're pushing them beyond their limits ("Captain Kirk, the warp drive is gonna blow if we keep this up . . ."). They crank them suckers up to 100% or more at the start of the launch, then ease them back to 75 or 80% as they are going thru "Max-Q" or the region of maximum dynamic pressure, so as to ease loads. They will then go back to the 100%+ range at about T+ 65 seconds. With 51L, that was only 5 seconds or so before the explosion. Alot of the early speculation right after the accident by the newstypes, centered around the "throttle up" call, and what effects it might've had. This brings to mind, though, what Mission control might have done had they noticed the burn thru. Would they have notified the crew and risked that chance of them doing something stupid, or would they just keep quite and hope that they could ride it out. -- *** mike (powered by M&Ms) smithwick *** "ever felt like life was a game, and someone gave you the wrong instruction book?" [discalimer : nope, I don't work for NASA, I take full blame for my ideas]
osmigo@ut-ngp.UUCP (Ron Morgan) (10/18/87)
[105% throttle settings]
I'm not a Space Cadet, but a few questions keep throbbing:
1. Is it *necessary* to launch at 105% power? If not, it seems that such
settings create an unnecessary risk, sort of like routinely accelera-
ting your car with the pedal on the floor. On the other hand, if it
IS necessary, that suggests that the launch device was deliberately
"underdesigned." What's the story here?
2. How closely related is rate of burn-through (assuming it occurs, which it
did) to power level? If power levels had been set within the limits
*intended by the design* would it still have burnt through, perhaps
later in the launch path?
(R)eply at your own risk...
--
Ron Morgan ARPA: osmigo%ngp@sally.utexas.edu
University of Texas at Austin UUCP: {ihnp4,seismo,allegra,sally}!ngp!osmigo
Dept. of Speech Communication osmigo@ngp.UUCPallen@mmm.UUCP (Kurt Allen) (10/19/87)
In article <340@ablnc.ATT.COM> rcpilz@ablnc.ATT.COM (Robert C. Pilz) writes: > >I heard the commands to the >crew to be at 105% throttle. They acknowledged that. Then they >were told to go to 80% (or so). They acknowledged that. Then >they were told (very quickly) to go back to 105%. At this point, >the explosion happened. The space shuttle engines were originally rated for less power, but the max power rating was raised. I'm not sure if the increase in maximum power was an engineering modification, or if the original restrictions were relaxed, but the later flights of th shuttle typically used 100+ percent power. I believe the maximum was 109 % of the originally rated maximum power. Before the accident I had heard of plans to modify the engines to handle even greater thrust. So, to answer your question, the power up that occured was normal and scheduled. It was not an emergency action in reaction to the SRB burnthrough. -- Kurt W. Allen 3M Center ihnp4!mmm!allen
henry@utzoo.UUCP (Henry Spencer) (10/19/87)
> 1. Is it *necessary* to launch at 105% power? If not, it seems that such > settings create an unnecessary risk... Well, no, it's not *necessary*, it's just a matter of how much payload you want to get into orbit. Just off the top of my head, you could probably run the shuttle engines at 90% and still reach orbit... with a payload consisting of two toothbrushes and a comb. The engines would probably be more reliable and need less maintenance at 90%, too. Of course, you couldn't do much with the shuttle that way. Engines, like lightbulbs, generally do not have a single specific setting for which they are designed, in the sense that all other settings work poorly or not at all. It's a continuous tradeoff: the harder you push it, the more you get out of it, and the shorter its lifetime. Where you set the normal operating point is a judgement call, not a clear-cut and obvious triviality. The "100%" level of the shuttle engines is an arbitrary number, not a point beyond which reliability deteriorates sharply. NASA PR guano aside, there is no such thing as perfect safety: it's just a matter of how much risk you are willing to take for a given end result. How "unnecessary" the risk is depends on how much you care about the result. > 2. How closely related is rate of burn-through (assuming it occurs, which it > did) to power level? ... Since the burn-through was in the SRBs, not the main engines, there is not much correlation, especially given that the problem was a seal failure, not a normal phenomenon to which a "rate" could be assigned. -- "Mir" means "peace", as in | Henry Spencer @ U of Toronto Zoology "the war is over; we've won". | {allegra,ihnp4,decvax,utai}!utzoo!henry
djr@scdpyr.UUCP (10/21/87)
Since the throttle up call on the Challenger mission has become a hot topic for discusion I thought I would throw in my 2 cents worth. Not too long ago I did some research on the shuttle program. Two of my biggest sources were the Rogers commission report on the Space Shuttle Challenger accident and Jane's all the Worlds Aircraft 1986-87. According to my sources the Space Shuttle Main Engines (SSME) have a power range from 0-109%. The performance for these engines was based on the original design sepcifications. The engines used on STS-1 were different from the ones currently in use. The engines on Columbia could only go to 100%. Improvments in the design for later missions is the reason for the change in the performance ratings. Normal procedures are to ignite the SSME's 6 seconds prior to launch. At this point the SSMEs are at 100%. As soon as the Shuttle has cleared the tower the SSMEs are throttled up to 104%. When the Shuttle reaches MAX-Q the engines are reduced to 65%. MAX-Q is the period of maximum aerodynamic force on the Shuttle. MAX-Q occurs at Mach 1.(Gee, I wonder why? ;-)). After MAX-Q, the engines are throttled up to 104%. The engines are not used at anything above 104% even though they are capable of 109%. I can't recall the specific numbers right off of my head. If someone hadn't swiped NCAR's copy of "Jane's" I would run and look them up. If anybody is curious about the specifics I suppose I could find my notes. Hope this didn't add too much to the confusion. -- "Hey laser lips, your momma was a snow blower!" -- Number 5 Dave Rowland at NCAR Boulder, Colorado djr@scdpyr.UUCP
campbelr@hpisof0.HP.COM (Bob Campbell) (10/22/87)
SSME Specifications (Full power level)
Thrust:
At seas level 417,300 lbs
In vacuum 512,300 lbs
Pressures:
Hydrogen pump discharge 7,040 psia
Oxygen pump discharge 8,070 psia
Combustion pressure 3,260 psia
Flowrates:
Total 1,130 lb/s 22,557 gpm
Hydrogen 160 lb/s 16,436 gpm
Oxygen 970 lb/s 6,121 gpm
Power:
High Pressure Pumps
Hydrogen 77,310 hp
Oxygen 29,430 hp
Weight: 6,900 lbs
Length: 14 ft
Diameter: 8 ft
--------------------------------------------------------------------------
Bob Campbell Some times I wish that I could stop you from
Hewlett Packard talking, when I hear the silly things you say.
hplabs!hpda!campbelr - Elvis Costellohenry@utzoo.UUCP (Henry Spencer) (10/23/87)
> ... The engines are not used at anything above 104% > even though they are capable of 109%. Actually, before Challenger there were plans to start using 109% for heavy payloads. Those plans are now officially cancelled, although they could re-surface later if things go well and various minor engine improvements do indeed make the things more durable. -- PS/2: Yesterday's hardware today. | Henry Spencer @ U of Toronto Zoology OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry
stu@splut.UUCP (Stu Cobb) (10/24/87)
In article <340@ablnc.ATT.COM>, rcpilz@ablnc.ATT.COM (Robert C. Pilz) writes: > > One thing about the crash has been haunting me. When I saw > the replays of the explosion, I heard the commands to the > crew to be at 105% throttle. They acknowledged that. Then they > were told to go to 80% (or so). They acknowledged that. Then > they were told (very quickly) to go back to 105%. At this point, > the explosion happened. OK. There is sort of a relationship here, but you've got it backwards. There's no cover-up. Aerodynamic pressure on a booster is a function of speed and atmospheric density. As you launch, speed increases and density decreases. Obviously, there is a point at which aerodynamic pressure ("q-bar") is a maximum. This point is called "Max Q." For the Shuttle, this happens between 40 and 70 seconds after launch. During Max Q, the aero stresses on the vehicle are at a maximum. This is one of the design points for the vehicle. Lots of turbulence, vibration, etc. Because of the great aero stresses, the Shuttle flight planners do what they can to remove additional stresses. Among other things, they throttle the engines back during that period. This happens automatically; it's built into the flight software. In fact, the SRB's also "throttle back." That's done by proper design of the grain when the booster is poured. All this is thought out well in advance. Now then. The last call we heard to Challenger was "Go at Throttle-up." This meant, among other things, that Houston had seen all three engines throttle back up to 104% (I believe), after passing through the "thrust bucket" at Max Q. Nothing sinister about that. So why did I say they're related? It is thought by some that the O-ring, after partially opening at ignition, sealed itself again. During Max Q, Challenger passed through a particularly vicious wind shear. It is thought (again, by some) that some combination of this wind shear and the already intense vibration of Max Q opened the O-ring back up, causing the fatal leak. It took some ten to twenty seconds for the leak to destroy the vehicle, long enough for the shuttle to leave Max Q behind and bring the engines back up to full power. Thus, you heard the throttle-up call right before the explosion. They're both related to Max Q; they're not related to each other. > ... areas like this need to be addressed beyond fixing > the boosters. There need to be better plans to abort the mission. We're working on it. Unfortunately, you simply can't abort until you drop the solids. This is obviously not good, but there's no way to fix it without rebuilding the entire fleet. This would cost several billion dollars. Congress has decided (perhaps implicitly) that the lives of a few astronauts are not worth that kind of money. They may well be right -- I know the thing's not particularly safe (that much explosive never will be), but I'd ride it anyway... Stu
stu@splut.UUCP (Stu Cobb) (10/24/87)
In article <8796@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes: > Engines, like lightbulbs, generally do not have a single specific setting > for which they are designed, in the sense that all other settings work > poorly or not at all. Sorry, but I don't believe this is true. The Shuttle engine is rather rare in that it can be throttled over almost a 2:1 range. Most rocket engines are, in fact, designed to run at a particular setting. This is primarily because most rocket engines were designed before flexible control systems were available. They use pneumatic/hydraulic/fluidic controls (the pinnacle of late-50's technology) which are fussy about their set-points. This fact is what got the early OSCARs into space. They couldn't throttle down their boosters to match the weight of the primary payload; instead, they had to carry ballast. Don Stoner had the bright idea to carry interesting ballast, which led to the OSCARs. Stu
grandi@noao.arizona.edu (Steve Grandi) (10/24/87)
In article <8822@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes: >Actually, before Challenger there were plans to start using 109% for heavy >payloads. Those plans are now officially cancelled, although they could >re-surface later if things go well and various minor engine improvements >do indeed make the things more durable. This "downscoping" of the shuttle thrust is causing major problems for heavy payloads such as the Hubble Space Telescope. Rumor hath it that the current worst case projection (depending on the amount of solar activity during the upcoming solar maximum which affects orbital drag) gives a lifetime in orbit for HST after a 100% thrust launch of around 5 months! It has always been planned to "reboost" HST during subsequent shuttle missions, but no one quite thought it would have to be so soon! -- Steve Grandi, National Optical Astronomy Observatories, Tucson AZ, 602-325-9228 UUCP: {arizona,decvax,hao,ihnp4}!noao!grandi or uunet!noao.arizona.edu!grandi Internet: grandi@noao.arizona.edu SPAN/HEPNET: 5356::GRANDI or DRACO::GRANDI
weltyc@nysernic (Christopher A. Welty) (10/27/87)
In article <201@splut.UUCP> stu@splut.UUCP (Stu Cobb) writes: >In article <8796@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes: >> Engines, like lightbulbs, generally do not have a single specific setting >> for which they are designed, in the sense that all other settings work >> poorly or not at all. > >Sorry, but I don't believe this is true. The Shuttle engine is rather rare >in that it can be throttled over almost a 2:1 range. Most rocket engines >are, in fact, designed to run at a particular setting. This is primarily >because most rocket engines were designed before flexible control systems >were available. They use pneumatic/hydraulic/fluidic controls (the >pinnacle of late-50's technology) which are fussy about their set-points. > In fact, as I recall, the X-15 was the first vehicle with "throttleable" rocket engines. I saw a clip once of that famous guy...ummm...Scott Crossfield (something like that, I can't believe I don't remember...) testing the X-15 engines new throttle. It blew up - big explosion, too. Scotty was OK, walked away in fact. Turned out to be a faultly valve. Anyway, it ended up working. The camera view of the test was right in back, so you could see the rocket engine throttling up and down as he announced the current rate. They were able to take it from 0% up to 100%. I believe the shuttle *main* engines are based on the same idea. Of course the SRBs are not dynamically "throttleable", although I believe you can preset a certain thrust pattern by playing with the solid fuel. Anyway, this was not Henry's point. He is correct in that certain thrust settings are very inefficient, and that most engines are built for optimal performance at a specific thrust. For the Enterprise, I believe this was Warp 6 (although it is arguable that *warp* drive generates any thrust). Christopher Welty --- Asst. Director, RPI CS Labs weltyc@cs.rpi.edu ...!rutgers!nysernic!weltyc
mike@thumperbellcore.com (Michael Caplinger) (10/27/87)
I remember vividly from my Caltech days two specific missions that *had* to use 109% thrust because of their mass -- Galileo and the Hubble Space Telescope. The former may have gotten lighter, but the HST is just as heavy as it ever was. It scared me even in 1981, and it scares me even more now. Mike Caplinger mike@bellcore.com
philb@maya.gwd.tek.com (Phil Biehl) (10/27/87)
In article <5270001@hpisof0.HP.COM> campbelr@hpisof0.HP.COM (Bob Campbell) writes: >SSME Specifications (Full power level) . . >Flowrates: > Total 1,130 lb/s 22,557 gpm > Hydrogen 160 lb/s 16,436 gpm > Oxygen 970 lb/s 6,121 gpm Does the above flowrates indicate tha actual combustion mixture present at the combustion chamber? If so then why is there a greater ratio than 2 parts H to one part O? I would think that the extra hydrogen would be unconsumed making the engine less efficient than it could be... What am I missing? Phil Biehl usenet: ...!{decvax,hplabs,zehntel,reed,uw-beaver}!tektronix!orca!philb arpanet: philb%orca%tektronix@csnet-relay.arpa csnet: philb%orca@tektronix.csnet work ph: (503)685-2122 mail: Tektronix Inc., IDG MS 61-028, POB 1000, Wilsonville OR 97070
henry@utzoo.UUCP (Henry Spencer) (10/29/87)
> ... I don't believe this is true. The Shuttle engine is rather rare > in that it can be throttled over almost a 2:1 range. Most rocket engines > are, in fact, designed to run at a particular setting... However, modest changes in the setting are generally feasible if thought about and done ahead of time. The engine control systems may well need alterations to get the setting you are after, but the guts of the engine are usually happy over a moderate range of thrust settings. Case in point: the Chinese run their Long March engines at about 80% of their "rated" thrust, for greater reliability. -- PS/2: Yesterday's hardware today. | Henry Spencer @ U of Toronto Zoology OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry
campbelr@hpisof0.UUCP (10/29/87)
>>Flowrates: >> Total 1,130 lb/s 22,557 gpm >> Hydrogen 160 lb/s 16,436 gpm >> Oxygen 970 lb/s 6,121 gpm > >Does the above flowrates indicate tha actual combustion mixture present at the >combustion chamber? If so then why is there a greater ratio than 2 parts H to >one part O? I would think that the extra hydrogen would be unconsumed making >the engine less efficient than it could be... What am I missing? > >Phil Biehl While the temperature of the products will be lower because of the energy used to heat the excess hydrogen, the SSME is not the most efficient at a 2 to 1 mixture ratio. Not getting involved in discussions on units, the performance of a rocket is measured by its specific impulse. F Vexit I = --- ~ ------- s . g w . Where F is thrust, Is is specific impulse, w is the mass flow rate, and Vexit is the nozzle exit velocity of the gases. Trusting me that exit velocity is proportional to the square root of combustion temperature over the molecualr weight of the exhaust gas, by lowering combustion temperature you are also lowering the molecular weight (Water = 18, Hydrogen = 2, Both mixed < 18) and at some point is the ideal mixture ratio. If you really want the equation for Vexit, let me know. It is rather larger than the one above :-) Bob Campbell Some times I wish that I could stop you from Hewlett Packard talking, when I hear the silly things you say. hplabs!hpda!campbelr - Elvis Costello
jenks@uiucdcsp.UUCP (10/29/87)
> [...] why is there a greater ratio than 2 parts H to > one part O? I would think that the extra hydrogen would be unconsumed making > the engine less efficient than it could be... What am I missing? > Molecular weight? Sure, you need two H atoms per O atom, but O weighs in at ~16 and H is ~1. > > > Phil Biehl I know how it goes -- I got a C in Freshman Chemistry, too. ;} -- Ken Jenks, MS: Aero/Astro Engineering, BS: Computer Science, UIUC Looking for job in space. Help, anyone? jenks@p.cs.uiuc.edu {ihnp4!pur-ee}uiucdcs!uiucdcsp!jenks
johng@ecrcvax.UUCP (10/29/87)
In article <9298@tekecs.TEK.COM> philb@maya.UUCP (Phil Biehl) writes: >In article <5270001@hpisof0.HP.COM> campbelr@hpisof0.HP.COM (Bob Campbell) writes: >>SSME Specifications (Full power level) >. >. >>Flowrates: >> Total 1,130 lb/s 22,557 gpm >> Hydrogen 160 lb/s 16,436 gpm >> Oxygen 970 lb/s 6,121 gpm > >Does the above flowrates indicate tha actual combustion mixture present at the >combustion chamber? If so then why is there a greater ratio than 2 parts H to >one part O? I would think that the extra hydrogen would be unconsumed making >the engine less efficient than it could be... What am I missing? > >Phil Biehl True, some of the hydrogen goes unconsumed. There are two problems with the perfect 8:1 mass ratio: 1) The metals of the engine cannot handle the temperature of the reaction. So the hydrogen acts as a coolant. 2) *I'm not positive of this one* The temperature of the 8:1 reaction is enough to disassociate some of the water back into hydrogen and oxygen therefore *lowering* the efficiency. *Anybody know for sure?* John Gregor
henry@utzoo.UUCP (Henry Spencer) (10/30/87)
> ... why is there a greater ratio than 2 parts H to > one part O? I would think that the extra hydrogen would be unconsumed making > the engine less efficient than it could be... What am I missing? 1. The mixture has to be hydrogen-rich to protect the engine walls against attack by hot high-pressure oxygen. (For the same reason, the hydrogen and oxygen supplies would not run out at the same instant if the engines burned to fuel exhaustion [normally they shut down a bit before that] -- the hydrogen tank is deliberately filled a little fuller than it has to be, to make *sure* the oxygen runs out first no matter what.) 2. Adding hydrogen lowers the average molecular weight of the exhaust, so a modest excess of hydrogen can improve performance even though it does not add energy. 3. Adding hydrogen lowers the exhaust temperature, which reduces energy absorption by thermal dissociation of H2O. Again, in small doses this can be a net win. -- PS/2: Yesterday's hardware today. | Henry Spencer @ U of Toronto Zoology OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry
alastair@geovision.UUCP (Alastair Mayer) (11/04/87)
In article <9298@tekecs.TEK.COM> philb@maya.UUCP (Phil Biehl) writes: >In article <5270001@hpisof0.HP.COM> campbelr@hpisof0.HP.COM (Bob Campbell) writes: >>SSME Specifications (Full power level) >. >>Flowrates: >> Total 1,130 lb/s 22,557 gpm >> Hydrogen 160 lb/s 16,436 gpm >> Oxygen 970 lb/s 6,121 gpm > >Does the above flowrates indicate tha actual combustion mixture present at the >combustion chamber? If so then why is there a greater ratio than 2 parts H to >one part O? I would think that the extra hydrogen would be unconsumed making >the engine less efficient than it could be... What am I missing? Rocket engine "efficiency" is more than just optimizing the mix ratio of the fuel and oxidizer. It is also related to the exhaust velocity of the combustion products. Having the SSMEs run H2 rich means that the average molecular weight of the exhaust is less than that of straight H2O, hence for a given combustion temperature will reach a higher exhaust velocity. (If I had my textbook handy I could quote formulas at you). The *thrust* isn't as great - you need a relatively heavy exhaust for that, but the efficiency (in terms of specific impulse) is better. One way of looking at it is comparing the engine with, say, NERVA, which used a nuclear heat source to heat hydrogen propellant. MW of the exhaust was just 2 - very efficient, except that the thermal coupling of the reactor to propellant was poor. The SSME uses a chemical heat source to heat the hydrogen propellant, said source being the H2-O2 combustion. Thermal coupling is excellent! There are a couple of lesser reasons for running the engines H2-rich - you want to maintain the exhaust as a reducing gas rather than an oxidizing gas, and trying to run the engines right at the stoichiometric mix ratio it would probably oscillate between reducing and oxidizing with interesting effects on the materials in the combustion chamber and engine bell... -- Alastair JW Mayer BIX: al UUCP: ...!utzoo!dciem!nrcaer!cognos!geovision!alastair "What we really need is a good 5-cent/gram launch vehicle."