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Author
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Topic: SRB thrust reduction at SSME throttle-down
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taneal1
Member Posts: 230
From: Orlando, FL
Registered: Feb 2004
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posted 05-16-2006 12:22 AM
   
Can anyone explain to me exactly how the SRBs reduce thrust at SSME throttledown? The best answer I can find so far is that the propellant charge "is shaped" to provide a thrust reduction beginning at L+55 seconds. I probably have some of the facts wrong because I don't understand how this works. The SRB igniters are located at the top of the SRB. At ignition they shoot down the flame tunnel (the hollow inner core of the solid propellant) and the heat ignites the propellant along the full-length of the SRB. The propellant then burns from the inside-outward, along the full length of the flame tunnel. As the propellant burns, the opening in the center grows in diameter until it reaches the inside of the SRB casing and thrust ceases. I've found sources that state the propellant is ignited at the top and burns to the bottom. This would make it easy to create a thrust reduction if true by providing a ring of propellant that produces less thrust. Does anyone know if either of the above is correct? I'm guessing that the *shape* of the hole in the center of the propellant is what they are referring to. A circular hole would produce less thrust than a hole shaped like an asterisk, for example, as the circular hole has less surface area to burn. The only other way that I can think of would be to put less potent propellant in a ring-shape the full-length of the SRB, but I've not found any mention of this. As far as I know they mix the propellant with the binder (which produces the pencil eraser appearance of the solid fuel) to produce a homogenous propellant to fill each segment. They are then connected by factory joints and shipped to KSC for final assembly by bolting the field joints together. Any thoughts? Tom [This message has been edited by taneal1 (edited May 16, 2006).] |
KSCartist
Member Posts: 2896
From: Titusville, FL USA
Registered: Feb 2005
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posted 05-16-2006 06:55 AM
Hi Tom-Welcome back, haven't seen you post in a while.
You're correct about the shape of the hole. I saw a display of the SRB at KSC and at the top the shape is a "star" or "asterisk" for the very reason you stated.
Hope it helps. Tim |
nasamad
Member Posts: 2121
From: Essex, UK
Registered: Jul 2001
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posted 05-16-2006 10:51 AM
Hi Tom, The propellant is distributed as you say in a star shape, but there are also differences in the wall thickness of the propellant along the length of the SRB. The propellant is thicker at the forward end of the SRB than at the aft end, so the propellant will run out at the aft end first, and so reduce thrust. Imagine standing a cone inside a cylinder, then fill the space around the cone with clay. Once the cone is removed, what is left is an exaggerated version of the inside of an SRB. Hope this helps...Adam |
mjanovec
Member Posts: 3811
From: Midwest, USA
Registered: Jul 2005
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posted 05-16-2006 11:39 AM
Do the SRBs reduce thrust throughout the flight? Or do they temporarily reduce thrust when the Shuttle goes through max Q and then resume "full" thrust. If I recall correctly from launch footage, the announcer will explain that the SRB is coming back to full thrust (along with the SSMEs) after max Q. Or maybe I misheard what they were saying.If so, I would have to think that a portion of the propellant would have to be composed of material that produces lesser thrust. Otherwise, the cone shaped interior would only explain reduction of thrust throughout the flight...but not a resumption of full thrust after max Q. Or am I misunderstanding the whole concept here. Maybe I need a diagram to figure this out.  |
nasamad
Member Posts: 2121
From: Essex, UK
Registered: Jul 2001
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posted 05-16-2006 01:27 PM
You are right Mjanovec, from 50 to about 75 seconds the thrust does climb again ! I understood the lowering of thrust to be caused by both the shape of the cone and the change in propellent shape, from star to circle as it burns making less surface area. I thought the increase in speed after Max Q was due to the decreased weight of the stack as more fuel was burnt from the SRB's and the ET, but there is a diagram on Wikipedia that shows that there is an increase of thrust. Maybe some of our more knowledgeable members will be able to explain how the SRB thrust is increased, as I haven't found any data to explain it yet. Adam [This message has been edited by nasamad (edited May 16, 2006).] |
spaceheaded
Member Posts: 147
From: MD
Registered: Feb 2003
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posted 05-16-2006 04:14 PM
quote:
Originally posted by nasamad:
I understood the lowering of thrust to be caused by both the shape of the cone and the change in propellent shape, from star to circle as it burns making less surface area.
Adam
...and then the diameter of that circle gradually increases as the SRB heads toward burnout, thereby involving more and more surface area, hence more thrust. A pure guess. Bill |
mjanovec
Member Posts: 3811
From: Midwest, USA
Registered: Jul 2005
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posted 05-16-2006 04:17 PM
quote:
Originally posted by spaceheaded:
...and then the diameter of that circle gradually increases as the SRB heads toward burnout, thereby involving more and more surface area, hence more thrust.A pure guess.
Sounds plausible to me! |
nasamad
Member Posts: 2121
From: Essex, UK
Registered: Jul 2001
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posted 05-16-2006 05:29 PM
Sounds good to me as well. But what would be the effect of length of the cone decreasing as the propellant burnt away, would it counteract the increase in diameter ? Adam |
mjanovec
Member Posts: 3811
From: Midwest, USA
Registered: Jul 2005
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posted 05-16-2006 05:53 PM
If the cone was at a gradual enough angle, I would imagine that you wouldn't see much thrust reduction until the point shortly before SRB separation, where the cone started to shorten. I imagine the SRBs need to reduce thrust significantly before separation or else they may outrun the Shuttle...instead of the Shuttle outrunning the boosters. |
nasamad
Member Posts: 2121
From: Essex, UK
Registered: Jul 2001
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posted 05-17-2006 05:24 PM
Mjanovic, I have looked at the cutaway diagrams for the SRB's and the taper of the cone is a very shallow angle, so you are right. I doubt there would be very little effect of the cone truncating until just before burnout. I wonder if there are different cone profiles for different flight profiles ? Adam |
katabatic
Member Posts: 72
From: Oak Hill, VA, USA
Registered: Jun 2005
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posted 05-19-2006 06:08 AM
The following explanation is from T.R. Heppenheimer's "Development of the Space Shuttle 1972-1981," p. 176:"The forward portion of each propellant charge was cast with a central perforation in the form of an eleven-point star, with the star's points being long and narrow. This allowed the thrust to vary as planned. The star initially exposed a large burning surface area, for peak thrust after liftoff. The burn spread from the points, widening them and reducing this burning surface. This lessened the thrust and hence the buildup in flight velocity, to prevent overstressing the vehicle through excessive aerodynamic pressure. the SSME's also helped by throttling back to 65 percent of rated power, to further diminish this pressure. Then, ascending amid thinner atmoshpere, the shuttle was free to accelerate anew. The star was completely consumed at fifty-two seconds, leaving a cylindrical perforation that would widen, for a useful increase in thrust." |
nasamad
Member Posts: 2121
From: Essex, UK
Registered: Jul 2001
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posted 05-19-2006 09:33 AM
Cheers for that katabatic, I saw that book on ebay this week. I knew I should have bought it !Adam |
taneal1
Member Posts: 230
From: Orlando, FL
Registered: Feb 2004
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posted 05-20-2006 01:37 PM
quote:
Originally posted by katabatic:
The following explanation is from T.R. Heppenheimer's "Development of the Space Shuttle 1972-1981," p. 176:
Thanks, Katabatic! That explains SRB "throttledown" completely. Good old Heppenheimer... Thanks to Adam and all the others who replied. Hi Tim, thanks for the welcome back. Does everyone agree that following ignition the propellant is actually burning the entire length of the SRB? When the star-shape has been consumed the thrust would begin to increase as the diameter of the central "hole" increased. The thrust would continue to increase until Max thrust is achieved at burnout as the "hole" reached max diameter, i.e. the interior diameter of the SRB steel casing. Thanks again to everyone for the responses! Tom
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mjanovec
Member Posts: 3811
From: Midwest, USA
Registered: Jul 2005
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posted 05-20-2006 03:22 PM
quote:
Originally posted by taneal1:
Does everyone agree that following ignition the propellant is actually burning the entire length of the SRB? When the star-shape has been consumed the thrust would begin to increase as the diameter of the central "hole" increased. The thrust would continue to increase until Max thrust is achieved at burnout as the "hole" reached max diameter, i.e. the interior diameter of the SRB steel casing.
Tom - Not quite from how I understand it now. Here is what I think based on what this topic has shown us so far: At ignition, the entire length is ignited. The star shape of the propellant has a higher surface area than a simple circular shape. Therefore a greater area is burning and high thrust is produced at liftoff. Once the star shape has been consumed, you're left with a roughly circular shape (but still with a somewhat narrow diamter). At that point, thrust decreases because there is less surface area burning. As the burning expands the diameter of the circular shape, the area increases thus gradually increasing thrust again (timed to occur after max Q). Thrust continues to increase until the lower end of the propellant is burned out to the edge of the casing, due to the gradual cone shaped of the interior diameter. This occurs shortly before burnout. The remaining area if propellant rapidly decreases as the cone shortens so that near seperation, there is only propellant burning in the upper part of the SRB. This decreased thrust at the point of SRB sep which allows the Shuttle orbiter and main ET to ourrun the boosters. Without the decrease of thrust from the cone shaping of the propellant, the SRB would go from max thrust to nothing in a very rapid fashion, making timing of the SRB sep difficult (especially if one SRB burned at full thrust a fraction of a second longer than the other). Someone correct me if I got any of that wrong. |
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