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Throttling SRB's



 
 
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  #1  
Old September 17th 03, 02:46 PM
Alan Pretre
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Default Throttling SRB's

How are the SRB's throttled? I was reading the Rogers report and it says
this:

"Both the Shuttle main engines and the solid rockets operated at reduced
thrust approaching and passing through the area of maximum dynamic pressure
of 720 pounds per square foot. Main engines had been throttled up to 104
percent thrust and the Solid Rocket Boosters were increasing their thrust
when the first flickering flame appeared on the right Solid Rocket Booster
in the area of the aft field joint."

http://history.nasa.gov/rogersrep/v1ch3.htm

TIA.

-- Alan


  #2  
Old September 17th 03, 03:15 PM
John Maxson
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Default Throttling SRB's

Alan Pretre no@spam wrote in message
...

How are the SRB's throttled? I was reading the Rogers
report and it says this:

"Both the Shuttle main engines and the solid rockets operated at reduced
thrust approaching and passing through the area of maximum dynamic

pressure
of 720 pounds per square foot. Main engines had been throttled up to 104
percent thrust and the Solid Rocket Boosters were increasing their thrust
when the first flickering flame appeared on the right Solid Rocket Booster
in the area of the aft field joint."


Via innovative engineering design, i.e., by molding the
shape of the solid fuel for different flight regimes.

--
John Thomas Maxson, Retired Engineer (Aerospace)
Author, The Betrayal of Mission 51-L (www.mission51l.com)


  #3  
Old September 17th 03, 03:17 PM
Ian Stirling
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Default Throttling SRB's

Alan Pretre no@spam wrote:
How are the SRB's throttled? I was reading the Rogers report and it says
this:

"Both the Shuttle main engines and the solid rockets operated at reduced
thrust approaching and passing through the area of maximum dynamic pressure
of 720 pounds per square foot. Main engines had been throttled up to 104
percent thrust and the Solid Rocket Boosters were increasing their thrust
when the first flickering flame appeared on the right Solid Rocket Booster
in the area of the aft field joint."


Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
If God hadn't intended us to eat animals,
He wouldn't have made them out of MEAT! - John Cleese
  #4  
Old September 17th 03, 03:25 PM
Doug Ellison
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Default Throttling SRB's


"Alan Pretre" no@spam wrote in message
...
How are the SRB's throttled?


It's not something actually done in real time, it's the construction of the
SRB themselves that, I presume, have less propellant at that stage of
burning than at other times

Doug


  #5  
Old September 17th 03, 03:26 PM
Alan Pretre
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Default Throttling SRB's

"Ian Stirling" wrote in message
...
How are the SRB's throttled?


Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.


Thanks. I thought perhaps they diluted the fuel in places with nonfuel
matter. Does the burning in the cylindrical hole happen all along the
length of the SRB or does it work its way from one end to the other?

-- Alan



  #6  
Old September 17th 03, 03:38 PM
Alan Pretre
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Default Throttling SRB's

"Doug Ellison" wrote in message
...

"Alan Pretre" no@spam wrote in message
...
How are the SRB's throttled?


It's not something actually done in real time, it's the construction of

the
SRB themselves that, I presume, have less propellant at that stage of
burning than at other times


Thanks. I'm wondering then, that if you had a SME failure very early in the
launch sequence but after SRB ignition (ie can't abort launch yet), that the
shuttle would be at a lower altitude than it should be at a given T+x
instant. Is it possible, with the SRB's thrust profile timed for 3 working
engines, can the the SRB profile get out of whack, that is, for the boosters
to get out of sync with respect to where the shuttle is in max Q? Or
perhaps it is not possible for the shuttle to reach max Q with an engine
out, even with the SRB's at full thrust.

-- Alan


  #7  
Old September 17th 03, 08:56 PM
Ian Stirling
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Default Throttling SRB's

Alan Pretre no@spam wrote:
"Ian Stirling" wrote in message
...
How are the SRB's throttled?


Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.


Thanks. I thought perhaps they diluted the fuel in places with nonfuel
matter. Does the burning in the cylindrical hole happen all along the
length of the SRB or does it work its way from one end to the other?


The SRB burns from the inside out.
One advantage of this method is that the fuel insulates the casing until
the last of it burns, so you only have to have insulation on the inside
that can cope with the very end of the burn and any asymmetries in burning,
rather than that which can cope with (at the nozzle end) several minutes.

There is a complex "starter", which IIRC is basically a small rocket
engine in the nose of the SRB pointing down the core, engineered for
utter reliability.

You can have combinations, where the core burns outwards from the middle
but only half-way up the rocket and then transitions to end burning, so
that you get a large burst of thrust at the beginning then a long sustained
burn.

(also, the propellant does not burn fast enough to allow a "end burner" of
the SRBs size to generate enough thrust.)

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
"The theory of everything falls out trivially." -- Etherman, sci.physics kook.
  #8  
Old September 17th 03, 10:41 PM
Richard Cochran
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Default Throttling SRB's

"Alan Pretre" no@spam wrote in message ...
"Ian Stirling" wrote in message
...
How are the SRB's throttled?


Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.


Thanks. I thought perhaps they diluted the fuel in places with nonfuel
matter. Does the burning in the cylindrical hole happen all along the
length of the SRB or does it work its way from one end to the other?


The try and make the fuel as uniform as they can, and go to fairly
significant effort to ensure uniformity -- no part of the fuel is
intentionally "diluted" or otherwise weaker than any other part.
The burning is essentially simultaneous along the entire length of
the booster (I believe the igniters are at the top, IIRC). It
burns from the center toward the outside casing. So all the
control of the thrust profile is done by the geometry of
the initial hole down the middle.

--Rich
  #9  
Old September 17th 03, 11:40 PM
Ian Stirling
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Default Throttling SRB's

Richard Cochran wrote:
"Alan Pretre" no@spam wrote in message ...
"Ian Stirling" wrote in message
...
How are the SRB's throttled?

Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.

snip
burns from the center toward the outside casing. So all the
control of the thrust profile is done by the geometry of
the initial hole down the middle.


There are however designs of solid rocket that can vary the thrust
in other ways.

Burning rate typically depends on pressure. Exactly how much depends on which
propellant.
The nozzle throat can be designed to erode, and reduce the pressure in
the casing over that of an uneroding nozzle.

You can also open large vents, and practically shut down the burn.
Depending on what sort of propellant you are using, it might even stop burning
altogether.
In theory if you could reclose the vent, you could then restart the motor.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
The fight between good and evil, an epic battle. Darth vader and Luke,
suddenly in the middle of the fight, Darth pulls Luke to him, and whispers
"I know what you'r getting for christmas!" Luke exclaims "But how ??!?"
"It's true Luke, I know what you'r getting for christmas" Luke tries to ignore
this, but wrenches himself free, yelling "How could you know this?",
Vader replies "I felt your presents" -- The Chris Evans breakfast show ca. 94
  #10  
Old September 17th 03, 11:52 PM
Julian Bordas
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Posts: n/a
Default Throttling SRB's

Alan Pretre wrote:

"Ian Stirling" wrote in message
...

How are the SRB's throttled?


Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.



Thanks. I thought perhaps they diluted the fuel in places with nonfuel
matter. Does the burning in the cylindrical hole happen all along the
length of the SRB or does it work its way from one end to the other?
Along the entire length. At the same time. As the propellant burns, the

interior radius and therefore the exposed surface increases therefore
the thrust increases. BTW as the propellant is burned the mass of the
SRB decreases so accelleration goes up

Julian

 




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