SRB Thrust Profile

I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in thrust
as fuel burns. So as the vehicle becomes lighter, thrust increases.

Why did they chose this pattern? Others are possible, so why is this
pattern used? Finally, does anyone know the variation between thrust
at launch and thrust just before burnout?

-Thanks

"Charles Talleyrand" wrote in
:

Finally, does anyone know the variation between thrust
at launch and thrust just before burnout?

Page 3-38 of the CAIB/NAIT Working Scenario has a graph of SRB thrust vs.
time. This document can be downloaded from:

http://www.caib.us/news/working_scenario/default.html


--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

In article ,
"Charles Talleyrand" wrote:

I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in thrust
as fuel burns. So as the vehicle becomes lighter, thrust increases.

Actually, as the vehicle becomes lighter, the thrust trends downward to
keep acceleration below a specified maximum. There are many graphs and
charts of the SRB thrust profile on the web. Google is your friend. ;-)

Why did they chose this pattern? Others are possible, so why is this
pattern used? Finally, does anyone know the variation between thrust
at launch and thrust just before burnout?

-Thanks


Again, the thrust profile was chosen to provide maximum thrust early in
the ascent when vehicle mass is greatest. This decreases over time to
keep the stack accelerations below that which the vehicle can tolerate.

--
Herb Schaltegger, B.S., J.D.
Reformed Aerospace Engineer
"Heisenberg might have been here."
~ Anonymous

Herb Schaltegger wrote in message ...
In article ,
"Charles Talleyrand" wrote:

I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in thrust
as fuel burns. So as the vehicle becomes lighter, thrust increases.



Again, the thrust profile was chosen to provide maximum thrust early in
the ascent when vehicle mass is greatest. This decreases over time to
keep the stack accelerations below that which the vehicle can tolerate.

That makes sense. But how is the thrust profile tailored? My
impression is that thrust profile adjustments are done by shaping the
fuel, and in this case the shape seems to imply an increase in thrust
(which is not what happens).

-Thanks

"Charles Talleyrand" wrote in message
om...
Herb Schaltegger wrote in message
...
In article ,
"Charles Talleyrand" wrote:

I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in
thrust
as fuel burns. So as the vehicle becomes lighter, thrust increases.



Again, the thrust profile was chosen to provide maximum thrust early in
the ascent when vehicle mass is greatest. This decreases over time to
keep the stack accelerations below that which the vehicle can tolerate.

That makes sense. But how is the thrust profile tailored? My
impression is that thrust profile adjustments are done by shaping the
fuel, and in this case the shape seems to imply an increase in thrust
(which is not what happens).

Not necessarily.

The initial shape is a star (11 points as I recall). This gives a lot of
surface area. As the points are burned away, the shape becomes more
cylindrical which has less surface area than the original star, hence
decreasing thrust. This is a grossly simplified version btw.



-Thanks

"Greg D. Moore (Strider)" wrote:
"Charles Talleyrand" wrote:
Herb Schaltegger wrote:
"Charles Talleyrand" wrote:

I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in
thrust
as fuel burns. So as the vehicle becomes lighter, thrust increases.



Again, the thrust profile was chosen to provide maximum thrust early
in
the ascent when vehicle mass is greatest. This decreases over time to
keep the stack accelerations below that which the vehicle can
tolerate.

That makes sense. But how is the thrust profile tailored? My
impression is that thrust profile adjustments are done by shaping the
fuel, and in this case the shape seems to imply an increase in thrust
(which is not what happens).

snip

Only the forward SRM segments contains the "eleven point star". The star
burns out at about 21-23 seconds and the overall SRM thrust then drops off
creating the SRB "thrust bucket." At around 52 seconds or so the thrust
increases again. This second thrust increase (about 25 seconds duration)
coincides with the geometry of the two center SRM segments. The center SRM
segments burn in an ever increasingly larger cylinder and also burn from the
bottom of the segment forward except where a thick rubber inhibitor protects
the outermost portion of each aft segment around the base of each segment.
The aft segment is a tapered cylinder. When the base of the taper burns out
the thrust of the SRB begins its final decrease (around 75-80 seconds.)
That is my understanding from my notes of a discussion with a NASA engineer
familiar with the thrust profile and a Thiokol presentation to the AIAA
which has been cited here recently.


--

Daniel
http://www.challengerdisaster.info
Mount Charleston, not Charleston, SC

Charleston wrote:

"Greg D. Moore (Strider)" wrote:
"Charles Talleyrand" wrote:
Herb Schaltegger wrote:
"Charles Talleyrand" wrote:

I understand that the main part of the SRBs are a hollow core of
fuel such that the SRBs burn from the interior towards the casing.
I also understand that this burn pattern produces an increase in
thrust
as fuel burns. So as the vehicle becomes lighter, thrust
increases.



Again, the thrust profile was chosen to provide maximum thrust early
in
the ascent when vehicle mass is greatest. This decreases over time
to keep the stack accelerations below that which the vehicle can
tolerate.

That makes sense. But how is the thrust profile tailored? My
impression is that thrust profile adjustments are done by shaping the
fuel, and in this case the shape seems to imply an increase in thrust
(which is not what happens).

snip

Only the forward SRM segments contains the "eleven point star". The star
burns out at about 21-23 seconds and the overall SRM thrust then drops off
creating the SRB "thrust bucket." At around 52 seconds or so the thrust
increases again. This second thrust increase (about 25 seconds duration)
coincides with the geometry of the two center SRM segments. The center
SRM segments burn in an ever increasingly larger cylinder and also burn
from the bottom of the segment forward except where a thick rubber
inhibitor protects the outermost portion of each aft segment around the
base of each segment.
The aft segment is a tapered cylinder. When the base of the taper burns
out the thrust of the SRB begins its final decrease (around 75-80
seconds.)

And, this is g-limiting in first stage. Much the same as the
g-limiting occurs in second stage by throttling the SSME down.

That is my understanding from my notes of a discussion with a
NASA engineer familiar with the thrust profile and a Thiokol presentation
to the AIAA which has been cited here recently.

A star shape molded in the propellant exposes greater surface area resulting in
higher thrust at launch. The star shape burns away. The resultant round shape
generates less thrust during "max-Q" so that the aerodynamic limits of the
structure are not exceeded.