Shuttle question: SRB thrust confusion

[astrowikizhang]

Hello, I have a question about SRB thrust.<br />Many technical info, e.g. http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html state that each booster has a sea level thrust of approximately 3,300,000 pounds at launch. Two SRBs contribute to 71% thrust at liftoff and during first-stage ascent.<br /><br />The confusion is three SSMEs have a total thrust of nearly 1.2 million lb. But:<br /><br />(3.3*2)/(3.3*2+1.2)=85%<br /><br />Some reports say each SRB has a liftoff thrust of 2.8 million lb, Then:<br /><br />(2.8*2)/(2.8*2+1.2)=82%<br /><br />Where the number of 71% comes from? I believe it has its reason to exist and NASA would not made such a mistake. Please help me out of this confusion.<br /><br />More questions about thrusts: what is the difference between sea-level thrust, vaccum thrust, and peak thrust? Is the 3.3 million lb the vaccum thrust of SRB or peak thrust?<br /><br />Thanks,<br /><br />zhang

 

[edkyle98]

SRB thrust varies during its burn, from 3-3.3 million lbs at and shortly after liftoff to 2.5 million lbs at Max-Q before tailing off to about 2 million lbs shortly before burnout. Average thrust is 2.395 million lbs. Peak thrust is 3.3314 million lbs.<br /><br />SSME thrust also varies, increasing with altitude. Liftoff (sea-level) thrust at the 109% rating is 1.252 million lbs for all three SSMEs total. Vacuum thrust is 1.54 million lbs. The engines throttle back at Max-Q. <br /><br />71% of the shuttle's liftoff thrust is about 5.575 million lbs, or about 2.8 million lbs of thrust for each SRB. That sounds close to an average thrust number, or perhaps it is the raw initial thrust at T-0, but in order for it to be 71% of the total, the SSMEs must produce an extra 1 or so million lbs of thrust that they don't produce! My guess is that the 71% came about because someone, once upon a time, divided one number out of context (average SRB thrust) by another number (total liftoff thrust). <br /><br /> - Ed Kyle

 

[astrowikizhang]

Here is the SRB sea-level thrust graph found on wikipedia: <br /><br />http://en.wikipedia.org/wiki/Image:Srbthrust2.jpg <br /><br />Maybe the vaccum thrust does not decrease that much when the vehicle skyrockets.

 

[astrowikizhang]

"Both the SRBs ( through propellant grain shape,) and the SSMEs reduce their thrust during part of the boost. The 71.4% figure probably relates to the total energy each type imparts to the stack during their joint burn time?"<br /><br />That is my idea too. The 71% number gets some dispute on:<br />http://en.wikipedia.org/wiki/Talk:Space_Shuttle_program#SRB_thrust_percentage_at_takeoff<br /><br />I still believe NASA has its reason to use this number. Maybe a SSME thrust/time graph will be helpful.