Perforations in solid propellant to reduce output
 

So I was doing a little reading online about MaxQ and I read something on Wikipedia that drew my interest and want to pose a question here, if anyone wants to take it on.

This is what I read about Max Q as it relates to the Space Shuttle.

"During a normal Space Shuttle launch, for example, max Q occurred at an altitude of approximately 11 km (35,000 ft).[1] The three Space Shuttle Main Engines are throttled back to about 70% of their rated thrust as the dynamic pressure approaches max Q;[2] combined with the propellant perforation design of the solid rocket boosters, which reduces the thrust at max Q by 1/3 after 50 sec of burn, the total stresses on the vehicle are kept to a safe level."

Neat huh? This is my question. I can't picture in my mind how they engineered in perforations into the solid propellant to reduce output without creating small combustion chambers deep inside the solid rocket motor that would ordinarily lead to a CATO?

Then my next question of course is, did they engineer in a throttle up on the boosters back to 100% after max Q or did the boosters continue to burn at 1/3 reduced output?

Not that this new found knowledge will make me a better model rocket enthusiast. Just curious. How did they figured out that they could do that?

David

They do it with a star core at the top and a tapered core for most of the motor. The various tapers and star shapes were engineered to deliver that particular thrust curve. Here's one I did.



http://en.wikipedia.org/wiki/File:Srbthrust2.svg

Having a regressive thrust trace, like an Aerotech C-Slot motor, it not only lowers the aerodynamic drag as compared to the alternative, it also lowers the G forces as the rocket looses propellant mass, perhaps keeping acceleration on a Space Shuttle under 3.5 G's.

Tech Jerry

I believe the wording here, and use of the word " perforation " is indeed in reference to grain design, the CORE being referenced as ' Perforation' .

Core design - ie: burnout of the star shaped core into a cylindrical Bates type core WOULD significantly reduce thrust from that point onward.

A hole ( literal perforation ) in the grains would actually INCREASE surface area and hence thrust level.

I understand what the article means now. Thank you to both of you for the clarification. Much easier to visualize now.

David

The first pic is a drawing showing a cutaway view of the SRB.

The second pic is a cross-section of the propellant grain. The star pattern (called finocyl) increases the burn area surface from the standard hole in the center.

The last pic is the SRB thrust curves for STS-107 (last flight of shuttle Columbia). You can see the obvious drop off of thrust during Max-Q.

Greg

Cool Pictures and Graphs GregGleason. Thanks for sharing. :)

That first attached image is the one I was looking for. But I found the thrust curve, and even made one MYSELF!