The ONLY way you can get the 1284 shuttle to NOT arc-over in flight is via using old C5-3's, VERY old 18mm Cox D8-3's, or AeroTech 18mm D10's, D13's, D21's, and D24's.

On the TURD-O thrust-profile Estes C6-3 it ALWAYS "arcs-over" even in ZERO wind.

ALMOST the same flight profile exists for the Estes/Semroc Mars Lander kits.

Both the 1284 Space Shuttle and Mars Lander should have featured 24mm mounts as standard.
I have seen both "converted" and they fly much better on C11's and D12's with the proper nose weight added.

Estes should have NEVER dropped the C5-3 (and B8) motors from production.
There is NOTHING the C6 and B6 Estes motors can do that the C5 and B8 motors do not do BETTER.
The Estes B6-6 should have been discontinued instead of the B4-6 upperstage motor.

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !