apache segmentation fault but no core dumps

when I start /etc/httpd/httpd -k start it works fine. But when I try with apachectl start, I see lots of [httpd] defunct process that come and go and a bunch of messages in the logs:

[core:notice] [pid 15126:tid 139966078363520] AH00052: child pid 15517 exit signal Segmentation fault (11)

I tried adding CoreDumpDirectory /tmp/apache-cores in the httpd.conf but never got any core dumps. I'm using Apache 2.4.6.

Presuming you restarted Apache after you added CoreDumpDirectory directive, I guess you may be hitting a problem with SELinux stopping httpd from writing a core file.

Try to change context of the destination dir:

chcon -Rv -t httpd_sys_content_rw_t /tmp/apache-cores

or temporary disabling SELinux by changing policy from enforcing to permissive in /etc/selinux/config (this requires system restart, and I don't recommend you keeping it in permissive mode).