Commercial Fixed Wing
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Confusion; Communication Breakdown
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Environment - Non Weather Related; Human Factors
We were assigned the AALLE 1 RNAV arrival into DEN.  We heard Denver Center clear us direct to FLAIL and to descend via the AALLE 1 arrival into Denver and that's what the First Officer read back to ATC without them questioning us.  He programmed that into the FMC and I confirmed it; then set 11;000 feet in the MCP for the lowest altitude.  A few minutes later ATC queried where we were going as he showed us off course and we should have been going to AALLE instead.  The First Officer re-programmed that into the FMC and I confirmed it.  11;000 was still a good lowest altitude and the flight path vector showed us not to our top of descent yet so we were on path.  I'm not sure ATC knows how to pronounce AALLE properly because the way he said it; it sounded like FLAIL.  Both AALLE and FLAIL sound very similar; especially when you are in a loud cockpit. The waypoint FLAIL needs to be changed on the AALLE 1 RNAV Arrival because AALLE and FLAIL sound way too similar.  Especially when you are in a loud cockpit.
Captain reported a navigation error due to similar sounding STAR names.