How fast can rapiers go, anyway?

[Laie]

Not going to make this a challenge because I expect the answer is reasonably well-known: how fast can you go on rapiers in air-breathing mode?

Right now I'm flying at 24km and the engines will flame out at ~1740m/s. I wonder if I could go much faster flying lower, but as that would require a different plane.... I thought I'd just ask.

[AHHans]

The wiki entry says: "Top speed is ~2,100 m/s in Kerbins atmosphere (mach 6.2)"

[IronMaiden]

Its speed is limited by the speed of sound, flaming out just over mach 6. So the lower you are in the atmosphere, the faster the speed of sound is and the faster you can go before the engine flames out. This will require some aero and thermodynamic loveery but you can go over 2000m/s at sea level.

[Laie]

47 minutes ago, AHHans said:

The wiki entry says: "Top speed is ~2,100 m/s in Kerbins atmosphere (mach 6.2)"

Wha.... hold on. My screenshot says 1736m/s@24km is Mach5.75 ... *slaps forehead*  Well yes, speed of sound decreases with altitude.

12 minutes ago, IronMaiden said:

Its speed is limited by the speed of sound, flaming out just over mach 6.

*checks data file* The thrust curve is designed to hit zero at Mach 6, and on my flight it flames out even before. I wonder if that's because the "Engine stops under 33%" according to the part description. Whatever those 33% refer to, I don't know. In the config file, it's called "ignitionThreshold".

Edited December 19, 2019 by Laie

[IronMaiden]

I believe the 33% is referring to the propellant requirement, e.g. if your intake results in only 80% intake requirement being met it will still be operational albeit at less thrust, but if you keep climbing without accelerating that will continue to drop until the engine flames out at 33%. Usually it's 100% if you're going fast enough and have enough intakes, but I'm not sure what exactly all goes into the equation at high altitudes. However, you're right, my eyes must've been slow last time I was playing around with this, the RAPIER will flame out at mach 5.95 which is over 2100m/s at sea level, but I'm guessing I didn't notice until I had coasted up a bit higher, gaining altitude faster than losing speed resulting in >mach 6. If you really exploit the aerodynamic model you can get it up to over mach 6.5. Without gross exploitation mach 5.75 is just about all you're going to get out of RAPIERs in open cycle before drag=thrust. 

Edited December 20, 2019 by IronMaiden