How to determine best fuel efficency?

[Jestersage]

I am designing a plane with 3 engines. I have to possible configuration:

1. 1x Whiplash + 2x Panther
2. 3x Whiplash

If I want to create a plane that works for both low and high speed, would it be better with option 1 (switching to dry mode when needed), or option 2 and lower speed?

EDIT: per experimenting, In my build (which involves Mk3 parts), 3x Whiplash is much better. 1x Wheezy is not enough to generate what I need tbh.

Edited March 18, 2019 by Jestersage

[bewing]

Flying at low (subsonic) speed for extended periods of time is almost never useful. The only reason to use panthers in dry mode is if your plane has low-enough drag that those two engines will make it go supersonic anyway and get to 650 m/s, or if your trip is so short that the cost of climbing to 20km altitude is wasted.

Beyond that, I can't imagine any plane where this engine tradeoff is going to work. Either a single whiplash will be enough to get it to 1200 (or 1400) m/s, or 1 whiplash and 2 panthers is going to be so underpowered that it won't even be able to go supersonic.

So I think you're going to actually have to build it and try it out.

As far as "best fuel efficiency" goes, you get that when you are zooming along in the upper atmosphere at 3 times the speed of sound and your engines have flamed out because your altitude is so high.

 

 

 

Edited March 17, 2019 by bewing

[Jestersage]

Thank you. part of the reason I was planning to use two Panther is, aside from its RL inspiration, is also my mistaken understanding that, when I drop to subsonic/Mach 1 and use Panther-Dry, it will be more efficent for fuel.

Not to mentioned when I am approaching a landing zone, it have to be subsonic.

I guess I will stick with the 3x Whiplash design for now.

[Aegolius13]

1 hour ago, Jestersage said:

Thank you. part of the reason I was planning to use two Panther is, aside from its RL inspiration, is also my mistaken understanding that, when I drop to subsonic/Mach 1 and use Panther-Dry, it will be more efficent for fuel.

Not to mentioned when I am approaching a landing zone, it have to be subsonic.

You are correct on this in some situations -- the Panther dry has a much higher ISP than Whiplash, so it can work well if you want to do a low-speed approach, or troll for science biomes, or whatever.  But in the grand scheme of things, the total difference in fuel usage is probably going to be pretty minor - a Whiplash is not going to burn all that much fuel when throttled down low for a landing approach.  

On the other hand, as I believe @bewing was alluding to, high altitude is inherently good for fuel consumption.  Since there's less drag higher up, you need less thrust to counteract the drag, which means you can end up using a lot less fuel, even with a lower ISP engine.  

1 Panther / 2 Whiplash could also be workable, if you do want to do some low-speed cruising and the dry-mode Panther has enough thrust to keep you above stall speed.  Or at that point, you could even look at the Wheesley or something.  

[bewing]

I hope that when you are approaching a landing, you have your engines off.

But yes, the point is that to actually fly subsonic, you need to be low enough in the atmosphere to have actual lift at subsonic speeds. Which means you are down there with all the nasty drag. So the Isp boost from 4000 to 9000 for panther dry mode doesn't help you a bit, since you are fighting all those nasty air molecules the whole time.

So yeah, to fly very long distances with excellent fuel efficiency with a whiplash -- you want to be flying somewhere between 24km and 28km altitude, at 1400 m/s if you can manage it. Just be careful not to blow up your plane's nose from overheating. When you are getting close to your landing zone, turn off your engines and glide as high as you can for as long as you can. Then do a nice steep dive toward your landing point.