Trouble with building high altitude planes in FAR

[Wovejulio]

Hi all,

 

I'm on my first career mode file for KSP, so I'm rather new, but I don't know why I can't seem to make a high altitude plane. I do use FAR, but I don't use B9 since I want to get familiar with the stock parts first.

Here's a picture of my most successful craft:

http://imgur.com/Or0SB0w

Even after hours of watching tutorials, I can't make it go higher than about 15 km or faster than about 350 m/s. I'd like to get it up to 18 km to collect high altitude science. Speed is secondary, but I'm also confused why it won't go faster. The thrust starts dropping off the higher I go, which makes sense and all, but I've seen plenty of planes go higher and faster on just two engines like these. I thought it might be lack of intake air, but adding air intakes seems to do nothing. I have no stability problems as far as I can tell, and the FAR graph seems to suggest I could go higher, but I think my lack of speed is killing me.

 

Is there any way to access the thrust v. altitude data for each engine? Additionally, what suggestions do you have for me? More engines? More wing? More intakes? Different shape? I don't really know where to go from here.

 

Thanks in advance!

Edited February 17, 2016 by Wovejulio

[Streetwind]

What engines are you using? The Panther or the Wheesley? It's not immediately apparent from the screenshot. The Wheesley will not go any faster than you're getting it, period. For the Panther, you'll want to switch on the afterburners to get the thrust you need.

What is apparent however is that you're using way too many intakes. One engine should rarely need more than one intake, and every one you add increases your plane's drag for little to no benefit, making it slower. In particular, get rid of the highly draggy, poorly performing structural intakes. Yes, all four of them, and any others that I can't see as well. In fact, since you have those large ramp intakes available to you, stick with the pair of them you have, and get rid of the mk1 divertless intake pods as well - just add regular fuel tanks in there if you need the fuel. Structural fuselages if you don't.

Finally, you'll want to pay attention to the area ruling. That's something that FAR models but KSP doesn't. If your area ruling is bad, you get massive wave drag in the transsonic regime and literally "get stuck in the sound barrier". Since your top speed is right around mach 1, that is a strong indicator.

 

Edited February 17, 2016 by Streetwind

[Chaos_Klaus]

Drag is the number one speed limiting factor here. Get rid of anything that you don't need. The strucktural intakes are crap, so are the inline ones. One of these ram intakes per engine is fine. Shock cones are even better, because they produce less drag.

 

Also consider the behaviour of the engines. Jet engines vary their thrust with velocity. That means if you climb to early or too steep, you will never gain enough speed and thus your engines will never reach their optimal performance. Each engine has a speed where thrust peaks. I thing you can see that in the part menu when you right click the engine.

Also, engines lose thrust when you get to certain altitudes. This service ceiling is also engine specific. 

Check out the FAR user interface. You can turn on some overlays in the SPH. It will show you two lines. Yellow and Blue. Let's see if I remember that correctly ... maybe i mix up yellow and blue. Havn't used FAR in a while. 

The yellow one represents the cross section area, plotted along the main axis of the craft. You want this curve to be as smooth as possible. If it wiggles arount too much, you will get a lot of wave drag at hypersonic speeds. 

How smooth it is, is illustrated by the blue line. This is the second derivative of the yellow curve and thus shows the change in slope of that curve. So you want this curve to be as low as possible. That will automatically translate to the yellow curve beeing as smooth as possible. 

This is called area ruling. 

 

EDIT: I just reinstalled FAR ... and guess what ... I'm completely wrong.

Click the FAR button in the SPH, click "static analysis" and choose "transonic design" ... there you have the options to turn on the different overlays. green is the cross section area ... yellow is the curvature of that function ... doh.

Edited February 17, 2016 by Chaos_Klaus

[WhiteKnuckle]

Your resource screen shows that you're carrying Oxidizer, unless you're launching a probe out of that cargo bay you shouldn't be carrying that.

Aside from that, just what other people have suggested: less drag, more thrust.  

[Wovejulio]

Streetwind: Oh, my apologies, that's not the best picture. I am using the Panther, though. However, I had no idea it had two modes - that'll probably push me over the edge, so thanks for that! I'll get rid of the intakes though. Thanks for the rule of thumb there. I also know nothing about area ruling - never even heard of it. I'll have to read up on that. 

 

Chaos_Klaus: I also didn't know that how quickly I ascended would affect my performance at the high point of my flight. Thanks for the little FAR tutorial you typed, too! That should really help a lot.

 

WhiteKnuckle: Ha, stupid me. I didn't even notice I was carrying oxidizer. Thanks!

 

 

Thanks guys! That should really help a lot. I truly appreciate it! Your responses were quite thorough - I should have no problem getting up there now.