How to get air-breathing engines balanced?

[g00bd0g]

So I am trying to build a re-useable launch vehicle using Rapiers. I have built several successful ones, but as of late, I have been having big problems.

I have yet to build a craft that "balances" the air to the engines properly. i.e. I would like each successive "ring" of engine to flame out together. Instead I usually end up with one "arm" going 1st, but I have had crazy distributions with seemingly random flame outs across all engines.

See "VTOL 7" for example.

https://drive.google.com/file/d/0B4zLBUio4kInZlJ2Vy1CbWdVRXc/view?usp=sharing

Of course this results in an uncontrollable vehicle and subsequent death.

Is there a "proper" way to built a craft like this? i.e. build out one "arm" completely and then apply symmetry? Or apply symmetry to each part as I am building?

Are there "rules" to generating proper air distribution?

Thanks!

[Othuyeg]

I do not think you can get them to flame out at the same time. Use action groups and change the propellant before they flame out.

[waterlubber]

Sort of. Fuel flow rules are VERY VERY weird.

As seen in this post, the bottom section applies to jets.

Essentially, symmetry doesn't matter in fuel flow rules.

Here's the important bit:

• The ship is scanned starting with first placed part and ending with last placed part.
  
• If the part is an air intake, its contents is added to a pool of intake air available.
  
• If the part is an air consuming engine, amount required by the engine is removed from the pool and given to the engine. If there is not enough air in the pool at the moment, the engine gets what is left in the pool.
  
• If there is any intake air left in the pool after scanning all parts, the scan runs again starting from first part of the ship. In this second run, leftover air is distributed over engines which don't have their demands satisfied yet - again in order in which they were attached to the ship.
  

If I remember correctly, this means that if you place your engine, then intake, then engine, each engine will last much longer before flaming out, and when they do, they do it all at once.

Also, if you point the engines at the CoM, flameouts wont matter. i.e if you set one thrust limiter to 0 and aim the other one with shift+WASD until the CoT is pointing through the CoM.

[manni01]

From the top of my head, it definitely helps to place intakes induvidually (no symetry), and put a some torque on it (put 1 or 2 asas modules on the sstos) to counteract flame out and throttle decrease. I haven't checked your crafts out, but I'm assuming you're using the SP+ parts, so just add some drone cores if you care about looks. I would know about this, i make sstos that reach orbital speeds in the upper atmosphere.

[Red Iron Crown]

I wrote a post with a fairly simple example of the ideal placement of air intakes and engines, based on the rules in the more detailed post linked above. It's not as detailed but can help with grasping the placement rules.

There's also a mod being developed that can optimize the intake/engine layout after building the craft, in this thread.

Note that using these techniques does not prevent asymmetric engine flameout, instead it merely delays it as long as possible. Careful monitoring of engine thrust and throttling back at the onset of air starvation is necessary to get the most out of airbreathers. You can also disable groups of engines via action group to make it easier to manage.

Edited November 28, 2014 by Red Iron Crown

[numerobis]

This script:

https://github.com/numerobis/KSP-scripts/blob/master/reorder-jets.py

Reorders jets and intakes in a .craft file. It can only handle one kind of jet, and if the number of intakes isn't evenly divisible, I don't really know what happens. Also, it might mangle the output; I make it output into a second file just to be safe, but you must verify that things are working before you overwrite your first file.

This doesn't stop asymmetric flameout; it just means all the jets weaken at the same rate. What I've been doing lately is building my plane with engines on the wing, run my script, load the new craft, and as a last step, add an engine on the tail. When the tail flames out, that means you've got about 10km (IIRC -- or is it 5km?) before an asymmetric flameout. Turn off all the wing-mounted engines before that happens, at which point the tail engine will fire back up.

[HoY]

You can use mechjeb to prevent flameouts, or there is a seperate mod that makes flameouts happen symmetrically.

Edit; the b9 engines switch over to LFO automatically, do the stock engines not do that? I've never had the urge to try them myself

Edited November 29, 2014 by HoY

[LordFjord]

Red Iron Crown already mentioned the mod I am working on, it can already do the balancing of the intakes to engines pretty well. Seriously, go give it a try, I did not receive any testing feedback on it so far.

What the mod can't do (and probably never will): cover several engine groups.

Edited December 1, 2014 by LordFjord
removed some advice that doesnt always work