damerell

Airbreather fuel flow logic

Recommended Posts


  • Generally quite pleased with 1.0, but we've just made an interesting discovery. Airbreathing engines now appear to use the monoprop fuel flow logic. This causes them to drain all "nearby" fuel tanks equally, to better enable aircraft to maintain balance in flight. Hooray!
  • However, this also causes them to feed fuel across parts which do not normally permit fuel flow - for example, decouplers with fuel flow turned off. (I haven't checked, but it presumably would also permit fuel flow across parts where it would be highly implausible). This is not so good; if I turn off fuel flow on a part, I really don't want fuel to flow through it.
  • I submit that a third fuel flow logic would make more sense - "like monoprop, but respects crossfeed being disabled". You could produce the same results if desired by turning on crossfeed, except that you would only be able to do so if the part permitted crossfeed at all.
  • The forum keeps turning the paragraphs in my posts into walls-of-text and I don't know why! Sorry about the listification.

Edited by damerell

Share this post


Link to post
Share on other sites

Known issue, bug already filed. No ETA for a fix, if there is a fix at all ...

And yes, it is stupid behaviour ( IMHO, of course ) and at odds with what was promised for the jet engines rebalance ( they would drain equally from the tanks in the same level​ IIRC and obviously not from forbidden tanks ;) ).

Share this post


Link to post
Share on other sites

I think we might avoid words like "stupid" but it does seem to be undesirable.

Share this post


Link to post
Share on other sites

Intresting, but what about intakes? how many do i need in an aircraft with four rapier for example? Air hogging is a thing of the past or slamming a lot of intakes on a craft still works?

Share this post


Link to post
Share on other sites
Intresting, but what about intakes? how many do i need in an aircraft with four rapier for example? Air hogging is a thing of the past or slamming a lot of intakes on a craft still works?

AFAIK intakes work as they always did, that is , as monoprop or electric charge: from source to consumer, disregarding decoupler and toher stuff that doesn't allow fuel flow.

Oh, and @ damerell ... well, as it is my opinion, I am entitled to use any words I find fancy as long as I don't break forum rules ( that I didn't, since I called the behaviour stupid , not who enabled the behaviour ;) ). And IMHO the behaviour is stupid because a) SQUAD made that some parts could not allow fuel flow long long time ago for various and good reasons ( like avoiding unintentional draining of fuel from upper stages ) and B) this change makes that the UI objectively lies regarding fuel flow rules. For those reasons I believe this behaviour is stupid, but is someone is offended, let us call it counterintuitive and incoherent in game context ;)

Share this post


Link to post
Share on other sites
Intresting, but what about intakes? how many do i need in an aircraft with four rapier for example? Air hogging is a thing of the past or slamming a lot of intakes on a craft still works?

I learned a most-valuable thing from a NathanKell post:

If your engines ever flameout asymmetrically, then you don't have enough air intakes. If you do have enough air intakes, your engines will flameout symmetrically when you get too high.

Share this post


Link to post
Share on other sites
Intresting, but what about intakes? how many do i need in an aircraft with four rapier for example? Air hogging is a thing of the past or slamming a lot of intakes on a craft still works?

I have had zero trouble with 1 intake/jet. At high altitude the thrust drops off faster than the air, so there isn't a problem.

Share this post


Link to post
Share on other sites

This is a problem for drop-tank users (well, unless you TACFB or something).

Share this post


Link to post
Share on other sites

What if you turn off fuel flow on the tanks you won't want drained instead of the decoupler between them and the rest of the vehicle? Does that stop it?

Share this post


Link to post
Share on other sites
What if you turn off fuel flow on the tanks you won't want drained instead of the decoupler between them and the rest of the vehicle? Does that stop it?
It certainly does.

And then drop-tank users have to fiddle around with the clumsy right-click fuel interface trying to enable tanks while flying.

Share this post


Link to post
Share on other sites

Hmmm, I would suggest a fourth option: treat jet fuel like rocket fuel, however, include a fuel balancer part. Quite simply, a part that, when active, automatically transfers fuel between all tanks which are directly connected via crossfeed to the balancer. That way, you can choose the behavior which best suits your design, be it for balanced weight distribution, drop tanks, etc... tank segments separated by non-crossfeed parts or decouplers would need to have their own balancer. Balancers would be able to manage/reverse the flow of fuel lines when active. As a bonus, we could apply the same logic to xenon and regular rocket fuel. One or two balancer parts could have balancing turned off/on for different fuel types, allowing the default behavior for all fuels to be the same.

Except monopropellant. That should still ignore crossfeed rules for the most part, but maybe still only draw from the active stage?

Shouldn't this really belong on the suggestion forum?

It certainly does.

And then drop-tank users have to fiddle around with the clumsy right-click fuel interface trying to enable tanks while flying.

Fuel tanks and other crossfeed parts should be controllable via action groups.

Edited by Colonel_Panic

Share this post


Link to post
Share on other sites
Fuel tanks and other crossfeed parts should be controllable via action groups.
Now that would be something nice for stock. In the meantime, you've given me an idea.

Share this post


Link to post
Share on other sites
I learned a most-valuable thing from a NathanKell post:

If your engines ever flameout asymmetrically, then you don't have enough air intakes. If you do have enough air intakes, your engines will flameout symmetrically when you get too high.

Unless they've changed the behavior for air intakes, this is somewhat correct. I am not sure if the old behavior is still valid, but it would be an artifact of the order of attachment for intakes to engines. The pool of intakes to engines would look like this if you attached two intakes, an engine, another intake, and an engine:

AIR1 > AIR2 > ENG1 > AIR 3 > ENG2

So in effect, air collected by 1/2 intakes is first pushed to ENG1, then past and pools leftover with AIR 3 which goes into ENG2. What would traditionally happen is that as you went higher up, ENG2 would flameout first as AIR3 would be exhausted and only the scraps of AIR1/2 get pushed into ENG two, whereas ENG1 is consuming two intakes entirely by itself. Excess air from AIR3 that ENG2 isn't using also does wrap around back to AIR1/2 pool.

A math example of this: (AIR produces, ENG consumes)

AIR1 - 5

AIR2 - 5

ENG1 - 3

AIR3 - 5

ENG2 - 3

This would result in 10 air going to ENG1 with 7 leftover air going to ENG 2 (total of 8 to ENG 2) leaving a total of 9 air in the pool.

As you went up in altitude, the intakes produce less, so the next model:

AIR1 - 1.5

AIR2 - 1.5

ENG1 - 3

AIR3 - 1.5

ENG2 - 3

Here we can see that while ENG1 still has just enough to continue to operate, ENG2 is receiving no 'spare' from AIR1/2 and thus ENG2 starves and flames out. In order to balance this, it was always a factor of putting an even number of intakes before each engine to ensure that the pool fed correctly into each engine. Please note my math examples aren't actual figures, they were made up.

The new model does help in terms of starvation, so it's rare that you will need more than one intake her jet as mentioned by Arq, but these rules still apply if you are using more than one intake. Never place your engines and intakes in symmetry mode on planes without remembering this as it can cause asymmetric flameout.

Edited by Quiana
Formatting on my table attempt

Share this post


Link to post
Share on other sites
Unless they've changed the behavior for air intakes, this is somewhat correct. I am not sure if the old behavior is still valid, but it would be an artifact of the order of attachment for intakes to engines. The pool of intakes to engines would look like this if you attached two intakes, an engine, another intake, and an engine:

AIR1 > AIR2 > ENG1 > AIR 3 > ENG2

So in effect, air collected by 1/2 intakes is first pushed to ENG1, then past and pools leftover with AIR 3 which goes into ENG2. What would traditionally happen is that as you went higher up, ENG2 would flameout first as AIR3 would be exhausted and only the scraps of AIR1/2 get pushed into ENG two, whereas ENG1 is consuming two intakes entirely by itself. Excess air from AIR3 that ENG2 isn't using also does wrap around back to AIR1/2 pool.

A math example of this:

Order AIR1 AIR2 ENG1 AIR3 ENG2

Produces 5 5 5

Consumes 3 3

This would result in 10 air going to ENG1 with 7 leftover air going to ENG 2 (total of 8 to ENG 2) leaving a total of 9 air in the pool.

As you went up in altitude, the intakes produce less, so the next model:

Order AIR1 AIR2 ENG1 AIR3 ENG2

Produces 1.5 1.5 1.5

Consumes 3 3

Here we can see that while ENG1 still has just enough to continue to operate, ENG2 is receiving no 'spare' from AIR1/2 and thus ENG2 starves and flames out. In order to balance this, it was always a factor of putting an even number of intakes before each engine to ensure that the pool fed correctly into each engine.

The new model does help in terms of starvation, so it's rare that you will need more than one intake her jet as mentioned by Arq, but these rules still apply if you are using more than one intake. Never place your engines and intakes in symmetry mode on planes without remembering this as it can cause asymmetric flameout.

If this is correct, would that mean that you could avoid flameout by always placing all your intakes before placing any of your engines? I'm not sure if I understand what the implication here is.

Share this post


Link to post
Share on other sites
If this is correct, would that mean that you could avoid flameout by always placing all your intakes before placing any of your engines? I'm not sure if I understand what the implication here is.

Incorrect. If you did that, it would look like this:

AIR1 - 1

AIR2 - 1

AIR3 - 1

ENG1 - 2

ENG2 - 2

The air for 1-3 goes into ENG1, pushing the necessary 2 air. The leftover 1 unit of air is pushed into ENG2, which is not enough to sustain it and it flames out.

Also check my last post, I edited because the formatting threw it off. order is top down, and it wraps back up to the top at the end of the cycle.

Edited by Quiana

Share this post


Link to post
Share on other sites
Incorrect. If you did that, it would look like this:

AIR1 - 1

AIR2 - 1

AIR3 - 1

ENG1 - 2

ENG2 - 2

The air for 1-3 goes into ENG1, pushing the necessary 2 air. The leftover 1 unit of air is pushed into ENG2, which is not enough to sustain it and it flames out.

Also check my last post, I edited because the formatting threw it off. order is top down, and it wraps back up to the top at the end of the cycle.

So in that case, how WOULD you prevent asymmetrical flameout?

Share this post


Link to post
Share on other sites

Set them up like this, placing in the same order:

AIR1

AIR2

ENG1

AIR3

AIR4

ENG2

If at X height each AIR produces 1 unit, and the ENG consumes 2 units, then they're evenly matched and you have no 'waste'. Beyond X height (which is now the maximum height of flight for this example setup) you'd flame out because both engines would starve within the same second and thrust would be cut evenly.

The concept scales up the same way, so if you wanted to have a four-engine you could setup this way and still have symmetric flame-out:

AIR1

ENG1

AIR2

ENG2

AIR3

ENG3

AIR4

ENG4

Now as a complex example of four engines, we have this which LOOKS like it would be okay, but doing an example shows the problem:

AIR1 - 1.5

AIR2 - 1.5

ENG1 - 2

ENG2 - 2

AIR3 - 1.5

AIR4 - 1.5

ENG3 - 2

ENG4 - 2

This would be just a bit above X height (maximum sustainable flight) for all engines. ENG2 would immediately flameout, BUT, the leftover 1 air from AIR1/2 would be passed on to AIR3/4 making a total of 4 air, which would leave ENG3/4 running. This would cause asymmetric flameout and rotation.

Share this post


Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.