Two-Stage Missile Confusion

[381654729 (OP)]

I'm trying to make a two-stage missile. I add one engine and one submunition with zero everything else as a test. Strangely, the range (not Overall Range), despite the first stage carrying no fuel, is 39.3m km, not zero as expected. Changing the separation distance, even to beyond the second stage's maximum range by itself, has no effect on this number.

Now I add another submunition but still no fuel. The first-stage range jumps up to 54.4m km, which puzzles me even more. Again, changing the separation distance does nothing.

Confused, I put one engine, one unit of fuel, and one submunition. The first-stage range shows up as 86.4m km. I add another submunition, and the first-stage range increases to 87.0m km.

There must be some fundamental thing I am not understanding about two-stage missiles. Either that or there's some weird bug going on. My questions:

1. What is going on?
2. When the first stage is in flight, are the engines on the second stage also on?
3. What's a typical process to design a two-stage missile? Like if I want to design box launcher fighters, I first design the missile, then the launcher, then the MFC, fighter, and finally the carrier, so I'd start from the "missile end" and work my way to the "carrier end". Which "end"  do I start if I want an effective two-stage missile system?

EDIT: spelling errors.

[Brian Neumann]

1. What is going on?
2. When the first stage is in flight, are the engines on the second stage also on?
3. What's a typical process to design a two-stage missile? Like if I want to design box launcher fighters, I first design the missile, then the launcher, then the MFC, fighter, and finally the carrier, so I'd start from the "missile end" and work my way to the "carrier end". Which "end"  do I start if I want an effective two-stage missile system?

EDIT: spelling errors.

1.  Not sure
2.  Not in previous versions
3.  For a multi stage missile you need to start with the final stage, create that missile (finish research on the design) and then use that as the sub-munition for the next stage.  You can in theory do this multiple times.  I don't think I have seen more than 2 stages in actual play however.  When you do this pay attention to the release range.  For optimal performance you want the final stage to release far enough out that the defenders don't get to shoot down the 1st stage before it releases.  I usually want to set it at least 5 million km from the targets.

Brian

[NihilRex]

The fuel from the submunitions is available to the missile bus, I suspect.

[381654729 (OP)]

The fuel from the submunitions is available to the missile bus, I suspect.

That is... bothersome. That means if the bus actually reaches its max range, the fuel for the submunitions has already been burnt and the submunitions won't move. This does not help me design matching fire control and sensor systems, unless I'm missing something here. I could still do it with some math, but why the extra convolution?

[Iranon]

Sounds like a good thing to me: more forgiving if you design something silly, and it avoids inefficiency when your desired released range is lower than the maximum range of your submunitions (e.g. you're reusing regular short-range missiles). Also useful at the end of the range, as the bus will usually be a good deal more fuel-efficient.

[381654729 (OP)]

So I just have to be careful when launching it, to ensure that the second stage will have enough fuel to reach their target. Not exactly ideal in my mind, but I can work with it.

[Steve Walmsley]

It's a bug.

Every stage should be self-contained. During flight the fuel from one stage should not be affected by other stages.

What is happening in v6.41 (and all earlier version) is that during missile design the fuel from the second stage is being included in the calculation for the endurance of the first stage. This means that fuel is effectively being used twice, once for the first stage and then again by the second. The reason this hasn't been spotted before is because it is only obvious in the missile design window if you have an engine but no fuel. With no engine or fuel, there is no endurance because the code only checks for fuel if there is an engine. Also, it's usually the case that the second stage has a lot less fuel than the first so players were not seeing missiles with ranges that looked a lot longer than they would expect.

I'll include this in the next bug-fix patch.

[381654729 (OP)]

Thanks Steve, your great game is about to get better.

I still have a question: when I fire the missile, is the second-stage fuel actually used twice for more range than actually possible with the fuel and engines on board, or will the second-stage fuel only be used once, for a shorter range than displayed?

[Steve Walmsley]

Thanks Steve, your great game is about to get better.

I still have a question: when I fire the missile, is the second-stage fuel actually used twice for more range than actually possible with the fuel and engines on board, or will the second-stage fuel only be used once, for a shorter range than displayed?

Missiles don't actually use fuel in flight. Instead, the endurance of the missile is calculated during the design stage. Endurance is measured in time. Once launched, the missile will be destroyed when it attacks its target, if it loses tracking and has no on-board sensor or when its endurance is exhausted.

The second stage missiles behave perfectly normally, based on their endurance calculated at design time. The first stage missile (which includes the second stage sub-munitions) will also behave normally based on the endurance shown at design time. The only issue is that the endurance of the first stage is calculated based on the total fuel in all stages, not just the fuel for the first stage.

Or put another way, whatever the missile specifications say during design time - that is how the missile will behave. The only bug is in the design process, resulting in the first stage having greater endurance that it should. There is no bug affecting what happens after launch.

[381654729 (OP)]

Very clear explanation, thank you!