burn time not in sync with time to node causing planned burns to fail

[Vanadiumn]

Hello, i am trying to do my first Eeloo mission, and whenever i set up my maneuver node and divided the burn time, i see the maneuver node is not in sync whatsoever and the timers are off.  The burn time is 2 seconds for 1 second,  and Node in t- is normal. If you don't get what I'm talking about, here's an example

It says I will reach the node in T- 2 minutes.
I plan to start my burn in 4 minutes, because of my burn time. But since this bug or glitch or whatever, it will actually be 8 minutes because of this double second, and then throw me off the planned burn.
 

I'm using 6 nuclear engines.

 

If i could please know what's wrong, thanks. I want to use this craft and get to Eeloo.
 

[linuxgurugamer]

If you have a 4 minute burn, you should start 2 minutes BEFORE the node 

[Vanadiumn]

5 hours ago, linuxgurugamer said:

If you have a 4 minute burn, you should start 2 minutes BEFORE the node 

I probably just wrote it wrong, but I use the in-game setting to calculate it for me. But that's beside the point, that the seconds it takes for 1 second to the node is 2 seconds for burn time.  Is my game just broken? 

Edited May 13, 2021 by Vanadiumn

[Vanamonde]

Is the game timer showing yellow digits? Do you have a lot of parts on the craft? 

[Blaarkies]

Is the burn calculator considering engines that are not active?

Basically, are you using a simple {capsule<>tank<>engine} rocket, or does it have complex stages with many engine types onboard (and weird fuel flow settings) ?

Edited May 13, 2021 by Blaarkies

[Guest]

In your example what is the actual burn time given? You mention 4 mins and 8 mins based on burn time but how have you calculated these?

Just to clarify, if you make a node that needs x amount of dV, the game will give a burn time and a time to node. You should half the burn time and start the burn at this figure before the node (the other half completed after the node).

So burn time for node = 6 minutes.

time to node = 10 minutes.

6 / 2 = 3 mins either side.

10 - 3 = 7 mins.

so you start the burn in 7 mins and finish in 13 mins.

 

I’m therefore I’m wondering if the burn time is being given as 8 mins but it’s saying to start the burn 4 mins before the time to node and therefore appearing to be doing 2:1?

[AlexinTokyo]

To give what @FruitGoose says some visual reference, here are some explanatory images:

This is the default burn-time indicator.  It shows the time to the node (T - 8 minutes), and the duration of the burn (Burn Time of 2 minutes).
As in FruitGoose's example, you want to split the 2 minute burn so that about 50% (i.e. 1 minute) is on either side of the node.  So, in the above expample you would start burning in 7 minutes (1 minute before the node time of 8 minutes)

 

There is also an "Extended Burn Indicator" (this image) which displays a countdown to the start of the burn ("Start Burn in") as well as the above node countdown and burn duration.
In this case, you can simply start burning when that countdown reaches 0.  You can activte this extended mode from the in-flight settings menu (Esc menu) as below.

 

Note that both "Node in" and "Start Burn in" are count downs, while "Burn Time" should not change once you have fixed the burn Δv.  If the "Burn Time" is changing, then either your ship is experiencing some acceleration (engine on at low thrust?) or there's something strange going on.
When you start burning, the Burn Time does count down; you can stop the burn when it reaches 0, at which time (in the above example) the "Node in" countdown should be at about T + 1 minute.

[Vanadiumn]

4 hours ago, AlexinTokyo said:

To give what @FruitGoose says some visual reference, here are some explanatory images:

This is the default burn-time indicator.  It shows the time to the node (T - 8 minutes), and the duration of the burn (Burn Time of 2 minutes).
As in FruitGoose's example, you want to split the 2 minute burn so that about 50% (i.e. 1 minute) is on either side of the node.  So, in the above expample you would start burning in 7 minutes (1 minute before the node time of 8 minutes)

 

There is also an "Extended Burn Indicator" (this image) which displays a countdown to the start of the burn ("Start Burn in") as well as the above node countdown and burn duration.
In this case, you can simply start burning when that countdown reaches 0.  You can activte this extended mode from the in-flight settings menu (Esc menu) as below.

 

Note that both "Node in" and "Start Burn in" are count downs, while "Burn Time" should not change once you have fixed the burn Δv.  If the "Burn Time" is changing, then either your ship is experiencing some acceleration (engine on at low thrust?) or there's something strange going on.
When you start burning, the Burn Time does count down; you can stop the burn when it reaches 0, at which time (in the above example) the "Node in" countdown should be at about T + 1 minute.

I use this setting when I did that burn, but it's just straight-up wrong for me.  Maybe the nuclear engines i have don't provide enough thrust to work with the manuver nodes? I have 6 though, that should be enough

7 hours ago, Blaarkies said:

Is the burn calculator considering engines that are not active?

Basically, are you using a simple {capsule<>tank<>engine} rocket, or does it have complex stages with many engine types onboard (and weird fuel flow settings) ?

This is the old design, the only thing I changed was the way the inner capsule was facing and added more nuclear engines.  The nuclear engines aren't connected to that inner lander stage, its a separate stage, neither is that tank in the bottom. Everything is seperate here.

Edited May 13, 2021 by Vanadiumn

[Vanadiumn]

11 hours ago, Vanamonde said:

Is the game timer showing yellow digits? Do you have a lot of parts on the craft? 

All green, and you can look at the pic above to see roughly the part count, i don't know exactly

[Vanadiumn]

5 hours ago, FruitGoose said:

In your example what is the actual burn time given? You mention 4 mins and 8 mins based on burn time but how have you calculated these?

Just to clarify, if you make a node that needs x amount of dV, the game will give a burn time and a time to node. You should half the burn time and start the burn at this figure before the node (the other half completed after the node).

So burn time for node = 6 minutes.

time to node = 10 minutes.

6 / 2 = 3 mins either side.

10 - 3 = 7 mins.

so you start the burn in 7 mins and finish in 13 mins.

 

I’m therefore I’m wondering if the burn time is being given as 8 mins but it’s saying to start the burn 4 mins before the time to node and therefore appearing to be doing 2:1?

 

 

Game also decided to remove the extra 1,000 m/s i had

Edited May 13, 2021 by Vanadiumn

[Guest]

4 hours ago, Vanadiumn said:

 

 

Game also decided to remove the extra 1,000 m/s i had

Ok top picture is correct for me; 8 min burn, node in 4 so 4 min before node and 4 after.

2nd one, depends on the context, I’m assuming it’s the same manoeuvre? It basically saying you missed the start of the burn by 11 mins and passed the node 10m 48s ago (ties in with 22.5 s burn before and 22.5 s burn after the node).

I’m thinking because you’re using the nervs the twr is quite low? It might be you’ve overshot the burn (not your fault directly) and ended up changing sphere of influence before the burn was complete which would force it to recalculate I think?

Tbh the low twr engines are a pain for long burns. Normally, after a while the dV starts climbing again so you have to loop back around to the Pe and start again (ie split the burn into 2 or 3 smaller burns) and take advantage of the Oberth effect.

Regardless though, it appears as though the in game calculations are correct for the context anyway.

[king of nowhere]

so, the burn time went off, by much more than was planned.

that's not uncommon. I often have cases when nerv engines have miscalculated burn times.

there is also another option, though; maybe the bottom of your ship is blocking the exhaust, making you lose power? does your ship look like this?

the above image is wrong engine mounting; engines are getting blocked

the above image is normal

 

everything seems a regular case of low twr. your burn ended 10 minutes after the node, the node was at periapsis, when you move away from periapsis you lose speed, the speed that was "removed" is just that. You also have lots of inefficiency, your burn is not going to be very effective because it is so long, there are ways to reduce this problem but too long to explain right now.

[Vanadiumn]

1 hour ago, FruitGoose said:

Ok top picture is correct for me; 8 min burn, node in 4 so 4 min before node and 4 after.

2nd one, depends on the context, I’m assuming it’s the same manoeuvre? It basically saying you missed the start of the burn by 11 mins and passed the node 10m 48s ago (ties in with 22.5 s burn before and 22.5 s burn after the node).

I’m thinking because you’re using the nervs the twr is quite low? It might be you’ve overshot the burn (not your fault directly) and ended up changing sphere of influence before the burn was complete which would force it to recalculate I think?

Tbh the low twr engines are a pain for long burns. Normally, after a while the dV starts climbing again so you have to loop back around to the Pe and start again (ie split the burn into 2 or 3 smaller burns) and take advantage of the Oberth effect.

Regardless though, it appears as though the in game calculations are correct for the context anyway.

2nd one is the same maneuver, i started the burn exactly as it told me and it ended up not working, and i split it over 3 burns, last transfer burn my apoapsis was past minimus

49 minutes ago, king of nowhere said:

so, the burn time went off, by much more than was planned.

that's not uncommon. I often have cases when nerv engines have miscalculated burn times.

there is also another option, though; maybe the bottom of your ship is blocking the exhaust, making you lose power? does your ship look like this?

the above image is wrong engine mounting; engines are getting blocked

the above image is normal

 

everything seems a regular case of low twr. your burn ended 10 minutes after the node, the node was at periapsis, when you move away from periapsis you lose speed, the speed that was "removed" is just that. You also have lots of inefficiency, your burn is not going to be very effective because it is so long, there are ways to reduce this problem but too long to explain right now.

The fairing piece at the bottom does look like it's somewhat blocking the engines, does it look like it to you? 

[king of nowhere]

28 minutes ago, Vanadiumn said:

 

The fairing piece at the bottom does look like it's somewhat blocking the engines, does it look like it to you? 

try to turn on the engines and take a pictures. if you see the effect with the sort of halo of light like in the first picture i posted, it is blocking. if you don't see the effect, it's all ok.

also, what is the mass of your ship? do you have just those 4 nervs, or more that we can't see?

4 nervs are pushing for 240 kN, if we know the mass of the ship it is easy to make a rough estimate of how long the burn should actually take, and then we can diagnose if there is something wrong with the burn indicator

 

 

[Vanadiumn]

47 minutes ago, king of nowhere said:

try to turn on the engines and take a pictures. if you see the effect with the sort of halo of light like in the first picture i posted, it is blocking. if you don't see the effect, it's all ok.

also, what is the mass of your ship? do you have just those 4 nervs, or more that we can't see?

4 nervs are pushing for 240 kN, if we know the mass of the ship it is easy to make a rough estimate of how long the burn should actually take, and then we can diagnose if there is something wrong with the burn indicator

 

 

That is an old design of the rocket, MK2 of this had 2 more engines equating to 6 total engines, and again with the newer design, the same problem. 

Here is a picture of the engines in flight, im using the waterfall effect so the "halo" your talking about might not be the same


Ill get the mass of the ship in just one second 

[Vanadiumn]

2 hours ago, Vanadiumn said:

That is an old design of the rocket, MK2 of this had 2 more engines equating to 6 total engines, and again with the newer design, the same problem. 

Here is a picture of the engines in flight, im using the waterfall effect so the "halo" your talking about might not be the same


Ill get the mass of the ship in just one second 

A little longer than a second, but the mass is 211 tons, 204 parts, with 8 nuclear engines now, not 6, sorry.

[AlexinTokyo]

OK, let's do some maths.

With a starting mass of 211 t, and a Δv of 1348, you have:

Δv = g0.Isp.ln(m0/m1)
m1 = m0/e^(Δv/(g0.Isp))
   = 211 / e^(1348.0 / (9.81 x 800))
   = 177.7

Δm = m0 - m1
   = 211 - 177.7
   = 33.3

Ft = g0.Isp.mbar
mbar = Ft/(g0.Isp)
     = 60 / (9.81*800)
     = 0.007645

So each engine will consume 7.6kg (0.0076t) of fuel per second, and your total mass change is 33.3t (in fuel consumed).

This gives:

t = Δm/(mbar*engine_count)
  = 33.3 / (0.007645 * 4) = 1088.94 (18m 9s)
  = 33.3 / (0.007645 * 6) =  725.96 (12m 6s)
  = 33.3 / (0.007645 * 8) =  544.47 (9m 4.5s)

So these are the expected burn times for 4, 6, and 8 LV-N engines.  I'm including all three, because I'm now confused as to which times from screenshots above refer to which version of the craft and therefore number of engines.
Note of course, that the starting mass will (I assume) be slightly different for the differing variants, so the above will not be fully accurate for all iterations.

So if you're seeing figures wildly different to these for the Burn Time indicated on the nav ball, then the game is having a problem correctly calculating the times.  Using the above method, you should be able to calculate exactly the time required, then (as has been said) split it 50/50 before and after the node time.
Since you're using KER, note that it has a burn time calculator (I think in the Vessel menu???) which I have found to be more accurate than the KSP native one.  Maybe see what that says and if it agrees with KSP or with the values above.

 

One thing that I noticed hasn't been mentioned: I don't suppose you are dropping into the atmosphere at any point during the burn (as you're starting off pointed well below the horizon)?

[Vanadiumn]

12 hours ago, AlexinTokyo said:

OK, let's do some maths.

With a starting mass of 211 t, and a Δv of 1348, you have:

Δv = g0.Isp.ln(m0/m1)
m1 = m0/e^(Δv/(g0.Isp))
   = 211 / e^(1348.0 / (9.81 x 800))
   = 177.7

Δm = m0 - m1
   = 211 - 177.7
   = 33.3

Ft = g0.Isp.mbar
mbar = Ft/(g0.Isp)
     = 60 / (9.81*800)
     = 0.007645

So each engine will consume 7.6kg (0.0076t) of fuel per second, and your total mass change is 33.3t (in fuel consumed).

This gives:

t = Δm/(mbar*engine_count)
  = 33.3 / (0.007645 * 4) = 1088.94 (18m 9s)
  = 33.3 / (0.007645 * 6) =  725.96 (12m 6s)
  = 33.3 / (0.007645 * 8) =  544.47 (9m 4.5s)

So these are the expected burn times for 4, 6, and 8 LV-N engines.  I'm including all three, because I'm now confused as to which times from screenshots above refer to which version of the craft and therefore number of engines.
Note of course, that the starting mass will (I assume) be slightly different for the differing variants, so the above will not be fully accurate for all iterations.

So if you're seeing figures wildly different to these for the Burn Time indicated on the nav ball, then the game is having a problem correctly calculating the times.  Using the above method, you should be able to calculate exactly the time required, then (as has been said) split it 50/50 before and after the node time.
Since you're using KER, note that it has a burn time calculator (I think in the Vessel menu???) which I have found to be more accurate than the KSP native one.  Maybe see what that says and if it agrees with KSP or with the values above.

 

One thing that I noticed hasn't been mentioned: I don't suppose you are dropping into the atmosphere at any point during the burn (as you're starting off pointed well below the horizon)?

Okay, ill try it. And the lowest I've gone is 78,000, so i am still above the Karman line

Edited May 14, 2021 by Vanadiumn