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[1.7.x] Anatid Robotics / MuMech - MechJeb - Autopilot - [2.8.4] [14 June 2019]

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17 minutes ago, Foxster said:

I've never understood what differential throttle is. What is it and when might I want to use it?

It throttles the engines independently of each other to steer the ship. So if you're steering to starboard it will throttle back the starboard engines. The port engines are providing more thrust so the ship turns to starboard.

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I found a case that MechJeb gets confused about.

I ended up putting a rocket engine, a separator and another rocket engine.  Crossfeed is enabled on the separator.  Normally this would be an inefficient design as the tankage that fed those first engines would normally be dropped.  However, this is a skycrane hauling a fuel tank and every other bit of tankage on the rocket is either for mechanical or aerodynamic reasons (No point in hauling an empty nose cone when I could put fuel in it.)

MechJeb correctly displays two stages.  I have no problem with it assigning all the delta-v to the first stage, it doesn't know I plan to drop the atmosphere engines once they've done the heavy lifting and continuing the mission on the space engines that will then be uncovered.

However, I do have a problem with not having a TWR for the second stage.

 

Obviously, no logs as this is behavior I do not like, not a bug.

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2 hours ago, Loren Pechtel said:

However, I do have a problem with not having a TWR for the second stage.

Obviously, no logs as this is behavior I do not like, not a bug.

How would it even know what value to assign to TWR for the second stage? What would that logic even LOOK like? Maybe that makes sense to you but it doesn't make sense to MJ and quite frankly it makes no sense to me either. You've created a situation where the second stage might very well have NO fuel in it when staged therefore it HAS no thrust and therefore can't assign a sensible value to TWR. You yourself indicate that MJ has no way of knowing what you're really going to do with that craft.

It can only assume that you're going to thrust until all available fuel has been exhausted, including fuel from the second stage across your crossfeed enabled separator. 

Seriously, what should the logic look like? Even if you can't code it, can you at least provide a valid set of logical rules for MJ to follow for crossfeed enabled designs that doesn't break logic for existing default designs with decouplers that block crossfeed?

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5 minutes ago, Starwaster said:

 

How would it even know what value to assign to TWR for the second stage? What would that logic even LOOK like? Maybe that makes sense to you but it doesn't make sense to MJ and quite frankly it makes no sense to me either. You've created a situation where the second stage might very well have NO fuel in it when staged therefore it HAS no thrust and therefore can't assign a sensible value to TWR. You yourself indicate that MJ has no way of knowing what you're really going to do with that craft.

It can only assume that you're going to thrust until all available fuel has been exhausted, including fuel from the second stage across your crossfeed enabled separator. 

Seriously, what should the logic look like? Even if you can't code it, can you at least provide a valid set of logical rules for MJ to follow for crossfeed enabled designs that doesn't break logic for existing default designs with decouplers that block crossfeed?

You don't need fuel to figure a TWR.  You need mass & engine power.

I don't expect MJ to get the delta-v.  It has no way of knowing when the boosters go bye-bye and since the delta-v doubles when that happens any calculation would be very, very wrong.  I would like to ensure I can land at my target, though.

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10 hours ago, Loren Pechtel said:

You don't need fuel to figure a TWR.  You need mass & engine power.

 

And a thruster with propellant. If it projects that the thruster will have no propellant at the start of that stage then it has no thrust and therefore no TWR

Edit: You acknowledged that it has no way of knowing what the delta-v is going to be for that stage so you know that it has no way of knowing how much propellant is going to be available, so how much mass would it even use for the TWR calculation? Propellant has mass and is part of that calculation. 

Edited by Starwaster

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3 hours ago, Starwaster said:

And a thruster with propellant. If it projects that the thruster will have no propellant at the start of that stage then it has no thrust and therefore no TWR

I disagree.  You can compute the TWR without looking at the fuel tanks.  There is no reason to omit the calculation and multiple cases where it might be of value.

There are rockets like mine where there are multiple engines that feed from the same tanks, and there are rockets that will obtain their fuel during the mission.  There are even a couple of mods that have engines that don't need fuel.

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1 minute ago, Loren Pechtel said:

I disagree.  You can compute the TWR without looking at the fuel tanks.  There is no reason to omit the calculation and multiple cases where it might be of value.

There are rockets like mine where there are multiple engines that feed from the same tanks, and there are rockets that will obtain their fuel during the mission.  There are even a couple of mods that have engines that don't need fuel.

It's not a question of whether there are multiple engines feeding off of the same tanks. It's that decoupler/separator that is crossfeed enabled. That design is capable of completely draining those tanks before it even gets to the stage that you want it to report TWR for. If that happens those engines have no thrust. So no TWR.

Or do you think it makes sense to report TWR for a stage with empty tanks? Because that's what you're proposing when you suggest not looking at the tanks. The calculation is automatically wrong no matter which way you look at it because either you're reporting thrust for engines that have no thrust or because you assume empty tanks when in actuality they have propellant in them.

Propellant has mass.

That mass is part of the TWR equations. Ignoring the tanks would break behavior for existing designs because all TWR would be wrong. Including for your specific cited situation it would definitely be wrong. You cannot ignore the tank's contents.

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1 minute ago, Starwaster said:

 

It's not a question of whether there are multiple engines feeding off of the same tanks. It's that decoupler/separator that is crossfeed enabled. That design is capable of completely draining those tanks before it even gets to the stage that you want it to report TWR for. If that happens those engines have no thrust. So no TWR.

Or do you think it makes sense to report TWR for a stage with empty tanks? Because that's what you're proposing when you suggest not looking at the tanks. The calculation is automatically wrong no matter which way you look at it because either you're reporting thrust for engines that have no thrust or because you assume empty tanks when in actuality they have propellant in them.

Propellant has mass.

That mass is part of the TWR equations. Ignoring the tanks would break behavior for existing designs because all TWR would be wrong. Including for your specific cited situation it would definitely be wrong. You cannot ignore the tank's contents.

I disagree.  The tanks are empty, the engines will burn for zero seconds.  That does not mean you can't figure out how hard they would push in that situation.

The number will be a bit deceptive because the fuel to actually make them burn would add some weight and thus the actual TWR would be slightly lower.  However, TWR is only a snapshot anyway, it always changes as the engine burns.  It's not like you have any use for a high precision value for the TWR, we look at TWRs to see if it's reasonable for the mission in question--and the calculation I am talking about would do this.

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44 minutes ago, Loren Pechtel said:

I disagree.  The tanks are empty, the engines will burn for zero seconds.  That does not mean you can't figure out how hard they would push in that situation.

The number will be a bit deceptive because the fuel to actually make them burn would add some weight and thus the actual TWR would be slightly lower.  However, TWR is only a snapshot anyway, it always changes as the engine burns.  It's not like you have any use for a high precision value for the TWR, we look at TWRs to see if it's reasonable for the mission in question--and the calculation I am talking about would do this.

That will horribly break reporting for designs that have boosters that aren't dropped. What if someone mounts SRB and doesn't jettison them (literally because they have no decouplers) and they are present in later stages? TWR will be wrong in those stages.

And it will still not meet the criteria created when you stated that you wanted to ensure that you could land at a given target because you won't know if the real TWR is high enough to allow landing until you actually get there.

Edited by Starwaster

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Feature request:

The distance input box in the maneuver planner is in km.  Can we have the units changeable?  Sometimes you get a lot of zeroes, especially if you're doing deep space operations.

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Is there any good solution for using MechJeb to send multiple craft in a single launch window?

I have unmanned birds loitering around Kerbin waiting for their launch windows.  I have another bird I also would like to send but MechJeb will schedule them close enough together that one will still be burning when the next one is ready.

In the real world if the second burned an orbit later it wouldn't matter but I don't see how to make MechJeb do it.

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4 minutes ago, Loren Pechtel said:

Is there any good solution for using MechJeb to send multiple craft in a single launch window?

I have unmanned birds loitering around Kerbin waiting for their launch windows.  I have another bird I also would like to send but MechJeb will schedule them close enough together that one will still be burning when the next one is ready.

In the real world if the second burned an orbit later it wouldn't matter but I don't see how to make MechJeb do it.

Go into map view, right-click the node, and click the plus sign to make it eject one orbit later.  A few days either way won't matter much.  Because you will end up doing a fine-tune burn along the way in almost all cases.

My burn strategy is:

  1. Setup the transfer burn using the Porkchop plots
  2. Execute the transfer burn
  3. Wait until I escape Kerbin's SoI
  4. Use "fine-tune" to setup a maneuver node
  5. Using Precise Maneuver Editor, I adjust the fine-tune burn to put me where I want at the target

So my goal with the transfer burn is to be about 95-98% accurate, I get the rest once I get out of Kerbin's SoI.

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1 hour ago, WuphonsReach said:

Go into map view, right-click the node, and click the plus sign to make it eject one orbit later.  A few days either way won't matter much.  Because you will end up doing a fine-tune burn along the way in almost all cases.

My burn strategy is:

  1. Setup the transfer burn using the Porkchop plots
  2. Execute the transfer burn
  3. Wait until I escape Kerbin's SoI
  4. Use "fine-tune" to setup a maneuver node
  5. Using Precise Maneuver Editor, I adjust the fine-tune burn to put me where I want at the target

So my goal with the transfer burn is to be about 95-98% accurate, I get the rest once I get out of Kerbin's SoI.

What am I missing?  Clicking the Plus does nothing.

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Just now, Loren Pechtel said:

What am I missing?  Clicking the Plus does nothing.

It changes the node to fire on the next orbit (watch the "time until burn" change).

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2 hours ago, WuphonsReach said:

It changes the node to fire on the next orbit (watch the "time until burn" change).

That's exactly what I was looking for, no change.  Are you perhaps running a mod that gives you that?

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15 minutes ago, Loren Pechtel said:

That's exactly what I was looking for, no change.  Are you perhaps running a mod that gives you that?

Ninja'd :(

This is stock functionality. Maneuver nodes have two modes, if you will.  The first you get when you left-click on a node and you get the prograde/retrograde, normal/anti-normal, etc. handles.  If you then right-click on the node, you get buttons which either delete the node (the red x) or advance/retard the number of orbits until burn.  Initially you can only advance the number of orbits until burn, obviously.  Right-clicking toggles between the two modes.

Edited by Brigadier

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40 minutes ago, Brigadier said:

Ninja'd :(

This is stock functionality. Maneuver nodes have two modes, if you will.  The first you get when you left-click on a node and you get the prograde/retrograde, normal/anti-normal, etc. handles.  If you then right-click on the node, you get buttons which either delete the node (the red x) or advance/retard the number of orbits until burn.  Initially you can only advance the number of orbits until burn, obviously.  Right-clicking toggles between the two modes.

Ok, now I understand.  Left click then right click.  I was just doing right click.  It changes the color but doesn't respond to +

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9 hours ago, Loren Pechtel said:

Is there any good solution for using MechJeb to send multiple craft in a single launch window?


Use the porkchop plot to select different departure times.  That's all you need.

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On 1/26/2017 at 2:25 PM, rsparkyc said:

I'm having a similar issue with maneuver nodes getting executed immediately in RO, and I'm wondering if it has to do with more that one part having a mechjeb module. EDIT: that doesn't seem to be the problem :(

Also had this in RO, which led to a little digging.  Went to prep some screenshots but couldn't duplicate.  However, think this is related to MJ2+RealFuels, specifically the ullage requirement / propellant state.  For "Very Stable" MJ2 correctly calculates "Node Burn Length" and "Node Burn Countdown."  If the propellant is not "Very Stable" those numbers become INF.  I think this is some fighting between RealFuels (RealFuels/SolverRF ?) and MJ2/FuelFlowSimulation.cs?

http://imgur.com/a/vTzAh

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Two things I've been wondering about:  what is the best way (while using Mechjeb) to get off of Mun/Minmus and ready for a return from moon maneuver?  Is there anything that can be useful in the advanced tab of the ACS?  Usually I just 'eye-ball' or get into some kind of ecliptic orbit, but I can't help but think there might be a better way.

 

Also, what do you all use for 'max Q' while launching? 

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8 minutes ago, Sarxis said:

Two things I've been wondering about:  what is the best way (while using Mechjeb) to get off of Mun/Minmus and ready for a return from moon maneuver?  Is there anything that can be useful in the advanced tab of the ACS?  Usually I just 'eye-ball' or get into some kind of ecliptic orbit, but I can't help but think there might be a better way.

 

Also, what do you all use for 'max Q' while launching? 

Ascent guidance isn't only for Kerbin, although it doesn't seem to handle the gravity turn too accurately when you call for a very low orbit.  (Launching from Minmus you likely have a very high TWR.  Combine this with the low gravity and your optimal flight path is to tip over almost immediately, burning horizontal so low that you need to keep terrain in mind.)  While it's given me some eggish orbits it's never given me a dangerous one.

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3 minutes ago, Sarxis said:

 

Also, what do you all use for 'max Q' while launching? 

1800 with 1.4 TWR depending in the design.

 

5 minutes ago, Sarxis said:

Two things I've been wondering about:  what is the best way (while using Mechjeb) to get off of Mun/Minmus and ready for a return from moon maneuver?

Ones in orbit,just execute "return from a moon",you'll find it in "Maneuver Planner".I usually leave it to 100 km and fine adjust when i leave SOI.
Then again you can edit the node once plotted.

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2 minutes ago, sebi.zzr said:

1800 with 1.4 TWR depending in the design.

 

Ones in orbit,just execute "return from a moon",you'll find it in "Maneuver Planner".I usually leave it to 100 km and fine adjust when i leave SOI.
Then again you can edit the node once plotted.

1800? As in 18kPa? 

 

Getting into orbit from a moon isn't a problem.  Just wondering if there was a way to do a more efficient 'direct assent' out of SOI back to Kerbal.  Sometimes, depending where we land, 'up' will put us straight out of SOI into a return trajectory, but I didn't know if there was some way to use Mechjeb to orient the craft right from the surface into an efficient trajectory back home.  Unfortunately I can't seem to use 'Kerbal' as a target, otherwise I could use the Smart SAS to do some advanced orientation stuff. 

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