NERV rocket use stupid from low Kerbin orbit? (Jool)

[JoeSchmuckatelli]

I had a brilliant plan: use the purportedly most efficient rocket to get a flyby of Jool.  Launched a small probe with an orange body gas tank into low Kerbin orbit, waited for a transfer window then hit the gas on my oh-so-carefully planned launch to Jool.

...

I then watched as my AP barely moved during my first burn.  I'm going into my 3d burn at PE (two minutes per) - but getting nowhere fast (have not yet raised AP to Munar distance as yet...).

Did I goof up - should I have kept the burn going and aimed at the blue crosshair for the entire time until I reached the necessary escape velocity - or is multiple burns at PE the right way to do this?

[MagicFireCaster]

Well the slow time to burn is due to your TWR (Thrust-to-weight ratio), also make sure to ge rid of your oxidizer since nerv engine doesn't need it.

I would recomend getting a bit higher to orbit( around 7500m or more) and make your burn from 3 or 2 minutes before the maneuver node

[JoeSchmuckatelli]

Can I dump oxidizer mid flight?  ('cause I didn't know about that until after I'd launched the ship...)

[numerobis]

Lay down a maneuver node -- how long does it say your burn will take? Or in map view, click the 'i' button -- what's your maximum acceleration? Or get KER or mechjeb for other readouts. Or do the math by hand.

In any of those ways you'll find that the LV-N only provides a small amount of thrust. It's pretty normal to have long burns. You might want to bring a couple extras to get your burn time down. You can drop some of them into Jool since you'll have burned off roughly half your mass by then.

[JoeSchmuckatelli]

I had a maneuver node for the first burn - I think it wanted 16 minutes or thereabouts... I don't remember.  It got deleted in frustration after the second pass.

[numerobis]

16 minutes is just three passes of five minutes. I've done worse. Use physics acceleration and it's 8 minutes of actual burn. Or triple the engines and it's just two burns of three minutes of so.

Stock you can't vent oxidizer, sorry. There's plugins that will let you do it, but probably best just to bail on this flight.

[JoeSchmuckatelli]

So doing passes is the right idea.  Well, at least I had that much going for me.

 

I'm in the learn by busting my head against the wall stage - it's the knuckledragger in me showing itself once again!

[MagicFireCaster]

revert to a save before launching your ship if you can, and remember to remove oxidizer on the vehicle assembly before launching

[JoeSchmuckatelli]

Well - I would... but I launched this back when I had a 'free' NERV from a 'test NERV at Launchpad' mission.  So it's been orbiting for a long time waiting for this window.  Gonna have to chalk this one up to experience.

 

On a lighter note - by increasing my burn times - I now have a Jool intercept coming up - after a 1675 m/s burn at my next node - should be doable

[Plusck]

My first burn to Jool went fine - it was a tiny probe.

My second burn, a few game-years later, had people and landers on it... and it worked out almost exactly as you described. Now I start with chemicals and end with LF drop tanks, and sod the pristine space environment.

[Snark]

So, yeah.  Major goof putting an LFO tank on it.  Not only are you lugging around a whole lot of oxidizer you don't need, you also have less than half the fuel capacity you thought you did, and you're also lugging an extra couple of tons of fuel tank that aren't getting used.  Totally hosed.

Normally, I'd say "just bail on the flight."  It's seriously mucked up.

That said:  Do you have a docking port on the ship?  (Or have you researched the Klaw?)  If you do, you could send up an empty tanker, dock to it (or Klaw it), and drain out the oxidizer.  The best way to do this is before you've done any of your burns, when you're still in circular LKO; there's no point wasting any fuel accelerating that oxidizer mass out of LKO.  Do you have a quicksave available from before you started your ejection burn?

And yes, doing multiple passes the way you're doing is exactly the way to swing it.  Good job. 

If you don't have a save handy from before you started burning, you can still pull this out of the hole, I think.  Stop burning for now, and leave your ship parked in its current elliptical orbit (with Pe in LKO and cis-munar Ap).  Send up a tanker that can dock to it (or Klaw it), drain off all the oxidizer, and then go ahead and refuel it with LF to replace what you've burnt thus far.  Then send it on its way.

 

24 minutes ago, MagicFireCaster said:

revert to a save before launching your ship if you can, and remember to remove oxidizer on the vehicle assembly before launching

That would be better than the current situation, but still not good-- it wastes a lot of mass, it would be carrying a lot of empty fuel tank around.  Far better to just build the ship with LF-only tanks, rather than LFO tanks with the O drained out.

[JoeSchmuckatelli]

Here's a silly question:  I set my node for the burn (I'm on an escape vector out of Kerbin's SOI) that gets me the encounter with Jool.  Just for kicks I hit the gas for a second - and it estimates a 2 minute burn.

That doesn't seem correct, given the NERV's low thrust.

Now - I do have a couple of Thud's onboard (yes, this is an ill though out mission).

 

Would the Thuds (even though they are currently deactivated) be causing the computer to estimate the burn time with the NERV and the Thuds?  Meaning the projected burn time is way off?  Or does 2 min sound correct for a NERV to get to 1675?

 

DERP - the maneuver now needs 14 minutes.  Guess that answers that.

Edited January 12, 2016 by JoeSchmuckatelli

[Plusck]

No - and no.

Just twitching the throttle like that can work, but often doesn't. I don't think it is really counting the other engines on board (though you never know, it might be), but instead the ship is getting bounced by the acceleration and it completely messes up the calculation.

1675 m/s, from what you've been saying, is going to be another 10-minute burn.

[JoeSchmuckatelli]

• 17 minutes ago, Snark said:

So, yeah.  Major goof putting an LFO tank on it.  Not only are you lugging around a whole lot of oxidizer you don't need, you also have less than half the fuel capacity you thought you did, and you're also lugging an extra couple of tons of fuel tank that aren't getting used.  Totally hosed.

Normally, I'd say "just bail on the flight."  It's seriously mucked up.

That said:  Do you have a docking port on the ship?  (Or have you researched the Klaw?)  If you do, you could send up an empty tanker, dock to it (or Klaw it), and drain out the oxidizer.  The best way to do this is before you've done any of your burns, when you're still in circular LKO; there's no point wasting any fuel accelerating that oxidizer mass out of LKO.  Do you have a quicksave available from before you started your ejection burn?

And yes, doing multiple passes the way you're doing is exactly the way to swing it.  Good job. 

If you don't have a save handy from before you started burning, you can still pull this out of the hole, I think.  Stop burning for now, and leave your ship parked in its current elliptical orbit (with Pe in LKO and cis-munar Ap).  Send up a tanker that can dock to it (or Klaw it), drain off all the oxidizer, and then go ahead and refuel it with LF to replace what you've burnt thus far.  Then send it on its way.

 

That would be better than the current situation, but still not good-- it wastes a lot of mass, it would be carrying a lot of empty fuel tank around.  Far better to just build the ship with LF-only tanks, rather than LFO tanks with the O drained out.

I heard this a while ago:

• Smart people learn from other's mistakes,
• Average people learn from their own mistakes,
• Stupid people never learn.

 

Once more - I am proving myself to be exceptionally Average!

Edited January 12, 2016 by JoeSchmuckatelli

[Snark]

11 minutes ago, JoeSchmuckatelli said:

Here's a silly question:  I set my node for the burn (I'm on an escape vector out of Kerbin's SOI) that gets me the encounter with Jool.  Just for kicks I hit the gas for a second - and it estimates a 2 minute burn.

That doesn't seem correct, given the NERV's low thrust.

Now - I do have a couple of Thud's onboard (yes, this is an ill though out mission).

Would the Thuds (even though they are currently deactivated) be causing the computer to estimate the burn time with the NERV and the Thuds?  Meaning the projected burn time is way off?  Or does 2 min sound correct for a NERV to get to 1675?

The game's stock "estimated burn time" indicator is notoriously flaky.  Lots of ways it can go wrong.  May I suggest a better alternative?    i.e. one that will tell you actually how long the burn will take, and doesn't ever say "N/A" at you.

Yes, it's lying to you, if it's saying you're going to do a 1675 m/s burn in two minutes.  That would be about 14 m/s acceleration.  For a NERV to pull that much acceleration, with its paltry 60 kN thrust, would mean that your total ship mass is 4.29 tons.  (That's including all the fuel.  And 3 tons for the NERV itself.)

The game's stock burn time indicator is very simpleminded-- it just remembers the max acceleration your ship has undergone in the current flight session, and assumes that you'll do that.  It only looks at "what have you done since loading the ship," and has not the slightest clue about "what is the ship actually capable of doing now."  So for example, if you were accelerating with a Skipper or something at 14 m/s, and then staged it away, the burn time indicator doesn't know that your next burn (with a much lower-thrust engine) will take a lot longer.

Edited January 12, 2016 by Snark

[Plusck]

You seem to be implying that we are all distinctly average.

[Teilnehmer]

A Jool flyby needs about 2000 m/s of Δv. You don’t need a large tank to do one.

A single 1.25 m MK1 Liquid Fuel Fuselage with a Nerv engine can give you 2000 m/s of Δv when your payload mass is less than 3.6 t.

Your acceleration will be greater than 8.7 m/s². So the burn time will be less than 3.5 min. You definitely can perform this with one pass.

If your payload is a probe core, some scientific devices ans solar panels, then you can fill the MK1 LF Fuselage with only a half of fuel, and your burn time will be even less — about 2 min.

BTW, the Nerv is too heavy for such a mission. A Spark engine or a cluster of 3—6 Ants do the job as well and make much less total mass (thus a smaller rocket is needed to bring the thing to LKO).

Edited January 12, 2016 by Teilnehmer

[JoeSchmuckatelli]

Okay - based on my very hard mistakes made - I'm following the forum's advice: reverting to just before I began the first burns - unfortunately I cannot revert this mission any earlier due to having completed other missions since launch.  (and I REALLY don't want to have to do all that over again...).

 

So - I'll be flying up a fuel tank with a grabber up to it and offloading oxy and uploading LF.

 

Will try this all again tomorrow.  Now I'm going to go drink a toast to 'another learning experience'.

Edited January 12, 2016 by JoeSchmuckatelli

[Plusck]

I know it is probably not what you want to hear, having planned the mission for so long, but you could also just give up on it and use LFOx engines.

If you're capable of getting a full orange tank + LV-N up to orbit, you'll certainly be able to get a very decent rocket up to orbit that can do plenty in the Jool system: 40 tons of rocket is far bigger than some of the designs you've certainly launched from KSC to get to Minmus and back, so that easily gives you at least 5000 m/s dv if you just bring up a similar staged design to LKO on the back of whatever you lifted the jumbo tank up with... And since it would be a vacuum-only design, with a lower TWR, you can multiply that even further.

 

Actually no. After tinkering a touch in the VAB I think that getting rid of the oxidiser and retrying your LV-N would be the best way to go. I didn't think that the figures would be so one-sided, since you're carrying empty tank and can't drop it in parts. I could maybe do better but my test design with a 7-ton lander and science, plus 33 tons of LF-Ox rocket underneath, was actually slightly less capable in dv and only slightly better in TWR than the same lander + orange tank without oxidiser + LV-N...

Edited January 12, 2016 by Plusck

[Teilnehmer]

1 hour ago, JoeSchmuckatelli said:

So - I'll be flying up a fuel tank with a grabber up to it and offloading oxy and uploading LF.

Actually, you should offload both

Given your payload mass is m, you need only 0.3 × (m + 7 t) of fuel. (7 t is the mass of Nerv (3t) plus empty orange tank (4t); 0.3 is the fuel to dry vessel ratio for 2100 m/s of Δv). The less excess fuel you have in your tank, the shorter burn you have.

So if your payload is 1 t, you need only 2.4 t of liquid fuel (no oxidizer of course), and the burn time is about 4.5 min.

Edited January 12, 2016 by Teilnehmer

[MagicFireCaster]

9 hours ago, Snark said:

That would be better than the current situation, but still not good-- it wastes a lot of mass, it would be carrying a lot of empty fuel tank around.  Far better to just build the ship with LF-only tanks, rather than LFO tanks with the O drained out.

Well i would be better, but it depends on the desing he wants for his ship and his current tech level, i would recommend using the multiple tank adapter to  carry more fuel if you still want a big ship, is a shame you can use it to connect the engine to the tanks without using fuel ducts, but you can still make it work

[JoeSchmuckatelli]

FEEDBACK TIME:

 

Okay - thanks to everyone who responded! 

I followed Snark's advice and installed the 'correct burn time' mod.  That's a real game-changer; = more confidence in what I have to do.

I also flew up a similarly configured ship, no science but a grabber on board.  Grabbed the Jool Mission ship, dumped the oxidizer and filled the tank completely with LF.  I was able to escape Kerbin and get on a good flight path in two burns.  (Total burn time recommended was over 12 minutes).  I burned for 4:30 on the first pass, and intended to do the same thing twice more (refining the final pass to get the insert I wanted...).  Turned out on the second pass I reached escape velocity.  Thus, I had no choice but to complete the burn.  I kept it prograde for 7/8th of the way, and pointed at the blue marker for the last bit - and got a good encounter (Good meaning 'I got one' at all).

The best thing?  I have more fuel on the ship (now, after the Jool burn) than I did when I started this whole mess yesterday.  Insertion to orbit burn looks to only be about 6 minutes (thank god for the mod!) so I'm anticipating having quite a bit of fuel left over to explore the neighborhood. (Fingers crossed).

I've got panels extended (Derp: next time I'm sending passive panels on everything - even if I do have extendable ones - also learned that the hard way.) three Science Jr.'s, three Goo containers and the repeatable science stuff, and an alarm set.

 

Thanks again for the help folks!

[Plusck]

9 minutes ago, JoeSchmuckatelli said:

The best thing?  I have more fuel on the ship (now, after the Jool burn) than I did when I started this whole mess yesterday.  Insertion to orbit burn looks to only be about 6 minutes (thank god for the mod!) so I'm anticipating having quite a bit of fuel left over to explore the neighborhood. (Fingers crossed).

 

What! No, no. Don't waste fuel on an insertion burn. Tweak your insertion when you're about three-quarters of the way there (or elsewhere if you have to do a plane change) then maybe again if you don't get something very neat to start with. Without a plane-change, it'll cost you a few tens of m/s at most.

What you want to do is play with radial in/out and prograde/retrograde, in pairs. By doing this, you can vary the time that you arrive over dozens of days.

So you tweak until you almost touch the outside of Tylo's orbit, and you tweak a bit more to make sure that Tylo is there when you get there. Place another manouvre node by Tylo and you should see yourself in Jool orbit without spending a drop of fuel.

You could do much the same thing with Laythe, but that brings you straight down deep into Jool's gravity well, which might limit your possibilities for further exploration.

[Snark]

Congratulations, glad it worked out!

17 minutes ago, JoeSchmuckatelli said:

I followed Snark's advice and installed the 'correct burn time' mod.  That's a real game-changer; = more confidence in what I have to do.

Glad you found it useful! 

18 minutes ago, JoeSchmuckatelli said:

Insertion to orbit burn looks to only be about 6 minutes (thank god for the mod!) so I'm anticipating having quite a bit of fuel left over to explore the neighborhood. (Fingers crossed).

Note that you can save a metric crapload of dV (and burn time!) at Jool by setting up a reverse gravity assist with Tylo (or Laythe).  Not to mention having a fun piloting challenge and learning more about orbital mechanics. 

You can set up such an assist very cheaply, as long as you do it long before you get to Jool, so that just a tiny tweak to your trajectory can make a big difference to your arrival time and/or your path past Jool.

Basically, what you want to do is to set up your Jool approach so that you skim past Tylo.  You want the lowest possible Tylo periapsis, and you want your path to be cutting right in front of Tylo as it orbits (i.e. in Tylo's prograde direction).  This will cause Tylo's gravity to slow you relative to Jool.  If you play your cards right (and your incoming velocity isn't too high), you can capture to Jool with just the gravity assist, don't even need to burn engines to capture.  (You'll still need to burn engines to adjust your orbit after the assist, but this can take a big chunk out of your total dV requirements.)

22 minutes ago, JoeSchmuckatelli said:

I've got panels extended (Derp: next time I'm sending passive panels on everything - even if I do have extendable ones - also learned that the hard way.)

I assume by "panels" you mean radiators?  The so-called "passive" ones are okay, but be aware that they draw electric power to function, just like the foldable panels-- but unlike the foldable panels, there's no way to turn them off, they're always on and always draining power even if you have nothing hot that needs cooling down.  The way to avoid this problem is either to use the foldable panels (they shut off and stop using power when you retract them), or else to install this other little mod I wrote, which gives you a toggle option for the static panels. 

[JoeSchmuckatelli]

 @Plusck @Snark I cannot express the way I'm laughing right now.  Here I am, successfully knuckledragging my way into an encounter - and yet you guys tell me that with just a bit of tweaking I can do things the smart way.  And have more fun and learn something more in the process.

Ach, this game!

 

Okay - I have about 3 game years before I get there - so I probably have time to do this the right way. 

I will admit my ship is a god-awful cobbled together ill planned mess - but at least it has an engine and fuel and science.  The panels I was talking about were the extendable solar panels 'Gigantor XL' or something (I do have two large radiators, as well - which seem a waste - but I was afraid that without them I'd overheat the nuke engine and explode).  I've already had to revert to saves several times due to not extending the solar panels and running out of power - so that's why I thought it important to mention 'panels extended'.  The solar panels are extended, but the 2 radiator panels (yes, the big ones) are retracted.  I am dragging a LOT of dead weight on this (two turned off Thuds as well) CharlieFoxtrot of a mission. 

 

I had (up till I read your posts) put this mission on the far-back burner - after heaving an enormous sigh of relief.  Guess I'll be going back to it to see if I can find a way to skim Tylo...

 

Ach - my head!

 

Edit: looks like I'm coming in for a massively large polar orbit of Jool - guess I'll be tweaking my path a lot to get within the orbital plane of Jool's moons.

Edit # 2: Hmmmph.  Due to the SOI change - I can't get the game to give me maneuver nodes.  Is there any way to force it without burning to create a solar orbit (thus eliminating the encounter / SOI change?

Edit # 3: having saved the game from the space center - I now have maneuver nodes available when I returned to the craft...

Edit # 4: Whelp - it looks like with a 30 second burn at the descending node, I can get a Laythe encounter.  Then a second burn at Laythe PE puts me in a circular orbit parallel to the orbital plane.

 

Whodathunkit?

 

Tango Yankee guys.  This game (and your help) just keeps getting better and better.

Edited January 12, 2016 by JoeSchmuckatelli