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MPDerksen

Leaving Duna (and elsewhere)

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background:  I can do interplanetary transfers.  I can land in a teacup.  I can dock.  I can EVA.  But now I'm faced with actually LEARNING.  My Career has progressed to Duna and Eve.  Instead of just copying ships from various YouTube videos, I want to actually understand how to build something intelligently, rather than just trial and error.  We can use leaving Duna as the example, since that is actually what my next mission will need.  But this time I want to better understand the design.  On to the specific question:

I have designed a lander.  I know it will sit on Duna.  I know what it weighs.  In the dV map, I see that I need 1,450 to achieve orbit, and another 740 to get back to Kerbin.  I know that I'm a terrible pilot, so I will plan on an additional 20%.  So I need enough fuel, and an engine that creates 2,628 dV.  This is what I need to see once it's landed.  So far, am I basically correct?  Fancy things, like refueling in orbit, leaving the Science Jr. behind etc. help, but at this point, let's stay basic.

How do I select an engine (or set of engines) that are appropriate for a given gravity and sufficient to lift what I want back up?  I think, if I can better understand this, I will make ALL my rockets better.  Sometimes I put together a lifter that seems fine, but then takes FOREVER and burns WAY more fuel than it should to get going off the launchpad.  

I might not be asking this correctly, but I think there is more than just the thrust to consider from the engine description in the VAB (and atmosphere vs. vacuum).  Start with small words to match my small brain :)

Michael

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11 minutes ago, MPDerksen said:

My Career has progressed to Duna and Eve.  Instead of just copying ships from various YouTube videos, I want to actually understand how to build something intelligently, rather than just trial and error.

Good luck @MPDerksen getting to those two planets! I am really new, just a bout a month now and will be looking to just get to the mun or minmus at most soon. To be honest Im just getting to the point of any kind of smooth circular orbit around Kerban. lol

So right, to spend the time and learn it yourself! I think thats my plan also. Good luck and godspeed!

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5 minutes ago, Saturn5tony said:

Good luck @MPDerksen getting to those two planets! I am really new, just a bout a month now and will be looking to just get to the mun or minmus at most soon. To be honest Im just getting to the point of any kind of smooth circular orbit around Kerban. lol

So right, to spend the time and learn it yourself! I think thats my plan also. Good luck and godspeed!

I was just getting my Duna skills when Breaking Ground launched and I had to start a fresh Career.  Now I have the tech tree done (just from Kerbin, Mun and Minmus), I have a small station, I have a relay network, I have multiple Sentinels and got my fly-by science of Duna done (and about $10M in the bank).  All that for the second time, but I STILL have no idea what ISP actually is....  I'm just tired of copying someone else's design from a video to get missions done.  Making my own stuff is part of the fun, right?

Michael

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Delta-V wise, you've got it right. 1450m/s for Duna is already generous, you probably won't need a 20% safety margin on top of that. I typically need 1300m/s -- with a high-twr, needle-shaped vessel, well under 1200m/s are possible.

Duna is rather friendly, with an atmosphere that slows you down for landing, but is so thin that it's not much of a hassle on the way up. You can use parachutes to great effect: while a soft landing would require dozens (too heavy!), one or two chutes will slow you to well under 100m/s. Engine power can take care of the rest, and the landing will be a lot less frantic than on the airless Mun. Such a landing usually requires under 100m/s. Bring 200m/s and you're safe. However, you will need a drogue chute as well, or the main chutes cannot be deployed in time.

You've got a wide choice of engines. The air is thin enough that most vacuum engines still perform well.

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7 hours ago, MPDerksen said:

How do I select an engine (or set of engines) that are appropriate for a given gravity and sufficient to lift what I want back up?  I think, if I can better understand this, I will make ALL my rockets better.  Sometimes I put together a lifter that seems fine, but then takes FOREVER and burns WAY more fuel than it should to get going off the launchpad.  

 

When you are in the VAB, there is a tab for Dv in the bottom right row of buttons. If you click on it, it allows you to select the planet as well as altitude for it to calculate the Dv of your ship that is displayed on the right while building. So set it to whatever planet you are heading too and build your lander. Once you attach the lander to whatever is carrying it, you can reset the DV calculations for each stage. There are also mods like Kerbal Enginer Redux which can also give you all the info, which were almost necessary before the recent updates. The mod is still useful, but not needed.

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1 hour ago, Driftin said:

When you are in the VAB, there is a tab for Dv in the bottom right row of buttons. If you click on it, it allows you to select the planet as well as altitude for it to calculate the Dv of your ship that is displayed on the right while building. So set it to whatever planet you are heading too and build your lander. Once you attach the lander to whatever is carrying it, you can reset the DV calculations for each stage. There are also mods like Kerbal Enginer Redux which can also give you all the info, which were almost necessary before the recent updates. The mod is still useful, but not needed.

Lander:  
20.9Tons
TWR 3.85 (for Duna)
ISP 332s
dV 2776

I'm guessing I want a TWR >1?

I just put it in Duna orbit at 55K.  I used aerobraking (setting my Pe at 15K and crossing my fingers).  I have about 700 dV left in the transfer stage.  My contract calls for NE Basin, Northern Shelf or East Canyon to find "Blueberries"? and bring them home.  I can only find the Shelf in the biome map I can see online, so I think I'll do some low orbit EVAs first.  Like mentioned above, I have 2776 dV in the lander, and that has to count for a gentle touchdown too.  Probably going to go just fine.  

But I still am puzzled if just TWR is the key number, and use that with the dV map?

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6 hours ago, MPDerksen said:

I'm guessing I want a TWR >1?

Yep. In duna you can land on parachutes alone, but to get in orbit again you need to fight the gravity, so TWR > 1 is a must, otherwise you will stand still on ground. I'd say more than 1.5, and near 3.0 if possible.

 

Blueberries: Check this link to see where these (and others) are: https://wiki.kerbalspaceprogram.com/wiki/Surface_features

 

Also, the lander alone is returning to Kerbin? If so your dv is NOT ENOUGH. When approaching Kerbin you need to brake around it. Since your pilloting skills are lacking, more dv is due.

1400ish from ground to Low Duna orbit

300 - 400 to High Duna orbit and beyond

800 - 1000 to Kerbin

400 - 600 to brake around Kerbin (orbit may vary depending of how good you planned the maneuvers, let's say elliptical?)

50-100 to position yourself for aerobraking.

 

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20 hours ago, MPDerksen said:

I STILL have no idea what ISP actually is....

11 hours ago, MPDerksen said:

But I still am puzzled if just TWR is the key number, and use that with the dV map?

For someone who can go interplanetary and land in a teacup, that's surprisingly basic questions.

ISP denotes the efficiency of a rocket motor: a higher ISP means that you will get more impulse out of your propellant, someone once called it "more buck from your bang". In first approximation, a vessel with a more efficient motor will have more delta-V; however, delta-V also depends on your wet/dry mass ratio, and a efficient but heavy motor doesn't always pay off.

TWR means "thrust to weight ratio". With a TWR < 1, you cannot even take off: engines running at full throttle, fire and smoke, but you're not going anywhere. With a TWR that's slighty greater than one, you can take off, but very very slowly. Things quickly get better with increasing TWR, 1.5 is usually sufficient. More doesn't hurt, though  personally I find anything above TWR=2 to be rather excessive.

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6 hours ago, Fierce Wolf said:

Yep. In duna you can land on parachutes alone, but to get in orbit again you need to fight the gravity, so TWR > 1 is a must, otherwise you will stand still on ground. I'd say more than 1.5, and near 3.0 if possible.

 

Blueberries: Check this link to see where these (and others) are: https://wiki.kerbalspaceprogram.com/wiki/Surface_features

 

Also, the lander alone is returning to Kerbin? If so your dv is NOT ENOUGH. When approaching Kerbin you need to brake around it. Since your pilloting skills are lacking, more dv is due.

1400ish from ground to Low Duna orbit

300 - 400 to High Duna orbit and beyond

800 - 1000 to Kerbin

400 - 600 to brake around Kerbin (orbit may vary depending of how good you planned the maneuvers, let's say elliptical?)

50-100 to position yourself for aerobraking.

 

Dang it. That would have been good to know before Jeb and Co. were in Duna orbit. See what I mean about missing some basics?  And here I was so proud of my first aerobraking success....

So I'll get them down and back, then maybe we need a rescue. Too bad I didnt add a docking port or I could send a refueller. Can I transfer fuel with the grabber unit?  Could be a fun mission right there...

1 hour ago, Laie said:

For someone who can go interplanetary and land in a teacup, that's surprisingly basic questions.

ISP denotes the efficiency of a rocket motor: a higher ISP means that you will get more impulse out of your propellant, someone once called it "more buck from your bang". In first approximation, a vessel with a more efficient motor will have more delta-V; however, delta-V also depends on your wet/dry mass ratio, and a efficient but heavy motor doesn't always pay off.

TWR means "thrust to weight ratio". With a TWR < 1, you cannot even take off: engines running at full throttle, fire and smoke, but you're not going anywhere. With a TWR that's slighty greater than one, you can take off, but very very slowly. Things quickly get better with increasing TWR, 1.5 is usually sufficient. More doesn't hurt, though  personally I find anything above TWR=2 to be rather excessive.

Right, so I either grossly overbuild or fail completely. My lander being TWR of 3+ means I could select smaller than the poodle for this?

Next it would be appreciated if someone clarified the benefits of using the nuclear and ion engines. 

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On 11/3/2019 at 9:27 PM, MPDerksen said:

I think there is more than just the thrust to consider from the engine description

RAW thrust is hardly important at all in space.  It is only really important for launching and powered landing.  The two main figures are thrust and ISP, as you say.  They are the rate of acceleration and the fuel efficiency figures of an engine although the exact performance you get from each depends on the mass of the vehicle they are pushing.  Thus thrust becomes TWR and ISP becomes dV once you attach an engine to a rocket (with fuel).

To relate TWR and dV to the mundane world, consider an aircraft flying across the Atlantic between the USA and Europe.  It will be designed (and fuelled) so that it can fly at, say, 300 miles per hour for 10 hours, giving it a total range of 3,000 miles.  Do you think it is more important for it to travel at a certain speed, for a particular amount of time or for it to go far enough?  I think you'll agree that speed and duration are just a trade-off and neither is as important as actually getting all the way across the ocean.  In spacecraft terms, the dV is vital but the TWR is not.

Specifically, as you said, your vehicle must have TWR (acceleration rate) > 1 for the body from which it is launching.  After that you can forget about TWR.  Any maneuvers in space, after launching, will take longer with a lower TWR but even a tiny amount of thrust can, eventually, accelerate the vehicle to any speed.  The Dawn engine is the perfect example of a very low-thrust engine intended to be fired for a very long time (and only to be used on very light vehicles).  TWR more or less takes care of the speed/duration trade-off in the aircraft analogy above.

For every other consideration dV is the important consideration and it depends entirely on the engines' ISP and what proportion of the mass of the whole vehicle is fuel.  A more efficient engine (higher ISP) with more fuel will be able to accelerate a vehicle more.  Accordingly, the best way to select engines for your vehicles is to choose those with the highest ISP that give you the minimum TWR that you need.  https://wiki.kerbalspaceprogram.com/wiki/Parts#Liquid_Fuel_Engines  Apart from the launch the ISP in vacuum will be important so the high-ISP choices are the Dawn and Nerv engines.  Unfortunately neither of them is likely to give you sufficient TWR to launch so work down the list for an appropriate size - Poodle, Terrier, Dart, etc.  Experience the joy there is in discovering that none of the engines you actually want to use is good for launching, as a general rule.  Recognise you'll have to cripple your in-space performance in order to get there in the first place.  Realise why people build single-purpose launch vehicles, with separate space-transfer stages and landers - like Apollo.  Of course, it's not always necessary to go that far because your launch-TWR/space-ISP compromise may be 'good enough' but at least now you know exactly why that design pattern comes up frequently.

The final wrinkle, just when you think you've found "the" right engine, is its own mass might hold you back.  Sometimes a lighter engine means a specific vehicle is so much lighter and the fuel-ratio so much greater that it delivers more dV even though its 'raw' ISP is not as good on paper.  The most important advice, however, is that after you've messed about choosing between engines a few times you'll very quickly learn which ones are appropriate, which ones you like to use and what their possible alternatives are in different circumstances.  It can take a lot of experimentation and swapping-around in the VAB to final decide which to go with but how long you spend on that is up to you.

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7 hours ago, Pecan said:

RAW thrust is hardly important at all in space.  It is only really important for launching and powered landing.  The two main figures are thrust and ISP, as you say.  They are the rate of acceleration and the fuel efficiency figures of an engine although the exact performance you get from each depends on the mass of the vehicle they are pushing.  Thus thrust becomes TWR and ISP becomes dV once you attach an engine to a rocket (with fuel).

To relate TWR and dV to the mundane world, consider an aircraft flying across the Atlantic between the USA and Europe.  It will be designed (and fuelled) so that it can fly at, say, 300 miles per hour for 10 hours, giving it a total range of 3,000 miles.  Do you think it is more important for it to travel at a certain speed, for a particular amount of time or for it to go far enough?  I think you'll agree that speed and duration are just a trade-off and neither is as important as actually getting all the way across the ocean.  In spacecraft terms, the dV is vital but the TWR is not.

Specifically, as you said, your vehicle must have TWR (acceleration rate) > 1 for the body from which it is launching.  After that you can forget about TWR.  Any maneuvers in space, after launching, will take longer with a lower TWR but even a tiny amount of thrust can, eventually, accelerate the vehicle to any speed.  The Dawn engine is the perfect example of a very low-thrust engine intended to be fired for a very long time (and only to be used on very light vehicles).  TWR more or less takes care of the speed/duration trade-off in the aircraft analogy above.

For every other consideration dV is the important consideration and it depends entirely on the engines' ISP and what proportion of the mass of the whole vehicle is fuel.  A more efficient engine (higher ISP) with more fuel will be able to accelerate a vehicle more.  Accordingly, the best way to select engines for your vehicles is to choose those with the highest ISP that give you the minimum TWR that you need.  https://wiki.kerbalspaceprogram.com/wiki/Parts#Liquid_Fuel_Engines  Apart from the launch the ISP in vacuum will be important so the high-ISP choices are the Dawn and Nerv engines.  Unfortunately neither of them is likely to give you sufficient TWR to launch so work down the list for an appropriate size - Poodle, Terrier, Dart, etc.  Experience the joy there is in discovering that none of the engines you actually want to use is good for launching, as a general rule.  Recognise you'll have to cripple your in-space performance in order to get there in the first place.  Realise why people build single-purpose launch vehicles, with separate space-transfer stages and landers - like Apollo.  Of course, it's not always necessary to go that far because your launch-TWR/space-ISP compromise may be 'good enough' but at least now you know exactly why that design pattern comes up frequently.

The final wrinkle, just when you think you've found "the" right engine, is its own mass might hold you back.  Sometimes a lighter engine means a specific vehicle is so much lighter and the fuel-ratio so much greater that it delivers more dV even though its 'raw' ISP is not as good on paper.  The most important advice, however, is that after you've messed about choosing between engines a few times you'll very quickly learn which ones are appropriate, which ones you like to use and what their possible alternatives are in different circumstances.  It can take a lot of experimentation and swapping-around in the VAB to final decide which to go with but how long you spend on that is up to you.

This is SUPER helpful, so thanks for the detailed response.  Let me look at a specific example from the Wiki.  Let's compare a Reliant vs. a Swivel.  I've typically just used the Swivel so I have the gimbal control at launch.  Seems I give up about 18% of the Thrust (atm) for that.  But right next to that, there are columns for T/W ratio for atm and vac.  How is that even helpful?  So do 4 Vectors = 1 Mammoth?  

And can someone explain the Atomic/Nuclear option?  It has a vac thrust of only 60, but an Isp of 800???  A terrier also has a vac thrust of 60, but an Isp of only 325.  Still vague on the difference between them.

I did get my Duna Lander home.  Cost me 1,300 dV to get to a 52K Duna Orbit, 775 to get my Kerbin capture, and another 300 to lower the Pe to about 30km for a nice firework display of my fuel tanks.  I came home with multiple missions accomplished, earned about $800K and 1,000 science points.  Plus all 3 member of the crew are now 3 stars.  All that time allowed my Mun Science Station to build up another 500 science too.  Next, I want to put a rover down on Duna for a scanning contract. 

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So the ΔV equation is:

ΔV = Isp * g0 * ln(m0/mf)

Which basically means that ΔV depends on your ISP and the % of your ship which is fuel. The higher your ISP, the higher your ΔV for the same amount of fuel. And the higher the % of your ship that is fuel (and of course the more fuel you have), the higher your ΔV is.

So while the Terrier and the Nerv are both low-thrust engines, the Terrier is lighter and therefore can have more ΔV on small ships, and better TWR on landers, and on the other hand the Nerv is very heavy but has a gigantic ISP and so on medium ships a single or pair of Nervs can give you 4-5000m/s, and if you have a large Mk3 ship with four or six - or even more - Nervs you can also haul massive payloads. The Nerv is a common engine to push large payloads interplanetary and for deep space manoeuvres where you don't need large TWR.

However, if you do have an efficient engine with a large TWR, like a Terrier or Poodle, in some cases that can reduce the ΔV you need. This is because these engines have a high TWR and so you can execute your manoeuvre in a shorter time. This means that you waste less ΔV on correcting the fact that you are moving along your orbit - so it can be better to launch low TWR ships interplanetary for high orbit.

The columns for TWR of the engine on its own are helpful as in a small craft the weight of the engine can actually decrease ΔV by a noticeable amount - try putting a Nerv on a small probe. Also, TWR in a vacuum matters because of Tylo, that evil moon with 0.8g and no atmosphere.

Edited by fulgur

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1 hour ago, MPDerksen said:

Let's compare a Reliant vs. a Swivel.  I've typically just used the Swivel so I have the gimbal control at launch. 

So did I for a short while until I almost exclusively started to use the Reliant in favor of the Swivel: higher thrust and less weight allows more payload/fuel, plus it's a little cheaper (savings here tough are usually more than eaten up by the need for active fins).

1 hour ago, MPDerksen said:

at launch.

Over time, my typical rockets evolved as 3+ -stage:
1st stage comprised of SRBs only, bring the vessel up to about 10-12 km altitude with a good vertical speed of 350-400 m/s. Reasoning: SRBs are cheap, and close to the end of their burn time have a  quite high TWR, giving the vessel a good vertical speed boost.
2nd stage is typically using a Reliant (or the respective engines for other diameters) with a TWR starting around 1.2, and moves the rocket close to circularization, ideally AP 80-100 km, PE positive but below 25 km. Reasoning for using a Reliant instead of a Swivel see above. The active fins are mounted on this stage in order to provide steering for 1st and 2nd stage. Separation after stage is burned leaves the PE below 25 km in order to avoid space junk (Kessler syndrome).
3rd stage is typically using either a Terrier (or the respective engines for other diameters, see above) for vessels needing below 4k dV, e.g. to Mun or Minmus. Or a NERV or similar engine for targets needing more than 4k (e.g. Jool moons or Moho). Reasoning: in vacuum, ISP becomes the dominating parameter, see @Pecan 's detailed post above.

If required, there might be additional stages as well, e.g. the final stage for landing at Tylo needs a high TWR, hence that stage typically is LfOx based with engines (depending on the payload mass) like Skipper or even Mainsail.

As a rule of thumb, the dV for any given vacuum stage should be numerical between 8 and 12 times the engine's ISP for optimal efficiency with an acceptable acceleration, I usually aim for an acceleration above 2.5 m/s^2

Edited by VoidSquid

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2 hours ago, MPDerksen said:

And can someone explain the Atomic/Nuclear option?  It has a vac thrust of only 60, but an Isp of 800???  A terrier also has a vac thrust of 60, but an Isp of only 325.  Still vague on the difference between them.

An engine like the NERV has a very high ISP, meaning a very high mileage (dV) per amount of fuel. Try building a vacuum stage for a rocket with say a Mk1 Command Pod as payload, that has a dV of no less than 8k, one build with a Terrier, one build with a NERV.  You'll easily see ;) 

@fulgur mentioned the rocket equation, from it it also follows that for typical KSP tanks, the maximum theoretical dV for any engine is 21.57 x ISP

If you're interested in further reading:

https://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet 

Edited by VoidSquid

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On 11/5/2019 at 6:40 AM, MPDerksen said:

Let's compare a Reliant vs. a Swivel.  I've typically just used the Swivel so I have the gimbal control at launch.  Seems I give up about 18% of the Thrust (atm) for that.  But right next to that, there are columns for T/W ratio for atm and vac.  How is that even helpful?  So do 4 Vectors = 1 Mammoth?  

And can someone explain the Atomic/Nuclear option?  It has a vac thrust of only 60, but an Isp of 800???  A terrier also has a vac thrust of 60, but an Isp of only 325.  Still vague on the difference between them.

  • TWR atm and vacc - this is related to the nerv/terrier issue but is a pretty esoteric detail in engine choice.  It's not one I ever think about.
  • 4 vectors/1 mammoth - Yes!  Structural/mass considerations aside they have the same thrust and ISP so, yes, they're exactly the same :-)
  • Nerv/terrier - Both engines have the same thrust but the Nerv is much, much more fuel-efficient, especially in space.  You'd want to use it all the time then, when that tiny thrust is enough.  The problem is that it's a heavy engine so on small, light vehicles that aren't going very far - such as a lander - the extra engine mass adds so much to the vehicle that you get less performance at the end.  Heavy engines need to be used long-distance with lots of fuel, where their efficiency more than makes up for their mass.  To put it another way; do you care whether your car does 40 or 100 miles per gallon if you're just going to the local shops once?  Well, yes, but not if the 100mpg car is lousy in the first mile while it warms up.

I think I detect a hint in your replies that you still think thrust and TWR are more important than they are.  Really, they mean next to nothing in space.  A higher thrust, and therefore TWR, will mean you can make a maneuver more quickly which is nice for accuracy but not critical.  Think of how long you burn your engines on a Kerbin-Mun transfer burn.  Does it matter if you can do it twice as quickly?  Not really, the important thing is you have enough fuel for the 800-odd m/s it requires.  (In my earlier analogy - it doesn't matter how long it takes the plane to cross the Atlantic, as long as it gets all the way across).  My vehicles are usually designed with a space-operation TWR of 0.2+ but only because I don't have the patience for 10-minute burns and the inaccuracy causes its own problems.  0.4 - 0.6 is plenty and 1 means you're definitely carrying heavier/higher-thrust engines than you need and would get better dV (the important bit) with lighter ones.  Except, of course, that you might need the extra thrust for launch/landing so you're back to the compromises.

The Swivel/Reliant question is actually one I love, since they are both starter engines that 'seem' very similar but there are lots of differences that should make people think more than they do.  Gimballing, battery-charging alternator, price, etc. are not directly related to performance but are worth thinking about.  For what it's worth the starter rockets I make usually use a swivel and then transition to reliant + active steering fins.

More important and useful than thinking of 'whole mission' engines is thinking of phases - launch, in-space, landing, etc.  Design specific stages for phases and there's more often a clear engine choice.  It always comes back to compromises though.

Edited by Pecan

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You can have a TWR of 0.01 as long as you are in orbit, that's the deal of The Dawn ion engine. It's only useful there, it can't put a rocket in space, nor land a lander, it will just slooooooowly push a vessel, very very efficiently.

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On 11/4/2019 at 9:00 PM, MPDerksen said:

Too bad I didnt add a docking port or I could send a refueller. Can I transfer fuel with the grabber unit? 

I might have missed the answer to this question reading through the other reactions, but yes, you can transfer fuel throug the Advanced Grabber Unit. Ideal part for refueler crafts/missions.

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4 minutes ago, Epicdreamer said:

but yes, you can transfer fuel throug the Advanced Grabber Unit

Depends on the difficulty setting ""Resource Transfer Obeys Crossfeed Rules"" though, iirc.

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1 hour ago, VoidSquid said:

Depends on the difficulty setting ""Resource Transfer Obeys Crossfeed Rules"" though, iirc.

You're right, thanks. I forgot about that ticbox in the difficulty settings menu.

Edited by Epicdreamer
Typo

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6 hours ago, Epicdreamer said:

I might have missed the answer to this question reading through the other reactions, but yes, you can transfer fuel throug the Advanced Grabber Unit. Ideal part for refueler crafts/missions.

Turns out I got them home with the onboard fuel, but this is great to know for the future!  Thanks.

I'm going to give myself a test, and post the results here.  Making up a mission, I'm going to deploy a science lab on Ike.  The dV map says I need 5,280 just to get there (I think I can improve that will some aerobraking around Duna on the way in...).  I'm still not super clear how to read the map in reverse.  But if I got back from Duna with 2,775, I bet I can do it from Ike with 1,800, plus ~500 for the landing, so 2,300 total in the lander?  I'll try out the Nerv to get from Kerbin to Ike orbit.  

I'll give the design a go, and talk through my choices here to see if I really understood all the great information.  

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