Moho and ion engines

[Hotel26]

Does anyone have any experience and/or tips regarding the use of ion engines when voyaging between Kerbin and Moho?

(Pardon me if this has been discussed elsewhere.  Search seems inop at the moment.)

[Foxster]

A good combination to power the ions is to use fuel cell arrays. 

You will need one array per two engines. Then you will need LF+O for the fuel cells. You will want amount of Xenon x 0.02 of LF (and matching O). 

Edited July 30, 2016 by Foxster

[Snark]

Moving to Gameplay Questions.

Sure, they work great. Especially since sunlight is so strong at Moho; solar panels give lot of electricity for ion engines.

Just two things to bear in mind.

First, prepare yourself for some very long burns. Going to Moho takes a metric crapload of dV, which on the one hand makes ion engines attractive, but on the other hand is going to take hours of burn with them.

Second, Moho's gravity is on the high side for a vacuum world, so you need a higher TWR than you would need on, say, the Mun. Ion engines aren't necessarily a great choice for landing there. So ion engines make a great transfer stage for going to and from Moho, but you may want to leave them in orbit and use a conventional engine for going down to the surface.

9 minutes ago, Foxster said:

A good combination to power the ions is to use fuel cell arrays.

That might be an option for the outer solar system, where sunlight is weak and solar panels don't work well... but why would you use them for going to Moho? Solar panels work great that close to the sun. And lugging the fuel to power the fuel cells will cut your effective Isp to less than a third of using purely xenon (IIRC, the effective Isp of a fuel cell powered ion engine is 1200-1300ish, compared with 4200 for pure xenon).

[Foxster]

25 minutes ago, Snark said:

... but why would you use them for going to Moho? Solar panels work great that close to the sun...

Well, solar panels have mass too, especially the Gigantors. But, basically, I just don't like relying on solar power. I have been let down a few times with sudden darkness, so now prefer a reliable power source. 

[Snark]

2 minutes ago, Foxster said:

Well, solar panels have mass too, especially the Gigantors. But, basically, I just don't like relying on solar power. I have been let down a few times with sudden darkness, so now prefer a reliable power source. 

Yes, but solar panels have a lot less mass than the fuel you'd need to use.  And fuel cells have mass, too.  Heck, the mass of a single fuel cell array is equivalent to 14 of the small folding solar panels... which is enough to provide power to two ion engines.  In other words:  You can accomplish the same work with less mass with solar panels.  Even without taking into account all that fuel mass you save, cutting your Isp to only a third of what you could be getting.

And saving mass is important when going to Moho.  That's a lot of dV.

And sure, in general, darkness can be a problem with solar-powered ion craft.  But that's just a matter of planning.  I mean, it's not as if the sun suddenly and unexpectedly turns off once in a while.  You know when you're going to be in darkness, so simply arrange your mission plan so that you don't need to do any lengthy ion burns while on the night side of a planet.

And besides, going to Moho, "lack of sunlight" simply never happens.  You need to do, at most, three ion burns:

1. Leaving Kerbin.  But for a Moho trip, this burn is going to be on Kerbin's daylight side, so it simply isn't an issue.
2. (Possibly) Doing an inclination-matching burn (if you didn't leave Kerbin at its AN/DN with Moho's orbit).  But that happens out in interplanetary space where sunlight is everywhere, so again, not an issue.
3. Arriving at Moho.

Of those, #3 is the only one where there's any chance of being in darkness.  But even there, it's really a non-issue.  First of all, it's trivially easy to arrange to arrive on Moho's daylight side.  Of course, in most cases where you're going from an outer planet to an inner one, your arrival would be on the inner planet's night-time side, assuming you want to end up in a prograde orbit.  However, that doesn't really apply with Moho and ion engines, for a couple of reasons.  First, Moho has such a ludicrously slow rotation time that it might as well not be rotating at all; the surface is only moving at centimeters per second, there's not really any dV benefit to orbiting prograde versus retrograde.  So one option is simply to arrange to arrive on the sunlit side, and brake into a retrograde orbit.  The other option, if you still prefer prograde, is to do most of your Moho-arrival burn before you get into the planet's shadow.  Normally that would be a bad idea for planetary arrivals in general (you want to do as much of your burn right at Pe as possible, to maximize Oberth benefit).  However, the TWR of an ion craft is so low that you really can't make much use of Oberth anyway, and Moho is small enough that Oberth benefit's not all that great.  So even if you plan to arrive at Moho's night-time side, you can still arrange to do most of your burn outside its shadow.

...What it boils down to:  The whole point of somebody using ion engines is to get their high Isp so they can manage really high dV.  A pure-xenon-powered ion craft gets three times the dV as running off fuel cells.  So in this case, I'd say that fuel cells would really not be a great choice for the OP, here.

[Hotel26]

1 hour ago, Snark said:

And besides, going to Moho, "lack of sunlight" simply never happens.  You need to do, at most, three ion burns:

1. Leaving Kerbin.  But for a Moho trip, this burn is going to be on Kerbin's daylight side, so it simply isn't an issue.
2. (Possibly) Doing an inclination-matching burn (if you didn't leave Kerbin at its AN/DN with Moho's orbit).  But that happens out in interplanetary space where sunlight is everywhere, so again, not an issue.
3. Arriving at Moho.

 

Thanks for this, as it's along the lines of where my rumination is going.  I'm not planning ion burns for 1 or 3 because they simply take too long.  But maybe I'm wrong about that!?

I certainly am going to have nukes for the critical burns.  And my lander is conventional and designed for everything lo-grav (<= 2.5m/sec).

I'd aim at getting the inclination burn done with the transit burn (only when Kerbin is aligned with the Moho orbit nodes).

But it seems like there may be a 2b and 2c burn if one is doing a multi-phase transition?  2b would be to circularize (or bring into similar eccentricity) at helio-periapsis.  And 2c to set up the intercept.  And possibly a series of
2d burns to fine-tune the intercept trajectory?  (I'm thinking out aloud, but it's good to get ideas and then try them out and then report back, because then the subject's on the table for perusal by others.)

I know people are "inclined" to go direct in one shot but could ion engines make this incredibly efficient, fairly simple and not too much longer in duration??

As an afterthought, I could be wrong about not using ion engines for 1, too.  Use nukes, say, to get escape; then ion thrust to lower the periapsis...  The only time-critical burn is stopping at Moho for capture?

Edited July 30, 2016 by Hotel26
spelling

[mikegarrison]

One thing that has only been alluded to here is that TWR can be an issue when trying to get into Moho orbit. Some of the times I've used electric engines to get to Moho, my orbital insertion burn has taken so long that I have actually shot all the way through Moho's SoI.

[Foxster]

2 hours ago, Snark said:

...What it boils down to:  The whole point of somebody using ion engines is to get their high Isp so they can manage really high dV.  A pure-xenon-powered ion craft gets three times the dV as running off fuel cells.  

I'm not sure that's quite right. I do agree it is less efficient but not by that much. It's more like 2/3 of the dV with fuel cell arrays and even that's assuming you are using the minimum numbers of solar panels with no allowance for not getting 100% out of all of them at Kerbin. 

 

Edited July 30, 2016 by Foxster

[Meithan]

2 hours ago, Snark said:

Yes, but solar panels have a lot less mass than the fuel you'd need to use.  And fuel cells have mass, too. [...]

Great post, Snark. I was just about to start designing an unmanned trip to Moho in my career save and was beginning to consider using ion engines simply because 1) they're cool (dat Isp), 2) there's a lot of sunlight at Moho. Your post pretty much resolves the worries that were starting to crop up about such a mission. The retrograde orbit suggestion is one I hadn't thought of, too. Good one!

Edited July 30, 2016 by Meithan

[Laie]

Ahem...

AFAIK the most common experience / problem among Moho first-timers is that you woosh past Moho at ridiculous speed and cannot effect a capture in the short time you spend in it's SOI.

Solutions:

• either bring enough TWR so you can kill your relative velocity quickly (this pretty much rules out ion engines),
• and/or start matching velocities before you even enter it's SOI.

The recommended way of getting to Moho is to ignore the usual transfer planners. Instead, you do the following:

1. start while Kerbin crosses it's AN/DN with Moho.
2. make it so that your orbit around the sun barely touches Moho's. Bonus points if you can mostly match Moho's inclination while leaving Kerbin.
3. don't worry if Moho will be there by the time you arrive.
4. Once you get to the point where you touch Moho's orbit, you decelerate. In such a fashion that Moho will be there on the next round.
5. encounter and capture.

Steps 2&4 should be familiar to anyone who ever set up a rendezvous. The guy with the ape avatar will probably be around shortly, and post helpful screenshots.

Edited July 30, 2016 by Laie

[Snark]

7 hours ago, Hotel26 said:

I'm not planning ion burns for 1 or 3 because they simply take too long.  But maybe I'm wrong about that!?

I certainly am going to have nukes for the critical burns.

Except that 1 and 3 are exactly the reason why you should have ions, precisely because those burns are so huge dV.  If you're not going to use ions for those, why even bother?

This is one of those cases where trying to give someone advice really bumps up against the beauty of KSP, namely, there are many different ways to accomplish a given goal.  Which of those ways are "better" than the others depends completely on the individual player's goals and values, which are going to be very different from person to person.

Are you trying to optimize for cost?  For lowest tech required?  For ship mass?  For calendar time?  For how cool the ship looks?  How patient are you?  And so forth.

Ions are good for getting to Moho because of their ridiculously high Isp, which in turn means they can pack lots and lots of dV.  But you largely obviate that benefit if you lug along lots of heavy secondary engines of lower Isp.  A nuke might make sense as an initial orbit-departure stage that you jettison when you start spending xenon, but if you keep the nuke and its fuel along for the ride, personally I'd say "what's the point".

Really, with ions, it comes down to a matter of personal choice:  how patient are you?  Specifically, are you willing to put up with stupidly long burns that are about as exciting as watching paint dry?  If yes, then ions are awesome, go for it!    If not... best not to bother with them at all.  You're only getting your value out of them when you're burning them for really long burns.

Bear in mind that engines are dead weight.  It's generally "best" (in terms of dV and mass efficiency) not to have multiple different propulsion types, i.e. hybrid ships that sometimes use one engine and sometimes another, because it means they're lugging along more dead weight than they have to.  You can absolutely make a hybrid nuke/ion ship work for a mission to Moho, if that's what you want to do and you like it that way.    But personally, I'd just go with either pure ions or pure nukes.

7 hours ago, Hotel26 said:

And my lander is conventional and designed for everything lo-grav (<= 2.5m/sec).

..Okay, just be careful.  Bear in mind that Moho's surface gravity is 2.7 m/s².

7 hours ago, Hotel26 said:

I'd aim at getting the inclination burn done with the transit burn (only when Kerbin is aligned with the Moho orbit nodes).

If you launch when Kerbin is at the AN/DN, you have a couple of options.  You can launch at an angle, to try to adjust inclination to match Moho.  Or, just don't bother and launch equatorial.  Doing that will mean that when you arrive at Moho, you'll be at a very non-equatorial inclination, but honestly that doesn't matter much-- Moho rotates so slowly that it might as well not be rotating at all, so it really doesn't matter whether you're prograde or retrograde or polar or whatever.  The dV from orbit to surface will be essentially the same.

7 hours ago, Hotel26 said:

But it seems like there may be a 2b and 2c burn if one is doing a multi-phase transition?  2b would be to circularize (or bring into similar eccentricity) at helio-periapsis.  And 2c to set up the intercept.  And possibly a series of
2d burns to fine-tune the intercept trajectory?

If you're planning on launching when you're at the AN/DN, rather than when you're at the right phase angle for an intercept, then yes, there would be one additional burn.  (This is not a bad plan, it'll save you dV, it just means you'll take more calendar time to get to Moho.)

The one additional burn would be executed when you reach solar periapsis at Moho's orbit, right at the AN/DN.  This burn would be to somewhat lower your Ap to adjust your orbital timing, in order to get a Moho intercept.  (You never "circularize" your solar orbit-- when you finally meet Moho, you'll simply be at Pe of an elliptical orbit.)  The amount you need to adjust your Ap by in order to get a Moho intercept will depend on where Moho is in its orbit relative to you, and also how many orbits you're willing to wait until you meat up with Moho.  No matter how unlucky you may get about Moho's placement, you can easily get an intercept without too much of a burn, if you're setting up an intercept for several orbits in the future.  A small adjustment of your Ap now makes a big difference in your placement several orbits from now.

The good news is that doing such a burn isn't really "wasting" any dV:  the more you spend there to lower your Ap, the less you'll need to match velocities with Moho and enter orbit when you finally get an intercept.  You're basically just doing part of your orbital-injection burn a bit early. 

7 hours ago, Hotel26 said:

I know people are "inclined" to go direct in one shot but could ion engines make this incredibly efficient, fairly simple and not too much longer in duration??

Ion engines are a great tool for building really high-dV ships that aren't too huge, but they're not a magic wand. My advice would be to pick a reasonably efficient navigational path that is compatible with your level of patience (both in terms of calendar time and your personal time), and go with that.  The navigation path you use is pretty much irrelevant to your choice of propulsion.

 

7 hours ago, Hotel26 said:

As an afterthought, I could be wrong about not using ion engines for 1, too.  Use nukes, say, to get escape; then ion thrust to lower the periapsis...  The only time-critical burn is stopping at Moho for capture?

No, there's no time-critical burn at all.  You can enter Moho orbit with ions just fine.  Heck, you've just spent months or years getting there, you've got all the time in the world.

The only thing you can't do with ions in a Moho mission is to land on it.  Everything else works just fine with ions.  (Before someone calls me on that:  yes, it's technically possible to land on Moho with pure ion power.  But you'd be operating really close to the theoretical limit of what they can do, and would have to have an extremely lightweight ship that can mass no more than 740 kilograms per ion engine, bearing in mind that the engine itself is 250.  And you'll be operating at a very low TWR, which makes for dicey landings.)

7 hours ago, mikegarrison said:

One thing that has only been alluded to here is that TWR can be an issue when trying to get into Moho orbit. Some of the times I've used electric engines to get to Moho, my orbital insertion burn has taken so long that I have actually shot all the way through Moho's SoI.

Well, it's only an issue if it's not planned. You've got all the time in the world, after all.  If you need a 2-hour burn... or a 3-hour... or even a 24-hour... go for it.  It'll be dull, sure, but other than testing your patience, there's no engineering reason why you can't have a burn that's as long as you like.

Of course, if it's a really long burn, you'll have to start your burn before you enter Moho's SoI, lest you shoot right through it as you describe.

8 hours ago, Snark said:

The whole point of somebody using ion engines is to get their high Isp so they can manage really high dV.  A pure-xenon-powered ion craft gets three times the dV as running off fuel cells.

 

7 hours ago, Foxster said:

I'm not sure that's quite right. I do agree it is less efficient but not by that much. It's more like 2/3 of the dV with fuel cell arrays

No, I'm pretty sure it's right.  Here, let's do the math.

Spoiler

Ion engine uses 8.74 EC/s of electricity, and 0.486 units/second of xenon.  Xenon is 0.1 kg/unit, so the xenon usage is 0.0486 kg/s.

Fuel cell array generates 18 EC/s of electricity, and uses 0.02025 units/s of LF and 0.02475 units/s of oxidizer.  Adding LF+O and multiplying by 5 kg/unit, that gives us a mass consumption rate of 0.225 kg/s.  To make an apples-to-apples comparison with the ion engine's needs, let's express that in terms of how many kg/s you need to feed an ion engine:  2.25 kg/s * (8.74/18) = 0.10925 kg/s.

So, an ion engine running on pure electricity spends 0.0486 kg/s (that's all xenon).  An ion engine that's running off fuel cell power spends 0.0486 kg/s of xenon, plus 0.10925 kg/s of LFO, for a total of 0.15785 kg/s.

Thus:  running on fuel cell power gives it (0.15785 / 0.0486) = 3.25 times the mass consumption rate, without giving any additional thrust.  That means its effective Isp is only 4200 / 3.25 = 1293 s.

...Yep, I recalled correctly, I was right.  The Isp of a fuel cell powered ion engine is a hair under 1300, or less than 1/3 that of running on pure xenon.

7 hours ago, Foxster said:

and even that's assuming you are using the minimum numbers of solar panels with no allowance for not getting 100% out of all of them at Kerbin.

Nope.  Whether you're running the fuel cells in concert with solar cells, or alone-- doesn't matter.  The fact is... every bit of electricity you get out of the fuel cells is costing you in mass.  Purely from the standpoint of dV, you're always much better off not using the fuel cell approach; you're throwing away over 2/3 of your dV.

The reason for using fuel cells would be if you had some mission scenario that requires you to do long burns in the dark, or if 1. you're in the far reaches of the outer solar system, where solar cells aren't super effective, and 2. you don't have the patience for running an ion engine off a low-power source like an RTG.

[Foxster]

1 hour ago, Snark said:

 

  Hide contents

Ion engine uses 8.74 EC/s of electricity, and 0.486 units/second of xenon.  Xenon is 0.1 kg/unit, so the xenon usage is 0.0486 kg/s.

Fuel cell array generates 18 EC/s of electricity, and uses 0.02025 units/s of LF and 0.02475 units/s of oxidizer.  Adding LF+O and multiplying by 5 kg/unit, that gives us a mass consumption rate of 0.225 kg/s.  To make an apples-to-apples comparison with the ion engine's needs, let's express that in terms of how many kg/s you need to feed an ion engine:  2.25 kg/s * (8.74/18) = 0.10925 kg/s.

So, an ion engine running on pure electricity spends 0.0486 kg/s (that's all xenon).  An ion engine that's running off fuel cell power spends 0.0486 kg/s of xenon, plus 0.10925 kg/s of LFO, for a total of 0.15785 kg/s.

Thus:  running on fuel cell power gives it (0.15785 / 0.0486) = 3.25 times the mass consumption rate, without giving any additional thrust.  That means its effective Isp is only 4200 / 3.25 = 1293 s.

...Yep, I recalled correctly, I was right.  The Isp of a fuel cell powered ion engine is a hair under 1300, or less than 1/3 that of running on pure xenon.

Nope.  Whether you're running the fuel cells in concert with solar cells, or alone-- doesn't matter.  The fact is... every bit of electricity you get out of the fuel cells is costing you in mass.  Purely from the standpoint of dV, you're always much better off not using the fuel cell approach; you're throwing away over 2/3 of your dV.

The reason for using fuel cells would be if you had some mission scenario that requires you to do long burns in the dark, or if 1. you're in the far reaches of the outer solar system, where solar cells aren't super effective, and 2. you don't have the patience for running an ion engine off a low-power source like an RTG.

OK. Let's do a practical comparison because I'm not convinced of that difference in dV. 

You outfit this with solar panels that will always produce enough power for the engines to run continuously at 100% at Kerbin regardless of the angle to the sun (except when in the shadow of a planet)...

Then we'll see what MJ or KER says it has for dV. I'll then do the same using a fuel cell array and LF+O and we'll compare them. 

BTW - zero monopropellant in the capsule. 

 

Edited July 31, 2016 by Foxster

[Laie]

1 hour ago, Snark said:

The good news is that doing such a burn isn't really "wasting" any dV:  the more you spend there to lower your Ap, the less you'll need to match velocities with Moho and enter orbit when you finally get an intercept.  You're basically just doing part of your orbital-injection burn a bit early. 

Strictly speaking, you're losing the Oberth effect from doing it near Moho. Then again, Moho's gravity is so low that Oberth doesn't amount to much in this case. Your losses will be very small indeed. In terms of dV saved, trying to match Moho's inclination while leaving Kerbin will be a lot more worthwhile than angling for oberth near Moho.

 

 

[Meithan]

2 hours ago, Laie said:

AFAIK the most common experience / problem among Moho first-timers is that you woosh past Moho at ridiculous speed and cannot effect a capture in the short time you spend in it's SOI.

Solutions:

• either bring enough TWR so you can kill your relative velocity quickly (this pretty much rules out ion engines),
• and/or start matching velocities before you even enter it's SOI

I just captured this ~1400 kg spacecraft around Moho using a single ion engine, after a standard midcourse-plane-change interplanetary transfer. Acceleration was about 1.5 m/s^2, so the ~3500 m/s capture burn took 40 minutes, or about 10 minutes at 4x physical timewarp. It requires some patience, yes, but it's certainly doable.

Edited July 31, 2016 by Meithan

[Hotel26]

Great responses, thanks, and a banquet of food for thought.

>This is one of those cases where trying to give someone advice really bumps up against the beauty of KSP, namely, there are many different ways to accomplish a given goal. 

I really appreciate listening to how other people do and think about things precisely because KSP is so rich in ways and means.

The time that xenon may save me in pumping fuel is going to be much more than the length of the "burns"...   

[juanml82]

But if you want to land a kerbal, you need to carry fuel and a chemical engine, so I''m not sure if a ions can carry the weight of a separate lander and retain enough trust to capture in less than 30-40 minutes.

Maybe lowering the solar apoiapsis with a gravity turn at Eve would help before intercepting Moho would help? I did it with nukes, but I don't think I've saved too much dV to bother. However, at that time, I was looking at the total dV and not how much is needed to circularize

[Hotel26]

I said I'd report back on this.  Omega 4, pictured below, has 24 Dawn ion engines and 22x of the large 5,250-unit xenon tanks, plus 32x Gigantor sails.  It weighs in at about 50 tons.

I'm pretty much sold on ion-drive!

As for the long "burns"..........   a good time to read the KSP Forum.

It's designed to be 'ganged' for bigger payloads, too.  This shot shows just a 16-pax cabin mounted as payload.

The 'sail' masts are mounted vertically for lift-off.  Once in space, they are detached and redocked in the 'wing' position.  Toggle them out and it's magic watching them unfurl. 

[Incidentally, an apology: I posted this topic while a perfectly good topic, Ion Engine burns for interplanetary transfers, was open concurrently.  Sorry about that.]

Edited August 3, 2016 by Hotel26
about ganging for bigger payloads

[W. Kerman]

I used an ion engine for my Moho trip. It was two 1-hour burns. That made me unhappy. Nuke engines for the win!

[Hotel26]

Of course, then I see this: bring xenon tank mass ratios to standard values and, yeah, 22x0.41 = 9 tonnes of my 50 are xenon tanks (dry weight).  Well, maybe it will get fixed.

Nevertheless, the thing that bothers me most about that thread are the comments that ion-drive is marginal because it's only useful for lightweight probes.  Well, let's see how many Omegas I can gang up to push my heaviest payload.

The longest burn suffered so far was 45 minutes.  The longest nuke burn I've ever had to do was around 15 and, if I recollect, I had to break that into two periapsis burns to allow engines to cool off.

[There is, actually, a better configuration for my 'sails' but it would be hard to get 32 that way.  In addition, like any sailcraft, when pointed directly into the sun during part of a burn, it's practical I have found, to "tack", just like a yacht.  Hold off from sunward by a few degrees enough to expose all panels and then "come about" onto the opposite tack and then continue prograde.]

[NachoAyala]

Yo! I made a small probe as a lifeboat to come back from Moho to Kerbin (Sorry is this is not what you are asking for). Of course, in "real life" a Kerbal should die due to lack of oxigen and food, but I kinda imagined that he was cryogenized (spelling?). I will download life support and cryo mods to fake this propperly ASAP. Ah! And it works great, the only problem is that the injection burn is so long that I needed like an hour (Or was it half an hour?) to direct me to Kerbin, so, in 10 or 12 minutes after the beginning of the injection I was directly aiming to Moho's surface. The sollution was to start the burn at a height of 400 kms. No probs after that. Using Ions to move bulkier spacecrafts? I wouldn't recommend it. I don't even recommend nukes for Kerbin to Moho, since Moho's SoI is so small that it's difficult to be propperly adressed after such a long burn. I did it with chemical engines partially mounted in orbit.

[Kryxal]

I don't think this was mentioned ... if you're leaving from Kerbin at the Moho AN/DN, you want to arrive at Moho periapsis ... a more expensive initial burn, and you'll be going faster Kerbol-relative, but Moho is going faster still, so it saves delta-v as long as you matched inclinations on departure.

[Biggen]

6 hours ago, Kryxal said:

I don't think this was mentioned ... if you're leaving from Kerbin at the Moho AN/DN, you want to arrive at Moho periapsis ... a more expensive initial burn, and you'll be going faster Kerbol-relative, but Moho is going faster still, so it saves delta-v as long as you matched inclinations on departure.

This. It takes about 1500m/s dV to brake and enter Moho orbit doing it from this method. Ive gone to Moho plenty of times and its the only way I do it. I dont mess with Ions.  Not enough TWR and Im not patient enough for long burns.

[A_name]

On 30/7/2016 at 10:36 AM, mikegarrison said:

One thing that has only been alluded to here is that TWR can be an issue when trying to get into Moho orbit. Some of the times I've used electric engines to get to Moho, my orbital insertion burn has taken so long that I have actually shot all the way through Moho's SoI.

In these cases, most of the time you can brute-force a new encounter by continuing to burn directly at the target marker (assuming you've set Moho as your target).

[Kryxal]

34 minutes ago, A_name said:

In these cases, most of the time you can brute-force a new encounter by continuing to burn directly at the target marker (assuming you've set Moho as your target).

You'll have to REset Moho as your target ... you lose it as a valid target when you enter the SoI

[Blaarkies]

On 7/31/2016 at 2:33 AM, Foxster said:

OK. Let's do a practical comparison because I'm not convinced of that difference in dV. 

You outfit this with solar panels that will always produce enough power for the engines to run continuously at 100% at Kerbin regardless of the angle to the sun (except when in the shadow of a planet)...

Then we'll see what MJ or KER says it has for dV. I'll then do the same using a fuel cell array and LF+O and we'll compare them. 

BTW - zero monopropellant in the capsule. 

 

Do you even Math? 

@Snark just provided proof, so now you want to argue with Mathematics itself? If you really want practical evidence, dont even listen to MJ/KER since they also just do calculations (that's why you added the RTG at the top right?).

Instead, barely leave Kerbin SOI. Write down the current velocity. Start the burn, the whole burn and nothing but the burn. When the xenon tank is empty, write down the final speed. The difference is your dv. *

You can then reverse the rocket equation to find the effective Isp.

* Sure you will lose some speed over a few hours due to Kerbol's gravity keeping you from ascending, but it won't be much more than 100m/s dv over a 10 hour burn