Help Me Plan My First Manned Comet Rendezvous?

[takethecake]

Hey guys! I'm about 200-300 hours into my first KSP career mode, and I recently got a contract to take a surface sample from a comet my Sentinels found. Like most comets (from what I gather), it's really far away and in an inclined, eccentric orbit. What I've been doing so far to plan my missions is a basic DV chart - here's an example of one that worked (just to see how it's *supposed* to work), and then my attempt for this comet mission:

Minmus Ferry/Recoverer Mk1
Start	End	dV	dV + 15%	Stage
Kerbin Surface	Kerbin LO	3200	3680	I
Kerbin Surface	Kerbin LO	200	230	II
Kerbin LO	Minmus Apoapsis	930	1069.5	II
Minmus Plane Change		300	345	II
Minmus Apoapsis	Minmus LO (10km)	160	184	II
Rendevous & Capture		200	230	III
Minmus LO	Kerbin Capture	160	184	III
Minmus Apoapsis	Kerbin LO + Aerocapture	300	345	III
Stage Totals
I	3680
II	1828.5	5508.5	(I/II)
III	759
Total	6267.5

Tanner Comet Lander Mk1
Start	End	dV	dV + 15%	Stage
Kerbin Surface	Kerbin LO	3200	3680	I
Kerbin Surface	Kerbin LO	200	230	II
Kerbin LO	Kerbol (Sun) Apoapsis	950	1092.5	II
Kerbol (Sun) Apoapsis	Inclination Change	250	287.5	II
Comet Rendezvous & Land		2350	2702.5	II
Comet Rendezvous	Transfer to Kerbin LO + Aerocapture	1000	1150	III
Stage Totals
I	3680
II	4312.5
III	1150
Total	9142.5

 

Originally my stage 2 was ~2700 m/s delta-V, which I realized after a few launch attempts wasn't going to be nearly enough, so I upped it to the 4300 it's at now, but I still don't seem to have enough fuel to make the rendezvous. My biggest issue troubleshooting this is not knowing if I'm doing the maneuvers inefficiently causing me to need more fuel, or if I just need more dV. That reason is why I started doing these charts in the first place, but when I'm not navigating between bodies listed on the kerbal "cheat sheet" where the dV values are roughly known, I'm a bit lost at what dV values to put in my chart. For instance, once in a solar orbit, the inclination change takes ~1000 m/s at the AN/DN on the way to apoapsis, but only ~400 m/s if I wait all the way until I hit apoapsis and come around to the other AN/DN - which is right? 

If I do this sequence of burns, I run out of fuel: Leave Kerbin LO (~2000 m/s to get apoapsis near comet's), Inclination change so AN/DN=0 (~1000 m/s), then prograde/retrograde burn about halfway from last node to apoapsis (~600), gives me about 800 m/s left to rendezvous, but my relative speed is still usually 700-800 m/s, so I burn all the rest of my fuel just to stay within, say 200,000 km of the comet and can't get any closer. These dV values are from my maneuver nodes I've set up during these attempts. 

Is it best practice to launch into an inclined orbit from Kerbin, or just do a standard 90? When I've tried launching into an inclined orbit, I can't figure out the relationship between Kerbin LO inclination and solar inclination, so my Kerbin inclination attempts of 10-45 degrees all resulted in a solar inclination of ~5 degrees. My target comet has around a 10 degree solar inclination.

Also, *when* do you launch to intercept a comet? I think I want the comet to be about 15-20 degrees behind Kerbin at launch but would appreciate some confirmation.

Last, just some notes on my chart/design for the comet lander - the 3400 for Kerbin LO is split up so I dump a stage ~68km and then fully reach orbit with my second stage - not leaving junk in space is a big objective for this career. Since there isn't a planet to crash a booster stage into around here, I designed the lander to leave the second stage attached to the comet and just detach the third stage to return home (though I have absolutely no friggin idea how much dV that'll take). Also, what engines would you think to use for this mission? Thought about using six xenon engines for second stage & a single terrier for the third stage, but I couldn't really get my TWR above 0.6 with the xenon's which I figured would make a rendezvous difficult.

Thanks for any help guys!

[Wobbly Av8r]

Love the planning and approach to the problem, @takethecake !!

I would highly recommend launching directly into the correct inclination rather than adjusting to it in LKO (Low Kerbin Orbit); this is a combination of time-of-day as well as actual inclination for your target. [ The path of the comet creates an arc which 'slices' through Kerbin - you must wait until the Launch site is located upon that 'slice' before launching into proper inclination ]

In general, for most orbital mechanics-type problems, the range of solutions vary from quick & expensive (high dV) to slow & efficient. I don't have your situation in front of me, but when I've undertaken these kinds of ventures, in order to do it within reasonable constraints has required a lot of very specific timing to eliminate the extra dV's required to adjust for more random timing.

As an example, for an asteroid intercept mission, I've first launched a probe into a very specific inclination that aligns with the asteroid's path passing Kerbin and then waited for 50+ days to "push out" into an orbit that would allow me to capture the asteroid on the trip back TOWARD Kerbin and Kerbin Pe to minimize velocity differences, keeping in mind that the closer you can orient your vessel's path to that of your target at intercept will allow for a greater disparity of velocity to be compensated for. Hope that helps...

Edited September 12, 2020 by Wobbly Av8r
Grammar, inclination explanation

[Wobbly Av8r]

1 hour ago, takethecake said:

Is it best practice to launch into an inclined orbit from Kerbin, or just do a standard 90? When I've tried launching into an inclined orbit, I can't figure out the relationship between Kerbin LO inclination and solar inclination, so my Kerbin inclination attempts of 10-45 degrees all resulted in a solar inclination of ~5 degrees. My target comet has around a 10 degree solar inclination.

One way to think about what's happening is that once you 'pop out' of Kerbin's SOI, you are *IMMEDIATELY* in Kerbol's SOI with all of the properties of Kerbin's orbit around Kerbol which have become your vessel's, plus or minus whatever you were doing around Kerbin. As a result, the dV requirement can become extreme to alter that path. The low-dV solution to this is to not only launch with proper inclination at time-of-day but time-of-year as well (when the arc of the comet through Kerbol intercepts Kerbin's orbit around Kerbol). [ In many cases you may not be able to launch at the optimal time-of-year, but in doing so you have to recognize that the dV requirement increases proportionately ]

Edited September 12, 2020 by Wobbly Av8r

[takethecake]

Wow thank you for such a comprehensive reply so soon! This is really great feedback - I think that the biggest things that will help will be trying to time my launches better (that slice visualization makes perfect sense). The visualization for accounting for time-of-day as well as time-of-year is a little trickier for me though - "[launch when] the arc of the comet through Kerbol intercepts Kerbin's orbit." When I read that I immediately thought "but wait, isn't the comet orbit always intersecting Kerbin's orbit?", remembering that when I would set the comet as my target, the A.N. and D.N. would pop up pretty darn close to where it looked like the orbits intersected. But now that I think about that, I think there was still a "little" gap - am I interpreting it correctly that at a certain point, that "gap" will close up and the orbits will "actually" intercept? Maybe I should just get in the tracking station and play around with time warp, though I'm not sure how easy it would be to see.

And a related question - I noticed in your second reply you didn't specifically mention where the comet physically is in it's orbit relative to Kerbin at a "reasonably optimal" launch time - is that because it's a function of one of the other "timing" things you've already suggested, because it doesn't matter, or something else?

Thanks so much for all the help man  gonna go do some followup research on timing launches. I guess I've been able to get by so far with the interplanetary transfer calculator to give me transfer angles - that tool makes going to the planets a lot easier than something like this!

[Wobbly Av8r]

It's one of the 'charms' of KSP that it hands out contracts that may be damn near impossible (well, nothing's impossible in KSP but you get my gist...) but some folks like a challenge so most players quickly establish an understanding of which contracts will help progress their game and which contracts will end up giving them an aneurysm... (I'll leave the classification to you on chasing this comet) There are definitely harder and easier ways to make Kerbucks...

The short answer to your question...

11 hours ago, takethecake said:

"but wait, isn't the comet orbit always intersecting Kerbin's orbit?", remembering that when I would set the comet as my target, the A.N. and D.N. would pop up pretty darn close to where it looked like the orbits intersected. But now that I think about that, I think there was still a "little" gap - am I interpreting it correctly that at a certain point, that "gap" will close up and the orbits will "actually" intercept?

...is that when two objects orbit a common body, yes, at two points the PLANES of their orbit will cross. For adjusting one body's orbit to another, THAT is the most efficient spot to make that adjustment but that doesn't mean YOU are any closer to the other object - just at the lowest dV plane-adjustment point. The orbital plane of Kerbin crosses the orbital plane of Eeloo twice per orbit, but that doesn't mean you're any closer to Eeloo after a plane change than when your were sitting on the Launch pad.

Because comet orbits are highly eccentric and inclined, they seem to be more "in reach" but in general, the possibility of capture is in the timing of the closest approach rather than a "transfer window" as it applies to other bodies orbiting Kerbol. Launching into the proper inclination is one thing, pushing the Ap out efficiently to make contact is highly dependent on the relationship between Kerbin's orbit and the comet's position relative to that orbit (meaning time of year) - IF you're on a "dV budget".

[ Edit: After reviewing my posts for clarity, I may have insinuated that the Launch itself is critical to all intercept orbit aspects, but it is not immediately critical to time-of-year, as that aspect of the intercept can be most easily dealt with by performing your "push out" burn at the correct position/time-of-year after establishing the proper inclination and orientation (longitude of ascending node) around Kerbin, i.e time your launch so that the initial orbit itself is parallel to the comet's orbit and maintain that until Kerbin's orbit around Kerbol places your orbit in the same PLANE as the comet's. Hope that clears it up... ]

Edited September 13, 2020 by Wobbly Av8r

[king of nowhere]

i can't really figure out what kind of trajectory you are trying to get, but i have a couple tips to add.

1) you mention minmus, i assume you're passing through it. I assume you have a refueling station on minmus? anyway, minmus look like a great place to launch a interplanetary mission, already on the edge of the sphere of influence, low gravity to make it very cheap to escape... i made that mistake myself.

Minmus is horrbile as a starting place to go anywhere. And the reason is oberth effect. i thought the difference would be mild, but it can take upwards to 5 times the fuel to make the same manuever. this soon overtakes the advantage on not having to climb out of a gravity well. A mission starting on minmus going anywhere except eve is more expensive than the same mission starting in LKO.

Mun is a much better place. it is only mildly more expensive, but you get better oberth effect. going to jool starting from mun or LKO is about the same cost, it is convenient for going closer.

the real best option, though, is to refuel on mun and leave by making a gravity assist around kerbin. this way you get both the discount on leaving the gravity well, and the oberth effect. You can do that from minmus too, but the slow and inclinated orbit makes it more difficult. it's difficult to set up this gravity assist perfectly, getting it good enough to save some fuel is not too complex.

So, if you were making your big burn to escape kerbin SoI in minmus orbit, you can save a lot by either doing it in LKO, or by using a gravity slingshot around kerbin.

2) there is no mention of where you are going to intercept the comet; time concerns favor intercepting it at periapsis, when it is closer. but it's much cheaper to intercept it at apoapsis. it's generally somewhere around eeloo's orbit, meaning 2000 m/s from LKO to reach. but once you are there, both you and the comet are moving so slowly, you only need a few hundred m/s to adjust for any difference of speed and inclination. similarily, getting back on an intercept to kerbin is also relatively cheap.

you'll intercept kerbin at high speed though, so aerocapture won't generally be possible.

[takethecake]

2 hours ago, Wobbly Av8r said:

It's one of the 'charms' of KSP that it hands out contracts that may be damn near impossible (well, nothing's impossible in KSP but you get my gist...) but some folks like a challenge so most players quickly establish an understanding of which contracts will help progress their game and which contracts will end up giving them an aneurysm... (I'll leave the classification to you on chasing this comet) There are definitely harder and easier ways to make Kerbucks...

The short answer to your question...

...is that when two objects orbit a common body, yes, at two points the PLANES of their orbit will cross. For adjusting one body's orbit to another, THAT is the most efficient spot to make that adjustment but that doesn't mean YOU are any closer to the other object - just at the lowest dV plane-adjustment point. The orbital plane of Kerbin crosses the orbital plane of Eeloo twice per orbit, but that doesn't mean you're any closer to Eeloo after a plane change than when your were sitting on the Launch pad.

Because comet orbits are highly eccentric and inclined, they seem to be more "in reach" but in general, the possibility of capture is in the timing of the closest approach rather than a "transfer window" as it applies to other bodies orbiting Kerbol. Launching into the proper inclination is one thing, pushing the Ap out efficiently to make contact is highly dependent on the relationship between Kerbin's orbit and the comet's position relative to that orbit (meaning time of year) - IF you're on a "dV budget".

[ Edit: After reviewing my posts for clarity, I may have insinuated that the Launch itself is critical to all intercept orbit aspects, but it is not immediately critical to time-of-year, as that aspect of the intercept can be most easily dealt with by performing your "push out" burn at the correct position/time-of-year after establishing the proper inclination and orientation (longitude of ascending node) around Kerbin, i.e time your launch so that the initial orbit itself is parallel to the comet's orbit and maintain that until Kerbin's orbit around Kerbol places your orbit in the same PLANE as the comet's. Hope that clears it up... ]

I think I get it now - it definitely helps to consider the individual steps/maneuvers, breaking it up makes it seem less complicated. I'm definitely going to try launching into LKO at the correct inclination and orientation first and then waiting for the time-of-year/plane intersection to leave the Kerbin SOI - will the relationship between my Kerbin inclination and Kerbol inclination be 1/1 if I launch at the correct orientation (or is the ratio dependent on doing the maneuver at the plane intersection)? 

Also, I meant to add in my first reply that in general, my philosophy on the time<->efficiency spectrum: at this point in my career I decided I'm not in a rush haha and that aiming for the most efficient maneuvers will a) make it easier to design ships and b) help me learn best practices, though I guess it leaves less room for error. I've been enjoying the challenge thus far!

1 hour ago, king of nowhere said:

i can't really figure out what kind of trajectory you are trying to get, but i have a couple tips to add.

1) you mention minmus, i assume you're passing through it. I assume you have a refueling station on minmus? anyway, minmus look like a great place to launch a interplanetary mission, already on the edge of the sphere of influence, low gravity to make it very cheap to escape... i made that mistake myself.

Minmus is horrbile as a starting place to go anywhere. And the reason is oberth effect. i thought the difference would be mild, but it can take upwards to 5 times the fuel to make the same manuever. this soon overtakes the advantage on not having to climb out of a gravity well. A mission starting on minmus going anywhere except eve is more expensive than the same mission starting in LKO.

Mun is a much better place. it is only mildly more expensive, but you get better oberth effect. going to jool starting from mun or LKO is about the same cost, it is convenient for going closer.

the real best option, though, is to refuel on mun and leave by making a gravity assist around kerbin. this way you get both the discount on leaving the gravity well, and the oberth effect. You can do that from minmus too, but the slow and inclinated orbit makes it more difficult. it's difficult to set up this gravity assist perfectly, getting it good enough to save some fuel is not too complex.

So, if you were making your big burn to escape kerbin SoI in minmus orbit, you can save a lot by either doing it in LKO, or by using a gravity slingshot around kerbin.

 

 

Oh I just added the Minmus ship as an example of my planning spreadsheet since I wasn't sure if my Comet spreadsheet would make sense on its own. Maybe that was more confusing haha. But no I haven't been trying to use any gravity assists from the Mun or Minmus or doing any refueling - I figured that would make things more complicated than they already are even if it'll save me 300-400 dV (or is it more, that's just a guess). I do have an orbital station at a 20km orbit around the Mun with 4000 units of liquid fuel (+ oxidizer) that doesn't have any plans. Would it make sense to refuel at the Mun, wait until on the opposite site of Kerbin and burn prograde to escape the Mun, then do an inclination change at the AN/DN to match the comet? If I do that do I still launch from Kerbin at the same orientation or do I have to do a maneuver somewhere to compensate?

Quote

2) there is no mention of where you are going to intercept the comet; time concerns favor intercepting it at periapsis, when it is closer. but it's much cheaper to intercept it at apoapsis. it's generally somewhere around eeloo's orbit, meaning 2000 m/s from LKO to reach. but once you are there, both you and the comet are moving so slowly, you only need a few hundred m/s to adjust for any difference of speed and inclination. similarily, getting back on an intercept to kerbin is also relatively cheap.

Okay 2000 m/s is a good rough reference point - that seems to match up with what I've seen on my attempts so far. And the one video I've seen so far - the guy also intercepted at apoapsis and that seems to be the right approach (I watched his maneuvers quite closely but just couldn't replicate them for anywhere near the same dV cost - now I think that's been a function of my launch/maneuver timing). But isn't it inefficient to do an inclination burn at apoapsis (instead of AN/DN)? If the inclination burn isn't done before say halfway to apoapsis it seems I'd end up far enough away from the comet that any efficiency savings would be canceled out by the dV needed to close the greater distance. Maybe I'm coming at this from the perspective of needing to adjust about 10 degrees after leaving Kerbin SOI in my attempts so far - from Wobbly Av8r's help it sounds like if I can make the right adjustments to my launch orientation/inclination and perform my burn to leave Kerbin SOI at the right time I can get that required inclination adjustment down to a few degrees? And maybe then it doesn't matter so much where the inclination adjustment happens?

Quote

you'll intercept kerbin at high speed though, so aerocapture won't generally be possible.

How much dV do you think I'd need - not the full 3400 m/s or so right? I budgeted 1000 m/s plus 15% safety factor for 1150 for launching off the comet, adjusting to intercept, and landing on Kerbin - do you think that's reasonable? I'd say I'm okay with up to 20 or so orbits around Kerbin before capture if necessary lol.

Thanks guys!

[Wobbly Av8r]

1 hour ago, king of nowhere said:

Minmus is horrbile as a starting place to go anywhere. And the reason is oberth effect. i thought the difference would be mild, but it can take upwards to 5 times the fuel to make the same manuever. this soon overtakes the advantage on not having to climb out of a gravity well. A mission starting on minmus going anywhere except eve is more expensive than the same mission starting in LKO.

Not trying to presume anything with regard to your experiences, but it usually takes less than 100 dV to leave a Minmus orbit to escape Kerbin's SOI, compared to ~4300+ for Kerbin; the Mun and Minmus are similar in advantage with the main difference in orbital period (how long it takes to get back to a specific postion with the Mun 'winning') and extra dV to both establish the orbit and then leave it again (with Minmus 'winning'). Once outside of Kerbin's SOI you have the orbital velocity of Kerbin around Kerbol (9,000+ m/s), give or take, to go/do whatever you want.

Many folks want to think of Minmus as a tall mountain on Kerbin that they can jump off of to other places but are stunned when they encounter the issues you have; the key to using Minmus is to understand that you have to FIRST escape Minmus' SOI and THEN escape Kerbin's SOI. Attempting to do both in one burn, depending on where you're going AFTER the burn, can diminish or completely remove any advantage you might have had operating from a Minmus orbit.

There's a saying among pilots that there are old pilots and bold pilots, but there are no old, bold pilots. The same could be said about space travel - there are quick solutions and there are efficient solutions, but there are no quick, efficient solutions; here you are faced with the potential of two burns - maybe 30 days apart - to leave Kerbin's SOI from Minmus orbit efficiently or some much, much higher dV burn to leave Minmus' SOI, enter Kerbin's SOI in a disadvantageous position in which you cotinue to burn inefficiently to escape Kerbin's SOI. It's similar to launching straight up and then making a 90 deg turn to establish orbit vs. a gravity turn.

Edited September 13, 2020 by Wobbly Av8r

[king of nowhere]

6 minutes ago, Wobbly Av8r said:

Many folks want to think of Minmus as a tall mountain on Kerbin that they can jump off of to other places but are stunned when they encounter the issues you have; the key to using Minmus is to understand that you have to FIRST escape Minmus' SOI and THEN escape Kerbin's SOI. Attempting to do both in one burn, depending on where you're going AFTER the burn, can diminish or completely remove any advantage you might have had operating from a Minmus orbit.

 here you are faced with the potential of two burns - maybe 30 days apart - to leave Kerbin's SOI from Minmus orbit efficiently

I don't get it.

So, you make a small burn that would put you out of minmus SoI, but still around kerbin. then what? you make a burn from there? you're still going to be pretty slow, around 5-700 m/s. which is still a strong improvement over minmus, granted. But it's still no more efficient than launching directly from Mun orbit. i doubt it could come close to the efficiency of a kerbin gravity assist

Unless i've misunderstood what you're suggesting. Anyway, mun orbit to kerbin gravity assist has the right balance between efficiency and complication for my tastes.

[Spricigo]

On 9/12/2020 at 12:14 PM, takethecake said:

For instance, once in a solar orbit, the inclination change takes ~1000 m/s at the AN/DN on the way to apoapsis, but only ~400 m/s if I wait all the way until I hit apoapsis and come around to the other AN/DN - which is right? 

Inclination changes are cheaper when your speed is low, If you have a choice  do it near the apoapsis. Notice that you can only get inclination to zero at AN/DN.

On 9/12/2020 at 12:14 PM, takethecake said:

If I do this sequence of burns,

Sorry, I'm not even attempting to follow all that without some images. But it's a rendezvous like the ones you did in Low Kerbin Orbit and the same principles apply (just takes a lot more deltaV and time). 

If you are behind the target you need to lower your orbit to catch up, if you are ahead of the target raise your orbit and let the target catch up with you. If is your inclination that is still off, you need to wait for the AN/DN to zero it out.

On 9/12/2020 at 12:14 PM, takethecake said:

When I've tried launching into an inclined orbit, I can't figure out the relationship between Kerbin LO inclination and solar inclination,

There is no simple relationship between inclination in Kerbin orbit and inclination in solar orbit, inclination is just one of six parameters that define your trajectory. the game takes your whole trajectory when you leave the previous SoI and translate it to your trajectory in the new SoI. If you are not crossing the SoI boundary there is no relationship at all, for all purposes your solar orbit is the orbit of the planet you are orbiting.

Notice , by the time you reach the SoI boundary, the small inaccuracies in the while launching and at LKO resulted in not so small deviations. Some player take this as a hint to not bother with  correct inclination at the launch and others are confident to keep it to manageable levels, in any case later corrections are expected.

On 9/12/2020 at 12:14 PM, takethecake said:

Also, *when* do you launch to intercept a comet? I think I want the comet to be about 15-20 degrees behind Kerbin at launch but would appreciate some confirmation.

You want the comet just about to enter Kerbin SoI by the time you launch. Unfortunately, it's not gonna happen. 

Anyways, is the usual rendezvous thing again.

For what is worth, I have a slight preference  for chasing the target in a lower phasing orbit and I'd take as much time as allowed by the contract deadline to save on the deltaV budget. 

On 9/12/2020 at 12:14 PM, takethecake said:

Also, what engines would you think to use for this mission? Thought about using six xenon engines for second stage & a single terrier for the third stage,

Since you are concerned about deltaV budget, there is 3 way to increase it: Moar Fuel, Moar Isp and less everything else (including engines you are not using)

Don't make sense to have a heavier terrier** being carried around by more efficient ion engines. It may the case that the ion engines lack the power for all the trip*, in which case is ok to use the terrier (or something else) to bring the ion engines to where they can take over.

 

*It may also be the case that ion engines are painfully weak and you agree with me about not using it except maybe in the lightest probes.

 

**that thing weight half a ton!! 

 

6 hours ago, king of nowhere said:

Minmus is amazing as a starting place to go anywhere. And the reason is Oberth Effect.

Fixed that t you.

You are correct that you don't get any noticeable boost to depart from Minmus. You don't need it at this moment anyway. 160m/s to get in orbit, 180m/s to escape and we are underway.

Then we let Kerbin accelerate us, te get to the apoapsis with full tanks and a nice boost from Oberth Effect to make our transfer burn.

[Wobbly Av8r]

...

Edited September 15, 2020 by Wobbly Av8r