# How far can you take an orange fuel tank?

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Hi! Quincunx here. I'm a physics PhD student and long, long time watcher, first time writing. I have a deep love of orbital mechanics and rocket science in general, so KSP was the perfect fit for me.

I know this is daring for a first post, but I have a "mission idea" fueled by so much curiosity it compelled me to make an account. The one thing that always got to me about KSP was the Rockomax Jumbo-64 fuel tank. It can hold an awful lot of liquid fuel, and if you design your rocket appropriately, that much dV can get you far. So I wondered: How far, exactly?

To my delight, there was a reddit challenge and Scott Manley video featuring this very question. The challenge was, using only an orange tank, stock parts, and no RCS, to bring a probe with the accelerometer, thermometer, magnetometer and gravioli detector for a Munar flyby and return safely. Hard mode was to land the probe safely on Duna instead. And, of course, Scott executed "hardererest mode" with a landing on Duna, then on Ike, and back home (almost) safely (and Desolution did even better) - but he also mentions having pondered a possible visit to Minmus as part of the trip.

I checked out this old question on the most efficient way to get to Duna, and it seems like the consensus (among at least two people) is that the minimum dV is expended by getting a gravity assist from Kerbin after a Minmal encounter (when Minmus is closer to Kerbol than Kerbin is). This makes sense, since one gets the benefit of the Oberth effect from executing a transfer burn inside Kerbin's gravity well. And, of course, we can do the reverse with Ike and Duna to come home, going the other way around to get kicked into a lower Kerbolar orbit. I am also aware that, when the Mun is properly positioned, a gravity assist from it can save fuel en route to Minmus.

All this to say: Is the following "itinerary" theoretically possible with a crewed single-orange-tank craft, even if it takes a master to perform?

1. Take off from Kerbin, and get into LKO.
2. Burn for a Mun encounter, and get a flyby gravity assist to Minmus.
3. Land on Minmus. (Wait there until Minmus is in the right position and a transfer window to Duna opens up.)
4. Take off from Minmus, and burn at the Kerbin encounter to transfer to Duna.
5. Aerocapture, aerobrake, and land on Duna.
6. Take off and land on Ike. (Wait there until the transfer window back to Kerbin opens up.)
7. Get one final gravity assist off Duna and return to Kerbin. (Land safely.)

Now, I'll admit I haven't done the dV estimation, though not for want of trying - while I'm familiar with the celebrated dV map, I just can't find an application to calculate dV for gravity assists, and I don't know how to make KSPTOT 1.6.7 do the maths for me. However, by my estimate, if 1-4 is not actually cheaper than a direct burn to Duna from LKO, it should only be slightly more expensive.

So is this insane mission even possible with a single orange tank? If necessary, it would be okay to drop the scientific gear, use a probe, and even nix the Minmal landing if absolutely necessary. I'd be happy with a definitive answer based on how much dV you can squeeze out of an orange tank with nuclear and jet engines. And if anyone could actually fly the mission, they would be Jebediah simply amazing.

Sorry for the text wall. :)

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A better way to do it would be to use multiple Mun assists to eject into a 6:5 resonance of Kerbin and then one more to raise the orbit to Duna. This is very significantly cheaper than what you had in mind.

This is an interesting challenge - I might give it a try.

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Ok, part 1 - taking the orange tank to orbit

Craft in VAB. The restriction of the orange tank leads to some less conventional design choices, at least for high performance SSTO craft. However, I still make use of the fairing occlusion and node occlusion properties of KSP to get nice and streamlined, as well as putting a 5 degree incidence on the wings. This gives a better lift to drag ratio.

On runway. The craft has one Rapier engine and one nuclear engine. It struggles off the runway because of the Rapier's low static thrust but that's okay since I can pick up speed in the lower atmosphere.

At 400 m/s I begin climbing to get out of the thick part of the atmosphere as early as possible.

At 20 km I level off and accelerate to 1600 m/s with the Rapier before turning on the Nerv. The Rapier lasts me until 1700 m/s before flaming out, at which point the single Nerv pushes to orbit.

At this point, I'm at the sweet spot where lift, gravity and centripetal force balance out almost exactly. I stay this way for the rest of the ascent. Burning horizontally like this in the upper atmosphere reduces gravity, Isp, and drag losses.

I'm in orbit with almost 4900 m/s left. This is enough to fly by any destination, and definitely enough to go to Duna.

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7 hours ago, camacju said:

A better way to do it would be to use multiple Mun assists to eject into a 6:5 resonance of Kerbin and then one more to raise the orbit to Duna. This is very significantly cheaper than what you had in mind.

This is an interesting challenge - I might give it a try.

Thank you, camacju, for your participation!

To be perfectly honest, rather embarrassingly, I am still on KSP 0.22 and I did not even realise the Rapier is stock... I've been trying to do this using two nuclear engines and a pile of jet engines with a whole stack of intakes (plus aerospikes if they help). Since you roundly crushed the original challenge, are you (or is anyone else) up for hardereresterestest mode with these engines only?

As for your suggestion, let me confirm what you proposed:

LKO - Mun encounter - Kerbin orbit (?) (with high eccentricity) - Mun encounter - ... - Kerbol orbit (6:5 resonance with Kerbin) - Duna transfer

Versus what I had in mind:

LKO - Mun encounter - Minmus orbit - Minmus surface - Minmus orbit - Kerbin encounter - Duna transfer

The main issue I had with the approach you advised is that I thought a prograde Munar flyby (not the kind used for free-return trajectories) would eject me from Kerbin's SoI and then it would be very hard to get further GAs. If one Munar flyby only kicks me into an eccentric Kerbin orbit (without burning at periapsis) then that definitely makes sense.

I guess my real question is this: What is the minimum amount of dV needed to get to Duna from LKO? Apparently there is a chart somewhere that shows dV for GAs but I have been searching for it to no avail.

Referring to the community dV map, which I'm assuming is not outdated, a direct burn to Duna starting in LKO would cost about (930+130)m/s or 1060 m/s (direct insertion).

According to the thread I linked before, a Mun - Duna GA would cost 414 m/s while a Minmus-Kerbin-Duna GA costs 306 m/s. Now, I'm not sure where either of those start from, but Slashy indicates LMO to Duna costs 467 m/s for him, and looking at the chart it's already 860 m/s to get a Munar encounter in the first place. So except for that really rare scenario where you set up a burn in LKO that encounters the Mun and goes on to encounter Duna (with maybe a few small corrections), it seems that a Munar GA to Duna is more expensive than a direct burn. In any event it's not possible to save more than 200 m/s, the delta between dV to Duna (direct) and dV to a Munar encounter (d^2V?).

Not that it's easy, but I'm starting to think that in terms of dV the most efficient path to Duna is through repeated Mun encounters. The perfect no-burn Munar GA definitely saves some dV, but not the full 200 m/s because the burn in LKO also has to line up the Duna encounter afterwards. If repeated flybys can get you to a 6:5 orbit for the price of a Munar encounter, that may be better.

Honestly, I don't know. Is the aforementioned GA dV chart still around? I don't think my question is much about actually carrying out the mission so much as theory now.

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13 minutes ago, Quincunx said:

To be perfectly honest, rather embarrassingly, I am still on KSP 0.22

I gotta ask, purely from curiosity, why?

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Just now, Gargamel said:

I gotta ask, purely from curiosity, why?

Because I'm a cheapskate who got KSP back in the bad old days and don't know how to update it.

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3 minutes ago, Quincunx said:

Because I'm a cheapskate who got KSP back in the bad old days and don't know how to update it.

Well then... I was too... I started on .17 myself.     Just log into your KSP store account (you might have to go through the lost password schtick) and then just download the latest version.    Don't even bother updating.   You'll have to download like 24 patchers to get that to happen, even if you can update.  So just get the latest and greatest 1.11.2, and install that fresh.

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Thanks for the advice, Gargamel. In the meantime, though, I would still really like to ask my theoretical question: What is the minimum amount of dV required to get to Duna from LKO?

I'm envisioning several scenarios based on the above discussion:

1. Kerbin-Duna (because...yeah)
2. Kerbin-Mun (burn at PE)-Duna (this seems to cost dV because we lose out on the Oberth effect)
3. Kerbin (burn in LKO perfectly)-Mun-Duna (this seems good, but potentially hard?)
4. Kerbin-Mun-Kerbin (burn at PE)-Duna (get the benefit of the Oberth effect again)
5. Kerbin-Mun-Minmus-Kerbin (burn at PE)-Duna (does this eject me from Kerbin's SoI altogether? If not, should be better than 4.)
6. Kerbin-Mun-Kerbin-Mun (burn at PE)-Duna (if one is good, two must be better)
7. Kerbin-Mun-Kerbin-Mun-Kerbin (burn at PE)-Duna (I'm not sure if I'm still in Kerbin's SoI at this point)
8. What he suggested-Duna

Again, if anyone knows where to find the chart that has these data, that would be stellar.

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Take a look here:

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Thanks VoteSquid, but I already looked at this chart and don't see any mention of GAs (which is the rather salient point here). Please correct me if I'm wrong.

Also, referring to this thread, it seems I may have this all wrong. Maybe I want an Eve GA for these purposes.

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2 minutes ago, Quincunx said:

Please correct me if I'm wrong.

You're not. All dv maps I know about are not including any GA assisted trajectories, I'm sorry.

Naturally, the higher the mass, the  greater the GA effect of a given CB can be, for the Kerbol system that would be, apart from Jool, Eve, Kerbin and Duna.

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Posted (edited)
1 hour ago, VoidSquid said:

You're not. All dv maps I know about are not including any GA assisted trajectories, I'm sorry.

Naturally, the higher the mass, the  greater the GA effect of a given CB can be, for the Kerbol system that would be, apart from Jool, Eve, Kerbin and Duna.

No worries, I guess this chart is lost to the aether.

Now I'm starting to wonder if GAs in the reverse direction can be done to get to Eve, for a follow-up GA to Duna.

Clearly there are a lot of possible mission plans to consider. Some of them make sense (i.e., they are provably better than a direct insertion), and some of them don't (they are provably worse). Most of them look really difficult, though.

Perhaps a challenge could be to attempt some of these GAs to Duna and see how much dV each of them costs. Anyone interested?

EDIT: On page 16 of the community dV map thread, in response to Benton:

Quote

Does the map include possible gravity assists?

Snark replies:

Quote

Generally dV maps don't do that-- mainly because gravity assists are so problematic in KSP.  The amount of dV you get from them is fairly small... and also it's so hard to aim when setting up a gravity assist that the extra amount wasted when (almost inevitably) you have to do a correction burn tends to more than offset the savings.  Plus, how much assist you'd get would seriously depend on circumstances, e.g. what angle you approach from, how low a periapsis you can manage, etc.

In practice,

• it would be really hard to have any meaningful info on a static "dV map" that would accurately capture gravity-assist benefits, and
• in practice, gravity assists tend to be so rarely used by KSP players that it would be of marginal benefit anyway to try to add it to the map.

So, essentially, "no".

In addition to those problems, GA-based dVs are basically Markov chains - previous states (such as whence you come) affect how much dV they cost. Still, it would be nice to estimate them for these missions. Also, before anyone comments on it - I am fully aware that lining up multiple GAs not only is extremely difficult, but also requires the assisting bodies to be configured correctly to permit the assist in the first place. Since right now the discussion is theoretical, I'm assuming that all assisting bodies are correctly positioned for whatever GAs that mission requires.

Incidentally, does anybody know how to calculate multiple GAs with KSPTOT? Maybe there is a tutorial somewhere, but I haven't found it yet.

EDIT 2: Looks like FF does what I need.

EDIT 3: Wait no. I don't have info on Munar/Minmal GAs. I guess I'll still have to do that with KSPTOT.

...

I'm obsessed with gravity assists.

-Qx

Edited by Quincunx
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2 hours ago, Quincunx said:

Now I'm starting to wonder if GAs in the reverse direction can be done to get to Eve, for a follow-up GA to Duna.

Kerbin-Eve-Duna and Kerbin-Duna take about the same delta-v but Kerbin-Eve-Duna involves greater Duna-relative velocity - in other words, it requires more aerobraking or a greater capture burn. Not worth it IMO.

Also, it's accepted that the lowest delta-v way to get anywhere involves spamming Kerbin and Mun assists until you wind up enough energy to go to Duna or Eve.

I'm going to post part 2 of my mission soon btw

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Part 2 - Going to Duna

Periapsis kick burn 1 of 2 - TWR is too low to effectively do the Mun transfer in one orbit

Periapsis kick 2 of 2 to get a Mun assist. This Mun transfer is over 850 m/s which isn't optimal but I have enough fuel to spare.

The suboptimal Mun assist means that I can take an easier gravity assist route. A Mun assist puts me onto a 1:1 resonance with Mun, which gives me another assist that gives me a plane change (to scrub off the residual orbit inclination) and puts me into a loiter orbit high above Kerbin.

A third Mun assist ejects me radially from Kerbin, where I can encounter Kerbin again in one orbit.

In the Kerbin encounter, I get another Mun assist to get more energy, since just a Kerbin encounter won't help me any. Using Kepler's third law, I try to get into a sun orbit with the sum of apoapsis and periapsis equal to 3.243 * 10^10, since that represents a 4:3 orbital resonance. This correction burn is very suboptimal but again I've got more than enough fuel.

View of Kerbin assist - the burn was precise enough that my subsequent Kerbin-Mun assist was already plotted

Mun assist

This brings me to an orbit touching Duna's orbit which reduces velocity relative to Duna

View of Duna encounter

Aerobraking

Scrubbing off velocity

Landed on Duna - with more than half my fuel remaining!

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Posted (edited)

My Kod, camacju. That is simply amazing. [:O]

I bet I can learn quite a bit from those images you posted, too.

EDIT: You say your first Mun encounter was suboptimal as in > 850 m/s. I'm looking at your remaining dV and it seems you went from 4859 m/s dV to 4008 m/s dV, i.e. an 851 m/s burn. Am I missing something or were you just being that literal?

And you got from there to Duna in an additional 21 m/s. Wow. That's even more incredible to me considering the route you took was...let's see... K-M-M-M-K-M-D?

The original mission plan I had in mind was to land on Ike, get a Duna flyby, and return to Kerbin, but as it is you could probably land on the Mun and maybe Minmus with fuel to spare!

Another thing I'm wondering - if you start this mission using air-breathing engines, can you dump oxidizer to get into orbit with even more dV to spend?

EDIT 2: Contemplating about a Mun landing on the way back got me thinking: How fast are you moving on a return from Duna? Should one attempt to get a Mun encounter instead of a Kerbin encounter, or is it more efficient to AB around Kerbin and get a Mun encounter that way?

All things considered, I'm still a newbie, so forgive the potentially dumb questions!

Edited by Quincunx
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4 hours ago, Quincunx said:

What is the minimum amount of dV required to get to Duna from LKO?

By the way - the minimum amount of dV required to get anywhere from LKO is 830 m/s. I had a discussion about this a while ago.

830 m/s gets you a Mun encounter, then chain Mun assists together until you escape Kerbin (multiple are required here because of the low initial energy)

Then spam resonant orbits and Mun assists until you get to Duna or Eve

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11 minutes ago, camacju said:

By the way - the minimum amount of dV required to get anywhere from LKO is 830 m/s. I had a discussion about this a while ago.

830 m/s gets you a Mun encounter, then chain Mun assists together until you escape Kerbin (multiple are required here because of the low initial energy)

Then spam resonant orbits and Mun assists until you get to Duna or Eve

Do you have the thread link? I suspect such a discussion would have addressed many of my questions (including ones I don't yet know I have).

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This was a Discord discussion, not on this forum. Sorry!

I could DM you an invite link

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Thanks, that would be awesome!

I don't know if your mission is still running, but if you're taking requests, would you be interested in flying around some more (Ike, Mun, Minmus or whatever)? Your posts in this thread have been both informative and entertaining.

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Yeah, I've got some more destinations in mind

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Haven't quite landed at my next destination yet, but here are some more images:

Ascent from Duna, pitched up to maintain altitude in the thin atmosphere. TWR means I can afford to do this

Picking up speed

Duna orbit - I've got a bunch of fuel left. Let's go somewhere else

Ike assist

Kerbin assist, lining up DN with Eve's orbit

Eve assist sets up another Kerbin assist

Eve flyby

Jool encounter using K-E-K-K-J route, no need for a second Eve assist this time. Tylo flyby gets a captured orbit intersecting Laythe's orbit

Approaching Laythe

The Laythe landing will probably be later today

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Laythe landing!

Getting toasty

Aerocaptured

Aerobraking

Landed at Laythe

Laythe ascent - much easier than Duna

Suboptimal ascent because I need to avoid this mountain

Going suborbital on just Rapiers

Circularizing

Laythe orbit

Vall and Laythe assists

Tylo assist

Tylo flyby

Kerbin and Eve assists

Aerocapture

Aiming for KSC

Aerobraking

Landed

Mission summary:
Landed on Duna and Laythe before coming back to KSC
Flybys of Mun, Ike, Eve, Jool, Tylo, Vall

I think this counts for "harder-er-est-er" mode (:

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@Quincunx Sorry I missed some of your other questions.

10 hours ago, Quincunx said:

Am I missing something or were you just being that literal?

I didn't mean to imply that 850 was optimal. Optimal is 830-835, so 851 isn't too good.

10 hours ago, Quincunx said:

That's even more incredible to me considering the route you took was...let's see... K-M-M-M-K-M-D?

I do correction burns two or three assists ahead of time (usually) since that tends to make them small. The only exception is the Kerbin-Mun-Kerbin assists because KSP doesn't let me see that far ahead. The route was K-M-M-M-K-M-K-M-D (I bounced off Kerbin and Mun twice).

10 hours ago, Quincunx said:

but as it is you could probably land on the Mun and maybe Minmus with fuel to spare!

I definitely could. Laythe is more fun of a destination however

10 hours ago, Quincunx said:

can you dump oxidizer to get into orbit with even more dV to spend?

Newer versions of KSP have tweakables so I can remove the oxidizer in VAB. Also the Nerv doesn't use oxidizer anymore.

10 hours ago, Quincunx said:

Should one attempt to get a Mun encounter instead of a Kerbin encounter, or is it more efficient to AB around Kerbin and get a Mun encounter that way?

The best way is to aerobrake directly into a Mun encounter but that's really hard. I prefer aerobraking into a high Kerbin orbit and then raising the orbit at apoapsis for a Mun encounter at periapsis. This pseudo-ballistic capture gives a smaller correction burn

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14 hours ago, camacju said:

Laythe landing!

Getting toasty

Aerocaptured

Aerobraking

Landed at Laythe

Laythe ascent - much easier than Duna

Suboptimal ascent because I need to avoid this mountain

Going suborbital on just Rapiers

Circularizing

Laythe orbit

Vall and Laythe assists

Tylo assist

Tylo flyby

Kerbin and Eve assists

Aerocapture

Aiming for KSC

Aerobraking

Landed

Mission summary:
Landed on Duna and Laythe before coming back to KSC
Flybys of Mun, Ike, Eve, Jool, Tylo, Vall

I think this counts for "harder-er-est-er" mode (:

Apart from Moho, Eeloo, Bop, and Pol, basically a grand tour of the old Kerbolar System. On one orange tank.

...

"You, sir, are a steely-eyed missile man."

If this were a (n active) reddit challenge, you definitely would get some sort of flair thingie. I guess the best I can offer you is a quote (for your sig if you like): Camacju, you took a single orange tank on the greatest road trip of its life. You have beaten hardereresterestest mode.

... actually, now that I think about it, I could probably learn a lot from your mission, especially in terms of dV.

Can you clarify the order in which you visited (regardless of assist/flyby/landing status) the bodies? I'd like to analyze the dV savings with your GAs relative to what is expected according to the dV map.

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Posted (edited)

This mission wasn't even optimal IMO. I should have visited Laythe first so I could drop the jet engine, then landed on Duna. If I flew this mission better I'd be able to fit in a Minmus landing too.

The itinerary:
Launch to Kerbin orbit
Two periapsis kick burns to a Mun transfer
Three Mun assists to eject radially into a 1:1 resonance with Kerbin
Assist off Kerbin and Mun to rotate Kerbin relative velocity prograde, ejecting into a 4:3 resonance
Assist off Kerbin and Mun to raise orbit up to Duna
Duna aerobrake and landing
Launch to Duna orbit
Single Ike assist to a Kerbin transfer
Kerbin assist drops orbit to Eve and gives me a plane change
Eve assist raises my orbit and gives me more Kerbin relative velocity
Two Kerbin assists raise my orbit to Jool
(Note: Kerbin-Eve-Kerbin-Kerbin-Jool is a pretty standard gravity assist route to get to Jool cheaply. However, it won't always work, if your initial Eve velocity is too low. Kerbin-Eve-Kerbin-Eve-Kerbin-Kerbin-Jool is more reliable but harder to execute).
Tylo assist to capture in Jool orbit
Laythe aerocapture
Laythe aerobrake and landing
Launch to Laythe orbit
Transfer to Vall, dropping my orbit and increasing Laythe relative velocity
Laythe assist to bring orbit high above Jool
Tylo assist to escape Jool and get another plane change
Kerbin assist, dropping orbit to Eve
Eve assist, reducing Kerbin relative velocity
Kerbin aerocapture (I didn't go all the way to landing here. I could, but I was trying to see if I could fit in a Minmus landing too).
Kerbin aerobrake and landing

(Also I didn't get a Minmus flyby, or Dres)

Kerbin orbit to Duna landing took 885 m/s, Duna orbit to Laythe landing took 557 m/s, and Laythe orbit to Kerbin landing took 695 m/s.

Optimal value for Kerbin->Duna is 831 m/s, optimal for Laythe->Kerbin is 590. Not sure what optimal for Duna->Laythe is but I almost certainly lost some efficiency there also. Now that I see these numbers I definitely could have fit in a Minmus landing had I flown this mission better. Maybe I'll try it again if someone does something more than what I did

Edited by camacju

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