Delta-V difference in order of visit to the Mun and Minmus

[Shrike99]

I'm sure I saw a video by Scott Manley that answered this, but I cant track it down, in the meantime I shall ask: if I am performing a mission to land on both the Mun and Minmus in one flight, what order is more efficient?

I would think that I should go to Minmus first, so that the craft is lighter when i get to the Mun, reducing gravity drag?

I should add that the craft in question only has a TWR of ~0.4 on kerbin

[ShadowDragon8685]

Visiting the Mun and Minmus both in one trip is going to be a trick and a half. I'd visit the Mun first - changing inclination to match Minmus will require a lot of thrust, but changing inclination to meet the Mun from a Minmus launch will be harder still. Best you get rid of the weight of your Mun ascent stage/fuel first. (Unless of course, you're going to be fueling up when you get to the Mun by Kethane mining or something, but I'd still go with Mun first.)

I'd be worried about that TWR when it came to landing on the Mun, though. The Mun is surprisingly unforgiving of a low TWR. More than one of my (thankfully un-Kerbbed,) Mun landings have ended in hilarity when I found my craft had enough dV and TWR to kill horizontal speed, but not enough (of one, either, or both,) to kill vertical speed. If you watch Scott Manley, he tends to use a rig with about five Rockomax 48-7S engines, four radially mounted around the fifth. That set-up will give you a thrust of 150 for the weight of half a ton, which is much better than even a KW Rocketry Vesta. (If you're going with a radial engine setup like that, just remember to make sure the fuel stacks all have equal amounts of fuel, otherwise you're liable to find your outboards or your inline motor flaming out suddenly. It's not a disaster if that's how you planned it, but if you didn't, it will be. Don't count on fuel lines, they'll vamp the inline engine's fuel to keep the outboards going.)

In general, though... I'm not sure why you're doing that, other than to say you did. I'd just send two missions, one to each of the Mun and Minmus simultaneously. Easier requirements, you can bring back Science! from both without lugging dead weight down to a surface and back both ways coming and going, etcetera. If you're just looking to plant a different flags on two bodies in one mission for bragging rights, though, you'll want to at least have a two-kerb lander, since I think Kerbs get one flag per mission. (Or you can transfer between lander and mothership I suppose.)

In the end, though, I'm not sure which will cost less dV, and it probably depends largely on the alignment of both the Mun and Minmus. You could always launch two identical missions as near as simultaneously as you can get and record the DV they both have remaining when they're making their final run for home.

[Shrike99]

A bit of background info: i've been meddling with Sstos, and have been able to got to the Mun or Minmus ,land, and get back to the ksc with 1400m/s left, so i thought, heck lets try and do them both in one go, so i need to figure out how to squeeze as much juice out as i can.

p.s, i know what low twrs are like, it took me until me 8 goes before i manage to land on the Mun with my first lander, which had a Twr of .2, i must have skimmed halfway around the Mun, slowly killing my speed before i touched down

[Kasuha]

Honestly, the difference is small so do it in whatever order you like.

If you want to save as much dv as possible, I'd say go to Minmus first, with gravity slingshot off Mun. And don't do any mid-space inclination changes, they're waste of dv.

[Keymaster89]

Aside of personal goal in creative, doing both landing in one mission without resupply have no sense IMO.

BTW i think u are right, as u said starting with Minmus will be a good choice, if u try also Mun landing be lighter will sure help.

I don't know what 0.4 TWR on Kerbin corresponds on Mun, but if it is <1 then for me is a little low, i know that many people can land with really low TWR (kinda 0.1-0.2), but i personally prefer to have my TWR >2 even on Mun, i feel better in terms of response, so i can do really fast manouvers and get precisely in the point where i supposed to land. Not to mention that Minmus with that TWR is kinda ultra easy to land on.

Also, u can do 2 free re-entry manouvers if u are precise with the transfers, to get back to kerbin more easily.

Edited January 27, 2014 by Keymaster89

[sjwt]

I would go with Min first then the Mun, plain changing wont use too much fuel compared to getting captured by Mun, Landing and taking back off..

[allmappedout]

i know that many people can land with really low TWR (kinda 0.1-0.2)

I want to meet these people a TWR of <1 on the body you are orbiting means you will fall towards the surface - gravity is pulling you down more than your thrust is pushing you up, so you will not slow down on your descent to the planet, until you burn enough fuel off to reduce your weight. a 0.1 TWR on Kerbin would not be sufficient on the Mun, which is about 1/6th the gravity. You would need at least 0.2TWR on Kerbin to land on the Mun successfully (and the burn would be very inefficient and a lot of the dV would be used to counteract gravity, as you would have to burn for longer).

[Keymaster89]

I want to meet these people a TWR of <1 on the body you are orbiting means you will fall towards the surface - gravity is pulling you down more than your thrust is pushing you up, so you will not slow down on your descent to the planet, until you burn enough fuel off to reduce your weight. a 0.1 TWR on Kerbin would not be sufficient on the Mun, which is about 1/6th the gravity. You would need at least 0.2TWR on Kerbin to land on the Mun successfully (and the burn would be very inefficient and a lot of the dV would be used to counteract gravity, as you would have to burn for longer).

Obviously i meant 0.1-0.2 TWR on Kerbin ^^ and as u pointed, 0.1 on Kerbin will be enough on the Mun only if it raise to near 1. And that's also why i advice him to go on Minmus first, in order to lose some mass.

Sorry if i can't explain myself clearly

Edited January 27, 2014 by Keymaster89

[Smidge204]

Visiting the Mun and Minmus both in one trip is going to be a trick and a half. I'd visit the Mun first - changing inclination to match Minmus will require a lot of thrust, but changing inclination to meet the Mun from a Minmus launch will be harder still.

I can't remember the last time I returned from Minmus that didn't have a Mun encounter, at least briefly. Mun's SOI is pretty big and thanks to being in a lower orbit means you're very likely to encounter it.

Minmus' inclination is only 6 degrees. This is not enough to avoid Mun's SOI:

(Drawn to scale, BTW)

It would absolutely be easier to visit Minmus first, in just about every situation. As stated, it would be easier to deal with the fuel required fro a Mun landing on Minmus than the fuel required for a Minmus encounter/Landing/Return on Mun, so burn that fuel early. There's no need for course corrections if you launch at the proper angle; Launch 6 degrees north or south of due east at the right time and you'll be in the proper inclination right from the start.

=Smidge=

[Kasuha]

I want to meet these people a TWR of <1 on the body you are orbiting means you will fall towards the surface - gravity is pulling you down more than your thrust is pushing you up, so you will not slow down on your descent to the planet, until you burn enough fuel off to reduce your weight. a 0.1 TWR on Kerbin would not be sufficient on the Mun, which is about 1/6th the gravity. You would need at least 0.2TWR on Kerbin to land on the Mun successfully (and the burn would be very inefficient and a lot of the dV would be used to counteract gravity, as you would have to burn for longer).

I believe he meant Kerbin-relative TWR. You only need 0.17 Kerbin-relative TWR to land on Mun.

Launch 6 degrees north or south of due east at the right time and you'll be in the proper inclination right from the start.

Easiest method to avoid inclination problems is to set up the intercept at one of inclination markers.

Edited January 27, 2014 by Kasuha

[Mmmmyum]

The few times I've done it I went from Minmus, shot around Kerbin and got caught by the moon on my way up towards apoapsis. (first few times I did this was a mistake though.)

[Champ]

Visiting the Mun and Minmus both in one trip is going to be a trick and a half. I'd visit the Mun first - changing inclination to match Minmus will require a lot of thrust, but changing inclination to meet the Mun from a Minmus launch will be harder still.

Are you serious ? I very often find an encounter with the mun while trying to get back from minmus, without changing planes. The Mun has a so big SOI, it's hard to miss it !

If you're just looking to plant a different flags on two bodies in one mission for bragging rights, though, you'll want to at least have a two-kerb lander, since I think Kerbs get one flag per mission.

It may be because of a mod, but my kerbals get a new flag every time they enter the capsule.

[Taki117]

Are you serious ? I very often find an encounter with the mun while trying to get back from minmus, without changing planes. The Mun has a so big SOI, it's hard to miss it !

It may be because of a mod, but my kerbals get a new flag every time they enter the capsule.

Kerbals have an unlimited number of flags in the capsule, but they only ever carry 1 on EVA.

[Claw]

Yes, Kerbals have access to unlimited flags by returning to the capsule.

I played with both approaches since I hadn't done a Mun/Minmus landing.

-The dV difference seemed pretty small. My Mun->Minmus run took a bit more than my Minmus->Mun run. Although I wasted some dV when landing on the Mun first and then didn't have enough to return from Minmus.

Minmus->Mun run seemed much more forgiving to mistakes and was easier to correct.

-When transitioning from Minmus to the Mun, I was able to set up a trajectory that took me back to Kerbin if I didn't have enough dV. I think that's a little less unnerving than the other way around.

-Minmus->Mun: I found I wasn't lacking Mun TWR, and didn't have excessive TWR on Minmus. (Since I was heavier on Minmus and lighter on the Mun.)

--Plus, I suppose with the same ship, you'll be fighting less gravity drag when carrying less fuel weight at the Mun.

-Minmus->Mun: It was easier to adjust orbital inclination to get a quick Minmus intercept, and was negligible to adjust midcourse on the way to Mun.

-Minmus->Mun: Seemed easier to find transfer orbits from Minmus->Mun. Also, Mun->Minmus took several correction burns to keep on track after timewarp.

All-in-all it seemed much more forgiving to do Minmus first, then land on Mun. Took me roughly 3600 dV.

[numerobis]

Kerbin-Mun is slightly cheaper via a bielliptic transfer than via a Hohmann. That makes me think it's cheaper to go out to Minmus first and Mun second. The difference is not huge (less than 50 m/s).

With infinite number of gravity assists you can get arbitrarily close to matching Minmus orbit for free after you leave Kerbin and fly out to Mun's orbit -- assuming you have perfect accuracy and no numerical errors. The single gravity assist, boosting off Mun, will be the biggest reduction in cost to get to Minmus, and it's actually plannable via a maneuver node at Kerbin.

As Kasuha mentions, ideally you rendezvous with Minmus at the ascending/descending node of Minmus with respect to Kerbin's equator, and stick around until the next crossing. That way you needn't change inclination.

[LethalDose]

I think I'd go to the Mün first, then Minmus. I'd orbit the Mün, and then land with a lander that would take off and transfer to Minmus. The Minmus lander would be much smaller than than Münar lander, and would land and the whole thing would head back to Kerbin.

Basically, I would get the "heavy lifting" out of the way first. I feel like the loss of dV due to carrying all the mass of a Münar lander out to Minmus and then back to the Mün to land would be greater than the loss of dV doing an inclination change from the Mün to Minmus (which can totally negated by doing the transfer at a asc/desc node, btw).

If you're using an Apollo-style LCS (Lunar-Command-Service Module) variant, though, Minmus first may be more efficient. I think it would rely heavily on craft design, which would, in turn, rely heavily on personal preference.

[Claw]

I think I'd go to the Mün first, then Minmus. I'd orbit the Mün, and then land with a lander that would take off and transfer to Minmus. The Minmus lander would be much smaller than than Münar lander, and would land and the whole thing would head back to Kerbin.

Basically, I would get the "heavy lifting" out of the way first. I feel like the loss of dV due to carrying all the mass of a Münar lander out to Minmus and then back to the Mün to land would be greater than the loss of dV doing an inclination change from the Mün to Minmus (which can totally negated by doing the transfer at a asc/desc node, btw).

If you're using an Apollo-style LCS (Lunar-Command-Service Module) variant, though, Minmus first may be more efficient. I think it would rely heavily on craft design, which would, in turn, rely heavily on personal preference.

It would seem that way, but the gravity of Minmus is so low that carrying that extra Mun weight to Minmus balances out the burden of carrying the Minmus weight to the Mun. Also, the transfer windows seemed a lot easier to manage leaving Minmus since you have to mess with inclination much less during transfer to the Mun. This way you don't have to sit around and wait for the orbit ascending/descending nodes. (I did my flight from Minmus->Mun when Minmus was at it's lowest point of inclination in orbit.) Once you are in the Mun SOI, inclination changes (due to Minmus transfer) to return to Kerbin are pretty much not required.

Edited January 28, 2014 by Claw

[MajorThomas]

Agreed, I think Minmus first is just the easiest way to go.

As Smidge says, it's easy to encounter the Mun on your way back from Minmus.

To those who think it's easier to go from Mun to Minmus...have you ever tried going from Mun orbit up to Minmus? It's not easy. Playing with the maneuver node, you'll see you can't easily fly "up" from the Mun to Minmus. What I saw available was to go to Minmus from the Mun when it was on the opposite side of Kerbin. However, if you start at Minmus and head back to Kerbin, on your way home, you could stop off at the Mun easily.

BTW, I would not bother to make orbital inclination changes while in Kerbin orbit before going to Minmus. Just a waste of fuel. Instead do a Hohmann transfer to Minmus altitude. When most of the way there, you'll find your velocity drop to something suitably minuscule, like 100 or even 50 m/s. Then do a very low Delta V burn to adjust your path so that it intersects Minmus, simples.

Edited January 29, 2014 by MajorThomas
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