lowest deltaV Mun transfer for landing?

[herbal space program]

Howdy,

I've been trying for some time now to build a SSTMAB (single stage to Mun and back) space plane, and after endless optimization of the design and flight to orbit, I am *almost* there with this honking Mk II-based ship:

Unfortunately, the best I have managed to do so far is get back to low Munar orbit with this puppy before running out of go-juice. I estimate I need maybe about another 300m/s or so of deltaV from where I run out of gas to get all the way back home. I feel like there must be someplace I can squeeze that out if I optimize everything.

So my question is: What is the most efficient trans-munar insertion trajectory for both getting captured by the Mun, and once captured landing with the least deltaV? My guess is that it would be to come in to the Mun's SOI low and from behind, so that the gravity well accelerates the ship both outwards and prograde to match velocities. Has anybody worked this all out for KSP physics? If so, is there a tutorial or general procedure for setting up a maneuver node to do this? TIA!

Edited October 27, 2014 by herbal space program

[Empiro]

The delta-V map as presented in many places (http://i.imgur.com/UUU8yCk.png) are essentially correct. If your numbers are close to that map, then you just need to carry more fuel. You might be able to squeeze out a bit more savings if your orbit around the Mun is less than 10km (you can even skim the surface and land all in one burn), but it probably won't save that much.

[Taki117]

The absolute minimus dV to the mun is about 860 (From experience), you can get back from LMO (about 10km) for about 200m/s.

[Yakky]

From your post and the other commentators' posts, it sounds like you're pretty close to optimal already. You might squeeze out a few dozen delta-v from trajectory optimizations, but getting more than 100 is probably tough. Here are the pointers I can think of that may help you, many of which you probably already know:

1. Go for lowest possible Kerbin orbit, right at 70km, when you do your trans-Munar injection burn. In fact, you might even consider keeping your periapsis down in the atmosphere slightly-- around 60km or so, since the speed and energy pickup from the greater Oberth Effect will probably outweigh the very slight frictional loss from the atmosphere at that altitude. (That takes real guts!)

2. And of course do your entire Kerbin-Munar injection burn very close to periapsis (at the point of max speed). Split it across multiple orbits if necessary rather than doing any significant part of it away from perapsis.

3. Any mass you can possibly shave off your craft will have an exponentially beneficial effect on Delta V. Even little things. For example, I see you're using the medium strength landing struts instead of the micro landing struts. Save a few pounds there. And use a single extendable ladder rather than multiple ladder rungs behind the cockpit. And.... parachutes? Bah!

4. Consider the lighter and more efficient LV-909 engines instead of the LV-T30's or -45's that you're using. That will save you up to two tons of mass, and give you better efficiency, at the expense of lower thrust, but you might be able to get away with less thrust since you're probably burning your jets almost all the way into space anyway. If you need more thrust for the final Mun landing, consider adding some very light 48-7S engines (toggled on only when necessary using action groups) to use for those thrust-critical parts of your Munar landing.

5. I think you have the trajectory about right, but remember that whatever trajectory crosses into the Mun's SOI at the lowest possible speed (as measured from within the Mun's SOI just after you enter) is going to be the lowest energy approach. Crossing in with excess velocity just means you arrive with more kinetic energy that ultimately has to be dissipated via thrusting. So find some trajectory that makes you just barely drift in.

6. Then from there do the closest thing to a pure suicide burn that you feel comfortable handling. You want to minimimze the amount of time you are thrusting against the pull of the Mun's gravity. This would be the time where a bunch of little 48-7S engines could pay for themselves by helping you decelerate very quickly. You need a high enough thrust to weight ratio that you don't have to burn for a ridiculously long time. The theoretically most efficient approach would be to free-fall all the way to the Mun's surface, then apply a massive burst of thrust at the very last possible second to slow you to a stop right at the surface. This would cut your gravity losses to zero.

7. Make sure you are getting as much out of your jets as possible. Gain all possible speed in the atmosphere before switching to rockets. And of course, don't carry extra jet fuel... If you find you're reaching space with unburnt jet fuel, then take off with the tanks only partially full. Less mass!

8. Related to 7, if you're really gutsy then don't carry any jet fuel for the return landing on Kerbin. With proper planning, you can glide in directly from your return trajectory to a dead-stick (gliding) landing. That's of course the most fuel efficient way but it takes amazing piloting skills... especially after you remove those parachutes.

What you're trying to do is very, very hard. Hope these help, and good luck! Let us know if you're able to get it working.

Edited October 26, 2014 by Yakky

[herbal space program]

The absolute minimus dV to the mun is about 860 (From experience), you can get back from LMO (about 10km) for about 200m/s.

Although I am grateful for these replies, this is not really what I am asking. I don't want to know *how much* the minimum deltaV is for these maneuvers, I want to know how to set it up to actually hit those numbers. IOW, where exactly do I put my transfer burn apoapsis relative to the Mun in order to get the minimum deltaV for landing, i.e. ahead or behind, inside or outside? As I said above, I think behind and inside is the correct answer, but I'd love some guidance on this from somebody more knowledgeable.

[r_rolo1]

Technically the least dV for a Munar mission is when you set your munar transfer burn in a way that your Munar Pe will be at surface level in the retrograde side of the Mun ( in theory 0 m , in practice most likely above 1-2 km ) ... and when you get to the munar Pe you burn retrograde until your speed is zero Also the place in LKO where you should do the burn is near the place you see half of the Mun above the horizon.

OFC this is almost surely not feasible in practice, but that is how you get the lowest dV It also means that there is just one place in the Mun you can get with that exact dV, because of the Munar tidal locking with Kerbin

Edited October 26, 2014 by r_rolo1
various adds ...

[Master Tao]

1. Make your LKO --> Mun transfer burn when the Mun is about 40 degrees ahead of you in its orbit.
   
2. Set the burn to give you a low altitude PE around the Mun – 30 km is plenty low enough and fairly safe for timewarp errors.
   
3. Circularize your orbit around the Mun by burning retrograde at PE.
   
4. When you're ready to land, burn retrograde to lower your PE to about 3 km, but watch where you land – some common terrain features go as high as 5 km.
   
5. As you approach PE, burn retrograde to kill horizontal velocity.
   
6. Angle the engine toward the ground as much as necessary to avoid crashing.
   

Technically, you can lower your delta-v expenditure even more by:

• Burning to the Mun directly from launch.
  
• Setting the initial Munar PE lower.
  
• Not circularizing around the Mun.
  
• Using higher thrust/lower I_sp engines for landing. You'll burn more fuel, but use less delta-v.
  

None of these ideas are practical. The big key is the horizontal landing – any vertical component is just wasting fuel fighting gravity.

[bakanando]

Although I am grateful for these replies, this is not really what I am asking. I don't want to know *how much* the minimum deltaV is for these maneuvers, I want to know how to set it up to actually hit those numbers. IOW, where exactly do I put my transfer burn apoapsis relative to the Mun in order to get the minimum deltaV for landing, i.e. ahead or behind, inside or outside? As I said above, I think behind and inside is the correct answer, but I'd love some guidance on this from somebody more knowledgeable.

This can't be very specific, as different sized orbits will have different angles and delta-v requirements. Try using a maneuver node at about 120 degrees behind and add prograde burn of about 820 delta-v and modify it from there. At Mun capture, it should still be somewhat behind you in it's orbit. If done reasonably well, you'll need around 200 delta-v for a 100km orbit around Mun.

[herbal space program]

From your post and the other commentators' posts, it sounds like you're pretty close to optimal already. You might squeeze out a few dozen delta-v from trajectory optimizations, but getting more than 100 is probably tough. Here are the pointers I can think of that may help you, many of which you probably already know:....

What you're trying to do is very, very hard. Hope these help, and good luck! Let us know if you're able to get it working.

Thanks so much for that detailed reply! You have given me a lot of food for thought, and maybe if I optimally address to all the points you raised I can get there!. By the numbers:

1-2) I think I had a pretty good parking orbit the last time, but it could have been better. I boosted from very close to the periapsis of a 110/70km orbit, but probably not right on it. I had also not considered the idea of dropping down into the 60's for that little bit of extra speed. From what you said, it sounds like I could have my initial apoapsis higher and do the burn on the second go-round. I will try to set that up next time. I guess what would be ideal would be if I could set up my timing so that I can do my TMI straight from the ground and not even bother with a parking orbit, but that will be seriously difficult...

3) Good point about the ladder. Even more than the weight of those individual rungs, I think their combined drag might be reducing my top speed on the jets. As to the landing struts, I 'll look into that, but I think I might not get enough height out of the smaller ones. Also I think the micro struts might break under that ponderous beast of a ship.

4) Actually, I'm using the KW Vesta engines, which are the only non-stock part on the whole craft AFAIK. They have 120kN thrust, weigh only 0.6 tons, and have an ISP of 400, which is to say they are a bit cheatsy compared to all stock. They also have a fair bit more thrust than I actually need, so I might be able to get away with the LV-909's instead. I don't think this will make ascent to orbit too difficult, but as you mentioned you ideally want to do the shortest, hardest burn possible for the Munar landing, and having underpowered engines will force you to burn longer. Ultimately, the nuclear option may be the best thing, because I can probably easily carry up several tons less fuel due to their high ISP, which should easily offset their weight.

5) Based on what you said here, it seems like the best thing I could do is shoot for a Kerbin apoapsis behind and inside of the Mun, so that I'm actually moving away from it initially. If I hit it just right, I could theoretically then rise to a motionless Munar apoapsis right at the trailing boundary of the SOI, then fall vertically down to the surface from there. For leaving, It seems like boosting straight up from a spot a bit inside of the most trailing part of the MUN would get me the Lowest Kerbin periapsis at Munar escape, but I'll have to mull that one over a bit more.

6) This will require some thought, as I have to balance the best ISP and TWR against not wanting to have to burn for too long to land. I may just have to try a few different configurations and see...

7) Right now I'm topping out at just about 1900 m/s at 30km altitude before engaging the bipropellant engines, at which point I'm generally using my last drop of extra jet fuel. I think I could do better than this though. Currently, I have a Communotron, 4 scientific instruments, a 70 unit monoprop tank, and 4 parachutes on board, none of which I actually need. Of course for a multi-functional spaceship you'd want all these things, but what I'm going for here is bragging rights not science. I also have a docking port on the front which I obviously don't really need to do this particular challenge. More than even the weight of all these things, the drag they produce is what I suspect might really be killing me on my initial trip to orbit. If I could get to 2200 m/s on the same amount of jet fuel, I'd have 300 more deltaV to play with right there. I will therefore pitch all of that stuff over the side and see how things improve for me.

8) That ship is so light when it is empty of fuel compared to at takeoff that landing it deadstick is not very hard. So long as I'm not trying to actually hit the KSC runway that is, but empty that plane has such a low stall speed that I can easily land it on the grass. My only worry is that when completely empty the plane also has the COL and COM right on top of each other, so I'm afraid it might get flippy on me.

Anyway, thanks again for all the ideas. If I am ever successful, you can be sure I'll be back with pictures to crow about it!

Edited October 26, 2014 by herbal space program

[Laie]

Eventually, you want to go around the Mun in the same direction that the Mun is going around Kerbin.

The apoapsis of your transfer burn (that you won't reach because you're entering into the Munar SoI before you get to your Kerbin apoapsis) should be barely touching the Munar SoI on the side facing Kerbin. It may be a little ahead of the Mun; but see to it that you end up going the right way around the Mun, with the lowest possible periapsis. This will ensure that you get the maximum gravity assist from the mun; low periapsis is also important for the Oberth effect.

But really... The nuclear option may be the easiest solution. I've got a vessel that's basically 4x(Shock Cone / FLT-800 / Jet), a single LV-N, and about 5t of other stuff. It has easily enough dV to get to the Mun and back. Maybe even Mun *and* Minmus.

[Pecan]

*Raise Shields*

(narrowcast) Tx: I regret the possible affront but why do you care? If you're dragging all that messy, massy, awkward load of wings, wheels, etc. etc. to Mun WHY are you worried about fuel? The first thing you should do is leave the SSTO in Kerbin orbit. If you don't know how to calculate and design for a Mun landing, why inflct a SSTO approach on yourself?

(broadcast) Tx: attacks are now expected, do it to make the point, the shields can suck it up.

*Brace For Impact*

[herbal space program]

*Raise Shields*

(narrowcast) Tx: I regret the possible affront but why do you care? If you're dragging all that messy, massy, awkward load of wings, wheels, etc. etc. to Mun WHY are you worried about fuel? The first thing you should do is leave the SSTO in Kerbin orbit. If you don't know how to calculate and design for a Mun landing, why inflct a SSTO approach on yourself?

(broadcast) Tx: attacks are now expected, do it to make the point, the shields can suck it up.

*Brace For Impact*

Why? Because it's a challenge! I've logged well over 800 hours on KSP. I've landed on the moon with regular rockets dozens of times. In fact, I've landed Kerbals on and returned them from every body in the Kerbolar system except for Eve and Eloo. I've beaten the current career mode into a pulp, with millions of Roots in the bank and millions more pending from already deployed missions. I've also built humongous space stations with closed frameworks of trusses that were the Devil Himself to dock correctly. I've flown SSTO's refueled at those stations all over the place. In fact, as it stands I'm starting to feel like I've run out of things to do! This, however is something truly difficult that I haven't done yet and THAT is why I'm doing it. Flying that thing all the way to the Mun and back successfully will require doing so with the absolute minimum deltaV possible, and getting to that level of piloting efficiency is not at all a trivial matter, as all the knowledgeable replies I've received in this thread indicate. Anyway, why anybody would need an explanation for that is a mystery to me.

Edited October 27, 2014 by herbal space program

[numerobis]

Pecan: OP has decided to get to the mun and back in a single stage not because it is easy, but because it is hard.

You can save about 40 m/s by using a bi-elliptic transfer from Kerbin 70km orbit into the Mun SOI, rather than the usual Hohmann transfer.

You can save another little bit (I forget how much, but a bit less than 40 m/s) by using a gravity assist off Mun to boost you to the very high apokee that you need for a bi-elliptic transfer.

You can save yet more by landing on wheels, which can handle a bit stronger jolt than can landing struts. There's another 10-30 m/s for you, depending on how risky you feel.

And if you don't need the struts, you can leave them at home and save fuel all flight long. Aircraft wheels are massless, whereas struts actually have mass.

[Pecan]

...In fact, as it stands I'm starting to feel like I've run out of things to do!...

Which is rather my point. You think you not only can do it all but have done it all; yet you design and build a ship which can't go to Mun then ask how to get it there?

Shouldn't you a) be telling us or, designing a better ship?

The thing is, Mun is a straightforward transfer and there's either Hohmann or bi-elliptic transfer. Beyond that, what can anyone tell you? If you want a better ship I'll design one for you - for a start I would make it Mun-worthy. If you want a better flight-path you'll have to argue with physics (KSP implementation, thereof).

[numerobis]

Actually, landing on wheels can be an extra 100-200 m/s if you use much more skill than I've ever had. If you can land in a flat spot and keep your vertical speed under control so that you stick the landing, you can then use the brakes to slow down.

I've heard of people doing this, but I can barely land in an atmosphere.

[cantab]

Coming back to the core matter.

For a straightforward transfer from 70 km LKO to 10 km Low Munar Orbit, you should be using 860 m/s for the trans-Munar injection and 270 for the orbital capture. Small changes in these orbital altitudes won't make much difference. If you're using more for your ejection burn, don't. With that 860 m/s set, I suspect you'll only get one burn point in your orbit for a low Mun periapsis.

You can save up to 90 m/s off your capture burn by using a Munar gravity assist. Small beer really. Basically you want to make one flyby of the Mun that puts you in an orbit similar to the Mun's, then go round and when you approach the Mun again you can capture for less delta-V.

As for landing, the most efficient approach is a small deorbit burn to put periapsis level with the ground at your landing site, then a large "suicide burn" as late as possible to land. A good landing can save plenty of delta-V compared to a bad one. You can lop a bit more off by lithobraking with tough parts like the aeroplane landing gear.

All told, I reckon one could get LKO-Mun surface-Kerbin down to 2450 m/s. That's using Mun gravity assist to reduce capture requirements, a 50 m/s landing, and Mun gravity assist again to return to Kerbin.

While harder to put in delta-V terms, fuel can also be saved in the aircraft ascent. And excess fuel or oxidizer once in LKO, if there's any, should be dumped before the trans-Munar-injection.

[Kryxal]

Say, would a touch-and-go be good enough, or do you want full stop? For that matter, I wonder if this wouldn't be the most efficient takeoff you could pull off on the Mun ... go orbital on the surface (until you hit a bump!)

[Pecan]

Pecan: OP has decided to get to the mun and back in a single stage not because it is easy, but because it is hard.

You can save about 40 m/s by using a bi-elliptic transfer from Kerbin 70km orbit into the Mun SOI, rather than the usual Hohmann transfer.

You can save another little bit (I forget how much, but a bit less than 40 m/s) by using a gravity assist off Mun to boost you to the very high apokee that you need for a bi-elliptic transfer.

You can save yet more by landing on wheels, which can handle a bit stronger jolt than can landing struts. There's another 10-30 m/s for you, depending on how risky you feel.

And if you don't need the struts, you can leave them at home and save fuel all flight long. Aircraft wheels are massless, whereas struts actually have mass.

I'm sorry, this crossed with my last post.

So - you can potentially save, by your estimate, up to 110m/s deltaV overall. How much would you save by not carrying the mass of the wings, etc. all the way to Mun and back or a very slightly different ascent to orbit? How much would you lose on a plane-change for the designated landing-site or a less-than perfect landing?

SSTMu isn't hard - a Mun round-trip is roughly 3km/s deltaV. Flying a bad ship can be hard, but if you're making things hard for yourself just for the sake of it, where's the challenge? Yes, 3% better flying isn't to be sneezed at, but a 10% better ship will be 10% better every time. If someone wants to single-stage to Mun or somewhere that's their business but the best efficiency saving would still come from a fit vehicle.

Aircraft wheels (small gear bays) are NOT massless.

[herbal space program]

Say, would a touch-and-go be good enough, or do you want full stop? For that matter, I wonder if this wouldn't be the most efficient takeoff you could pull off on the Mun ... go orbital on the surface (until you hit a bump!)

Cool idea! I suppose that grazing the Mun with my wheels and then taking off again, if I could manage it without things getting all 'splody, would in fact be the lowest-energy way to do it, but somehow that feels insufficient to me. I want to plant the flag and at least take a surface sample, even if I have to leave all those other draggy science bits behind. The idea of stopping with brakes after making a rolling touchdown however is quite intriguing to me. I actually made a SSTO plane that had front-mounted, downward-pointing engines that allowed it to stand up onto rear-mounted lander legs, even when fully fuelled on Kerbin. I made a sequence of screencaps that I will post some time soon with the title "stupid space plane tricks". No doubt this has been done many times before, but it was certainly amusing for me when I came upon this poor man's VTOL solution. Anyway, the weight and drag of all that was way too much for what I'm attempting here, but on the Mun, even one such engine would probably be enough to pitch me up sufficiently to take off without air. Then I could also drop the lander legs, saving a bunch in mass and drag....

[Yakky]

Thanks so much for that detailed reply! You have given me a lot of food for thought, and maybe if I optimally address to all the points you raised I can get there!. By the numbers:

7) Right now I'm topping out at just about 1900 m/s at 30km altitude before engaging the bipropellant engines, at which point I'm generally using my last drop of extra jet fuel. I think I could do better than this though. Currently, I have a Communotron, 4 scientific instruments, a 70 unit monoprop tank, and 4 parachutes on board, none of which I actually need. Of course for a multi-functional spaceship you'd want all these things, but what I'm going for here is bragging rights not science. I also have a docking port on the front which I obviously don't really need to do this particular challenge. More than even the weight of all these things, the drag they produce is what I suspect might really be killing me on my initial trip to orbit. If I could get to 2200 m/s on the same amount of jet fuel, I'd have 300 more deltaV to play with right there. I will therefore pitch all of that stuff over the side and see how things improve for me.

Try different trajectories within the atmosphere to get that extra speed. Even several extra minutes running the jets to pick up an extra 100 m/s won't really consume that much more fuel. The efficiency of jets is so high compared to rockets that any extra speed you can collect under jet power is almost "free" from a fuel and mass perspective. Optimally flying a jet to get every last drop of speed is a real art, but the gist of it is to level out near the top of the engines' operating envelopes (probably around 28-32 km depending on the plane) and just fly level there until you've gained all the speed you possibly can. It's a slow and tedious process because the engines are weak up there and the plane may be hard to control.

8) That ship is so light when it is empty of fuel compared to at takeoff that landing it deadstick is not very hard. So long as I'm not trying to actually hit the KSC runway that is, but empty that plane has such a low stall speed that I can easily land it on the grass. My only worry is that when completely empty the plane also has the COL and COM right on top of each other, so I'm afraid it might get flippy on me.

I have gotten to love the relatively new Trajectories mod, which shows you in map view where you're going to end up after re-entering the atmosphere, taking atmospheric drag and planet rotation into account. Extremely useful for landing at or near the KSP space center. Consider giving it a try if you like mods.

[Yakky]

Say, would a touch-and-go be good enough, or do you want full stop? For that matter, I wonder if this wouldn't be the most efficient takeoff you could pull off on the Mun ... go orbital on the surface (until you hit a bump!)

This would be really tricky given the Mun's bumpy surface, but I've been wondering if it would be possible to do on the perfectly flat Minmus flats. In theory you could put your periapsis right at zero on the flats, touch down on wheels, and just use wheel braking (and a little downforce) to kill your forward velocity. That would be the ultimate no-burn suicide burn.

Does anyone know if this has ever been done?

Edited October 27, 2014 by Yakky

[herbal space program]

I'm sorry, this crossed with my last post.

So - you can potentially save, by your estimate, up to 110m/s deltaV overall. How much would you save by not carrying the mass of the wings, etc. all the way to Mun and back or a very slightly different ascent to orbit? How much would you lose on a plane-change for the designated landing-site or a less-than perfect landing?

SSTMu isn't hard - a Mun round-trip is roughly 3km/s deltaV. Flying a bad ship can be hard, but if you're making things hard for yourself just for the sake of it, where's the challenge? Yes, 3% better flying isn't to be sneezed at, but a 10% better ship will be 10% better every time. If someone wants to single-stage to Mun or somewhere that's their business but the best efficiency saving would still come from a fit vehicle.

Aircraft wheels (small gear bays) are NOT massless.

With all due respect, just telling me that I should build a better space ship is unhelpful. What do you mean by that? If you have actual design advice for my self-imposed SSTMAB challenge, then please give it to me. Are you saying that using wings to fly out of Kerbin's atmosphere is a waste of mass? Then tell/show me what you built instead! I have no doubt that others have come up with better solutions than I have. I'm sure I could go look them up if I really wanted to, but the whole point of the game is to figure it out yourself as much as possible, isn't it? The reason it's such a great game is that really smart people find it challenging. In my book, it ranks up there with (net)Hack, Half-Life, and Civilization among the best computer games ever created. In this instance, I was sufficiently confused by KSP's version of multi-body physics that I was unsure how to optimize my approach to Mun, and didn't want to spend countless hours figuring it out by trial and error. To my gratification, I got a great deal of very helpful advice and ideas from a bunch of friendly people here (thank you!). From you, not so much. Why?

[herbal space program]

I have gotten to love the relatively new Trajectories mod, which shows you in map view where you're going to end up after re-entering the atmosphere, taking atmospheric drag and planet rotation into account. Extremely useful for landing at or near the KSP space center. Consider giving it a try if you like mods.

I admit I have greatly desired this, but outside of some KW and NP parts I am trying to stick with stock. They really should implement this in stock though. Modeling Kerbin re-entry trajectories as if it were an airless body is pretty much useless.

[tater]

You don't need rungs or ladder at all. You can use EVA pack to get to the hatch.

[herbal space program]

You don't need rungs or ladder at all. You can use EVA pack to get to the hatch.

Absolutely right. It feels a bit desperate to dispense with that, but if I have to I most certainly will

Edited October 27, 2014 by herbal space program