worst-case scenario for an omni-lander?

[Colonel_Panic]

Hi,

I know Tylo is a very challenging planetoid to land on due to its high gravity and no atmosphere, and so requires a lot of delta v and TWR to land on successfully. Exactly how much is recommended for this, and to reach orbit again?

Now for the other part of my question, disregarding eve and laythe which require staging or splaceplanes to ascend, or jool which is just a kraken swamp, are there any other bodies that are particularly difficult to land on and return to orbit again? Assume the challenge of reaching them and entering orbit will be accomplished by a tug or other means... what special considerations would I need for a single-stage lander that can touch down on and return to orbit from every planet and moon in the kerbol system?

[Temstar]

Tylo needs about 3000m/s to land, then 2500m/s to take off again. The tricky thing is this has to be done with pretty high TWR, else gravity drag is going to further eat into your delta-V.

Trouble with an universal lander, Eve aside is Moho. Moho requires up to 7000m/s to reach from LKO. It will be exceedingly difficult to bring a Tylo-capable lander to Moho consider the size of a single stage Tylo lander. To actually land on Moho and return to orbit is about as difficult as Vall - ie not very difficult. So to get that 7000m/s with a reasonable sized tug you want your Moho lander to be small.

[tavert]

Ignoring mountains for a second, if you were able to orbit Tylo at 0 altitude you would be going 2170 m/s orbital. Subtract Tylo's rotation speed of 17.8 m/s to get surface velocity, assuming a prograde equatorial orbit. So at a bare minimum you'll need at least 4304 m/s delta-V to land and take off again. Now taking gravity losses (you'll need to start from an orbit higher than the mountains) and steering losses into account, you should add on anywhere from 500 to 1500 m/s margin, more if your TWR is low or you want extra loiter time. Bare minimum TWR at landing (so after burning at least half your fuel) would be 80% relative to Kerbin, but I wouldn't recommend going that low.

Why disregard Laythe? You can SSTO from Kerbin without jets, so you can SSTO from Laythe without em. Laythe is 80% of Kerbin gravity with 80% of Kerbin's atmosphere.

Next hardest to take off from is Duna, with 30% Kerbin gravity and 20% of Kerbin's atmosphere.

Vall and Moho are pretty closely matched after that (so including landing, tougher than Duna). Moho has 27.5% Kerbin's gravity, Vall has 23.5%. But Vall has a larger radius, so the orbital speed at the surface of Vall is 831.6 m/s (adjust for rotation speed of 17.8 m/s), vs Moho orbital speed of 821.2 m/s (very slow rotation, 1.3 m/s).

Temstar: Single stage Tylo landers aren't necessarily that big. This design should be able to do it at only 21 tons: http://imgur.com/a/7slrW#0 (though I haven't dragged it out to Tylo to absolutely confirm that yet)

You could try something similar with 1 aerospike and 13 or 14 tons of fuel, that might also work.

Edited May 29, 2013 by tavert

[magnemoe]

Laythe is easier than Tylo as you can aerobrake in.

One idea is to make an SSTO lander for Vall and Moho, then dock an extra stage for Laythe and Tylo

I made an grand tour with an SSTO kerthane miner who could reach orbit from Kerbin, if it mined after landing it could reach orbit with more fuel everywhere except Tylo.

[Rhyunix]

As crazy as it may sound, all these delta v calculations might not be necessary. Say for example, your first mission was to gilly, then to dres. Why don't you swap the engines? Make each engine design roughly the same weight and just keep a decent ascent stage? It sounds like it might work but i haven't put it on paper yet. As for the relevance to this, i'm pretty sure there is none

[John FX]

I would say tylo is one of the hardest landings. I recently did the grandmaster challenge and had to just land on every body so I made a universal lander capable of simply landing everywhere. The lander coped fine everywhere except tylo where I had to keep the interplanetary thruster stage connected almost to the ground to eat up DV during descent and still have enough (lander has ~2.5kDv) to land.

[John FX]

I would say for landing, Tylo is hardest and for liftoff Eve is hardest so if you make something that can land on Tylo (About 3KDv) and launch from eve (IIRC about 9K Dv) then it can land and take off from everywhere. It would be a universal lander/SSTO. If you make one I would be very interested in the craft file. I need a good universal station service vehicle to take kerbals from orbit down to the planet and back systemwide. I`m putting in the stations now and planned to design the lander next. If you want any help in design I`d be up for it.

Edited May 29, 2013 by John FX

[Colonel_Panic]

Tylo needs about 3000m/s to land, then 2500m/s to take off again. The tricky thing is this has to be done with pretty high TWR, else gravity drag is going to further eat into your delta-V.

Trouble with an universal lander, Eve aside is Moho. Moho requires up to 7000m/s to reach from LKO. It will be exceedingly difficult to bring a Tylo-capable lander to Moho consider the size of a single stage Tylo lander. To actually land on Moho and return to orbit is about as difficult as Vall - ie not very difficult. So to get that 7000m/s with a reasonable sized tug you want your Moho lander to be small.

I'm not really worried about the dv to reach the planet. I plan to use some clever orbital hackery and gravity assists along with ideal windows to peg the moho transfer with a "minimal" 4000-5000-ish dv, but even that aside the omni-lander is basically going to be a 4-part integrated vessel that can be assembled in orbit, consisting of a main ship (tug), lander module, heavy lander addon, and rover. The mission profile for the vessel will be to transfer to the desired planet via the tug, descend with the lander, heavy lander addon and rover, land, and then return to orbit again (either with the entire thing, or, in the case of places like Tylo or Duna, with just the main lander component, leaving the rest behind, to rendezvous with the tug and return to kerbin and/or another convenient nearby orbital resupply and refit station before going on to the next mission. The tug will be fairly large, probably 2 orange tanks worth of fuel but minimal dry weight (it will have only 2 LV-N engines, which it will augment with the heavy lander addon's own engines to save weight by reducing unused engine mass.)

More of what I was wondering about was other requirements, such as, I'd heard moho could pose an overheating problem for engines, etc... are there for example certain engines I have to rule out of using in my lander?

Ignoring mountains for a second, if you were able to orbit Tylo at 0 altitude you would be going 2170 m/s orbital. Subtract Tylo's rotation speed of 17.8 m/s to get surface velocity, assuming a prograde equatorial orbit. So at a bare minimum you'll need at least 4304 m/s delta-V to land and take off again. Now taking gravity losses (you'll need to start from an orbit higher than the mountains) and steering losses into account, you should add on anywhere from 500 to 1500 m/s margin, more if your TWR is low or you want extra loiter time. Bare minimum TWR at landing (so after burning at least half your fuel) would be 80% relative to Kerbin, but I wouldn't recommend going that low.

Why disregard Laythe? You can SSTO from Kerbin without jets, so you can SSTO from Laythe without em. Laythe is 80% of Kerbin gravity with 80% of Kerbin's atmosphere.

Next hardest to take off from is Duna, with 30% Kerbin gravity and 20% of Kerbin's atmosphere.

Vall and Moho are pretty closely matched after that (so including landing, tougher than Duna). Moho has 27.5% Kerbin's gravity, Vall has 23.5%. But Vall has a larger radius, so the orbital speed at the surface of Vall is 831.6 m/s (adjust for rotation speed of 17.8 m/s), vs Moho orbital speed of 821.2 m/s (very slow rotation, 1.3 m/s).

Temstar: Single stage Tylo landers aren't necessarily that big. This design should be able to do it at only 21 tons: http://imgur.com/a/7slrW#0 (though I haven't dragged it out to Tylo to absolutely confirm that yet)

You could try something similar with 1 aerospike and 13 or 14 tons of fuel, that might also work.

For the heavy lander addon I plan to aim for a 6000dv with 1-1.2 TWR (in Kerbin Gs) fully fueled with 50%+ wet weight, so it should be around 1.6-1.7 TWR on landing with half the dv (and therefore about 2/3 the fuel mass) expended. If I can't go that high, I'll probably keep the TWR profile and use the lighter lander core for return to orbit when on tylo.

I'm disregarding laythe because I plan to set up an extensive base network there with SSTO spaceplanes, orbital assembly docks, and rovers to take advantage of the oxygenated atmosphere, so I won't need my exploratory lander to set down there on its own.

Here's my other question though: disregarding for a moment tylo, what's the -next greatest- minimum TWR required to safely land on a planetoid or rocky body? I basically want to design my lander so that with the heavy lander addon it can land on tylo, but without it I want to conserve engine mass by making it lower TWR for the other bodies. So what kind of minimum number should I aim for, for that?

Laythe is easier than Tylo as you can aerobrake in.

One idea is to make an SSTO lander for Vall and Moho, then dock an extra stage for Laythe and Tylo

I made an grand tour with an SSTO kerthane miner who could reach orbit from Kerbin, if it mined after landing it could reach orbit with more fuel everywhere except Tylo.

This is pretty in-line with what I plan to do. I don't plan to set it down on laythe though, just tylo. I want to use the heavy lander stage for all landings I can, but to reserve jettisoning it for tylo, or as an emergency contingency under other circumstances.

I would say for landing, Tylo is hardest and for liftoff Eve is hardest so if you make something that can land on Tylo (About 3KDv) and launch from eve (IIRC about 9K Dv) then it can land and take off from everywhere. It would be a universal lander/SSTO. If you make one I would be very interested in the craft file. I need a good universal station service vehicle to take kerbals from orbit down to the planet and back systemwide. I`m putting in the stations now and planned to design the lander next. If you want any help in design I`d be up for it.

I don't plan to visit Eve with this ship. Doing so would require a special mission profile focused only on the eve landing and ascent (and landing on Eve is indeed very difficult if you're planning to do it with a ship capable of taking off from Eve again..) Furthermore I have no reason to believe that any form of SSTO is possible from Eve. Anything even close to having the TWR and dv necessary would need many stages, and lacking some sort of eve-atmosphere-breathing engine, it's not going to happen.

[tavert]

Moho overheating is an old thing, not the case anymore. You can use pretty much whichever engines you feel like, though there are reasons to rule out a handful: the large radial engine and the poodle just aren't very good stats-wise, the ant and ion engines have too low thrust to be useful most places, etc. The LV-N has great vacuum efficiency as you know, but can be physically awkward to use in a lander just due to its length. There are workarounds for that though.

Next hardest to land on after Tylo is probably Moho, with Vall close behind. Moho needs higher TWR, Vall has slightly higher orbital speed due to its larger radius. And there's Duna (and Laythe, Kerbin, and Eve), if you are doing powered landing for that last bit to brake from terminal velocity, or need the engines to help augment any chutes. There's a big gap in the Kerbol system at the moment between Vall/Moho/Duna gravity and Tylo/Laythe. I wouldn't be at all surprised if the next new-planets update comes with a moon in the 0.5-0.6 g range.

We haven't seen an Eve ascent SSTO yet, it's certainly extremely difficult. Maybe impossible, but OTOH someone might eventually pull it off.

On the subject of Tylo, I tried my aerospike + 14 tons of fuel idea, and it wound up being more than enough. I tried again with 12 tons of fuel, again more than needed. This surprised me, but it worked with only 8 tons of fuel: http://imgur.com/a/lGWM5#0 A higher TWR returns substantial dividends when it comes to Tylo landing.

Edited May 29, 2013 by tavert

[Colonel_Panic]

Moho overheating is an old thing, not the case anymore. You can use pretty much whichever engines you feel like, though there are reasons to rule out a handful: the large radial engine and the poodle just aren't very good stats-wise, the ant and ion engines have too low thrust to be useful most places, etc. The LV-N has great vacuum efficiency as you know, but can be physically awkward to use in a lander just due to its length. There are workarounds for that though.

Next hardest to land on after Tylo is probably Moho, with Vall close behind. Moho needs higher TWR, Vall has slightly higher orbital speed due to its larger radius. And there's Duna (and Laythe, Kerbin, and Eve), if you are doing powered landing for that last bit to brake from terminal velocity, or need the engines to help augment any chutes. There's a big gap in the Kerbol system at the moment between Vall/Moho/Duna gravity and Tylo/Laythe. I wouldn't be at all surprised if the next new-planets update comes with a moon in the 0.5-0.6 g range.

We haven't seen an Eve ascent SSTO yet, it's certainly extremely difficult. Maybe impossible, but OTOH someone might eventually pull it off.

On the subject of Tylo, I tried my aerospike + 14 tons of fuel idea, and it wound up being more than enough. I tried again with 12 tons of fuel, again more than needed. This surprised me, but it worked with only 8 tons of fuel: http://imgur.com/a/lGWM5#0 A higher TWR returns substantial dividends when it comes to Tylo landing.

Less is more with landings. Lighter craft need less thrust to negate the effects of gravity. Curious why you'd go with an aerospike though, as I'd think the only real benefit would be atmospheric ascent.

[tavert]

Indeed, I just thought 4800 m/s wouldn't be enough until I tried it and it worked!

Mainly playing with this spreadsheet: https://docs.google.com/spreadsheet/ccc?key=0AvPJpCCRbRI3dG5aTTVUd2ZmczlOQXlrYkY3ejF2TlE#gid=0

Ignoring payload, a rocket that's just fuel + one engine with TWR = 1 has the most vacuum delta-V with a LV-T30, but that rocket's nearly 22 tons. The comparable aerospike rocket with TWR = 1 has a little less vacuum delta-V, but is only 18 tons. That's where my thought process started, but you can play around with the planetary surface gravity, maybe put in a payload mass and see how that changes things.

[Colonel_Panic]

Indeed, I just thought 4800 m/s wouldn't be enough until I tried it and it worked!

Mainly playing with this spreadsheet: https://docs.google.com/spreadsheet/ccc?key=0AvPJpCCRbRI3dG5aTTVUd2ZmczlOQXlrYkY3ejF2TlE#gid=0

Ignoring payload, a rocket that's just fuel + one engine with TWR = 1 has the most vacuum delta-V with a LV-T30, but that rocket's nearly 22 tons. The comparable aerospike rocket with TWR = 1 has a little less vacuum delta-V, but is only 18 tons. That's where my thought process started, but you can play around with the planetary surface gravity, maybe put in a payload mass and see how that changes things.

Well the real trick is determining at what mass using LV-Ns becomes feasible, because for lower mass payloads and engines, you can actually get more dv AND more twr just by going lighter... but with a heavy enough lander, the engine weight is less important than thrust and efficiency.

EDIT: not sure how accurate that spreadsheet is. I know I've gotten 11k dv out of an orange fuel tank and an LV-N...

[tavert]

Scott Manley had a Tylo lander that was just an LV-N, fuel, a probe, and some ladders. It's in the last one of his "Tylo or Bust" videos. It didn't have a great TWR though. And Kerbals on EVA have been made 3x heavier in 0.20, so that'll cost some delta-V too. If you're going to go the newfangled seat approach, I think what might work better is 5 tons of fuel and 2 LV-909s. Should get over 5 km/s with almost as good TWR as my aerospike landercan one.

[Colonel_Panic]

Scott Manley had a Tylo lander that was just an LV-N, fuel, a probe, and some ladders. It's in the last one of his "Tylo or Bust" videos. It didn't have a great TWR though. And Kerbals on EVA have been made 3x heavier in 0.20, so that'll cost some delta-V too. If you're going to go the newfangled seat approach, I think what might work better is 5 tons of fuel and 2 LV-909s. Should get over 5 km/s with almost as good TWR as my aerospike landercan one.

I'm planning a lot heavier than that... but yeah, will see.

[tavert]

EDIT: not sure how accurate that spreadsheet is. I know I've gotten 11k dv out of an orange fuel tank and an LV-N...

Not with TWR = 1 you haven't.

Edit: 5 t fuel and 2 LV-909's was also more than necessary, about 350 m/s extra. So I did what I do best, downsized even more! Behold, 4.1 ton Tylo throne: http://imgur.com/a/NKaJU#0

Edited May 30, 2013 by tavert