Need help with Tylo mission

[Kelderek]

I'm trying to build a vessel to land on Tylo and return back to orbit. Thus far I am only working on the lander itself, so I am starting in Tylo orbit with a vessel capable of doing a de-orbit burn, landing on Tylo, ascending back up and orbiting again. I'll worry about a transfer stage and Kerbin launcher later. The DV map I am using says that I will need 3070 m/s for both landing an ascending, so my vessel currently has a bunch of stages totaling about 6500 m/s. So far though, my tests with hyperedit are showing that it will take a lot more DV to land than 3070 m/s and I am trying to figure out where I am going wrong.

1. What orbit would be considered "low orbit" for Tylo? The range for "in space near Tylo" extends to a whopping 250km according to the wiki, but since Tylo has no atmosphere any orbit that can clear the mountain peaks might be sufficient.

2. What orbit would you recommend I establish before attempting to land? How much of a difference will my orbit altitude make for the necessary landing DV at Tylo? Since Tylo has decent gravity I am expecting changes in orbit matter a lot.

3. It's possible that I am doing the powered landing wrong or poorly. I have to kill more than 2000m/s of orbit speed if I start at 50km altitude, so I just burn hard until I get down to about 200m/s and then I throttle it down so that I get to roughly 100m/s around 10km altitude and then let the throttle stay low but high enough to bleed off that speed as I descend, aiming for about 20m/s at 1km and <5m/s inside 100m. Using this method took me almost 4500m/s of DV to land from 50km so I'm sure this is all wrong, lol.

Here's the VAB shot of my lander as it stands so far:

I can use all the help I can get, thanks.

EDIT: I also tried to use MechJeb for landing and that fails horribly. I'm guessing that it has a hard time with using staging during the landing as every time I try it, I crash hard into the ground at high speed.

Edited February 4, 2015 by Kelderek
answered

[Laie]

From relatively recent experience, ascent from Tylo to 50km takes ~2500m/s. Theoretically, the descent should be in the same ballpark, but I want to have at least 3300m/s to feel safe.

Other than that... you are aware of the constant-altitude thing? Kosmo-Not has made a nice video to explain the concept.

EDIT

also, this: http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

Edited February 4, 2015 by Laie

[Tank Buddy]

1. Low orbit would most likely be considered the lowest orbit that is stable (clearing atmosphere and terrain. For Tylo this would be from 12000m - 40000m

2. As low of an orbit as possible. Plan to intercept Tylo so that your periapsis is low over the surface. This way your burn to decelerate into orbit will require less Delta V, since the orbital Velocity is higher and your operations will take place in LTO (Low Tylo Orbit). I think your problem is your method of decent. You should avoid descending straight from high altitudes, and should first transfer to an orbit that has a low Periapsis. If i read correctly, you are de-orbiting at 50 Km, then you fight the acceleration all the way to the surface. This is an extremely inefficient method of landing. You have to fight gravity and will have monumental costs in Delta V because of it.

3.

You should perform your de-orbit burn and landing at as low of an altitude as possible. Once you decrease your horizontal velocity you begin to "drop" out of orbit. You will no longer have sufficient horizontal velocity to orbit, and therefore your altitude will begin to decrease and your vertical velocity will increase (in the downward direction). You then will spend all of your DV on staying afloat.

This is my best strategy:

1. Lower Periapsis to the lowest altitude so that the terrain will not interfere with your trajectory. (Your periapsis should be over your target landing site)

2. Time warp to near the periapsis, then depending on TWR, begin killing your horizontal velocity a distance from periapsis, anywhere between 2 minutes and 30 seconds from Pe. (maintain an attitude near the horizon, unless you need to point up to control the v-rate). You should be as close to the surface as possible while clearing the terrain.

3. Enter vertical decent, performing a suicide burn (throttling to 100% at the last minute to decelerate to 0 m/s) or something close to one. You should spend as little time as possible counteracting the full force of gravity, especially on Tylo. Every second you are burning vertical, you expend 9.8 m/s of DV on top of your apparent acceleration upwards. You should be doing this no more than 500 meters above the surface.

Good luck!

Also, you should consider using fewer engines with a higher Isp and thrust, like Aerospikes instead of the 48-7S's.

Edited February 4, 2015 by Tank Buddy

[TheSaint]

My take is that you're making your lander needlessly complex. I've done a couple of Tylo landers, they've all been two stages, the first stage runs about 2,500 m/s, the second runs about 4,000 m/s. Don't let MechJeb handle your entire landing, you're correct, it can't. It calculates the suicide burn point based on the TWR of your first stage, which will almost always be much higher than your second stage. My Tylo landing profile has been this:

1. Start from about 50km (although this isn't too important, my last one started from about 90km and it worked just fine)

2. Burn the first stage to cancel almost all of the orbital velocity. This should leave you with about 300-500 m/s left in the first stage.

3. Coast down until your surface speed/vertical speed is higher than the remaining dV in the first stage. But don't get too close to the suicide burn time.

4. Burn the remaining fuel in your first stage retrograde to cancel some of your velocity, then jettison the first stage.

5. Land your lander like any other moon landing, either manually or just click "Land Somewhere".

If you do it right, you should have between 2800 and 3000 m/s left in your second stage, which is plenty enough to get you back into orbit.

[Slugy]

As others said >2500m/s dV and a low burn that keeps you just above terrain for most of it will work best. I think 15km is a safe orbit height.

The other thing that makes landing (and take off) more efficient is a high TWR - 1.6x is ok but 2x or more for all stages at all times will save (a bit) more - although that does mean you carry more weight ofc. Your descent stage is around 3mins of full throttle burn - that is all time that gravity loses are happening.

Useful links: Tavert's Landing and Takeoff Delta-V vs TWR and specific impulse

Really handy travel guide

[Kelderek]

Also, you should consider using fewer engines with a higher Isp and thrust, like Aerospikes instead of the 48-7S's.

If I try to keep my TWR in roughly the same range, I lose about 300-500 m/s DV when I tried switching to aerospikes. That doesn't seem like it would help, as I have a good enough TWR already.

[Randazzo]

4500 seems like an awful lot for a landing. Were you in a retrograde orbit, traveling in the opposite direction of surface rotation? I imagine that would increase the Delta V cost just as entering a planets SOI traveling the opposite direction vastly increases your relative velocity compared to approaching it from "behind".

[Laie]

2. What orbit would you recommend I establish before attempting to land? How much of a difference will my orbit altitude make for the necessary landing DV at Tylo?

3. It's possible that I am doing the powered landing wrong or poorly. I have to kill more than 2000m/s of orbit speed if I start at 50km altitude, so I just burn hard until I get down to about 200m/s and then I throttle it down so that I get to roughly 100m/s around 10km altitude and then let the throttle stay low but high enough to bleed off that speed as I descend, aiming for about 20m/s at 1km and <5m/s inside 100m. Using this method took me almost 4500m/s of DV to land from 50km so I'm sure this is all wrong, lol.

2. check the wiki for which altitude allows what timewarp. If memory serves, you get max. 10x at 50km and no warp at 30km. I always started at the lowest altitude that allowed 50x. Wherever that is, from there to "sea level" is only 60-70m/s.

3. It's not quite clear from your description, but I suspect you're doing it wrong. Ask you preferred search engine for constant altitude landing -- it comes up on these forums about every two weeks. If those posts don't help you, try Kosmonot's video (certainly linked from one of the posts).

The data tables I linked above give you an indication how TWR affects your delta-V requirements.

My take is that you're making your lander needlessly complex.

Agreed. Landing on Tylo is busy enough as it is. You don't want to watch your staging during the last 1000m/s or so.

4500 seems like an awful lot for a landing. Were you in a retrograde orbit, traveling in the opposite direction of surface rotation?

Tylo's rotation is so slow, it barely makes a difference.

[-Velocity-]

6500 m/s dV is overkill; you only need something like 5000-5100 m/s dV for a Tylo lander, though I always have at least 5500 m/s just to be on the safe side. You always want a considerable margin for safety sake. Your TWR can start fairly low, like 1.3, as in the beginning of the descent you are not acting against gravity, but the TWR needs to rapidly climb as you slow down.

With the fully reusable Tylo landers I build, I usually start with a TWR of like 1.4 - 1.5, I usually have a dV of like 5700-5900 m/s.

Perhaps the way you're descending towards planetary surfaces is your problem. You want to burn retrograde, continuously at 100% throttle, until you touch down. It's basically an inverse gravity turn. Now, because that's very difficult to time, especially on Tylo, you start your burn a little early. Consider if it takes 5100 m/s to land and take off, and your lander has 5800 m/s dV, then you've got up to like 90 seconds of hover time on Tylo. That's quite a cushion for error.

Now, what's the right time to start this burn? IRL, they solve a calculus problem and figure it out. In KSP, we have something that is far easier and far less time consuming- quick save and quick load. Do you really think that someone would have the technical expertise to build a liquid rocket engine, but not calculate the right time to start a de-orbit burn?

- - - Updated - - -

Also, you should consider using fewer engines with a higher Isp and thrust, like Aerospikes instead of the 48-7S's.

No, aerospikes are a very bad idea for Tylo, and the 48-7S is the best lander engine for Tylo, bar none. The TWR is incredibly high, and the ISP is respectable. The high TWR means that almost none of the mass of your lander is engines. The aerospikes are awful in comparison, and should never be used unless you're in a thick atmosphere. TWR is critically important on Tylo because you have to have very high thrust to efficiently land, and the 48-7S allows you to achieve that thrust without weighing your lander down with a huge engine mass. IMO, the 48-7S is so good that it needs to be nerfed for gameplay balance. It basically represents a nearly massless way to fairly efficiently convert fuel into a high amount of thrust.

Edited February 4, 2015 by |Velocity|

[Jouni]

My typical Tylo landing is roughly the following:

1. Leave the mothership above 60 km, as it's the lowest altitude that allows reasonable time warping. Otherwise rendezvous is going to be really boring.
   
2. Before landing, move the lander to a 30 km orbit. Time warping is still possible, but you'll have less altitude to lose during the actual landing.
   
3. 1/3 to 1/2 orbits before the intended landing site, burn retrograde until your trajectory hits the ground a bit after the site.
   
4. 1/5 orbits (?) before the landing site, burn retrograde to reduce the speed to around 1000 m/s. Your trajectory should still hit the ground a bit after the site.
   
5. Now you're really starting to lose altitude. Don't allow the descent rate get above 100 m/s, or the landing is going to be harder than necessary. If you have to burn to keep the descent rate under control, use it as an opportunity to lose horizontal speed.
   
6. Kill horizontal speed whenever it feels appropriate, as long as you're still going to land roughly at the right area. The final kilometer or two of the descent should be mostly vertical.
   

This is not the most efficient way to land, but it's close enough. My Jool-5 lander used around 3000 m/s for the landing.

[-Velocity-]

Oh and a final note, you might want to separate your crew quarters and your lander.

What I mean is, Kerbals always wear spacesuits in space, so why do they need a pressurized living area on their lander and ascent spacecraft? Just throw a 50 kg command chair and that, plus the (40 kg?) EVA'd Kerbal means you only have to have a mass of (90 kg?) for each Kerbal. No need for a crew cabin that weighs tons. If you want a living area for your Kerbals to live in, then separately land a living quarters that stays behind on the planet's surface after the Kerbals leave. Heck you can even revisit it later if you want.

Substituting command chairs for pressurized crew cabins, you can make some incredibly small Tylo landers. I'm trying to remember the weight of my smallest one, it might have been only 10 tons. It's also possible to make some really nice and cheap Eve sea-level-to-orbit spacecraft this way. I made one that could take four Kerbals from Eve sea level to like a 500 km circular orbit, and only massed 300 tons or so. I did put a "windscreen" (a 2.5m - 1.25 m adapter) over their heads to shield them from the atmosphere during ascent.

Do be warned though, there is a problem though with quick-saving and quick-loading while Kerbals are in command chairs. Sometimes they get kinda screwed up, and when you try to disembark from the chair, the Kerbal can clip into the vessel and be ejected from the area at a few hundred m/s, or even cause the vessel to break up and explode. However, if you do some experimenting back at Kerbin, utilizing the save game and load game features, you can make spacecraft that are largely immune to this problem.

Oh and also, if the mothership that launches the lander has RCS, there's no reason to put RCS thrusters and fuel on the lander. Omitting RCS can give you a few hundred additional m/s of dV.

Edited February 4, 2015 by |Velocity|

[Kelderek]

My take is that you're making your lander needlessly complex. I've done a couple of Tylo landers, they've all been two stages, the first stage runs about 2,500 m/s, the second runs about 4,000 m/s. Don't let MechJeb handle your entire landing, you're correct, it can't. It calculates the suicide burn point based on the TWR of your first stage, which will almost always be much higher than your second stage. My Tylo landing profile has been this:

1. Start from about 50km (although this isn't too important, my last one started from about 90km and it worked just fine)

2. Burn the first stage to cancel almost all of the orbital velocity. This should leave you with about 300-500 m/s left in the first stage.

3. Coast down until your surface speed/vertical speed is higher than the remaining dV in the first stage. But don't get too close to the suicide burn time.

4. Burn the remaining fuel in your first stage retrograde to cancel some of your velocity, then jettison the first stage.

5. Land your lander like any other moon landing, either manually or just click "Land Somewhere".

If you do it right, you should have between 2800 and 3000 m/s left in your second stage, which is plenty enough to get you back into orbit.

I found this method to be very useful, it worked for me to do it this way on my first try. I did simplify my lander to make it only 3 stages (1 for landing, 2 for ascent - mostly because I want my final vessel to be as small as possible once back in orbit). I started with 6448 m/s DV at 50km altitude, used roughly 3200 +/- 50m/s to land and it only took about 2600 gain a 50km orbit again. So I can refine the design some more to trim off the excess DV.

I was definitely doing my descent burns wrong before. I don't think I understood the suicide burn part correctly, I would just burn retrograde all the way down adjusting my throttle here and there until I landed. After this discussion and all the mentions of "suicide burn" I took notice of the "suicide burn distance" and "suicide burn DV" values displayed in my KER vessel window. I think I understand this to mean that the distance ticks on down close to 0 at which point I would need to use the DV amount to stop in time without crashing.

When I tried the method TheSaint mentioned, I killed my orbital speed down to about 150m/s at which point I noticed that my suicide burn was about 100m/s more than the remaining DV in my landing stage. I coasted until the suicide distance ticked down to about 100-200m and then burned full throttle, ditching the landing stage* and using about 100 or so m/s DV of my ascent stage to finish the landing (I turned on MechJeb for the final bit). This seemed to work well enough that I am probably almost ready to design the transfer stage and Kerbin launcher for this mission.

Thank you all for the advice and assistance.

EDIT: When I say ditching the landing stage, I only mean the tanks and engines that helped me land, I'm not quite dumb enough to get rid of my landing gear before landing

Edited February 4, 2015 by Kelderek

[Kelderek]

I managed to finish my mission, you guys rock! Thanks again for the help.

I had a funny almost-disaster on my way back home from Tylo. I had used one of my larger tugs as a transfer stage with the plan that I would refuel it at my Jool fuel station if necessary. This was a tug design I had made a while back, so it was not custom tailored for a trip to Tylo. As you can see from the second image, I just BARELY made it to the fuel station with about 62 liquid fuel remaining from tanks that once held over 14,000 liquid fuel. So next time, I should probably be a bit more careful with my planning. My Tylo lander worked great though, so thanks again for all the tips.

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[TheSaint]

No worries, glad it worked out for you. First time I landed on Tylo I wound up in back in orbit with about 5 m/s left, nothing but fumes in the tanks. Jeb and Bill had to sit tight while Bob brought the mothership around to pick them up.

[-Velocity-]

No worries, glad it worked out for you. First time I landed on Tylo I wound up in back in orbit with about 5 m/s left, nothing but fumes in the tanks. Jeb and Bill had to sit tight while Bob brought the mothership around to pick them up.

One time, running low on my overall fuel budget, I decided to use some of my fuel on my lander just to reduce the altitude of my orbit around Tylo. This put me further behind on my total delta-V budget than I realized. On ascent, when my lander flamed out, I still had like 200 m/s delta-V left to go till orbit, and no saved games from before I made my landing! Jeb had to bail out, grab all the science, and use his EVA pack to take him the rest of the way into orbit.

[KerikBalm]

So far though, my tests with hyperedit are showing that it will take a lot more DV to land than 3070 m/s and I am trying to figure out where I am going wrong.

...

2. What orbit would you recommend I establish before attempting to land? How much of a difference will my orbit altitude make for the necessary landing DV at Tylo? Since Tylo has decent gravity I am expecting changes in orbit matter a lot.

3. It's possible that I am doing the powered landing wrong or poorly. I have to kill more than 2000m/s of orbit speed if I start at 50km altitude, so I just burn hard until I get down to about 200m/s and then I throttle it down so that I get to roughly 100m/s around 10km altitude and then let the throttle stay low but high enough to bleed off that speed as I descend, aiming for about 20m/s at 1km and <5m/s inside 100m. Using this method took me almost 4500m/s of DV to land from 50km so I'm sure this is all wrong, lol.

Its called gravity drag... every second you are burning perpendicular to the surface, Tylo is robbing you of roughly 8 m/s of dV.

IIRC, orbital velocity for Tylo is about 2,000 m/s. In theory, you could do the landing with that much dV.

The ideal landing would be to lower your perapsis right down to the surface and then kill your velocity right at PE.

Of course... you need a stupidly high TWR to pull that off, so you set your PE a bit above, yout burn retrograde before your reach PE because the burn takes time, and as your PE lowers into the groun, you pitch a bit to control your vertical descent speed.

If done right, it doesn't even take 2,500 m/s.

You also don't need 3070 to get into orbit. 2,500 should get you to adecently high orbit.

All of the above assumes you have a pretty good TWR... lets say at least 3:1

[Phil deCube]

This is my tried and tested Tylo Lander:

This is the whole craft on the launchpad.

]

This is the interplanetary stage, lander and return craft after boosters have done their bit.

This one is the interplanetary drive stage.

When I get to Tylo this is the remaining spacecraft.

The bottom section of this is the return ship to take me back to Kerbin. This stays in Tylo orbit at about 60km until needed.

Above that is the lander/return to orbit combo.

Which looks like this on the pad (with Jeb clibing the ladder to the return to orbit stage).

And finally, here's Jeb testing the return to orbit stage over Kerbin.

Once back in orbit and within 50 metres of the return to Kerbin ship, Jeb just EVAs over, boards and it's Home Sweet Home.

[Kelderek]

That's pretty cool, thanks for the pics. My lander wasn't quite as efficient, but it got the job done just fine. I never use the exterior command seats for anything but rovers, as personally it just doesn't suit my playstyle to use them for landers or orbiters. But I do like your design, especially that last pic - the look on Jeb's face is fitting

[KerikBalm]

Ermmmm... Jeb has no helmet on.... No bueño

[Phil deCube]

Ermmmm... Jeb has no helmet on.... No bueño

Yeah... I think that has something to do with the Renaissance Compilation mod that I'm using. It's probably decided that Jeb is *in* a craft, so doesn't need it on. Something like that.

It's no wonder he has that look on his face though, in those circumstances.

[gogozerg]

15 km orbit around Tylo, then I separated the lander.

I needed slightly more fuel than the 4 small outer tanks for landing (Poodle). Then back to the orbiting ship.

Refueled the lander, waited for the transfer window, and a quick burn for the flight back to Kerbin with the lander only (still twice the fuel you need).

Not optimized, too much fuel.