Manned Duna Mission - Docking and Construction Woes

[Branjoman]

I'm planning to create a 3-piece mother ship to perform a manned Duna mission (My first!). However I'm also having some worries and troubles with the scheme.

Firstly, I for the life of me cannot do docking. I tried to make a space station a while back in 0.24 and got

some good intercepts, but every time I tried to match our velocities I just zipped by. Does anyone have any helpful pointers on the matter?

Secondly, I'm not 100% sure if my setup can get me to Duna and back. My propulsion stage is one of the Kerbodyne tanks with 2 L-VN's on the bottom. Is this viable or should I scale the tank down a bit?

Finally, I'm a bit confused on how to build the lander - specifically how much Delta V I need. I'm assuming a mix of LV-909's and Rockomax 48-7S are good enough. Do I even need parachutes?

Thanks for the help in advance.

EDIT - I do have KER so I can see Delta V, TWR etc. and I am NOT using transfer windows. I haven't gotten the hang of them yet, and I'm just too impatient! (I also still need to get the hang of using Imgur)

Edited January 22, 2015 by Branjoman

[wibou]

Rendez-vous can be hard but you get used to it very quickly... There is some good turorial about it on youtube; something like https://www.youtube.com/results?search_query=scott+manley+rendezvous might be what you are looking for (aim for tutorials).

Kerbodyne tank (which one, the big big one, really?) and two LVN seems A LOT, if you can put that monster into orbit you can probably go anywhere (burn with LVN will take a while but it shouldn't matter)... But it would really help to see the actual setup, you could post an image of it. You could also install some mod (MechJeb?) as it would give you the delta-V of each stage, you could then check on the wiki if that's enough.

The lander... well a picture would help, again. Duna has about 1/3 of the gravity of Kerbin plus much much less atmosphere. If you can raise your lander from 0km to 42km on kerbin, then you will definitely have enough on Duna to reach space... if you plan a bit more to circularize then you should be fine (it's probably too much, even?).

Parachute ARE useful, since there is some atmosphere. But don't expect them alone to land you safely, you will probably have to do a short burn just prior touching ground.

Edited January 19, 2015 by wibou
(added note on parachute)

[Janos1986]

If you practice it enough, docking even becomes a chore.

Seriously, master docking, it will take you places AND save money if you are creative enough with your tugs/engine sections/multipurpose vectors.

[goldenpsp]

If you aren't opposed to mods, get docking port alignment indicator. IMO it should be one of those UI elements that should be in the stock game.

[Skorpychan]

It's like planetary encounters. Match speeds at closest point. You may need to set up maneuver nodes to give you an idea of when to start the burn.

My best advice is to download mechjeb and watch it do the rendezvouses. Once you're THERE, the docking is pretty simple with a properly-designed ship.

[wibou]

Once you're THERE, the docking is pretty simple with a properly-designed ship.

Riiiight, that part is VERY important. If your RCS are NOT around your center of mass, you WILL have hard time docking.

Also make sure you have (at least) 4 RCS thrusters. Less is a pain since you need to ROTATE your ship to strafe a different direction.

If you are super heavy, you might want to add more than 4 thrusters, but unless you have more than ~75 tons in the part you are trying to dock, 4 should be fine.

[OhioBob]

I've done only one manned (or is it kerballed?) landing and return from Duna. I agree that a Kerbodyne tank sounds like overkill for a LV-N rocket. I was able to complete my mission with nothing more than a X200+16 + X200-8. You can see my spacecraft in the image below. The little vehicle on top is an Ike lander that I separated and flew to Ike after entering Duna orbit. The Mk1-2 command pod served a multi-function role as space habitant, Duna lander, and Kerbin reentry vehicle. My mission launched all on one launch vehicle and required only one docking - in Duna orbit between the command/service module and the unmanned propulsion stage. Perhaps the screenshot can give you some ideas for constructing your own mission.

Fortunately I still have my notes from when I flew that mission. Below is my pre-flight ÃŽâ€V budget:

Launch ≈ 4550 m/s

Trans-Duna injection ≈ 1060 m/s

Mid-course corrections(s) ≤ 30 m/s

Duna orbit insertion (75 km) ≈ 640 m/s

Site selection/plane change ≤ 300 m/s

Descent orbit insertion ≈ 40 m/s

Landing, final braking ≈ 40 m/s

Ascent to Duna orbit (50 km) ≈ 1400 m/s

Plane change (if needed) ≤ 100 m/s

Rendezvous & docking ≈ 85 m/s

Trans-Kerbin injection ≈ 560 m/s

Mid-course corrections(s) ≤ 50 m/s

Some of these numbers - i.e. trans-Duna injection, Duna orbit insertion, and trans-Kerbin injection - will vary depending on your specific launch window. The others are estimates and contingencies. As you can see, landing takes very little ÃŽâ€V as most of the braking is done with parachutes. I used four parachutes for Duna landing, which are the one you see mounted to the decoupler. (The parachutes mounted to the command pod were for landing back at Kerbin.) I didn't fire my engine until I was 60 meters above the ground. Of course I selected a landing site that sat at a low elevation to take maximum advantage of the atmosphere. Landing at a higher elevation will require more propulsion because parachutes will be less effective in the thinner air. I don't think I used any of the 300 m/s contingency for "site selection/plane change". This was just in case I miscalculated and got myself into an orbit from which I couldn't reach a good landing site.

- - - Updated - - -

Firstly, I for the life of me cannot do docking. I tried to make a space station a while back in 0.24 and got

some good intercepts, but every time I tried to match our velocities I just zipped by. Does anyone have any helpful pointers on the matter?

Definitely watch some of the tutorials on YouTube. Understanding and mastering use of the Navball is critical. I figured out rendezvous pretty quickly, but docking was a pain until I really learned how the Navball fit into the whole equation. Now 90% of the rendezvous and docking it done entirely with the Navball. You also have to learn to maneuver with RCS. Once you've closed to within a few hundred meters of your target, it's all RCS.

(ETA)

Something else I forgot to mention about the ÃŽâ€V budget. It gets a little tricky because the different maneuvers are performed in different configurations. Some maneuvers are performed by the lander and some by the propulsion stage. And some of those performed by the propulsion stage are performed before landing (with a fully fueled lander attached), and some are performed after landing (with a nearly empty lander attached, or perhaps no lander). When I use KER to find the ÃŽâ€V of the propulsion stage, it gives me the number based on it always being attached to a fully fueled lander. The actual ÃŽâ€V that I'll get out of the propulsion stage in the order that the maneuvers will actually be performed is higher than what KER tells me. I found it best to perform all the ÃŽâ€V calculations by hand to make sure it was getting done correctly.

Also be careful to check the fuel in your tanks before you separate and land. In my case when I burned the engines on the propulsion stage, the first fuel tank that it drew from was the topmost tank that's part of the lander. Before separating I had to transfer fuel back into that tank. Had I not checked it, I would have landed on Duna with an empty fuel tank. You might want to also top off your RCS tanks.

Edited January 19, 2015 by OhioBob
added comments

[Scarecrow88]

My last mission to Duna used the same craft I used to go to Mun. The only change I made was to add some parachutes to the lander. The flight profile was slightly different; I used aerobraking at Duna to establish my original orbit, parachutes on the lander took care of most of the braking prior to landing so very little fuel was used during the descent and the 2-stage lander took off as a single unit and the 'descent' stage was only jettisoned when it ran out of fuel.

By using aerobraking and parachuting to save fuel, the dV required to go to Duna isn't so different to going to Mun, so you shouldn't have too many problems.

A thread showing the craft I used can be seen here, if you are interested.

http://forum.kerbalspaceprogram.com/threads/107486-Going-to-Duna-Apollo-style?p=1673449#post1673449

[OhioBob]

I used aerobraking at Duna to establish my original orbit...

I typically use aerobraking at Eve and Jool, but for my above referenced mission to Duna I decided against it. The spacecraft wasn't all that big and it didn't require all that much fuel using an LV-N engine. I could still launch the whole mission on one rocket. Had my spacecraft been bigger I probably would have elected to do an aerocapture.

A thread showing the craft I used can be seen here, if you are interested.

http://forum.kerbalspaceprogram.com/threads/107486-Going-to-Duna-Apollo-style?p=1673449#post1673449

I saw that thread the other day. Pretty cool! Nice looking rocket and spacecraft.

Edited January 19, 2015 by OhioBob

[Frank327]

I also did a Duna + Ike apollo mission, here's what it looked like in LKO.

I used only LV909 and the Poodle engine for most of this mission, but it doesn't have a lot of room for mistakes. LV-N's on the carrier would obviously have made it easier, but I didn't have them unlocked yet. You can make it a lot lighter if you take out the science lab and habitation module. If you get the phase angles right (ksp.olex.biz) then getting to duna should only take some 1100m/s. What I did was not circularize at duna but instead get into an equatorial elliptical orbit. That way I didn't have to burn a lot of fuel to get to Ike after returning from Duna.

So once you're in an elliptical orbit with periaps around 60km you undock the lander at apoaps and reduce periaps to around 10km to land. Pack enough parachutes and you won't need to burn a lot of fuel (on this lander I used 5). When you return from duna, get into a circular orbit that touches the orbit of the main ship. Now you'll be at the same place as the main ship, you just have to do a burn to make sure you'll be there at the same time. When you're at the touching point and on the next pass the main ship will be just behind you, burn towards apoaps until your orbit takes long enough for both ships to arrive at the same time. From there it's a normal rendezvous.

As for docking, don't bother placing RCS thrusters all over your main ship. It's much easier to do the docking with your lander. Just make sure you can control your main ship and keep it stable.

Ike should be a bit easier than Duna. You can do it with the same lander, just refill your fuel from the main ship.

[KerikBalm]

Firstly, I for the life of me cannot do docking. I tried to make a space station a while back in 0.24 and got

some good intercepts, but every time I tried to match our velocities I just zipped by. Does anyone have any helpful pointers on the matter?

That is a thread unto itself, and asked many times, I'd need more information on what you're doing to give a good answer.

Secondly, I'm not 100% sure if my setup can get me to Duna and back. My propulsion stage is one of the Kerbodyne tanks with 2 L-VN's on the bottom. Is this viable or should I scale the tank down a bit?

Its viable, burns will be long, but PE kicking should be able to handle that.

Note that the kerbodyne tanks are among the worst liquid fuel tanks. Their Full:emtpy mass ratio is 8.2:1 - In coparison, the Rockomax tanks, and the 1.25 meter (FL-T100 to FL-T800) tanks have a 9:1 ratio (the tiny fuel tanks are the worst though). A single orange tank is probably best.

Its also worth noting that the new Mk3 spaceplane parts are the best tanks in the game, and I new use them even on pure rocket stages.

They have an full:empty ratio of 9.333:1 (using them gets you closer than ever to a SSTO rocket from eve, which is to say... not that close ) - this includes the mk3->2.5m and mk3-> 3.75m adaptors

Finally, I'm a bit confused on how to build the lander - specifically how much Delta V I need. I'm assuming a mix of LV-909's and Rockomax 48-7S are good enough. Do I even need parachutes?

Parachutes are a big help... but of course you don't "need" them. You can after all, in a single stage, land on Tylo, and then ascend again.

They make the finale landing sequence much easier -> no horizontal drift, no major suicide burn requirement.

However, "less is more" - particularly if you want a reusable single stage lander. Every additional parachute you put on it is added weight to haul all the way to duna, (and if its a single stage design, added weight to haul up to duna orbit). Going from a 20m/s parachute descent speed to a 10 m/s parachute descent speed required 4x the weight in parachutes. You quickly lose more dV due to dry weight, than you gain from a softer landing.

So, add a parachute, and do a powered descent to slow down for a soft touchdown.

terminal velocities with current stock aero are here:

http://wiki.kerbalspaceprogram.com/wiki/Duna#Lifting_off

You'll probably be landing somewhere at about 1,000 to 2,000 meters (no place is exactly zero, the lowest is under 200m though)

So a parachute will save most of the 140 m/s or so that remains after you've reached terminal velocity, which will pay off the cost of taking the chute there.

Also note that due to the small scale height... you'll likely be coming in a lot faster than terminal velocity, even from aerobraking, establishing orbit, and then doing as shallow of a deorbit as possible.

For lift off, you want about 1,500 m/s to be on the safe side (assuming a mood margin for orbital rendevous)

If repeat landings arent needed, you can stage off the struts and chutes for the ascent stage.

[OhioBob]

I've only landed on Duna a handful of times, so I'm by no means a well-seasoned pro. However, from what I've experienced, the one thing that I'd warn about is this: If you plan to land at a fairly high altitude and/or with few parachutes, chances are you won't have killed all your horizontal velocity by the time you have to go to propulsion. It's likely you may need to cancel out some horizontal speed with your engines during you final descent or else you could touchdown with some lateral motion and topple over. This certainly doesn't mean an exceedingly difficult landing, but it is something to be prepared for. You don't want to mess up your landing because you were caught off guard by something you weren't expecting (like I was once).

For my first manned Duna landing I decided to play it safe. I included enough parachutes and landed at a low enough elevation that I was almost assured to be descending vertically with negligible horizontal velocity. I calculated beforehand what my descent speed would be (about 15 m/s) and what my final burn had to be to assure a nice soft landing. Basically all I had to do was hit a button when I reached to right altitude (60 m) and I landed Soyuz-style without difficulty.

I certainly don't disagree with anything KerikBalm wrote, but his method sounds a bit more challenging. If you're up for the challenge then by all means go for it.

[MircoMars]

seriously, you need to learn how to dock. It's not that hard and opens soooo much possibilities.

You wrote that you can rendevouz flyby, that's good!

Now you need to get rid of that nasty relative velocity and get close to your target.

1. select target as target (...duh), the new big round symbol on your navball is "towards target" the small three legged is the "away from target"-marker.

2. click on orbit-velocity to change to target mode (if that didn't happen automaticly). Pro- and Retrograde markers work now relative to target.

3. nose towards retro marker and a short burn to get a feeling for your deceleration. You also want to align the retrograde- and retrotarget-markers, here comes how:

retrograde always moves away from thrust vector, prograde moves towards it. Keep your velocity at about distance/20s [1000m / 20s = 50m/s; 200m / 20s = 10m/s etc.]. if you get to 50m, brake down to <0.2m/s, and take a deep breath.

intermission:

0. while placing your RCS in the VAB, detach all lower stages and drain as much fuel as would be used in the mission at the time of docking. activate the CoM-marker and place a set of 4 linear thrusters "around" it. this is your middle line. now start putting normal RCS-ports at equal distance above and below your middle (still 4 times). for a small ship 8 thrusters are enough, bigger ships need more. Test your RCS placement on launchpad with hacked gravity. if you can use HNIJKL without turning the ship, it's perfect.

4. kissing ships...

argh, real life interferes. to be continued if you say so....

[OhioBob]

I'd like to add one more thing to what MicroMars wrote... At some point as you draw close to the target, say a few hundred meters out, you should change focus from your active ship to the target ship. After making the switch, you'll need to select the approaching ship as the target's target. Adjust your attitude so that the docking port is facing the approaching ship. Assuming the docking port is on the nose of the target, turn the ship until the nose is pointed at the center of the "towards target" marker on the Navball. Now switch back to the active ship and continue the docking.

If the target has no ability to turn, that means you'll have move the active ship into a position that lines it up with the target's docking port. This greatly increases the difficulty. It always a good idea to include some form of attitude control on target if for no other reason than to assist in the docking.

[KerikBalm]

I've only landed on Duna a handful of times, so I'm by no means a well-seasoned pro. However, from what I've experienced, the one thing that I'd warn about is this: If you plan to land at a fairly high altitude and/or with few parachutes, chances are you won't have killed all your horizontal velocity by the time you have to go to propulsion. It's likely you may need to cancel out some horizontal speed with your engines during you final descent or else you could touchdown with some lateral motion and topple over. This certainly doesn't mean an exceedingly difficult landing, but it is something to be prepared for. You don't want to mess up your landing because you were caught off guard by something you weren't expecting (like I was once).

...

I certainly don't disagree with anything KerikBalm wrote, but his method sounds a bit more challenging. If you're up for the challenge then by all means go for it.

Yes, I prefer to have an ample number of chutes. It makes landing far easier. I've had some squat designs that landed with no propulsion at all... aerobraking and then lithobraking.

The landing would be "hard" and collapse a landing strut or three, but then I just send out a kerbal to fix the struts while the chutes are repacked... and good as new.

I've also experimented with cutting the chutes before landing, and doing a full propulsive landing. The thing about chutes is that once you pop them... you are coming down and can't really change where... if it happens to be over a steep slope, it will not end well -> but if you treat it like the Mun with higher gravity, you can choose where you land.

However, careful timing of when you pop your chutes can largely avoid that.

I only mentioned the minimum chute number because he was asking about doing it with no chutes at all - I guess he thinks with the lower gravity, you can do it just like landing on Mun -> which you can do if you wish. Aerobraking with no chutes can still save you several hundred m/s of dV, so that your propulsive landing stage doesn't need to be much more than what you'd use on the Mun (to land something as heavy as your 1,500 m/s dV ascent stage) -> but that scale height is really small, and that atmosphere is really thin, to make much use of it without parachutes, you need to come in very shallow (which makes for large landing elipses).

Before tweakables, chutes could only pop at 500meters, and there was much incentive to use a droguq, because you'd easily come in going a few hundred m/s, and the chutes would often either tear the ship apart, or not stop in time. I like to use a single drogue (actually, a pair of non-stock radial drogue equivalent to 1 stock drogue) and 2 pairs of radial chutes on my landers.

I like to make SSTO landers with orbiting fuel depots, and I try to design them to be fuel optimal (so I need to refuel the depot less often), and I was experimenting with a single mk-16 chute on a single LV-N powered lander ... but while it consumed the least fuel, it also tipped over a lot, and wasn't reliable without quickloads. So I added the drogues which open sooner, and more main chutes to slow it down more (it already had a wide footprint made with cubic struts, landing legs, and strut connections).

The added weight wasn't costing that much extra fuel considering it was meant to take 2 kerbals, 2 mat bays, and 2 goo pods up and down from the surface. Extra reliability was worth a bit of extra fuel use.

For your first landing... just stick a load of chutes on there, it will make the landing much easier.

LV-Ns work well on Duna, the atmosphere isn't thick enough to severely impact their ISP, and the gravity is weak enough that the LV-N has a good enough thrust to weight ratio - you just have to deal with the lumpy terrain and a tall engine.

*edit* Also, for career mod contracts, I accomplish the land on duna and transmit data with just a single detached* QBE probe core with some science instruments, coms, stat panels, and a mk16 parachute (it also has that tiny size decoupler still attached).

For such a light weight "lander", it doesn't even destroy the tiny decoupler that I intended as a "shock absorber"

*what it detaches from is an ion probe that then goes to Ike, and can land on Ike.

Edited January 20, 2015 by KerikBalm

[Scarecrow88]

The number of chutes required is surely dependent on the mass of the lander. My last few missions to Duna used a lander that only required one drogue chute and one main chute to land safely, with a small amount of engine braking in the last 100m or so.

[OhioBob]

My first Duna landing (maybe two) was just a land and transmit mission, which one came off without a hitch. The next one was an unmanned land and return mission. This was before I figured out how to make a precision approach to the planet. I came in way too far north an ended up in a high inclination orbit. Rather than wait for my intended landing site to rotate into my orbital plane, I picked another site, but unfortunately it was at much higher elevation than I believed. I was still almost horizontal when the chute fully deployed. That left me with almost no time to react and get slowed down. Needless to say, it was not one of my better moments. Fortunately I had done a quick save, but the save was at a point in which I was already committed to the landing. On the replay I was able to slow down enough to land, but barely. I didn't have a big fuel margin to work with so I had to be miserly in how much propulsion I used. It was definitely sweaty palms time. In the end I was able to complete the mission and gain some good experience. For my next manned mission I had a much better plan in place.

- - - Updated - - -

The number of chutes required is surely dependent on the mass of the lander.

I have a spreadsheet that let's me calculate the terminal descent velocity of a lander with different numbers of parachutes. Using that spreadsheet I produced the following table. These numbers are, of course, for Duna. Across the top is the altitude in meters above Duna 'sea level' (0 to 3000 m). Down the left side is the metric tons per parachute, including the parachute mass (1 to 10 t/chute). This is based on a Mk16 or Mk2-R parachute. A Mk16-XL can support twice the mass. The numbers that populate the chart is the terminal velocity in meters per second when the parachute is fully deployed.

[TABLE=width: 500]

[TR]

[TD][/TD]

[TD=align: center] 0 [/TD]

[TD=align: center] 500 [/TD]

[TD=align: center]1000[/TD]

[TD=align: center]1500[/TD]

[TD=align: center]2000[/TD]

[TD=align: center]2500[/TD]

[TD=align: center]3000[/TD]

[/TR]

[TR]

[TD]1[/TD]

[TD=align: center]6.3[/TD]

[TD=align: center]6.9[/TD]

[TD=align: center]7.4[/TD]

[TD=align: center]8.1[/TD]

[TD=align: center]8.8[/TD]

[TD=align: center]9.5[/TD]

[TD=align: center]10.3[/TD]

[/TR]

[TR]

[TD]2[/TD]

[TD=align: center]8.9[/TD]

[TD=align: center]9.7[/TD]

[TD=align: center]10.5[/TD]

[TD=align: center]11.4[/TD]

[TD=align: center]12.4[/TD]

[TD=align: center]13.4[/TD]

[TD=align: center]14.6[/TD]

[/TR]

[TR]

[TD]3[/TD]

[TD=align: center]10.9[/TD]

[TD=align: center]11.9[/TD]

[TD=align: center]12.9[/TD]

[TD=align: center]14.0[/TD]

[TD=align: center]15.2[/TD]

[TD=align: center]16.4[/TD]

[TD=align: center]17.8[/TD]

[/TR]

[TR]

[TD]4[/TD]

[TD=align: center]12.6[/TD]

[TD=align: center]13.7[/TD]

[TD=align: center]14.8[/TD]

[TD=align: center]16.1[/TD]

[TD=align: center]17.5[/TD]

[TD=align: center]19.0[/TD]

[TD=align: center]20.6[/TD]

[/TR]

[TR]

[TD]5[/TD]

[TD=align: center]14.1[/TD]

[TD=align: center]15.3[/TD]

[TD=align: center]16.6[/TD]

[TD=align: center]18.0[/TD]

[TD=align: center]19.5[/TD]

[TD=align: center]21.2[/TD]

[TD=align: center]23.0[/TD]

[/TR]

[TR]

[TD]6[/TD]

[TD=align: center]15.4[/TD]

[TD=align: center]16.7[/TD]

[TD=align: center]18.1[/TD]

[TD=align: center]19.7[/TD]

[TD=align: center]21.3[/TD]

[TD=align: center]23.2[/TD]

[TD=align: center]25.1[/TD]

[/TR]

[TR]

[TD]7[/TD]

[TD=align: center]16.6[/TD]

[TD=align: center]18.0[/TD]

[TD=align: center]19.6[/TD]

[TD=align: center]21.2[/TD]

[TD=align: center]23.0[/TD]

[TD=align: center]25.0[/TD]

[TD=align: center]27.1[/TD]

[/TR]

[TR]

[TD]8[/TD]

[TD=align: center]17.7[/TD]

[TD=align: center]19.2[/TD]

[TD=align: center]20.9[/TD]

[TD=align: center]22.7[/TD]

[TD=align: center]24.6[/TD]

[TD=align: center]26.7[/TD]

[TD=align: center]29.0[/TD]

[/TR]

[TR]

[TD]9[/TD]

[TD=align: center]18.8[/TD]

[TD=align: center]20.4[/TD]

[TD=align: center]22.1[/TD]

[TD=align: center]24.0[/TD]

[TD=align: center]26.0[/TD]

[TD=align: center]28.3[/TD]

[TD=align: center]30.7[/TD]

[/TR]

[TR]

[TD]10[/TD]

[TD=align: center]19.8[/TD]

[TD=align: center]21.5[/TD]

[TD=align: center]23.3[/TD]

[TD=align: center]25.3[/TD]

[TD=align: center]27.4[/TD]

[TD=align: center]29.8[/TD]

[TD=align: center]32.3[/TD]

[/TR]

[/TABLE]

[FleshJeb]

Apologies for short post, I'm at work.

KSP Parachute Calculator:

http://ksp.freeiz.com/