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Foxster

Parachute-only Duna landing

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Posted (edited)

Considering that Duna is commonly the next planet that most people visit after Kerbin, it can be surprisingly easy to crash there due to the thin atmosphere and so the ineffectiveness of parachutes. 

So I thought I'd see what was involved with getting down to the surface with just a de-orbit burn and maybe a blip of throttle near the surface i.e. without loads of engine braking that wastes dV that could be better used for the return journey. 

Here is what I have so far...

sRhfOEG.png?1

'Chutes won't open until a minimum speed is reached and there seem to be no stock settings to override this. It is ~510m/s for drogues and 220m/s for main 'chutes. You can change the minimum pressure and altitude that they open at though, as long as the minimum speed is reached. 

So I changed the settings so the drogue chutes open at 4000m and the mains at 2000m to get them working for longer. 

Then there was the matter of getting the speed down enough so that the chutes would deploy at all. The no-engine solution for this was to use some air-brakes, which did the job nicely. Earlier in the game before these are unlocked I suppose some wing parts could be used instead, though they will probably need to be dumped on Duna's surface before lifting off from there. 

With air-brakes, then drogues and then main chutes, the 9t craft above landed OK without any engines braking in one of Duna's wide valleys at near sea level. 

Thoughts?

Edited by Foxster
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7 hours ago, Foxster said:

So I thought I'd see what was involved with getting down to the surface with just a de-orbit burn and maybe a blip of throttle near the surface i.e. without loads of engine braking that wastes dV that could be better used for the return journey. 

In my experience (and mostly by feeling) 1 drogue and 1 regular parachutes is enough for a small lander (orbit->land->orbit) .  The final burn seems to use less fuel weight than the weight of extra parachutes.  

For one way trips I try to have enough deltaV for deorbit and touch down in ant+Oscar or 2 spider+Oscar combos.  

In the end the question is not how many parachutes you need to land without engines,  but the minimum overall weight of propulsive +parachute landing. Not to mention lithobreaking... 

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Having a thread for this is a good idea.  

I am still (since April 2016) playing my first career game, in which I've had Kerbals land and return from every planet and moon except Duna.  My Duna probe landed hard, and I seem to have never looked back to Duna.

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I've tried doing this for landing my rover on duna. The rover is mounted inside the fairing, which is equipped with airbrakes, drogue chutes and main chutes. Once it's cleared off duna atmosphere, the fairing deploys, while airbrakes slows it down before the drogue chutes deployed. After it's slowed down low enough by rogue chutes, the rover are decoupled mid air while deploying the main chute. It lands on duna, but it seems it depends where and when you land. Earlier attempts seems ended with my rover's wheel being broken/ exploded on contact with surface (though I'm finally able to land it safely by delaying the decoupling long enough so that my rover is able to be decoupled at lower speed than usual)

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I know it's obvious, but I've found the angle of approach makes all the difference on Duna. I don't use engines once I'm suborbital, but I try to move horizontally through the atmosphere as long as I can. I just keep a high Pe and let Duna do the rest. You've already mentioned that drogues are the key. Once they pop, you know your mains will open in time. I use larger landers than your example, but the concept is the same. I just need to tap the gas right before touchdown to get below the 6m/s threshold I feel is safe for the landing legs.

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21 minutes ago, Cpt Kerbalkrunch said:

I know it's obvious, but I've found the angle of approach makes all the difference on Duna...

Good stuff. I'll have a play with that. 

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Once suborbital, this one with 2 drogues and 1 main chute only needed 16 units of fuel to land safely (I don't know how much delta v that is). Threw it together real quick for the Duna lander question thread. Works pretty well.

 

screenshot402.png

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I'd rather burn a small amount of fuel instead of taking all those extra parts to slow me down.

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Haven't been to Duna for a while but what I remember:

1. Chute min pressure in VAB isn't enough - min for chute/drogue is 0.04/0.02, but you can set them to 0.01 in flight. From what I remember, VAB default min isn't enough. To be on the safe side, if I'm just targeting a landing without location requirement, I just slide pressure to min and altitude to max. If I need a precise landing (like landing something near my base), then these settings are my in-flight controls.

2. Without engine, it's more likely to hit the ground at a higher speed. Duna isn't that flat, and if, say, you hit a slope at 10+ or even 20m/s - you'll get in a small trouble. One possible solution is to use retracted airbrakes as "bumpers" that are designed to pop upon touchdown, to get rid of extra mass as well as soften the landing.

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I once did a rover mission to duna, it used only a stack of seperatrons for suicide burn.

Then during the descent I dropped the frog he chutes. 

Then there goes the main chutes.

And upon touchdown I am using 6 RCS fuel containers (empty) to lithobrake.

The only thing is that I was in a fairing.

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6 hours ago, FancyMouse said:

1. Chute min pressure in VAB isn't enough - min for chute/drogue is 0.04/0.02, but you can set them to 0.01 in flight. From what I remember, VAB default min isn't enough. To be on the safe side, if I'm just targeting a landing without location requirement, I just slide pressure to min and altitude to max. If I need a precise landing (like landing something near my base), then these settings are my in-flight controls.

This. Very much this.

 

On 6/16/2017 at 1:34 PM, Spricigo said:

In the end the question is not how many parachutes you need to land without engines,  but the minimum overall weight of propulsive +parachute landing. Not to mention lithobreaking... 

Also very much this. Note that parachutes will gain effectiveness if you have an engineer to repack them and attempt multiple landings with the same lander. For a single landing and then back to Kerbin, a couple of drogues can help a lot (depending on mass, ofc) and you should otherwise not worry too much about fuel expenditure for the landing.

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I always set drogues to .37, roughly the middle of the slider. It's still going to pop 5kms above the surface. At that altitude, even Duna has some atmosphere. A .1 setting would be practically airless. The only reason I've ever seen one fail to open is excessive speed. Once you drop below 700 m/s, it's going to open. A drogue's purpose (at least in KSP) isn't necessarily to open at a higher altitude, but at a higher velocity. I don't see how setting the slider lower would hurt you, but it isn't necessary.

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1 hour ago, Cpt Kerbalkrunch said:

I always set drogues to .37, roughly the middle of the slider. It's still going to pop 5kms above the surface. At that altitude, even Duna has some atmosphere. A .1 setting would be practically airless. The only reason I've ever seen one fail to open is excessive speed. Once you drop below 700 m/s, it's going to open. A drogue's purpose (at least in KSP) isn't necessarily to open at a higher altitude, but at a higher velocity. I don't see how setting the slider lower would hurt you, but it isn't necessary.

How does that work? 0.37 (I assume you're not off by a decimal point, since you can't set 0.037 to chutes) is greater than Duna ASL pressure (~0.067), which means your chute is never going to have semideployment stage - it goes right into full deployment when it hits radio altimeter value. 0.37 looks like a value that I would use on Kerbin, but absolutely not on Duna. How is it useful?

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duna.jpg

Winglets, deployed at 150% in alternating directions, can slow a minimal lander to 400m/s, and full-size parachutes partially deploy at that speed on Duna.  We can land on those winglets at 12m/s, whereas early-career landing legs are not stiff enough to protect the engine.  Using the reaction wheel (with a pilot or probe-core that has SAS) is important to avoid tipping on the touchdown bounce.  One could use the engines just before touchdown, but it is hard to judge distance to the ground when you are new at the game.  

This lander has enough delta-v to return to Kerbin, using six aero-braking passes to reach the surface. The engineer (or pilot, depending on the rules) needs to repack the chutes after use at Duna.

Atmospheric pressure is only 0.07 atmospheres on the surface of Duna, so we do need the 'pressure' slider down to 0.04 atm if we want the parachutes to partially-deploy before their full-deploy altitude.  A partial deployment reduces the g-forces on the pilot.

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15 minutes ago, FancyMouse said:

How does that work? 0.37 (I assume you're not off by a decimal point, since you can't set 0.037 to chutes) is greater than Duna ASL pressure (~0.067), which means your chute is never going to have semideployment stage - it goes right into full deployment when it hits radio altimeter value. 0.37 looks like a value that I would use on Kerbin, but absolutely not on Duna. How is it useful?

I honestly couldn't say. But it works. Maybe it pops at the altitude or pressure setting, whatever it hits first? Provided you're below the minimum speed, that is. Someone with more knowledge than me might be able to give a definitive answer, cuz I'm not exactly sure. I just set every drogue to that setting (Duna, Laythe, or Eve) and they always open.

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1 minute ago, Cpt Kerbalkrunch said:

I honestly couldn't say. But it works. Maybe it pops at the altitude or pressure setting, whatever it hits first? Provided you're below the minimum speed, that is. Someone with more knowledge than me might be able to give a definitive answer, cuz I'm not exactly sure. I just set every drogue to that setting (Duna, Laythe, or Eve) and they always open.

Then you're only relying on your full deployment. This is exactly why you're seeing excessive speed problem - because you didn't semi deploy at all. The point of semi deployment is exactly to avoid being too fast at low altitude.

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2 minutes ago, FancyMouse said:

Then you're only relying on your full deployment. This is exactly why you're seeing excessive speed problem - because you didn't semi deploy at all. The point of semi deployment is exactly to avoid being too fast at low altitude.

I see what you're saying but, it seems to be a problem unique to Duna. I don't worry much about it because they will still open (had to double-check just now to see what settings I used, and they were indeed set to .37, while the main was set to .75; both opened). I just come in almost completely horizontal. I don't attempt to slow down in any way. Without the drogues, I'd hit the ground with my chutes still in the red. But the drogues pop, the main pops, and I feather the throttle a bit just before touchdown. It's actually pretty exciting. When the chutes pop I'm still horizontal and very low. I dig it.

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Forgot to stage the chutes (wasn't multi-tasking at the time and not paying attention), so I didn't pop 'em at the normal 5km. Noticed and staged 'em at 3km instead. Still worked just fine.

 

screenshot412.png

 

Staged the main chute just after.

 

screenshot413.png

 

It's just above the surface, but not too low. Still have to float for a few seconds before I need to slow down a bit for the landing.

 

screenshot415.png

 

Nothin' to it.

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Lots of good advice above-- as has been observed, there are many different ways to do this.  :)

One additional technique that I haven't seen mentioned here:  you can use lift to your advantage.

I don't mean you need to build a full-fledged airplane, but just including some stubby winglets means you can plow into atmosphere at an angle (i.e. with an AoA of 30 degrees or so, not straight prograde/retrograde) to generate lift as you slow down.  This allows you to set your reentry Pe fairly low, without worrying so much that you're going to faceplant while still going too fast for the chutes to open.  Even with very small wings, you can use AoA-based lift to maintain altitude long enough for the atmosphere to slow you down to the point that the drogues can open.

Obviously this is not a necessary technique (plenty of good examples above that don't need it), just one more tool in the ol' toolbox to consider.  :)

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3 hours ago, Snark said:

I don't mean you need to build a full-fledged airplane.

On top of that: no one is saying a full-fledged airplane is not an option.

Incidentally, if one will rely on wings (/stubby winglets) also may consider landing gears for a vertical landing.

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13 minutes ago, Spricigo said:

On top of that: no one is saying a full-fledged airplane is not an option.

Yup, building a Duna airplane is definitely doable, and can be a fun change of pace to try.

The main challenge is getting the landing speed low enough not to break up on landing.  Duna's atmosphere is so thin that even spamming lots of wing area, it can be tricky to go really slow without stalling; and also, Duna doesn't have a lot of really flat areas.  It tends towards gently rolling hills everywhere.  Those gentle hills are no problem at all when you're coming down on parachutes, but they're a major pain in the posterior when you're gliding along at nearly 100 m/s looking for a place to land.

I've had best results when I mount a couple of Vernor engines on the underside of the plane (one in the front, one in the back), with an action group toggling them on/off.  They don't have to be strong enough to lift the plane (I'm not building a VTOL); just enough to provide a bit of assistance. I leave them off most of the time, and turn them on only when I'm about to land.  This gives a few benefits:

  • By providing some lift, it allows me to slow to a more manageable speed for landing.
  • Since I tip the nose up when I flare for landing, that points the Vernors somewhat forward, which means that their thrust helps to slow me down.
  • When it's time for takeoff, it helps me get airborne sooner because the one in the nose helps shove the nose off the ground and get me rotated for takeoff.

 

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Plane on Duna:

Doable, yes.

Fun, maybe.

Interesting enough to make actually design one, not yet. I'm mostly a rocket guy.

 

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One trick I discovered is that an open shielded docking port acts like a mini-airbrake. If you've got one on the nose of your craft, make sure it's open during a Duna atmospheric entry.

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8 hours ago, Norcalplanner said:

One trick I discovered is that an open shielded docking port acts like a mini-airbrake. If you've got one on the nose of your craft, make sure it's open during a Duna atmospheric entry.

How well does it work? I know it has nice high heat tolerance.

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2 hours ago, qzgy said:

How well does it work? I know it has nice high heat tolerance.

Not a huge amount.  In my testing during Kerbin reentry, it would generally increase the G load by 1 (e.g. going from 5 to 6 Gs).  If I had to guess, I'd say it's equal to maybe a third of a single airbrake.  But if you already have one on your craft anyway, just keep it open.  Here's an example I from my 3.2x GPP career, when I was landing on Niven:

 cxgLP8x.png

In this case, it also helped to keep everything oriented correctly to have a bit of extra drag at the back.  It also doesn't get very hot in this sort of configuration, as it's shielded by the rest of the craft.

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That looks familiar, from the GPP loading screens...

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