Precision landing on bodies with an atmosphere

[SunlitZelkova]

Hello,

Background:

My Duna lander had an extremely delicate design, and as I was tired, I made a mistake causing the lander to explode. The first attempt failed, and I am now preparing to launch a second rescue mission. There is heightened importance to the mission as the stranded cosmonauts are three of the original four.

Question:

How does one make a precision landing on a celestial body with an atmosphere? Specifically Duna, although how to do so in general with an atmosphere would be good to know for the future. I am already capable of making a precision landing on an airless body.

I am trying to land within reasonable walking distance of the crash site.

During the first rescue mission, I basically entered a similar orbit to the one the original lander used, roughly estimated how much the orbit/trajectory would change in map mode to where the rescue spacecraft would intercept the crash site, and then of course landed as usual. The rescue craft came within about 10 km of the crash site. I am wondering if there is a better way to do this.

EDIT- I am playing stock.

Edited February 18, 2021 by SunlitZelkova
Forgot a major detail

[Echo__3]

I use the mod Trajectories to help calculate these kinds of landings.

[Stone Blue]

If you are not averse to installing a simple plugin mod, theres Trajectories, or MechJeb, which has a Landing Guidance module in it. However, there have been mixed results with it.

In either case, *MAKE A BACKUP OF YOUR SAVE FILES, BEFORE INSTALLING*, other than that, there should be no issues installing/running either mod.

[StarStreak2109]

Another option is:

[1.9-1.10] Throttle Controlled Avionics - Add-on Releases - Kerbal Space Program Forums

[king of nowhere]

my way has always been to save and reload until i got there. even then, it's difficult. only with a lot of effort i can even hit the biggest island on laythe.

but if you don't mind wasting some fuel, there is a shortcut: make a high speed pass in the atmosphere. then, when you approach the target, burn your rockets.

[Guest]

It's tricky. I use aircraft on bodies with atmospheres; even gliders can be landed quite precisely. With Duna the issue with them is that your airspeed is pretty high because of the thin atmosphere so unless you have S/VTOL capability (tilt-rotor, tilt-rocket, hover-rocket...) you're likely to end up lithobraking pretty hard. 

With ballistically delivered craft, I can only get consistent results if I practice in "simulated" missions with the specific craft I'm working on. They need some kind of aerodynamic controls as well. I've had best results designing them to be pretty slippery, but equipping them with a ring of airbrakes: this gives me enough control over the trajectory that if I plan an overshoot, I can brake to a stop usually within a couple of km of my intended target.

[Spricigo]

Well, as most things in KSP:

• practice
• there is a mod for that

44 minutes ago, Brikoleur said:

With ballistically delivered craft... ... They need some kind of aerodynamic controls as well. I've had best results designing them to be pretty slippery, but equipping them with a ring of airbrakes: this gives me enough control over the trajectory that if I plan an overshoot, I can brake to a stop usually within a couple of km of my intended target.

I suppose it can be done with control surfaces and airbrakes, but I just use engine(s) and MoarFuel^tm  

[Guest]

1 minute ago, Spricigo said:

I suppose it can be done with control surfaces and airbrakes, but I just use engine(s) and MoarFuel^tm  

Yeah with Duna engines work too, you can burn retrograde to control your descent instead of using airbrakes. With Laythe or Kerbin ... ehh, it depends on the design of the craft really, and which end forward you design it to enter the atmosphere.

[Spricigo]

1 minute ago, Brikoleur said:

Yeah with Duna engines work too,

As I see, It's more a case of aerodynamic flight, despite of the elegance,  not working that well with Duna. Engines are not without their own issue but It seems more straightforward to deal with for me. The checklist goes as:

1. there is enough thrust?
2. thee is enough fuel
3. can keep the right attitude?

Of course, there is my bias in it. The way I play planes don't have much use except for Laythe and Eve.

[bewing]

Either come in high enough that you still have attitude control, point retrograde, burn to a stop directly above your target, and fall straight down.

Or, put wings and control surfaces on your craft, fly down just above your target, pull up into a vertical stall, pop your parachutes (make sure they make your engines point down), and use a bit of thrust to create a soft landing.

 

[Guest]

43 minutes ago, Spricigo said:

As I see, It's more a case of aerodynamic flight, despite of the elegance,  not working that well with Duna. Engines are not without their own issue but It seems more straightforward to deal with for me. The checklist goes as:

It does work well with Duna, you just need to design the aircraft for it. They need more wing, more control authority, and some way to land in one piece. If you get that, Duna planes are terrific. It's also an interesting design challenge IMO.  It's certainly more work than a rocket, but then planes usually are. The relatively low dV to orbit and the low atmospheric drag mean that it's really perfect for rocket planes of all types.

 

[Spricigo]

1 hour ago, Brikoleur said:

It does work well with Duna, you just need to design the aircraft for it.

Oh! I'm in perfect agreement that good Duna planes can be designed.

But, I'd rather not design such kind of vessels. Just a preference.

[Guest]

8 hours ago, Spricigo said:

Oh! I'm in perfect agreement that good Duna planes can be designed.

But, I'd rather not design such kind of vessels. Just a preference.

Heh, my situation is that I do know that precise landings with rockets on Duna are possible, but I’d just rather make a plane.

[Clamp-o-Tron]

I would use the aerodynamics of the atmosphere to your advantage.

That's a Falcon 9 coming through the transonic regime for the landing burn. What you see is the massive angle of attack there- they use the entire rocket like a wing to adjust their landing site, instead of having to do continuous engine burns like you would on a vacuum world to trim your trajectory. Just angle your rocket away from the direction you want to go, and you'll move that way.

To illustrate how this works, even on real life Mars with a fraction of Duna's atmospheric pressure, go watch this simulation of the Mars 2020 EDL earlier today. Notice how the capsule isn't pointed exactly retrograde or towards its target at Jezero Crater? Again, that's using aerodynamics. Scott Manley illustrated something similar here, again. Although both examples are capsules, it works just as well with thinner spacecraft as long as they aren't just atmosphere-penetrating pencils.

Design is less important than you'd think. The stock landers work fine for atmospheric descent and control on Duna, but it would help a lot to add airbrakes. Wings in a lifting-body configuration would greatly assist your crossrange, as said above.

The important part here is pinpointing your trajectory- this atmospheric control can only do so much. Just remember how you would target a landing on a vacuum body, then move that point where your trajectory intersects the ground a specific amount that can only really be obtained with fiddling. Since you know how to make a 10 km accurate landing, I would suggest going back to the conditions before your descent, quicksaving, then iterating the descent until you get it close enough to use aerodynamic control.

Hope this helps!