Duna space-planes

[Nuclear_Wizard]

So I've been preparing my Duna program, I have 4 ships in orbit ready to go, and I want to add a space-plane to the mix. The problem is, I have no idea how the planes will react to lower gravity and thinner atmosphere. I have a plane built that flies pretty well on Kerbin (rocket powered, of course) but I don't know if it'll get enough lift to fly properly on Duna.

Is there a good rule of thumb for designing planes and testing them on Kerbin? Or more helpfully, how does the atmospheric thickness contribute to lift?

[SGT_RIZZO300]

I am not sure however I think that the lower atmosphere (less lift) is made up for by the lower gravity. I have only tested it with a tiny RC jet so far.

[zekes]

I tried with a little RC jet and it worked on kerbin but failed on Duna. Now from what i see the lower gravity does not make up for the lack of lift, so expect high landing speeds (100 m/s).

Personally, I think landing at Duna is a waste of fuel. What i do is use a space plane to orbit the planet (and rendezvous with a station/refuel depot) and then use smaller (vertical) shuttles to take men and supples to and from the surface. It saves fuel and is a lot easier than landing a large spaceplane....

[Eric S]

Yeah, I made an electric UAV for the heck of it. It flies great on Kerbin, is in the air before it hits 20 m/s. Has to hit about 80 m/s to get airborne on Duna. I have yet to manage to land it without damaging it on Duna. Think I'm replacing the UAV I was planning on sending with either a tethered balloon, or a probe lander sitting on Ike, either way with a telescope pointing at the colony.

[bsalis]

If you are just interested in getting to the surface and back to orbit, then you can try using legs...

[RoboRay]

The surface pressure at the datum on Duna is roughly the same as that on Kerbin 10km above sealevel. So, if your plane flies well at 15km or higher in testing out of KSC, you've got a usable Duna Flyer. If you can only ascend to 12-14km, you can fly above the lowlands but will need to dodge the mountains and avoid the highlands.

I prefer airships for Duna.

Slower, yes, but much safer than planes and even safer than rovers. Faster than rovers, too. If you're building a permanent Duna base, check out the Hooligan Labs Airships mod.

Edited April 30, 2013 by RoboRay

[Nuclear_Wizard]

Related question: what is the comparison between parachuting on Kerbin and Duna? I want to air-drop in a bunch of rovers, and I'm not sure how slow they need to descend on Kerbin to correspond to a safe descent on Duna. I don't want to bomb the surface, I want to explore it.

EDIT: Even better, if anyone knows the equations they use to model air resistance in terms of atmospheric density, I could just figure it all out myself.

Edited April 30, 2013 by Nuclear_Wizard

[RoboRay]

You can't test parachutes on Kerbin because the unreefing is controlled automatically by height above the surface.

You'll want a lot of parachutes for landing on Duna, though. Here's an example of me landing a heavier rover there...

I slowed down a lot with retrorockets before the parachutes opened, and still used a little rocket thrust a few meters above the surface to cushion the landing more.

[Nuclear_Wizard]

RobotRay, that is a cool truck and landing.

My question is, more specifically, if my rovers are currently landing at around 5 m/s (assuming they are adequately slowed from orbit) what speed will they land on Duna? What is the rule of thumb, and is there some equation I can use?

[RoboRay]

Thanks!

I don't really know a correlation between landing speeds from parachutes on Kerbin to Duna. I've got a gut feeling that 5m/sec may not be enough, though.

I rarely attempt parachute-only landings on Duna; I usually go for parachute-assisted powered landings, letting the 'chutes do most of the work and hold the craft upright while I throttle the landing engines to hit the target descent speed.

[Nuclear_Wizard]

Yeah, that's looking more and more like the best option here. I wanted to have a ship in low orbit with 8 or so mini rovers on it that are just fire and forget, so they auto parachute and land themselves. Might need to get more involved with the landings now.

[RoboRay]

There's a mod for inflatable airbags, but I can't recall the name. You could always do a bouncy Mars-rover-style landing with them to cushion the jolts from the parachute descent.

That might work well for small, lightweight mini-rovers.

[Nuclear_Wizard]

Thanks, I'll look it up.

Worst case scenario is I manually land them, and a few of them crash. Expensive but no lives lost.

[mewp]

If you're looking to fly on Duna, it should be relatively similar to Kerbin. The handling characteristics of your plane should be similar to when you're in the upper atmosphere of Kerbin - you will have a little less dynamic control authority from your flaps / canards / movable wings (that is to say if you wanted to up and turn your plane around using your controls, it would take a little longer), and you should have nearly identical static stability of the plane (the natural tendency to your plane - does it flip over itself when you let go of the controls? or does it glide nicely?).

The one thing to keep in mind is if you're burning fuel from your plane to get to Duna, of course your center of gravity can shift significantly. So although you've tested your plane out and it fly's fine on Kerbin on a full tank of gas, it may not be stable or as stable on Duna with a half a tank of gas.

If you're interested I did a quick video on aircraft stability (and rocket stability - they're similar topics):

[Nuclear_Wizard]

That was a good video, mewp, I didn't actually know that about planes. I assumed I needed the COM on top of the COL.

[Rockhem]

To simulate flying at a regular duna altitude say, 3km. that should be around 13km or so to simulate that. although the lower gravity cant be simulated but this could be close!

[mewp]

Thanks! And you bring up another good point about stability of things while they're flying through air. The placement of the COM and the COL laterally along the plane (in the direction of intended travel) matters significantly in how well the plane fly's (really how well the plane controls its pitch), however the COM and COL placement along the height of the plane does matter as well. The height-wise placement of the COM and the COL dictate your roll stability - having the COM above your COL would act in the reverse pendulum effect, and the reverse for having the COL above the COM.

The physics behind a plane being more stable in roll with the COL above the COM is a bit complicated, as well, the height-wise placement of the two points affects other stability parameters. That being said there are some interested examples to note - significantly is what is called "wing dihedral", which is how much the wings are rotated up or down with respect to the long axis of the plane. Planes with a positive dihedral are trying to bring the COL up, and the planes with negative dihedral are trying to bring the COL down. A good illustration of a positive dihedral plane, as well as a quick preview of how the physics make the plane more stable in roll can be summed up simply by this image http://history.nasa.gov/SP-367/fig144.jpg

Hope this hasn't put you to sleep!

[Tristavius]

I've been doing a lot of Duna landings with various sized craft recently. My first few designs used lots of parachutes, but that was the only source of descent and all but the tiniest designs failed to land.

For landing any kind of size, you really want a bit of engine power as well... doesn't need to be much at all, almost a token effort. Last night I was dropping about 30 tons on 4 high atmosphere and 4 radial parachutes. They were able to brake effectively from whatever speed I was doing on insertion with no structural failure and between them would bring my decent to about 30-40m/s reaching the surface. From there it really is just a few puffs from a liquid engine during the last 300m or so to bring it down at a safe 5-10m/s.

May try and do some experiments with different weight and parachute combinations and what speed they should bring you down to.

[Nuclear_Wizard]

Hope this hasn't put you to sleep!

As a physics student, I find this all quite interesting. Maybe I'll read up on the physics of planes on the weekend, when I have some time. The COM/COL height thing makes sense, I can see it effecting the stability in ways I can't imagine.

They were able to brake effectively from whatever speed I was doing on insertion with no structural failure and between them would bring my decent to about 30-40m/s reaching the surface. From there it really is just a few puffs from a liquid engine during the last 300m or so to bring it down at a safe 5-10m/s.

This is good to know. As long as I don't have to slow down from orbital velocities with engines, it should all be pretty easy to upgrade the rover landers.

[xzxvrx]

Did you mean this mod?

http://kerbalspaceprogram.com/cp-airbag-system/

[RoboRay]

Did you mean this mod?

http://kerbalspaceprogram.com/cp-airbag-system/

Yeah, that's the one I played around with a while back. I think there's another one out there with bigger bags, but that one should work.