Testing Duna planes on Kerbin

[Teilnehmer]

I’ve decided to make an ion glider for Duna.

My research shows that the Isp and thrust values of the ion engine on Duna are ≥ 80% of those in vacuum. So it seems feasible.

Now I want to make proper wings for it.

The atmospheric pressure on Duna (0.2 atm) is equal to that at 16 km altitude on Kerbin. The Dunian surface gravity is 2.94 m/s² vs 9.31 m/s² at 16 km over Kerbin (3.2 times lower).

When a plane flies, four forces are applied to it:

 gravity mg (pointing down)

 thrust Ft (pointing forward)

 lift Fl ~ v² (pointing up)

 drag Fd ~ v² (pointing backwards)

Their sum equals zero when flying at constant speed and altitude.

If I replace the Ion engines with Spark rocket engines and xenon tanks with tiny LF+Ox tanks, I can get a plane of the same shape and mass but with thrust 3.2 times greater (assuming the fuel and throttle levels can be adjusted). With such a plane at 16 km over Kerbin, all 4 forces are balanced at 3.2 times greater values.

So if I manage to fly it horizontally at a constant speed over Kerbin, I will be able to fly the ion one on Duna at a speed √3.2 = 1.8 times lower.

Are my speculations valid, or am I mistaken in something?

Edited June 3, 2015 by Teilnehmer

[Foxster]

I'd use Hyperedit either to make Kerbin have the characteristics of Duna and test your craft that way or else I'd hyperedit the craft to Duna and try it there. Or is that a bit too cheaty for you?

[Teilnehmer]

My question is mostly about physics. Is that a valid way to simulate other planets’ conditions for plane testing or not?

[Wanderfound]

No idea about your calculations, but: flying Duna isn't that tricky. As you noted, Duna sea level is roughly equivalent to moderate altitude Kerbin. If it can fly slowly at a moderate height on Kerbin, then it'll fly just fine on Duna.

The hard part of Duna planes isn't the flying, it's the landing. Wide and long wheelbase, tailstrike guards, Vernor VTOL, retrothrusters and drag chutes are all recommended.

If you shift to FAR, the analysis data allows you to select speed, altitude and planet.

[Teilnehmer]

Thanks for the advice!

[Chaos_Klaus]

You'd have to calculate your drag and lift forces depending on angle of attack. That's not so easy as these forces depend on the shape of your craft. You will propably have better results when testing it, instead of doing the math. I'm all for doing the math, but here it's not worth the effort, i think.

[Teilnehmer]

You'd have to calculate your drag and lift forces depending on angle of attack. That's not so easy as these forces depend on the shape of your craft.

I assume the shape and the attack angle of the Kerbal-testing plane are identical to those of the actual Duna plane. The velocities are a bit different though.

[Snarfster]

You could create a lifter craft that lifts your Duna glider to the equivalent pressure on Kerbin; then fly the glider at that altitude. This should give you a good indication of whether it will work on Duna.

(AFAIK this should work with ions as well.)

[Teilnehmer]

You could create a lifter craft that lifts your Duna glider to the equivalent pressure on Kerbin; then fly the glider at that altitude. This should give you a good indication of whether it will work on Duna.

The different gravity is the problem. The lift force can be sufficient to fight the gravity of Duna but insufficient for Kerbin.

That’s why I’m trying to scale the thrust (and therefore lift and drag) along with the gravity.

If I make a glider fly on Kerbin without any scaling, it will be a big overkill for Duna. I do not want to deal with 3 times as many ion engines as I actually need.

Edited May 27, 2015 by Teilnehmer

[GoSlash27]

I’ve decided to make an ion glider for Duna.

My research showed that the Isp and thrust values of the ion engine on Duna is ≥ 80% of those in vacuum. So it seems feasible.

Now I want to make proper wings for it.

The atmospheric pressure on Duna (0.2 atm) is equal to that at 16 km altitude on Kerbin. The Dunian surface gravity is 2.94 m/s² vs 9.31 m/s² at 16 km over Kerbin (3.2 times lower).

When a plane flies, four forces are applied to it:

 gravity mg (pointing down)

 thrust Ft (pointing forward)

 lift Fl ~ v² (pointing up)

 drag Fd ~ v² (pointing backwards)

Their sum equals zero when flying at constant speed and altitude.

If I replace the Ion engines with Spark rocket engines and xenon tanks with tiny LF+Ox tanks, I can get a plane of the same shape and mass but with thrust 3.2 times greater (assuming the fuel and throttle levels can be adjusted). With such a plane at 16 km over Kerbin, all 4 forces are balanced at 3.2 times greater values.

So if I manage to fly it horizontally at a constant speed over Kerbin, I will be able to fly the ion one on Duna at a speed √3.2 = 1.8 times lower.

Are my speculations valid, or am I mistaken in something?

Teilnehmer,

This seems about right to me, but transsonic drag is liable to mess the whole thing up.

Rather than multiplying the thrust to compensate for the gravity difference, why not run it at .64 atm (about 3km)? Your takeoff speed on Duna would then match whatever speed you cruise at. I *think*...

The thrust vs drag would balance out at 16km.

HTHs,

-Slashy

[Teilnehmer]

OK. Tests showed the method to be more or less working.