Orbital Altitude around Duna

[957Chatterton]

I have searched for the answer to this and can't even find a discussion pertaining to it. Here's the scenario.

I have two Orbital station modules around Duna, one core and one fuel. They're at an altitude of about 575Km, not yet docked. I was thinking of descending them to about 50Km before docking. What I'm using them for is a refuel waypoint for my Duna Surface base while mining kethane on Duna and on Ike. I want a low orbit for simply "dropping" a ship into the atmosphere after refueling. This would conserve as much fuel as possible that I'm delivering to the surface base.

Is a 50Km orbit really ideal or do I have something wrong? Also, a 575Km orbit is the most fuel efficient orbit for returning to Kerbin after refueling. Excess fuel can be dumped into a refueler around Kerbin for later use.

Edited April 20, 2014 by 957Chatterton

[Geschosskopf]

A 50km orbit is good if parachutes play an important role in your landing strategy. Duna's atmosphere is not only thin but also very shallow (it doesn't become significant until about 12km), so for parachutes to be most effective, you have to come in with long, flat descent profiles which maximize their time in the air. Starting from 50km is good for this if your LZ is about 1/4 of the way around Duna from where you do the deorbit burn. The problem with this type of landing, however, is that it's a bit difficult to land closer than a few km to something already on the ground, so you need a way to bring the things together once they're down.

However, Duna's atmosphere is so thin that if you're landing anything more than about 20 tons, the number of chutes required for them to do any real good is so big that they start really interfering with the rest of the ship's design. In such cases, it's a lot simpler just to land on rockets alone, perhaps with a few chutes to save some fuel if you really want to. For this, a higher orbit in the 75-100km area will do and helps improve landing accuracy.

[Jimbimbibble]

You might want to increase the altitude a bit, maybe 80km. If you're too close to the atmosphere it can be hard to rendezvous with an orbiting craft.

[957Chatterton]

So, if I were to land with engines only, is it the higher the altitude then the more precise the landing?

[Gauss H2K]

So, if I were to land with engines only, is it the higher the altitude then the more precise the landing?

Well a higher orbit means you'll have less radial velocity, but it means you're fighting gravity more. Since the atmosphere is so thin around Duna, it might not be a bad idea to use basic kinematics formulas to figure out the range at any altitude since air resistance will be very little. If you're not opposed to mech jeb, it has a module that predicts landing sites.

[MarvinKitFox]

When landing anywhere, but especially for planets with atmosphere, landing from a high orbit is easier and more accurate, but wastes more delta-v.

This is because from a higher orbit, you ratio of horizontal to vertical movement decreases, making aim easier but braking harder.

The ultimate expression of this difference would be to completely stop your horizontal velocity and drop like a stone to the ground.

You can get your landing accurate to millimeters, at the expense of a small mountain of fuel used.

[957Chatterton]

Thanks everyone for your input! I've decided to go to 100Km. Since I'm dropping fuel to the surface with 7 of the huge tanks from 23.5, it makes sense to save some DV. I've already tested it and it works fine.