What is the Best Mothership Orbiting height for an apollo style mission on Mun?

[DarthRevan]

Or is there no best height because too low and the landing craft has to kill a lot of horizontal velocity and too high and the landing craft has to kill to much vertical velocity.

E: ya i had vertical and horizontal switched

Edited March 6, 2014 by DarthRevan

[Taki117]

I generally park at about 10km, this is a happy medium because what you don't use going down can be used coming back up. Also that's about as low as you can get without smashing into anything. It also takes less dV to get into a lower orbit than it does to get into a higher one.

[Kryxal]

Do you have those backwards? Anyway, the usual takeoff/landing profile for no particular target is to get your periapsis as low as you can afford and brake while maintaining altitude ... or the reverse, get all your horizontal speed ASAP. If you time it well, you can do a direct ascent, if not, you'll want at least SOME difference in height to set up for a transfer orbit. Don't forget about time warp too ... it's a pain if you can't warp as fast as you want, but 1000x is probably overkill for this.

[Streetwind]

That's an interesting question actually. Does the combined velocity (horizontal orbital speed plus whatever vertical speed you would gain from falling straight down from where you are now) increase or decrease as you lower your orbit? The former decreases with height, the latter increases. Both scale with the gravity of the orbited body, but only the former scales with its physical size - and quite substantially so, looking at RSS. This would suggest that the answer in KSP may turn out to be different than IRL, since KSP has much smaller celestials and thus smaller vertical velocities.

[Greenspan]

It depends on how much Delta-V your lander has. You would want it as high as possible so that your lander can land and return to the same orbit plus a safety margin.

The higher your Mun Orbit, the more Delta-V it will take for your lander to get down and back up; however the lower the orbit the more Delta-V will be required for your return craft (i.e. to get out of Mun's gravity well)

The amount of Delta-V going from an X km orbit to landing on Mun surface is identical to launching from Mun surface and going to an X km orbit. Lower orbits require less Delta-V to land from, and reach orbit to.

[Kasuha]

That depends on what you consider best. If it is best in the sense of saving fuel, then best parking orbit is a highly elliptical one with periapsis at safe height above terrain (7500 m) and apoapsis almost at the SOI boundary. Of course the lander must have enough dv to be able to land from this orbit and return to it. It's about 1/2 more than the necessary minimum for landing from and returning to low mun orbit. You get as much as you can from Oberth effect and don't move unnecessary mass to low orbits.

[Vanamonde]

A point to consider: the lower the orbiting ship is, the smaller the window for a rendezvous launch. If you're not good at rendezvous, or just don't want to have to rush, put the mothership higher and give yourself more time to maneuver and a more forgiving margin for error.

[regex]

A point to consider: the lower the orbiting ship is, the smaller the window for a rendezvous launch. If you're not good at rendezvous, or just don't want to have to rush, put the mothership higher and give yourself more time to maneuver and a more forgiving margin for error.

That shouldn't matter because you can circularize the lander above the mothership and wait for it to come around rather than chase it. If you're not going to launch into an intercept the thing to do is make one circular orbit higher (and thus slower) than the lower (faster) orbit. Then plan your intercept accordingly. If the mothership is that low circularizing the lander at a higher orbit isn't going to take that much more delta-v than a direct intercept.

E: Hell, you could also come in at the mothership from above during a launch intercept if it's in a very low orbit to give yourself more maneuvering room. Which means that any orbit you're comfortable with is the best orbit for your mothership.

Edited March 5, 2014 by regex

[maccollo]

That's an interesting question actually. Does the combined velocity (horizontal orbital speed plus whatever vertical speed you would gain from falling straight down from where you are now) increase or decrease as you lower your orbit?

It always decreases if you lower your parking orbit.

You can just think about it in reverse. Starting from the surface, does it take more or less delta V to reach a higher orbit?

That depends on what you consider best. If it is best in the sense of saving fuel, then best parking orbit is a highly elliptical one with periapsis at safe height above terrain (7500 m) and apoapsis almost at the SOI boundary. Of course the lander must have enough dv to be able to land from this orbit and return to it. It's about 1/2 more than the necessary minimum for landing from and returning to low mun orbit. You get as much as you can from Oberth effect and don't move unnecessary mass to low orbits.

I was gonna say this wouldn't work, but then I realized you can just wait until Mun has done half an orbit around Kerbin. It takes less than a day I think.

[DocMoriarty]

I put my mother, father and brother and sisterships usually at 40km above the Mun. That leaves me with 30km below it for a lower orbit if i have to catch up with it. dV doesn't really matter since thats peanuts anyway at the Mun.

[Callisto]

I did a recent Kerpollo mission to the moon, requiring a rendezvous with the mothership orbiting mun...

I used sepratrons to get the craft upto a decent height, then used RCS to circularise and rendezvous... actually worked really well

[MarvinKitFox]

As high as you can, that your lander can safely reach.

Almost by definition, your lander is the smaller, lighter of the two vessels.

You want to minimize required rocket mass, so you want as much of the delta-v of the mission to be done by the lightest possible component.

If you lander is capable enough, the ideal Mun orbit for your mothership is low kerbin orbit.

For that matter, if your lander is good enough, the best orbit for your mothership is in feasibility studies, only.

For me, a nice compromise is about 1m higher than the highest peaks it will encounter.. But that's because my landers suck, at least in mass-efficiency. My typical Mun lander is 120tons++

[MarvinKitFox]

That's an interesting question actually. Does the combined velocity (horizontal orbital speed plus whatever vertical speed you would gain from falling straight down from where you are now) increase or decrease as you lower your orbit? The former decreases with height, the latter increases. Both scale with the gravity of the orbited body, but only the former scales with its physical size - and quite substantially so, looking at RSS. This would suggest that the answer in KSP may turn out to be different than IRL, since KSP has much smaller celestials and thus smaller vertical velocities.

Lower orbit will require less delta-v to land from and to ascend to at takeoff. No question or exception to this rule, equally valid in real life and KSP.

However, and especially for a *very* low orbit around an airless body, it does sharply increase the minimum TWR your vessel must possess.

In addition, i find it much easier to time a "suicide burn" descent when coming in near vertical, rather than near horizontal. Heck, i find any form of landing easier if I dont also have to worry about orientation, rate of rotation, skew of thrust as I turn, etc. A dead drop downwards is always easier to execute.

[DarthRevan]

Thanks guys for all of your input, from all of this and testing i realized that a low orbit is best because the vertical delta V does not increase by a significant amount while the horizontal delta V does the higher up you need to land from and get back up to.