Soft landing on the moon discussion

[Roadam]

There has been a suggestion for a thread like this so I an taking initiative.

Lets say that our spacecraft is orbiting the moon at some height (the less the better, well unless you hit a crater ring . So now we have to initiate deorbiting burn and land somehow.
Options to do that:

Airbag landing: dispersing kinetic energy with rolling on lunar surface

Apollo-like landing: controlled descent with thrusters for deacceleration

Hard landing: from uncomplete deacceleration into "arrow" mode where spacecraft sticks into lunar surface (regolith not only optional ). Cargo itself would have to survive of course, but there is still a lot of stress involved. Maybe if thruster tanks would be used as collapsable front as in cars...

And of course making a fresh crater...

Anything to add?

[Qfwfq]

The lower the altitude of the orbit, the harder it is to have a soft landing.

[freeztar]

Originally Posted by Qfwfq

The lower the altitude of the orbit, the harder it is to have a soft landing.

So where is the "sweet spot"?

[Janus]

Originally Posted by Qfwfq

The lower the altitude of the orbit, the harder it is to have a soft landing.

In what way do you mean?

Orbital speed just skimming the crater tops: 1681m/s

This is the velocity you need to shed to make a soft landing.

Starting at an orbital altitude 100km. First you do a 230m/s burn to put you in a moon grazing elliptical orbit. At perigee, your speed will be 1704m/sec, which is the velocity you need to shed. total delta v = 1934 m/sec, more than the value above.

[Roadam]

Does it really matter?
Escape veilocity on the moon is 2.4km/s. And that is the delta v which needs to be fired away. To land of course.

[Turtle]

My gut feeling is that the airbag route is safer, albeit less accurate. From lunar orbit, decelerate with retro-rocket then deploy the bag(s) for the drop.

[Buffy]

Does anyone know if the Apollo Lunar Module's legs had shock absorbers? I know there were very flimsy sensors extending down from the pads to trigger engine shutoff, but the legs sure don't look like there's anything under the mylar than aluminum poles....

At any rate, I'd think that single-use shock absorbers could be built without too much additional weight, possibly less weight that those big rubber bags....

Shocking,
Buffy

[Pyrotex]

Originally Posted by Buffy

Does anyone know if the Apollo Lunar Module's legs had shock absorbers? ...

Yes. I know.

The landing pads had six inches of crushable aluminum honeycomb material. The legs themselves contained something similar between the telescoping segments.

[Roadam]

When using shock absorbers you have to point them...or land on your head

There is a question how much delta v coud an airbag survive.

[Buffy]

Originally Posted by Pyrotex

Yes. I know.

I just *knew* you would!

Originally Posted by Pyrotex

The landing pads had six inches of crushable aluminum honeycomb material. The legs themselves contained something similar between the telescoping segments.

That's single use! And light! At least some "delta v" to be absorbed there!

Originally Posted by Roadam

When using shock absorbers you have to point them...or land on your head

There is a question how much delta v coud an airbag survive.

Obviously the thrusters necessary to "keep you upright" for those "pointed" shock absorbers could be traded off against the all-around padding of an unoriented airbag-protected craft--that is, the latter would require airbags on all sides if you wanted to eliminate the need for the thrusters.

If you're going unoriented though, I'd think you'd need to worry also about the rigging you need to keep the eggs from breaking *inside*. It would have to absorb equally well in all directions too...

An ounce of wall-adhesive is worth a pound of all-the-king's-men,
Buffy