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 :eek_big:). 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]

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

 

So where is the "sweet spot"?

[Janus]

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. :eek: :esmoking:

[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]

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

Yes. I know. :esmoking:

 

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 :esmoking:

 

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

[Buffy]

Yes. I know. :eek:

I just *knew* you would! :esmoking:

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!

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

[DougF]

I thought you might like to see some diagrams, so here you are, I'll put my two cents in where I can.

 

Project Apollo Diagrams

Apollo Lunar Module

Apollo Lunar Module - Wikipedia, the free encyclopedia

[Qfwfq]

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.

That certainly isn't the only way and sure isn't the easiest.

[Roadam]

Apollo lander has designated delta v of 2470m/s as a descent with rockets, go get figure.

[Pyrotex]

Informational presentation at :

 

Idaho National Laboratory - Nuclear Energy

 

Click on "Space Nuclear" in the frame at far left, about 4 inches down.

 

Various types and sizes of Radioactive Thermoelectric Generators (RTGs).

 

Useful for Lunar missions -- when you don't wanna hafta go to sleep when the Sun goes down!!

[Roadam]

Various types and sizes of Radioactive Thermoelectric Generators (RTGs).

 

Useful for Lunar missions -- when you don't wanna hafta go to sleep when the Sun goes down!!

 

Usefull, but not really needed, maybe on lander. Those things have low power output compared to their weight, although they last long.

 

Recently I looked into the three default rockets and it turned out that only rockot has reignitable upper stage. Also it has the best isp of all three, 310 in vacuum. Surely its third stage could be extended and maybe even used as a lander...

[Janus]

 

Recently I looked into the three default rockets and it turned out that only rockot has reignitable upper stage. Also it has the best isp of all three, 310 in vacuum. Surely its third stage could be extended and maybe even used as a lander...

 

Checking the specs, the net mass of the upper stage is 1820 kg. Even if the entire payload (1850 kg) was extra fuel, you wouldn't have enough to reach a translunar trajectory.

 

Gotta dump that last stage, it represents too much dead weight.

 

One draw back for the Eurockot that hasn't been mentioned yet is that the payload value given is for an orbit with an inclination of 63 degrees. That's going to increase the delta v needed to make a proper moon intercept, as it involves an additional course correction.

[Roadam]

Gotta dump that last stage, it represents too much dead weight.

 

One draw back for the Eurockot that hasn't been mentioned yet is that the payload value given is for an orbit with an inclination of 63 degrees. That's going to increase the delta v needed to make a proper moon intercept, as it involves an additional course correction.

 

You are right of course. Smaller engine would be better. As for inclination problem: Does inclination really needs to be zero if one wants to make ideal moon approach? If I am not mistaken there are some companies that launch from equator, or close to it at least. Maybe even a ride on one of the Arianes?

 

Quick thought: Maybe the whole 1.6tonnes could still be one last stage, if we could find an engine with thrust that just still allow earth departure, and also strong enough to kill orbital velocity and make gravity turn landing on the moon. Combine it with fuel tanks that can be dumped in transit...

 

On the hunt:)