Small rocks under the engine.

[Jairo]

In picture AS11-40-5921 it´s possible to see some tiny rocks under the LM engine, and a HB said they shoud have been blown away by the gas jet.

I think they can be not as loose as it was suggested, but what is your explanation for that?

[margamatix]

[pzkpfw]

There is good info here: www.clavius.org/techcrater.html

My "simple answer" is:

When stopping your car, you don't jump on your brake and go from 100 km/h to 0 in no time at all, right at the point where you wish to stop. You slow down gradually.

Likewise (well, it's probably a really bad analogy) the lunar lander descent rocket is not at full power all the way down to touch-down. I think it's not even "on" for last metre or two.

[frenat]

Also the rocket plume spreads out quickly in a vacuum as opposed to in an atmosphere. The overall pressure of the thrust on the ground is not very much at all.

[margamatix]

frenat said:

Also the rocket plume spreads out quickly in a vacuum as opposed to in an atmosphere. The overall pressure of the thrust on the ground is not very much at all.

But surely it would be enough to move a piece of 10mm pea shingle?

[frenat]

matix said:

frenat said:

Also the rocket plume spreads out quickly in a vacuum as opposed to in an atmosphere. The overall pressure of the thrust on the ground is not very much at all.

But surely it would be enough to move a piece of 10mm pea shingle?

Is that your professional opinion? Do you have any math to back that up matix? Because I have found this

the engine nozzle was about 54 inches across (from the Encyclopaedia Astronautica), which means it had an area of 2300 square inches. That in turn means that the thrust generated a pressure of only about 1.5 pounds per square inch! Moreover, in a vacuum, the exhaust from a rocket spreads out very rapidly. On Earth, the air in our atmosphere constrains the thrust of a rocket into a narrow column, which is why you get long flames and columns of smoke from the back of a rocket. In a vacuum, no air means the exhaust spreads out even more, lowering the pressure.

I found that quoted on another forum and unfortunately do not know the original source. (I suspect it originated with JayUtah though) The math is sound though. With a maximum pressure of 1.5 pounds per square inch and then the pressure being lower than that as it spreads out in the vacuum, it is not surprising that some small rocks were not moved.

[frenat]

My bad. the original quote came from badastronomy.com from this page
www.badastronomy.com/bad/tv/foxapollo.html#crater

[margamatix]

frenat said:

Do you have any math to back that up matix? .

Only Newton's Third Law.

[Al Johnston]

Which needs numbers to be of any use.

If you don't know the action you can't know the reaction.

[frenat]

Perhaps you should start with how much less than 1.5 pounds per square inches of pressure should move. Your handwaving won't work here. (not that it has worked anywhere else.)

[margamatix]

frenat said:

Perhaps you should start with how much less than 1.5 pounds per square inches of pressure should move. Your handwaving won't work here. (not that it has worked anywhere else.)

If you can put this message into a form that I can understand, then I will try to answer it.

[frenat]

English not your first language matix? how about this? Strike one word and add another.

Perhaps you should start with how much will less than 1.5 pounds per square inches of pressure should move. Your handwaving won't work here. (not that it has worked anywhere else.)

"less than 1.5 pounds per square inch of pressure" is what the thrust was after coming out of the nozzle after which it greatly decreased due to spreading out in a vacuum. Handwaving is what you've been doing when you say something is obvious but you don't have anything to back it up.

Edit: Anybody know why the bolding doesn't work inside a quote on this post? I ended up underlining the word I wanted bolded as well because the bolding would show up.

[DaiHoss]

How do we know that they are loose on the surface (and thus movable) and not physically attached to the bedrock (and thus immobile) ?

[Al Johnston]

Merely from inspection of the photograph, we don't.

[DaiHoss]

I didn't mean to post my last one - I hadn't finished it yet

Anyway I did mean to continue that they could be immobile if they were pahoehoe lava flows which are not flat but instead have pillows, lumps and bumps sticking up. Any dust could then fill the gaps between these bumps.

Not a perfect explanation but I also haven't studied geology for 15 years