Why is there no dust on the Lunar Lander's footpads?

[JayUtah]

...the exhaust as you get further from the source would expand very rapidly and the effect on the dust would quickly become insignificant.
After this point the dust particles would follow nicely parabolic trajectories. With a hefty initial velocity...

This is a physically correct description of what happens.

[Grashtel]

AS11-40-5920 is a very good example...
AS14-66-9261 onward are also excellent examples, what were you expecting to see exactly?

I think AS11-40-5921 is an even better example, at the right there is a highly visible ray pattern formed by the exhaust (if you look closely it is present in other areas but most visible there due to the angle of the Sun) and to me at least it seems that there is a slight depression under the engine bell itself.

[Echnaton]

You're such a tease, Trebor! Tell me, what am I missing? Don't be rude, now.

This is the trouble. You say there was a 'broad, shallow crater'. I have looked at photos of the area below the lander, and I do not see any type of crater - even shallow. You say tomato...

You have looked at photos of all the landings and not seen any depression of any kind caused by the LM?  You need to look closer.  There are obvious examples of dust being swept from the surface by the approaching LM and very modest broad and shallow indentations caused by the final LM landing.  Go back and check again. 

[raven]

Also worth noting is that, if it was fake, every little grain, ever little crack and granny, ever exhaust ray, would have to be sculpted and shaped by hand.
So they put all this attention to detail, meticulous to the point of superlative, yet forgot to do something as simple as dig a crater?
<Yoda voice> Much contradiction I see, hmm?</Yoda voice>

[Jason Thompson]

True. I should not have used the word 'float', as of course dust would only 'fall' on the moon.

Concession noted. Since you concede that things will not float, will you also concede that there is no way for the dust to be billowing up and settling down on the footpads, which was your original question?

Jason Thompson refers to the 'rather large and powerful rocket engine' blowing dust away from the lander's footpads.

On the scale of dust grains, the LM engine is rather large and powerful. It's nearly five feet across at the nozzle end, and generates 3-10,000 lb of thrust when it is on, depending on the throttle. In the whole scheme of rocketry it's not that powerful, especially when put next to something like an F-1 engine, but it's still a force to be reckoned with.

Yet it is often mentioned that the reason for no crater (or even indentation) having being created below the lander, was due to the exhaust not being powerful enough on descent to blow any dust away - thus not leaving even the slightest crater.

It is never stated that the LM engine is not powerful enough to blow away any dust. This is where hoax believers and the rest of us differ: we know how ridiculous it would be to claim that no dust was blown away because it is evident from the film of all the landings and from the pictures that dust was indeed blown away. There is, however, a world of difference between blowing away surface dust and carving a crater in compacted regolith. It's much the same as the difference between a Hawker Harrier landing in desert terrain and blowing up a large cloud of sand and dust and not carving out a huge crater as it lands. (The Harrier, incidentally, generates more thrust than the LM during vertical take off and landing, and yet it doesn't carve craters out of the ground.)

Too weak on descent to form even an indentation in the dust below the lander - yet 'large and powerful' enough to blow the dust away from the footpads. Which is it?

That's a false dilemma, as I have just explained. It doesn't have to be large and powerful to blow dust away from footpads, does it?

[smartcooky]

On the scale of dust grains, the LM engine is rather large and powerful. It's nearly five feet across at the nozzle end, and generates 3-10,000 lb of thrust when it is on, depending on the throttle. In the whole scheme of rocketry it's not that powerful, especially when put next to something like an F-1 engine, but it's still a force to be reckoned with.
-----
It is never stated that the LM engine is not powerful enough to blow away any dust. This is where hoax believers and the rest of us differ: we know how ridiculous it would be to claim that no dust was blown away because it is evident from the film of all the landings and from the pictures that dust was indeed blown away. There is, however, a world of difference between blowing away surface dust and carving a crater in compacted regolith. It's much the same as the difference between a Hawker Harrier landing in desert terrain and blowing up a large cloud of sand and dust and not carving out a huge crater as it lands. (The Harrier, incidentally, generates more thrust than the LM during vertical take off and landing, and yet it doesn't carve craters out of the ground.)

I agree with all of the above. I have personally watched a Harrier GR3 land on bare ground (i.e. hard dirt with no grass) when I was at RAF Akrotiri in Cyprus back in the mid 1980's. The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust) distributed through four vectored nozzles, all of which are much narrower than the bell on the LM motor.

After it had taxied away, all you could see was a big "blast patch" where the loose dirt had been blown away. There was definitely no crater.


Photo of a GR9 landing on grass at RAF Wittering. NOTE: It is not making a crater!

[Rob260259]

You've got a lot to read Edwardwb1001. And some questions to answer as well. However, something tells me you will not answer most comments and remarks. And please don't comment on these few lines.

[sts60]

Not to be too nanny-ish - aw, who am I fooling?  to be totally nanny-ish - can we hold off on the snark a little longer?  Let's give Edwardwb1001 a little more chance to study the difference between exhaust stream entrainment and wheel ejection, not to mention examine the clear evidence of dust being blown out from under the DPS engines. 

At best the metacommentary doesn't advance the discussion; at worst it provides an excuse to avoid substantive discussion.  Just because Edwardwb1001 has started out in a familiar way doesn't mean he is bound to follow the familiar path.

[Echnaton]

Just because Edwardwb1001 has started out in a familiar way doesn't mean he is bound to follow the familiar path.

True!  Perhaps it's been too slow around here and everyone is wanting to get in the beginning of the discussion.  i am happy to see if he can come forward with anything new or interesting.

[theteacher]

A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?

It is not a discrepancy. It's a difference.

[smartcooky]

A straightforward question but one to which I have not yet received a straightforward answer.  Photos of Apollo 11's landing pads whilst resting on the moon show them to completely dust free, the gold foil covering them shimmering and gleaming - with not a speck of lunar dust or soil present.

Even if the LM's exhaust did not produce a strong enough blast to form a crater, and even if the landing site was quite hard and rocky, there surely would have been a certain amount of dust/soil which would have billowed up to float down and settle on the abovementioned landing pads?

Other landers show decidedly dusty footpads whilst resting on the surface of other worlds. The 2008 Phoenix Mars lander of 2008 shows a completely dust-covered footpad in photos taken.  Why would there be this discrepancy?

It is not a discrepancy. It's a difference.

....and its a difference with a reason.

Mars has an atmosphere... Its not much of one, but it is enough to allow the existence of weather events and phenomena such as clouds, a coloured sky, dust storms and dust devils. The very finest particles will billow and float.

It is a BIG mistake to think that Mars is just a "Big Moon", and that any physical phenomena we see on the Moon will automatically be the same on Mars 

[JayUtah]

The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust)...

And close to ten times the thrust of the DPS as throttled for terminal descent.  Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust and thus quantitatively unconnected to plume impingement.

In terms of rocket engines for, say, launch vehicles the DPS is small.  It's on par with the shuttle's OMS engines -- the little ones in the pods in back.  We routinely operate jet engines in commercial aviation that, as you say, have an order of magnitude more thrust.

The velocity of the exhaust gas is the key value in the entrainment problem, not the thrust rating or throttle setting of the engine.  However the overall thrust at that moment is just a little less than half an OMS engine and far, far less than the typical jet engine for a large airframe.

[Glom]

But let's compare like with like. How does the thrust of the DPS compare to a Robinson R-22?

[smartcooky]

The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust)...

And close to ten times the thrust of the DPS as throttled for terminal descent.  Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust and thus quantitatively unconnected to plume impingement.

In terms of rocket engines for, say, launch vehicles the DPS is small.  It's on par with the shuttle's OMS engines -- the little ones in the pods in back.  We routinely operate jet engines in commercial aviation that, as you say, have an order of magnitude more thrust.

The velocity of the exhaust gas is the key value in the entrainment problem, not the thrust rating or throttle setting of the engine.  However the overall thrust at that moment is just a little less than half an OMS engine and far, far less than the typical jet engine for a large airframe.

The Rolls Royce Pegasus engine produces between 20,000 and 24,000 lbs of thrust depending on the type of aircraft it is installed in. This is more than many fighter engines even with full afterburners running. Its also equipped with water injection, which gives it the ability to attain peak thrust on hot days or at altitude. The reason it is so much greater is simple; most other aeronautical jet engines are designed the push an aircraft along a runway until aerodynamic lift gets it airborne, but with the Pegasus, we asking the engine to "dead lift" the full weight of the aircraft at takeoff (or more correctly, liftoff) and to support its full weight during descent and landing.

The exhaust velocity also varies between models, but the one in the photo I posted earlier is a GR9 fitted with the Mk.107 engine, and that delivers an exhaust velocity of over 950 fps through the rear nozzles, and somewhat less (about 450fps) through the front nozzles, in full thrust. How does that compare with the LM descent engine?

[nomuse]

The Pegasus engine generates over 20,000 lb of thrust (more than twice the LM landing motor at full thrust)...

And close to ten times the thrust of the DPS as throttled for terminal descent.  Total LM thrust just before touchdown is around 2,500 lbf, of which up to 40% is pressure thrust and thus quantitatively unconnected to plume impingement.

In terms of rocket engines for, say, launch vehicles the DPS is small.  It's on par with the shuttle's OMS engines -- the little ones in the pods in back.  We routinely operate jet engines in commercial aviation that, as you say, have an order of magnitude more thrust.

The velocity of the exhaust gas is the key value in the entrainment problem, not the thrust rating or throttle setting of the engine.  However the overall thrust at that moment is just a little less than half an OMS engine and far, far less than the typical jet engine for a large airframe.

The Rolls Royce Pegasus engine produces between 20,000 and 24,000 lbs of thrust depending on the type of aircraft it is installed in. This is more than many fighter engines even with full afterburners running. Its also equipped with water injection, which gives it the ability to attain peak thrust on hot days or at altitude. The reason it is so much greater is simple; most other aeronautical jet engines are designed the push an aircraft along a runway until aerodynamic lift gets it airborne, but with the Pegasus, we asking the engine to "dead lift" the full weight of the aircraft at takeoff (or more correctly, liftoff) and to support its full weight during descent and landing.

The exhaust velocity also varies between models, but the one in the photo I posted earlier is a GR9 fitted with the Mk.107 engine, and that delivers an exhaust velocity of over 950 fps through the rear nozzles, and somewhat less (about 450fps) through the front nozzles, in full thrust. How does that compare with the LM descent engine?

That sounds like something that should be on the radio show "Says You!"  "Panel, for our next question, define the difference between 'takeoff' and 'liftoff.'"