The lunar module's descent engine should have dug a huge crater in the lunar surface.

I have yet to see a conspiracist who has given any kind of quantifiable justification for this belief. We could simply ask, "Why do you expect a crater?" and probably be done with it. A few have made vague references to other vehicles in other situations that produce some kind of visible interaction with the soil underneath them. But none can explain why that ought to be immediately generalized to include the lunar module.

The Lunar Landing Training Vehicle, for example, didn't produce any craters. And it directed even more downward thrust than the lunar module. Harrier jets and large helicopters routinely produce vast amounts of downward thrust without leaving large craters behind.

The rocket engine's thrust was focused on one point for quite some time. Surely there would be a significant visible effect.

Not necessarily. It's difficult to tell from the landing film footage just how high above the surface they were. But until the very last few seconds, the approach profile for the lunar module called for some forward motion. The exhaust probably wasn't focused on any one spot for very long.

The notion that it was focused at all displays some misunderstanding of how rocket engines behave in a vacuum. Watch very carefully at the next rocket launch. As the rocket climbs higher and higher, the exhaust plume spreads out. Because the surrounding air gets thinner as the rocket climbs, there is less air pressure to impede the dispersal of the exhaust gasses.

The lunar module's descent engine produced 10,000 pounds (4,550 kgf) of thrust. Surely 10,000 pounds of pressure is enough to dig a very large hole.

Basic Newtonian physics solves this problem.

"Weight" is simply the force of gravity between two masses. If something weighs a certain amount on earth, that's the same as saying a force of that amount exists between the earth and the object. The force of gravity is computed partly by multiplying the masses of the two objects in question. The moon has only a fraction of the mass of the earth, and so exerts much less gravity. The force between the moon and that same object would be only 1/6 as much.

Galileo's principle lets us treat force, weight, and acceleration as identical concepts when dealing with gravity. A falling object accelerates downward because gravity imparts a constant force resulting in a constant acceleration. This acceleration produces an increase in downward velocity.

So if you want to descend at a constant rate you have to precisely negate that gravitational force so that your acceleration along the vertical axis is zero. This means the net force along the vertical axis must also be zero. So if you can apply a force exactly equal to the force of gravity, but in the upward direction instead, you can achieve that constant velocity. (Hovering is the same principle, but with the constant velocity being zero in that case.)

The Apollo 12 lunar module, for example, had a mass of 33,325 lbm (15,148 kg) fully loaded. On earth gravity would exert a force of 33,325 lbf on that spacecraft. But near the end of the descent it was not fully loaded. Most of the descent engine (DPS) propellant had been burned away. Fortunately there are ample references to how much DPS propellant was consumed. We can therefore calculate the weight of the lunar module very accurately as it neared touchdown. According to telemetry, 705 lbm (320 kg) of DPS propellants remained from an initial load of 18,226 lbm (8,285 kg).[Reports12] This means at touchdown the lunar module had shed at least 17,521 lbm (7,964 kg) by burning its descent fuel. Subtracting this from the launch mass gives a landing mass of 15,804 lbm (7,184 kg).

Earth's gravity would exert a force of 15,804 lbf on that mass, but the moon's gravity exerts only one-sixth that much: 2,634 lbf.

So in order to negate the downward force of 2,634 lbf we merely have to apply an upward force of the same magnitude. Therefore a thrust of 2,634 lbf was required to hover or descent at a constant rate.

Yes, it really is that easy.

This describes the situation seconds before touchdown. The initial descent was of course very fast. And so to slow the rate of descent it would have been necessary to apply a larger thrust that surpasses the force of gravity. This amount of thrust was applied at high altitude where it did not affect the lunar surface.

By comparison, a fully-loaded Harrier jump jet produces 27,000 lbf thrust at liftoff -- ten times more than a lunar module. Yet you typically do not see a crater under a Harrier. This is because popular intuition dictates that a rocket engine of any size is automatically more powerful than a jet engine of any size. In fact, most jet engines are more powerful than the lunar module's rocket engines.

The published strength of the lunar module descent engine is 10,000 pounds, not 3,000 pounds. With weight at a premium on the lunar module, the designers wouldn't have specified an engine larger than necessary. Therefore it's wrong to say that only 3,000 pounds of thrust was applied. [Aulis]

The published capacity of the lunar module descent engine (DPS) is indeed just under 10,000 lbf (4,550 kgf), and weight certainly was at a premium. But managing the descent and hovering over the lunar surface just prior to touchdown wasn't the DPS's only task. It was also used to perform orbital maneuvers prior to the landing. The lander was bloated with fuel and supplies at the start of the descent, and orbital maneuvers are very time-critical. Having a large engine ensured they were carried out precisely with short burns, not sloppily with long burns from a weaker engine. Further, should the astronauts have needed to abort the landing and ascend, the engine would have to produce much more thrust than the force of gravity.

Physics is obviously a mystery to the folks at Aulis. They're clearly grasping at straws. With 10,000 lbf of thrust applied upward, a constant rate of descent would have required an equal force of lunar gravity applied to the lander in order to produce zero net force and therefore no acceleration. Since gravity is six times stronger on earth, this means the lander would have massed 60,000 lbm on earth -- nearly twice its published takeoff mass. Aulis is only looking at the published lunar lander data that supports his theory. Then they apparently hope the physics will all work itself out.

They don't.

When I worked at Rocketdyne I saw tests of engines as powerful as the lunar module descent engine. They can move boulders across canyons. The engine should have dug clear down to bedrock on the moon. [Bill Kaysing]

Thrust of Common Engines Engine Thrust (lbf) Thrust (kN) Space shuttle (one SSME) 518,000 2,300 German V-2 160,000 714 Boeing 747-300 (one Pratt & Whitney JT9D-7R4G2) 54,750 241 F-16N jet fighter (in afterburner) 27,000 119 Boeing 737-700 (one GE CFM56-7B) 24,200 108 Apollo LM DPS (25% throttle) 2,600 11 Marquardt steering jet 100 0.5

Table 1

Mr. Kaysing is clearly exaggerating, or is perhaps confused. Since he is not a trained engineer and was merely a spectator at any tests he may have witnessed at Rocketdyne, he may have not known the rating of the engines he saw tested. Rocketdyne would eventually build the most powerful rocket engines in the Apollo program, the F-1, and was the clear choice to design and build large rocket engines. Perhaps Mr. Kaysing saw one of those being tested. Since Mr. Kaysing never specifies what projects at Rocketdyne he actually worked on, we simply have to decide whether to take him at his word.

10,000 lbf is not a very powerful engine as engines go. As noted above, people intuitively believe that any rocket engine is automatically more powerful than any jet engine. In fact they produce thrust in exactly the same way: by ejecting high-velocity gas from the rear nozzle. Many jet engines are in fact quite a bit more powerful than the lunar module descent engine. And Kaysing also seems unaware that the LM engine would have to be throttled back to about 25% -- 2,634 lbf -- in order to land.

Table 1 compares the thrust of some common engines, both rocket and jet. The Boeing 747 certainly has a tremendous thrust, and care must be taken when those engines are operated near airport equipment. The Boeing 737 is a more common aircraft and many air travelers have seen and felt its engines operating at various thrust levels around airport personnel and equipment. The lunar module descent engine at 25% throttle is about the same as taxi thrust (5%) of a 737, the amount of thrust used to get the aircraft moving after it has pushed back from the gate. You don't see it throwing baggage carts or workers across the ramp. It is hard to imagine it digging down to bedrock.

The exhaust plume was very hot, about 5,000 F. It should have melted the lunar surface. Yet no there is no sign of melting in the photographs.

The exhaust gas was 5,000 F in the combustion chamber, where most of the combustion took place. At the nozzle exit the temperature was about 2,800 F. And as the plume expands into the vacuum of space, it cools very rapidly, down to 1,000 F or so. By the time it strikes the lunar surface it is not hot enough to melt it.

The lunar surface is composed of rock and dust. It takes a tremendous amount of heat concentrated on such material for a long period of time to melt it. We collected some desert rocks and dust and heated them with an oxy-acetylene torch (5,700 F) for five minutes. They did not melt, and they were only slightly discolored. Photographs of the area under the Apollo 11 descent engine nozzle (Fig. 2) show an apparently discolored surface.

Is there any evidence in the photographic record of the effect of the lunar module's descent engine?

Fig. 1 - Closeup of the lower left corner of AS11-40-5920 (396 KB). The ground shows unmistakable signs of fluid erosion. The DPS plume would have swept the surface from lower left to upper right. (NASA)

In Fig. 1 the exhaust plume can be seen to have swept the surface from lower left to upper right. The DPS exhaust nozzle is out of frame to the lower left.

Fig. 2 - The lunar surface directly beneath Apollo 11's descent engine. The spot directly beneath is discolored and the surrounding area shows radial patterns of fluid erosion and signs of sooting. (NASA: AS11-40-5921, 316 KB).

Fig. 2 shows the area directly beneath the engine. In the hi-res version the erosion pattern from the exhaust can be clearly seen. The area directly beneath the nozzle, which would have been subjected to the most heat, is discolored slightly red. This could be a thermal effect, or a chemical effect from the nitrogen tetroxide used as oxidizer.

Note carefully the lines of erosion that spread out in a radial pattern away from the point of impingement.

The conspiracists seem disappointed that a more dramatic result was not produced. Unfortunately this is what we expect to see under the lunar module. The exhaust plume is simply not powerful enough to dig holes in the tightly-packed regolith.