The lunar module's
descent engine should have dug a huge crater in the lunar
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
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
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
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.
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
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.
Thrust of Common Engines
(one Pratt & Whitney JT9D-7R4G2)
F-16N jet fighter
(one GE CFM56-7B)
|Apollo LM DPS
|Marquardt steering jet
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
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
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.