Is my math correct? LM descent engine force at nozzle

[Allan F]

So the throat of the engine was about 11 cm in diameter?

Funny, how details after details keep popping up. Once you get into the workings, layer upon layer of complexity reveals itself. That is also one of the problems the conspiracists struggle with. The inability to grasp that complexity, that somebody actually has worked all these details out.

[ka9q]

The DPS engine diameter is given as 59 inches, and since the nozzle seems to be the widest part I assume that's the diameter of the nozzle exit. That corresponds to an exit area of 1.76 m^2.

The expansion ratio, which always refers to the areas, is given as 47.5:1. (I don't know offhand if that's for the J or pre-J models.) That implies a throat area of 1.76 / 47.5 = .03713 m^2. That in turn corresponds to a throat diameter of 21.74 cm. Your 11 cm would be the radius of the throat, half the diameter.

[Allan F]

Math in the head. Not always the best way.

Seeing into the engine bell on A17's ascent, the 11 cm seemed a little on the narrow side. 22 is much better.

The J-missions 15-16-17, did they have a different/extended nozzle? Maybe to compensate for the extra load of the LRV and consumables?

[ka9q]

The J missions had a longer nozzle on the descent engine to handle the larger load. The longer nozzle increases the expansion ratio and the efficiency of the nozzle in turning heat energy into kinetic energy.

Apollo 15's DPS nozzle was crunched on landing, though this was not unexpected.

[smartcooky]

Yeah, I keep pointing that out. The Saturn V was something like 2/3 LOX by weight at launch. They never seem to have an answer for that. In fact, they never seem to have an answer for a lot of questions that would back them into a corner.

Oh yes they do....flouncey flouncey!!

[Daniel Dravot]

Yeah, I keep pointing that out. The Saturn V was something like 2/3 LOX by weight at launch. They never seem to have an answer for that. In fact, they never seem to have an answer for a lot of questions that would back them into a corner.

When they talk about rockets pushing against the air, it causes me to imagine one with something that looks like a steamboat paddle attached to the back.

Maybe if I have some time, I'll do a photoshop

[Donnie B.]

Yeah, I keep pointing that out. The Saturn V was something like 2/3 LOX by weight at launch. They never seem to have an answer for that. In fact, they never seem to have an answer for a lot of questions that would back them into a corner.

When they talk about rockets pushing against the air, it causes me to imagine one with something that looks like a steamboat paddle attached to the back.

Maybe if I have some time, I'll do a photoshop

I like that idea.  The Robert J. Fulton rocket.

It's absurd on the face of it.  What is that stream of hot gas supposed to push against?  Are the atmospheric nitrogen and oxygen molecules supposed to just hang around and push back?  Or solidify into a sturdy column as the rocket rises?  It makes you wonder if these lackwits have ever heard of something called "wind".  Come to think of it, the only way their concept would work was if the rocket engines were mounted on the ground firing upward, so as to blow the rocket into space...

We have technology that "pushes against the air" -- it's called a propeller.  Can you imagine the size and/or number of propellers required to get a Saturn V off the ground -- without even the benefit of wings? 

[Noldi400]

Math in the head. Not always the best way.

Seeing into the engine bell on A17's ascent, the 11 cm seemed a little on the narrow side. 22 is much better.

The J-missions 15-16-17, did they have a different/extended nozzle? Maybe to compensate for the extra load of the LRV and consumables?

Not having much of a head for physics (biology fits much better), I'm way behind both of you, but NASA has a page that I've found helpful:

http://www.grc.nasa.gov/WWW/k-12/airplane/lrockth.html

It has a link to a nozzle simulator applet where you can change the nozzle parameters and see the effect it has on thrust and other factors.

Speaking of HBs who think a rocket pushes against the air, it seems to me that another common characteristic of a lot of them is a preference for believing what their "common sense" or what they think of as "intellect" tells them rather than doing some real-world research.

The question of "why don't you hear engine sounds on the voice recordings" comes to mind...  they never seem willing to take the simple step of listening to voice tapes of other people in the cockpits of aircraft with noisy engines, they just stick with their preconceptions.  Or the old "blast crater" chestnut - insisting that there should be one despite the fact that Harriers, with a much higher thrust/cm² than the LM, don't make craters when landing on dirt surfaces.

Or - one of my personal favorites - Hunchbacked's insistence that the LM should have landed by coming to a complete stop horizontally, then descending vertically, even though any pilot of a helicopter or other VTOL craft will tell you that (1) if at all possible they want to be moving forward during landing (you can't see what's directly under you)  and (2) holding a hover with no horizontal movement is one of the hardest things they have to do.  Not to mention that hovering with a rocket engine is the most horrendously fuel-wasting thing you can do.

[smartcooky]

Or - one of my personal favorites - Hunchbacked's insistence that the LM should have landed by coming to a complete stop horizontally, then descending vertically, even though any pilot of a helicopter or other VTOL craft will tell you that (1) if at all possible they want to be moving forward during landing (you can't see what's directly under you)  and (2) holding a hover with no horizontal movement is one of the hardest things they have to do.  Not to mention that hovering with a rocket engine is the most horrendously fuel-wasting thing you can do.

I can vouch for that. I have tried doing this in a real helicopter; a Bell 47G (for those who don't know this helicopter, think M.A.S.H.).

Picture this.

You have your left hand on a lever that is rather like a handbrake between the bucket seats of a small car. This is the "collective" lever; lifting it increases the pitch of the main rotor. On this lever is a twist grip like a motorbike throttle. Twisting it to the left increases the rpm of engine.

You have your feet on two pedals on the floor in a similar position to the pedals on your car. They are connected to the tail rotor and change its pitch. pushing on the left pedal pushes the tail to the left (therefore rotating the helicopter clockwise). Vice versa for the right.

Finally, your right hand is on the control stick, Pushing the control stick on any direction tilts the helicopter in that direction.

Now, even in dead-calm conditions, the hovering chopper will drift in a direction, say, left. When you try to correct it by pushing the control column right, if you make the slightest input forward or back when trying to centralise the control column, it will start to drift in that direction too. Also, when the helicopter drifts, it begins to lose altitude, because the down-draft is no longer pushing directly downwards, so you have to lift up the collective lever slightly and increase the throttle a notch, but doing that will cause the helicopter to counter rotate against the torque of the increased main rotor speed, so you have compensate for that with the pedals. As the helicopter comes upright, the increased main rotor pitch and speed causes the helicopter to climb, so you need to drop the collective and rpms a little, which of course, affects the counter rotation. 

Add to that the gyroscopic effects of the main rotor, and you can see that hovering a helicopter is no easy task.

...and this is what will happen if you don't know how all this works...

[Allan F]

How did he get in there without an instructor? No way he was ready to solo.

Edit: My old highschool had work done on it's roof. 4 large aircondition/ventilation units, size of a small van, had to be lifted and placed on the roof. Due to shape of the landscape, no crane was able to reach up and in to put them in place. Maersk Air provided a helicopter (for a price), and the pilots and crew put the gear in place with milimetre precision. I stood on the roof, a couple of dozen meters away, and was quite amazed by the precision they were capable of. I could see into the cockpit, and could clearly see the pilot's minute, barely visible, control inputs on the cyclic.

[smartcooky]

How did he get in there without an instructor? No way he was ready to solo.

Edit: My old highschool had work done on it's roof. 4 large aircondition/ventilation units, size of a small van, had to be lifted and placed on the roof. Due to shape of the landscape, no crane was able to reach up and in to put them in place. Maersk Air provided a helicopter (for a price), and the pilots and crew put the gear in place with milimetre precision. I stood on the roof, a couple of dozen meters away, and was quite amazed by the precision they were capable of. I could see into the cockpit, and could clearly see the pilot's minute, barely visible, control inputs on the cyclic.

Yep, in flying a helicopter, there really is there is no substitute for experience.

[Allan F]

It is all reaction programming. One's reflexes are 3-4 maybe even 10 times faster than deliberate conscious decisions. That's why a secretary can type 600 letters/minute, and others use the biblical approach on the keyboard. That's why those who practise martial arts are able to deflect and throw bigger and stronger opponents, race car drivers can do extreme things with cars.

[Allan F]

Talking about rockets and air, there's something I have been wondering about. That is: Is it possible to improve the efficiency of a rocket engine, firing in atmosphere, by adding a ring around the nozzle? In a way, so there is room between the ring and the nozzle, and the ring extends from somewhere above to somewhere below the nozzle. I have nothing to back this up, but my theory is, that the exhaust will pull ambient air into the direction, increasing the mass flow of the exhaust. Somewhat similiar to the way a propeller on a boat is more efficient, if it is ducted?

[Daniel Dravot]

...and this is what will happen if you don't know how all this works...

Ouch.

So if I am interpreting what I am seeing correctly, the pilot-in-training emerges from the wreckage and stalks off rather sheepishly.

I'm surprised he didn't realise he was in trouble while still skipping across the ground, and cut power - the outcome might not have been so bad in that case.

He did manage to provide some free lift for the wing of that craft in the foreground at the end.

[Allan F]

Once he was off the ground, the smart thing (I suspect) would be to gain altitude, sort himself out, get stable, and then work from there.

It looked like he had his left and right pedal confused. And once the tail rotor touched the ground, all control was lost.