Is my math correct? LM descent engine force at nozzle

[ka9q]

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?

I don't think so, because it wouldn't affect the forces on the inside of the rocket engine where all the thrust is generated. It might have some second-order effects on the pressures around the base of the rocket, but offhand I can't say what they'd be. It might just add drag.

If you used the ring to extend the nozzle, this would increase performance only if the ring were flared to permit additional expansion, and only if the nozzle isn't already overexpanded. A nozzle is never overexpanded in vacuum, but in air it will be overexpanded if the exit pressure is below ambient.

Some engines have nozzle extensions that can be deployed in flight, and they look much like your ring except that it's flared and moves. It's launched with the extension retracted and during flight it is lowered into place with jack screws until it contacts the nozzle and effectively makes it longer.

This would be used on a ground-lit engine to allow it to avoid overexpansion on the ground while increasing the expansion ratio after reaching altitude to reduce underexpansion and improve performance. I don't know how well it works.

[ka9q]

Picture this.

To this non-pilot engineer, this seems like an application crying out for modern control technology. Use an IMU (inertial measurement unit) plus GPS in a closed-loop control system to set the throttle, collective and rotor tilt so as to provide whatever state (orientation, position and velocity) the pilot wants as indicated by pair of joysticks. Or an autopilot.

No doubt the purists would consider this "cheating", but I point out that our favorite helicopter-like flying machine, the Apollo LM, worked in much this way. Only the specific mechanisms for producing the reactive forces on the vehicle were different.

I understand that not all states are immediately accessible, e.g., you cannot start translating from a hover without changing your attitude at least momentarily so the pilot would still have to take this into account.

Don't those small hobby helicopters and other flying machines already have systems like this? If so, why not the big machines too?

[Daniel Dravot]

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 looks like that might be what he tried to do, although obviously without success.

[Daniel Dravot]

To this non-pilot engineer, this seems like an application crying out for modern control technology. Use an IMU (inertial measurement unit) plus GPS in a closed-loop control system to set the throttle, collective and rotor tilt so as to provide whatever state (orientation, position and velocity) the pilot wants as indicated by pair of joysticks. Or an autopilot.

No doubt the purists would consider this "cheating", but I point out that our favorite helicopter-like flying machine, the Apollo LM, worked in much this way. Only the specific mechanisms for producing the reactive forces on the vehicle were different.

I understand that not all states are immediately accessible, e.g., you cannot start translating from a hover without changing your attitude at least momentarily so the pilot would still have to take this into account.

Don't those small hobby helicopters and other flying machines already have systems like this? If so, why not the big machines too?

As a pilot whose air time is better measured in minutes than in hours, and someone who was an engineer a long time ago, I've wondered about this.  Once outside of Sydney, I watched the boat I was on slide more or less sideways to the dock, which seemed impressive to me.  So I wondered, have these control systems become "intelligent," where rather than telling each individual control surface what to do, the captain tells the boat what to do, and the boat figures out how to do it?  Or was it just a highly skilled captain at work?

When Airbus started to produce airplanes which would adjust the control surfaces without asking the pilot's permission first, that seemed to be controversial.  I don't know how it is now.

[smartcooky]

Picture this.

To this non-pilot engineer, this seems like an application crying out for modern control technology. Use an IMU (inertial measurement unit) plus GPS in a closed-loop control system to set the throttle, collective and rotor tilt so as to provide whatever state (orientation, position and velocity) the pilot wants as indicated by pair of joysticks. Or an autopilot.

No doubt the purists would consider this "cheating", but I point out that our favorite helicopter-like flying machine, the Apollo LM, worked in much this way. Only the specific mechanisms for producing the reactive forces on the vehicle were different.

I understand that not all states are immediately accessible, e.g., you cannot start translating from a hover without changing your attitude at least momentarily so the pilot would still have to take this into account.

Other things you have to consider with a helicopter are outside environmental influences. For example, I don't think there is any likelihood of a sudden gust of wind as you are landing your LM in the Fra Mauro highlands.

Also, there is a nasty little phenomenon called "ground resonance". There is no automatic way to resolve this issue, just pilot know-how

Ground resonance will demolish a helicopter very effectively if not dealt with. Here is a short video with an example and explanation.

Don't those small hobby helicopters and other flying machines already have systems like this? If so, why not the big machines too?

Most of those hobbyist RC helicopters get around the main rotor torque and reaction issues in two ways.

1. they dispense with the swashplate assembly that allows main rotor blade pitch angle change and overall rotor tilt control to be combined into a single hub. Instead the main rotor blades are fixed pitch and the rotor only tilts. Lift is controlled using the throttle to vary the main rotor rpm. 

2. The torque problem is overcome using two stacked contra-rotating main rotors.



EDIT: Some helicopter types have something called "force trim", which basically allows the pilot to set a new "zero" position for the cyclic control, so for example, he's lifted off and pushed the cyclic forward and is flying forwards at 100 kias. If he were to let go of the cyclic, it would normally come back to the neutral position, however, if he presses the "force trim" button, a new "zero" is set in whatever position the cyclic is in, and if he takes his hand off, it will remain there.

[Daniel Dravot]

Other things you have to consider with a helicopter are outside environmental influences. For example, I don't think there is any likelihood of a sudden gust of wind as you are landing your LM in the Fra Mauro highlands.

Not outside the craft!

Also, there is a nasty little phenomenon called "ground resonance". There is no automatic way to resolve this issue, just pilot know-how

Ground resonance will demolish a helicopter very effectively if not dealt with. Here is a short video with an example and explanation.

Ooh!  The washing machine will do that too, if it is loaded incorrectly.

As a person with precisely zero rotary-wing experience (and very, very slightly more fixed-wing experience), I am surprised they say the correct action is to get back into the air.  I would have thought that when you're on the ground, the corrective action for most bad things that could happen would be, shut it off, now.  Would that not have solved the problem illustrated in the video?

[Allan F]

I see  it as the landing gear bumping on the ground, causes the rotor blades to flex unevenly, which causes a blade to stall, which causes the landing gear to bump on the ground which causes . . . . .

Either get away from the ground, or push the collective down in order to cancel the lift, so the resonnance effect stops. Am I correct?

[Noldi400]

I had a good friend killed - on TV, no less - when the helicopter he was in crashed while trying to hold a stable hover during a body recovery (they were over a partially demolished water tower; the tail rotor contacted part of the structure and the helo did a 180o pitch-down and hit the ground rotor-first.

Most of my own experience is with military pilots assisting with rescues in rough terrain where the best way to get the patient out is by winch, and they all very much prefer not to try to hold a hover any longer than necessary.  Even without wind, it's just hard to keep everything nulled out and I suspect the same is true in a LM. You remember even Armstrong felt he found himself with some backward and left translational movement just before touchdown and "arrested this backward rate with some possibly spasmodic control motions, but I was unable to stop the left translational rate".  I guess if it was easy, anyone could do it.

[Daniel Dravot]

I had a good friend killed - on TV, no less - when the helicopter he was in crashed while trying to hold a stable hover during a body recovery (they were over a partially demolished water tower; the tail rotor contacted part of the structure and the helo did a 180o pitch-down and hit the ground rotor-first.

Short version.  From the moon, the Earth is generally within 5^o of directly overhead at 0^o lat and long. Apollo 17 landed at 20.2^o North, 30.8^o East, which should put Earth at about 20^o from local vertical.

But, all the references from the flight say Earth was at about 45^o elevation.  What am I missing that would put the Earth that low in the sky?

Assuming I'm doing this right, the landing spot is almost 29 degrees away from 0-0.  But even if the earth's "wobble" was 5 degrees the other way, that still only gets us to about 34 degrees.

Was the 45 degree figure an actual measurement, or just someone eyeballing it?

[ka9q]

Don't forget that the A17 site was ringed by mountains that put the horizon well above the horizontal in some directions. That's why the earth seems to be at such a low elevation angle in some of their photos; you don't have your inner ear telling you which way is down.

[Noldi400]

Short version.  From the moon, the Earth is generally within 5^o of directly overhead at 0^o lat and long. Apollo 17 landed at 20.2^o North, 30.8^o East, which should put Earth at about 20^o from local vertical.

But, all the references from the flight say Earth was at about 45^o elevation.  What am I missing that would put the Earth that low in the sky?

Assuming I'm doing this right, the landing spot is almost 29 degrees away from 0-0.  But even if the earth's "wobble" was 5 degrees the other way, that still only gets us to about 34 degrees.

Was the 45 degree figure an actual measurement, or just someone eyeballing it?

From the AS-17 Surface Operations Manual:



And comments in the ALSJ Transcript, talking about orienting the LM high-gain:

115:21:22 Parker: Okay. And, Challenger, that should be a Pitch of 21 and a Yaw of minus 45.

[Cernan - "The high gain (antenna) had a pitch axis and yaw axis. I don't think it had a roll axis. I don't think they were particularly related to the inertial axes of the spacecraft."]
 [Gene's memory is correct. From the LM, Earth is at an elevation of 45 degrees and at a bearing 30 degrees south of west; therefore, these pitch and yaw numbers are not simply related to the direction of Earth in the local sky.]

Actually, I think I figured it out by using Orbiter, of all things. 20^o N latitude moved Earth's position down about (duh) 20^o at 0^o longitude; then moving to 30^o E longitude moved it down into the right position.

I do wish that my spatial perception skills were better; it seems like I have to draw myself a picture to get the right perception every time.

[Allan F]

On a related note to my original question:

How much did the nozzle of the descent engine heat up during use? I understand the underside and the legs of the descent stage were protected by the heat reflecting kapton and that the radar antenna used to measure distance and closure speed relative to the surface also was protected. The nozzle must have been quite hot. Or did the ablative cooling inside the nozzle prevent the outside of the nozzle to heat up?

I have no idea how to calculate this. The answer would obviously be different relative to the throttle setting and runtime of the engine - and how much of the ablation layer had been burned off. There has to be an equilibrium curve between engine use and temperature, right?

Edit: Also the different parts of the nozzle would have different temperature, as the exhaust cools as it expands.

[ka9q]

Short answer: a lot.

Quite a few in-flight videos of hypergolic engines similar to the LM's show the nozzle glowing orange. The area near the throat glows the brightest, because as you said the exhaust cools as it expands.

A shield was added between the landing radar antenna and descent engine to protect the antenna from the nozzle's thermal radiation. I've also seen it claimed that it was to block a false return from the engine, but I tend to doubt that given how the radars worked - they were both of the CW type, so they already see a lot of their own transmitted signal.

[Allan F]

The LM's descent and ascent engines (and the SPS) were pressure-fed, with supercritical helium supplying the pressure. I understand this system was limited in it's longivity, because heat seeped into the helium tank, raising it's pressure. And when the burst disk blew, the engine was dead.

I Wonder how a similar fueled engine could be kept viable for longer duration missions (Mars). How to put pressure in the tanks without the restraint of keeping helium from heating up and eventually have to be vented. Big plastic bags inside the tanks supplying mechanical pressure? 

Cryo-fuels are fine when you have a big industial complex to supply fresh fuel, but in Space, that's not an option - I think. Can a gas be kept cool enough for a long time? Actively cooled?

[raven]

You might need to resort to pumps. If you are going for active cooling, why not go for a full cryogenic propellent setup? Higher ISP (even higher if you go nuclear) that way, which  might help alleviate the the mass of the pumps. You still have to deal with the higher complexity factor though.