So, who wants to win 1 million Euro?

[ka9q]

Bob, this makes me wonder. Can you make a thrust-free (or nearly thrust-free) "cycler" orbit that would continually fly around the moon, then fly close to earth (without re-entering) and then back to the moon?

Buzz Aldrin discovered the first of a class of orbits that do this with Mars and Earth, so it seems like it should be possible with the moon.

[Bob B.]

Bob, this makes me wonder. Can you make a thrust-free (or nearly thrust-free) "cycler" orbit that would continually fly around the moon, then fly close to earth (without re-entering) and then back to the moon?

I don't know; I've never considered the problem before.  If the moon's orbit were circular, I'd say it might be possible.  However, considering the moon's ellipticity, I believe that mid-course corrections will probably be required each trip to altered the trajectory for the next cycle.  Not only is there the ellipticity problem, but the moon's orbit also has quite a number of cyclical variations and perturbations.  It seems like it should be possible, though.  If each cycle were roughly six days in duration, we'd make five trips each lunar orbit.  Every pass around Earth would advance the next outbound leg 72 degrees on average.  I might be able to derive a hypothetical solution using a simplified circular and coplanar model, but I certainly don't have the time or inclination to work out a real world solution (if I'm even capable).

[Peter B]

... I therefore suspect that Heiwa believes the spacecraft had to gradually change attitude during the burn to keep the engine pointed in the direction of travel.

Well, it didn't. The spacecraft was aimed in a particular direction for the burn, and remained pointed in that direction for the duration of the burn.

In post #786 the question was about how much fuel was used (to produce a force) in order to just slow down a space ship to get into Moon orbit or to the speed up the space ship to get out of Moon orbit. Nobody seems to know the answer as no replies have turned up.

Saying that over and over doesn't make it true.

Anyway, Noldi400 at post #819 has the definitive answer, using your own data. Do you have any comments?

Another question is evidently in what direction this force is applied during the speed change maneuver. And for how long.

To get into Moon orbit you must, apart from slowing down, change course from a straight one into an elliptical/circular one around the Moon and Moon gravity will assist. Peter B suggests that during a 6 (or eight) minutes burn (30 kg/s fuel burnt) applying 97 400 N force in one particular direction suffices.

It is a complicated maneuver in 3-D. The inertia force of the space ship is applied in one direction, Moon gravity pulls in another direction (I assume the trajectorey is already curved due to Moon gravity) and then you apply a third force - the brake force - in a third, particular direction while losing 10 800 kg mass in the SM ... that you keep steady all the time? If the brake force is not in the direction of travel, it will evidently also change the direction.

That's true.

There is then quite a number of factors to keep an eye on. Just how to keep the space ship stable in one - the right - direction during 6 minutes braking/turning, while you loose 10 800 kg mass (30 kg/s) is complicated. Imagine if the space ship tips over and goes away in the wrong direction ... and you have no fuel left or means to correct it.

The business of keeping a rocket stable had been worked out by the Germans with their V-2 rockets back during World War Two. It was hardly a mystery to either the Americans or the Soviets. I can think of at least three methods which can be used to keep a spacecraft pointed in a desired direction while its rocket engine is firing. Two of the three can also be used when its rocket engine isn't firing.

1. Redirect the flow of the rocket engine exhaust. On the V-2 this was done by placing directable vanes in the rocket exhaust. On the Saturn V rocket, and the CSM engine and the LM Descent Engine, it was done by swivelling the engine bell. It was also done with the Space Shuttle: watch some footage of the Space Shuttle Main Engines before and during ignition; watch them swivel.

2. Retro-rockets. On the Apollo Command and Service Module, those cross-shaped rocket engines on the Service Module (which others have already pointed out to you) could be used to change the direction the spacecraft was pointing.

3. Gyroscopes. These are used on the Hubble Space Telescope so it can change the direction it faces without needing to use rocket engines.

Using methods 1 or 2 to keep the spacecraft pointed in the right direction seem straightforward to me: accelerometers at 90 degrees to each other measure movement on three axes; if an accelerometer measures anomalous movement then a combination of retro-rockets fires to counteract the movement.

And anyway, why should the spacecraft suddenly topple over? The fuel being burned in the SPS engine was stored in tanks which were symmetrically located in the Service Module. So as the two tanks drained, the spacecraft's mass remained evenly distributed. It's not rocket science - if you were wearing a beer hat what do you think would happen if you drank all the beer from one can first?

So I am happy, Peter B, that you have pointed out that the brake force cannot be applied in the direction of travel but in another particular direction where it is held steady when the space ship changes direction and looses mass and slows down. You are a genious.

Ouch, the sarcasm.

Now tell me how much fuel you need to slow down!

I refer you again to Noldi400's post #819.

[ka9q]

I don't know; I've never considered the problem before.  If the moon's orbit were circular, I'd say it might be possible.

Mars' orbit is also highly elliptical. It's also somewhat out of the ecliptic.

The Aldrin cyclers have periods equal to integer multiples of the synodic period of Earth and Mars (about 2 years). He proposes two separate cyclers, one with a quick Earth-Mars transfer and another with a quick Mars-Earth transfer.

[ka9q]

2. Retro-rockets. On the Apollo Command and Service Module, those cross-shaped rocket engines on the Service Module (which others have already pointed out to you) could be used to change the direction the spacecraft was pointing.

You mean reaction control systems (RCS). They could be used as retrorockets if the required delta-V was small enough, though when the CSM came back from earth orbit the retro burn was usually performed with the SPS.

3. Gyroscopes. These are used on the Hubble Space Telescope so it can change the direction it faces without needing to use rocket engines.

There are two kinds: momentum wheels and control moment gyros. The first vary their speed to change the stored angular momentum, and the second operate at constant speed but are turned around by other motors to redirect that angular momentum vector relative to the spacecraft.

[Peter B]

2. Retro-rockets. On the Apollo Command and Service Module, those cross-shaped rocket engines on the Service Module (which others have already pointed out to you) could be used to change the direction the spacecraft was pointing.

You mean reaction control systems (RCS). They could be used as retrorockets if the required delta-V was small enough, though when the CSM came back from earth orbit the retro burn was usually performed with the SPS.

Sorry, yes, that's what I meant.

3. Gyroscopes. These are used on the Hubble Space Telescope so it can change the direction it faces without needing to use rocket engines.

There are two kinds: momentum wheels and control moment gyros. The first vary their speed to change the stored angular momentum, and the second operate at constant speed but are turned around by other motors to redirect that angular momentum vector relative to the spacecraft.

Oops, okay, I didn't know about the CMGs, only the momentum wheels, which is what I understand Hubble has.

[ka9q]

Oops, okay, I didn't know about the CMGs, only the momentum wheels, which is what I understand Hubble has.

Control moment gyros are generally used on large spacecraft because they're more energy-efficient than momentum wheels, but (and I'm not sure) Hubble uses momentum wheels because they provide finer pointing control at the expense of speed and energy efficiency.

Control moment gyros are even used on some ships for roll stabilization. They're set up to produce an angular momentum vector of variable magnitude (including negative) along the ship's roll axis.

[Daggerstab]

Björkman has changed his page again. And again, only the most significant changes follow:

By the skillful insertion of a single word, the Apollo 11 mission report is now "approved by" Low, instead of only "by" him.

In several places, he has tried to provide citations more specific than "[1]", which include something I presume are section numbers. There are also a few quoted paragraphs from the report.

Apollo 11 with three asstronuts aboard launched from Cape Kennedy on July 16, 13.32.00 G.m.t, 1969 fitted on top of a hugh, 100 + meter tall three stages rocket or fire works looking like something right. Three minutes later the launch escape three motors system on top the CM was jettisoned ... one way or another. Why it was  fitted in the first place is not clear.

*jaw drops* Why am I still surprised, after the RCS debacle... Heiwa, was it that hard to look up "launch abort system" on Wikipedia?

An entirely new paragraph:

Just prior to powered descent the LM crew managed the following important manual check on intertial platform drift at 1 500 m/s speed:

"Just prior to powered descent, the angle between the line of sight to the sun and a selected axis of the inertial platform was compared with the onboard computer prediction of that angle and this provided a check on inertial platform drift."[1-4.10.2]

Imagine that - manually checking the computer calculations!

They checked the result of the computer's calculations against reality. They didn't have to re-do the calculations. Also, I'm curious why Heiwa thinks the "speed" of the craft makes any difference.

The section on lunar samples got this long insert:

"Collecting the bulk sample required more time than anticipated because the modular equipment stowage assembly table was in deep shadow, and collecting samples in that ares was far less desirable than taking those in the sunlight. It was also desirable to take samples as far from the exhaust plume and propellant contamination as possible."[1-4.12.4]

or ... another version:

"Approximately 20 selected, but unphotographed, grab samples (about 6 kilograms ) were collected in the final minutes of the extravehicular activity. These specimens were collected out to a distance of 0 to 15 meters in the area south of the lunar module and near the east rim of the large double crater. ... During bulk sampling, rock fragments were collected primarily on the northeast rim of the large double crater southwest of the lunar module".[1-11.1.5]</p>

Strangely enough the asstronuts didn't measure the temperature of the samples. Maybe it was too hot?

No gravity experiments were carried out, e.g. to drop a piece of rock from the LM platform down to ground, distance 3.61 meters, and film it. The drop would take exactly 2 seconds (compared with 0.86 seconds on Earth). But why drop it? Throw it upwards instead. It will really go far!

My jaw is already starting to hurt... Someone really didn't do their homework on Apollo...

There's also a paragraph on "cosmic particles" citing the report.

Some of the numbers in the "calculations" section have been changed, with cites to specific sections of the A11 report. Someone is learning. Very, very slowly...

[cjameshuff]

Control moment gyros are generally used on large spacecraft because they're more energy-efficient than momentum wheels, but (and I'm not sure) Hubble uses momentum wheels because they provide finer pointing control at the expense of speed and energy efficiency.

I'm not sure there's a big difference in energy efficiency, but CMGs are more complex. You need gyroscopes on motor-driven mounts, while momentum wheels can just be anchored to a fixed frame. The precision argument is probably correct...CMGs require fine control of absolute angular position, which means high precision rotary encoders, stepper motors with lots of gear reduction, etc. Momentum wheels only require fine control of rotation rate, which can be done simply by using synchronous motors controlled from a stable clock.

[Zakalwe]

I wonder will he (Heiwa) come back on here after he has had his a*se resoundably handed to him in quite such a manner.

[ka9q]

I'm not sure there's a big difference in energy efficiency, but CMGs are more complex.

No question, CMGs are more complex but they're also much more energy efficient. Creating torque with a momentum wheel requires applying (or taking) power to/from the wheel, while creating it with a CMG requires only forcing a constant-speed wheel to precess. That takes little power. The difference can be several orders of magnitude.

[ka9q]

They checked the result of the computer's calculations against reality. They didn't have to re-do the calculations.

The purpose wasn't to check the computer or its calculations. As it says, the purpose was to check for drift of the inertial reference platform. This was practically an obsession during every Apollo mission, particularly before major burns as platform drift would cause the computer to point the burn in an equally wrong direction.

[cjameshuff]

I wonder will he (Heiwa) come back on here after he has had his a*se resoundably handed to him in quite such a manner.

I'm betting he will. He doesn't even recognize that he's so thoroughly lost.

I'm reminded of Ronald Satz, a recent crank who showed up in BAUT/Cosmoquest and Rationalwiki who was able to selectively fail to comprehend simple high-school algebra. At one point, after I posted a voltage plot demonstrating clearly with no room for doubt that he was wrong, he accused me of refusing to take the measurements that voltage plot represented. (He himself never gave similar measurements supporting his own theory, despite repeated prompting and claims to have made such measurements.)

[cjameshuff]

No question, CMGs are more complex but they're also much more energy efficient. Creating torque with a momentum wheel requires applying (or taking) power to/from the wheel, while creating it with a CMG requires only forcing a constant-speed wheel to precess. That takes little power. The difference can be several orders of magnitude.

Peak power requirements might be higher for the momentum wheel while increasing the rotation rate when already spinning at high speed (similar to rocket engines, stored energy is proportional to the square of rotation rate while angular momentum is directly proportional), but you'll sometimes be decreasing the rotation rate, which takes no power. And when you're holding orientation, odds are you're maintaining a lower rotation rate for the momentum wheels than you would for CMGs, which need to be running at high speed constantly...higher speeds, higher losses, higher power requirements.

[Glom]

He doesn't know what the LES is?

We're beyond Dunning-Kruger at this point.