[X] My Assistant

[Phil Stooke]

Luna 16 flew before Apollo 14. Its flight profile would have been understood at once even without CIA help. The Luna sample return capability only entered the ASSB deliberations (at least in the minutes) at the last meeting, 11 February 1972 which was also after Luna 18 and only 2 weeks before Luna 20. The exact area accessible might have depended on details as you suggest, but the general area (S or SW of Crisium) was known well enough.

Phil

[Phil Stooke]

I should have added, there is a map in the last ASSB meeting minutes showing the remote sensing coverage... and it looks like there never was a site ALS-5R until the backup for Apollo 12 was chosen.

Phil

[Guest_BruceMoomaw_*]

"So: how did the Apollo-era planners know what the capabilities of the Luna system were before they flew? The unclassified CIA material which I've seen is actually quite vague in terms of payload weights, upper stages etc for Proton, and just a few percentages out would make a helluva difference in terms of actual capacity to change trajectory out at the Moon."

I believe it had to do mostly with the limited number of Soviet ground stations capable of communicating with the craft -- something which was always an enormous hindrance to the Soviet spce program. In any case, this belief that the Soviets were going to concentrate on the near-Crisium region is explicitly mentioned in the final Apollo 17 science summary report's discussion of the landing site selection process. (I have no inside documents on this subject.)

[Phil Stooke]

The near-Crisium region is defined by orbital dynamics and the chosen flight prifile.

Imagine you lift off from the Moon at slightly over escape velocity. At a certain distance above the Moon you are leaving it with a bit of excess speed. If you are going in the direction of the Moon in its orbit around Earth, your spacecraft is now in Earth orbit, but going faster than the Moon, so it's heading into a higher Earth orbit... obviously not what you want if you're heading home.

But if you leave the Moon with that bit of excess velocity, but going 'backwards' along the Moon's orbit - backwards relative to the Moon - now you are in Earth orbit, but going slower than the Moon. So you 'fall' into a lower orbit. To get home all you have to do is plan the excess velocity so it's enough to bring you back.

This could be made to work two ways... lift off, go into orbit, then when your orbit vector faces the right way burn out of lunar orbit and fall towards Earth. Or, just lift directly off the lunar surface, not needing a second burn. But that only works if you are in the correct location on the Moon. In the longitude 60 East region, going upwards points you in the right direction and all you have to control is the velocity.

This is the simplest way to get home. Landing anywhere else, you have to do the partial orbit plus second burn method to get home. This was clear as soon as the design of the return capsule was known - it had no trajectory correction or second burn capability - well, certainly not second burn.

Phil

[Bob Shaw]

Phil:

There must be more than that to it, as the Luna sample-retrieval capsules had to be targetted to Soviet territory. OK, you could leave the Moon with a trajectory which was Earth-impacting, but to leave with one which would only hit a certain longitude and latitude within the northern hemisphere... ...more complicated. I can think of a certain amount of targetting as a function of time of launch vs speed (the Earth rotates under the orbit), but unless the Luna system's verniers could produce mid-course corrections, how would that allow a return to the USSR itself?

Bob Shaw

[Phil Stooke]

Well, Bob, yes, there's more to it, but that's the basics. But really, all you need to control is the plane of the orbit and the exact timing. And the Soviet Union was quite a big target. I confess I'm not sure if there was absolutely no correction ability, or a little bit, but it wasn't much if any. The point is, details aside, the procedure in question works for a limited area on the moon and that was well known.

Phil

[Bob Shaw]

Well, Bob, yes, there's more to it, but that's the basics.  But really, all you need to control is the plane of the orbit and the exact timing.  And the Soviet Union was quite a big target.  I confess I'm not sure if there was absolutely no correction ability, or a little bit, but it wasn't much if any.  The point is, details aside, the procedure in question works for a limited area on the moon and that was well known.

Phil

Phil:

So it was all done as a single-impulse trajectory, presumably with the ascent stage hard-engineered in such a way as to 'seek' a particular trajectory, wth the actual aiming done by time of launch and (perhaps) a rotation during ascent (as they couldn't be 100% sure of the alignment of the lander)? That fits with the philosophy behind their original satellite launchers, which used rotating bases (Soyuz uses the system to this day) and biased launch vehicles just like the V2 did.

I always wondered why there were only two vernier motors in the illustrations of the ascent stage - if they only had to deal with a strictly limited range of inputs to the trajectory then that's explained!

Really, very clever!

Bob Shaw

[dvandorn]

Yes, Bob, that's exactly how the Soviets did it. They set up landing sites where their probes had to only lift straight up, angle slightly to the right declination, and cut off the motor at a precisely pre-planned time, and if the lift-off time was calculated *precisely*, the sample return capsule would land on Soviet soil. There are only a few places on the Moon that are located in the right spot to do that, and Mare Fecundatitis and the surrounding highland plateau are a couple of the places where that is possible.

The Luna sample returns happened at landings on Mare Fecunditatis and the surrounding highland plateau.

Coincidence? I think not.

-the other Doug

[tedstryk]

"The Luna sample returns happened at landings on Mare Fecunditatis and the surrounding highland plateau."

Also Mare Crisium (Luna 24), which is immediately adjacent.

[Phil Stooke]

Applied Space Resources (RIP) planned a mission called Lunar Retriever, a commercial sample return mission. It would fly a similar flight profile to the Lunas, but with different details, so its landing area could be further west. They were intending to go to Mare Nectaris.

I think they were done in by expense. If I recall, the mission would cost about $160 million and they couldn't raise it. I expect a microsat approach today might bring the cost down a lot below that. though I know nothing about such matters. Come on Jay! You can do it!

Phil

[Phil Stooke]

Since we have been talking about the Luna sample return flight profile and landing sites... (even if it is off topic)

There is another side to this: for operational reasons it was often desirable to descend to the Moon's surface vertically rather than obliquely. For instance, Rangers 3,4,5 needed to take nested images (with small slow cameras, not the Ranger 7 type cameras) to locate the landing site. Any horizontal movement would stop the nesting and the process would not work. Ditto Surveyor 1 with its descent imager.

The easiest way to descend vertically is to get into an elliptical orbit with its high point near the Moon, at just the time the Moon is nearby, and then when the orbit vector is parallel to the lunar surface, burn a retrorocket to kill forward velocity along the orbit. The s/c then just falls down a radius to the surface. A gross oversimplification, but that's the basic idea. This was the procedure planned for the early Rangers, for Surveyor 1, for the early Luna landers. And it works best in a limited area as well, near the equator on the western half of the earthside, a mirror-image of the sample return situation. Differences in detail of the trajectories made the Lunas go to a different place than the Rangers or Surveyor 1, but all in the basic area. Early (very early) Apollo planning was briefly directed at this area too.

Phil Stooke

[Guest_BruceMoomaw_*]

Huh? The retrorocket on the Block 2 Rangers would only have slowed the lander capsule -- not the main craft, which carried the TV camera. A near-vertical approach trajectory was desired -- not only for the reason you state (which makes sense, though I've never heard it before), but also to minimize the total velocity that the capsule retrorocket had to null out.

Similarly, Surveyor 1 was coming down at an angle of only 6 degrees from the vertical BEFORE its retrorockets ignited. I believe the main reason for given for that -- at least on the first mission -- was to maximize the ease with which the multi-beam RADVS radar could lock onto the lunar surface, since that was supposed to lock on before the main solid retromotor had finished firing and so the craft was still coming down somewhat off vertical when the RADVS turned on. As early as the unsuccessful Surveyor 2, however, they were ready to increase the off-vertical pre-retrofire approach angle to 23 degrees.

[Phil Stooke]

Bruce is quite right and what I said was unintentionally quite misleading. I initially described a hypothetical, generic method for getting to the Moon, and I would have been better off leaving it out. If you plan that initial very elliptical path out from Earth correctly, as the s/c arrives in the Moon's gravitational influence it is pulled in on a near-vertical descent path IN THAT SPECIFIC AREA of the Moon. That's the easiest way to get there - the trajectory brings you in just as you want to come in, vertically.

So we'll forget what I said about braking at a distance, that was not done on those missions. It would have been on some versions of Apollo flights without lunar orbit rendezvous. The point is, though, there is a specific vertical descent area just as there is a simple 'return to Earth' area.

The vertical descent was needed by early Ranger to enable image nesting. It was also needed by the first Surveyor for that reason, though in the end they didn't use the descent imager, and it was dropped from flights after Surveyor 2. As Bruce said, on that first test flight the vertical descent also made it easier to get the radar to work, but that was not essential for its operation and would not apply later.

Surveyor 1's landing site was actually chosen to meet three criteria: it had to be in the (near-) vertical descent region, in the Apollo zone, and at the smoothest place they could find in the intersection of those two areas. USGS and JPL had drawn up lists of suitable sites in 1965 and 1966, and that particular site at Flamsteed was pretty much the only one that worked for it. After that they allowed the 'unbraked impact angle' (angle between incoming trajectory and surface if no braking is done) to increase to 25 degrees for Surveyor 2, and to the outer limit of 45 degrees for Surveyor 5.

Who says the Internet has no quality control? Thanks for picking up my gaffe, Bruce.

Phil

[Guest_BruceMoomaw_*]

Actually, I'm relieved that I was correct -- the only one of my statements I was absolutely certain about was the lack of a retrorocket on the Ranger Block 2 main spacecraft. In quoting the approach angles for Surveyors 1 and 2, I was working off a 39-year-old memory. (I knew that Surveyor 5 had the most extreme approach angle of all, but couldn't remember what it was.)

Given my ignorance of fundamental orbital mechanics, was I correct in saying that a near-vertical approach also minimized the total approach velocity for the Block 2 Rangers and thus allowed lightening of the retrorocket? (By the way, that retrorocket was one of the few things salvaged from the Block 2 fiasco -- it worked very well as the high-powered kick motor to put the early Vela nuclear-test-monitor satellites into their circular orbits 60,000 miles from Earth starting in late 1963.)

[dvandorn]

I have the (in)famous Ranger issue of National Geographic, but it's somewhere in storage right now and I don't have it at hand.

What I don't recall is how the hard-lander "ball" was to be controlled in attitude during the retro-rocket firing. Was it spin stabilized? I sure don't remember it having anything like a sophisticated RCS. Perhaps a control-moment gyro system? A CMG wouldn't have to be all that heavy, since the ball was fairly light.

But I do recall that the lander only really had the one-axis seismometer, a transmitter and a battery. No camera, no other particles and fields sensors that I recall off the top of my head.

-the other Doug