something strange about the apollo program...

[JtPB]

The bottomline is that it's all a matter of thrust-to-weight ratio: the higher it is, the closer you can get to the optimal landing trajectory (in terms of delta-v). While it's possible to land with a TWR less than one, it becomes rather expensive.

You're in luck: some time ago Kosmo-not posted a very nice video showing how to land using a "constant-altitude trajectory":

As you can see, it's not a vertical suicide burn. It's rather much like a "horizontal takeoff" in reverse. The fundamental idea is to control your vertical speed by pitching your spacecraft. At the start you'll need very little pitch to keep your descent speed low and thus will be thrusting almost horizontally, which means your thrust is used almost entirely to decelerate instead of fighting gravity.

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so i saw there that rule of thumb at non-atmospheric landing is to keep my movement direction as parallel to the horizon as possible. and TWR of above 1.2 is enough.

the only problem now is how do i land exactly at the certain area that i chose...

This is only (approximately) true for very small changes in altitude. Otherwise, the acceleration due to gravity is not constant. (g® = GM/R^2.) I'm guessing you probably know this but just misspoke at the time. Either way, calculating impact velocity based on change in gravitational potential energy, which is what I think you did, is correct.

yep, that what i did...

[Meithan]

the only problem now is how do i land exactly at the certain area that i chose....

Yep, that is usually a problem, and the practical solution is to spend a bit more delta-v in order to have better control of your landing site. The more vertical your trajectory, the easier it is to pinpoint the landing spot. So try a compromise between the two methods (efficient "horizontal" landing vs. precise "vertical" landing). Streetwind's and Wanderfound's suggestions are good.

I've done precision landings a few times (landing about 100 m from my target on the Mun) and trust me, they were nowhere near optimal in terms of spent delta-v. So my main suggestion would be: make sure your craft has a lot of delta-v to spare.

Edited November 5, 2014 by Meithan

[Bill Phil]

The LM was extremely light, especially compared to the CSM.

Designing the mission to be Dv cheap generally means that you want the lowest altitude when you're coming from outside the system. This way the Oberth effect helps you out, and you have to slow down a lot less. So, it's not really strange, it's just physics.

[tg626]

AFAIK, the SM/LM lowered it's PE to somthing very low before the LM was "cut loose" to begin it's descent. Not sure if the CM then raised it's PE to wait for the LM's return or not...

[rdfox]

OK, for the record, the standard Apollo lunar parking orbit for the CSM was 60 nautical miles, circular. (Don't blame me for the units, they're what NASA used. That said, if you just use them as kilometer values, you end up with a fairly decent Munar situation!) The initial Lunar Orbit Insertion burn (LOI1) was done to a nominal orbit of Ap 160nmi, Pe 60nmi. (By way of reference, for Apollo 11, the post-LOI1 orbit was 160.9 by 60.9, referred to as a "damned accurate automatic burn" by the crew.) They then circularized the next time around.

On the earlier missions, undocking would come in this 60nmi circular orbit, and, after a separation maneuver, the LM descent engine would be used for a Descent Orbit Insertion burn that would lower Pe to about 8 nmi (50,000 feet), some distance before the planned landing site. On later missions, the CSM would use its big engine (which had far, far more delta-V than was necessary for the mission) to enter this orbit before undocking from the LM, then would recircularize into parking orbit. (Returning to the standard parking orbit would eliminate the need to rewrite the ascent guidance software for both of the LM's computers--which was particularly problematic for the minimalist Abort Guidance Computer, which had minimal memory.)

Either way, the LM would make an orbit or two before, shortly before Pe., using the descent engine for Powered Descent Initiation, the big braking burn that would gradually lower the Pe to zero--at which the computer would go to the next program, which automatically started to transition from orbital descent to vertical landing. If memory serves, this transition occurred at only about 3000 feet above the surface, and landing was vertical. From the start of PDI until the six-foot-long foot probes touched the surface, triggering the cockpit's contact light that told the crew to shut off the descent engine, the descent engine burned continuously, though the throttle varied throughout the burn, depending on what the requirements were. NASA decided against a "suicide burn" type landing (of the sort MechJeb does so well) partly out of conservatism, and partly to allow for situations like Apollo 11, where Armstrong saw that the automatic guidance was going to land them in a boulder field and had to take over manual control to fly past the boulders to a safe landing site.

[Meithan]

To add to rdfox's informative answer about the Apollo deorbit & landing profile, you can directly browse a version of the original Apollo 11 Mission Report. On page 30 (of the document, page 42 of the PDF) you can see a diagram of the first type of LM separation and deorbit program rdfox was talking about. The next page has nice diagrams of LM attitude vs. distance to landing site. On page 39 (51 of the PDF) there's a more detailed graph of pitch attitude vs. time. That report contains a wealth of information about all aspects of the mission, by the way. I'm very glad NASA has made it publicly available.

As you can see their landing profile was quite conservative, probably far from optimal for safety reasons. The Apollo descent engine had an available delta-v budget of 2500 m/s, and the mission plan was to use 2063 m/s during landing (so they had 21% more than needed). However, Apollo 11 used more fuel since Armstrong had to fly longer to avoid rough terrain, but I couldn't find exactly how much extra delta-v they used. It was pretty close, though. For reference, the ascent back to orbit required about 1850 m/s of delta-v, and the ascent stage had something like 2200 m/s total.

[rdfox]

Very belatedly (life has been kicking my butt recently!), but the time-remaining calls heard from CAPCOM during the landing phase on Apollo 11 referred to the length of time remaining in powered flight before the crew would have to either land or abort immediately, as the abort plan at that point used the descent engine to arrest the LM's downward velocity before the ascent engine fired, which means that they couldn't run the descent engine completely out of fuel before aborting. IIRC, post-mission analysis indicated that they had 20 seconds of fuel remaining before they would have hit the "must commit now" point when they landed.