GoSlash27

Ah, but Kerbin's gravity will take over regardless. Does it take over in such a way as to return you to the mun? Actually, nevermind. LD has already confirmed that my method was in line with his model. Best, -Slashy

I read all of this as "wharrgarbl you must be lying". Of course, anybody can reproduce my results. The vehicle has exactly 8 parts. 1 radioisotope generator 1 command pod (full RCS propellant) 1 RCS tank loaded to 70% for ballast 1 FL-T100 tank (full) 1 48-7S engine set to 16.5% thrust limit 3 cubic octagonal struts to serve as legs. Any impartial bystander can see that this craft lacks the means to dispose of or add propellant, as all the parts are tallied in the map view and visible in the craft view. The monopropellant is merely to add mass to total 2 tonnes. Sorry dude, but I've got better things to do than mislead you. Your model is inaccurate. I invite anyone who's interested to try it for themselves. -Slashy

Numerobis, Thanks for the explanation, but now I'm afraid I'm at a loss. If 773 gets you to the edge of SOI with zero velocity and 807 causes an escape, what happens in between? Say, at 790 m/sec? If you leave the SOI with 17 m/sec velocity, why would you not continue to drift away since there's no gravity to bring you back? Scratchin' mah head -Slashy

(emphasis mine) Valid point. It could be KSP that's off. But since the entire point of this is to provide a useful lower bound for KSP, it's clearly the model at fault. I've tested Arkie's model everywhere from Gilly to Tylo at Isps ranging from 290 to 4,200 and haven't recorded any errors in it beyond the margin of error. If it's wrong, it's wrong in a way that accurately predicts the results in KSP. Best, -Slashy

Arma, If the model is correct, I should not be able to "fall short" of it under any circumstances. It should always take me more DV than the model predicts. never less. Especially with terrain in the way. As for the difference between escape velocity and velocity sufficient to escape SOI, I'd need a clarification on the difference. I was always under the impression that they were the same thing. Best, -Slashy

No, Sir (and merry Christmas!) The discrepancy between the two models isn't at an altitude where the reduction in gravity is a significant factor. It's in the early phase of the launch where low t/w penalizes the most. Best, -Slashy

LD's prediction 893m/sec <-- "lower bound" Total DV to escape 871m/sec <-- *actual result* under less than ideal circumstances That's a worthwhile resource...

Arkie, Truth through empirical testing. I built a ship as close to spec as I could make it and had a go at 1.5:1 t/w. Any test involving a "sea level" launch on the mun necessitates terrain avoidance, so my results were far from ideal. Anywho... Initial mass = 2.01t initial t/w =1.51 launch altitude 822m elapsed time 5:40 Total DV to escape 871m/sec LD's prediction 893m/sec Arkie's prediction 856 m/sec LD's model doesn't predict that I can achieve 871m/sec to escape at all, let alone with terrain in the way. Arkie's does, so that oughtta settle that. Screen caps Merry Christmas, -Slashy

Greep, This is a whole 'nother subject, but actually... no. You don't need a high t/w ratio to land, and a suicide burn is actually the least efficient way to do it. Scope out my tutorial here for a technique that's more efficient, easier, more precise and safer than the suicide burn and only requires 1.2:1 t/w ratio. Best, -Slashy

arkie, the series of Mun tests were conducted using the same vehicle in the same location. It was an RCS tank with selectable banks of O-10 engines. I landed it, created a save point, and then repeatedly launched it into orbit at various t/w ratios. There was no change in the initial state that would account for the discrepancy. Best, -Slashy

Arkie, The contour plot you posted earlier agrees with with my findings. If I'm reading it right, it predicts a high 50% efficiency for t/w=1 and TVR of 5.2. That's in line with my result. *edit* And also the predicted efficiency for Mun at 1.0 TWR with 290s Isp vs. Mun at 1.0 TWR with 4,200s Isp shouldn't be anywhere near each other, yet your table upstream predicts about 48% efficiency for both cases. The way I read your contour plot, 290s Isp should get me high 50% efficiency, while 4200s Isp should get me high 30% efficiency. I'm thinkin' there was a transcription error somewhere. Best, -Slashy

K^2, All due respect, but we're not doing this for the entertainment value. We're doing this to answer questions. Best, -Slashy

My low TWR result at 290s Isp launch out-performing the model troubles me. Just to verify, it was to an orbital velocity of 547m/sec. I wonder what's behind this... *scratches head*

For Mun tests, I can't go perfectly horizontal for climbout since there's terrain. That would explain the inefficiency there. And also, higher t/w ratios become increasingly difficult to control precisely in the early part of the launch. I hadn't thought about it before, but that sets a practical upper bound on increased efficiency from higher t/w as well. Not any way that I know of to integrate that into a formula. The Mun test with ions really needs to be taken with a grain of salt. Total fuel consumed in that test was only 2.5% of total vehicle mass, and with the margin of error for final mass, it could've been anywhere between 23% and 80% efficiency. The result at Isp=290s is a legit result and should be accounted for. Incidentally, LD is trying to derive a solution for the same problem. You may want to check in with him. Best, -Slashy

It is as The_Rocketeer sez. You have to select the first part so that it knows which part you want to be the new root. You have to select the second part because the first part may have many parts attached to it and it needs to know which piece is the trunk. Best, -Slashy

As an engineer, I don't have a problem with that. I can reason that it's optimal and don't require proof. I can understand if you do as a mathematician, but he didn't set out to prove that the strategy is correct, he set out to show the effects of t/w on the strategy's efficiency. Merry Christmas, -Slashy

K^2, I thought *you* were doing that? Best, -Slashy

arkie, I have conducted tests in the following situations: Location/ Isp/ TWR / efficiency Mun/ 290/0.91/ .526 Mun/290/1.84/ .883 Mun/290/3.68/ .921 Mun/290/7.37/ .928 Mun/4200/1.01/ .565* Tylo/290/0.95/ .738 Tylo/290/3.78/ .970 *test results have an unacceptably high margin of error. If you'd like me to test anything, just let me know. Best, -Slashy

Step 4: Final approach This is pretty much the same as all visual approaches. You just fly to correct your drift while aiming for your desired touchdown spot, and gradually reduce your sink rate as you approach the surface. This particular approach placed me within 1.5 Km of my target point. I didn't overshoot or undershoot, but ended up right. Normally I can parallel- park this approach precisely on the spot, but snapping pics like a tourist kinda ruined my finesse. Still... a klik- and- a- half ain't nuthin' to sneeze at. That's all there is to it. Time to go and science! There's really nothing demanding about this technique, which is why I use it. I don't have to practice it or figure out any timing. So long as the ship is properly built for it, I don't have to worry about missing my target or hitting terrain. This technique works the same way on all airless bodies. Best, -Slashy

Step 3: Powered descent We are mid-way between apoapsis and touchdown, and it's time to begin our powered descent. We will keep the nose a little above the retrograde marker and increase throttle so that we remain halfway between apoapsis and touchdown. If we are low (closer to the surface than apoapsis), we increase throttle. If we are high (closer to apoapsis than the surface), we decrease throttle. If we are long (overshooting the landing site), we pitch toward the retrograde marker. If we are short (falling short of the landing site), we pitch more vertical. Once we can't clearly see where we're at, it's time to transition to the final visual approach.

Step 2: Deorbit burn We are at the leading edge of the maneuver node, so it's time to begin. Go to full throttle retrograde and burn until your flight path intersects the landing area, then throttle down to cutoff. If you have a reference on the surface, you no longer need the maneuver node. It's okay to get rid of it at this point. You can keep the maneuver node up if you want it to provide you a target for landing. We will free- fall until we are halfway between our apoapsis and the surface. At this point we will begin our powered descent.

Step 1: Setting up So here we are in orbit around Tylo. First order of business is to make sure that we're ready to shoot an approach. Engines should be armed and throttled down. Nose is pointed retrograde and we are rolled upside- down according to the nav ball. Next, we identify our landing site. Now we create a maneuver node. It will be set up for a retroburn of roughly 1/2 our orbital speed. Then you drag it into position so that it drops into the landing area. This maneuver node is your guide for when to begin your approach. Once you are over the front intersection with the surface, it is time to begin your retro burn. But before we do that... We want to make sure our view is where we want it (looking forward at the top of the ship with the surface on the left) and look everything over one last time. When we are over the leading edge of our maneuver node curve, it's time to begin our approach.

I've been doing a lot of testing with landers this past week, and would like to share a landing technique that I've figured out in the process. Not sure if it's already commonly used or has a different name. If so, please let me know. The two most common landing techniques are the "suicide burn" and "zero descent rate" approaches. The suicide burn is a reverse vertical ascent-then-circularization burn. On the plus side, it can drop you in the neighborhood of where you want to be, but relies on perfect timing, high thrust to weight, and is hugely wasteful of fuel. The zero descent rate approach is the most efficient approach and doesn't require a high thrust to weight. It's the reverse of an immediate prograde burn to orbit. But on the downside, it's highly inaccurate AFA final touchdown point, and may smack you into terrain if you're not careful. I call this the "reverse gravity turn approach" and it has advantages over both of these techniques. It has medium efficiency, works with low t/w ratios, is very easy to execute, and will drop you right where you want to be with no fuss. I will demonstrate what you need to use this technique, starting with the vehicle: We are going for Tylo in this example, and this is our lander. DV is twice the "perfect" DV for Tylo according to WAC's DV chart In the case of Tylo, it says 2270 m/sec, so we'll double that to 4,540. T/w should be at least 1.2 at the body you wish to land on. No harm in going a little higher to start with, but keeping a low t/w will make your touchdown less twitchy because you have finer throttle control.

Actually, it's a hybrid between the two techniques. The zero- vertical velocity technique is the reverse of the horizontal departure. Very good for efficiency, but bad for terrain avoidance and landing accuracy. The "suicide burn" is the reverse of vertical ascent with circularization burn. It'll put you into your desired ballpark, but is perilously dependent on precise timing, hogs fuel, and requires a very high t/w ratio. What I'm doing is sort of a reverse gravity turn technique. medium efficiency, doesn't rely on high t/w, easy to execute, and will drop you precisely where you want to be every time. I'll be writing up a tutorial shortly. *edit* tutorial is up here. After I'm done with that, what situations would you like me to test out? Best, -Slashy

As an aside, I've been using the experience I've gained over the past week to develop a new landing technique that's not only more efficient than suicide burning, but also much safer and more accurate. As a side- bonus, it doesn't require a crazy t/w ratio to work. This lander has had good results at just 1.2:1 initial t/w. As you can see here, it works pretty good! *edit* New variant is confirmed. http://i52.photobucket.com/albums/g13/GoSlash27/TyloLander1_zpse6f985f2.jpg Mass for the entire assembly is 13.85 tonnes and price is 15,791. Pretty respectable for Tylo. I'll pass on my test results from the ascent portion. Best, -Slashy