GoSlash27

I'm thinking the same. Of course...Tylo has a sidereal rotation of 17.8 m/sec and I wasn't exceeding the prediction there at extremely low t/w. Then again, that would be a much smaller fraction of my orbital velocity as well... Best, -Slashy

arkie, yeah, it is... but that's not the point. The point is I'm outperforming the model at very low t/w and I shouldn't be. Especially not with terrain in the way (which is very tough to dodge at low t/w) Gotta be something the model's not accounting for. Maybe initial launch altitude above "sea" level, or sidereal rotation? What do you think? Best, -Slashy

arkie, Oh, wait... I think I screwed that up. I shouldn't be using Vo, huh? *facepalm* You never posted the minimum DV for your final orbit, but it's got to be more than your final Vo, right? Confusin' myself again, -Slashy

Yeah, it's pretty hilarious to watch (and difficult to control). At first it just sits there. Then it slowly picks up every leg but one, and drags/bumps around the surface trying to tip over. Eventually it rises barely off the surface and I can *very gingerly* begin to slowly accelerate east while maintaining altitude. The 72% would be the LFO numbers. v_0/deltaV= 571.8/797 = 71.7% efficiency. Referring to your contour plot, TVR=4.98 and t/w=1.1 should only yield about 68% efficiency. Please run the numbers and confirm this if you don't mind. Maybe I'm reading the chart incorrectly? Thanks, -Slashy

I'm glad that everybody's in agreement. I personally don't care about absolute precision AFA what I'm planning on doing with this info, but I am bothered when my empirical results aren't predicted by the model. To that end... my empirical results are exceeding *both* of these models on smaller bodies and extremely low t/w ratios. There's something else at work, and I don't know what it is or how significant it is... but it's there and neither model is accounting for it. For example, Using o-10 engines (Isp=290s) from the Mun and an initial t/w of 0.91, I'm establishing 10km orbit with .526 efficiency. Neither model would predict that's possible, especially with the necessity of clearing terrain on the way. And if my results are suspect, arkie actually confirmed it on one of his launches as well; predicted maximum efficiency 69%, actual result 72%. (arkie, please correct me if I'm mistaken here) Best, -Slashy

Jouni, Not quite. It would be the "perfect DV to achieve your desired orbit"/ "how much DV you actually expend". As an example, referring to the Mun in my table above, it would theoretically take 580 m/sec DV to achieve a stable 10kM orbit. At 50% efficiency, you would actually expend 1,160 m/sec. I disagree, although I said essentially the same thing. It requires more engine mass, but only marginally more fuel (the only added mass is the engine itself). Were engines massless, you could double the t/w and not require any additional fuel to make the same DV. Best, -Slashy

arkie, The above graph is lacking range in the y axis for smaller bodies and higher Isp engines (unless my math is off). It doesn't extend far enough for LV-N launches from any body smaller than Vall and doesn't accomodate PB-ION engines anywhere. It also can't accomodate the use of any engines (no matter what the Isp) on tinier planets like Pol. -Slashy

Jouni, I've been volunteering gofer work on this project and can answer these questions. Yeah, that had me scratching my head as well at first. It is the ratio of exhaust velocity to orbital velocity, or 9.82Isp/Vo. An intuitive description... As your Isp goes up, TVR goes up proportionally. As your orbital velocity goes up, TVR goes down inversely. Here's a handy table I knocked together: So as an example, using 48-7S engines (Isp= 350) to launch from the Mun (Vo= 547) would give you a TVR of 350*9.82/547= 6.3 It's the ratio of "perfect" Dv expenditure to how much Dv you'd actually expend to achieve orbit. It's useful because it allows us to define where adding "Moar boosters" is no longer beneficial due to cost and weight penalty. Using the example above, we can see that using the contour plot at DVR= 6.3, we achieve about 58% efficiency at t/w=1 and about 90% efficiency at t/w=2. If the fuel and tankage costs and weighs less than the engines, it's not worth it to double the t/w. Best, -Slashy

Hope that doesn't butcher it! Best, -Slashy

Arkie, Wow, I see what you mean. I didn't realize the numbers would fall off so sharply under 1:1. In that case, there is a problem. I am outperforming the model at very low t/w ratios on the Mun. This would be a problem... BTW, I really like that expanded plot. I'm gonna see if I can't make it just a little more user friendly and repost it here. Best, -Slashy

Pretty much this. I built a model of a Tylo lander for development purposes using o-10s before creating a 48-7S lander for the exact same payload. Here they are side by side on the surface of Tylo: The O-10 lander is $28,600 and weighs 26 tonnes (not counting the unnecessary engines) The 48-7S lander is $16,000 and weighs 14 tonnes. The price is a big deal in career mode, but not sandbox. the *mass*, OTOH, is a potential mission killer. Best, -Slashy

Yeah, pretty much this. I don't give a flip *which* model is more accurate (I'll shamelessly apply either one to my efforts), but LD and Arkie were fighting over it and I provided a data point. Arkie's model (for whatever reason) serves as a lower bound for my results and LD's doesn't. LD wants to reject it, that's his business... but if I were in that situation and didn't trust other's results, I'd run the test myself. Merry Christmas, -Slashy

No Sir, that's not what I'm saying... Hang on a sec; let me try something... What I'm saying is that these numbers do not agree with these numbers If you go with the values from the contour plot, then I'm not out- performing the model at 290s Isp. I am at 4200, but the margin of error is so huge that my result may well be in conformance with the model and the resolution is masking it. I need a way to get a more accurate read for ions. The expended mass is so frustratingly tiny... Hope you had a great Christmas! -Slashy

Good question... It's much simpler to execute the test yourself than it is to invent reasons to reject other people's results. Merry Christmas, -Slashy

Darkfine, I'm glad it worked out for you. A few tips for the future: 1) Try to make the plane so that the fuel tank is as close to the CoM as possible. 2) Drain the fuel tank to see where your CoM shifts 2a) If it shifts back, leave the tanks empty while you build. 2b) If it shifts forward, leave the tanks full while you build. 3) Attach and balance all wings such that the CoL is in the middle of CoM. Finally, refill the tanks if necessary. That's it. You'll have a nicely- balanced plane every time. Merry Christmas, -Slashy

Constans, Since KSP only models one SOI at a time, this technique would not work. Sorry and Merry Christmas, -Slashy

Vlad, While the o-10/ Rockomax RCS tank combo does make an SSTO lifter, the lifter winds up being heavier and more expensive than a rocket/ LF+O tank for the same performance. What you gain from having weightless engines you lose in empty tank mass, poor Isp, and expense of parts. Best, -Slashy

Thank you, Viktor! Best wishes to you and yours! -Slashy

Outstanding! I look forward to seeing the results! Merry Christmas, -Slashy

No, Sir, *your* table. It doesn't go down that far, but I'm interpolating. Point being that low TWR shouldn't be anywhere near the same in the two instances I cited according to the contour plot, but are nearly identical in the table. I think the table is hosed, 'cus the contour plot agrees with the empirical results, plus the table disagrees with your math. Best, -Slashy

FL-R25. *edit* Hmm... now that you mention it, I've got 2 different RCS tanks in my list with that name. One has a capacity of 100 (.55t), while the other has a capacity of 250 (1.15t) Must've been a cleanup error during an update? Shouldn't matter so long as you have a 2t ship with a 48-7S set to 16.5% throttle. Best, -Slashy

What LD will accept <> "credible". "Repeatable" is the scientific standard. All anyone has to do to confirm or reject my results is repeat the experiment.

False, and this is the crux of the matter. When going orbital velocity, you "weigh" nothing. With zero angular velocity, you "weigh" appropriately to the gravitational field. Between the two, you "weigh" less in proportion to the angular velocity. If what LD states is true, we'd have lots of pictures of astronauts walking around on the ISS instead of floating. The misconception is a core part of his model, and is why it doesn't work.

^ "Grudging" So there we have it. Grudging concurrence. The model doesn't predict the outcome, and is therefore flawed. It doesn't serve as a lower bound if empirical results wind up under it. Arkie's model has not been outperformed by empirical results (at least not within the margin of error) and is therefore superior. Really all that needs to be said on the subject. -Slashy

Arma, My burn was done long before I had reached 5kM altitude. As I said, the inefficiency of low t/w happens at launch, not at altitude. LD will (grudgingly) concur on this point. The error is in how the craft behaves in the early stages, not the latter. The problem is that his model doesn't take into account the centrifugal force generated by angular velocity. Arkie's does, and that's what makes the difference. Arkie's model doesn't take into account the sea level radius of the body or effects of launching above/ below that altitude, and neither model takes into account the body's axial rotation. (unless I missed something in the math) But that centrifugal force is a pretty big deal. That's why arkie's model works and LD's doesn't. Best, -Slashy