GoSlash27

True, but Eve also has an atmosphere, which overrides TVR as the primary driver of T/W choice. In all cases over the entire range of airless bodies and engines, it doesn't appear that going over 2:1 t/w is going to yield meaningful efficiency gains. Very carefully! In the case of Tylo, I just wait until it's supporting itself, then adjust pitch to maintain zero climb rate until I've got some speed established. As fuel burns off, it gets easier. In the case of Gilly, orbital velocity is only like 30 m/sec, so it's very difficult to crash. More likely to just bump up against something at like 3 m/sec. Shoulda seen that monster; a full Jumbo 64 getting nudged along by a single LV-1. It looked like it had sprung a pinhole leak! Apologies, but I didn't record time of flight for my testing. I'll include that info in my next round. Thanks! -Slashy P.S., that log layout is pimpin'! Much easier to read the trends that way IMO. Would you be able to double the contour lines for resolution? I think that'd help tremendously.

Arkie, Could you do me a favor and post this contour plot slightly differently? y axis as a log scale from 1.5 to 1500 and x axis linear from 1 to 4? Unless my math is off, there's no situation in KSP that exceeds these bounds (please confirm that). I'm assuming the worst case is ions on Gilly and the best case is 0-10 engines on Tylo. Thanks, -Slashy

Arkie, When I was running the last series of tests, they were at 1:1 t/w ratio. I was trying to confirm the effect of TVR on efficiency. The problem is that your FMR winds up being so tiny in those situations that it's hard to read the change in mass with sufficient accuracy. Feel free to incorporate the test results I've provided upstream. I'm testing a Tylo lander and will relay the findings. That's kinda the info that I'm after. At some point, the improvement in efficiency is no longer worth the increase in mass and cost associated with adding engines. Best, -Slashy

Duxwing, Once we've got this pinned down, users will be able to more accurately predict how much fuel their ascent vehicles will consume. They will also be able to figure out the mass- optimal t/w ratio for a particular engine on a particular mission, thus enabling them to build lighter lifters, which makes for lighter ascent vehicles... which makes for lighter descent stages and so on. Arkie was looking at the theoretical aspects of it, but the practical upshot of all this is already proving useful. Best, -Slashy

arkie, Lookin' good, and DVR is clearly not a thing (which is what I was sayin') Can you extend the plot out to beyond 4:1 t/w? That way I can plot my empirical results against it and see how it stacks up. Thanks, -Slashy

Will do, but, uhh... where is it? Best, -Slashy

Sorry, what I mean to say is this: For massless engines, the optimal point is clearly infinity. For engines with mass, there's a point at which adding an engine adds more mass to the vehicle than the additional fuel and tankage you need for being underpowered. If seeking the lightest lander package (which is a common engineering goal) these curves would suggest that the ideal t/w for most launchers is a good deal lower than I had previously thought. Even more so when looking at overall vehicle cost; fuel is much cheaper than engines. Overall cost, you're better off at 1:1 t/w at launch, even with massless engines. What's the curve look like for a 4,200 m/sec Isp engine operating on Gilly? That would have to be the worst case scenario for a low- thrust lifter. Thanks, -Slashy

The problem isn't the size of the lander, but rather the resolution of the math. No result is more accurate than the resolution of the lowest resolution factor that feeds into it. That's why I limit my results to the first 3 significant digits. If I ran a ginormous lander, I'd still have the same problem; the model predicts results to a much higher resolution than testing can confirm or refute. But that said, there's nothing here that refutes anything your model has predicted, so I definitely call that a win. If it's off, it's off by less than anyone would be able to notice. So... what does this say about optimal t/w from an overall vehicle mass/ cost perspective? I think it shows that the optimal point is much lower than we all thought it was. A gee- and- a- half looks to be plenty efficient to get the job done according to the curve. Best, -Slashy

High Isp test is in, and it's a bit inconclusive. Same issue with the low Isp launcher on a light planet, the fuel mass is so low that it's hard to get an accurate read on the change in mass. It also acted *exactly* like the low- Isp launcher on a light planet; poor inertia control and it spent nearly the entire launch pointed somewhere other than where I wanted to go. Not because the mass was so high, but because the fuel was such a tiny percentage of it. It stands to reason, since in the rocket equation treats Isp and Go exactly the same. The result: Launch from Mun using PB Ion at 1 G: Thrust = 2kN Initial mass-= 1.22t T/w at launch= 1.01 Final mass= 1.19 t Expended DV= 1,026 m/sec Efficiency = 56.5% <-- that last reading is highly suspect due to granularity. Again, the penalty in DV is high (as was expected), but it's a high percentage of a very tiny amount of fuel. I think the model is good to go. Best, -Slashy

Okay, test #2 was definitely a bust. Even at a low Isp, It just didn't show enough mass consumption to get an accurate read on DV. Nevertheless, I was able to see the reasons why a large planet penalizes underpowered lifters less than small planets: #1) Inertia. Thrust- to- mass is the name of the game for changing the direction of an object, and the low gravity of a small body means a ridiculously low thrust- to mass. #2) Lack of centripedal lift. I spent the entire push to orbit from Gilly at a high pitch angle, whereas on Tylo it was a small portion of the entire launch stuck in the "this lifter is an underpowered pig" regime of the launch. While the smaller gravity well penalized more heavily (presumably) for being underpowered, it was a larger percentage of a miniscule DV budget, so not really worth discussing. If a "perfect" launch is 30 m/sec, who cares if you wind up spending 60 m/sec? I think these tests confirm that the model is sound in this area. Next up, varying Isp. If this test pans out, then I think it's safe to confirm this model with empirical testing. I will build a munar lifter at high Isp and confirm that the model predicts it's behavior. I expect it to be penalized more severely than the low Isp lander for being underpowered.. Best, -Slashy

arkie, Yeah it is! Great job on the model! I decided to change up the second test. I'm afraid that an ion powered flight from Gilly would use so little fuel that I won't see a change in mass, so I'm using an LV-1 instead. I'll go low Isp and low gravity and see how that shakes out. Best, -Slashy

First test results are in, and it definitely supports what the model predicts, so proof-of-concept dance! Test #1: Comparing the effect of orbiting a large planet on a low Isp launcher's efficiency loss due to low t/w Mi- 3.77 tonnes Isp= 290 s Launch from Tylo surface to 60kM orbit [TABLE=width: 500] [TR] [TD][/TD] [TD]Thrust (kN)[/TD] [TD]T/W initial[/TD] [TD]Mo (tonnes)[/TD] [TD]ÃŽâ€V expended (m/sec)[/TD] [TD]Efficiency (%)[/TD] [/TR] [TR] [TD]Test1[/TD] [TD]28[/TD] [TD].946[/TD] [TD]1.28[/TD] [TD]3,076[/TD] [TD]73.8[/TD] [/TR] [TR] [TD]Test2[/TD] [TD]112[/TD] [TD]3.78[/TD] [TD]1.66[/TD] [TD]2,340[/TD] [TD]97.0[/TD] [/TR] [/TABLE] As the results show, yeah the initial state of being underpowered hurts you, but not as much as I would've expected. Running the same test on the Mun yielded a 50% drop in efficiency for a 1G launch using the same engine, so Tylo itself is responsible for this improvement. Lookin' good so far! -Slashy

My point being (I guess... I've confused myself so badly by this point...) is that we wouldn't bother plugging in FMR as a global constant or display results as varying with it. We have already been given the FMR when we input the t/w, Isp, and Go. We just assume that we have *enough* fuel at the outset and whatever mass we lost was fuel. Accordingly, there's no need to display results in the y axis as FMR. So long as you have enough fuel, the results don't vary with it. In the case of Ve/Vo, This is definitely a parameter that impacts things, but I'm not understanding how it concludes that launching from Tylo, (for example) would dramatically improve the efficiency of a grossly underpowered launcher as compared to (say) Gilly. I'll see if I can confirm this through an empirical test; I'll launch 2 vehicles from Tylo to 60km orbit. One will launch at 1G (or at least as close as I can get it), while the other will launch at 4G. Isp will be 290. I'll post the test results here. Then later, I'll try the same sort of test from Gilly using PB-ion and see what that shows. Best, -Slashy

Aha! Now I'm pickin' up what you're puttin' down This model assumes that to be true even at extremely low t/w ratios, but does not go out of it's way to prove it. I have no doubt that the zero vertical velocity landing is the most efficient (since it's the most efficient launch), but it's also the least accurate option for landing at a specific location and most prone to smack you into a crater if you're not careful. Best, -Slashy

grom, I'm sorry, but I'm having a hard time following what you're saying here in some parts. The problem isn't figuring out when a launched rocket will intersect the orbit of a station, but rather figuring out how much time it takes for your rocket to achieve that orbit and where it will be when it does. This problem is compounded by the launch mode that you're using. A highly- efficient ion glider might take most of an hour to achieve orbit and be on the other side of the planet when it finally does. A high thrust-to-weight rocket will only take a few minutes. Both examples require completely different flight profiles. All you can do really is trial and error in sandbox until you get a feel for where the station needs to be at launch for that particular rocket. Sorry, -Slashy

Oh, wait! I think I just spotted a disconnect. You said "(Mw-Md)/Mw" rather than "Mw/Md". Is there a particular reason you phrased it that way, or was that a typo? -Slashy

I can prove that my definition is equivalent. e(ÃŽâ€V/IspGo)= Mw/Md. <-- Rocket equation. You can freely substitute one for the other. I'll derive this form if you'd like. ÃŽâ€V would be "expended" in this case, since that's that's the context you're using it in. *edit* unless it's not. In which case... it's uhh... not (sheepish grin)... It's in whichever sense you intended the other form of it, 'cuz they really both mean the same thing. Negatory, Kind Sir. Putting more fuel in the stage than it will burn doesn't mean that it will burn it, it just means that the leftover fuel and structure is dead weight (i.e. "payload"). Unburned fuel has already been accounted for as weight in the t/w ratio. Consider a tanker lifting a payload of Kethane from Tylo to orbit. It doesn't matter whether you're lifting a ton of kethane and tankage or a ton of lead. All that matters is that it's a ton. Agreed. -Slashy

Hehe... IRT K^2's question, I read it as launching at precisely 1.0:1 T/W ratio. If that's the case, then no. It could arguably be efficient for SSTOs in atmosphere (I actually do that myself), but on an airless planet you are better- off launching at a higher t/w because there's no such thing as terminal velocity. This is actually what arkie is trying to pin down. Best, -Slashy

Okay, I think I just got it for sure: FMR by your definition (7) is really just e(ÃŽâ€V/IspGo) Likewise, TVR is really just IspGo/Vo e is a constant ÃŽâ€V and Vo are inputs we already have. Therefore, neither of these values need definition. In this model, for each FMR there can only be one TVR, and each of these have already been defined by the values we plugged in to start. It would vary with Isp and Vo ,but not with FMR or TVR. Pretty sure that's right, -Slashy

Oh, wait... Light bulb just went on (I think)... I believe eq.7 should not be defined as " (Mw-Md)/Mw ", but rather " e(ÃŽâ€V/IspGo) ". In this case, the fuel consumed would be a function of the Isp, thrust, and time, not the amount of fuel we put in it to start. Likewise, we would have already defined Go when we set our t/w, so I suspect this may be an unnecessary variable. Yes? No? -Slashy

Ah... Okay, I'm following you now. But then how does having a higher Isp or leaving a more massive planet help you achieve improved efficiency at 1G initial thrust? I would expect that to be the opposite... I mean, if you have a high Isp, then it would also mean that you have a low FMR, which I would expect to hinder efficiency at 1G rather than help it since your mass isn't reducing as much to help you. Still confuzzled -Slashy

What the... what?? *D'OH!* Typo. I'll fix it. It's definitely a reasonable approximation. But my model predicts it varies with FMR and TVR as well. We should probably continue this exclusively in the science lab post before people get confused. I define it the same way: ideal i.e. minimum deltaV/used i.e. required deltaV I think the way you wrote it is backwards since ideal deltaV <= achieved deltaV always.

If you're running stock, you'd want to have the sum of the lift coefficients of your wing sections to equal the aircraft mass in tonnes. That gives a good balance between lift and drag. example: a 75 tonne spaceplane would need the lift coefficients of it's wing panels to total 75. You'll still be okay if you're over this; it doesn't have to be exact. The large wing connecters and delta sections have a lift coefficient of "2" per panel, so you'd want at least 38 of these panels total, or 19 panels per wing. Check out my basic spaceplane tutorial from 0.25 here: http://forum.kerbalspaceprogram.com/threads/102182-So-you-want-to-build-a-space-plane If you're running FAR, I have no idea. Good luck! -Slashy

Yeah, you're right. Brain-fart on my part. Disregard that; it's a bad idea. Best plan (without slingshots) I can propose would be to: launch prograde from KSC , Escape burn while pointed at the sun to set your solar periapsis Once at periapsis, extend your apoapsis and finally once at apoapsis, do your inclination change. You really want to be going as slow as possible when doing inclination changes. They hurt. Now... if you don't mind gravity assists, you can aim for the trailing edge of Eve to help speed you up on your way to periapsis, and if you can, try to grab the leading edge of another planet on your way up to help slow you down. Just a thought. I'll model that real quick and make sure it'd work... Sorry! -Slashy

*scratch that* Totally bad advice