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Found 2 results

1. ## Experiments with several different rotors to determine Lift to weight ratio

TLDR I did some tests with various rotors to determine which yields the highest lift/weight ratio for heavy payloads on either Kerbin or Eve. In the static tests I’ve performed on the ground of Kerbin and Eve, I noticed that the large helicopter Type S blades provide the most lift per tonnes rotors+motor . In my flawed tests, 8 rotors per motor work best on Kerbin and 4 rotors per motor work best on Eve. I was also able to roughly determine the maximum mass of a craft at which it would still fly. Long story I’ve been running tests with various sizes of propellers to determine the a correlation between vertical lift and mass. I’m especially looking for optimum mass to lift ratio’s for heavy crafts and trying to find a way to calculate the amount of propellers I need to get something in the air. I’ve been trying to approach it scientifically but eventually just tried some stuff at random. Literature study None.. just felt like messing around with the propellers. There are probably a bunch of better articles describing how to calculate the amount of lift on a given planet at a certain altitude. My initiial guess is that the lift is calculated something like: Lift = Function [ rotating speed, angle of attack, air density, rotor blade type] Experimental setup I performed two experiments: 1. On Kerbin , using a rigid clamped setup with 3 heavy motors in series. I tested 3 types of rotors with 8, 4 or 2 blades per motor 2. On Eve, using a massive s4 fuel tank setup ( 1,200 tonnes) with 4 motors. Testing 2 rotors, with either 8, 6, 4 or 2 blades per motor Results The current measurements have been done under static conditions on kerbin and Eve. Below you find the results. Propeller type Amount of props on motor Total weight of props (ton) Weight Motor (ton) Max RPM optimal Angle of attack Planet Lift (kN/prop) Total lift (kN) Lift/total weight motor + rotors (kN/ton) Heli S-Type 8 1.44 2,2 440 5 Kerbin 270 2160 593 Heli S- type 4 0.72 2.2 445 8 Kerbin 335 1340 459 Heli S- type 2 0.36 2.2 450 8 Kerbin 346 692 270 Propeller S type 8 0.96 2,2 460 82 Kerbin 34 272 86 R-25 fan 8 0.96 2,2 460 84 Kerbin 37 296 94 Heli S-type 8 1.44 2,2 253 5 Eve 403 3224 886 R-25-fan 8 0.96 2,2 450 84 Eve 163 1304 413 Heli S-type 6 1.08 2,2 253 3 Eve 500 3000+ 915 Heli S-type 4 0.72 2,2 418 3 Eve 700 2800 959 Heli S-type 2 0.36 2,2 450 3 Eve 1100 2200 859 I changed the angle of attack to find the optimum angle at which most lift was created. From these short tests, I found that the Heli S-type blades performed the best when you look at lift per weight ratio. I also tested if less rotors would yield in better performance on either Kerbin or Eve. From what I’ve measured I could state that: 1. On kerbin 8 Type S Helicopter blades performed better than less blades 2. On Eve, the optimum lift/ton motor+rotors weight seems to be 4 rotos per large motor Discussion One of the things I wanted to find out If I could calculate the maximum weight a certain setup would be able to lift. From what I’ve conducted. It’s fairly easy to calculate: m = max mass (in tonnes) = F * G F = Lift (kN) G= gravitational force ( 9,81 for kerbin, and 16,5 on Eve). For a two motor helicopter with 4 large helicopter type s blades per motor, this would mean that it could carry: 2800kN / 16,5 m/s^2 * 2 motor with 4 props = ~336 tonnes I tested this and It seems to check out. I also wondered if I could translate tests on kerbin to match the results on Eve. It seems that the there is a correlation between air density, RPM and lift on Kerbin. However, the maximum amount of RPM is different on Eve, compared to kerbin, so I think I need to add torque and drag into the equation for this. I didn’t have time to work this out though. Btw, this was a very flawed study and many additional tests need to be done before any hard conclusion can be made. Several things come to mind: - Does the lift vary with different vertical velocities (anything else than 0 m/s)? - Does the optimum angle of attack vary as the density changes (it does) - What is the effect of the angle of the ship on the optimal angle of attack of the blades - Etc.. Anywho… It was a nice experiment. I’m now able to calculate how many rotors I need to carry a certain payload.
2. ## Landing Delta-V Calculation

I've been working on an program to calculate combined takeoff and landing delta-v from a wide range of planet sizes (comets through superearths) and atmospheric thicknesses (vacuum through supervenuses). I'm reasonably happy with the takeoff delta-v calculation - a two-burn Hohmann transfer from surface to orbit assuming a vacuum, plus a term to approximate atmospheric drag. It's not perfect - it makes several assumptions including unlimited TWR on the rocket - but it's a decent first approximation. The landing delta-v calculation involves a deorbit burn and then a braking burn. Deorbit is easy enough - just reverse the circularization burn to bring the periapsis back to the surface. But the braking burn is more involved, because I'm looking to land a rocket capable of taking off back to orbit (not just a capsule). We can set certain limits. Braking delta-v can be as low as 0 m/s (super-thick atmosphere and/or tiny comet where descent to the surface is very slow) or as high as 110% of the takeoff delta-v (vacuum descent with unlimited TWR, allowing 10% safety margin). Between these two values - where the atmosphere is thick enough to slow descent but not to a safe landing speed - is where I could use some ideas on how to proceed. The rocket we're landing will vary greatly in mass depending on the surface gravity and thickness of the atmosphere we're dealing with. My initial thinking is to find the terminal velocity at the surface and use that to deduce the braking delta-v. This won't be the same as the terminal velocity on ascent though, because on descent there'll be more drag (rocket travelling rear-end first). Also, any parachutes will have much more of a drag effect on low-mass rockets than heavy ones. Clearly there's a lot going on here. I'm not looking for an exact solution, but a decent approximation. How do we estimate landing delta-v for a rocket - across a range of planet sizes - when there's not enough atmosphere to land safely without a braking burn? Any thoughts are welcome!
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