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@Snark, I thought about what you've said and I have to agree. Still, that means for a burn over an arc you have: Always follow prograde - zero cosine loss, some gravity loss Always follow horizon - zero gravity loss, some cosine loss Always follow a point in between (eg node marker) - combination of cosine and gravity loss I wonder if it's possible to do analysis on each of these, to see if the loss in all cases are equal or is there some local optimum solution. Regardless for this particular case it may be easier to put the interplanetary ship into Low Mun Orbit. Fully fuel the stack in Mun orbit (via ISRU or otherwise) then when the transfer window comes up eject back into Kerbin with a low PE, reach the PE then complete the interplanetary burn. That way the outbound burn would only be around 1400m/s of delta-V divided into two relatively small burns, instead of the monster 1930m/s single burn directly from LKO.

You don't have to follow the node marker, the burn can also be completed by following the prograde marker so you never deorbit yourself. When you follow the node marker for the entire burn you get zero cosine loss but some gravity drag, due to having some component of the burn in or against the direction of gravity. When you follow the artificial horizon on the navball for the entire burn, you get zero gravity drag, but some cosine losses because some of your burn has radial component relative to the node. You start the burn before the node so the radial is in one direction which is cancelled out by the radial component in the opposite direction from the part of the burn that's perform after you've passed the node When you follow the prograde market on the navball for the entire burn, you get some gravity drag since prograde starts to move away from the artificial horizon once you start your burn, you also get some cosine loss because prograde does not follow the node exactly. The sum of gravity drag and cosine loss should be lower than either the "follow the node" method or the "follow the horizon" method.

The dV cost to orbit can be broken up into four things: orbital speed, gravity losses, aerodynamic losses, and steering losses. For launching from stock Kerbin with typical TWRs (1.3 to 1.5) and a reasonably pointy rocket, orbital speed is about 65%, gravity losses are 25%-30%, aerodynamic losses are 5%-10%, and steering losses are one or two percent at most. You cannot reduce orbital speed, but you can reduce the losses. Out of the three, gravity losses are the largest by far. Gravity losses are constantly incurred whenever a rocket engine is fired not perfectly perpendicular to the gravity well. The more it is firing towards the gravity well, the worse the losses are. Directly at launch, when the vehicle is perfectly vertical, gravity eats up 9.8 m/s worth of dV for every second the engine fires. 9.8 m/s, every single second, just gone. That's why gravity losses are the biggest loss factor. So how do you reduce it? Well, if you could turn over harder and sooner, you'd be firing your engine further away from the direction of the gravity well sooner, and for longer. Also, if you could just fire your engine for a shorter time, fewer losses would accumulate. And both of these things have one thing in common: they require you to have higher acceleration. In other words: a higher TWR. This is what TWR does for you, beyond having enough of it to lift off; it makes your ascent more efficient by minimizing gravity losses. Which is what @Entropian already correctly pointed out, just with an actual explanation attached There are two takeaways from this: one, if you lose more dV to additional engine mass than you save in gravity losses with the thrust from those engines, all you did was waste money; and two, TWR largely ceases to matter once you are in orbit. (I say "largely", because stretching a burn over too large of a section of the circular orbital path starts incurring you significant cosine losses as you thrust off-prograde for minutes on end both ahead of and after passing the maneuver node. And if your thrust gets so low that you have to fly constant-thrust spiral trajectories, like ion engines require you to do IRL, then the cost of transfers can rise drastically - up to 2.4 times that of an ideal Hohmann transfer, given an infinite SoI and infinitessimally low acceleration.)

When Claw put in that little mod to make the chutes prettier by spreading them out so they don't clip, I don't think there was any more logic put into it than that. I'm sure the loss of efficiency is just a cosine loss. The parachutes create a drag force along their axis, and if that axis is not vertical, you get a cosine factor. The more chutes, the bigger the angle, and the bigger the cosine loss. Which is fairly realistic, so I don't think anyone will find it upsetting.

Am I right? Is that or isn’t that the most powerful vacuum engine ever manufactured? It’s more than twice the thrust of the J-2, and even the SSME’s vacuum thrust is slightly less. The F-1 had more vacuum thrust, of course, but it wasn’t a vacuum engine. I wonder if it is dual-bell. It doesn’t look dual bell but it is hard to tell. I do not think we have a sea level compensating bell like the RS-25. As others have said, it won’t — both because the nozzle is not as thin, and because it will not gimbal. They could not use a radiatively cooled nozzle extension like the MVac because the engine is not directly exposed during the burn, due to Starship’s skirt/heat shield. The Raptor will be fixed in place and likely be in contact with the skirt. I assumed 15% gimbal range because that’s what Elon said. Cosine loss can go up to 3.41%, but that’s from the minimum throttle setting already which does not help as much. In other words, that’s 3.41%, not 3.41 percentage points. Not quite enough to get below one gee.

Note that on planets with an atmosphere, you wind up optimizing for a lot of variables. An ideal path should only burn along prograde, any used to turn the rocket are considered "cosine losses" (your loss is proportional to the cosine of the angle between your thrust vector and prograde). I'd call what you are asking for "hypotenuse losses". An ideal burn would calculate the vertical delta-v and horizontal delta-v, add them up in right angles and then burn along the hypotenuse. Of course in worlds with an atmosphere, you want to minimize aero losses so you start by heading straight up. Presumably NASA computers work out the amount of aero loss and "hypotenuse" loss and work out a trajectory that minimizes both (actually I'm sure they do a single burn to minimize the thrust needed from the second stage).

I'm assuming the above is with superheavy vertical, and without any thrust vectoring? There is the option of thrust vectoring which should get you a little closer to hovering, even with superheavy upright. Taking the centre diagram you should be able to tilt the 40-55% pair away from each other. 8 degrees of thrust vectoring mean the cosine loss is about 1%, which would allow the third engine to throttle lower. That gets you a little closer to hovering. There is also the possibility of hovering with superheavy canted over. Taking the centre diagram, tilt SH so its COM is offset slightly towards the bottom pair of engines. If you have the right amount of tilt, then the third engine should be able to throttle to match the bottom pair. (I'm not sure how much tilt that is, but I'm guessing it is around 5 degrees). This should also give you the full throttle range of all 3 engines, so 2640-6600kN. (A 2 engine hover should also be possible).

Yes. You let the cosine loss occur while focusing in reducing gravity loss. While in a gravity turn you let the gravity loss occur while focusing In reducing cosine loss. Something to notice: in a constant descent trajectory the resulting force (thrust - weight) is aligned with velocity. You use 'just enough' of your thrust to counteract weight and any extra to reduce horizontal speed. Another point of constant descent ftrajectory is that while you velocity is high enough (orbital) you are constantly falling and missing the ground(orbital freefall) . Ideally you are in a trajectory with a periapsis barely above the ground and all your thrust will be used entirely to kill your orbital velocity at the periapsis. Requires instantaneous deltaV expenditure, thus impossible in practice. What you do is come as close as possible to this, dealing also with the 'details' like terrain and piloting errors.

With a single VTOL thruster, CoM below the point of thrust is really the only way to increase stability, other than adding other types of stability like reaction wheels and RCS. But if you have multiple VTOL thrusters, angling them outwards from CoM will improve stability, but cost you a little in cosine losses. And that is why it works. Let's say you have 4 VTOL thrusters, front, back, left and right, and they're angled 5° out. If your craft pitches forward, then the forward thruster will start pointing more straight down, reducing its cosine loss, while the rear thruster gets even more angle, increasing its cosine loss. The net effect is that the craft will try to pitch back until the thrusters balance again.

I think of it in terms of energy, always. Burning prograde adds energy, if gravity is reducing the vessel's acceleration then that is the gravity loss. If the vessel is pointing away from prograde then steering losses occur (sometimes this is desirable, as when changing inclination). A vessel coasting toward apoapsis in vacuum is losing speed but not losing energy, I don't think of that as gravity loss. With the idealized two-instantaneous-burn ascent, no gravity loss occurs. What you are describing as steering loss here I think of as a subset of cosine losses. Though the vessel doesn't need to rotate to experience cosine losses, of course.

Well, that varies with the type of engine. In real life, the Oberth effect depends not only on the velocity of the ship, but on the mass flow rate of the burned fuel. Thus, it does a lot more good for low-ISP engines than for high-ISP engines. IOW, it helps chemical rockets a lot, thermal nuclear engines somewhat, but has no real benefit for ion engines. But that's real life. KSP factors in the Oberth effect on burns automatically and completely under the hood, so I have no idea if it takes this into account or not. That said, however, there's more going on with using a higher parking orbit than a low one. For instance, the higher you are, the longer your orbital period, so the less likely you are to reach your ejection burn node at the optimal time for the cheapest transfer. Depending on how high you are, you might miss the best transfer window by days to a week, which increases the cost of the burn over what it could have been if you'd been lower and thus left at a more optimal time. Plus, of course, the higher you are, the less Oberth. Because of these inefficiencies, the only real reason to use a high parking orbit is if your rocket has a very low TWR so your transfer burn will be very long. When you're down at 80-100km or so, you should never burn more than about 5 minutes at once. Otherwise you lose a lot of efficiency due to cosine loss. But the bigger your orbital radius, the longer burn you can do with acceptable cosine loss. And if you have such a low TWR to want to do this, then you probably have way more dV than you really need, so you don't really care about the loss of efficiency from non-optimal departure times and lack of Oberth effect. This.

Fission fusion antimatter ion drive The Fuel Pellet The conductor of the fuel pellet is superconducting Li6H2 This forms a solenoid, a capacitor and a switch. Dialectric filler material is decaborane. B10 10 H14 2 The fuel pellet is charged a few seconds before firing with as much current as possible. It assumes the shape of LR circuit where R is close to zero. The fuel pellet is discharged from the ship with a gas gun. Current in the fuel pellet can be terminated in a variety of ways. This is accompanied by a loss of superconductivity and a total chemical explosion. Voltages greater than 1 million volts are created between the top and bottom of the fuel pellet by magnetic quench. The Fission System One or more fission reactors are used to generate neutron beams and generate electricity using Kr78 as the primary heat exchanging fluid. The radiators also serve as gaseous diffusion isotope separators which separate Kr78 from Kr79 The Kr79 is fed to small feeder tanks for the ion engines which may operate either continuously or in relatively frequent pulses. Kr79 has a half life of 1.46 days. It emits a positron and becomes Br79. Kr-80 and Br-79 impurities arise both in the radiator and the ion drive tanks, they may continue as coolant or propellant or be removed for some other system. Ship Electric Drives Standard ion drives for Kr 79 are arranged in a radially symmetric manner pointed inward at 1 degree. This entails 1/1000 cosine losses of momentum, but greatly increases the power achievable by converging on the fuel pellet. Electron guns are paired with the ion drives, also converging near the same point, perhaps leading slightly. These may afford a greater angle of radial convergence. Lasers and electrically powered neutron sources can also be used at even greater angles of radial convergence. The Zone of Reactions The chance that Kr 79 ion will release a positron at a time and place where that positron can interact with the fuel pellet is far less than 1 in a million, perhaps far less than 1 in a trillion. But it is not unreasonable to think that trillions of ions could be made to nearly converge in a single pulse, with at least a few positrons reaching the fusion fuel pellet. This is one of possible ways to initiate magnetic quench and the destruction of the pellet. It is also possible that a neutron could initiate the quench. Or an internal switch of the pellet could initiate destruction, though this is probably unnecessary. The fusion reaction is only barely contained by the external magnetic field. Therefore it is questionable what fraction of the deuterium in the pellet will undergo fusion. What fraction of other nuclear reactions will occur? And how much useful momentum can be harnessed by the spacecraft? In the case where zero fusion occurs and zero positrons provide useful momentum we have an engine that is very expensive in terms of cost, but not bad in terms of weight. Krypton ion engines are good by themselves. Decaborane and lithium hydride in a magnetic quench will all become monatomic ions even if fusion does not occur. Somewhere between 1% - 10% fusion burnup rate could justify the weight and expense of extra complicated radiators, fission power plants, and highly processed input materials. It is not necessarily wasteful of natural resources either, because Li 7 H 1 is a good superconductor with which to make a giant solenoid hull. And various B 11 compounds are good for shielding and chemical explosives. Pulse engines of necessity demand shock absorbers for the crew and payload. If our hull is a tube and we carry gas, then our payload capsule ought to be a cylinder within a giant piston. Protection from electromagnetic radiation involves a superconducting Faraday cage around the payload. Protection from neutrons requires gadolinium film in critical areas, also gas and hull materials help a lot.

I'm slightly surprised that the vacuum Raptors won't gimbal. If everything is working properly, that isn't an issue. They can use either differential throttle and/or rcs for steering during burns, assuming all three engines are operating. But if a vacuum raptor were to fail that would only work if the vacuum Raptors are aligned through the centre of mass, and not parallel to the hull. Aligning them through the centre of mass would mean a tiny cosine loss (probably negligible overall; it might even be smaller than the dV gained from the weight savings from a lighter engine that doesn't gimbal) but would also mean that they couldn't use differential throttle to steer, and would have to rely to rcs. Source? Also were those studies for 6-8 month trips, or for 2-3 year trips?

Hello, and welcome to the forums! There are two main potential sources of "loss" when you're climbing to orbit from the surface of Kerbin, i.e. ways that you "waste" dV on your rocket: Gravity losses, i.e. fuel you spend fighting gravity rather than accelerating your rocket. Aerodynamic losses, due to drag. (There's a third category that can happen, cosine losses, if you spend significant time thrusting any direction other than . However, assuming you're doing a reasonably well-executed gravity turn, you can keep cosine losses close to zero, so I'll assume those aren't an issue here and will ignore them.) Ever since the new aerodynamics model arrived in KSP 1.0, it's been the case that for most "normal" rockets, gravity losses are much bigger than aerodynamic losses. Therefore, your piloting strategy should focus on the gravity losses. You minimize your gravity losses by being high TWR, at least during the initial part of your ascent when you're going mostly vertical. (Easy way to see this: Imagine if you had a TWR of 1.0001: you'd rise sloooooooowly off the launch pad, basically just hovering. Burning tons and tons of fuel, but going nowhere and getting no velocity out of it.) Climbing fast like that will mean you get more aerodynamic drag, sure. But since gravity losses tend to be so much bigger than aero losses, it ends up being a net win. The optimum-efficiency case for a vertical ascent is when your rocket is traveling right at terminal velocity all the way up (bearing in mind that terminal velocity rises with altitude, due to the thinning air). So if you built a rocket with stupid-crazy-huge TWR like 5 or something, then yeah, you'd be wasting way too much dV to aero losses and should slow it down. But if you did that... you'd be inefficient due to having far more engine than you need, and lugging around all that dead weight. Most rockets aren't designed like that. Most rockets have a saner TWR, either less than 2.0, or at least not a lot higher than 2.0. And for such a rocket: you should put the hammer down and accelerate at 100% throttle all the way. Why? Because you never catch up to your terminal velocity, which means "the faster the better" and higher throttle = higher efficiency. So that's the theoretical explanation of the results that you have already experimentally observed. Congratulations on your successful experiment. You're officially a rocket scientist!

Well, yes, which makes physical sense if you think about it. It's the same reason you can't lift yourself off the floor by pulling up on your pants. In the real world, no. Rocket exhaust needs to be aimed somewhere that it won't hit anything, because rocket exhaust is, well, unpleasant. In KSP, I believe that the answer is yes, there's a limit, though I think it depends on the engine-- how far that limit is will depend on the engine's model, IIRC. (For example, I think I read a post from someone saying that the ion engine has a limit of zero, i.e. you can put it inside something and it'll work just fine, obstructing the exhaust causes no problems.) In my own games, I generally build my ships so that the exhaust simply stays well clear of the rest of the ship-- both for the practical reason that you've run into, and also for the somewhat role-playing reason that it feels like "cheating" to me if I've built a physically impossible ship, even if the distance is such that the game doesn't care about my exhaust. I want each engine's exhaust to have a clear, unobstructed line from the nozzle to infinity. I usually accomplish this either by mounting the engines on outriggers so they're far enough out to one side that their exhaust doesn't hit the rest of the ship. Occasionally I'll rotate them slightly to point the exhaust outwards-- that means taking some cosine loss, but it's very minimal as long as you keep the angle small. For example, a 5-degree rotation will incur only a 0.4% cosine loss; 10 degrees will be only a 1.5% loss. (After that it starts to build up in a hurry, so I try to avoid a deflection of more than 10 degrees.) 5 degrees may not sound like much, but it can give you a fair amount of elbow room if the ship is long.

This is pretty much me. LKO rescues are always at approximately the same height, give or take a kilometer or two, which means it's pretty easy to eyeball: I launch my rescue ship to the pad, shift to map view, rotate the camera so that it's looking straight down at the south pole (so that orbits go from left to right) and zoom in a bit. When the target gets at the "right spot" (about five degrees or so above KSC's western horizon), I launch and do my usual gravity turn on ascent. As soon as my Ap gets high enough for the "target closest approach" markers to appear on the map, that tells me how well I estimated my launch timing. Did I hit it spot on, or am I slightly leading or trailing the target? If I'm leading the target, pitch slightly above as I continue to burn. If I'm trailing the target, pitch slightly below and continue to burn. (Yes, this means a bit of cosine loss, but it really is only slightly off prograde, and it's a fairly minor amount of dV, so this is fine.) Wait for the navball to auto-switch to target-relative mode (i.e. when the target gets within 60 km or so of me), and then just complete the rest of the rendezvous in the usual way by watching the relative positions of the and markers and burning appropriately. Works like a charm. The reason it's easy is that I always launch when the target is in the same spot over the western horizon, which makes it easy to eyeball. The reason why it's always the same spot is that, 1. the rescuees are always in pretty much the same orbit, and 2. I always design all of my rockets to have exactly the same launchpad TWR, which means my ascent profiles are pretty consistent. This lets me retrieve kerbals without even completing an orbit. I launch, directly rendezvous, EVA-transfer the kerbal, retro-burn, and re-enter. End up landing without even getting halfway around Kerbin from KSC (unless I deliberately delay my reentry burn so as to try to land the kerbal right next to KSC, as a pointless-but-fun bit of target practice, which I sometimes do.)

Ok I just want to set some things straight. 1. Reducing thrust to prevent going faster then terminal velocity. This is just wrong. Yes this can happen if your atmospheric TWR is greater then some ridiculously high value at launch. Some one did some research on it but I cant find the post. The fix is not throttling down but adding more fuel or using a smaller engine to get TWR down. Check MJ I believe it will tell you how much dv you have used fighting gravity, friction, and cosine losses. low TWR = Gravity losses, High speed = friction, bad gravity turn and REALLY bad TWR cause cosine losses. Gravity losses are the dominant cause of loss. The most efficient launches are basically fireballs until engines cut out and they coast to AP. 2. Second stage TWR of 2 or 2.5. This is very wrong if you are doing a gravity turn. If you are going straight up until 70km and then circularizing it would make sense. In reality this is very complicated but you can launch a craft with a .6 second stage TWR what is really important is what speed are you going when staged, what altitude, how much dv and time to burn. Almost all of my 20t+ rockets follow the same pattern. I put in a .6-.8 twr high altitude engine. I put in a low altitude stage and add fuel until the atmospheric TWR is .8. Then I add SRBs until atmospheric TWR is 1.5. Yes I know it is not optimal but it is easy, cheap, quick and it still does very well. In fact they generally out perform highly tuned lifters. For example if I have a 10t lifter that can lift the payload for 900 kredits/t and I use it to lift a 7.3t payload my funds per ton go up to 12,300 funds per ton. If I use my slap together method it will generally come out to 10,000-11,000 funds/t 3. Pushing MJ and KER asap. And this is all personal so to each his own. Learn the math do 2-3 launches maybe 1 mission on paper and then get the mods and never look back. dv=ISP*g*log(m0/m1) (I hope I got that right it would be really embarrassing if I flipped m0 and m1) g = 9.81 m/s^2 m0=starting mass m1=final mass 4. dv to kerbin orbit is not 3400-3600, 2450 is possible (but not practical and requires a ton of TWR and no payload) Ignoring friction and with infinite TWR 2289 I believe is the theoretical minimum. An interesting note that of the 2289 something like 2024 is just getting to orbital speed and 124 is the transfer from 0m to 70x70km. (don't quote me that is the best I remember) 5. Skipper is a Great engine and fills a HUGE gap between the reliant/swivel and mainsail. 5 mainsails would be overkill on TWR and a loss of dv 6. I think @GoSlash27 is wrong about SRBs there not cheap their VERY CHEAP generally SRBs will make up 1/3-1/2 the mass of my ships on the pad but only provide 1/5-1/3 the dv. 7. Burning main engines and boosters vs boosters then mains. Depends on a ship by ship basis. Yes burning booster first will make more dv because they have a bad ISP so the LFO engine is wasting energy accelerating it, BUT BUT BUT at liftoff you experience most of the gravity losses and higher TWR will generally make up for the loss of dv. I would argue a middle ground of mains full throttle until 250 m/s then 1/4 throttle until SRBs burn out.

No. It is not worth it in time, money, fuel, or hassle to build any sort of station within Kerbin's SOI for the purpose of assisting interplanetary ships. This has been discussed many times, based on all the various mod and stock refueling systems that have come out over the years, and the answer is still the same. Because the KSP universe is so small, the practical benefit from doing this is so small that the system will never pay for itself, nor be worth the trouble of using. You just never need more fuel than you can lift. And then, by the time you're doing interplanetary missions, lifter and transfer fuel is really a small fraction of the cost of a rocket---the payload is where you spend your money. If you can afford the payload, you can afford to lift it and shoot it out from Kerbin. Now, things might be different if you use RSS or some other rescaled solar system, for the same reason that such ideas have some traction in the real world. But in stock KSP, don't bother. ------------------------------- So, back to the thread's title of interplanetary parking orbits, the best ones are the lowest you can manage. Here's why.... To go to another planet, you need to reach Kerbin's escape velocity plus whatever extra is required to raise your Ap (or lower your Pe) to the altitude of your target. Therefore, the higher your velocity to start with, the less dV you need to reach the required transfer velocity, and you have a higher starting velocity the closer you are to Kerbin. In addition to this, the higher your velocity, the less fuel is required to create a given amount of dV, thanks to the Oberth effect. Thus, a ship in a higher orbit will require more fuel to create the same amount of dV than a ship in a lower orbit. Therefore, the higher your parking orbit, the more dV you need to create, and the more fuel that dV costs, to get to the same place. This means that the lowest possible parking orbit is the best---lower dV required and it's cheaper per unit of dV on fuel. HOWEVER, transfer burns take a non-zero amount of time, which means that at the start of the burn, your ship will be pointing off prograde and losing efficiency. In fact, it will be pointing at Kerbin somewhat. Thus, if your burn is more than about 5 minutes (which is about 20% of an orbit around Kerbin in LKO), you lose a lot of efficiency to cosine loss. And if your starting orbit is too low, you might even push yourself down into the atmosphere. Therefore, if your burn time is 5 minutes or less, start in about an 80km orbit. If it's 6-7 mintues, maybe start at 100km. If it's any longer than that, either do the burn in multiple passes or start in an orbit big enough to minimize the cosine loss and just accept the higher dV required and the reduced Oberth efficiency. Of course, if you've got a really long burn (10-20 minutes, or even more), then odds are you're using some really super-efficient but very low-thrust transfer engine. In which case, you've probably got enough fuel in the tank to go anywhere you want so don't care about the loss of efficiency from starting in a high orbit.

Ah, the days of tinfoil ships and iron kerbs, bucko mates and driving skippers, salt horse and hard tack, pressgangs and shanghaiing. Astronauts these days have life so easy. It's refreshing to take a nostalgic look back Even though I have to break out my magnifying glass to read them, I always enjoy your photo credits That's for the tutorial! Yeah, 10 minutes is about 1/3 of an orbit in LKO so you waste a LOT of dV in cosine loss and risk hitting the air as you have to start the burn pointed down. 5 minutes is really the most you should burn at any time in this situation. Nicely done, although I remain to be convinced that reusable rockets make any sense economically That doesn't take away from the technical prowess needed to do it, though, which is cool both in real life in KSP. If he's mad, might as well leave him there forever. What further harm can be done? Well, if you had, you'd have been right in "Aerobrake Canyon", which looks scarier than it really is. For some reason, that canyon always seems to be along the path of ships going a bit lower. You can see it in your pic. I've frequently thought my ships were below the mountaintops on either side but the last time I took a plane to Duna, I measured terrain altitudes and realized this was an optical illusion--my ships were always way above the terrain. Congrats on getting to this point. Good luck with the rest!

Well, I think your main problem is that this mission profile of wanting to reach a 100km parking orbit isn't suited to an ion-powered ship. As you know, ion-powered ships have minuscule TWRs and more dV than they'll ever use. This has several important implications that means you should do things differently with than than with other engines. First off, there's no benefit from ejecting from a low parking orbit. The Oberth effect depends not only on the velocity of the ship, but also on the mass flow rate of the burned fuel. Ion engines don't put out much in the way of exhaust mass so Oberth has no noticeable effect on them. Second, you should never burn more than about 5 minutes at a time in LKO. Otherwise, you end up losing a lot of efficiency due to cosine loss. But a transfer burn of only 5 minutes with an ion engine is unlikely unless the probe is very tiny. Therefore, if you start low, you'll usually end up having to break the total burn up into several sections of no more than 5 minutes each spread over several orbits, and this throws your timing off on doing the transfer burn, so you need a bigger correction down the line. So it's usually better to start from a higher parking orbit, where the cosine loss for a longer burn is still acceptable, and you can do the whole burn in 1 go. So here's what I'd do.... I'd burn the ion engine on the way up and accept the higher Ap you get. And I'd circularize at whatever that ended up being, to be in a nicely high orbit from which you can do the whole transfer burn in 1 go. This would be better in the long run than trying to keep the thing at 100km.

Lander legs have the lest slippage but you need to kill their springs and max their damping or bad things happen. And even then, they're kinda wonky. As an alternative, you can increase the foot's surface area. Even relatively slippery parts get reasonably good grip if you have a big enough shoe size. My fastest walkers have used pistons straight ahead and astern in a push-pull configuration to move the body forward, then hinges to raise the feet off the ground while the pistons reset. Meanwhile, another identical set of pistons takes the next inchworm step. Very ungainly but it works. Still, even this is limited by the speed of piston extension and that's only about 5m/s. In this case, there's no "cosine loss" from pushing an otherwise unpowered leg, but there's no gain from the lateral length of the leg, either. Still, there's only a small structural panel on the end of the each piston to prevent slippage. Thus, the mass the pistons are moving is reduced to the main body of the walker, and the other set of legs resetting during the step. This is less mass than the body of the rover plus longer legs with multiple joints, so the piston motion is faster. So, at the bottom line, complexity = slow for walkers. And walkers are already slow to begin with. Thus, if you're after speed, use rover wheels. But if you're after smooth, walking motions, then knock yourself out with multi-jointed walkers.

Recently, I was trying to do another mostly RCS build, and one thing I am curious about is the angle of the RCS thrusters. Specifically, perfect perpendicular vs 45 degree offset. Aside from the typical "4 thrusters pointing outward along the 45 degrees line", I am also experimenting with a 45-degree rotated 4-RCS block. So I am not certain: 1) If the RCS block is rotated by 45 degree such that it forms an X, then it will be weaker in thrust. Unknowne effect on RCS DeltaV but I suspect it will be worse. 2) If the RCS block is not rotated, but at 45 degree X, it will be not at precise in compared to perpendicular layout. Could someone confirm my observation? If so, is it because more RCS nozzle need to be used? EDIT: So other people can see how bad cosine loss is: Say 0 is the direction you want, and 90 is the perpendicular cos 0 deg gives 100% thrust cos 90 deg gives 0 percent thrust cos 45 deg (4 way symtery) gives 70% each cos 60 deg (6 way symmtery, furthest away from the near 90 degree) gives 50%. Note that due to how symetry works, you will typically have 1 right on the 90 degree cos 30 deg (6 way symmtery, closest from the near 90 degree) gives 86.6%. cos 22.5 deg (8 way symmetry further rotate offset, closest to 90 degree) gives 92%

Good point. Oddly, I tend to have trouble visualizing the cosine loss aspect, so Oberth makes more intuitive sense to me. I guess the cosine thing is akin to how it's cheaper to change your orbit via prograde and retrograde maneuvers than via radial maneuvers.

Maneuver nodes assume instantaneous change in velocity. With that in mind at each burns of a hohmann transfer the maneuver node is exactly in the same direction of prograde/retrograde. However in real maneuvers change in velocity its not instantaneous and you will lose efficiency anyway, if you are pointing in direction of maneuver node there is a difference in direction between velocity and burn, if you are holding pro/retrograde there is change in direction of your burn. For what I know both cases are equivalent in regard of efficiency, if something holding pro/retrograde its more prone to inaccuracies due to piloting/SAS. Its the same situation for a constant descent trajectory, for the part of our thrust we are not using to counteract gravity. Not enough data to know (from my part at least). But seems plausible a combination of both that end with a better overall efficiency (eg we start following a constant descent trajectory and finish a gravity turn) More to the point if there is a atmosphere you want to reduce drag what you archive reducing the cross-sectional area perpendicular to the airflow. In a gravity turn you are already keeping the thurst pointed in the (opposite) direction of the airflow so you get two benefits(the other being minimal cosines loses) with the same trajectory. Without delving in maths or experimenting we don't know which one is better for a airless body . To me it seems that if the gravity losses are a major concern constant altitude is better, if steering losses are a major concern gravity turn is better. But there is several catches: 1.It may be the case that we are just trading one kind of loss for another in similar amounts. It helps nothing if to reduce gravity loss in 200m/s we lose the same200m/s due steering. 2.We may be doing a efficient maneuver in a inefficient way, (eg not burning at full throttle, letting the craft wobble) 3.Other variables may be a greater concern than either gravity or cosine loss (eg low control authority). 4.Its possible that the ideal trajectory is part constant descent, part gravity turn.

Sorry, but I can't get this part. Won't you get cosine loss from misalignment of thrust and velocity, resulting in less energy loss? Moreover, as gravity turn let vertical velocity build up, it will have more speed overall resulting in more energy loss. What part of this is wrong? Also, I can't find the dv of gravity turn landing from the article you referred to. Would you tell me where I can get it?