Empiro

It depends on the mass of the satellite and the amount of delta-V you need after LKO. In general, the more delta-V you need and the heavier the satellite, the more the equation favors engines with the best ISP, even if they're heavier. One of folks on the forum made a pretty nice tool for making this determination:

For interplanetary intercepts, the big thing to look at is where the ascending / descending nodes are with respect to your close encounter point. The encounter will never be close unless that node is right at your intercept point. Even a .1 degree difference in inclination will result in no encounter except at the ascending / descending nodes. The second thing to watch out for is that a slight difference in orbit (e.g you cross your target's orbit) will result in a large difference in encounter velocity, and it will cost you a lot of fuel to match the target's orbit, so try your best to get the orbits to just barely be touching each other if possible. If you're in a rush then this might not be possible. The final thing to note is that your encounter distance can be in the thousands of kilometers and still be successful. You simply need to pay attention 1-2 hours before the close encounter, and push the retrograde velocity marker toward the target retrograde as soon as you notice it diverge.

Yep, that sounds like it could be the issue. Your rocket looks good -- you can try to take out the reaction wheel. It might be weakening the joint of your rocket a bit. If you take that out, and directly connect the tank to a larger fairing base, I bet your rocket will be rock solid. The only other minor issue I could see is that the TWR is a tad high on both your first and second stages. The easiest way to fix that would be to add more fuel to the second stage, but that's completely optional -- I think the rocket should fly great either way.

There are many causes of unstable rockets, but the keys are proper design and piloting. I never tinker with thrust vectoring or control surfaces, and I rarely add reaction wheels. When designing your rocket: The top of the rocket should be as aerodynamic as possible. Putting a fairing on could be a huge help I always have fins at the bottom of my rockets. They're usually ones with control surfaces Don't have a TWR above 1.5 on the pad. Speed will amplify any small aerodynamic issues The rocket should be aerodynamically stable. This means to test it by flying it, turning off SAS, and giving it a slight nudge in a direction. It should return to pointing to prograde after a few seconds. When flying your rocket: Make only small course corrections (press Caps Lock for fine controls). Don't ever let your nose get pointed far from your velocity vector That means that your gravity turn should be gradual and smooth. I use a 5-10 degree turn at 50 m/s for medium to large rockets Don't use SAS too much on big rockets -- it tends to cause oscillations. I usually turn it off after my gravity turn, or set it to prograde and turn it on in short spurts If your rocket starts to wobble or bend, don't try to correct it. Instead, turn off SAS if it's on, and throttle back slightly

Ah, okay I misunderstood you. If you only need to get into orbit around Kerbin, then 1800 m/s should be more than enough to let you make orbit, make any corrections for rendezvous, and de-orbit. That should make the design much easier. In the hanger, you can right click on each engine to switch them to rocket mode. If you switch everything, it'll show the rocket-only stats. The extra wrinkle is that by the time you switch when piloting the craft, you'll have burned away some of your liquid fuel, so you'll need to drain one of the tanks a bit.

The biggest thing to do is pare down on any mass you don't need. My first suggestion would be to remove the mono-prop section, saving you a bit in mass. The docking port has mono-prop built-in, and it should be enough to do docking. Try to use slightly smaller elevons for pitch control. You shouldn't need something that big for a mk2 craft. I don't know the particulars of the engine (which I will assume to be similar to the RAPIER), but it looks like you have more TWR than necessary. You could try removing one of the engines. Do the pre-coolers actually do anything? If not, you can probably take it out. You can also reduce drag a bit by removing some of the intakes. Generally, you only need 1 per-engine. Finally, make sure that you're flying the craft well. Ideally, you'd get up to at least 1200 m/s at 20+km before switching over to rocket mode, and you'll want to have just burned all of your liquid fuel. In stock aerodynamics, you'll want your velocity to be pointed at about 10+ degrees switch over, because increasing your AoA creates tons of drag. I can't see the delta-V statistics for how much you have in rocket mode, but your rocket-only mode needs about 2600 m/s delta-V at the minimum. About 1400 will be needed to get into orbit around Kerbin. Then another 1100 to transfer to Duna, and another 100 for corrections around Duna. I'd want to have a bit more just to be sure. Your craft shouldn't be close to the point of really diminishing returns, so make sure that the fuel is properly fed into the engines.

I hope the idea takes off. One thing I like about the idea is that after the initial design phase, it can be done incrementally. It's not an all-or-nothing endeavor. Maybe we don't immediately build a giant array that allows a probe to get to 20% light-speed. Maybe we don't build a probe that is small enough to weigh 1 gram. We can instead build a modest sized array, and a larger probe, and maybe propel it to .1% light speed. That would still be sufficient to explore the outer planets, perform flybys of 90377 Sedna and many other KBOs (like the possible Planet 9) within months or years instead of decades.

Basically it says how fast your rocket can go if you burn all of your fuel, starting from a complete stop, and assuming no other forces like air drag or gravity. It's very helpful in your rocket design to understand why the delta-V is calculated the way it is, but if you get a mod, you won't need to do any of the math yourself. How much delta-V you have is the fundamental way to measure where you can go and what you can do. A delta-V map like the ones linked above are like subway fare maps. You add up the total cost to go from point A to point B, and your delta-V is essentially how much money you have.

In older versions of KSP, some parts had problems when you decouple them. They're considered to be a part of your ship, and the camera tries to follow the center of mass as that part moves away. I don't know if that's what's happening here, but a part pack may be to blame.

If the two are close, then pitching up too much can indeed cause you to lose control. Having the CoM even further forward should help you maintain control. Alternatively, have an even shallower re-entry so that you don't need such a large amount of pitch to avoid overheating.

I too also launch with a TWR of around 1.3 these days. Since the air quickly gets thin with altitude, between your engines increasing in thrust and your rocket getting lighter, you don't need a very high TWR on the pad to get good acceleration toward the end of your stage.

If you switch to Fine Controls mode by pressing "Caps Lock", it will help reduce the rotation when you try to translate your ship. I tend to only use the NavBall for docking (or a docking alignment mod). The camera is only used to confirm that I'm lined up, and if I need to make corrections, I go back to looking at the NavBall. One trick I like to use is that if you're a moderate distance (10-50m), and not moving very quickly relative to your target (like say, 0.2 m/s), instead of turning, use the IJKL keys to move your velocity vector around on the navball (it doesn't matter if you're looking at the retrograde or prograde marker). It's fairly intuitive because J/L will move it left or right, and I/K will move it up or down. Move the velocity vector around until the velocity vector is right under where your nose is pointing. Then press H/N to go forward or backward to completely arrest your relative motion. You can then do the point both ships at each others' docking port trick as above, and then just press H to move in.

One thing to keep in mind is that using less fuel and using less delta-V aren't always the same thing (and using less fuel isn't necessarily cheaper if you need to spend more money on engines). Depending on your design and how you fly it, it may very well be the case that your craft spends less delta-V by doing a direct ascent. However, by changing both your design and your piloting, you can probably improve on the overall cost. I remember that someone here has done a pretty detailed analysis with mechjeb and multiple designs (I can look for the original post if you're interested). The most efficient (in terms of cost) were crafts that had low TWR (~1.3 on launch, meaning fewer expensive and heavy engines) but lots of fuel. You fly it by doing a gravity turn, but completing the transfer to the Mun while you're still inside the atmosphere. You're essentially going into "orbit" at an altitude of 50km, taking as much advantage of the Oberth Effect as possible while balancing that against gravity drag and atmospheric drag losses. However, at the end of the day, the saving is pretty minimal compared to just going into a stable orbit of 80km, which is why most people just do that.

Since you need to use the Thud in pairs, there's almost no reason to prefer it over the Poodle, which has more thrust than the pair of Thuds at less mass and a far better ISP. The only time you might want to use the Thud is if you absolutely need a radial attached engine, which is pretty rare.

You can do that, but the question is why would you want to? Other than the rare case of having to balance out an asymmetrical payload (and even then, try to shape your payload so that it is symmetrical), there's no reason to carry dead weight around.

Try wiggling either craft a bit. Normally, the magnetic force would twist the ships into position in space, but here, there might be some friction preventing the crafts from rotating.

Others have already given advice on making your craft lighter, but there's also nothing wrong with having a larger craft if you want to go for a mission that more resembles real-life missions. When I do Mun landing missions, it is always with at least a 2-person crew, and I tend to use the lander-can, or even the inefficient the MK-2 pod. It should be entirely possible given your current tech-level. While I haven't seen your initial designs, I think the biggest error I see newer players making is having a too-high TWR in space. Having an initial TWR of more than 0.5 (5m/s/s) in space is completely unnecessary. I usually start with around 0.3 for any longer mission.

Yes, according to the delta-V maps, though it's cutting it pretty close unless your landing is near perfect.

I don't know if this would be acceptable for a "true" Grand Tour, but it seems to me that as soon as you make LKO, you'd want to split off your ship and send fuel tanks off separately. You don't want to be hauling fuel for exploring all of Jool while you're messing around Eve and Moho, so just send that fuel off to Jool from Kerbin. You'd naturally want to go to Eve first to get that out of the way, and then at every step of the way you dock with fuel tanks sent from LKO, each of which has just enough fuel to refill your lander to land on whatever body you're around and transfer you to the next destination. This way, you're not carrying fuel around that you won't immediately use, and you won't run into the worst parts of the rocket equation. You'd still need a huge launch craft, but it won't be nearly as big as trying to get 20+km/s worth of delta-V.

Press F3 to see the exact cause. You have a very large craft, so it could be some sort of breakage, though it's pretty hard to do that in stock.

It may take some time to properly get the trajectory set up. You need to do it early, and it may require you to speed up or slow down slightly. However, if you're passing within 9km of Tylo, and your path within Tylo's SOI looks like a straight line, then you're moving way too fast (i.e. bad transfer). If your path does bend a lot in Tylo's SOI, then it's just a matter of entering at the right time and place. Don't forget that Laythe can also be a good target for a reverse assist.

It depends on your burn time. I think the general rule is that you don't want to burn more than 1/6 of your orbital period (of a circular orbit). As stated, you don't need to burn all at once, and you can raise your AP through several passes.

The Oberth effect is true whether or not you're in the presence of a gravity field, but using gravity gives you another way to look at things. Gravity wells are considered to be conservative fields. In particular, when you fall toward a body, you lose gravitational potential energy but gain the same amount of kinetic energy. The Oberth effect says that the faster you're moving (i.e. more kinetic energy you have), the more energy you gain from spent delta-V (it comes from the square in the kinetic energy equation: 0.5mv^2). Therefore, burning at periapsis or deep within the gravity well works best: you've traded all your gravitational energy for kinetic energy.

[quote name='Tatonf']Thanks for all the replies, but I still don't understand why I gain more "kinetic energy" from burning in LKO rather than in Sun's orbit. You say that I can't compare my velocity from LKO and the velocity from my Sun orbit, but what does matter at the end ? My velocity relatively to the Sun ? That would mean that, if I go orbiting the Sun at an altitude where my velocity is greater than 9284.5 + 3431.0 = 12 715.5 m/s, the Oberth effect I'll get from the Sun will be better than the one I get from LKO ?[/QUOTE] Indeed, if you drop to a lower periapsis around Kerbol, then your speed will be higher, and you'd get a bigger benefit out of the Oberth effect. The gain, however, would be offset by your lower total orbital energy. There are lots of different factors at play, which is why sometimes, it's good to drop your periapsis and then burn at periapsis, and sometimes, you want to do a direct burn. The easiest way to test out the Oberth effect is to do the following: do one burn in LKO at 1000 m/s so that you're just barely escaping Kerbin in the prograde direction. Wait until you exist, then burn another 1000 m/s prograde. Look at where your Apoapsis is. Then, do a second test where you burn all 2000 in LKO so that you're ejected in Kerbin's prograde direction. Note where your Apoasis is. In the second case, your AP will be around Jool's orbit, while in the first case, it won't be close. You can also repeat the test in the retrograde direction.

Depending on where you are, 8-9km is actually a very close encounter. When I intercept asteroids in solar orbit, the initial close encounter is often hundreds km apart. When you get close, all you need to do is push the retrograde velocity marker on top of the retrograde target marker, and you'll keep heading toward your target.