OhioBob

Mathematically I can show that it takes less Δv from the 300 km orbit than from the 70 km. First the 70 km orbit, Vorb = (GM/r)0.5 = (3531600000000/670000)0.5 = 2295.9 m/s Vesc = (2GM/r)0.5 = (2*3531600000000/670000)0.5 = 3246.9 m/s V∞ = 2800 m/s (approximate average for Jool) Vbo = (V∞2 + Vesc2)0.5 = (28002 + 3246.92)0.5 = 4287.5 m/s Δv = Vbo - Vorb = 4287.5 - 2295.9 = 1991.6 m/s Now the 300 km orbit, Vorb = (3531600000000/900000)0.5 = 1980.9 m/s Vesc = (2*3531600000000/900000)0.5 = 2801.4 m/s Vbo = (28002 + 2801.42)0.5 = 3960.8 m/s Δv = 3960.8 - 1980.9 = 1979.9 m/s Now let's show that both have the same energy post burn... Etotal = Ekinetic + Epotential = mv2/2 - GMm/r We can divide through by m to cancel out the mass of the vehicle, giving Etotal = v2/2 - GM/r At 70 km, Etotal = 4287.52/2 - 3531600000000/670000 = 3,920,000 J/kg At 300 km, Etotal = 3960.82/2 - 3531600000000/900000 = 3,920,000 J/kg We can see that in both cases the energy is the same, as expected. We also see that the burn at 300 km required 11.7 m/s less Δv than the burn at 70 km, which is contrary to the popular belief. (ETA) Also note that Alex Moon's Launch Window Planner shows the same thing that my computations are showing. Find a good transfer to Jool and you will see the that ejection Δv is less at 300 km than it is at 70 km. Go much above 300 km and the Δv starts to go up, just like the graph in the OP.

I don't think you really mean AoA (angle of attack), I think what you are referring to is "flight path angle". Flight path angle is the angle that the velocity vector makes with the horizontal plane. In the example I gave, the flight path angle at the time of the extended burn is 3.086o. That is so close to horizontal that the losses are negligible. Also note that after I complete the burn I am effectively orbital, 37km x 250km, and I have nearly a half orbit to complete before I reach apoapsis (155o). What I'm doing is almost a Hohmann transfer from the burnout point up to the 250 km apoapsis. Maybe you are use to more highly lofted ascent trajectories?

I like one launch if practical, but I also don't like big humongous launchers. About the biggest thing I've ever put in orbit in one launch was about 90 tonnes (v0.90). If more than that, I typically break it up into smaller launches and assemble/refuel it in Kerbin orbit. I try to make my payloads as lightweight as possible.

I didn't take steering losses into consideration; all my calculations assume instantaneous Δv. However, it's been discussed and demonstrated in other threads that smaller the angle that the spacecraft sweeps through during a burn, the smaller the Δv losses. Obviously the higher the orbit, the less angle a spacecraft will sweep through during a burn of given duration. A five minute burn in a 75 km will have greater losses than a 5 minute burn in a 250 km orbit. High orbits should definitely be more tolerant to long duration burns than low orbits. I recently saw somebody post that the sweep angle during a burn should be limited to 1/6th of an orbit, i.e. 60 degrees. That's about 5 minutes in a low orbit, which sounds about right to me.

I've been recently performing some computations regarding the Oberth effect and I'd like to share the results. I've always thought the term "Oberth effect" was a bit overused around here and somewhat misunderstood. It is not the hard and fast rule that everyone seems to thinks it is. For example, the place where I usually hear the Oberth effect evoked is when the conversation is about interplanetary ejection burns and/or orbit insertion burns. It is usually stated that performing the burn closer to the planet is always better. However, for low orbits very close to a planet, the usual thinking isn't always correct. A transfer trajectory to another planet requires a certain hyperbolic excess velocity, denoted V∞. This is the velocity left over after a spacecraft escapes a planet's gravity. The V∞ required to reach a specific destination during a specific launch window is the same regardless of the orbital altitude from which the spacecraft is ejected. The equation for V∞ is, V∞2 = Vbo2 - Vesc2 where Vbo is the burnout velocity and Vesc is the escape velocity. Vbo is the initial orbital velocity plus the ejection Δv, i.e. Vbo = Vorb + Δv. Therefore, by substituting and rearranging the equation we get, Δv = (V∞2 + Vesc2)0.5 - Vorb Of course, Vorb and Vesc are both functions of the orbital altitude. Therefore, for any given orbital altitude and V∞, we can compute the Δv required to eject our spacecraft. Let's take the example of a spacecraft on a transfer trajectory to Duna where V∞ = 900 m/s. Computing Δv and plotting it on a graph, we get Note that it initially takes less Δv the higher the orbit (significantly less in this case). It is not until we pass an altitude of 8145 km that the Δv begins to increase (i.e. the Oberth effect). The shape of the curve varies dramatically with different values of V∞. For instance, lets' say we are going to Jool during a launch window where V∞ = 2800 m/s. The graph now looks like this, Again, the Δv initially decreases with increasing altitude, though the decrease is much less than the previous example. Also note that the Δv starts to increase just past an altitude of 303 km, which is much lower than the Duna case. What these results don't take into account is that, although it may require less Δv to eject from a higher orbit, it takes more Δv to reach the higher orbit in the first place. When we add together both the launch Δv and the ejection Δv, the lower orbit results in less total Δv. When planning a mission, you can always use Alex Moon's Launch Window Planner and adjust the orbit altitudes to see how the ejection and insertion Δv change. However, note that when arriving at a planet, the orbit that you want to enter into will be the one that produces the lowest overall mission Δv. You don't want to enter into an orbit just because it has a low insertion Δv if its going to make subsequent maneuvers more costly.

In the numerical example that Jason and I presented (independently but essentially identical), we're suggesting that the rocket follows the exact same ascent profile in both the low apoapsis and the high apoapsis scenarios (the AoA doesn't change). The only difference is that in the high apoapsis scenario, the rocket must burn its engine a little longer (probably about 6 seconds or so). This makes virtually no difference in the gravity losses because the extended burn comes near the end of the ascent after we're essentially already in orbit.

You are correct. Any time you can combine a plane change with an altitude change, you save considerable dV over performing the two maneuvers separately. It is simply a matter of how the vectors are added. (ETA) For example, let's say you are making a 100 m/s altitude change and a 100 m/s plane change. If you do them separately it's 100+100 = 200 m/s. If you combine them it's (1002+1002)0.5 = 141.4 m/s.

That's the same thing I do. I test everything that I can test under simulated conditions on and around Kerbin. What I can't test directly, I use physics/math to verify a design in principle. I've never used hyperedit, though that's not because I consider it cheating. I just prefer to devise other ways to test my craft.

Holy crap! Your example is virtually identical to mine. The only difference is that I figured an initial burnout altitude of 45 km while you used 40 km. It looks like I beat you to it by 6 minutes. But seriously, it's great that we got the same result and arrived at the same conclusions.

#1 is better but the difference is insignificant. Below is a numerical example. The effects of the atmosphere have been ignored and all ÃŽâ€v are assumed to be applied instantaneously. Let's say we're launching into a 71 km circular on way to eventually establishing a 250 km circular orbit. Let's say that during ascent we cut our engine at an altitude of 45 km, leaving us in a temporary orbit with an apoapsis of 71 km and a periapsis of 0 km. By the time we coast up to our apoapsis, we require a burn of 65.0 m/s to circularize at 71 km. We then perform a second burn to boast our apoapsis to 250 km, and then a third burn to circularize at 250 km. These second and third burns require ÃŽâ€v of 131.2 m/s and 123.7 m/s respectively. That's a total ÃŽâ€v of 319.9 m/s to establish the 250 km orbit, not counting what it took to get up to our initial 45 km cutoff point. Let's now say we travel up to the exact same cutoff point as before, but instead of stopping we continue to burn prograde until we boost the apoapsis all the way up to 250 km. This takes an additional ÃŽâ€v of 167.9 m/s, leaving us in a temporary orbit with Ap = 250 km and Pe = 37.3 km. After coasting up to apoapsis, we require a burn of 151.4 m/s to circularize at 250 km. That's a total ÃŽâ€v of 319.3 m/s. So you can see in this example that the difference is a negligible 0.6 m/s. I would recommend that you just do what is most comfortable to you and what best suits the circumstances. There is really no big advantage one way or the other in terms of ÃŽâ€v.

If I need to perform a mid-course correction, I usually like to do it when my spacecraft is 90 degrees from planet intercept. At this location we get the most effective use of normal/anti-normal velocity changes if needed. A MCC at the 90o location and another just inside SOI usually works pretty well for me.

I generally burn straight up until I a reach a vertical velocity of 50 m/s, then pitch over about 10-15 degrees (to a pitch of +75-80o). I then decrease pitch as needed to gradually lower my flight path angle. I want my flight path angle to be about +45o when I reach an altitude of 7 km, and about +20-25o when I reach an altitude of 20 km. Engine cutoff typically occurs at an altitude of about 20 km when the apoapsis altitude reaches 55-60 km. (You can burn longer if you want a higher orbit.) I then finish off orbit insertion with a burn at apoapsis. This profile should put you in orbit for under 1400 m/s.

The following is something I posted before that I think helps to answer this question. This test was performed prior to v1.0, though that really shouldn't matter. I just added the "burn time" column. As you can see, the losses are pretty small for burns lasting just a few minutes.

I don't know what other people do, but I usually target a Pe of about 42 km from LKO, about 30 km from Mun/Minmus, and about 25-30 km from deep space. The higher the entry velocity, the lower the Pe. These numbers came from experiments in which I sought to minimize the g-load and heating.

For Mun I typically set my maneuver node about 140 degrees to retrograde, and for Minmus about 100 degrees to retrograde.

You can always use the Launch Window Planner to see what the ÃŽâ€v difference is between orbits of different altitudes. However, that's only half the story. You also have to consider that it takes more ÃŽâ€v to get into a higher parking orbit in the first place. For low Kerbin orbits (up to ~200 km), a ballpark approximation is that is takes about 1 m/s more ÃŽâ€v to get to Duna for every 1 km increase in the parking orbit altitude. For Jool it takes about 1.5 m/s per 1 km.

I've noticed that that often happens when the velocity relative to target goes to zero or near zero. It usually comes back as soon as I give the vehicle a little impulse. Don't know why it happens or how to avoid it.

An ablator is a material, usually some type of resin, that burns away during atmospheric entry. As the ablator burns and vaporizes, it cares away with it the unwanted heat. The remaining ablator also insulates the spacecraft. Having a high reputation means that you are offered more lucrative contracts.

I don't remember what my high water mark was, maybe about 40-50 with most of those being flags. I typically don't like to have a lot of stuff in orbit because it clutters up the map view. I usually keep old stuff around only if I can get something useful out of it in the future, such as utilizing an old probe to fulfill a new contract for some quick and easy cash. Otherwise I like to terminate stuff to keep the clutter down.

Doing something that you are comfortable with is certainly important. For instance, I know that the method I use to land on airless bodies is not the most efficient (neither is it the least efficient), but I'm comfortable with it. When I feel relaxed and confident in what I'm doing, the chances of success are much greater. If I started out with a TWR of 2, like you, then that might be true. However, by starting out with a low TWR of, say 1.2-1.4, then I'm actually flying slower through the thickest part of the atmosphere and drag losses are low. My TWR is rarely above 3 by the end of stage burnout, and by that point the air is significantly less dense. Keeping below terminal velocity was important prior to v1.0, but with the new aero model, my launch vehicles never come close to terminal velocity, even at full throttle. I agree that stability at staging can sometimes be a problem, but I find that if I set SAS to prograde hold through the staging cycle, this keeps the rocket plenty stable. And if not, then I have a poorly designed launch vehicle and its back to the VAB for modifications. I generally have little problem getting a nice gravity turn and a modest circularization burn. I'm not trying to convince you to do anything different or to move outside your comfort zone. I'm just pointing out a competing design philosophy. The more ideas we can toss around the better. That does make a difference. If you are able to recover the cost of your engines then there is less need to design for economy. My launch vehicles are typically disposable, so I need to pinch pennies.

I agree. I rarely have a mission failure because I take the time to do the math and carefully plan things out ahead of time. My greatest satisfaction comes when I have a mission that goes exactly as I planned it.

I advocate that everyone play the game they want to play and have fun doing it. That being said, from a payload and cost perspective, I don't think you should ever throttle back a liquid fueled engine. If it is necessary to throttle back, that just means the engine is bigger than it needs to be. The biggest payload that a particular engine can lift into orbit comes when you load up on fuel to the point that you bring your liftoff TWR down to nearly 1.2. You then run at full throttle the whole way. The low TWR results in significant gravity losses, but with all that extra fuel (which is cheap) you can burn longer and lift a big payload for a low unit cost.

I purposely avoided mods for the first several months that I played the game because I thought they were kind of cheaty. However, after performing the same computation by hand for the hundredth time, there's not a whole lot left to prove anymore. Okay, I've proven to myself that I can plan, build and execute a mission without the assistance of mods, but that doesn't mean I want to go through that every time I play the game. I've since added a few mods to help alleviate the grind and make the game more enjoyable to play, most notably Kerbal Engineer Redux, Kerbal Alarm Clock, and Precise Node. I certainly do not consider it cheating. In fact, I would advocate making those three mods stock.

Don't get hung up on trying to minimize ÃŽâ€v; that's the wrong way to go about it. You should really set as your goal minimizing the cost to deliver a unit mass of payload to orbit. Below is a recent thread in which we discussed this; it should answer many of your questions. http://forum.kerbalspaceprogram.com/threads/134299-Launch-Vehicle-Optimization-Test-Results The most cost efficient launch vehicles are not the most efficient in terms of ÃŽâ€v. But who cares? Minimizing ÃŽâ€v doesn't get you anything.

Note that in science mode building upgrades are not applicable. Science mode beings with all the buildings at level 3. Building upgrades are required in career mode only.