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I assume you mean due to the cosine loss of slightly off-angle engines? You can angle them back to straight if you like - the "F" key while using the rotation widget ("3" key in the VAB/SPH) will help a lot. But cosine loss really is negligible under a few degrees anyway.

True, but the cosine loss due to gimbaling is microscopic (unless you're talking about something with a huge gimbal, like the Vector)-- the cosine loss due to a 2-degree deflection is just 0.06%, and even that much is only when it's actually deflected and not pointing straight. It's so tiny as to be utterly negligible. My own rockets are almost always three stages to orbit: SRBs, to lift off the pad High TWR liquid-fuel engine to get out of the atmosphere and start the circularization burn Low TWR, vacuum-efficient liquid-fuel engine to complete orbit. #1 can't gimbal at all, 'coz SRBs. #3, I usually disable gimbal completely, since it's a slow, low-TWR burn with no atmosphere to contend with and plenty of time to get pointed in the correct direction before I start the burn, and reaction wheels are plenty for doing any orientation adjustments. (I've been sorely tempted to use ModuleManager to tweak the Terrier and Poodle to have locked gimbal by default, since I almost always want it locked and equally frequently forget to do so in the VAB.) But I do like to have some gimbal in #2. It's a handy adjunct for maintaining stability during the high-speed atmosphere transition from around 10-15 km up out of the atmosphere, like having steerable fins. (And my 2nd stage is often constrained from having much in the way of fins, because it's up higher on the rocket when it's sitting on the launchpad, and putting fins would destabilize the rocket during stage #1. Engine gimbal is a way of gaining some stability without fins.) Emphasis on "some". Too much, and it thrashes madly and is really irritating, regardless of practical consequences. So I often find myself reducing gimbal, but it depends on the ship design-- specifically, very tall ships need less gimbal, because the engine is farther behind the CoM and therefore the gimbal produces more torque. A common stage-2 for my larger ships will be a Mainsail under a Big Orange Tank, and in those cases I almost always cut gimbal down to around 25-30%. For smaller ships (like a Swivel under the 4-ton LFO tank), I generally just leave gimbal as-is because it doesn't seem to hurt anything. The primary case where I deliberately leave gimbal cranked way up is if I'm launching something that has a really awkward payload that's impractical to streamline, so I know that my aero stability will be crap, in which case full-power gimbal helps to make up for the ugly ship design and just muscle the ship onto prograde.

I have never found any desirable outcomes for rockets using gimbal. The effect of gimbal is to make the running engine act as a boat's rudder, causing a curve in the trajectory while under thrust. This is normally counterproductive because burns are all vector addition and efficiency is maximized by burning in a single direction at the appropriate time. Any off-line burning results in "cosine loss". Thus, the only possible benefit of gimbal on rockets is to counteract a curve in the opposite direction (such as due to mounting the engine out of line with the rocket's CoM) so that the net thrust is in the desired direction, with the cosine loss accepted as the price for making the lop-sided design work at all. I never make lopsided rockets, and I don't make structurally wobbly rockets, either. As a result, for ROCKETS, I find gimbal useless for its intended purpose and have no need to turn it down to avoid structural wobbles. So most of the time I just ignore it completely I pick engines based on thrust and Isp, and either they have no gimbal or the amount of gimbal has no noticeable effect either positive or negative. Thus, I never think about gimbal at all most of the time. There are, however, a few engines that I might use for their combination of thrust and Isp, but which have stupid amounts of gimbal. Like the Vector. If you don't actually NEED the extreme gimbal to balance a lop-sided shuttle or something, it will throw even a solid, monolithic rocket around all over the sky during a burn. So with such engines, I always disable the gimbal entirely. The only other time I mess with gimbal is for SSTO spaceplanes. I find that allowing gimbal to work in the atmosphere makes it pretty much impossible to for the spaceplane to hold the desired heading and AoA, and even harder to force it back into the desired attitude once it gets off it. So I always disable the gimbal of spaceplane engines. Now OTOH, if I'm making an atmospheric stunt plane, I want lots of gimbal (aka thrust vectoring) to make the plane more maneuverable. But with these, I'm not concerned with holding a specific orientation (usually I'm not on a steady course at all except during takeoff and landing).

This is a statement whose foundational logic is in error. The ISS is at an inclination of 51.6 degrees and Cape Canaveral is at 28.6N Boca chica is at ~26'N. During every orbit the ISS crosses 28.6'N and 25'N once as it travels north and once as it travels south. At a semi-major axis of 404km in altitude (a = 6776000 m, v = 7669.76 m/s, P = 5550 sec, f = 31/day) the ISS, on average, crosses either launch site and any launch site between 56.0'N and 56.0'S passes within 5.8 degrees along the east west (or north as one approaches 51.6 degrees north). The reality however is the 51.6 - 28.6 = 23' or 51.6 - 26' = 25.6 means that the distance in absolute degrees is ~ 2.26' at CC and ~2.59' at BC. Given the earth is 40,000 km in diameter this is a distance from ISS course (DfIC) of 251 km for CC and 288 km for BC. However if one looked at windows every 2nd day or 4th day the DfIC would be half or a quarter. But lets just use the daily value. Lets say that 251 km puts the two vessels at 0.351' and 0.412' orbital inclination to each other. To correct this in orbit (the worst case scenarios) one would need a burn dV of 46.9 for CC 55.72 m/s for BC, a difference of only 8.73 dV higher for Boca Chica. However, the western launch from Boca chica has a higher contribution from Earth's rotation. 40,000,000/86400 = 462.6 meters per second on any east west departure at the equator. For CC its 406 m/s and for BC its 415 m/s, since both intercept trajectories will be traveling ENE or ESE to intercept there are cosine losses 373.3 for CC and 374.3 (and yes I am aware of the east launch restriction, but also don't forget due east is only restricted to 320 km downwind). Consequently we can calculate the dV required to reach orbit for any average day if we stipulate that drag and gravity losses equate to 1200 dV. Boca chica launch site is about 8 miles from the Mexican border so the maximum variation is 2.92' South which means at 200 miles it would only need to turn 23' South. SQRT(2 * (7669.7^2/2 + u/6371000 - u/6776000) ) + 1500 = 8142.3 + 1500 = 9642.3 dv. For CC it would be that + 46.9 - 373.4 = 9315.4 m/s for BC it would be 9323.72 a difference of only 8.32 dV. Remembering that we have to make an orbital inclination burn anyway of 0.412' orbital inclination burn, but we also use more momentum from the Earth. One could after traveling 183 miles eastbound heading south at 25.6 degrees, the horizontal orbital velocity at that point would be 3000 m/s so that the turn would cost 1296 m/s however the cost could be lowered if and agreement could be reached with Mexico allowing it to change course over the commercial zone. However in choosing an east to west launch one gets back 30 dV of the cost. The other alternative is to simply catch the ISS on the rise, since the first stage would be captured in the gulf of Mexico, the only down range risk is the failure of the second stage (which it will either fail at firing or quickly be out of risk for the continental US.). Space Law applies to any vessel above 100 km; however, and this has major importance on such a course change. Because the ISS is so high now, 404 km, one can choose a vertical course steeper, and essentially fly over Mexico's internationally recognized airspace of 100km with a surface velocity of 600 m/s (and 415 = 1015) Since. this will occur pretty close offshore, you could launch to the east get about 300 or so horizontal velocity and turn to 120' the cost of the 30' change would be a loss of 200 dV and (increased because of the loss of some of earths rotation that contributed with east to west take-off). In the grand scheme of things that would only be an increase in Boca-chica versus Canaveral of 2.2%. Ok that is cleared up. The same is also true for GSO orbits, since they are so high, there is no general benefit to try to cling to the Earth, However, Boca Chica is better launch point for Equitorial GSO than any launch site in the continental US. However because Space X has a barge technology they can simply barge a rocket from the Port of Brownsville, haul it to a launch site and launch it, this could be done from any port that has access to the rocket assembly technology. Finally, the Sun, the sun (tropic of cancer) approaches Boca-chica from the south, Once June 21st this orbit 3 33'86'' from the Boca Chica site at Midday, it can be also reached at Midnight on December the 21st. Both can be accessed from a due east flight and this allows for launches to other targets outside the solar system. During spring it is as great as 25.8 degrees to the south (or north if launched at midnight). Boca chica is a better launch site for orbits that travel out of the planetary system, it captures more of the Earths momentum (9 dV more and needs to make a smaller dV change to achieve a decent orbit for escape from our system, however because of the dynamics of the Earth moon system this needs to be precisely calculated. Boca chica is not suitable for polar orbits, those are best launched from Siberia or Alaska. Biggest problem with Boca Chica. The launch site they are building on is the flood plain of the Rio Grande river, its not particularly stable soil (I know this from very unpleasant personal experience). They will have to spend alot of time stabilizing the soil because of the large amount of sulfides and other unstable organic materials that have made the sand spongey. It is hard to imagine the amount of sediment at the site, but just consider the length of the rio grande and the terrain it has cut through.

Having an issue with the Thiokol (ATK) GEM 40s. They seem to subtract thrust rather than add it. When I attach them to my delta II core, when I have MechJeb set to include cosine losses, it shows the thrust as decreasing, and when I turn the cosine option off, the thrust goes back to what it should be like. This is also true on launch, in that the rocket won't leave the launchpad until the cosine loss included TWR goes above 1. Haven't tested this with the other boosters yet, but will look into it. Edit - Also, the node heights on the RS-27A seem to be a fair bit higher than they should. There's about a 30cm gap between the top of the engine and the fuel tank above it. Edit 2 - Turns out the node problem is with all KW engines. Doesn't seem to apply to any of the other engines.

The sources of inefficiency: Cosine losses Gravity losses Poor use of Oberth effect So to get the best efficiency, you want to optimize all three: Burn only surface-retrograde, so cosine losses are zero. Do a suicide burn, which minimizes gravity loss and maximizes Oberth effect. Approach the surface almost horizontally, which gives good Oberth effect. In practical terms: From a very low orbit, you only need a trivially small retro-burn to drop your trajectory to intersect the surface. Then just do a suicide burn to the surface. That's as good as you'll get.

As long as you don't change SoI it is always more efficient to do multiple burns. The efficiency loss you are alluding to is your choice of sub-optimal prograde burns away from Pe or cosine losses with an averaged burn. Peak efficiency is prograde exactly when velocity is highest. It is all about compromise. I've heard 1/6 an orbital period quoted and that seems a fairly reasonable compromise between cosine losses and burn time.

Well, sure, if you're doing the launch-straight-up thing. (Note that when I say "launch straight up", I'm also including your description of doing a little-bit-of-a-gravity-turn, since you're not accomplishing much difference by doing it just a little. Unless you're already going nearly horizontal by the time you leave Kerbin's atmosphere, you're missing most of the benefit of the gravity turn. For maximum effectiveness in getting to Jool, you want to be traveling Kerbin-prograde at the time you leave Kerbin's SoI. So if you're launching straight up (or nearly so), naturally time-of-day makes a big difference because that will affect the direction you eject from Kerbin. On the other hand, if you do a gravity turn and then circularize in orbit, you can set a maneuver node at your leisure, so that lets you get everything lined up just so and it doesn't matter what time of day you launched the rocket. On the third hand, if you do a gravity turn, and you're relying heavily on SRBs so you can't circularize and just burn until you eject, then yes, it would once again matter what time of day ...though in general the "right" time of day would be different from a straight-up launch. For a straight-up launch to go to Jool, the right time of day would be just before sunrise. The more you curve, the earlier you'd launch. For a gravity-turn-with-horizontal-departure-from-atmosphere, you'd want to launch a bit before midnight if you're not circularizing. Actually, if you're launching straight up (or doing just a slight turn, so you're still going mostly vertical when you leave atmosphere), then fighting gravity is exactly what you're doing. You're avoiding cosine losses by following prograde, but you're not avoiding gravity loss. The way to stop fighting gravity (and avoid the gravity loss) is to do a harder gravity turn so that you're spending most of your dV going mostly horizontal. If I'm reading your screenshot correctly, you're basically launching on 56 Kickbacks all at once (seven of them in a row, with 8-fold symmetry), with fairly neglible mass besides those (since the Kickbacks alone will be 1344 tons). I assume you've got their thrust cranked up all the way to 100%, which is the default. That would give you a TWR on the pad of about 2.5. If you're launching with a decent gravity turn, you can get by with a considerably lower TWR, and use additional staging to save. For example: Suppose you set up the staging (and add some radial decouplers) so that instead of firing all 7 Kickbacks in each row straight from the pad, you fire 5 of them. In other words, your rocket would be built with 8 decouplers around the central core (the way it is now), then each row would have 2 Kickbacks, then a radial decoupler, then the remaining 5 Kickbacks. With a scheme like that, you still have 56 Kickbacks, but you're not using all of them at launch; you'd only be using 40 of them off the pad. This would give you a TWR of 1.8, which is plenty good enough if you're doing a gravity turn. You'd nudge it to start the turn practically right off the pad. By the time the first set of 40 Kickbacks burn out, you'd already be going fast, high in the atmosphere, going mostly horizontal. When those first 40 Kickbacks burn out, you'd stage them away and then fire your 16 remaining Kickbacks, sending you on your way. You don't have to do the above, of course.... but you'll find that if you do, you'll get a lot more bang for your buck. Not only will you be saving scads of dV due to not fighting gravity, but you also won't be lugging the dead weight and drag of those first 40 Kickbacks all the way to space (that's significant-- that's 180 tons of dead weight you'd be shedding), and as an added bonus, your final 16 Kickbacks will be doing all their thrusting in a near-vacuum where their Isp is significantly higher than it is on the pad.

First of gravity losses: Mathematically, you are in a orbit while landed. It is just an elliptic orbit with you getting 1g of radial acceleration at Ap (orbital velocity equal to surface). Gravity losses are off prograde thrust (also known as cosine losses) and thrust inefficiencies when not burning at Pe (Oberth effect) or Ap (Kepler observations). All you need to do to minimize gravity losses is to efficiently transfer from that elliptic (sub-)orbit to a higher, circular one. The complication is that you have atmosphere and a relatively low TWR craft for the dV the manuver needs. Low atmo is full throttle unless your aerodynamics are a drag. Drag will sap around 600 m/s on a good rocket ascent! That's more than any reasonable cosine loss. When you GTFO of the low atmo like that, you tend to be on an overly high gravity turn trajectory. You want to reduce TWR so that your trajectory allows you to circularize your orbit without losing much speed from coasting. The best way is to stage to a lower TWR configuration, but SSTOs are stuck reducing throttle. My most efficient SSTO to date is a nuclear space plane. It has only enough oxidizer for the RAPIERs to bring Pe to approximately -200 km. That kick is just enough for the .4 TWR nuke to circularize orbit by the time it reaches Ap (60 km) while burning at full throttle (at which point the low TWR is efficient)

Here is a place where opinions differ. Looking upon that machine, I see a thing of great beauty. Also, I wouldn't be so worried about that thrust angle. Cosine losses only really get icky around 25 degrees, this looks to be maybe 15, which gives a loss of barely over 3.4%

I read through your whole description very carefully and it's quite cogent and comprehensible. I like this point (quoted above) because it's another technique in the armory to consider. And I think an experienced pilot will use an amalgam as circumstances indicate. My approach is not to keep the engine running, but to turn and use small thrusts to shepherd the markers where I want them. With experience, this can be done without over-controlling and, generally, the off-pitch angle ["cosine loss"] is not great. It keeps the approach moving faster until the end (which is good). It also becomes a bit more necessary when you're intercepting a target that is NOT on the equator. In my thinking, where you point the nose and use the throttle in combination repel the marker that indicates direction of flight. That one in turn repels the target location. You are the nose. Your travel is the sheepdog. The target is the sheep. And the "zenith" is the pen. Good teamwork does not chase the sheep all over hill and dale but gently coaxes its trend. Really appreciate your input and experience on this [fanatical] subject!

Not at the moment, no. Actually, the "correct" way to do it requires no trigonometry, it just needs all the thrust and deltaV related calculations to be done as vectors rather than scalars (because force and velocity are both vector quantities). Simply calculating a cosine loss against the primary axis of the vessel (or any fixed axis) assumes that all the stages are fired with the vessel pointing the same way. Doing it all as vectors allows it to easily account for very odd designs, e.g. with a first stage that points up, and a second that points sideways where you will change the orientation of the vessel between the stages, a simple cosine loss calculation will get 0 dV for the second stage. Doing it as vectors per stage makes a "smaller" assumption, that each stage is burned completely in one direction but each stage can be in a different direction. The calculated deltaV for each stage will be a vector and, when totalling the deltaV, the magnitudes of the individual stage vectors will be added. I plan to do this fairly soon though there are a number of other things I'll be doing first.

That's true, but it is, in practical terms, often much easier to get the airspeed feedback loop going first, when you're low down, because even if the engine's pressure curve says it can operate more efficiently higher up (on a thrust/drag basis), as you get higher up it has to compensate for the loss of lift (hence flying at higher AoAs and/or requiring higher control deflections, both of which are murder on your drag profile, and can cause cosine losses and impact your intakes as well). This is in my experience particularly noticeable with planes with relatively low liftoff TWRs, i.e ~0.3.

The amount of Oberth effect you get is proportional to reaction mass. Therefore, it really helps LFO rockets, is meh for nukes, and practically non-existent for ions. The OP is using nukes and ions, so Oberth is not of great concern to him, but cosine loss is.

Certainly that can be an issue if you just point and ignore as it changes. But long, low-TWR burns are less efficient even if you've got SAS set to hold (in which case there wouldn't be any cosine losses). The real (unavoidable) limit is Oberth loss.

Returning to the subject of engines for a true Minmus SSTO - Jet Engines I's a balancing act , because the more of these you fit, the faster/higher you can go on jet power before having to call in the comparatively thirsty rockets. RAPIER ISP (Airbreathing) 3200 NERV ISP 800 RAPIER ISP (Closed Cycle) 305 The problem is each RAPIER is 2 tons. That's a LOT of mass that has to be boosted from 20km to orbit and then on to Minmus. What's a good compromise ? At the very least, I think there should be enough jet power to reach mach 3.7. As you are probably aware, RAPIERs deliver more and more thrust in jet mode as you get faster, but there is a peak - mach 3.7. After that, thrust declines even if you stay at the same altitude. At mach 4.5 they loose 20% of their power compared with what they do at 3.7. After that the decline steepens considerably, thrust falling to zero at mach 6. To be honest, if mach 3.7 is your target you may find it hard to pass the sound barrier in the first place. I have to climb above 10km, then set Prograde on the autopilot and temporarily activate the nukes, to accelerate from 0.86 to 1.3 mach on some of my designs. Closed Cycle Power I find that with my 35 ton mark 2 designs, there's about 75kn of drag in the mach 4-5 speed range at 24-33km. At some point, the thrust output of your jet engines is going to decline so that it barely exceeds this. If your jet engines have four times the ISP of your nukes, but 75% (or more ) of their power is just being used to stand still, it's worth firing up the nukes - Jet power 100kn (one Rapier) @ 3200isp Nuke power (2 NERVS) 120kn @ 800isp Drag 75kn Excess power - 145kn "gravity loss" - 34% isp (averaged across all 3 engines) 1891 ISP This is a better situation than having only the jet running @ 3200 ISP but loosing 75% of it to gravity. Unfortunately as you get faster and higher, the jet engine produces less and less till it flames out completely @ 29km. Then you go through a tough patch where you have 120kn thrust @ 800 isp, with 75kn drag taking away more than half your gains. However, as you edge towards mach 5.5 and 33km the orbital freefall effect increasingly supports the craft's weight. The wings are able to lift you higher and higher into thinner and thinner air without requiring excessive and draggy AoA to do so. By this point your drag will be at least halved and things continue to get easier until your AP heads out of the atmosphere at around mach 6.6. So, at this point, you start asking if I really need 2 nukes. They are even heavier than RAPIERs, at 3 tons each. Our closed cycle propulsion system has two jobs to do - 1) boost your airbreathing speedrun 1250m/s to orbital velocity of about 2250 m/s (+1000 m/s speed increase) 2) lift the craft from LKO to Minmus and land on it(at least 1000 dv?) If 1) is easier with more nukes, due to reduced need for oxidizer/less gravity loss, 2) is merely penalized by the weight. On a low gravity body like minmus, one engine is more than enough! Obviously, the problem with cutting down on nukes is that you will end up with less thrust than drag once the jet engines fade. You will never be able to reach the magic 35km/mach 5.5 halfway point where drag starts falling rapidly. To get around this, people switch their RAPIERs to close cycle mode and start burning oxidizer. There's three problems with this - The RAPIERs only give 305 ISP in Closed Cycle mode which is just awful. You no longer benefit from the residual jet thrust from 22-23km all the way up to 29km @ 3200 ISP The vast increase in acceleration as you go from 60kn (or less) per engine to 180kn can cause you to go too fast too low and encounter excessive drag. Or you try pulling up hard to get around this, and create lots of drag with high AoA/G forces as well as cosine losses from thrusting directly downwards against gravity. As far as point 1) goes, the fuel and oxidizer burned as a result, will weigh more than a nuke engine. But, as Aegolius13 points out, you're only lifting this mass to 35km, not taking it to Minmus and back. He further states that this oxidizer was lifted by "cheap" jet power, though I'd like to point out that an optimised design is probably already lifting as much weight to 20km as it can with jet engines, so this oxidizer does not come "free". More oxidizer means bringing less LF for the Minmus insertion burn, or tacking on another RAPIER engine (another 2 tons to carry up to Minmus and back). To me, it seems there's an obvious solution to 2) and 3) - don't switch all of your RAPIERs over to closed cycle ! Imagine a somewhat larger 70 ton Ship, with 2 LV-N Nukes and 3 RAPIER - when the climb rate gets too sluggish, switch only the centreline RAPIER over to Closed Cycle. You'll have 180kn from the Closed Cycle engine and 60kn each from the two nukes, plus whatever dregs of power the outboard RAPIERs can still manage in air breathing mode. Since this craft is twice the size , drag will be double at 150kn - but we'll still have at least twice as much thrust as drag. 70 tons doesn't seem an unreasonable size given the weight of IRSU gear, and given the very low payload mass fraction possible to Minmus when fully SSTO.

Agreed, but gimbal can obviously deal with some or all of this. D2 was never going to take off from Mars, it was a 1-way trip, so TO not an issue. ITS(y) has engine gimbal, so it can cant them out for entry or intentional cosine loss as a form of throttling, or along centerline for TO. (why would autocorrect change along to alone? Sheesh)

Basically you can't. To launch into a given orbit you need to be below it at launch time. Which means the target orbit need to be inclined enough to reach the launch site's latitude. Which means launching from the pole the orbit will be always have 90° inclination. If you are at the north pole the only direction you can go to is south and vice-versa. The only way is a plane change, either during the launch (which means lots of cosine loss due change in direction of thrust) or after orbit acquired (which means a inefficient burn perpendicular to the velocity).

I'm not sure if that is exactly what you are planing to do but assuming it is, and based : 1.launching from kerbin: 3,4km/s 2.transfer to minmus: 930m/s 3.Minmus orbit: 160m/s 4.Refuel: -4490m/s 5.Transfer to kerbin: 160m/s 6.transfer to Eeloo: 200-300m/s (???I'm just guessing there) Actually its not difficult to find by yourself how much the transfer burn will cost. (if you have access to KSP, which unfortunately I don’t have right now) Put a small craft a 75kmx75km orbit around Kerbin , raise the apoapsis to the orbit of Minmus, plan the transfer at the periapsis. Off course Yu need to make all the usual considerations about timing, ejection angle, cosine loss.

On the other hand, for the original problem of satellite drifting, it can be solved without "cheating" like this - bring one Ant engine, set thrust limiter, use throttle, pointing to any 4 of the directions other than prograde/retrograde (maximizing cosine loss, essentially). Then it's just a matter of adjusting semi-major axis. I use KER, and with its way of displaying that number, I usually target 999,999.999m SMA for most of the bodies. You can try to calculate how's the drift around for example Minmus for such 1Mm+-0.0005m orbits. I'm surprised myself when I look at that number.

I have been playing KSP since 0.20. I've been to all the planets and back more times than I can count. And I have never once had a good experience with engine gimbal, so I always disable it. I do this for perfectly logical and valid reasons, and with only 2 exceptions I cannot find any logical or valid counterarguments in favor of using gimbal. And even those exceptions are minor special cases that can easily be handled without using gimbal. Thus, I am very curious as to why so many folks seem to think gimbal is a good thing, and why so many engines have it when it does so many bad things. Here is why I find engine gimbal to be not only useless but counterproductive in the vast majority of cases: First off, let's be clear on what gimbal is. It is the rotation of the engine nozzle off the centerline of the ship. As a result, gimbal creates off-axis thrust, which necessarily induces a rotational moment in the ship while the ship is already under thrust from the same engine. This causes the ship's trajectory to curve. Because of this, the game uses gimbal as a primary control mechanism accessible via user WASD and SAS, the same as it treats torque, aerodynamic control surfaces, and RCS thrusters. The question then arises: When is it desirable to curve your trajectory while you're under thrust? There are only 2 situations that answer. Doing a gravity turn during launch Doing aerobatics or ACM in a highly maneuverable airplane In ALL other cases, you want to your ship to be going in a straight line while under thrust. This includes all orbital maneuvers, the ascent of SSTO spaceplanes, and airplanes trying to cover long distances. Gimbal is counterproductive in these situations because SAS uses it like any other control system to handle the very minor course adjustments necessary to keep the ship pointed in the desired direction. This results in wobble due to the frequent application of off-axis thrust, which will amplify and be amplified by any structural wobble in the ship. While it's true that in 1.1.x you can now tune the amount of gimbal to reduce such wobbles, why even bother when you really don't want gimbal to begin with? It's far easier to just disable it and have enough other control authority (torque, RCS, control surfaces) for SAS to do its job. Besides, every time the engine gimbals, you lose fuel efficiency due to cosine loss, which is another reason not to use it. So now let's look at the 2 special cases: gravity turns and stunt-flying. In both cases, gimbal can be substituted for by other control systems. And in the case of gravity turns in particular, the excessive control authority of gimbal (at least without spending a lot of time tweaking it) is largely counterproductive again. So that leaves gimbal desirable only for stuntplanes. Now, if the game was smart enough to use gimbal as a TRIM mechanism instead of a primary control mechanism, my opinion would be very different. Then gimbal would be great for asymmetrical ships because it could keep the thrust always going through the CoM. But gimbal doesn't work that way so I absolutely hate it. So tell me, do you like gimbal and why?

Yup, point taken. Though there are some ways to mitigate this. First, you can (up to a point) split the burn. Instead of doing the whole burn in one whack, you can do a "reasonable" burn that's not too long (say, a minute, or a minute and a half) to boost your Ap somewhat, then coast around and do it again. You can't do that all the way, because there's only so much dV you can add before you're on an escape trajectory. But it does give you a place to invest a few hundred dV, thus reducing the size of the last burn. Also, it makes it easier to to do that last burn, because it's happening at the periapsis of a long elliptical orbit, so your orbital track is less curved, so you can do a longer burn without distortions/inaccuracies. The other thing is, even in those cases where you do have to do a really long burn, you don't do it by pointing at the maneuver node and burning. That would be inefficient (big cosine losses) and problematic (lower your Pe, possibly catastrophically). Instead, you point prograde (which will be higher than the node). It means a very dicey navigational job, since the ideal burn won't necessarily be a 50/50 split across the node, and judging that can be tricky. But it works well enough, especially with practice. What I usually do in such cases is do what I've just described, but stop burning after the burn's about 85% done, then kill the maneuver node and set up a new one a few minutes ahead of my craft. The savings in accuracy is generally worth the slight loss of Oberth benefit during those few minutes of coasting.

@nobodyhasthis2 fair enough; might as well leave it then. @Skalou in real life you go off-prograde all the time, indeed most modern LVs (and Saturn V when ascending to parking orbit) circularize after apogee. But in real life while the cosine loss for burning off prograde for 30s may be 50m/s, just as in KSP, you're talking about total steering losses of maybe 75m/s out of 9400m/s expended, vs KSP where that would be 75 out of 3200 or so.

[quote name='billkerbinsky']Just a random idea: Put the retrorockets around the perimeter, angled out slightly (at 30 degrees, you'll get (1-cos(30)) = about 14% cosine loss); the exhaust doesn't hit the upper stage, you get a nice pyro fountain effect, and if you put blow-off panels over the nozzles, a little confetti, too...[/QUOTE] See above, about why I didn't stick them on the outside. :) The problem with putting them interior at all, but close to the edge, is that they would then fire through whatever fairing sticks to the decoupler from an engine above. No good solution without making the ports completely external (in which case I'd angle them a little as you mentioned). I could still redesign it to do that. We'll see.

[quote name='NecroBones']For the MRS "low profile" decoupler, I've already added the solid motors, since that seems like a very stock-alike way to do it, which is the flavor of this pack. The nozzles are inside the ring, but exhaust damage is disabled for them. I thought about having external nozzles, but this is the "low profile" decoupler, meaning it shouldn't really have anything visible or protruding from the outside. For other sizes, and in SpaceY, I'll think over those options.[/QUOTE] Just a random idea: Put the retrorockets around the perimeter, angled out slightly (at 30 degrees, you'll get (1-cos(30)) = about 14% cosine loss); the exhaust doesn't hit the upper stage, you get a nice pyro fountain effect, and if you put blow-off panels over the nozzles, a little confetti, too...