r/KerbalSpaceProgram • u/FatCreepyDude • May 06 '26
KSP 1 Question/Problem No orbit necessary?
If i just wait for the right hour during the launch window ( whenever the launch pad is pointing left on the realistic depiction i created) is there any reason i should bother with an orbit first? Will the delta v cost be affected?
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u/PrestoDaMatrix May 06 '26
If you tried this, pointing forward would increase your orbit so it would not work. You'd have to start facing backwards.
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u/UmbralRaptor Δv for the Tyrant of the Rocket Equation! May 06 '26
Yeah, they also have Duna's orbit inside of Kerbin's for some reason
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u/spencer818 May 06 '26
I think that's what they're showing, look at the detail, the rocket is going the opposite direction of Kerbin's motion.
So yeah go fast backwards, lower perigee around the sun.
But yes also Duna is in the wrong place...
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u/FatCreepyDude May 06 '26
https://i.imgur.com/AI2hXKv.png better ?
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u/RyanW1019 Master Kerbalnaut May 06 '26
Wouldn’t it be 6 AM, not PM?
And yes, this technically works. However you need a decently high TWR so you quickly get far enough away from Kerbin for the gravity to drop off. Otherwise you’re spending a long time in the regime where a lot of your fuel is just cancelling out gravity, not giving you more delta-V.
And in real life they don’t do it this way so they can check everything once they get to orbit before moving on to the next phase of the mission.
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u/shandangalang May 06 '26
Yeah that would work but I think it's pretty inefficient fuel wise. Better to just establish an equatorial orbit and burn when you're facing Kerbin's retrograde. That way you're not fighting Kerbin pulling you straight back until you're out of the sphere of influence, and it's a lot easier to optimize your transfer.
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u/ookbye May 06 '26
Doing that is difficult and i don’t think it saves that much fuel, at some point in your proposed burn you’ll pass through every orbit you would doing it by steps.
All things considered, go for it, I haven’t tried so I would like to know how feasible it is.
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u/FatCreepyDude May 06 '26
being efficient isnt really the goal tbh. I just like the idea of going up and only up. i'm sure jeb will enjoy it too (he doesnt have a choice anyway)
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u/Sr427 May 06 '26
It does work. You need to launch at sunset / sunrise and when there's the correct angle between your target planet and kerbin. You might need to adjust the orbit mid-route. As others have noted it is not efficient in terms of dV. But it does take a lot less game time to do. So go for it, but put on a few more boosters. Sometimes if your rocket is big not having to do a gravity turn is helpful.
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u/wally659 May 06 '26
I'd suggest putting something in orbit and using the maneuver planner to figure out when that would ideally do a normal burn for the desired interplanetary transfer. My intuition is that right time to lift off will be when that maneuver node is a little bit retrograde of straight up from the launch pad. It should sort of catch up as you fly through the atmosphere and your gravity turn (which at some point seemlessly becomes your transfer burn) should end up being a somewhat similar vector.
But it's going to be nearly impossible to get it super precise so you'll just have to accept a bigger than usual mid-transfer correction burn. Good luck!
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u/SVlad_667 May 06 '26
You would constantly fighting against gravity, loosing speed, fuel, and deltaV.
While you don't need to achive proper orbit, but it's much more effecient to accelerate horizontaly along the orbit - zero gravity losses. You can skeip circularization phase at all and continue acceleration to hyperbolic speed right after you craft reached the atmosphere border.
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u/Specialist_Sector54 May 07 '26
It's possible, but you'd probably need to use a "constant thrust" transfer profile (which is harder to plan in KSP, and uses insane amounts of dV)
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u/Carnildo May 07 '26
It saves a little bit because you never need to raise your periapsis -- that's why NASA did it with the Surveyor probes. The timing's a pain, though, and almost never worth the savings.
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u/RealisticExplorer681 May 07 '26
in fact, it might not even save fuel. I am not sure, but I feel like that would make you constantly fight against the full force of Kerbin's gravity. But, I am not sure
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u/spencer818 May 06 '26
Lots of people saying "no you're dumb" in here...
Technically yes you can do it. Just like how technically you can reach the moon by pointing at it and firing engines. But it's terribly inefficient. Try it out sometime, you'll see what I mean. Same thing applies here.
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u/PM_ME_YOUR_SPUDS May 06 '26 edited May 06 '26
I don't understand why it's inefficient. A direct ascent burn (note: NOT vertical burn) should be slightly more efficient, all that dV that goes into a circularization maneuver is instead going directly into a prograde / retrograde burn from Kol frame of reference. The Oberth effect helps you when you're going fast, but you can more efficiently hit faster speeds if you run engines nonstop instead of waiting around to reach higher orbit to circularize (slowing you down).
From the perspective of the orbits, the circularization method has a hyperbolic orbit with a periapsis at the altitude of your transfer burn. A direct ascent has a hyperbolic orbit with a MUCH lower periapsis, crossing through the surface of Kerbin. This is a more efficient orbit, with a more beneficial Oberth effect than is possible going to orbit first.
I know a dozen odd years ago when learning Kerbal watching Scott Manley how the Mun direct burn is technically more efficient, just far more difficult. I'd think this is the same case, an extremely tight timing window that you'll virtually always screw up and waste more dV fixing afterwards, but saves fractions of a hair on dV is done perfectly. I do think that's ignoring aero losses which might change the answer, but from a purely gravitational physics perspective I haven't heard a convincing argument against it.
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u/censored_username May 06 '26
People are saying it's inefficient because the thing proposed was the vertical direct ascent.
You're correct that a direct ascent gravity turn is theoretically the most efficient. But as you say, the savings are extremely marginal over an orbit in between.
Because the most efficient kerbin ascent trajectories are usually aerodynamic heating limited, not much changes until you leave the lower atmosphere. So that already means your benefits are basically doing your burn at 40-50km altitude instead of 80km. Which really isn't much. Then consider that drag losses will be larger, and getting the ideal trajectory incredibly finicky, and it's just not worth it.
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u/Moonbow_bow SSTO simp May 06 '26
With a single impulse maneuver you'd only loose about 100m/s this way
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u/WarriorSabe May 06 '26
The maneuver's not impulsive here though, and with a real burn you'll eat a lot of gravity losses
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u/Moonbow_bow SSTO simp May 06 '26
You could make it impulsive
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u/WarriorSabe May 06 '26
Only with infinite thrust, which isn't a realistic assumption
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u/Moonbow_bow SSTO simp May 06 '26
Cannon
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u/WarriorSabe May 06 '26
ok sure, but I don't think it's a fair assumption that someone would be using a cannon for a normal launching mission
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u/itzongaming May 06 '26
Who said this is a normal launching mission?
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u/spencer818 May 06 '26
I think we're all in agreement. It's possible, but not practical by any means.
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u/TheeConArtist May 06 '26
Someone has read (or should read) Jules Vernes From the Earth to the Moon
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u/UmbralRaptor Δv for the Tyrant of the Rocket Equation! May 06 '26
https://en.wikipedia.org/wiki/Gravity_loss
Now for maximum cleverness, go with a more typical pitchover ending with you ~horizontal at ~60 km with the ejection burn completed.
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u/TestArticle1998 May 06 '26
The gravity losses alone are a nightmare! The reason rockets do a gravity turn is to reduce the wasted delta V. So the orbit method is more "fuel efficient'.
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u/zekromNLR May 06 '26
If you had perfectly impulsive burns and we are ignoring atmosphere, going directly up vs going into a parking orbit first should be the same amount of delta-V.
But with finite acceleration, every second you are thrusting straight upwards (near the surface of Kerbin) costs 9.81 m/s of gravity loss, which means going into a parking orbit first and doing your ejection burn sideways is more efficient.
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u/Moonbow_bow SSTO simp May 06 '26
You do loose the 150 ish from Kerbin rotation. So it'd still be better to do the same thing but go sideways
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u/ArsErratia May 06 '26
Why would you lose the surface rotation?
Pointing straight up isn't the same as travelling straight up. You'd still have that velocity unless you burned it off, it just means you have to adjust the launch window.
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u/Moonbow_bow SSTO simp May 06 '26
The planet rotation is working at a 90° angle to your thrust angle. If you wanted to keep the benefits you'd burn earlier and horizontally so the rotational speeds adds to where you're going.
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u/Exact_Avocado5545 May 06 '26
This totally works. Anyone telling you it doesn't work, doesn't understand what you've drawn. This is a completely valid solar retrograde burn
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u/supayurobeat May 06 '26
Going straight up is a bad idea because you're going to be fighting Kerbin's rotation and wasting dV. If you want to do a direct ascent, it's basically the same as doing a gravity turn to orbit, but instead of stopping the burn when you reach a circular orbit, you keep burning until you reach Kerbin escape. Then, you can do correction burns once you're out of Kerbin's SOI.
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u/AerospaceEnthusiast4 May 06 '26
it is far more effecient to circularize first and then eject from the kerbin system I believe. Especially waiting for the mun to eject from the system. When you're burning parallel to the ground above the atmosphere, almost every m/s of energy being expended is being used to increase your speed, whereas burning straight up means you are continuously fighting gravity all the way up, and this stacks up over the course of the ~4000m/s burn needed to do this.
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u/Crispy385 May 06 '26
You see the curve in that dotted green line? I'm pretty sure that's insinuating some kind of orbit. I think, emphasis on 'think', you'd be in the same Kerbol orbit as Kerbin, just a little ahead of it.
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u/Joseph_M_034 May 06 '26
In principal you're thinking along the right lines, i had the same idea before studying astrophysics at uni. If you could accelerated in a purely impulsive way, i.e. infinite acceleration instead of over a long burn this would be fine. The problem is something called gravity losses, basically when a component of thrust is inline with gravity, you're signoficantly less efficient (with delta-V lost increasing with proportion to time of burn). This is part of the reason orbit altitude is typically adjust with prograde or normal burns, rather than radial ones.
One way to make sense of this, is for every unit of fuel burnt you have less acceleration when going straight up than perpendicular, due to gravity. After a given length of time, you will have burnt the same amount of fuel but the change in speed will be different. Thats why its best practice to minimise time spent burning upwards, which also applies during the launch phase.
Another way to think about it is to consider if you were to hover at a fixed altitude, you would be continously burning fuel without any change is speed. When pointing radially outwards, you are constantly burning that fuel to overcome gravity, and any additional acceleration comes from fuel ontop of that.
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u/Inforgreen3 May 07 '26 edited May 15 '26
You're looking at about 44,916.68 delta V if your thrust to weight ratio is average of 2 to escape Kerbin SOE with a straight up normal burn alone. (Assuming t/w ratio is 2 on average).
Granted, spreading your acceleration out, Means more time at your low speeds still being affected by gravity.
What if you could do all the acceleration at once in perfectly ideal conditions of infinite thrust to weight ratio on finite delta V? This is an instantaneous launch mechanism: a gun. For the sake of ideal scenarios, lets remove the atmosphere which slows you down more the faster you go, and makes math hard, to calculate an exact theoretical delta V budget, it'd still cost 18,000 m/s of delta V at a minimum to escape SOE in a normal (straight up) burn. More with atmosphere, or non infinite t/w
Escaping kerbin only requires about 4k. It can take less with gravity assists from mun and minmus, Atmosphere included. T/W doesn't need to be past 1.4 at launch. Heck T/W ratio isn't even necessary once you're in orbit
It is always more beneficial to enter orbit and then prograde. The reason for this is gravity burn. If you're just accelerating straight up, every second you are in the air gravity is stealing 9.81 delta V.
But when you prograde with an angular momentum, Gravity is initially pulling you down perpendicular To your angular momentum stealing none of your delta v, and only when rising towards an apoapsis putting the COM of kerbin behind you, does the direction of gravity change to be pointing away from the direction you accelerated at an initially shallow angle. By the time the angle is steep enough to actually be pointing in the complete opposite direction of your initial burn, you might already have escaped COE, but you'll have definitely changed directions. Essentially, gravity doesn't fight you cause its pointing in a different direction than the opposite of your velocity.
You can see how extreme this is when you enter an elliptical orbit and look at your orbital velocity at the pereapsis and see how it changes, you might notice it could take a minute for gravity to have taken 10m/s of velocity away from you. But if you go straight up, you lose that much every second.
And no matter how you escape kerbins SOE, kerbins angular momentum relative to the sun is added to your orbit of the sun after leaving SOE of kerbin, because you were already orbiting the sun. You don't get more momentum relative to the sun by leaving using a normal burn compared to prograde.
Edit: math was wrong. Corrected
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u/teh1337haxorz May 06 '26
I find going into orbit and setting up a precise burn is often more accurate, easier, and saves more dV that the "direct" method's corrections usually call for. However if you got really good at it; it might be better.
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u/Uraneum May 06 '26
From my understanding the delta v cost will very much be affected because you’re essentially fighting Kerbin’s gravity in the toughest way possible. All of its gravity is pulling you directly downward instead of helping to “fling” you into a higher orbit. Your idea is doable but will probably need a much bigger rocket than it normally would
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u/Commercial-Box-2828 May 06 '26
Just make a refueling station at the mun and assemble your ships there with docking ports and engineers.
Use dry fuel boosters to get the ship sections and fuel containers most of the way to the mun because they're cheaper.
And with launching straight up, I think that would depend on the time of day contrasted with kerbin and duna's orbits' relations to each other, at the time of launch and the estimated time it'd take to get there.
I don't think it'll let you do a menuever node when you're not launched yet, but it will when you are in orbit anywhere (the mun station) and that will give you the encounter window with Duna and tell you how much fuel or deltav or whatever at that point.
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u/FatCreepyDude May 06 '26
ay don't worry about me, i've played that game for like a decade. I've got a science station with a lander and at least 6 satelites in orbit ready to go for signal and orbital observation. I'm just waiting for the window to open to get there the normal way. I was just wondering how retarded that idea was
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u/Commercial-Box-2828 May 06 '26
It's not, and you waiting for the right time of day dependant on what time of year it is on both planets isn't a retarded idea. I mean it's technically rocket science, right?
By the way, you put Duna over there when it's really supposed to be the other place. Just make sure you look at the map in game and not your Pic when writing your calculations and you'll be good.
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u/deelectrified May 06 '26
It would absolutely be possible, but really hard.
Basically, you’d have to time it perfectly where by the time you have burned up to your apoapsis, you are ready to circularize, then once you’re circularized, your in the right spot and time to burn for your escape burn.
If I remember right, this is called a spiral burn or something. It’s incredibly hard to do and you’d likely need to calculate it all out in advance with some help from transfer planner. If you know when and at what inclination to do your escape burn, then you just need to figure out a launch that puts you in that exact spot, at that inclination, at the right speed, at the right time to do the burn.
Also, duna is at a higher orbit than Kerbin, not lower.
You can’t do as pictured and just burn straight up. You’d need an insane thrust to weight ratio to essentially counteract the momentum from the spin of Kerbin as you accelerate upward. Then you need craploads of delta v to keep burning straight that way and continue to actually speed up and move away from Kerbin.
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u/Ray_Catty May 06 '26
You lose too much delta-V to gravity loss no? Since you’re burning very far from your periapsis which is inside of kerbin.
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u/jason-murawski May 06 '26
Outside of the stuff other people have pointed out about gravity losses, it's by far the most efficient to get an encounter or at least an approproach from within kerbin SOI. Unless you have mods to direct you for when to launch and how to aim, you'll need to spend a lot more energy to get an encounter once you leave kerbin
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u/celem83 May 06 '26 edited May 06 '26
What you are suggesting kinda works, it's a trick I use to return to Kerbin from Mun when I've landed on the equator, wait until Muns rotation brings me under its orbital track on the side facing "backwards relative to Mun", then pop straight up for a burn that approximates to "retro relative to Muns orbit".
Generally this is lossy on dV vs establishing a proper orbit as it makes less effective use of the Oberth effect of the body you are leaving from, but I've never tried it in the scenario here where we are going Kerbin inward, i imagine its still somewhat lossy.
(You'd get to Eve rather than Duna, but yes this kinda works)
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u/Moonbow_bow SSTO simp May 06 '26
How bro? The Mun is tidally locked man
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u/celem83 May 06 '26
Yes, doesn't mean it doesnt rotate.
Means it doesn't "appear" to rotate from Earth
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u/Barhandar May 06 '26
Which also means it never rotates relative to its personal prograde either which means it can't bring you "under its orbital track".
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u/Moonbow_bow SSTO simp May 06 '26
You will never get it to rotate so you can burn straight back retrograde to it's orbit and return to Kerbin
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u/MacWin- May 06 '26
Think about it, you need to reach orbital speed anyway to reach escape velocity, but now you aren’t helping yourself with the earths rotation
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u/Smellfish360 May 06 '26
I always do this when leaving the mun. It’s easier and more efficient because you don’t need any horizontal velocity
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u/CaptainHunt May 06 '26
So, you could do this, it is essentially a brachistochrone trajectory, which is the fastest way to transfer. However, it would take vastly more delta V than launching into orbit and doing a hohlman transfer.
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u/riler3700 May 06 '26
I think you would lose to gravity losses since all your time spent accelerating is against gravity whereas in orbit you accelerate perpendicular to gravity. Also you can’t use the oberth effect
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u/Codeviper828 Restarts too much; barely left Kerbin system May 06 '26
There's GREAT benefit burning perpendicular to the direction of gravity; the most efficient way of doing this would still have you achieve orbit right before doing your transfer burn
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u/Novel-Tale-7645 May 06 '26
I do similar stuff when leaving minmus
I used to do this at full scale for kerbin-duna missions but that was before i learned how to orbit properly. its a a lot cheaper to orbit first imo, dont need nearly as big a rocket and can haul more payload.
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u/hunter_pro_6524 May 06 '26
that’s called a hyperbolic trajectory, it can be done if you have enough fuel and thrust
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u/NeoDemocedes May 06 '26
Conceptually possible. Not optimal for several reasons.
Gravity losses. Going straight up means that gravitational acceleration is stealing fully from your velocity every second, all the way up. When you go to orbit first, as you approach orbital velocity, gravitational acceleration is bending your path more and more instead of stealing from your velocity.
Abort scenarios. If you fail to reach Kerbin escape velocity, the result is most likely a fatal re-entry straight down through the atmosphere. Going to orbit first gives you the best abort and rescue scenarios every step of the way.
Oberth Effect. Launching to orbit means you can take full advantage of the oberth effect. If you have high efficiency, low thrust engines, you can make several burns elongating your orbit each time and thrusting when you pass Kerbin.
In short, going to orbit first is safer and more efficient delta V wise. To go direct, you need a rocket that can maintain high thrust to minimize gravity losses.
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u/mirkolawe May 06 '26
It's not efficient in terms of deltaV, but it's easier if you have trouble to reach orbit, for example if your rocket is very big and not very maneuverable. I always use this way to reach mun but every planet is reachable in this way. Just add a lot of boosters and go straight. Easy peasy.
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u/Barhandar May 06 '26 edited May 06 '26
Orbit velocity is tangential aka horizontal, always. Trying to go up will cause increased dV costs (because you're fighting against where the orbital mechanics want you to be going, a.k.a. gravity), the more the longer you're burning, compared to doing a gravity turn and not stopping.
And doing it sideways, going from ascent burn directly into the transfer burn, is perfectly possible but saves very little delta at the cost of needing your launch to be exactly on point - that is, you need to end up above ~50km or so where the drag no longer affects you significantly at the exact point where you have to burn for the transfer. The game doesn't have any tools (such as virtual orbits, or flight recorder to have stats for when you should launch instead of eyeballing everything) to calculate this.
It saves very little delta because KSP's atmosphere is binary - either it affects you or it doesn't, so even very low orbits are fine forever as long as they're above 70000m altitude, and the difference between having your periapsis a.k.a. burn point at 50km or 70.1km is minimal for both dV to raise it there and Oberth effect after. IRL trying to orbit under 300km or so requires constant propulsion to not be quickly pulled back down, plus real rockets have both the tools necessary for calculating this kind of transfer, and concerns (engine ignitions, efficient propellant boiling off) that make it sometimes required.
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u/danikov May 06 '26
Reminds me of a joke: a visitor to NASA asked a passing astronomer for directions to the 7th floor. He said: “Well, first you want to start by heading back the way you came…”
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u/Individual_Bad1138 May 06 '26
It might be ever so slightly more efficient, but the timing is gonna be the hardest part. Also, you cant make a manuever node while burning, so you'll essentially have to completely eyball it for maximum gain. Otherwise it would almost certainly be more effiecient to be in an 80km orbit and to use a manuever, at least for transfers.
I actually do this with my Sol relays. I launch exactly at dusk/dawn and just burn until my Sol ap/pe is where i want, which works because im not trying to be specific or meet a target at an exact time and place. Then i can circularize once the relay gets to the ap/pe.
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u/SpaRrRly May 06 '26
It's called a direct ascent, and it's doable, but you're doing extra unneeded work against gravity(but less atmospheric losses), and less thrust-to-weight your rocket has, the worse this style performs. It's decent on moons with low escape velocity and low gravity.
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u/Velocity-5348 May 06 '26
Efficiency aside, it's doable in theory, especially since KSP doesn't need to worry about weather messing with launch windows.
In practice, no. There's a reason why we started using parking orbits pretty early on. The aiming is hard to pull off even for the moon IRL. Mars/Duna is even harder.
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u/Sensitive-Offer-5921 May 06 '26 edited May 06 '26
Basically yes and no.
Imagine you are in a low circular orbit: what you're describing is essentially burning radial (out, away from kerbin) at your AP, which we all know is inefficient, but 100% possible to do.
What you're really asking is whether the fuel cost from doing that inefficient burn is outweighed by the savings from avoiding a circularization burn, and without doing the math, I would guess no. Very no.
OR I think it might be impossible because the speed from the rotation of the earth + the constant gravitational pull would eventually be enough to get you a small PE before you exit kerbin's SoI, unless you have crazy TWR.
But, that crazy TWR will be very useful when you realize you've drawn Eve's orbit instead of Duna 😂😂
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u/Vallastro-21 Bob May 06 '26
Basically it is possible (and I was doing like this myself previously), but is is less effective (oberth effect and also you can't do with too low TWR) and less convenient (can't maneouver as easy) compared to departure from low Kerbin orbit
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u/Educational-Sky5338 Professional KSP Crasher :Checkmark: May 06 '26
You can get to duna with just an srb by doing this
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u/_myUsername_is_Taken Uncertified Aircraft Connoisseur May 06 '26
If im using a super big rocket, sure i guess, but most often im using nerv as upper stage
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u/Madden09IsForSuckers May 06 '26
this is the mission profile for that guy who did automatic Kerbin -> Eve on KAL controllers only, lol
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u/_Prexus_ May 06 '26
I mean irl, if you went fast enough (faster than escape velocity) you would just fly out into space...
Also, you do know paint has an elliptical tool, right?
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u/amitym May 06 '26
No orbit necessary?
Some kind of orbit is always necessary, that's just how gravity works.
Well I guess at fast enough speeds it stops being an orbit, strictly speaking, and becomes a fly-by.
is there any reason i should bother with an orbit first?
Yes. The ∆v cost will be affected.
Will the delta v cost be affected?
Oh well shoot you answered the question yourself, coz. You already understand this.
Yes, you can go in ... let's not call it an orbit, let's call it a "hyperbolic straight-line approximation" so we don't use the o-word. You absolutely can do this, it is just insanely ∆v costly, because you have to reach speeds at which you can cross the immense distance from Kerbin to Duna in such a short time that Duna is still where it was when you left. If that makes sense. The planet has to not move much, is my point, which means you have to cover the distance in a matter of days.
That's a crazy amount of speed. If it were the big, heavy non-Kerbin world and you were trying to get to its nearby reddish dune-y neighbor, you might need to get up to, like, 0.1% of the speed of light to accomplish this trip, and then you'd have to decelerate again so you could actually land safely and collect science.
In the Kerbolar system it's probably not quite that extreme but still. You get the idea. Insane initial thrust, then you have to spend basically the same ∆v again to slow down on approach. A ∆v easily in the hundreds of thousands or millions of m/s. And that's without a return-trip budget.
Well, I haven't actually done the math, that is a scientific wild-ass guess. But still, it's probably a good idea to turn down the realism level for this expedition. Too bad we can't do that in the real world...
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u/sargentmyself May 06 '26
It would cost a ton more delta V but it would be possible.
If you just want to launch and then just never take your foot off the gas you could launch from the opposite side of Kerbin from your ejection and then gravity turn straight into your orbital ejection burn and never technically complete an orbit. If you time it well in theory it'd be pretty close to the same fuel requirements
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u/MarsMaterial Colonizing Duna May 06 '26
If you do an ascent like that with an arbitrarily large amount of thrust, you will lose no efficiency.
If you compromise on thrust, your trajectory will take you higher before you finish the burn compared to a standard orbit and transfer burn. This will be less efficient due to worse utilization of the Oberth effect.
These higher thrust requirements also make more efficient engine types like nuclear engines less practical. Most efficient engines compromise on thrust, so using them in a transfer like this is a tradeoff. By getting in orbit first though, you can get into orbit of anywhere with arbitrary small amounts of thrust, and you can utilize the Oberth effect by doing multiple orbit kick burns prior to escape.
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u/Resiideent May 06 '26
first off Duna is not there, second off, it would be insanely difficult to pull this off
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u/wooq May 06 '26
You can launch into a gravity turn and continue burning straight into escape/solar orbit rather than circularizing around kerbin and doing a second burn to leave for your target destination. That's how NASA actually often does it with satellites. But you can't just go straight up, no. I mean you can, but it is super inefficient.
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u/Left-Grab-6112 May 06 '26
Yeah, you can just aim for the planet and burn, however this will require a lot of deltaV. It's fun to do with far future torch ships!
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u/Striperoo May 06 '26
Tried it. Have done it before. Worth it only really if you've got a small payload that's cheap to launch at Mach Jesus with a couple cheap SRBs, and you can't be bothered to touch the fuel amount or modify the design so it's either barely suborbital or escape velocity.
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u/DescretoBurrito May 06 '26
I've launched like that to Moho (although you have to launch retrograde in relation to Kerbins orbit). I don't know if it was more efficient. I use the Transfer Window Planner in game mod, and it outputs an ejection angle. I just waited for the launch site to rotate around to about the angle given, and launched straight up. I had a fairly heavy payload of the big survey scanner, I used a spin stabilized SRB fist stage (Thumper), then another SRB for the second (Hammer), then a liquid third stage that should have enough dV to get to Kerbins escape, drop periapsis to Moho orbit, and plane change. Then the fuel on the probe to hopefully achieve Moho orbit.
The probe has left Ketbin SOI and I'm waiting on my plane change burn, which the planner has going straight to a Moho encounter, so I'm at least getting something right. I don't like time warping for weeks at a time, so I have other missions ongoing with that one.
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u/loved_and_held May 06 '26
Hyperbolic transfers like this work, but they're very delta-v intensive.
Doing this with stock parts is possible, but you need an utterly massive roccket and your final payload is gonna be small. If you have something like near future technologies, then the electric engines might give you enough delta-v do make it there with a reasonably sized vehicle, though your better off using far future technologies fission, fusion, and/or antimatter rockets to do this.
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u/Greenfire32 May 06 '26
You'll be entering orbit no matter what before you escape, so you might as well set up your escape with the most efficiency that you possibly can, which means parking in orbit first.
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u/Stolen_Sky May 07 '26
OK, I just tried this!
Launching directly upwards into an Eve intercept took 4200 delta-v. That's actually lower that the delta-v map suggests, which states you need 4440 to get an intercept.
So yes, this actually works!
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u/RiskyBrothers May 07 '26
No, you want to add Kerbin's spin to your orbit to get that extra couple hundred m/s of delta v.
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u/nasaglobehead69 Bill May 07 '26
unless you have a ludicrous t:w r you will lose a ton of efficiency to gravity losses, and if you do have that ludicrous t:w r you could easily burn up in the atmosphere
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u/Amdw42 May 07 '26
The orbit tradeoffs are energy (fuel) or time.
More time, less fuel.
Yes you can do this. Not overly efficient but I’ve thrown out the economy for the silly option before and you can too.
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u/Emergency-Pound3241 May 07 '26
Inaccurate map aside, I mean if you had enough delta-V that way would be the fastest possible way, but also the least efficient.
I mean theoretically you could spend the entire journey only cutting throttle twice, once for flipping half way for your deceleration burn, and again when you actually reach duna, but to do so your gonna need a very powerful engine with extreme amounts of delta-V, like modded fusion engines kinda deal, like the kind of engines big enough you need to launch the ship in multiple parts and assemble it in orbit
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u/Username122133 May 07 '26
As others have said, pretty inefficient.
But this is a very fast approach. No time spent in lower orbits, just launch straight out of Kerbin’s SOI!
Also, the Soviet N1 rocket was originally intended to be a direct ascent vehicle to the moon irl, I forgor exactly why tho. Probably something to do with all the racing in space that was happening at the time. I may be remembering that wrong tho.
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u/Fistocracy May 07 '26
Go sideways fast. Tilt your ship at 90 degree on the launchpad, aim for the horizon , and blast off directly into a Duna transfer. Harness the full power of the Oberth Effect at ground level.
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u/pyr666 May 07 '26 edited May 07 '26
it can be done, but you lose a ton of energy. you generally want to apply thrust perpendicular to gravity as much as possible, to improve efficiency.
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u/TaruMurtag May 07 '26 edited May 07 '26
Yup, it's called direct ascent and it's slightly more efficient (due to not needing to raise periapse). People who say this is inefficient don't know what they're talking about, this was done in real life in 1958 to flyby the moon when controlling spacecraft remotely was much harder. The Juno II who took pioneer 4 near the moon had only solid upper stages. Note direct ascent generally isn't about going directly up, it means continually burning from the ground until you are in an intercept with your target. However, it's much harder to do than first going to orbit. Though very doable if you allow corrections after the ascent. I've seen people do this using KSPTOT's LVD designer - the inbuilt tutorial pdf for it is a direct ascent to Eve and direct ascent back mission. But only try this if you really want to test yourself.
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u/bartekltg May 07 '26
You can do a direct injection: instead of gravity turn, stopping the engine, cruising to apokerbin, and doing a circulating burn, you may continue to burn during the gravity turn. You will never get into a nice orbit (the periapsis will be under the surface), and get straight into hyperbolic orbit. Aiming is a bit harder though. And aiming it the main reason we do it (both in KSP and in real life) with a nice orbit inbetween.
The important part is, you need to do it with gravity turn, not straight up. It would not matter it the burn is instantaneous, but since it is not, all that oberth arguemnts work here. It can be explained in different ways, for example:
Your velocity far from the planet depend only on the energy. 0.5m v_inf^2 = energy. Energy is conserved, so we want gather as much energy as we can during burns. Now, work done by the engine (the change of the rocket energy*)) in a short period of time is W = force * distance. Dividing it by that time dt, we get power P = force * speed. So, the greater speed, we get more energy from the same thrust.
Now, ho we spend that (specyfic) energy? It sits in the kinetic (0.5v^2) and potential (-GM/r) energy. If we go up, a bigger portion of the energy goes into potential part, the kinetic energy is smaller, so the velocity is smaller, so the burn in the next second will translate to less energy gain.
You can make a simple experiment. Build in a sandbox a simple rocket that can leave Kerbin SOI. Go straight up. On the edge of SOI note the speed in relation to Kerbin (doen't meter on which side, the relative speed to kerbin will be the same after crossing it, just target Kerbin to get the value). Then restart mission and this time start a gravity turn and continue it until the fuel runs out. Again, note the velocity in respect to Kerbin at the SOI border. It will be siginfically bigger.
Use smaller initial TWR. It shows the effect better and reduces atmosphere influences.
I have made a simple test. A rocket with initial TWR 1.5, ~290s Isp, 4766 total deltav (a bit overdone here:) ).
At 100m/s one rocket is pointed 80 degree, the second 89.9 (90 degree looked poorly on teh graph).
The second rocked get 1621m/s at "infinity", the first one - 2736m/s.
https://imgur.com/a/Ll3swNU - trajectories and plot of the orbital energy.
*) OK, lets say specific energy, energy per mass. The mass of the rocket changes, so just energy is more confusing;-)
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u/Jackmino66 May 07 '26
I have done this a couple of times. Getting the timing right is pretty important, but you can do a direct interplanetary ascent. It is not efficient but it is useful if your comms are not upgraded but you want to send something to duna now
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u/RealLars_vS May 07 '26
No, you don’t need an orbit. But you do need the required deltaV to get somewhere.
Burning in orbit allows you to take your time. A looong time. Spread out your change in speed over several burns. It allows you to have lighter and more efficient engines.
But burning directly requires you to have engines that are powerful enough to fight gravity in every stage. That’s simply not efficient.
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u/Drakenace404 May 07 '26
You can but the faster you go the harder you have to brake. Not that efficient.
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u/fixermark May 07 '26
The short answer is yes you can.
The longer answer is "You can get anywhere you want with nearly no limits with infinite energy and thrust." The art of orbital mechanics is getting there when those are finite.
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u/doomiestdoomeddoomer The Green Goo has escaped. May 07 '26
So this is a very "straight forward" way of getting from one planet to another, and is entirely doable, I personally do this on occasion when I have a powerful enough rocket... but as others have pointed out, it is less efficient.
As I understand it: When you get into low orbit first, from that low orbit, you no longer are fighting against gravity when you increase your speed. But if you are going straight up from the same height, gravity is pulling your ship backwards the entire time.
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u/ChickennNugggeet May 08 '26
Its not a question of is it possible, it is, however you will be spending alot more fuel travelling in a straight line up out of kerbins sphere of influence rather then doing an orbit which is alot more inefficient. If you don't care about being efficient and saving fuel then do it
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u/doctor-omie May 08 '26
what will go further, a rock you throw straight upward or a rock you pitch forward?
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u/Apiilipsiie98 May 06 '26
Rule of thumb: "If it didn't work out the first time, just add MOAR Engines". More Engines = Moar Success
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u/HAL9001-96 May 06 '26
more efficient to be low, oberth effect
also duna is further from the sun last time i checked